2nd generation mobile networks used digital technology to transmit voice calls and basic data via SMS. GSM was the first digital standard, using either TDMA or CDMA. GSM offered teleservices like calls and voicemail, bearer services for data transport, and supplementary services like call forwarding. It operated using TDMA, dividing frequencies into time slots for multiple access. Voice was digitized and encrypted for features like error correction. Subscriber identity was based on SIM cards containing keys and identifiers. CDMA used spread spectrum and unique codes to allow multiple devices on the same frequency band simultaneously. GSM consisted of the base station system for radio access, the network switching subsystem for call processing, and the operation and support system for

1. 2nd
GENERATION:
The 2G secondgenerationmobilecommunicationsystemis digitalsystem.Itcannotnormallytransfer
data such as email orsoftware otherthanthe digital voice call itself,andotherbasicancillarydatasuch
as data and time.SMS messagingisalsoavailable asaform of data transmissionforsome standard.2G
cellulartelecomnetworkswere commerciallylaunchedonthe GSM(Global systemformobile
communication) standardinFinland. 2Gtechnologiescanbe dividedinto Time Division Multiple
Access (TDMA) basedandCode Division Multiple Access (CDMA) based standards depending on the
type of multiplexing used.
Three servicesare offeredbyGSM
1) Teleservices
2) BearerServices
3) Supplementary Service
TELESERVICES:
>Telephony
Codecsforvoice anddata
Encryptionof voice data
>EmergencyCalls
These callsmusthave the highestpriority
Directingyoutothe nearestconnectionpoint
>Voice Mail
>Fax Mail
>Facsimile Group3
>Short MessagingService (SMS)
Includingthe SMS – Cell Broadcast(SMS-CB)
Bearer Services:
>Bearerservicesare for the transportof data
9600 bitspersecondmaximum
Synchronousandasynchronous
Alternate dataand speech
>Transparent
Fixeddelayindatatransfer
No guaranteesof dataintegrity
>Non transparent
Variable Delay
GuaranteeddataintegrityviaAutomaticrepeatrequest(ARQ)
SUPLEMEMNTARYSERVICES:
User Identification
Call Redirection
Call Hold
Advice of Charge
Call Barring
Call Forwarding
Multipartycalls

2. GSMOperates:
Time DivisionMultiplexAccess(TDMA)
o Thisallowthe frequencytobe brokenupintoslots
 The frequenciesusedare GSM900 , GSM 1800 andGSM 1900
 Separate frequenciesare usedforthe uplinkanddownlink
 890-915MHz uplink,935-960MHz downlinkforexample
 UplinkandDownlinkare 45 MHz apart
 200KHz spacingon the frequency
 124 pairsof channels
 These channelsare thendividedinto8time slices
 For GSM each slice is 0.577 ms
 Total channelsavailable is8* 124 = 992 (ina theoretical context)
o Thismeansthat there iseighttimesthe capacityas before inone partof the
frequency
 Assumingthe same rate isused
One of the featureswhichallowedthe increase inthe numberof usersisdigitisationof voice
o The voice issampledusinga(analogue toDigital Converter) ADC
 8KHz / second,withan 8 bit result
o The voice informationisthentransferredusingone of these methods
 EnhancedFull Rate
 Bestquality speech
 Uses a full slot
 Marginal increase inprocessoroverhead
 Full Rate
 Improvedspeechquality
 Takesa full slottotransfer
 Half Rate
 Thismode allowsfora doublingof capacityto a base station
 Loss inspeechquality,butnotsignificant
 Half a slotto transfer
Digitisation
o Of voice allowsforanumberof features
o Takeslessbandwidthtotransferthe voice
o The data once digital
 Can be encrypted
 Can be recoveredevenwiththe attenuationeffects
 Additional bitscanbe addedtoallow forerrors withintransport
 Withoutthe needforretransmission –ForwardError Checking
(FEC)
 The digital voice datacan thenbe place inside of apacketfor transport
Time DivisionMultiple Access(TDMA)
 Guard Time:Interval betweenburstsusedtoavoidoverlapping
 Preamble: Firstpart of the burst
 Message:Part of burst that includesuserdata
Postamble:Last part of burst – usedto initialise followingburst

3. SubscriberIdentityModule(SIM) Card
o Essential forthe GSMnetwork
o The SIM islocatedwithinthe Mobile station(MS)
 Basicallythe phone handsetyouuse
o Contains
 SubscriberAuthenticationkey
 128 bitencryptionkey
 International MobileSubscriberIdentity(IMSI).
 A unique internationalnone dial able numberwhichisattachedtoyour
device.
 TemporaryMobile SubscriberIdentity .
 Mobile StationIntegratedService DigitalNetwork(MSISDN).
 BasicallythisisYOUR phone number.
 PIN to secure the card.
 SMS messages.
 Personal data,phone numbers,Phonesettings etc.
o Each one isunique
 Authenticationencryptionisusedcalledthe A3/A8.
Thisis usedtoauthenticate yourdevice usingchallenge/response
 A randomnumberissent.
 The A3/A8 algorithmthenworksonthe numberandreturnsa
32-bit response.
 If this matchesthe one whichthe networkhascalculatedthe
device isauthenticated.
 A5 Encryptionisusedfor the voice calls.
Code DivisionMultiple Access(CDMA)
o Multiple accesstechniqueusedbyamericanSystem(notusedbythe EuropeanGSM
system)
o Basedon the spread-spectrumtechnique:
 „Spreadspectrum“indicatesthatthe signal occupiesmore spectrumthanin
FDMA and TDMA system
 Transmissionmode wherethe transmitteddataoccupiesalargerbandwidth
than that requiredtotransferdata
o Accesstechnique realisedbeforetransmissionbyadditionof acode that is independent
of the data sequence

4.  code usedat the receive endwhichmustoperate synchronizedwiththe
transmitter,todespreadthe receivedsignal inordertorecoverthe initial data
o Allowsmanydevicestotransmitsimultaneouslyinthe same frequencyband
Principle:
Each MH is allocatedarandom sequence orcode – thismustbe differentandorthogonal or
quasi-orthogonal(i.e.decorrelated) fromall othersequences
o CDMA providesprotectionagainstmultipathfadinginterference,privacy,interference
rejection,anti-jammingcapability,low probabilityof interceptionandallows
macrodiversity
o Three basicspread-spectrumtechniquesare defined:
 Direct Sequence CDMA– DS-CDMA
 Fast FrequencyHopping CDMA – FH-CDMA
 Time HoppingCDMA – TH-CDMA
GSMconsistsof three major systems:
 Base-StationSystem(BSS)
 NetworkSwitichingSub-System(NSS)
 OperationandSupportSystem(OSS)
o The SwitchingSystemperformscall processingandsubscriberrelatedfunctions
o The systemcontainsthe followingfunctional units
 Home LocationRegister(HLR)
 Mobile SwitchingCenter(MSC)
 VisitorLocationRegister(VLR)
 AuthenticationCenter(AUC)
 EquipmentIdentityRegister(EIR)
 Base Station System(BSS)
o Thisis the part of the networkwhichconnectsthe MS to the networkviathe radio
interface
 Base Station(BS)
 Knowas (Base TransceiverStation(BTS)) inofficial GSMdocumentation
 Thisis the radiotransceiverthe areaof transmissionisreferredtoasthe
cell
Base stationcontroller
o Thisnode takesoverall control of a numberof base stationsa pointof control for the BS
 Otherfunctionality
 Trafficconcentration
 Switchingoperations
Data basedand voice
 Networkmanagementtoall BS attached
 Radiochannel managementforall BSattached
 Conversionof airinterface ratestonetworkapplicable rates
13 Kbpsto 64 Kbps
 Handovercontrol
Thisis forbase stationswhichithas control of
 NetworkSwitchingSub-System(NSS)
 Responsible forthe transferof informationfromthe handsetstothe correctpart of the
infrastructure

5.  Functions
 HandoversbetweenBSC’s
 Allowcommunicationbetweenmobile andPSTN networks
 Allowsforroamingbetweennetworks
 A numberof nodesare a part of the NSS
o NSSnodes consistof
 Mobile SwitchingCentre (MSC)
 Home LocationRegister(HLR)
 VisitorLocationCentre (VLR)
 AuthenticationCentre (AuC)
 EquipmentIdentityRegister(EIR)
o HLR isthe most importantdatabase
 Storage and managementof subscriptions
 Permanentdataincludes:
 Subscriber'sservice profile
 Subscriber'slocationinformation
 Subscriber‘sactivitystatus
 Subscribingtoa particularprovider‘sservice registersyouinthe HLR of that provider
o The MSC performsthe telephonyswitchingfunctionsof the network
 Controlscall to andfrom othertelephoneanddatasystems
 Alsoperformsfunctionssuchas
 Toll ticketing
 Networkinterfacing
 CommonChannel signalling
o VLR containsdata onvisiting(roaming)subscribers
 Integratedwiththe MSC
 Whena roamer entersthe service areathe VLRqueriesthe appropriate HLR
 If a roamermakesa call the VLR will alreadyhave the informationitneedsforcall setup
o The AUC verifiesthe identityof the userandensuresandensuresthe confidentialityof eachcall
 By provide authenticityandencryptionparametersforeverycall
 Protectsnetworkoperatorsfromfraud
 Assuresacertainlevel of securityforthe contentof eachcall
o The EIR isa database that includesinfosolelyaboutthe identitymobile equipment
 Preventscallsfrom stolen,unauthorisedordefective mobile devices
 OperationandSupportSystem
 Thispart of the networkallowsmonitoringof the network
 Ensuresthe smoothrunningandinvolvesmanagementtasks
 AccountingandBilling
 StatusReports
 Trafficmonitoring
 In additionif aBSC can not carry out all of itstasksthe OSSwill take control of
the BSC’s tasks
 The OSS islogicallyconnectedtoall equipmentinthe infrastructure.

6. GSMInfrastructure
Made by: Muhammad Ahad