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GSM
bull What is GSM
bull Evolution of GSM
bull Fundamental of GSM
bull Architecture of GSM
bull Establishment of Network
bull Fundamental of Radio Propagation
bull Call flow
History of Wireless Communication
bull 1906mdashHuman voice was successfully transmitted over RADIO
bull 1921mdashDetroit police dept used 2 Mhz frequency in vehicular mobile radio
bull 1930mdashAmplitude Modulation was inventedbull 1935---Frequency Modulation was invented (improved
audio quality)bull 1947mdashBell labs Envisions the cellular conceptsbull 1991mdashfirst digital cellular standard (GSM) is launchedbull 1998mdashNo of subscriber world wide has grown to over
200 million
GSM
bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European
Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in
three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible
ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)
ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360
billionyear worldwide
Performance characteristics of GSM
Communication mobile wireless communication support for voice and data
services
Total mobility international access Worldwide connectivity
High capacity better frequency efficiency smaller cells more customers per cell
High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone
calls at higher speeds (eg from cars trains)
Security functions access control authentication
GENERATION OF GSM
bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT
bull 2nd generation- digital mobile technologies - GSM CDMA
bull 25generation- Enhancement of GSM- GPRS
bull 3rd generation- Technologies coursed by ITU-IMT
The following table lists the key events in the GSM evolution
bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull What is GSM
bull Evolution of GSM
bull Fundamental of GSM
bull Architecture of GSM
bull Establishment of Network
bull Fundamental of Radio Propagation
bull Call flow
History of Wireless Communication
bull 1906mdashHuman voice was successfully transmitted over RADIO
bull 1921mdashDetroit police dept used 2 Mhz frequency in vehicular mobile radio
bull 1930mdashAmplitude Modulation was inventedbull 1935---Frequency Modulation was invented (improved
audio quality)bull 1947mdashBell labs Envisions the cellular conceptsbull 1991mdashfirst digital cellular standard (GSM) is launchedbull 1998mdashNo of subscriber world wide has grown to over
200 million
GSM
bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European
Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in
three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible
ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)
ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360
billionyear worldwide
Performance characteristics of GSM
Communication mobile wireless communication support for voice and data
services
Total mobility international access Worldwide connectivity
High capacity better frequency efficiency smaller cells more customers per cell
High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone
calls at higher speeds (eg from cars trains)
Security functions access control authentication
GENERATION OF GSM
bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT
bull 2nd generation- digital mobile technologies - GSM CDMA
bull 25generation- Enhancement of GSM- GPRS
bull 3rd generation- Technologies coursed by ITU-IMT
The following table lists the key events in the GSM evolution
bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
History of Wireless Communication
bull 1906mdashHuman voice was successfully transmitted over RADIO
bull 1921mdashDetroit police dept used 2 Mhz frequency in vehicular mobile radio
bull 1930mdashAmplitude Modulation was inventedbull 1935---Frequency Modulation was invented (improved
audio quality)bull 1947mdashBell labs Envisions the cellular conceptsbull 1991mdashfirst digital cellular standard (GSM) is launchedbull 1998mdashNo of subscriber world wide has grown to over
200 million
GSM
bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European
Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in
three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible
ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)
ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360
billionyear worldwide
Performance characteristics of GSM
Communication mobile wireless communication support for voice and data
services
Total mobility international access Worldwide connectivity
High capacity better frequency efficiency smaller cells more customers per cell
High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone
calls at higher speeds (eg from cars trains)
Security functions access control authentication
GENERATION OF GSM
bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT
bull 2nd generation- digital mobile technologies - GSM CDMA
bull 25generation- Enhancement of GSM- GPRS
bull 3rd generation- Technologies coursed by ITU-IMT
The following table lists the key events in the GSM evolution
bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull 1906mdashHuman voice was successfully transmitted over RADIO
bull 1921mdashDetroit police dept used 2 Mhz frequency in vehicular mobile radio
bull 1930mdashAmplitude Modulation was inventedbull 1935---Frequency Modulation was invented (improved
audio quality)bull 1947mdashBell labs Envisions the cellular conceptsbull 1991mdashfirst digital cellular standard (GSM) is launchedbull 1998mdashNo of subscriber world wide has grown to over
200 million
GSM
bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European
Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in
three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible
ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)
ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360
billionyear worldwide
Performance characteristics of GSM
Communication mobile wireless communication support for voice and data
services
Total mobility international access Worldwide connectivity
High capacity better frequency efficiency smaller cells more customers per cell
High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone
calls at higher speeds (eg from cars trains)
Security functions access control authentication
GENERATION OF GSM
bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT
bull 2nd generation- digital mobile technologies - GSM CDMA
bull 25generation- Enhancement of GSM- GPRS
bull 3rd generation- Technologies coursed by ITU-IMT
The following table lists the key events in the GSM evolution
bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM
bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European
Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in
three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible
ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)
ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360
billionyear worldwide
Performance characteristics of GSM
Communication mobile wireless communication support for voice and data
services
Total mobility international access Worldwide connectivity
High capacity better frequency efficiency smaller cells more customers per cell
High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone
calls at higher speeds (eg from cars trains)
Security functions access control authentication
GENERATION OF GSM
bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT
bull 2nd generation- digital mobile technologies - GSM CDMA
bull 25generation- Enhancement of GSM- GPRS
bull 3rd generation- Technologies coursed by ITU-IMT
The following table lists the key events in the GSM evolution
bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull GSMndash formerly Groupe Speacuteciale Mobile (founded 1982)ndash now Global System for Mobile Communicationndash Pan-European standard (ETSI European
Telecommunications Standardisation Institute)ndash simultaneous introduction of essential services in
three phases (1991 1994 1996) by the European telecommunication administrations (Germany D1 and D2) seamless roaming within Europe possible
ndash today many providers all over the world use GSM (more than 184 countries in Asia Africa Europe Australia America)
ndash more than 70 of all digital mobile phones use GSMndash over 10 billion SMS per month in Germany gt 360
billionyear worldwide
Performance characteristics of GSM
Communication mobile wireless communication support for voice and data
services
Total mobility international access Worldwide connectivity
High capacity better frequency efficiency smaller cells more customers per cell
High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone
calls at higher speeds (eg from cars trains)
Security functions access control authentication
GENERATION OF GSM
bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT
bull 2nd generation- digital mobile technologies - GSM CDMA
bull 25generation- Enhancement of GSM- GPRS
bull 3rd generation- Technologies coursed by ITU-IMT
The following table lists the key events in the GSM evolution
bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Performance characteristics of GSM
Communication mobile wireless communication support for voice and data
services
Total mobility international access Worldwide connectivity
High capacity better frequency efficiency smaller cells more customers per cell
High transmission qualityhigh audio quality and reliability for wireless uninterrupted phone
calls at higher speeds (eg from cars trains)
Security functions access control authentication
GENERATION OF GSM
bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT
bull 2nd generation- digital mobile technologies - GSM CDMA
bull 25generation- Enhancement of GSM- GPRS
bull 3rd generation- Technologies coursed by ITU-IMT
The following table lists the key events in the GSM evolution
bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GENERATION OF GSM
bull 1st generation- Analog mobile technologies - AMPS TACS amp NMT
bull 2nd generation- digital mobile technologies - GSM CDMA
bull 25generation- Enhancement of GSM- GPRS
bull 3rd generation- Technologies coursed by ITU-IMT
The following table lists the key events in the GSM evolution
bull AMPSmdashAdvanced Mobile Phone Systembull TACS----Total Access Communication Systembull NMT----Nordic Mobile Telephones
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
YEAR EVENT TOOK PLACE
1982 CEPT adopts WARC 79 recommendation allocating 890-915 MHz and 935-960 MHz for land mobile
GSM created to set standard
1985 EEC supports GSM standards throughout community
1987 Digital Technology standards set for TDMA speech coding channel and modulation method
Telecommunication carriers from 14 European countries sign Memorandum of Understanding (MOU) and agree to install
system in 1991
1988 Industrial development started
1989 Acceptance of GSM-1800 system with GSM as standard
1991 First system deployed (July)
1992 First GSM terminals receive interim type approval
1993 First GSM-1800 network launched
1995 First GSM-1900 network in US amp Canada
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM
EDGE
25 G
GPRS
2G
25 g+
UMTS 3G WCDMA
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Evolution
1G only Mobile Voice Services2G GSM middot CSD25G HSCSD middot GPRS 275G EDGEEGPRS3G UMTS 35G HSDPA middot 375G HSUPA middot 39G HSPA+ middot LTE (E-UTRA) also EV-DO4G LTE Advanced
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM Evolution for Data Access
1997 2000 2003 2003+
GSM
GPRS
EDGE
UMTS
96 kbps
115 kbps
384 kbps
2 Mbps
GSM evolution 3G
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull GPRS-Wireless Data Services bull EDGE(Enhanced Data rates for GSM Evolution)mdash
Provides 3 times the data capacity of GPRSbull 3G---(UMTS- universal mobile telecommunication
system)Uses WCDMA technologies Over Air interface (5MHz)
bull HSDPA ndash(High-Speed Downlink Packet Access) Typical downlink rates today 2 Mbits ~200 kbits uplink It creates a new channel within W-CDMA called HS-DSCH or high-speed downlink shared channel
bull HSUPA(High-Speed Uplink Packet Access )-HSUPA uses an uplink enhanced dedicated channel (E-DCH) HSPA family with up-link speeds up to 576 Mbits
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull HSPA+ (High-Speed Packet Access )- HSPA+ provides HSPA data rates up to 84 Mbits on the downlink and 22 Mbits on the uplink through the use of a multiple-antenna technique known as MIMO (for ldquomultiple-input and multiple-outputrdquo) and higher order modulation (64QAM)
bull LTE ndash (Long Term Evolution )-- The LTE specification provides downlink peak rates of at least 100 Mbps an uplink of at least 50 Mbps and RAN round-trip times of less than 10 ms LTE supports scalable carrier bandwidths from 14 MHz to 20 MHz and supports both frequency division duplexing(FDD) and time division duplexing (TDD)
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM STANDARDSGSM-900 Standard
The GSM-900 standard is a standard for digital voice transmission in the 900 MHz band This so called ldquoprimary band includes two sub bands of 25 MHz
GSM-1800 Standard
In GSM-1800 1800 MHz band was allocated for digital mobile telephone services which has frequency of 75 MHz This was three times the bandwidth allocated for GSM-900
GSM-1900 Standard
GSM-1900 is the standard for the 1900MHz band It includes the same network component as the GSM-900 or GSM-1800 The band width of this standard is 60 MHz
GSM ChannelsCarriersThe following table will show the channels and carriers of different GSM models
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
P-GSM(Primary-GSM)
900
E-GSM
900(Extended-GSM)
R-GSM
900(Railways-GSM)
GSM 1800
GSM 1900
Uplink frequency 890-915 MHz
880-915 MHz
886-915 MHz
1710-1785 MHz
1850-1910 MHz
Down link frequency
935-960 MHz
925-960 MHz
931-960 MHz
1805-1855 MHz
1930-1990 MHz
Channel spacing 200 kHz 200 kHz 200 kHz 200 kHz
200 kHz
Carrier Frequency
124 174 144 374 299
Duplex spacing 45 MHZ 45 MHZ 45 MHZ 95 MHZ
80 MHZ
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Spectrum
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM uses paired radio channels
0 124 0 124
890MHz 915MHz 935MHz 960MHz
UPLINK
DOWNLINK
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Technology
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull FDMA (Frequency division Multiple Access)
bull TDMA (Time division Multiple Access)
FDMA (Frequency division Multiple Access) In FDMA signals from various
users are assigned different frequencies Frequency guard bands are maintained between adjacent signal spectra to minimize crosstalk between channels
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Frequency multiplex
bull Separation of the whole spectrum into smaller frequency bandsbull A channel gets a certain band of the
spectrum for the whole timebull Advantages
ndash no dynamic coordination necessary
ndash works also for analog signalsbull Disadvantages
ndash waste of bandwidth if the traffic is distributed unevenly
ndash inflexiblendash guard spaces
k2 k3 k4 k5 k6k1
f
t
c
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
TDMA (TIME DIVISION MULTIPLE ACCESS)
In a TDMA system data from each user is conveyed in time
intervals called ldquoTime slotsrdquo Several slots make up a frame Each slot is made up of a preamble plus information bits addressed to various stations the functions of the preamble are to provide identification and incidental information and to allow synchronization of the slot at the intended receiver Guard times are used between each userrsquos transmission to minimize crosstalk between channels
Advantage of TDMA
1) TDMA permits a flexible bit rate
2) TDMA offers the opportunity for frame-by-frame monitoring of the signal strength and bit error rates
3) TDMA transmits each signal with sufficient guard time between time slots
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
f
t
c
k2 k3 k4 k5 k6k1
Time multiplexbull A channel gets the whole spectrum for a certain
amount of timebull Advantages
ndash only one carrier in themedium at any time
ndash throughput high even for many users
bull Disadvantagesndash precise
synchronization necessary
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
f
Time and Frequency Multiplex
bull Combination of both methodsbull A channel gets a certain frequency band for a certain
amount of time
t
c
k2 k3 k4 k5 k6k1
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
f
Time and Frequency Multiplexbull Example GSM bull Advantages
ndash Better protection against tapping
ndash Protection against frequency selective interference
ndash Higher data rates compared tocode multiplex
bull But precise coordinationrequired
t
c
k2 k3 k4 k5 k6k1
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Various Access Method
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
CellsCells is the basic geographic unit of cellular systemA group of cells is called a clusterThere are four different cell sizes in a GSM networkmdashbullmacro Macro cells can be regarded as cells where the base station antena is installed on a mast or a building above average roof top level bull micro - Micro cells are cells whose antenna height is under average roof top level they are typically used in urban areas bull pico - Picocells are small cells whose coverage diameter is a few dozen metres they are mainly used indoors bull femto - Femtocells are cells designed for use in residential or small business environments
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull Cell horizontal radius varies depending on
bull antenna height
bull antenna gain
bull geographical conditions(landscape)
bull propagation conditions from a couple of hundred meters to several tens of kilometers
bull The longest distance the GSM specification supports in practical use is 35 kilometers (22 mi)
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Representation of Cells
Ideal cells Fictitious cells
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Cell size and capacity
bull Cell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all users
bull Capacity within cell limited by available bandwidth and operational requirements
bull Each network operator has to size cells to handle expected traffic demand
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Cell structurebull Implements space division multiplex base station covers a certain
transmission area (cell)bull Mobile stations communicate only via the base stationbull Advantages of cell structures
ndash higher capacity higher number of usersndash less transmission power neededndash more robust decentralizedndash base station deals with interference transmission area etc
locallybull Problems
ndash fixed network needed for the base stationsndash handover (changing from one cell to another) necessaryndash interference with other cells
bull Cell sizes from some 100 m in cities to eg 35 km on the country side (GSM) - even less for higher frequencies
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
capacity
Capacity within cell limited by bull available bandwidth bull operational requirementsbull Each network operator has to size cells to
handle expected traffic demand
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Sectorizationbull The problem with employing omni-directional cells (Radiate waves
to 360 degrees) is that as the number of MSs increases in the same geographical region we have to increase the number of cells to meet the demand
bull To gain a further increase in capacity within the geographic area we can employ a technique called ldquosectorizationrdquo Sectorization splits a single site into a number of cellseach cell has transmit and receive antennas and behaves as an independent cell
bull This has a number of advantages firstly asbull we are now concentrating all the energy from the cell in a smaller
area 60 120 180 degrees instead of 360 degrees we get a much stronger signal which is beneficial in locations such as ldquoin-building coveragerdquo
bull Secondly we can now use the same frequencies in a much closer re-use pattern thus allowing more cells in our geographic region which allows us to support more MSs
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM IdentifiersIMEI ndash International Mobile Equipment IdentifierThe IMEI is an internationally-unique serial number allocated to the MShardware at the time of manufacture It is a kind of serial number It is registered by the network operator and (optionally) stored in the AuC for validation purposes Thus IMEI = TAC(6) + FAC (2)+ SNR(6) + SP(1) It uniquely characterizes a mobile station and gives clues about the manufacturer and the date of manufacturing(15-17) SP-SpareTAC-Type Approval Code FAC-Final Assembly Code SNR-Serial Number IMSI ndash International Mobile Subscriber IdentifierWhen a subscriber registers with a network operator a unique subscriberIMSI identifier is issued and stored in the SIM of the MS An MS can onlyfunction fully if it is operated with a valid SIM inserted into an MS with avalid IMEI IMSI(MCC) 3 +(MNC) 2 +(MSIN) Maximum 10
TMSI ndashTemporary Mobile Subscriber IdentityA TMSI is used to protect the true identity (IMSI) of a subscriber It is issuedby and stored within a VLR (not in the HLR) when an IMSI attach takes placeor a Location Area (LA) update takes place At the MS it is stored in the MSrsquosSIM The issued TMSI only has validity within a specific LA it can consist of up to 4x8 bits
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
MSISDN ndash Mobile Subscriber ISDN number
The MSISDN represents the lsquotruersquo or lsquodialledrsquo number associated with the subscriber It is assigned to the subscriber by the network operator at registration and is stored in the SIM It is possible for an MS to hold multiple MSISDNs each associated with a different serviceMSIDN =Country Code (CC) Up to 3 decimal places + National destination code(NDC) typically 2-3 decimal + subscriber number(SN) maximum 10 decimal places
MSRN ndash Mobile Station Roaming NumberThe MSRN is a temporary location-dependant ISDN number issued by the parent VLR to all MSs within its area of responsibility It is stored in the VLR and associated HLR but not in the MS The MSRN is used by the VLR associated MSC for call routing within the MSCVLR service area
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
LAI ndash Location Area IdentityEach Location Area within the PLMN has an associated internationally unique identifier (LAI) The LAI is broadcast regularly by BTSs on the Broadcast Control Channel (BCCH) thus uniquely identifying each cell with a in LA (CC) 3 +(MNC) 2 +(LAC) maximum 5 decimal
placesCI ndash Cell IdentifierThe CI an identifier assigned to each cell within a network However the CI is only unique within a specific Location Area When combined with the internationally unique LAI for its associated LA the Global Cell Identity (GCI) is produced which is also internationally unique
BSIC ndash Base Station Identity CodeEach BTS is issued with a unique identity the BSIC and is used to distinguish neighbouring BTSs
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM Architecture
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM Architecture
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Mobile Station MOBILE STATION (MS) - The mobile station (MS)
represents the terminal equipment used by the wireless subscriber supported by the GSM Wireless system Man machine interface Each MS is identified by an IMEI
The SIM may be a removable module while the equipment identity is not linked to a particular subscriber
Functions of a Mobile Station -bull Radio transmission termination bull Radio Channel Managementbull Speech EncodingDecodingbull Radio Link error Protectionbull Flow control of databull Mobility Managementbull Monitors the Power level and signal quantitybull Ms includes an equalizer that compensates for
multi-path distortion on the received signal
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Types of MS
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
SIMbull It is basically
ndash a removable smart card in compliance with the ISO 7816 standard
ndash a plug-in module (25 x 15 mm)ndash Micro SIM for apple I-Pod
bull It includes a microprocessor with all the subscriber-related information
bull IMSI MSISDN Ki Kcbull SIM (and consequently MS) is protected by a Personal
Identification Number (PIN)bull It has a PIN Unblocking Key (PUK) used to unblock it
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Information stored in a SIM card (1)
bull Serial numberbull International Mobile Subscriber Identity (IMSI)bull Security authentication and cyphering
information ndash A3 and A8 algorithmndash Ki Kc
bull Temporary Network information (LAI TMSI)bull List of services subscribed by the userbull Personal Identity Number (PIN)bull Personal Unblocking Number (PUK)
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
BASE STATION SYSTEM (BSS) - (BSC+BTS)The BSS consists of three major hardware components
1) The Base Transceiver Station ndash BTS
The BTS contains the RF components that provide the air interface for a particular cell This is the part of the GSM network which communicates with the MS The antenna is included as part of the BTS
2) The Base Station Controller ndash BSC
The BSC as its name implies provides the control for the BSS The BSCcommunicates directly with the MSC The BSC may control single or multipleBTSs
3) The Transcoder ndash XCDR
The Transcoder (XCDR) is required to convert the speech or data output from the MSC(64 kbits PCM) into the form specified by GSM specifications for transmission over
theair interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa)
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
BSS
bull BSS includes the network elements taking care of the radio cellular resources within the GSM network
bull On one side it is directly linked to the MSs through the radio interface (Air interface)
bull On the other side it is interconnected with the switches of the NSSndash its role consists in connecting MS and NSS and
hence in connecting the caller to the other usersbull It is controlled by the NMS (or OSS)
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Characteristics of the Base Station System (BSS) are
The BSS is responsible for communicating with mobile stations in cell areas
One BSC controls one or more BTSrsquos and can perform inter-BTS and intra-BTS
handover
- Each PLMN Should have at least one HLR
- One VLR is connected to several MSC but one MSC has one VLR
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
BSS
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
BTS Scheme
helliphellip
Combiner
BSC
A-bis Interface
PCM line or Radio system 2 Mbs
TXTX RX
Signal Processi
ngTRXControll
erTRX
one TX antenn
aSplitter
TXTX RX
Signal Processin
gTRXControll
erTRX
two RX antenna
s
helliphelliphelliphelliphellip
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
BTSbull BTS is a network element with transmission
and reception devices (transceivers) to and from the MS includingndash antennasndash signal processing specific devices for the Air
interface managementbull It can be considered as a complex radio
modem controlled by the BSCbull It is involved also in the transmission and
reception with the BSC through the A-bis interface
bull It has just executive functions (no management)
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
BTS Functionsbull Broadcastreceive tofrom the MS either
signalling and traffic signalsbull Perform source and channel codingbull ModulateDemodulate signals to be
broadcastedreceived through the Air interface radio channel
bull Multiplex the information to be transmitted over each carrier
bull Measure the quality of the signalling and traffic signals in the downlink and uplink channels
bull Transmitreceive signalling and traffic signals tofrom the BSC through the A-bis interface
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
BSC Functionsbull Control and supervise the BTSsbull Configure each cell with the allocation and
the release of traffic and signalling channelsbull Manage the paging operationbull Collect the signals quality measures acquired
by the BTSs over the downlink and uplink channels
bull Manage all the radio interfacesbull Manage the handover proceduresbull Transcode and Sub-multiplex the bit streambull Operate and sustain the whole BSS
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
TranscoderThe Transcoder (XCDR) is required to convert the speech or data output from the MSC (64 kbps PCM) into the form specified by GSM specifications for transmission over the air interface that is between the BSS and MS (64 kbits to 16 kbits and vice versa) Transcoder is a device that takes 13 Kbps speech or 36612 kbps data multiplexes and four of them to convert into standard 64 kbps data First the 13 kbps or 36612 are brought up to 16 kbps by inserting synchronizing data to make up difference between 13 kbps speech or lower rate data and four of them are combined in transcoder to provide 64 kbps channel with in the BSSbull This conversion from 13kbps to 64kbps is necessary as MSC
[exchange] understands only 64kbps
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
NSS Functions (1)
bull Call controlndash identification of the subscriberndash establishing a call and release of the connection after
the call is over
bull Mobility management ndash taking care of the location of the subscribers before
during and after a call
bull Collecting the charging information about a callndash number of the caller and of the called subscriberndash length and type of the provided servicesndash hellip
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
NSS Functions (2)
bull Transfer the acquired charging information to the
Billing centre
bull Signalling with other networks and BSS through
the different interfaces
bull Subscriber data handling
ndash Data storage permanently or temporarily in some
databases
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
MOBILE SWITCHING CENTER (MSC) -
Mobile services switching center (MSC)
performs the switching functions for all mobile stations located in the geographical area covered by its assigned BSSs
Functions of the MSC include -bull Management of MSC-BSS signaling Protocolbull Handling location registration and ensuring internetworking between mobile station
and VLRbull Call handling that copes with mobile nature of subscribersbull Exchange of signaling information with other system entities
HOME LOCATION REGISTER (HLR) -
The Home Location Register (HLR) contains
the identities of mobile subscribers their service parameters and their location information
Function of the HLR include-bull Identity of mobile subscriber bull Location information for call routingbull ISDN directory number of mobile station
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
VISITOR LOCATION REGISTER (VLR) -
The Visitor Location Register (VLR) contains the subscriber parameters and location information for all mobile subscribers currently located in the geographical area controlled by that VLR
Functions of the VLR include -
bull Identity of mobile subscriber
bull Any temporary mobile subscriber identity (TMSI) allocation
bull Location area where the mobile station is registered
bull Copy of the subscriber data from the HLR
AUTHENTICATION CENTER (AUC) -
The Authentication center (AUC)
bull Contains subscriber authentication data called authentication key (Ki)
bull Generates security related parameters needed to authorize service using Ki
bull Generates unique data pattern called a cipher key (Kc) needed for encrypting user speech and data
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
AUC Contdhellip
AUC sends RAND to MS AUC amp MS do some calculation based on RAND secret key of SIM amp A3 algorithm amp generate a number
If result sent by MS is same as AUC Then MS is authenticated
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
EQUIPMENT IDENTITY REGISTER (EIR) - The Equipment Identity Register (EIR) is
accessed during the equipment validation procedure when a mobile station accesses the system It contains the identity of mobile station equipment which may be valid suspect or known to be fraudulent
This contains bull White or Valid list ndash list of valid MS equipment identitiesbull Grey or Monitored list ndash list of suspected mobiles under
observationbull Black or prohibited list ndash list of mobiles for which service is barred
OPERATION AND MAINTENANCE CENTER (OMC) - The Operation and maintenance
Center (OMC) is the centralized maintenance and diagnostic heart of the base station system (BSS) It allows the network provider to operate administer and monitor the functioning of the BSS
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
IWF
bull The aim of the Inter Working Function [IWF] is to provide the interfacing capability with the various public and private data networks currently available because GSM providing wide range of data services to its subscribers Networks include
bull PSTN bull ISDN bull Circuit Switched Public Data Networks [CSPDN] bull Packet Switched Public Data Networks [PSPDN]
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
OTHER NETWORK ELEMENTS
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Other optional network elements that the MSC can interface include
Operations and Maintenance Support
Includes database management traffic metering and measurementmanndashmachine interface
Internetwork Interworking
Manages the interface between the GSM network and the PSTN
Echo Cancellor
An EC is used on the PSTN side of the MSC for all voice circuits Echo control isrequired at the switch because the inherent GSM system delay can cause anunacceptable echo condition even on short distance PSTN circuit connections
Billing Center and Service Center
Gateway MSC
The GMSC routes calls out of the network and is the point of access for calls entering the network from outsideGMSC--------PSTN+ISDN (other NW)
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
CHANNELS
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Traffic Channels (TCH)bull One physical channel (1 timeslot) can supportbull 1 TCHF or 2 TCHHbull TCHF 13 kbs voice or 96 kbs databull TCHH 65 kbs voice or 48 kbs databull Uplink Downlink SynchronisationThe MS transmit burst is delayed by 3 timeslots after the BTS burst This delay allows enables Use of the same UL and DL timeslot number in TDMA frame Avoids simultaneous TxRx requirement
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Broadcast Channels (BCH)bull BCH channels are all downlink and are allocated to timeslot zero BCH
channels includeFCCH Frequency control channel sends the MS a burst of all lsquo0rsquo bits which acts as a beacon and allows MS to fine tune to the downlink frequency and time synchroniseSCH Synchronisation channel enables TDMA-Frame number synchronisation by sending the absolute value of the frame number (FN) together with the
BTSrsquos BSICBCCH
Broadcast Control Channel sends network-specific information such as radio resource management and control messages Location Area Code etcBCH channels are all downlink and are allocated totimeslot zero
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Common Control Channels (CCCH)CCCH contains all point to multi-point downlink channels (BTS toseveral MSs) and the uplink Random Access ChannelRACH Random Access Channel is sent by the MS to request a resources from the network eg an SDCCH channel for call setupbull AGCH Access Grant Channel is used to allocate a dedicated channel (SDCCH) to the mobilebull PCH Paging Channel sends paging signal to inform mobile of a callbull CBCH Cell Broadcast Channel is an optional GSM Phase II implementations for SMS broadcast messages for example road traffic reports or network engineering messagesbull NCH Used for GSM Phase II voice services such as Voice BroadcastService (VBS) or Voice Group Calling Service (VGCS)
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Dedicated Control Channels (DCCH)
DCCH comprise the following bi-directional (uplink downlink) point to point control channelsbull SDCCH Standalone Dedicated CHannel is used for callset up location updating and also SMSbull SACCH Slow Associated Control CHannel is used for link measurements and signalling during a callbull FACCH Fast Associated Control CHannel is used (whenneeded) for signalling during a call mainly for deliveringhandover messages and for acknowledgement when aTCH is assigned
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
NMC
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
INTERFACE
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Um INTERFACE (MS- BTS)
The interface between the MS and the BSS
A-BIS INTERFACE (BSC - BTS)
When the BSS consists of a Base Station Controller
(BSC) and one or more Base Transceiver Stations
(BTS) this interface is used between the BSC and
BTS to support the services offered to the GSM
users and subscribers The interface also allows
control of the radio equipment and radio frequency
allocation in the BTS
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
B INTERFACE (MSC - VLR)
The VLR is the location and management data base for the mobile subscribersroaming in the area controlled by the associated MSC(s) Whenever the MSCneeds data related to a given mobile station currently located in its area itinterrogates the VLR When a mobile station initiates a location updatingprocedure with an MSC the MSC informs its VLR which stores the relevantinformation This procedure occurs whenever an MS roams to anotherlocation area Also when a subscriber activates a specific supplementaryservice or modifies some data attached to a service the MSC informs (via theVLR) the HLR which stores these modifications and updates the VLR ifrequired
C INTERFACE (HLR and the MSC)
The Gateway MSC must interrogate the HLR of the required subscriber toobtain routing information for a call or a short message directed to thatsubscriber
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
D INTERFACE (HLR - VLR)
This interface is used to exchange the data related to the location of the mobilestation and to the management of the subscriber The main service provided to the mobile subscriber is the capability to set up or to receive calls within the whole service area To support this the location registers have to exchange data The VLR informs the HLR of the location of a mobile station managed by the latter and provides it (either at location updating or at call set-up) with the roaming number of that station The HLR sends to the VLR all the data needed to support the service to the mobile subscriber The HLR then instructs the previous VLR to cancel the location registration of this subscriber Exchanges of data may occur when the mobile subscriber requires a particular service when he wants to change some data attached to his subscription or when some parameters of the subscription are modified by administrative means
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
E INTERFACE (MSC ndash MSC)
When a mobile station moves from one MSC area to another during a call ahandover procedure has to be performed in order to continue thecommunication For that purpose the MSCs have to exchange data to initiateand then to realize the operation After the handover operation has beencompleted the MSCs will exchange information to transfer A-interfacesignalling as necessary When a short message is to be transferred between aMobile Station and Short Message Service Centre (SC) in either direction thisinterface is used to transfer the message between the MSC serving the MobileStation and the MSC which acts as the interface to the SC
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
F INTERFACE (MSC and EIR)
This interface is used between MSC and EIR to exchange data in order that the EIR can verify the status of the IMEI retrieved from the Mobile Station
G INTERFACE (VLR ndash VLR)
When a mobile subscriber moves from a VLR area to another LocationRegistration procedure will happen This procedure may include the retrieval of the IMSI and authentication parameters from the old VLR
H INTERFACE (HLR - AuC)
When an HLR receives a request for authentication and ciphering data for a Mobile Subscriber and it does not hold the requested data the HLR requests the data from the AuC The protocol used to transfer the data over this interface is not standardized
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM Voice amp Channel Coding
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Here we will consider two forms of coding
techniques used within the GSM system
Firstly the process used to convert human
speech into a digital equivalent and
Secondly the coding processes for
compressing and protecting the data for
transmission over the air interface
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
GSM Voice amp Channel Coding Sequence
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Speech Coding
bull GSM transmits using digital modulation - speech must be
converted to binary digits Coder and decoder must work to
the same standard Simplest coding scheme is Pulse Code
Modulation (PCM)
bull Sampling every 125 μs
bull Requires data rate of 64 kbps
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull Coding is added to the information bits (redundancy) which enable the original to be reconstructed given a small number of random errors
bull Convolution Coding Rate of coding describes the amount of redundancy in the coded databull 12 rate code transmits twice as many bits as actual databull Data rate is halvedbull Convolutional codes cannot detect errors
The efficiency of the convolutional coding described above is based onAn assumption that errors will be randomly distributed However radiotransmission paths tend to be prone to frequency-dependent lsquoburstyrsquo type errors due primarily to fading
Inter leaving
So the data bursts are not sent in their natural order but areinterleaved among a set of timeslots within the multiframe
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Antenna typesDirectional antennas are useful for covering long streets and have the following advantages -Extra gain in the forward direction -Suppressed signal in the reverse direction this is a useful characteristic if the cell is a potential interferer with another cell located behind it-It is also worth noting that a directional antenna could be used to improve in-building coverage in specific buildings within the microcell areaOmni antennas are useful for covering open areas (for example squares plazas) In these areas it is desirable to have a clearly designated lsquobest serverrsquo cell to avoid excessive handovers and their attendant problems
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
HANDOFF
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
bull Procedure in which an MS releases a connection with a BTS and establishes a connection with a new BTS while ensuring that the ongoing call is maintained
Needs two mechanisms
1048790 Handover preparation detection of cell-border crossing Based on radio link quality measurements
1048790 Handover execution setup of a new channel in a cell and tear-down of a previous channel
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Types of Handoff
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
042123 101
IV Handover
bull radio links are not permanently allocated for the duration of a call
bull Handover (handoff) switching of on-going call to a different channel
bull Radio Resource layer performs execution and measurements required for
handover
bull 4 types of handover in the GSM system
1 transferring a call between channels (time slots) in the same cell
2 transferring a call between cells (BTS) under the same BSC
3 transferring a call between BTS under different BSCs but same MSC
4 transferring a call between Cells under the control of different MSCs
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Hard amp Soft handover
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
042123 103
internal handovers involve only one BSC
bull managed by the BSC without involving the MSC for efficiency
bull MSC is notified at the completion of the handover
external handovers are handled by the MSC(s) involved
bull original MSC (anchor) remains responsible for most call-related functions
bull new MSC (relay) handles subsequent inter-BSC handovers
MSC
BSC BSCMSC
BSC BSC
23 4
1
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Call Flow
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
CALL MANAGEMENT
The Following Steps Took Place while a Mobile call is originated and Terminated-
bull Mobile-to-Land Scenario
bull Land-to-Mobile Scenario
bull Mobile-to-Mobile Scenario
Mobile-to-Land CALL Scenario
The following table lists the phases of a Mobile-to-Land call
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
STAGE DESCRIPTION
1 Request for services the MS requests to setup a call
2 Authentication the MSCVLR requests the AUC for Authentication parameters Using these parameters the MS is Authenticated
3 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
4Equipment Validation the MSCVLR requests the EIR to check the IMEI for validity
5 Call Setup the MSC establishes a connection to the MS
6 Handover (s)
7 Call release the speech path is released
Mobile-to-Land CALL ScenarioMobile-to-Land CALL Scenario
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Land-to-Mobile CALL Scenario
The following table lists the phases of a Land-to-Mobile CALL Scenario
STAGE Description1 Routing Analysis the MS terminated call is routed to the visited MSC using information from
the HLR and VLR
2 Paging the MSC initiates a communication with the MS
3 Authentication the MSCVLR requests the AUC for authentication parameters Using these parameters the MS is authenticated
4 Ciphering using the parameters which were made available earlier during the authentication the uplink and the downlink are ciphered
5 Equipment Validation the MSCVLR requests the EIR to check the IMEI for validation
6 Call setup the MSC establishes a connection to the MS
7 Handover (s)
8 Call release the speech path is released
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Mobile-to-Mobile Scenario
Phases of a Mobile-to-Mobile Call
The Mobile-to-Mobile call phases can be sub divided in two parts
1 The Originating Mobile
2 The Terminating Mobile
ORIGINATING MOBILE
The phases of originating mobile arebull Request for services
bull Authentication (optional)
bull Ciphering (optional)
bull Call setup
bull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
Mobile-to-Mobile Scenario
TERMINATNG MOBILE
The phases of originating mobile arebull Routing analysisbull Pagingbull Authentication (optional)bull Ciphering (optional)bull Equipment validation (optional)bull Call setupbull Release
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