Module4-Cellular Overview - GSM

8/11/2019 Module4-Cellular Overview - GSM

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2006Confidential and Proprietary

Integrated Wireless Solutions Pvt. Ltd

Module 2

GSM Introduction

Integrated Wireless Solutions Pvt. Ltd.


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GSM Introduction

Introduction Cellular Structure GSM Architecture GSM Functions GSM Radio Interface

Frequency Allocation

Channel Structure Burst Structure

GSM Features Frequency hopping DTX Timing Advance Power Control Multi-path and Equalization

GSM Services


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Challenges

Initial Operator Challenge

Equipment limited to operate within the boundary of country

Market for each mobile equipment was limited (high cost)

GSM (Group Special Mobile) formed in 1982 by CEPT tostandardize the cellular system for use in Europe withemphasis on following features

Spectral Efficiency

International roaming

Low mobile and base station cost

Good subjective voice quality

Compatibility with other systems such as ISDN (IntegratedServices Digital Network)

Ability to support new services

Introduction


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Specifications

Responsibility for GSM specifications

passed from CEPT to ETSI in 1989 Aim of GSM specifications

Describe the functionality and the interface foreach component of the system

To provide guidance on the design of thesystem Guarantee the proper inter-working between

the different elements of the GSM system.

In 1990, the phase I of the GSM specificationswere published but the commercial use of GSM

did not start until mid-1991

Introduction


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GSM Events

Phase 2 of the GSM specifications Coverage of rural areas1995

Coverage of main roads GSM services start outside Europe1993

Enlargement of the countries that signed the GSM- MoU> Coverage of largercities/airports

1992

Commercial launch of the GSM service1991

Appearance of the phase 1 of the GSM specifications1990

The responsibility of the GSM specifications is passed to the ETSI1989

Validation of the GSM system1988

TDMA is chosen as access method (in fact, it will be used with FDMA) InitialMemorandum of Understanding (MoU) signed by telecommunication operators

(representing 12 countries)

1987

Field tests were performed in order to test the different radio techniques proposedfor the air interface

1986Adoption of a list of recommendations to be generated by the group1985

CEPT establishes a GSM group in order to develop the standards for a pan-European cellular mobile system

1982

EventsYear

Introduction


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GSM Architecture

MSC/VLR

HLR/AUC EIR SC/VM

OMC

BSC

BTS

PSTN

ISDN

PSPDN

MS

BSS

NSS

Um

Abi

s

BIE

A

F

C

H

MSC/VLR

E


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Mobile Station

Mobile equipment or terminal

Fixed terminals

20 Watt output power

Portable terminals

8 Watt output power Handheld terminals

2 watts to 0.8 watts (most popular)

The subscriber Identity Module (SIM)

Provides access to all the subscribed services

GSM Architecture


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Base Station Subsystem

Connects the MS and NSS

The Base Transceiver Station (BTS) or Base station The Base Station Controller (BSC)

BTS The BTS corresponds to the transceivers and antennas

used in each cell of the network. A BTS is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between one and sixteen transceivers

depending on the density of users in the cell.

BSC The BSC controls a group of BTS and manages their radio

resources. A BSC is principally in charge of handovers, frequency

hopping, exchange functions and control of the radiofrequency power levels of the BTSs.

GSM Architecture


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Network and Switching Subsystem

Its main role is to manage the communications between themobile users and other users, such as mobile users, ISDNusers, fixed telephony users, etc.

It also includes data bases needed in order to storeinformation about the subscribers and to manage theirmobility.

Mobile Switch Center (MSC) It is the central component of the NSS. The MSC performs the

switching functions of the network. It also provides connection toother networks.

Gateway MSC (GMSC) The GMSC is the interface between the mobile cellular network

and the PSTN. It is in charge of routing calls from the fixed network towards a

GSM user. The GMSC is often implemented in the same machines as the

MSC.

GSM Architecture


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Home Location Register (HLR) The HLR is a database that stores information of the subscribers belonging

to the covering area of a MSC. It also stores the current location of these subscribers and the services to

which they have access.

Visitor Location Register (VLR) The VLR contains information from a subscriber's HLR necessary in order to

provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR

associated to this MSC will request information about the new subscriber toits corresponding HLR. The VLR will then have enough information in orderto assure the subscribed services without needing to ask the HLR each timea communication is established.

The VLR is always implemented together with a MSC; so the area undercontrol of the MSC is also the area under control of the VLR.

The Authentication Center (AuC) The AuC register is used for security purposes. It provides the parameters

needed for authentication and encryption functions. These parameters helpto verify the user's identity.

GSM Architecture


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The Equipment Identity Register (EIR)

The EIR is used for security purposes. It is a register containinginformation about the mobile equipments. More particularly, itcontains a list of all valid terminals.

A terminal is identified by its International Mobile EquipmentIdentity (IMEI). The EIR allows then to forbid calls from stolen orunauthorized terminals (e.g, a terminal which does not respectthe specifications concerning the output RF power)

The GSM Inter-working Unit (GIWU) The GIWU corresponds to an interface to various networks for

data communications. During these communications, thetransmission of speech and data can be alternated.

The Operations and Support Subsystem (OSS) The OSS is connected to the different components of the NSS and

to the BSC, in order to control and monitor the GSM system. It is

also in charge of controlling the traffic load of the BSS.

GSM Architecture


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GSM Functions

OAM

Service carrier

CM

MM

RR

Subscriber

Transmission

Radio Resource Management

Mobility Management

Call Management

Operation Administration andMaintenance


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Transmission

The transmission function includestwo sub-functions:

Transmission of user information

MS, BTS and BSC perform this function

Transmission of signaling information

Above components and MSC, HLR, VLR,EIR perform this function

GSM Functions


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Radio Resource Management (RR)

The role of the RR function is to establish,

maintain and release communication linksbetween mobile stations and the MSC.

The RR is also responsible for the management ofthe frequency spectrum and the reaction of thenetwork to changing radio environmentconditions. Some main functions are: Channel assignment, change and release.

Handover.

Frequency hopping.

Power-level control. Discontinuous transmission and reception.

Timing advance.

GSM Functions


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Radio Resource Management (RR)

Handover The user movements can produce the need to change the channel or cell, specially

when the quality of the communication is decreasing. This procedure of changingthe resources is called handover. Four different types of handovers can bedistinguished:

Handover of channels in the same cell. Handover of cells controlled by the same BSC. Handover of cells belonging to the same MSC but controlled by different

BSCs. Handover of cells controlled by different MSCs.

Handovers are mainly controlled by the BSC and MSC. In order to perform the handover, the mobile station monitors continuously its

own signal strength and the signal strength of the neighboring cells. The list ofcells that must be monitored by the mobile station is given by the base station.The power measurements allow to decide which is the best cell in order tomaintain the quality of the communication link.

Types of Handover Quality Handover

When the quality of the transmission decreases (i.e the signal isdeteriorated), the power level of the mobile is increased. This is done untilthe increase of the power level has no effect on the quality of the signal.When this happens, a handover is performed.

Power budget handover This algorithm performs a handover, instead of continuously increasing the

power level, in order to obtain a good communication quality.

GSM Functions


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Mobility Management (MM)

The MM function is in charge of all the aspects related withthe mobility of the user, specially the location managementand the authentication and security.

Location Management When a mobile station is powered on, it performs a location

update procedure by indicating its IMSI to the network. The firstlocation update procedure is called the IMSI attach procedure.

The mobile station also performs location updating, in order to

indicate its current location, when it moves to a new LocationArea or a different PLMN. This location updating message is sentto the new MSC/VLR, which gives the location information to thesubscriber's HLR. If the mobile station is authorized in the newMSC/VLR, the subscriber's HLR cancels the registration of themobile station with the old MSC/VLR.

A location updating is also performed periodically. If after the

updating time period, the mobile station has not registered, it isthen deregistered. When a mobile station is powered off, it performs an IMSI detach

procedure in order to tell the network that it is no longerconnected.

GSM Functions


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Mobility Management (MM)

Authentication and Security The authentication procedure involves the SIM

card and the Authentication Center. It is usedto verify the authenticity of the user and theservices subscribed by the user.

Another security procedure is to check theequipment identity. If the IMEI number of themobile is authorized in the EIR, the mobilestation is allowed to connect the network.

In order to assure user confidentiality, the useris registered with a Temporary MobileSubscriber Identity (TMSI) after its firstlocation update procedure.

GSM Functions


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Communication Management (CM)

Call Control (CC)

The CC is responsible for callestablishing, maintaining and releasingas well as for selecting the type of

service.

Short Message ServicesManagement (SMS)

Connects to the Short Message ServiceCenter (SMSC)

GSM Functions


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Operation, Administration andMaintenance (OAM)

The OAM function allows the operator to monitor

and control the system as well as to modify theconfiguration of the elements of the system.

The components of the BSS and NSS provide theoperator with all the information it needs. Thisinformation is then passed to the OSS which is incharge of analyzing it and control the network.

The self test tasks, usually incorporated in thecomponents of the BSS and NSS, also contributeto the OAM functions.

The BSC, in charge of controlling several BTSs, isanother example of an OAM function performedoutside the OSS.

GSM Functions


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GSM Radio Interface

The radio interface is the interface

between the mobile stations and the fixedinfrastructure. It is one of the mostimportant interfaces of the GSM system.

One of the main objectives of GSM isroaming. Therefore, in order to obtain acomplete compatibility between mobilestations and networks of differentmanufacturers and operators, the radiointerface must be completely defined.


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GSM adopts TDMA/FDMA modechannel width: 200KHzeach channel has 8 timeslots

GSM adopts TDMA/FDMA modechannel width: 200KHzeach channel has 8 timeslots

User 1

User 2

User 3

Frequency

time

TDMA

GSM Radio Interface


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time

Frequency

BP

15/26ms

200KHz

interval

GSM Radio Interface

Timeslot and Frame Structure


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2006 Confidential and ProprietaryIntegrated Wireless Solutions Pvt. Ltd.

GSM900 :

up: 890~915MHz

down: 935~960MHz

duplex interval: 45MHz

bandwidth: 25MHz

frequency interval: 200KHz

GSM1800 :up: 1710-1785MHz

down: 1805-1880MHzduplex interval: 95MHzworking bandwidth: 75MHzfrequency interval: 200KHz

EGSM900 :

up: 880~890MHz

down: 925~935MHz

duplex interval: 45MHz

bandwidth: 10MHz

frequency interval: 200KHz

GSM1900MHz:up:1850~1910MHz

down:1930~1990MHzduplex interval: 80MHzworking bandwidth: 60MHzfrequency interval: 200KHz

GSM Frequency Allocation

GSM Radio Interface


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GSM Frequency Allocation

GSM Radio Interface

890MHz 960MHz

1 2 3 124Channels:

200KHz

915MHz 935MHzBTS Uplink/Rx BTS Downlink/Tx


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Propagation characteristic

Cell coverageradius :

Higher the propagation frequency

Higher the propagation loss

Smaller the cell coverage radius.

900MHz

1800MHz

1900MHz

Frequency Resource

We know

GSM Frequency Impact

GSM Radio Interface


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Single Band Network

1800MHz

1900MHz

900MHzWhich one?

General Priority

High

Low

Reason

For SubscriberFor Operator

Propagation

Characteristic

New Operator

Band Selection

GSM Radio Interface


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Due to frequency constraints in GSM900 band,many networks are dual band (GSM900 andGSM1800). Most mobiles support dual band.

1800MHz

1900MHz

900MHzSingle Band

Dual Band

Triple Band

Frequency ResourceGSM Radio Interface

Multi Band Networks


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0 2045 2046 2047321 2044

3210 4947 48 50

0 24 251

1 super high frame = 2048 super frame = 2715648 TDMA frame

1 super frame = 1326 TDMA frame6.12s

0 1 2524 504910

1 multiplex frame = 26 TDMA frames120ms 1 multiplex frame = 51 TDMA frame

0 1 765432

1 TDMA frame = 8 timeslot120/26 = 4.615ms

BCCH

CCCH

SDCHSACCH/TCH

FACCH

GSM Radio Interface

Timeslot and Frame Structure


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Channel Structure

Two Types of Channels

Traffic Channels (TCH) Used to transport speech and data information TCH for Uplink and downlink and separated by

3 bursts to make electronics simple

Control Channels Used for network management messages and

channel maintenance tasks Three types of control channels

Broadcast Channels Common Control Channels Dedicated Control Channels Associated Control Channels

Channel Structure


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channel

TCH

CCH

Voice CH

Data CH

FR Voice Traffic Channel (TCH/FS)

HR Traffic Channel (TCH/HS)

4.8Kbit/s HR TCH (TCH/H4.8)

9.6Kbit/s FR TCH(TCH/F9.6)

4.8Kbit/s FR TCH (TCH/F4.8)

BCH

FCCH (down)SCH (down)

BCCH (down)

CCCH

RACH (up)

AGCH (down)PCH (down)

DCCH

SDCCH

FACCH

SACCH

14.4Kbit/s FR TCH (TCH/F14.4)

Enhanced FR Traffic Channel (TCH/EFR)

Channel Type-Summary


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Control Channels

Common Control Channels (CCCH)

The CCCH channels help to establish the callsfrom the mobile station or the network. Threedifferent types of CCCH can be defined: The Paging Channel (PCH). It is used to alert

the mobile station of an incoming cal

The Random Access Channel (RACH), which isused by the mobile station to request accessto the network

The Access Grant Channel (AGCH). It is used,by the base station, to inform the mobile

station about which channel it should use.This channel is the answer of a base station toa RACH from the mobile station

Channel Structure


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Control Channels

Dedicated Control Channels (DCCH)

The DCCH channels are used for message exchangebetween several mobiles or a mobile and thenetwork. Two different types of DCCH can bedefined: The Standalone Dedicated Control Channel (SDCCH),

which is used in order to exchange signaling

information in the downlink and uplink directions. The Slow Associated Control Channel (SACCH). It is

used for channel maintenance and channel control.

Associated Control Channels The Fast Associated Control Channels (FACCH)

replace all or part of a traffic channel when urgentsignaling information must be transmitted. TheFACCH channels carry the same information as theSDCCH channels.

Channel Structure


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Burst Structure

Defined in a group of 26 TDMA frames called a Multiframe 24 frames are reserved for traffic 1 frame is used for Slow Associated Control Channel (SACCH) The last frame is unused. Allows mobile to perform other functions

such as measuring the signal strength of neighboring cells.

Channel Structure


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GSM Features

Frequency Hopping This is a method of transmitting radio signals

by rapidly switching a carrier among manyfrequencies

Signals become more resistant to interference

Can have tighter re-use for increased capacity Two type of frequency hopping

Base-band Signal is switched between the available carriers

Synthesizer Signal is switched between all the frequency band

available


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GSM Features

Discontinuous Transmission (DTX) The function of the DTX is to suspend the radio transmission

during the silence periods Helps reduce interference Increases battery life

Discontinuous Reception Paging Channel is divided into sub-channels Each mobile only listens to its sub-channel Helps conserve mobile battery power

Timing Advance (TA) Timing of bursts is important in GSM to avoid overlap between

different timeslots Delay in reception is dependent on distance of mobile from base

station

Mobiles further out from the base station, advance theirtransmission so that they are received at the same time as themobiles transmitting closer by

GSM Features


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GSM Features

Power Control

Both mobile and base station measure thequality and signal strength of the burst

If the quality of the received signal is not good,then either mobile or base station increase the

power of next burst

Multi path and Equalization

Radio waves reflect from buildings, cars etc

which can corrupt the received signal Equalizer estimates the corruption due to multi

path and corrects extracts the correct signal

GSM Features


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GSM Services Teleservices

Telephony

Fax Emergency calls

SMS

Voice mail

Bearer Services (DataServices) Asynchronous and

synchronous data Alternate Speech and data Packet Switched Data

(GPRS)

Supplementary Services Call Barring Call Hold Call Waiting Advice of charge Multi Party services Closed User Group Calling Line Identification

(CLI)


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3 Steps to 3G: The GSM Network Transition


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Hope that you enjoyed this course

Thank you

for your participation

Documents

Module4-Cellular Overview - GSM