Introduction Digital cellular systems have many features such as improved communication quality due to the use of various digital signal processing technologies, new services (e.g. non telephony services), improved ciphering, greater conformity with

Introduction
Digital cellular systems have many features such as improved
communication quality due to the use of various digital signal processing
technologies, new services (e.g. non telephony services), improved
ciphering, greater conformity with digital networks, and efficient utilization
of the radio spectrum.
The development of digital cellular system was triggered by
standardization efforts in Europe, which was home to many competing
analogue systems.
In Europe, analogue cellular systems were using different frequency
bands and schemes which made interconnection impossible across
national borders.
In 1982, the European Conference of Postal and Telecommunications
Administrations (CEPT) established the global system for mobile (GSM)
and developments were carried out under the leadership of the European
Telecommunications Standards Institute (ETSI).
The GSM-based services were launched in 1992.
• In the United States, the IS-54 was developed by the Electronic
Industries Association (EIA) and the Telecommunications Industry
Association (TIA).
• IS-54 services launched in 1993 were required to satisfy dual-mode
(both analogue and digital cellular) operations and adopted time
division multiple access (TDMA).
• Studies on CDMA inclusive of field tests had been carried out in a
vigorous manner from 1989 onwards and consequently the IS-95
standard based on CDMAtechnology was adopted in 1993.
• All multiple access techniques depend on the adoption of digital
technology.
• Digital technology is a standard for digital cellular systems where all
analogue calls are converted to digital form for transmission over the
backbone.
Digital transmission has a number of advantages over analogue
transmission:
• It economizes bandwidth.
• It allows easy integration with personal communication system (PCS)
devices.
• It maintains superior quality of voice transmission over long
distances.
• It is difficult to decode.
• It can use lower average transmitted power.
• The worldwide market figures for digital cellular networks as follows:
The most popular digital system is GSM with approximately 70 per
cent market share.
• The analogue advanced mobile phone system (AMPS) holds 3 per
cent and the Japanese PDC holds 5 per cent (60 million users).
• The remainder is split between CDMA (12 per cent), TDMA (10 per
cent) systems, and other technologies.
• In Europe, almost all users use digital GSM (over 370 million) with
no analogue systems left.
• The situation is different in the United States and some other
countries that have adopted US technology (e.g. South Korea,
Canada).
• Here, the digital market is split among TDMA, CDMA, and GSM
systems with 107 million TDMA, 135 million CDMA, and only 16
million GSM users (North America only).
• This chapter deals with GSM and its architecture, GSM
specifications, GSM operation, GSM channels, protocol stack
configuration of GSM, and its basic call flow.

1. Unit-6
Digital Cellular Data Access

2. Digitalcellularsystemshavemanyfeaturessuchasimproved
communicationqualityduetotheuseofvariousdigitalsignalprocessing
technologies,newservices(e.g.nontelephonyservices),improved
ciphering,greaterconformitywithdigitalnetworks,andefficientutilization
oftheradiospectrum.
Thedevelopmentofdigitalcellularsystemwastriggeredby
standardizationeffortsinEurope,whichwashometomanycompeting
analoguesystems.
InEurope,analoguecellularsystemswereusingdifferentfrequency
bandsandschemeswhichmadeinterconnectionimpossibleacross
nationalborders.
In1982,theEuropeanConferenceofPostalandTelecommunications
Administrations(CEPT)establishedtheglobalsystemformobile(GSM)
anddevelopmentswerecarriedoutundertheleadershipoftheEuropean
TelecommunicationsStandardsInstitute(ETSI).
TheGSM-basedserviceswerelaunchedin1992.
Introduction

3. •IntheUnitedStates,theIS-54wasdevelopedbytheElectronic
IndustriesAssociation(EIA)andtheTelecommunicationsIndustry
Association(TIA).
•IS-54serviceslaunchedin1993wererequiredtosatisfydual-mode
(bothanalogueanddigitalcellular)operationsandadoptedtime
divisionmultipleaccess(TDMA).
•StudiesonCDMAinclusiveoffieldtestshadbeencarriedoutina
vigorousmannerfrom1989onwardsandconsequentlytheIS-95
standardbasedonCDMAtechnologywasadoptedin1993.
•Allmultipleaccesstechniquesdependontheadoptionofdigital
technology.
•Digitaltechnologyisastandardfordigitalcellularsystemswhereall
analoguecallsareconvertedtodigitalformfortransmissionoverthe
backbone.

4. Digital transmission has a number of advantages over analogue
transmission:
•It economizes bandwidth.
• It allows easy integration with personal communication system (PCS)
devices.
• It maintains superior quality of voice transmission over long
distances.
• It is difficult to decode.
• It can use lower average transmitted power.
•The worldwide market figures for digital cellular networks as follows:
The most popular digital system is GSM with approximately 70 per
cent market share.
•The analogue advanced mobile phone system (AMPS) holds 3 per
cent and the Japanese PDC holds 5 per cent (60 million users).
•The remainder is split between CDMA (12 per cent), TDMA (10 per
cent) systems, and other technologies.
•In Europe, almost all users use digital GSM (over 370 million) with
no analogue systems left.

5. •ThesituationisdifferentintheUnitedStatesandsomeother
countriesthathaveadoptedUStechnology(e.g.SouthKorea,
Canada).
•Here,thedigitalmarketissplitamongTDMA,CDMA,andGSM
systemswith107millionTDMA,135millionCDMA,andonly16
millionGSMusers(NorthAmericaonly).
•ThischapterdealswithGSManditsarchitecture,GSM
specifications,GSMoperation,GSMchannels,protocolstack
configurationofGSM,anditsbasiccallflow.

6. GSM Network architecture
Overview
•GSMnetworksaremadeupofmobileserviceswitchingcentres
(MSC),basestationsystems(BSS),andMS.
•Thesethreeentitiescanbebrokendownfurtherintosmallerentities.
WithintheBSS,wehaveBaseStationControllers(BSCs),BTSs,
andTranscodersasshowninFigure(GSMNetwork)
.

7. •Nowwewillusethethreemajorentities.
•WiththeMSC,BSS,andMS,wecanmakecalls,receivecalls,
performbilling,andsoon,asanynormalPSTNnetworkwould
beabletodo.
•TheonlyproblemfortheMSisthatallthecallsmadeorreceived
arefromotherMSs.
•Therefore,itisalsonecessarytoconnecttheGSMnetworkto
thePSTN.
•MSswithinthecellularnetworkarelocatedin“cells”.Thesecells
areprovidedbytheBSSs.
•EachBSScanprovideoneormorecellsdependingonthe
manufacturers’equipment.
•GSMnetworkconsistsofseveralfunctionalentitieswhose
functionsandinterfacesaredefined.Figureshowsthedifferent
partsofGSMnetwork.

8. GSM Network Architecture

9. •TheMS
•Thebasestationsubsystem(BSS)
•Thenetworkswitchingsubsystem(NSS)
•Theoperationandmaintenance/supportsystem(OSS)
ThefollowingaretheprinciplecomponentgroupsofaGSMnetwork:
•Mobilestation(MS):Thisconsistsofthemobiletelephone,fax
machine,andsoon.Thisisthepartofthenetworkthatthe
subscriberwillsee.
•Basestationsystem(BSS): Thisisthepartofthenetworkwhich
providestheradiointerconnectionfromtheMStotheland-based
switchingequipment.
•Networkswitchingsystem(NSS): ThisconsistsoftheMSCand
itsassociatedsystem-controldatabasesandprocessorstogether
withtherequiredinterfaces.
•ThisisthepartwhichprovidesforinterconnectionbetweentheGSM
networkandthepublicswitchedtelephonenetwork(PSTN).

10. •
Operations and maintenance system (OMS):
This enables the network provider to configure and maintain the
network from a central location.
(
1)The mobile station
•TheMSconsistsoftwocomponentsasshowninFigure:the
mobileequipment(ME)andanelectronic“smartcard”calleda
subscriberidentitymodule(SIM).

11. •TheMEisthehardwareusedbythesubscribertoaccessthe
network.
•Thehardwarehasan identitynumberassociatedwithit,whichis
uniqueforthatparticulardeviceandpermanentlystoredinit.
•Thisidentitynumberiscalledtheinternationalmobileequipment
identity(IMEI)andenablesthenetworkoperatortoidentifyME
whichmaybecausingproblemsonthesystem.
•TheSIMisacardwhichplugsintotheME.
•ThiscardidentifiestheMSsubscriberandalsoprovidesother
informationregardingtheservicethatthesubscribershouldreceive.
•Thesubscriberisidentifiedbyanidentitynumbercalledthe
internationalmobilesubscriberidentity(IMSI).
•MEmaybepurchasedfromanystorebuttheSIMmustbeobtained
fromtheGSMnetworkprovider.

12. •TheSIMmaybecontainedwithintheMS,oritmaybearemovable
unitthatcanbeinsertedbytheuser.
•Itprovidespersonalmobilitysothattheusercanhaveaccesstoall
subscribedservicesirrespectiveofboththelocationoftheterminal
andtheuseofaspecificterminal.
•WithouttheSIMinserted,theMEwillonlybeabletomake
emergencycalls.Bymakingadistinctionbetweenthesubscriber
identityandtheMEidentity,GSMcanroutecallsandperformbilling
basedontheidentityofthe“subscriber”ratherthantheequipmentor
itslocation.
•TheMSalsoactsasareceptorforSMSmessages,enablingthe
usertotogglebetweenthevoiceanddatause.
•Moreover,themobilefacilitatesaccesstovoicemessagingsystems.
TheMSalsoprovidesaccesstothevariousdataservicesavailable
inaGSMnetwork.
•Thesedataservicesincludethefollowing:

13. •X.25 packet switching through a synchronous or asynchronous dial-
up connection to the PAD at speeds typically at 9.6 kbps.
• GPRS using either an X.25 or IP-based data transfer method at
speeds up to 115 kbps.
• High-speed circuit-switched data at speeds up to 64 kbps.
Mobile equipment (ME)
•The ME is the only part of the GSM network which the subscriber will
really see.
•There are three main types of ME. These are listed below:
• Vehicle mounted: These devices are mounted in a vehicle and the
antenna is physically mounted on the outside of the vehicle.
• Portable mobile unit: This equipment can be handheld when in
operation, but the antenna is not connected to the handset of the
unit.
• Hand portable unit: This equipment comprises of a small telephone
handset not much bigger than a calculator. The antenna is connected
to the handset.

14. •The ME is capable of operating at a certain maximum
power output dependent on its type and use.
•These mobile types have distinct features which must be
known by the network.
Subscriberidentitymodule(SIM)
•TheSIMasmentionedpreviouslyisa“smartcard”which
plugsintotheMEandcontainsinformationabouttheMS
subscriber,hencethenameSubscriberIdentityModule
(Figure19.10).
•TheSIMcontainsseveralpiecesofinformation:
•Internationalmobilesubscriberidentity(IMSI):This
numberidentifiestheMSsubscriber.Itistransmittedover
theaironlyduringinitialization.

15. •Temporarymobilesubscriberidentity(TMSI): Thisnumber
identifiesthesubscriber;itisperiodicallychangedbythesystem
managementtoprotectthesubscriberfrombeingidentifiedby
someoneattemptingtomonitortheradiointerface.
•Locationareaidentity(LAI):Identifiesthecurrentlocationofthe
subscriber.ThefieldsofLAIare(1)countrycode(CC):three
decimalplaces;(2)mobilenetworkcode(MNC):twodecimalplaces;
(3)locationareacode(LAC):maximumfivedecimalplacesor
maximumtwicethe8bitscodedinhexadecimal(LAC<FFFF).
•Subscriberauthenticationkey(Ki):Thisisusedtoauthenticatethe
SIMcard.
•Mobilestationinternationalservicesdigitalnetwork(MSISDN):
Thisisthetelephonenumberofthemobilesubscriber.Itiscomprised
ofacountrycode,anetworkcode,andasubscribernumber.
•TheSIMcardandthehighdegreeofinbuiltsystemsecurityprovide
protectionofthesubscriber’sinformationandnetworksagainst
fraudulentaccess.
•SIMcardsaredesignedtobedifficulttoduplicate.

16. •TheSIMcanbeprotectedbyuseofapersonalidentitynumber(PIN)
password,similartobank/creditchargecards,toprevent
unauthorizeduseofthecard.
•TheSIMiscapableofstoringadditionalinformationsuchas
accumulatedcallcharges.
•Thisinformationwillbeaccessibletothecustomervia
handset/keyboardkeyentry.
(2)Basestationsubsystem(BSS)
•TheGSMBSSistheequipmentlocatedatacellsite.Itcomprisesof
acombinationofdigitalandRFequipment.
•TheBSSprovidesthelinkbetweentheMSandtheMSC.Generally,
theBSScommunicateswiththeMSoverthedigitalairinterfaceand
withtheMSCvia2Mbpslinks.

17. •TheBSScontainsaBSCandoneormoresub-standingBTSs.
•TheBSSisresponsibleforallfunctionsrelatedtotheradio
resourceschannelmanagement.
•Thisincludesthemanagementofradiochannelconfigurationwith
respecttouseasspeech,data,orsignallingchannels,allocation
andreleaseofchannelsforcallset-upandrelease,controlof
frequencyhopping,andtransmittedpowerattheMS.
•TheBSSiscomposedofthefollowing:
•Basetransceiverstation(BTS)
•Basestationcontroller(BSC)
•Transcoder(XCDR)
•TheBTSandBSCcommunicateoveraninterfaceknownasAbis
interface.
•Thisinterfaceestablishescompatibilitybetweencomponentsfrom
differentsuppliersandallowsoperationsbetweenthem.

18. •The BSS consists of four to seven or nine cells and one or more
base stations. A high-speed line (T1 or E1) is established between
BSS and MSC as shown in Figure.

19. Basetransceiverstation
•TheBTScontainsaradiotransceiverandantennas.Power
transmittedfromtheBTSdefinesacellinthenetwork.
•AsshowninFigure,BTSisplacedinthecentreofcellandhandles
theradiolinkprotocolswithMS.
•AlargenumberofBTSsaredeployedinalargeurbanarea.Number
oftransceiverswitheachBTSdependsondensityofusersinthe
cell.EachBTSmaycontain1–16transceivers.

20. The BTS does the following functions:
•Frequencyandtimesynchronization
•Encoding,encrypting,multiplexing,modulating,andfeedingRF
signalstotheantenna.
•Voicethroughfull-orhalf-rateservices
•Timingadvances
•Transcodingandrateadaptation
•Decoding,decrypting,andequalizingreceivedsignals
•Randomaccessdetection
•Uplinkchannelmeasurements

21. Basestationcontroller
BSCisaswitchingdevice.ItactsasaradioresourcemanagerofBTSs
asitperformsthefunctionsoffrequencyhopping,radiochannel
setup,andhandovers(bothintercellandintracell).Italsodoesthe
followingfunctions:
•EstablishesconnectionbetweenMSCandmobile
•ManagesthetimeslotsandfrequenciesfortheMS
•Translatesthevoicechannel(13kps)overtheradiolinkto
standardchannel(64kbps)usedbyPSTNorISDN
•AllocatesnecessarytimeslotsbetweentheBTSandMSinitsarea
•ReallocatesfrequenciesamongBTSs
•Controlsfrequencyhopping

22. •Provides an interface to the operations and maintenance centre
(OMC) for the BSS
• Performs traffic concentration to reduce the number of lines from the
MSC
.
• Synchronizes frequency and time
• Does time-delay measurements of received signals from the MS
• Power management

23. The transcoder
•ThetranscoderisusedtocompactthesignalsfromtheMSsothat
theyaremoreefficientlysentovertheterrestrialinterfaces.
•AlthoughthetranscoderisconsideredtobeapartoftheBSS,itis
veryoftenlocatedclosertotheMSC.
•Thetranscoderisusedtoreducetherateatwhichthetraffic
(voice/data)istransmittedovertheairinterface.
•AlthoughthetranscoderispartoftheBSS,itisoftenfound
physicallyclosertotheNSStoallowformoreefficientuseofthe
terrestriallinks.
•ThetranscoderconvertsthevoiceordataoutputfromMSCintoa
formmentionedinGSMspecification,thatisitconvertssignals
from64kbpsto16kbpsandviceversa.

24. BSSconfigurations
•Aswehavementioned,aBSCmaycontrolseveralBTSs.The
maximumnumberofBTSswhichmaybecontrolledbyoneBSCis
notspecifiedbyGSM.
•Individualmanufacturer’sspecificationsmayvarygreatly.
•TheBTSsandBSCmayeitherbelocatedatthesamecellsite(“co-
located”),orlocatedatdifferentsites(“remote”).
•Inreality,mostBTSswillberemote,astherearemanymore
•BTSsthanBSCsinanetworkasshowninFigure.
•AnotherBSSconfigurationisthedaisychain.ABTSneednot
communicatedirectlywiththeBSCwhichcontrolsitanditcanbe
connectedtotheBSCviaachainofBTSs.
•Daisychainingreducestheamountofcablingrequiredtosetupa
networkasaBTScanbeconnectedtoitsnearestBTSratherthan
allthewaytotheBSC.

25. •ProblemsmayarisewhenchainingBTSs,duetothetransmission
delaythroughthechain.
•Thelengthofthechainmust,therefore,bekeptsufficientlyshortto
preventtheroundtripspeechdelaybecomingtoolong.
•Othertopologiesarealsopermitted,includingstarsandloops.Loops
areusedtointroduceredundancyintothenetwork.Forexample,ifa
BTSconnectionislost,theBTSmaystillbeabletocommunicate
withtheBSCifasecondconnectionisavailable.

26. (3) Network switching subsystem
•TheNSSincludesthemainswitchingfunctionsoftheGSM
network.
•ThemainpartofNSSisthemobileswitchingcentre(MSC)which
performstheswitchingofcallsbetweenthemobileandotherfixed
ormobilenetworkusers,aswellasthemanagementofmobile
servicessuchasauthentication(Figure).
•Italsocontainsthedatabasesrequiredforsubscriberdataand
mobilitymanagement.Itsmainfunctionistomanage
communicationsbetweentheGSMnetworkandother
telecommunicationsnetworks.
•ThecomponentsoftheNSSarelistedbelow:

28. •Mobile service switching centre (MSC)
• Home location register (HLR)
• Visitor location register (VLR)
• Equipment identity register (EIR)
• Authentication centre (AUC)
• Interworking function (IWF)
• Echo canceller (EC)
•Inadditiontothemoretraditionalelementsofacellulartelephone
system,GSMhaslocationregisternetworkentities.Theseentities
areHLR,VLR,andEIR.
•Thelocationregistersaredatabaseorientedprocessingnodes
whichaddresstheproblemsofmanagingsubscriberdataand
keepingtrackofaMS’slocationasitroamsaroundthenetwork.

29. •Functionally,theIWFandtheECsmaybeconsideredaspartsof
theMSC,sincetheiractivitiesareinextricablylinkedwiththoseof
theswitchasitconnectsspeechanddatacallstoandfromthe
MSs.
Mobileserviceswitchingcentre(MSC)
•TheMSCisincludedintheGSMsystemforcall-switching.Its
overallpurposeisthesameasthatofanytelephoneexchange.
•However,becauseoftheadditionalcomplicationsinvolvedinthe
controlandsecurityaspectsoftheGSMcellularsystemandthe
widerangeofsubscriberfacilitiesthatitoffers,theMSChastobe
capableoffulfillingmanyadditionalfunctions.
•ThecentralcomponentofthenetworksubsystemistheMSC.The
MSCperformstheswitchingofcallsbetweenthemobileandother
fixedormobilenetworkusers,aswellasthemanagementofmobile
servicessuchasregistration,authentication,locationupdating,
handovers,andcallroutingtoaroamingsubscriber.

30. •TheMSCwillcarryoutseveraldifferentfunctionsdependingupon
itspositioninthenetwork.
•WhentheMSCprovidestheinterfacebetweenthePSTNandthe
BSSsintheGSMnetwork,itisknownasa gatewayMSC.
•Inthisposition,itwillprovidetheswitchingrequiredforallMS
originatedorterminatedtraffic.
•EachMSCprovidesservicetoMSslocatedwithinadefined
geographiccoveragearea.
•ThenetworktypicallycontainsmorethanoneMSC.OneMSCis
capableofsupportingaregionalcapitalwithapproximatelyone
millioninhabitants.
•AnMSCofthissizewillbecontainedinabouthalfadozenracks.
•ThefunctionscarriedoutbytheMSCarelistedbelow:

31. •Callprocessing:Includescontrolofdata/voicecallsetup,supervision
andrelease,inter-BSS,andinter-MSChandovers,andcontrolof
mobilitymanagement(subscribervalidationandlocation).
•Operationsandmaintenancesupport: Includesdatabase
management,managementofradioresources,registration,location
updatingandtrafficmeteringandmeasurement,andaman–machine
interface.
•Internetworkinterworking:ManagestheinterfacebetweentheGSM
networkandthePSTN.
•Billing:CollectscallbillingdataHome location register (HLR)
•TheHLRisthereferencedatabaseforsubscriberparametersusedfor
storageandmanagementofnewandoldsubscriptions.
•Itstorespermanentdataaboutsubscriberswhichincludeservice
profile,activitystatus,andlocationinformation.Itistherefore
consideredasmostimportantdatabase.
•WhenapersongetsaSIM,thenallhisinformationmentionedin
subscriptionformisregisteredinHLR.

32. Usually,oneHLRisdevelopedforeachGSMnetworkforadministration
ofsubscriberconfigurationandservices
Besidestheup-to-datelocationinformationforeachsubscriber,whichis
dynamic,theHLRmaintainsthefollowingsubscriberdataona
permanentbasis:
•Internationalmobilesubscriberidentity(IMSI)
•Servicesubscriptioninformation
•Servicerestrictions
•Supplementaryservices
•Mobileterminalcharacteristics
•Billing/accountinginformation

33. Visitor location register
•TheVLRisadatabasethatlessenstheburdenonHLR.
•ItisintegratedwithMSCandcontainstemporaryinformationabout
subscribers.ThisinformationisrequiredbytheMSCinorderto
servicevisitingsubscribers.
•TheVLRwillgetthedataaboutaMSfromtheHLR,wheneverthe
MSroamsintoanewMSCarea.
•LaterthisinformationcontainedinVLRwillbeusedforcallsetup,
wheneverthatMSmakesacallwithoutinterrogatingHLR.
•TheVLRalsocontainsinformationaboutlocallyactivatedfeatures
suchascallforwardonbusy.
•TheadditionaldatastoredintheVLRislistedbelow:
•Mobilestatus(busy/free/noanswer,etc.)
•Locationareaidentity(LAI)
•Temporarymobilesubscriberidentity(TMSI)
•MobileStationRoamingNumber(MSRN)

34. Location area identity (LAI)
•Cellswithinthepubliclandmobilenetwork(PLMN)are
groupedtogetherintogeographicalareas.Eachareais
assignedaLAI.Alocationareamaytypicallycontain30cells.
•EachVLRcontrolsseveralLAIsandasasubscribermoves
fromoneLAItoanother,theLAIisupdatedintheVLR.Asthe
subscribermovesfromoneVLRtoanother,theVLRaddress
isupdatedattheHLR.
Temporary mobile subscriber identity
•TheVLRcontrolstheallocationofnewTMSInumbersand
notifiesthemtotheHLR.TheTMSIwillbeupdatedfrequently.
•Thismakesitverydifficultforthecalltobetracedand
thereforeprovidesahighdegreeofsecurityforthesubscriber.
•TheTMSImaybeupdatedinanyofthefollowingsituations:
•Callsetup
•OnentrytoanewLAI
•OnentrytoanewVLR

35. •Mobile subscriber roaming number
•Asasubscribermaywishtooperateoutsideits“home”systemat
sometime,theVLRcanalsoallocateaMSRN.Thisnumberis
assignedfromalistofnumbersheldattheVLR(MSC).
•TheMSRNisthenusedtoroutethecalltotheMSCwhichcontrols
thebasestationintheMSscurrentlocation.
•Equipment identity register
•TheEIRmaintainsinformationtoauthenticateterminalequipment
sothatfraudulent,stolen,ornon-type-approvedterminalscanbe
identifiedandservicecanbedenied.
•Theinformationisintheformofwhite,gray,andblackliststhatmay
beconsultedbythenetworkwhenitwishestoconfirmthe
authenticityoftheterminalrequestingservice.
•TheEIRdatabaseconsistsoflistsofIMEIs(orrangesofIMEIs)
organizedasfollows:
•Whitelist:ContainsthoseIMEIswhichareknowntohavebeen
assignedtovalidMSequipment.
•Blacklist:ContainsIMEIsofMSwhichhavebeenreportedstolen
orwhicharetobedeniedserviceforsomeotherreason.

36. •Graylist:ContainsIMEIsofMSwhichhaveproblems(e.g.,faulty
software).Thesearenot,however,sufficientlysignificanttowarranta
“blacklisting”.
•TheEIRdatabaseisremotelyaccessedbytheMSCsinthenetworkand
canalsobeaccessedbyanMSCinadifferentPLMN.
•AsinthecaseoftheHLR,anetworkmaywellcontainmorethanoneEIR
witheachEIRcontrollingcertainblocksofIMEInumbers.
•TheMSCcontainsatranslationfacility,whichwhengivenanIMEIreturns
theaddressoftheEIRcontrollingtheappropriatesectionoftheequipment
database.
Authentication centre (AuC)
•TheAUCisaprocessorsystem.Itperformsthe“authentication”function.It
willnormallybeco-locatedwiththeHLRasitwillberequiredto
continuouslyaccessandupdate,asnecessary,thesystemsubscriber
records.
•TheAUC/HLRcentrecanbeco-locatedwiththeMSCorlocated
•remotefromtheMSC.
•Theauthenticationprocesswillusuallytakeplaceeachtimethesubscriber
“initializes”onthesystem.

37. Interworkingfunction
•TheIWFprovidesthefunctiontoenabletheGSMsystemto
interfacewiththevariousformsofpublicandprivatedatanetworks
currentlyavailable.
•ThebasicfeaturesoftheIWFarelistedbelow:
•Datarateadaption
•Protocolconversion
•SomesystemsrequiremoreIWFcapabilitythanothers;this
dependsuponthenetworktowhichitisbeingconnected.

38. Physical and logical channels
•Thephysicalchannelisthemediumoverwhichtheinformation
iscarried.Inthecaseofaterrestrialinterface,thiswouldbea
cable.
•Thelogicalchannelsconsistoftheinformationcarriedoverthe
physicalchannel.
GSM physical channels
•AsingleGSMRFcarriercansupportuptoeightMS
subscriberssimultaneously.
•Thefigureshowshowthisisaccomplished.Eachchannel
occupiesthecarrierforone-eighthofthetime.
•Thisisatechniquecalledtimedivisionmultipleaccess
(TDMA).

39. Timeslots and TDMA frame

40. •Timeisdividedintodiscreteperiodscalled“timeslots”shownin
Figure.
•Thetimeslotsarearrangedinsequenceandareconventionally
numbered0to7.
•Eachrepetitionofthissequenceiscalleda“TDMAframe”.
•Thetimingofthetransmissionstoandfromthemobilesiscritical.
•TheMSorBSmusttransmittheinformationrelatedtoonecallat
exactlytherightmoment,orthetimeslotwillbemissed.
•Theinformationcarriedinonetimeslotiscalleda“burst”.
•Eachdataburst,occupyingitsallocatedtimeslotwithinsuccessive
TDMAframes,providesasingleGSMphysicalchannelcarryinga
varyingnumberoflogicalchannelsbetweentheMSandtheBTS.

41. GSMlogicalchannels
•Therearetwomaingroupsoflogicalchannels:trafficchannelsand
controlchannels.
•GSMtrafficchannels(TCH):Thetrafficchannelcarriesspeechor
datainformation.
•Thedifferenttypesoftrafficchannelsarelistedbelow:

42. Subtypes of traffic channels

43. Speechchannels:Speechchannelsaresupportedbytwodifferent
methodsofcodingknownasFRandEFR.EFRcodingprovidesa
speechservicethathasimprovedvoicequalityfromtheoriginalFR
speechcoding,whilstusingthesameairinterfacebandwidth.
EFRemploysanewspeechcodingalgorithminadditiontotheFR
channelcodingalgorithmtoaccomplishthisimprovedspeech
serviceasshownin.Figure-previousslide.

44. GSMcontrolchannelgroups
•Thecontrolchannelgroupsmaybedividedintobroadcastcontrol
channel(BCCH),commoncontrolchannel(CCCH),andthededicated
controlchannel(DCCH)groupsasshowninFigure.
•BCCHgroup: TheBCCHaredownlinkonly(BSStoMS)and
comprisethefollowing:
•BCCHcarriesinformationaboutthenetwork,aMS’spresent
cellandthesurroundingcells.Itistransmittedcontinuouslyasits
signalstrengthismeasuredbyallMSsonsurroundingcells.
•Thesynchronizingchannel(SCH)carriesinformationforframe
synchronization.
•CCCHgroup:TheCCCHgroupworksinbothuplinkanddownlink
directions:
•Randomaccesschannel(RACH)isusedbyMSstogainaccessto
thesystem.
•Pagingchannel(PCH)andaccessgrantedchannel(AGCH)operate
inthe“downlink”direction.TheAGCHisusedtoassignresources
suchasastand-alonededicatedcontrolchannel(SDCCH)totheMS.
ThePCHisusedbythesystemtocallaMS.ThePCHandAGCHare
neverusedatthesametime.
•Cellbroadcastchannel(CBCH)isusedtotransmitmessages(e.g.
roadtrafficinformation,sportingresults,andsoon)tobebroadcastto
allMSswithinacell
.

45. Subtypes of CCH

46. DCCHgroup:DCCHsareassignedtoasingleMSforcallsetupand
subscribervalidation.DCCHcomprisesofthefollowing:
•SDCCHwhichsupportsthetransferofdatatoandfromtheMS
duringcallsetupandvalidation.
•ACCHconsistsofslowACCHwhichisusedforradiolink
measurementandpowercontrolmessages.FastACCHisusedto
pass“event”typemessages,forexamplehandovermessages.Both
FACCHandSACCHoperateinuplinkanddownlinkdirections.
Broadcastcontrolchannel
•TheBCCHistransmittedbytheBTSatalltimes.TheRFcarrier
usedtotransmittheBCCHisreferredtoastheBCCHcarrier.
•TheinformationcarriedontheBCCHismonitoredbytheMS
periodically(atleastevery30s),whenitisswitchedonandnotina
call.BCCHcarriesthefollowinginformation(thisisonlyapartial
list):

47. •Location area identity (LAI)
• List of neighbouring cells which should be monitored by the MS
• List of frequencies used in the cell
• Cell identity
• Power control indicator
• DTX permitted
• Access control (e.g., emergency calls, call barring)
• CBCH description
TheBCCHistransmittedatconstantpoweratalltimes,anditssignal
strengthismeasuredbyallMSwhichmayseektouseit.“Dummy”bursts
aretransmittedtoensurecontinuitywhenthereisnoBCCHcarriertraffic.
•1.Frequencycorrectionchannel(FCCH):Thisistransmittedfrequentlyon
theBCCHtimeslotandallowsthemobiletosynchronizeitsownfrequency
tothatofthetransmittingbasesite.
•TheFCCHmayonlybesentduringtimeslot0ontheBCCHcarrier
frequencyandthereforeitactsasaflagtothemobiletoidentifytimeslot
0.
•2.Synchronizationchannel(SCH):TheSCHcarriestheinformationto
enabletheMStosynchronizetotheTDMAframestructureandknowthe
timingoftheindividualtimeslots

48. Subtype of BCCH

49. Commoncontrolchannels(CCCH)
•TheCCCHisresponsiblefortransferringcontrolinformationbetweenall
mobilesandtheBTS.Thisisnecessaryfortheimplementationof“call
origination”and“callpaging”functions(Figure).
•Itconsistsofthefollowing:
Randomaccesschannel(RACH): Usedbythemobilewhenitrequiresto
gainaccesstothesystem.Thisoccurswhenthemobileinitiatesacallor
respondstoapage.
Pagingchannel(PCH): UsedbytheBTStopageMS(pagingcanbe
performedbyanIMSI,TMSI,orIMEI).
Accessgrantcontrolchannel(AGCH): UsedbytheBTStoassignaDCCH
toaMSinresponsetoanaccessmessagereceivedontheRACH.The
MSwillmovetothededicatedchannelinordertoproceedwitheithera
callsetup,responsetoapagingmessage,locationareaupdate,orshort
messageservice.
Cellbroadcastchannel(CBCH): Thischannelisusedtotransmitmessages
tobebroadcasttoallMSswithinacell.TheCBCHusesaDCCHtosend
itsmessages.However,itisconsideredacommonchannelbecausethe
messagescanbereceivedbyallmobilesinthecell.
.

50. •ActiveMSsmustfrequentlymonitorbothBCCHandCCCH.The
CCCHwillbetransmitted
•ontheRFcarrierwiththeBCCH
SubtypeofCCCH

51. Multiple Access Techniques
Introduction
•Cellularsystemsdivideageographicregionintocells,wherea
mobilephoneineachcellcommunicateswithabasestation.
•Themainobjectiveofcellularsystemsdesignistohandleasmany
callsaspossible(i.e.,capacity)inagivenbandwidthwithreliability.
•Thesuccessofcellularnetworkdependsontheavailabilityofthe
radiofrequencyspectrum.
•Theradiofrequencyspectrumisafinitenaturalresourceandhas
greaterdemandsplacedoniteveryday.
•Thisfiniteresourceisusuallydefinedintermsofbandwidth.
•Toallowmanymobileuserstosharesimultaneouslyafiniteamount
ofradiospectrum,inamostefficientway,varioustechnologieshave
beendevelopedandthegoalbehindthesemethodsistohandleas
manycallsaspossibleinagivenbandwidth(i.e.,call-handling
capacity).

52. •Thisconceptiscalled“multipleaccess”(Figure).
•Thechoiceofmultipleaccess(MA)technologyistosharethe
availablescarcebandwidthefficientlyamongalargenumberof
userswhichcouldsignificantlyenhanceorlowertheservice
qualitydeliveredtoendusers.
Multiple access scenario

53. •Sharing the limited bandwidth efficiently among many users is one
of the main objectives of multiple access schemes.
•The analogy of highway with several lanes (Next Figure) gives the
simple example of multiple access methods such as SDMA, FDMA,
and TDMA. In this figure, the medium is highway, the users are
cars, and the interference is due to accidents.

57. •Spread-spectrumisatechniquethatincreasessignalbandwidth
beyondtheminimumnecessaryfordatacommunication;theband
spreadisaccomplishedbymeansofacodethatisindependentof
thedata,andasynchronizedreceptionwiththecodeatthe
receiverisusedfordespreadingandsubsequentdatarecovery.

59. Frequency division multiple access
•TheFDMAisthesimplestschemeusedtoprovidemultipleaccess
inanaloguetransmission.
•InFDMAsystems,theradiofrequencyspectrumisdividedinto
severalfrequencybandsseparatedbyacertainguardband.
•Eachfrequencybandcanbeusedsimultaneously

60. •Inthistechnique,thebandwidthisdividedintoanumberof
channelsanddistributedamonguserswithafiniteportionof
bandwidthforpermanentuseasillustratedinFigure.
•Inaddition,wecanobservefromFigurethatFDMApermitsonly
oneuserperchannelbecauseitallowstheusertousethechannel
100percentofthetime.
•Therefore,onlythefrequency“dimension”isusedtodefine
channels.
•Eachblockrepresentsadifferentuser.Frequencyguardbandsare
providedbetweenadjacentsignalspectratominimizecrosstalk
betweenadjacentchannels.
•InFDMA,thechannelhastwofrequencies,namelyforward
channelandreversechannel.
•WhentheFDMAtechniqueisemployed,aslongastheuseris
engagedin“conversation,”nootherusercanaccessthesame
spectrumspace.

61. NumberofchannelssupportedbyFDMAsystem
•Thenumberofchannelsthatcanbesimultaneouslysupportedin
FDMAsystemisgivenby(Figure)

63. Advantages
1.Acontinuoustransmissionscheme,andthereforeoflower
complexitythanTDMAscheme,forexample,synchronization
requirementsarenotsevere
2.Simpletoimplementfromahardwarestandpoint,becausemultiple
usersareisolatedbyemployingsimplebandpassfilters
3.Fairlyefficientwithasmallbasepopulationandwhentrafficis
constant
4.Nochannelequalizationrequired
5.Capacitycanbeincreasedbyreducingtheinformationbitrateand
usinganefficientdigitalspeechcodingscheme
Disadvantages
1.IfanFDMAchannelisnotinuse,thenitsitsidleandcannotbeused
byotheruserstoincreaseorsharethecapacity.Therefore,FDMA
implementationbecomesinefficientuseofspectrum.
2.FDMArequirestightRFfilteringtominimizeadjacentchannel
interference.
3. Network and spectrum planning are intensive.

64. 4.Frequencyplanningistimeconsuming.
5.Eventhoughnotwousersusethesamefrequencybandat
thesametime,guardbandsareintroducedbetween
frequencybandstominimizeadjacentchannelinterference.
Guardbandsareunusedfrequencyslotsthatseparate
neighbouringchannels.Thisleadstoawasteofbandwidth.
Whencontinuoustransmissionisnotrequired,bandwidth
goeswastedsinceitisnotbeingutilizedforaportionofthe
time.
ApplicationsofFDMA
1.Walkie-talkiesandmobilenetworksforclosedusergroups
oftenuseFDMA.
2.AnotherexampleofFDMAisAMorFMradiobroadcasting,
whereeachstationhasitsownchannel.
3.EarlycellulartelephonymostlyusedFDMAanalogue
transmission

65. •InGSM900,thefrequenciesforuplinkanddownlink
communicationareallottedbythebasestationasillustratedin
Figure

66. Timedivisionmultipleaccess
•Asthefrequencyspectrumexperiencesmoretraffic,spectrum
efficiencybecomesincreasinglyimportant.
•TDMAsystemsweredevelopedasFDMAsystemspectrum
efficiencybecameinsufficient.
•Indigitalsystems,continuoustransmissionisnotrequiredbecause
usersdonotusetheallottedbandwidthallthetime.
•Itallowsseveraluserstosharethesamefrequencybandbydividing
thetimescaleintodifferenttimeslotswhichareperiodicallyallocated
toeachmobileuserforthedurationofacall.
“TDMAsystemshavethecapabilitytosplitusersintotimeslots
becausetheytransferdigitaldata,insteadofanaloguedata
commonlyusedinlegacyFDMAsystems”.
•Digital2GcellularsystemsthatusedtheTDMAtechnologyare
GSM,IS-136,PDC,andDECTstandardforportablephones

67. TDMA principle of operation
“TDMAsystemsdividetheradiospectrumintotimeslotsandeach
userisallowedtoeithertransmitorreceiveineachtimeslots(i.e.,
differentuserscanusethesamefrequencyinthesamecellbutat
differenttimes)”.

68. •Figureshowstheprincipleofoperation.EachrowintheFigure
representsasinglechannelanddividedintothreetime-slots.
•CallsinaTDMAsystemstartinanalogueformatandaresampled,
transformingthecallintoadigitalformat.
•Afterthecallisconvertedintodigitalformat,theTDMAsystem
placesthecallintoanassignedtimeslot.
•NumberofuserssupportedbyTDMAillustratestheimprovementin
theefficiencyofTDMAsystem;letuscompareFigures(FDMA)and
Figure(TDMA).
•ItisobviousthattheFDMAsystemsupports4userswhiletheTDMA
systemsupports12userswithinthesamebandwidthastheFDMA
system
•NumberofuserssupportedbytheTDMAsystem=Numberof
channelsinthefrequencyspectrum×Timeslots/channel
=4×3=12(fromFigure)

70. TDD:Incellularcommunications,whenausermovesfromonecell
toanotherthereisachancethatusercouldexperienceacalllossif
therearenofreetimeslotsavailable.
TDMAusesdifferenttimeslotsfortransmissionandreception.This
typeofduplexingisreferredtoastimedivisionduplexing(TDD).TDD
doesnotrequireduplexers.TDDprovidestwosimplextimeslotson
thesamefrequencyasshowninFigure.
TDMA frame structure
•InaTDMAsystem,timeisdividedintoequaltimeintervalscalled
slots.Userdataistransmittedintheslots.Severalslotsmakeupa
frame.Guardtimesareusedbetweeneachuser’stransmissions
tominimizecrosstalkbetweenchannels(Figure).
•Thepreamblecarriestheaddressandsynchronizationinformation
thatboththebasestationandthemobilestationsusefor
identification.

72. EfficiencyofTDMA
•TheefficiencyofTDMAsystemisameasureofthepercentageof
transmitteddatathatcontainsinformationasopposedtoproviding
overheadfortheaccessscheme.
•Theframeefficiency“η”isthepercentageofbitsperframewhich
containstransmitteddata.

73. Advantages
•DataTransmissionisindiscretebursts
ExtendedbatterylifeoverFDMAandtalktime
Handoffprocessissimpler,sinceitisabletolistenforotherbase
stationsduringidletimeslots
•Moreefficientuseofspectrum,comparedtoFDMA
Willaccommodatemoreusersinthesamespectrumspacethanan
FDMAsystemwhichimprovescapacityinhigh-trafficareas,suchas
largemetropolitanareas
Efficientutilizationofhierarchicalcellstructures–pico,micro,and
macrocells
•Sincedifferentslotsareusedfortransmissionandreception,duplexersare
notrequired

74. Disadvantages
•
TDMArequiressynchronization.Ifthetimeslotsynchronizationislost,
thechannelsmaycollidewitheachother.
•Formobilesand,particularlyforhandsets,TDMAontheuplink
demandshigh-peakpowerintransmitmodethatshortensbattery
life.IfaTDMAinterfaceconsistsofnchannels,thenthetransmitted
powersare10logntimeshigherthaninanFDMAsystem.
•Networkandspectrumplanningareintensive.
•Droppedcallsarepossiblewhenusersswitchinandoutofdifferent
cells.
•Highercostsduetogreaterequipmentsophistication.
•Equalizationisrequired,sincetransmissionratesaregenerallyvery
highascomparedtoFDMAchannels.

76. Code division multiple access
•CDMAisthethirdmultipleaccesstechniqueusedincellular
systems.CDMAallowstransmissionstooccupytheentirebandwidth
atthesametimewithoutinterference.
•CDMAusesspread-spectrumtechniquetoincreasespectrum
efficiencyovercurrentFDMAandTDMAsystems.
•Aspread-spectrumsignalisasignalthathasanextramodulation
thatexpandsthesignalbandwidthbeyondwhatisrequiredbythe
underlyingdatamodulation.
Spread-spectrumcommunicationsystemsareusefulforthefollowing:
1.Suppressinginterference
2.Makinginterceptiondifficult
3.Accommodatingfadingandmultipathchannels
4.Providingamultiple-accesscapability

77. •Themostpracticalanddominantmethodsofspread-spectrum
communicationsaredirect-sequencemodulationandfrequency
hoppingofdigitalcommunications.
•CDMAcellulartechnologyisoriginallyknownasIS-95,which
competeswithGSMtechnologyfordominanceinthecellularworld.
•CDMAcellularsystemsoperateinthe800MHzand1.9GHzPCS
bands.QUALCOMMisthedeveloperoftheCDMAairinterfaceused
incellularsystems.ComparedtoGSMcellularsystems,
•CDMArequiresfewercelltowersandprovidesuptofivetimesthe
callingcapacity.
•CDMAalsoprovidesmorethan10timesthevoicetrafficofearlier
analoguesystem(AMPS)andisthebasisfor3Gdatatransmission
forGSMcarriers.
CDMA principle of operation
•CDMA uses unique spreading codes to spread the base band data
before transmission.
“CDMAassignstoeachuserauniquecodesequencethatisused
tocodedatabeforetransmission.Ifareceiverknowsthecode
sequencerelatedtoauser,itisabletodecodethereceiveddata”.

78. •Thecodesaresharedbythemobilephoneandthebasestation.
ThecodesarecalledPseudorandomcodesequences.
•Alltheuserscanaccesstheentirespectrumallocationallofthe
time(Figure).
•Auser’suniquecodeseparatesthecallfromallothercalls.The
signalistransmittedinachannel,whichisbelownoiselevel.The
receiverthenusesacorrelatortodespreadthewantedsignal,
whichispassedthroughanarrowbandpassfilter.
•Unwantedsignalswillnotbedespreadandwillnotpassthroughthe
filter.
•Codestaketheformofacarefullydesignedone/zerosequence
producedatamuchhigherratethanthatofthebasebanddata.
•Therateofaspreadingcodeisreferredtoas chiprateratherthan
bitrate.
•CDMAchannelscanhandleanunspecifiednumberofusers.The
capacityofthesystemdependsonthequalityofcurrentcalls.As
moreusersareadded,noiseisaddedtothewidebandfrequency,
thereforedecreasingthequalityofcurrentcalls.

80. •Eachuser’stransmissionpowerincreasesthelevelofthefrequency
spectrum’s“noisefloor,”andthereforedecreasestheoverallcall
qualityforallusers.
•Toeliminatethe“noisefloor,”CDMAmobilephonesandbase
stationsusethe minimumamountofpower requiredto
communicatewitheachother.
•Theyuseprecise powercontroltodecreaseusers’transmission
power.Bydecreasingauser’stransmissionpower,themobile
phonehasaddedbatterylife,increasedtalktime,andsmaller
batteries.
Advantages
•Greatestspectrumefficiency:capacityincreasesabout8to10times
thatofananaloguesystemand4to5timesthatofotherdigital
systems,whichmakesitmostusefulinhightrafficareaswithalarge
numberofusersandlimitedspectrum.
•CDMAimprovescallqualitybyfilteringoutbackgroundnoise,
crosstalk,andinterference
•“Softhandoffs”:becauseofthemultiplediversitiesinuse,handoffs
betweencellsareundetectedbytheuser.

81. Simplifiedfrequencyplanning:allusersonaCDMAsystemusethe
sameradiofrequencyspectrum.
–Engineeringdetailedfrequencyplansarenotnecessary.
–Frequencyre-tunesforexpansionareeliminated.
–Fewercellsarerequiredforqualitycoverage.
•RandomWalshcodesenhanceuserprivacy;aspread-spectrum
advantage.
•Precisepowercontrolincreasestalktimeandbatterylifeformobile
phones.
Disadvantages
•Backwardscompatibilitytechniquesarecostly.
•Currently,equipmentisexpensive.
•Difficulttooptimizetomaximizeperformance.
•Lowtrafficareasleadtoinefficientuseofspectrumandequipment
resources
.

82. Near-far problem
•AcommondifficultyencounteredinCDMAwirelesscommunication
systemsimplementedusingdirectsequencespreadspectrum
(DSSS)technologyisreferredtoasthenear-farproblem.
•Thenear-farproblemoccurswhentwoormoreDSSStransmitters
transmitthesignalstowardsthesameDSSSreceiverasshownin
Figure.

83. •InFigure,thepowerofthespread-spectrumsignalcomingfrom
DSSStransmitter1islowerthanthatcomingfromDSSStransmitter
2becausetransmitter1isfartherawayfromthereceiverthan
transmitter2.
•ThiscanbeaseriousproblemwhentheDSSSreceiverissetto
demodulatethespread-spectrumsignalcomingfromDSSS
transmitter1,becausethepowerlevelofthissignalislowerthanthat
ofthespread-spectrumsignalcomingfromDSSStransmitter2.
Space division multiple access
•Inadditiontofrequency,time,andcodedomains,thespatial
dimensioncanalsobeusedformultiplexingofdifferentdatastreams
bytransmittingthedatastreamsoverdifferent,non-overlapping
transmissionchannels.Theuseofspacedivisionmultiplexingfor
multipleaccessistermedSDMA.
“SDMAenablesuserstosharesimultaneouslythesamebandwidthin
differentgeographicallocations.SDMAsolvescapacityproblemof
wirelesscommunicationsystemsbyexploitationofthespatial
dimensionwhichmakesitpossibletoidentifytheindividualusers,
evenwhentheyareinthesametime/frequency/codedomains.
SDMAcanbeachievedusingbeamformingorsectorization”.

85. AdvantagesofSDMAtechnique
TheadvantagesofSDMAsystemincludethefollowing:
•Range extension: The coverage area of the antenna array is greater than
that of any single element as a result of the gain provided by the array.
When a system is constructed using SDMA, the number of cells required to
cover a given area can be substantially reduced.
•Interferencesuppression:Interferencefromothersystemsandfrom
usersinothercellsissignificantlyreducedbyexploitingthedesireduser’s
uniquechannelimpulseresponses(CIRs).In“noisy”areaswhererangeis
limitedbyinterference,spatiallyselectivetransmissionandreceptionresult
inrangeextension.
•Multipatheffectelimination:Thecopiesofthedesiredsignalthathave
arrivedattheantennaafterbouncingfromobjectsbetweenthesignal
sourceandtheantennacanoftenbemitigated.Incertaincases,the
multipathcanactuallybeusedtoreinforcethedesiredsignal.
•Capacityincrease:Capacityincreasecanbedoneintwoways:
•Channelreusepatternsincellularsystemscanbesignificantly
tighterbecausetheaverageinterferenceresultingfromco-channelsignals
inothercellsismarkedlyreduced(e.g.,movingfroma7-celltoa4-cell
reusepatternnearlydoublescapacity).

86. Separatespatialchannelscanbecreatedineachcellonthesame
conventionalchannel.
•Inotherwords,intracellularreuseofconventionalchannelsis
possible.
Compatibility:SDMAiscompatiblewithmostoftheexisting
modulationschemes,carrierfrequenciesandotherspecifications.
Furthermore,itcanbereadilyimplementedusingvariousarray
geometriesandantennatypes.

87. Comparison of SDMA, TDMA, FDMA, and CDMA

88. Random Access
Inrandomaccessorcontentionmethods,nostationissuperiorto
anotherstationandnoneisassignedthecontroloveranother.No
stationpermits,ordoesnotpermit,anotherstationtosend.Ateach
instance,astationthathasdatatosendusesaproceduredefined
bytheprotocoltomakeadecisiononwhetherornottosend.
ALOHA
Carrier Sense Multiple Access
Carrier Sense Multiple Access with Collision Detection
Carrier Sense Multiple Access with Collision Avoidance
Topics discussed :

91. Frames in a pure ALOHA network

92. Procedure for pure ALOHA protocol

94. Vulnerable time for slotted ALOHA protocol

98. CSMA: Behavior of three persistence methods

99. Flow diagram for three persistence methods

100. Flow diagram for the CSMA/CD

101. Energy level during transmission, idleness, or collision

102. Timing in CSMA/CA

103. In CSMA/CA, the IFS can also be used to
define the priority of a station or a frame.
Note

104. In CSMA/CA, if the station finds the
channel busy, it does not restart the timer of
the contention window;
it stops the timer and restarts it when the
channel becomes idle.
Note

105. Flow diagram for CSMA/CA