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MobileCommunicationSystemsChapter92021/10/101Outline2021/10/102CellularSystemMSCBSCBSCBTSBTSBTSBTSBTSBTS………MSHLRVLREIRAUCMSGatewayMSCMSC…PSTN/ISDNBaseStationSystemBaseStationSystem2021/10/103VLR/HLRVLRcontainsinformationaboutallvisitingMSsinthatparticularareaofMSC.VLRhaspointerstotheHLR’sofvisitingMS.VLRhelpsinbillingandaccesspermissiontothevisitingMS.2021/10/104

RedirectionofCalltoMSataVisitingLocationCallroutedaspercallednumbertoMSHomeMSCHLRVisitingMSCVLRBSMSCellwhereMSiscurrentlylocated2021/10/105Registration2021/10/106

UsingaMobilePhoneOutsidetheSubscriptionAreaVisitingBSMSBeaconsignalexchange1Requestforregistration2Authentication/reject5HomeBS3Authenticationrequest4Authenticationresponse2021/10/107StepsforRegistrationMSlistenstoanewbeacon,ifit’sanewone,MSaddsittotheactivebeaconkerneltable.IfMSdecidesthatithastocommunicatethroughanewBS,kernelmodulationinitiateshandoffprocess.MSlocatesthenearestBSviauserlevelprocessing.ThevisitingBSperformsuserlevelprocessinganddecides:Whotheuseris?Whatareitsaccesspermissions?Keepingtrackofbilling.HomesitesendsappropriateauthenticationresponsetothecurrentservingBS.TheBSapproves/disapprovestheuseraccess.2021/10/108HandoffChangeofradioresourcesfromonecelltoanadjacentone.Handoffdependsoncellsize,boundarylength,signalstrength,fading,reflection,etc.HandoffcanbeinitiatedbyMSorBSandcouldbeduetoRadiolinkNetworkmanagementServiceissues2021/10/109Handoff(Cont’d)RadiolinkhandoffisduetomobilityofMS. Itdependson:NumberofMSsinthecellNumberofMSsthathaveleftthecellNumberofcallsgeneratedinthecellNumberofcallstransferredfromtheneighboringcellsNumberofcallsterminatedinthecellNumberofcallsthatwerehandofftoneighboringcellsNumberofactivecallsinthecellCellpopulationTotaltimespentinthecellbyacallArrivaltimeofacallinthecelletc2021/10/1010Handoff(Cont’d)Networkmanagementmaycausehandoffifthereisdrasticimbalanceoftrafficinadjacentcellsandoptimalbalanceofresourcesisrequired.ServicerelatedhandoffisduetothedegradationofQoS(qualityofservice).2021/10/1011TimeforHandoffFactorsdecidingrighttimeforhandoff:SignalstrengthSignalphaseCombinationofabovetwoBiterrorrate(BER)DistanceNeedforHandoffisdeterminedby:SignalstrengthCIR(carriertointerferenceratio).2021/10/1012HandoffInitiationBSiSignalstrengthduetoBSjEX1SignalstrengthduetoBSiBSjX3X4X2X5XthMSPminPi(x)Pj(x)2021/10/1013Handoffinitiation(Cont’d)RegionX3-X4indicatesthehandoffarea,wheredependingonotherfactors,thehandoffneedstobeperformed.OneoptionistodohandoffatX5wherethetwosignalstrengthsareequal.IfMSmovesbackandfortharoundX5,itwillresultintoofrequenthandoffs(ping-pongeffect).ThereforeMSisallowedtocontinuewiththeexistingBStillthesignalstrengthdecreasesbyathresholdvalueE.Differentcellularsystemsfollowdifferenthandoffprocedures.2021/10/1014TypesofHandoffHardHandoff(breakbeforemake)ReleasingcurrentresourcesfromthepriorBSbeforeacquiringresourcesfromthenextBS.FDMA,TDMAfollowthistypeofhandoff.SoftHandoff(makebeforebreak)InCDMA,sincethesamechannelisused,wehavetochangethecodeofthehandoff,ifthecodeisnotorthogonaltothecodesinthenextBS.Therefore,itispossiblefortheMStocommunicatesimultaneouslywiththepriorBSaswellasthenewBS.2021/10/1015HardHandoffBS1BS2MSBS1BS2MSBS1BS2MS(a).Beforehandoff(c).Afterhandoff(b).Duringhandoff(Noconnection)2021/10/1016SoftHandoff(CDMAonly)BS1BS2MSBS1BS2MSBS1BS2MS(a)Beforehandoff(b)Duringhandoff(c)Afterhandoff2021/10/1017RoamingTomovefromacellcontrolledbyoneMSCareatoacellconnectedtoanotherMSC.BeaconsignalsandtheuseofHLR-VLRallowtheMStoroamanywhereprovidedwehavethesameserviceprovider,usingthatparticularfrequencyband.2021/10/1018

Roaming

BS1BS2MSBS1BS2MSHomeMSCVisitingMSCHomeMSCVisitingMSC2021/10/1019PSTNMSC4MSC3MSC2MSC1PagingArea1PagingArea2abcdeHandoffScenarioswithDifferentDegreeofMobilityMS2021/10/1020PossibleHandoffSituationsAssumeMSC1tobethehomeoftheMSforregistration,billing,authentication,etc.Whenhandoffisfromposition“a”to“b”,theroutingcanbedonebyMSC1itself.Whenhandoffisfromposition“b”to“c”,thenbi-directionalpointersaresetuptolinktheHLRofMSC1toVLRofMSC2.Whenhandoffoccursat“d”or“e”,routingofinformationusingHLR-VLRmaynotbeadequate(“d”isinadifferentpagingarea).ConceptofBackbonenetwork.2021/10/1021InformationTransmissionPathwhenMSHandsOfffrom“b”to“c”MSC1HLRMSC2VLRabcInformationtoMSbeingsentInitialpathofinformationtransferPathafterhandoffMS2021/10/1022IllustrationofMSCConnectionstoBackboneNetworkandRouting/ReroutingMSCRouterPagingarea1(PA1)Pagingarea2(PA2)MSC1(a,b)MSC2(c)MSC3(d)MSC4(e)(a,b,c,d,e)(a,b)(a,b,c,d)(d)R3R4R6R2R5R9R1R7R10R12R8R11R13Fromrestofthebackbone(c)(e)2021/10/1023BackboneNetwork2021/10/1024HomeAgents(HA),ForeignAgents(FA)andMobileIPTwoimportantsoftwaremodulesareassociatedwithrouters,homeagent(HA)andforeignagent(FA).MSisregisteredwitharouter,mostlyarouterclosesttothehomeMSCcanbeusedtomaintainitsHA.ArouterotherthanclosestonecouldalsoserveasanHA.OnceaMSmovesfromthehomenetwork,asoftwaremoduleinthenewnetworkFAassistsMSbyforwardingpacketsfortheMS.ThisfunctionalityissomewhatsimilartoHLR-VLR.2021/10/1025HomeMSCMSC1MSC2MSC3MSC4SelectedrouterformaintainingitshomeagentR3R4R6R9HomeMSCandHomeAgent(HA)forthePreviousNetwork2021/10/1026CallEstablishmentusingHA-FAWheneveraMSmovestoanewnetwork,itstillretainsitsinitialHA.TheMSdetectstheFAofthenewnetwork,bysensingtheperiodicbeaconsignalswhichFAtransmits.MScanalsoitselfsendagentsolicitation

messagestowhichFAresponds.WhenFAdetectsanewMS,itallocatesaCOA(careofaddress)totheMS,usingdynamichostconfigurationprotocol(DHCP).OnceMSreceivesCOA,itregistersitsCOAwithitsHAandthetimelimitbindingforitsvalidity.SuchregistrationisinitiatedeitherdirectlybyMStotheHAofthehomerouterorindirectlythroughFA.2021/10/1027CallEstablishment(Cont’d)HAconfirmsitsbindingthroughareplytotheMS.AmessagesentfromanarbitrarysourcetotheMSatthehomeaddressisreceivedbytheHA.Bindingischecked,theCOAoftheMSisencapsulatedinthepacketandforwardedtothenetwork.IfCOAoftheFAisused,thenpacketreachesFA,itdecapsulatesthepacketandpassestoMSatthelinklayer.InaninternetenvironmentitiscalledMobileIP.Afterbindingtime,ifMSstillwantstohavepacketsforwardedthroughHA,itneedstorenewitsregistration.WhenMSreturnstoitshomenetwork,itintimatesitsHA.2021/10/1028FA11”1’BeaconSignalIamnewhereOK,sendinformationHereismyHAandbindinginfomation.3COAorC-COAcreatedMSHA2(Anyonenew)44’SameasstepHereisCOAorco-locatedCOA(C-CoA)forthisMSAcknowledgeRegistration+binding44”Sameasstep4RegistrationProcessBetweenFA,MS,andHAWhentheMSMovestoaPagingarea2021/10/1029SourceToMSPayloadDataIncomingmessageforMSHAHACoA/C-CoASourceToMSPayloadDataEncapsulationFASourceToMSPayloadDataForwardingthroughintermediaterouterifCoAusedDecapsulationDecapsulationdoneatMSForwardingthroughintermediaterouterifC-CoAusedMSMessageForwardingusingHA-FAPair2021/10/1030RoutinginBackboneRoutersHowFAfindsHAoftheMS?OneapproachistohaveaglobaltableateachrouterofeachMSCsothattheroutefromFAtoHAforthatMScanbedetermined.Disadvantages:Informationtoolarge,onenetworkmightnotliketogiveoutinformationaboutallitsrouterstoanyexternalnetwork(onlygatewaysinformationisprovided).UseofDistributedRoutingScheme.

2021/10/1031PA1PA2PA3PA4PA5RouterXRouterWRouterYRouterZNetwork1Network2MSmovesIllustrationofPagingAreas(PAs)andBackboneRouterInterconnect2021/10/1032RoutetoPANexthopRoutetoPANexthopRoutetoPANexthopRoutetoPANexthop1X1-1X1Y2X2-2X2Y3X3Y3Z3-4X4Y4Z4-5X5Y5Z5-TableatrouterWTableatrouterXTableatrouterYTableatrouterZDistributedRoutingTableandLocationPAs2021/10/1033MulticastingProcessoftransmittingmessagesfromasourcetomultiplerecipientsbyusingagroupaddressforallhoststhatwishtobethemembersofthegroup.Reducesnumberofmessagestobetransmittedascomparedtomultipleunicasting.Usefulinvideo/audioconferencing,multipartygames.2021/10/1034Multicasting2021/10/1035MulticastingBi-directionalTunneling(BT)andRemoteSubscriptionapproacheshavebeenproposedbyIETFforprovidingmulticastoverMobileIP.InBTapproach,wheneveraMSmovestoaforeignnetwork,HAisresponsibleforforwardingthemulticastpacketstotheMS.InRemoteSubscriptionprotocol,wheneveraMSmovestoaforeignnetwork,theFA(ifnotalreadyamemberofmulticastgroup)sendsatreejoinrequest.2021/10/1036MulticastingRemoteSubscriptionbasedapproachissimpleandpreventspacketduplicationandnonoptimalpathdelivery.ItcancausedatainterruptiontilltheFAisconnectedtothetree.ItresultsinanumberoftreejoinandtreeleaverequestswhenMSareincontinuousmotion.Incontrast,intheBTapproach,theHAcreatesabi-directionaltunneltoFAandencapsulatesthepacketsforMS.FAthenforwardsthepacketstotheMS.2021/10/1037MulticastingBTapproachpreventsdatadisruptionduetothemovementofMS.ButcausespacketduplicationifseveralMSsofthesameHA,thathavesubscribedtothesamemulticastgroupmovetosameFA.AlsocausesTunnelConvergenceProblem,whereoneFAmayhaveseveralMSssubscribedtothesamegroup,belongingtodifferentHAsandeachHAmayforwardapacketforitsMSstothesameFA.2021/10/1038HAMulticastpacketsfromthemulticasttreeMS1MS2MS3FAMS1MS2MS3PacketDuplicationinBTTunnelApproach2021/10/1039MulticastpacketsfromthemulticasttreeHA1HA2HA3CoA(MS1)CoA(MS2)CoA(MS3)CoA(MS4)MS1MS2MS3MS4FATunnelConvergenceProblem2021/10/1040MulticastingToovercomeTunnelConvergenceProblem,MoMprotocolisproposedwhereintheFAselectsoneoftheHAs,calledtheDesignatedMulticastServiceProvider(DMSP),fromtheHAListforaparticulargroup.TheremainingHAsdonotforwardpacketstoFA.2021/10/1041MulticastpacketsfromthemulticasttreeHA1HA2HA3CoA(MS1)CoA(MS2)CoA(MS3)MS1MS2MS3MS4StopStopForwardDMSPSelectionFACoA(MS4)IllustrationofMoMProtocol2021/10/1042Transferthroughanopenairmediummakesmessagesvulnerabletovariousattacks.Onesuchproblemis“Jamming”byaverypowerfultransmittingantenna.Canbeovercomebyusingfrequencyhopping.Manyencryptiontechniquesusedsothatunauthorizeduserscannotinterpretthesignals.2021/10/1043EncryptionTechniquesPermutingthebitsinaprespecifiedmannerbeforetransmittingthem.Suchpermutedinformationcanbereconstructedbyusingreverseoperation.Thisiscalled“DataEncryptionStandard(DES)”oninputbits.2021/10/1044InputOutputSimplePermutationFunction1234567815263748WIRELESSWLIERSES2021/10/1045(a)Permutationbeforetransmission(b)PermutationafterreceptionBitPatternsbeforeTransmissionandafterReceptionusingDES2021/10/1046EncryptionTechniquesAcomplexencryptionschemeinvolvestransforminginputblockstosomecodedform.Encodedinformationisuniquelymappedbacktousefulinformation.Simplesttransformationinvolveslogicalorarithmeticorbothoperations.2021/10/1047EncodingTransmittedsignalsignalReceivedDecodingInformationblockEncodedsignalEncodedsignalInformationblock(Original)AGenericProcessofEncodingandDecodingattransmitterreceiverat2021/10/1048KeyK1ff++f+Input(64bits)InitialPermutation(IP)32bits32bitsLefthalf:L1Righthalf:R1Lefthalf:L1=R1R1=L1f(R1,K1)++Lefthalf:L16=R15R16=L16f(R15,K16)++Inverseinitialpermutation(IP–1)CodedOutputPermutationandCodingofInformation2021/10/1049AuthenticationMakingsureuserisgenuine.UsingaHashFunctionfromanassociateduniqueidentificationoftheuser(notfullproof)Anotherapproachistousetwodifferentinterrelatedkeys.Oneknownonlytosystemgeneratingthekey(privatekey),otherusedforsendingtooutsideworld(publickey).RSAalgorithm(bestknownpublickeysystem)2021/10/1050SystemUseri(1)ComputePublicKeyforUserifromitsprivatekey.(2)SendPublicKeySavePublicKeyUsepublickeytogeneratesignature.(3)ID,Signature(4)VerifyusingprivatekeyofUseri.(5)AuthenticationResultPublic/PrivateKeyAuthenticationStepsusuallydoneofflineSystemSystemUseriUseri2021/10/1051Authentication(RSAAlgorithm)InRSAmethod2largeprimenumbers(p,q)areselected.

n=p*qAnumbereisselectedtouse(n,e)asthepublickeyandistransmittedtotheuser.Userstoresthis,wheneveramessagem<nneedstobetransmitted,usercomputesc=me|mod