2G...All about second generation of cellular networks.
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.