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GSM OVERVIEWGSM OVERVIEW

AMEEN AKBAR

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GSM stands for Global System forMobile

Communication

1

st

totally digital cellulartelephone system

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SERVICESSERVICES

Teleservices (TS) Telephony, emergency calls, voice

messaging Bearer services (BS)

SMS and cell broadcast, 9.6kbit/s

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Supplementary Service (SS)

Barring outgoing call, Internationalcalls, roaming calls

Call forwarding under variousconditions

Call hold , Call waiting

Call transfer to a third party

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GSM ArchitectureGSM Architecture

B a se Tra n sce ive rStation ( )BTS

Base Transceiver( )Station BTS

Base Station( )Controller BSC

Abis interface

( )Base Station BS

Base Transceiver( )Station BTS

Base Transceiver( )Station BTS

Base Station( )Controller BSC

Mobile Stations

( )MS

Um interface

A interface

( )Base Station BS

Abis interface

CCITTSignalling

.System No 7( )SS7

interface

MobileSwitching

Centre( )MSC

GMSC

PSTN

VLR

HLR

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Elements of the NetworkElements of the NetworkSubscriber: user who pays

subscription charges for using mobilecommunication services.

Mobile Station: is a subscriber unitintended for use while on the move

at unspecified locations. It could be ahand-held or a portable terminal.Base Station: a fixed radio station

used for communication with MS. It

is located at the centre of a cell andconsist of Transmitters andReceivers.

Mobile Switching Centre : it

coordinates the routing of calls, dothe billing, etc.

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Mobile StationMobile Station MS consist of :

Mobile Equipment (ME)

Subscriber Identification Module

(SIM)

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SIM CardSIM Card Subscriber Identity Module (SIM) is

a smart card which storesinformation about thesubscription and feature ofservices.

Stored information including: Authentication Key Ki Encryption IMSI and TMSI

SIM card is protected by aPersonal Identity Number (PIN) ofthe user

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International Mobile SubscriberID(IMSI)

15 digit = 3 for country code, 3 for

mobile network code and 9 for mobile ID number. Temporary Mobile Subscriber

ID(TMSI) per-call basis ID for security reason

to avoid sending IMSI over the air

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Base Transceiver Station (BTS)Base Transceiver Station (BTS) BSS consist of two part :

Base TransceiverStation (BTS)

Base StationController

(BSC) BTS is a radio-end whichdetermine a cellcoverage and providelink with MS.

BTS include

Transmitters andReceivers, antennaand signal processingunit as well asinterface.

BTS communicate with

MS via Um (air)interface

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Base Station ControllerBase Station Controller

BSC handle radio-channel setup,

frequency hopping,and handoverwithin BSC

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Home Location Register (HLR)Home Location Register (HLR)

HLR containdatabase of users,including all the

subscriptionrecords HLR records the

update location of

every user formobilitymanagementpurposes

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Visitor Location Register (VLR)Visitor Location Register (VLR)

VLR is a temporarydatabase of user

VLR is used tohandle a roamer

connection. VLR could beaccessed by MSCfor every call setup.

Every MSC isconnected to aVLR, but a VLRcould beconnected to

several MSC

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A cce ss M e ch a n ismA cce ss M e ch a n ism

, ,FDMA TDMA CDMA

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Frequency multiplexFrequency multiplex

Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the

spectrum for the whole time :Advantages

works also for analog signals :Disadvantages

waste of bandwidthif the traffic is

distributed unevenlyinflexible

guard spaces

k

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k

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f

t

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i l i lTi l i l

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k

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k

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Time multiplexTime multiplex

A channel gets the whole spectrum for a certainamount of time

:Advantages only one carrier in the

medium at any time throughput high even

for many users :Disadvantages precise

synchronizationnecessary

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Code MultiplexCode Multiplex

Each channel has a unique code All channels use the same spectrum at

the same time :Advantages

Bandwidth efficient

No coordination andsynchronization necessary Good protection against

interference and tapping :Disadvantages

Lower user data rates More complex signal regeneration

Implemented using spread spectrumtechnology

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Various Access MethodVarious Access Method

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CellsCells

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Cell structureCell structure

:Implements space division multiplex base station covers a( )certain transmission area cell Mobile stations communicate only via the base station :Advantages of cell structures

,higher capacity higher number of users less transmission power needed ,more robust decentralized ,base station deals with interference transmission

.area etc locally :Problems

fixed network needed for the base stations ( )handover changing from one cell to another

necessary

interference with other cells , . .,Cell sizes from some 100 m in cities to e g 35 km on the

( ) -country side GSM even less for higher frequencies

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Capacity of a CellularCapacity of a Cellular

SystemSystem

-Frequency Re Use Distance The K factor or the cluster size Cellular coverage or Signal to

interference ratio

Sectoring

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Handover MechanismHandover Mechanism

BS1 BS2

A B

RS

L

Level at B

Level which HO occured

time

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Hexagonal Cell GeometryHexagonal Cell Geometry ,Define coordinate axes

U & V , at 60 o angles For given frequency

,reuse plan go istepsin Udirection and j

steps in direction Number of cells in

reuse pattern is

2 2 N i ij j= + +

U

V

(2,1)

(1,3) (u,v)

1 3/

i =2

j = 1

N= 7 reuse pattern

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The Cell Structure for K = 7The Cell Structure for K = 7

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=Cell Structure for K 12=Cell Structure for K 12

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Increasing cellularIncreasing cellular

system capacitysystem capacity

Cell sectoring Directional antennas subdivide cell

into 3 or 6 sectors Might also increase cell capacity by

factor of 3 or 6

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Increasing cellularIncreasing cellular

system capacitysystem capacity Cell splitting

Decrease transmission power in base andmobile Results in more and smaller cells -Reuse frequencies in non contiguous

cell groups :Example cell radius leads 4 fold

capacity increase

f h f

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Performance characteristics ofPerformance characteristics ofGSMGSM

Communication , ;mobile wireless communication support for voice

and data services Total mobility

, -international access chip card enables use ofaccess points of different providers

Worldwide connectivity ,one number the network handles localization

High capacity , ,better frequency efficiency smaller cells more

customers per cell High transmission quality

,high audio quality and reliability for wireless( . .,uninterrupted phone calls at higher speeds e g

, )from cars trains Security functions

, -access control authentication via chip card and PIN

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Disadvantages of GSMDisadvantages of GSM

/No full ISDN bandwidth of 64 kbit s to theuser

Reduced concentration while driving Electromagnetic radiation Abuse of private data possible High complexity of the system Several incompatibilities within the GSM

standards

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Thank YouThank You