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Mobile infocommunication systems

Mobile infocommunication

networks

- GSM/GPRS system –

- GSM/GPRS radio interface basics

www.hit.bme.hu

Fazekas Péter, PhD.

BME Dept. of Networked Systems and Services

Voice telephony basics

Setting the scene

end of 1980’s, beginning of 1990’s

voice telephony -> transformed to digital telephony

networks

– ISDN: integrated services digital network: defined the

digital end terminal (phone) and full digital network

GSM stepped in to extend ISDN to mobile environment

started in the beginning of 1990’s

Hálózati Rendszerek és Szolgáltatások Tanszék Budapesti Műszaki és Gazdaságtudományi Egyetem 3

Voice telephony

core of the network: telephony exchange offices, or

telephony exchanges

– large, high capacity switching centres capable of switching

hundreds of thousands of connections

PBX Private Branch Exchange: local exchange centers,

to interconnect the local telephony of a company, or a

building

signalling: SS7 (Signalling System Seven)

– the flow of messages and set of rules how the exchange

centres communicate to each other

– the goal is to set up a fix, permanent connection between

any two endpoint of the network, based on the called party

telephone number

Hálózati Rendszerek és Szolgáltatások Tanszék

Budapesti Műszaki és Gazdaságtudományi Egyetem 4

SS7 Signalling example

e.g. setting up a call. Messages like: IAM (Initial address message: the number of the

called; CPG: call in progress)

Network behind voice telephony

high capacity voice trunks, carrying a lot of calls together

switch/route the trunks

multiplexing / de-multiplexing groups of calls

finally to interconnect all exchanges

carry voice traffic and signalling


TDM

Plesiochronous Digital Hierarchy

– for digital telephony trunk lines

– designed together with SS7 and to carry 64 kbps voice

channels

– ISDN PRI: 30 voice channels + signaling : 2048 kbps

• E1

• in telecommunications: n*E1 capcities

TDM

Time division Multiplexing

– multiplexing: 1 byte voice sample for each connection

– + 1 byte link control, 1 byte signaling

TDM

SDH: Synchronous Digital Hierarchy

– high capacity core network technology, mainly over

optical fibers

– slight difference in the US

• SONET

can carry in „containers” the former E carriers

often implemented over microwave wireless point-point

links

TDM

basic bitrate is: 155 Mbps, STM1

– ez a szabványban definiáltan számos

kapcsolattípus összefogásával érhető el

other rates are: STM4: 622 Mbps; STM16:

2.5 Gbps; STM64: 10 Gbps, STM256: 40

Gbps

in all cases: a frame is 1/8000 sec

– a byte in a frame: a voice call

– a timeslot in the transmission link

TDM

• hierarchically built networks

• fix n*E1 capacities between points

TDM

• simplifed view • usually topology

is such that anything (except the last segments) can be reached via multiple routes

• more local loops/endpoints are aggregated at higher levels of TDM hierarchy

•

TDM

2G GSM networks

MS

PSTN

B S C

B T S

MS

B S C

B T S B S C

BTS

MS

MSC/VLR

IWF

MSC/VLR

IWF

AuC EIR

HLR

GSM network

GSM network

GSM system

GSM network consists of four subsystems

– Mobile Station (MS),

– Base Station Subsystem (BSS)

– Network Switching Subsystem (NSS)

– Operation SubSystem (OSS).

The main interfaces between the network elements are

– Um: radio interface (MS–BSS),

– Abis interface (inside BSS: BTS–BSC)

– A interface (BSS–MSC).

Mobile Station MS:

– it is a subsistem, containing the SIM card (Subscriber Identity Module) and

– Mobile Equipment ME: the equipment itself

BSS, Base Station Subsystem

this is the access network, containing

– devices for allowing mobile terminals to reach the network

– devices for controlling the radio network

three main functinal entities

– Base Transceiver Station (BTS),

– Base Station Controller (BSC)

– TransCoder (TC), often: Transcoding Unit


BTS: Base Transceiver Station

base stations are connected directly to the mobile stations, over the radio interface

– physical layer signal processing

main functions:

– channel coding and decoding: error correction

– interleaving and de-interleaving: mixing data

– encryption and decryption of voice streams

– radio processing

– measurement of channel quality

in summary: the digital voice stream is transformed to/from radio

waves by the BTS


BSC, Base Station Controller

main functions:

– configure and control the radio channel for all base stations and cells

under its control

– connection point towards the NSS

it controls and monitors

– allocation of radio channels (timeslots) for all user terminals, control and

traffic channels

– quality and strength of traffic channels

– finding the subscriber within its area (paging)

– handover of calls between cells

in summary: management of radio resources (channels) and

interconnection towards NSS

– the mobile phone’s signalling is connected to the BSC

GSM system


Transcoding and Rate Adaptation Unit (TRAU)

creating the voice stream of the radio interface (13 kbps) from the traditional telephony 64 kbps voice

adapt the bitrate of CS data services of GSM to/from ISDN data channels

originally: part of base station

– „normal” telephone trunk lines could be connected to the BTS

– this is TFO (Tandem Free Operation)

advanced solution

– to install TRAU „between” MSC and BSC

– lower bitrate voice streams could be carried in large part of the network

– extra computation

– extension of ISDN voice transmission: 16 kbps data channels

Transport network in GSM

TDM (Time Division Multiplex) networks (interfaces)

– PDH (Plesiochronous Digital Hierarchy)

– later SDH (Synchronous Digital Hierarchy)

– over microwave, copper or optical fiber

high bitrate, robust networking for telephony operators

TDM: voice calls are represented by sending a byte in a timeslot over the link

– a timeslot is periodically allocated for a given connection, end to end, for the whole duration of the call

signalling: SS7, with MAP (Mobile Application Part)

– special signalling for mobile network

GSM Network and Switching Subsystem

main role is to switch and maintain:

– voice connections between MEs in the network

– ME in the network and outside

two main parts are:

– switching centres (exchanges)

– data registers

auxiliary part for handling extra features

– IN Intelligent Network

– IT infrastructure for advanced services


MSC: Mobile Switching Centre

MSC is basically a telephone exchange

– enables setting up, switching, maintaining of voice calls

– registration, authentication, location updating, handovers, and

call routing to roaming subscriber

– collecting billing records

– value added services, controlled by IN

• call barring, forwarding, announcements,

Main difference between an MSC and an exchange is

– authentication

– MSC can find the subscriber in case of mobile terminated call


usually there are several MSCs in the NSS

some of them are configured to be Gateway MSC

find roaming customer and route call towards visited network

route call from outside network

Subscriber and terminal equipment databases

Home Location Register (HLR)

– contain permanent information of all subscribers of the

network

– their identity, number

– services the user can access

– contain location information

• at least: in which MSC/VLR more accurate position info

can be found



HLR

– if a mobile attaches to the network, the system checks its available services

– if there’s an incoming call, the (G)MSC queries the HLR where to route the call

– Internationa Mobile Subscriber Identity

– Mobile Station ISDN Number

Visitor Location Register VLR

– it is a separate functional entity, but in practice it is integrated within the MSC

– temporary storage (in memory) of subscriber data

– knowledge of location are of registered users

– Temporary Mobile Subscriber Identity


AuC Authentication Center

– It contains authentication and encryption keys for subscribers

– It runs the authentication algorithms to check the identity of the SIM card

– authentication may be required when registering to the networks or

– when initiating a call or

– when activating a service


EIR Equipment Identity Register

– contain information about the mobile devices

• not the subscribers

– International Mobile Equipment Identity number

– to track malfunctioning, or stolen devices


SMSC Short Message Service Center

– storing and forwarding device for SMS messages

• actually MSCs switch it

– not standardized in all details

– some implementations distinguish between centres for

accepting SMS from outside and for sending

– SMSC should be able to query the HLR whether the

mobile is available

• store the message if not

• detect if it becomes attached again

Operation Support Subsystem

OSS enables the operator to supervise and monitor the network

Telecommunications Management Network functions:

– collecting data of and managing:

– errors, configurations, performance, security, etc.

According to standard TMN concepts, the OSS is

– connected to all network elements

– provide machine-human interface for the operator

• this is the OMC or NOC

OSS measures user quaity parameters, such as dropped calls, failed handovers, etc.

To some extent remote intervention (e.g. restart) is possible through the OSS

Operation Support Subsystem

Operation and Maintenance Center OMC, Network Operation Center NOC

– this is the central office for network supervision

– alerts are shown, collected, managed here

Intelligent Network in GSM

ITU recommendation, for implementing value added services

– for telecomm networks in general, not just GSM

can be implemented in MSCs or in combination

IT infrastructure and logics, close interaction with MSCs

– televoting, telephone number portability, toll free calls, prepaid calling, virtual private networks (such as family group calling), virtual private branch excange

– mass-calling service, reverse charging, call-distribution, etc.

later evolved into TAS (Telecommunication Application Server) and, or application servers of the IMS

GSM air interface

GSM radio basics

medium access: TDMA/FDMA/FDD

FDMA: 200 kHz wide band around a carrier

frequency

TDMA: frame structure based on 8 timeslots /

frame

– a connection has a timeslot (out of 8) in both

directions

FDD: uplink and downlink communications are

in two separate frequency bands

GSM frequency

channels

UPLINK.

[MHz]

DOWNLNK

[MHz]

Number of

channels

GSM 450 450.4 -

457.6

or

478.8 -

486

460.4 -

467.6

or

488.8 -

496

35

P-GSM900 890-

915

935-

960

124

DCS1800

(GSM 1800)

1710-

1785

1805-

1880

374

E-GSM 880-

915

925-

960

174

R-GSM900 876-

915

921-

960

194

GSM 1900

(USA,

Canada)

1850-

1910

1930-

1990

299

GSM 850

(USA,

Canada)

824 - 849 869 - 894 124

GSM radio basics

Frame structure:

– 1 frame = 8 timeslots, each with length of approx. 577

s slot, frame length is 4,615 ms

– multiframe:

• traffic channel timeslots: 26 frames 120 ms

• control channel timeslots: 51 frames 235.36 ms

GSM radio basics

0

.

13. 12. 24. 25.

Multiframe: 26 frames: 120 ms

0…11 frame: TCH 13…24. keret: TCH

SACCH empty

physical bursts (in timeslots) 0. 1. 2. 3. 4. 5. 6. 7.

5.

TDMA frame 60/13 ms

3 bit 57 1 26 1 57 3 8,25

Normal burst

tail Data (voice)

data (Voice)

tail guard interval

flagbit flagbit

training sequence

GSM radio basics

Logical channels:

– a channel between the mobile and the BSC

• logical channel: for a given purpose, as if there

was a separate „wire” for this purpose

• but mapped and carried in the timeslot/frame

structure

– Traffic Channels - TCH carry user data (voice)

– Control Chanels - *CCH: control and signalling

informations

GSM radio basics

– dedicated channels: dedicated to a user, TCHs are

dedicated, can be dedicated CCH, used when mobile

is active

– common channels: used by more mobiles, used when

mobile is idle

TCHs:

– carry data or voice traffic

– a single timeslot and a normal burst in the timeslot, in

every frame, except the 13rd and 26th, which are

control and empty, respectively

GSM radio basics

Elements of the burst (see figure above)

Training sequence (26 bits)

– one of 8 standard sequences

– enables bit-level synchronisation of the receiver

– a known seqence (the receiver knows it) -> it enables the

estimation of the effect of the channel. Hence the data bits

can be detected correctly.

– tail bits (3-3): no information, this is needed to let the

electronic circuits set their operation point, when turning

on/of at the beginning/end of the burst

GSM radio basics

– simultaneous transmission/reception is not

possible, it would cause big problems

• not full duplex at radio level

– therefore there are 3 timeslots difference between

reception and transmission

• although full duplex at service level (voice call

is full duplex, both party can talk at the same

time)

• half-duplex at low, radio operation level

GSM radio basics

– guard interval

• due to the different distances of mobiles from

the base station, the propagation delay of

bursts is also different

• guard interval prevents bursts of different

users to collide when arriving to base station

GSM radio basics

Timing advance

– mobile syncronizes itself to the BTSs signal in

downlink

– according to the received frames at the MS, lets

suppose it would have to start transmission at the

beginning of a timeslot, at e.g. T0

– however, in downling, the has this info at T0+d/c,

where d is its distance, c is speed of light

– it starts transmitting, but on uplink it again has a

delay, hence arrives to the base station at T0+2d/c

– the burst may overlap with that of the user

transmitting in the next slot

GSM radio basics

Timing advance

– hence the system measures the delay of the burst’s

arrival

– orders the mobile to start the transmission earlier

– this is timing advance

• theoretically the mobile should start earlier by

2d/c, compared to when it thinks it shoud start

GSM radio basics

Basic procedure of finding the network

– MS turned on

– tuning over GSM frequencies and listing them according to received signal level

– searching special signal pattern (see later SCH) on the channel, to detect whether the frequency carries broadcast control channel (is it BCCH carrier?)

– If yes: • search for FCCH (Frequency Correction CH): tune its

frequency exactly to that of the base station

• search for SCH (Synchronisation CH): this carries special pattern for bit-synchronisation (-> clock of MS can be same as clock of BS) and reckognition of SCH; this also carry frame number (-> to sysncronise to frame structure); and a base station code: to enable distuinguishing between SCHs of neighboring BSs

– If no: continue searching with the next frequency

GSM radio basics

Basic procedure of finding the network

– MS searches the BCCH (Broadcast Control CH); it

contains:

• network code (operator); Location Area Code

(LAC); used frequencies by neighboring base

stations; configuration of contorl channels, other

system confguration informations

– if the MS finds its operator’s network (own network in

BCCH information), then it may initiate the registration

process

GSM radio basics

Basic procedure of registering to the network

– MS sends an access request on the RACH

(Random Access CH)

• how does it know, where is it (in which timeslots?) -

> its broadcasted in the BCCH, of course

• in the RACH burst the mobile sends the reason of

sending the RACH (-> „I want to register”)

• the MS then listens to the AGCH (Access Grant

Channel)

– the system sends on the AGCH the reply and assignes

an SDCCH (Standalone Dedicated Control CH) for the

rest of the process

GSM radio basics

Basic procedure of registering to the network

– over the SDCCH the authentication and

registration process (bi-directional exchange of

messages) is done

• also the mobile makes a Locatuion Update, that is it

sends the LAC to the system

• after succesful authentication, the MS is known to

the netwrok, the network stores the current LAC and

that the mobile is turned on and available

GSM radio basics

Basic procedure of „camping” (being idle) in the network

– the mobile is constantly listening to the

• FCCH, SCH, BCCH of the best serving cell

• to keep synchronised and check whether its LAC changed (->

the mobile may move to another LA)

– the mobile is listening to the PCH (Paging Channel) of that cell

• it is where incoming calls are notified

Basic procedure of location update

– the mobile finds a new LAC

– it initiates a RACH message (reason: „I want to make a location

update”)

– system answers on AGCH and assigns a SDCCH

– location update message is sent over SDCCH

GSM radio basics

Basic procedure of receiving a call

– when a call arrives, the system sends a notification to the

PCH of all cells in the LA, where the mobile is

– the mobile reads its identity in the PCH

– the mobile initiates a RACH message (reason:”I’m

answering to a call”)

– the system answers over the AGCH and assigns a traffic

channel (TCH)

• this is immediate assignment

• higher level signalling and then the actual voice data is

over the TCH

GSM radio basics

Meanwhile having a call

– every 13rd frame contain a timeslot for the mobile, that is not TCH

but SACCH (Slow Associated CCH), see figure above

– SACCH contains: power control commands, timing advance

commands, measurement reports and SMS, if it is during a call

– if the mobile goes out of the cell, while having a call: based on the

reports provided in the SACCH, the system decides that a

handover is necessary

– it sends the handover command over the FACCH (Fast

Associated CCH)

• FACCH: it is physically transmitted in the place of a voice

burst. The flag bit in the normal burst (see figure) ssignals the

fact if the FACCH has stolen the burst from the voice

– the mobile then knows that it should interpret the bits there

as FACCH message, not voice

GDSM radio basics

frame structure, timeslots and SAACH


GSM radio basics

mapping of logical channels to

timeslots

– for SACCH and FACCH we discussed

– timeslot 0 on a given frequency is for

control channels (combined

configuration)

– in case of heavy traffic, timeslot 1 is

used to carry dedicated control channels

and timeslot 0 for common control

channels (CCH)

– moe slots in uplink can be used if more

RACH slots are needed

– note SACCH in the figure -> it is

because every SDCCH also has a

SACCH

GSM radio basics

mapping of logical channels to timeslots

– non-combined channel allocations

HSCSD - (High Speed Circuit-Switched

Data)

Next step in GSM evolution

Goal is higher bitrates

How: allocate maxim 4 timeslots in parallel to a single

data connection

maximum 4 * 14.4 = 57.6 kbps bitrate same as dial-in

modems in fixed phone networks at that time

• Advantage: can be implemented in existing GSM

network, no need for new devices

• Naturally new mobile terminals are needed

• Disadvantage: still circuit switched

• the connection is alive and is billed based on the

time; regardless of actual data transmission

GPRS - (General Packet Radio Service)

Goals: – to support IP based packed data networking and communications, in the

whole mobile network

• not over dial-in modems

– to support higher bitrates

– least possible modifications is the network

• do not change radio interface and radio network

to achieve this – new devices needed in the core network, to support PS data

transmission/routing

• along with mobility support

– this, the GSM/GPRS core network will be able to be used as core network for the 3rd generation mobile system (UMTS)

– new, flexible mechanisms in the air interface

• channel allocation is only for the transmission, when it is needed

• charging is based on the amount of data transmitted, regardless the duration of the transmission

GPRS motivation

Do not touch the MS-BTS (Um) interface (just minimally)

therefore older (non-GPRS capable) phone remains

compatible

dynamic sharing of resources with „classical“ GSM

speech services

should provide packet switching service

internetworking with IP- and X.25 nets standardized

just establish new core network elements

14/06/2016 59 © Department of Networked

Systems and Services

GPRS network

Forrás: Ericsson

PCU

GPRS/UMTS network

GPRS network

new elements in the core network: SGSN, GGSN (Serving GPRS Support Node, Gateway -||-)

new element in BSS (Base Station Subsystem): PCU

PCU (Packet Control Unit): – practical implementation: a hardware extension for the BSC; or a distinct

device in the network (usually co-located with the BSC)

– interface between the GPRS core network and the BSS

• forwarding of packets to/from base stations from/to SGSN

– the PCU transforms the data into PCU frames (a special frame formats)

• similar to TRAU frames, format of the voice frames in GSM

– radio resource management for PS services

• allocation of timeslots, logical channels

• scheduling of user data: who can transmit/receive when

• setting up radio links for GPRS connections

– mobility management for GPRS services

• paging

• handover

GPRS network SGSN:

– packet routing and forwarding for the PCUs, other SGSNs and GGSNs

– router (not correct term: switch) or center for the packet swtched traffic

– take part in mobility management

• location update and querying of location database regarding the mobiles’ position

• mobility is tracked based on routing area

• mobile phones in ready state (meaning: its about to communicate) the location is tracked at cell level

– collection of charging records for packet data traffic

– helps setting up IP-level session between the mobile and GGSN

• PDP context, later

– security encription; optional data compression

GPRS hálózat

GGSN:

– collecting charging records for communications with the outside networks (e.g. internet)

– gateway to the internet and other mobile networks

• allocation of IP address to the mobiles

• anchor of mobility in the GPRS:

– wherever a mobile roams in the network, while having an active session, its traffic is going through a given GGSN

– conversion of IP addresses, if needed

– the mobile has an IP level session with the GGSN

• it is the PDP context (Packet Data Protocol)

• it contains: allocated IP address + IMSI (International Mobile Subscriber Id.) + other descriptiors of the traffic

GPRS network architecture



GPRS protocol stack



MAC: Medium Access Control

RLC: Radio Link Control

LLC: Logical Link Control

UDP: User Datagram Protocol

TCP: Transmission Control Protocol

BSSGP: BSS GPRS Protocol

GTP: GPRS Tunneling Protocol

SNDCP: Sub-Network Dependency

Convergence Protocol

IP: Internet Protocol

Előadás címe 67 © Előadó Neve,

Híradástechnikai Tanszék

Budapesti Műszaki és

Gazdaságtudományi Egyetem

GPRS protocol stack

note: we discussed GPRS protocols only at the GGSN-

SGSN interface, we discussed GTP protocol only

– the rest is for those who are interested

14/06/2016 68

GPRS protocol stack

from the GPRS mobile station via the BSS and the

SGSN to the GGSN

the Interconnection between the PLMN and external

Packet Data Networks or PDN's is achieved via the

GGSN

Note that the IP and UDP/TCP protocols between SGSN

and GGSN present the Intra-PLMN backbone as a

transport medium for the GPRS Tunneling Protocol

(GTP)

however, they do not represent the application or user

protocols that can be found at the very top of the

protocol stack

14/06/2016 69

MS-BSS interface (Um interface)

Layer 1: radio subsystem layer (physical layer)

– radio interface to the BTS is the same interface used by

the existing GSM network

MAC (Medium Access Control)

– MAC arbitrates access to the shared medium between

multiple MSs and GPRS network

– priority handling between data flows of one UE

– priority handling between UEs by means of dynamic

scheduling

RLC (Radio Link Control)

– error correction

– in-sequence delivery of SDUs

– duplicate detection 14/06/2016

70

MS-BSS interface (Um interface)

LLC (Logical Link Control):

– reliable logical link between MS and SGSN

– LLC messages are transparent to RAN

– C-Plane: attach, authentication and PDP activation

– U-Plane: carry actual data

SNDCP (Sub-Network Dependency Convergence

Protocol)

– Transparent over BSS

– Used only in U-Plane

– compression of user data

14/06/2016 71

Tunneling

14/06/2016 72

Tunneling

User data packets are sent over the GPRS backbone in

„containers”.

When a packet coming from an external packet network

arrives at the GGSN, it is inserted in a container and sent to

the SGSN.

The stream of containers inside the GPRS backbone

network is totally transparent to the user

– To the user, it seems like he/she is connected directly

via a router (the GGSN) to external networks

In data communications, this type of virtual stream of

containers is called a tunnel

GSNs are performing tunneling of user packets

14/06/2016 73

GPRS Tunneling Protocol

GTP packets are transported between the SGSN and the

GGSN

GTP is defined both for the Gn interface, that is, the

interface between GSNs within the same PLMN, and the

Gp interface between GSNs in different PLMNs

Two modes of operation of the GTP layer are supported

(and can support both modes simultaneously):

– unacknowledged (UDP/IP)

– acknowledged (TCP/IP)

allows end users of a GSM or WCDMA network to move

from place to place whilst continuing to connect to the

internet as if from one location at the GGSN

14/06/2016

74

GPRS Tunneling Protocol

14/06/2016 75

GPRS radio interface


minimal changes to GSM

modulation waveform is the same

frequency channels, timeslot and frame structure is

the same

basic unit of transmission: RB Radio Block (456 bits)

-> 4 physical bursts in 4 timeslots

novelties:


novelties for higher bitrates – allow the use of all 8 timeslots for a given user

• theoretical max bitrate per timeslot is 20 kbps, the total max is hence 8*20 kbps = 160 kbps

• for those instants, when the mobile is scheduled, not for long time usually

• in practice, averag e 30-40 kbps can be reached

novelties for flexibility (for efficient serving of PS data) – no need to allocate channel (timeslot) for a long time to a user, only should it be

scheduled when it is needed

• in contrast with voice, where the timeslot is allocated to a call until its end, regardless of actually having voice transmitted or not

– scheduling timeslots dynamically

– several users can share a timeslot, meaning not a particular slot in a particular frame; but e.g. timeslot 4 can be used by several mobiles, and the scheduler will decide in which frames which mobile will use timeslot 4

– uplink and downlink can be asymmetric (usually more traffic in downlink)

• -> ul and dl scheduled independently, based on the needs

– introduction of quality of service classes


new logical channels:

– with the same functionalities as GSM logical channels

– name is also P*CH, where *CH was the name of the

GSM logical channel (e.g. PCH, BCCH, AGCH, etc.)

– might use the same timeslots as GSM logical channels

– PNCH: packet Notification Channel: the GPRS defines

multicast sending

• multicast: a given transmission is for more then one mobile

• it is sent only once, on a single channel -> all the mobiles in

the multicast group should listen to

• PNCH tells when a multicast packet is coming


timing advance (TA) control

– the actual transmission of a mobile can be sporadic

– can travel physically between two transmissions

• data: no continuous transmission (compared to voice) ->

therefore TA cannot be measured continuously

– therefore: there is special TA slots in the GPRS frame, where

active mobiles send RACH burts

• below: GPRS frame. Note that this is mapped to a timeslot,

say timeslot 4 in the air frame structure (next slide)


GPRS frames


channel scheduling, downlink: – the system tells the MS which timeslots to listen to

• e.g. slots 1,2,3 and 4 should be listened to – in every timeslots there’s a stream of RBs, see previous figures – how does the MS know, which RB should be read?

• there is a label, address in the RB, called TBF Temporary Block Flow • when mobile sees its TBF in the block, it reads its content

– how does the MS know when it is finished? • there’s a bit FBI Final Block Indicator, this tells if the mobile’s flow has

ended channel scheduling, uplink:

– the system tells the mobile which uplink timeslot it may use, say timeslot 6,7 and 8

– the mobile must read the downlink slots 6,7 and 8 – there is one more label in the blocks, the USF (Uplink State Flag) -> label of

the mobile • if the mobile reads its own USF in any block in a timeslot (e.g. mobile

sees its own USF in block 215, in timeslot 6 in the downlink) • -> it can send in the same timeslot in the next block in uplink (e.g. the

mobile sends in the block 216 in uplink timeslot 6 – end of uplink flow: the system makes the scheduling decisions in advance, it

should know earlier, when the uplink flow will finish. So in uplink, there is a counter in the blocks, which are decreased in the last few blocks. The last block has counter value 0.

Enhanced Data rates for GSM

Evolution (EDGE)

extension of GSM/GPRS for higher bitrates

with the introduction of a new modulation waveform, the

bitrate can be increased to 60 kbps per timeslot, in case

of good channel conditions

using 8 timeslots, theoretical max of 480 is achievable

– in the instants of transmitting, receiving

in practice, roughly 80-100 kbps on average

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Mobile infocommunication systems Mobile …fazek/English_course_Mobile_communication_networks...Mobile infocommunication systems Mobile infocommunication networks ... – Abis interface