白皮书 lte 中的协议信令程序LTE正式版

WhitePaper

ProtocolSignalingProceduresinLTE

By:V.SrinivasaRao,SeniorArchitect&RambabuGajula,LeadEngineer

Overview

Theexplodinggrowthoftheinternetandassociatedserviceshasfueledtheneedformoreandmorebandwidth.Handhelddevicesaregrowingexponentiallyandthustheneedfortheservicesonthemovehasincreasedtremendously.Current3Gtechnologyisabletocopewiththedemandtosomeextentbutunabletosatisfytheneedscompletely.

LongTermEvolution(LTE)promiseshigherdatarates,100Mbpsinthedownlinkand50MbpsintheuplinkinLTE’sfirstphase,andwillreducethedataplanelatencyandsupportsinteroperabilitywithothertechnologiessuchasGSM,GPRSandUMTS.Plus,LTEhassupportforscalablebandwidth,from1.25MHzto20MHz.AllthesefeaturesmakeLTEaveryattractivetechnologyforoperatorsaswellasthesubscribers.

InthispaperwebrieflytouchupontheproceduresexecutedbyLTEuserequipment(UE)andthevariousLTEnetworkelementsinordertoprovidetheservicesrequestedbytheUE.

CONTENTS

NetworkArchitecturepg.2BearersinLTEpg.2

SystemInformationBroadcastingpg.3

TrackingAreaUpdatepg.7

Mobile-OriginatedDataCallpg.8

MobileInitiatedDataCallTerminationpg.9Pagingpg.9

MobileTerminatedDataCallpg.10

Conclusionpg.10

Referencespg.11

ProtocolSignalingProceduresinLTE|RadisysWhitePaper

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NetworkArchitecture

InFigure1,thearchitectureoftheLTEnetworkisgivenwiththevariousinterfacesbetweenthenetworkelements;GERANandUTRANnetworksareshownforcompleteness.

Thefunctionsofthevariousnetworkelementsare:

eNodeB:RadioResourceManagementfunctions,IPheadercompression,encryptionofuserdatastreams,selectionofanMME,routingofuserplanedatatoS-GW,schedulingandtransmissionofpagingmessage.

MME:NASsignaling(eMM,eSM)andsecurity,ASsecurity,trackingarealistmanagement,PDNGWandS-GWselection,handovers(intra-andinter-LTE),authentication,bearermanagement.

S-GW:Thelocalmobilityanchorpointforinter-eNodeBhandover;downlinkpacketbufferingandinitiationofnetwork-triggeredservicerequests,lawfulinterception,accountingonuserandQCIgranularity,UL/DLchargingperUE.

P-GW:UEIPaddressallocation,packetfilteringandPDNconnectivity,ULandDLservice-levelcharging,gatingandrateenforcement.

Figure2showstheprotocolstacksatvariousLTEnetworkelements.

Figure1.LTENetworkArchitecture

Figure2.LTEControl/UserPlaneProtocolStacks

BearersinLTE

InLTE,end-to-endbearersarerealizedbytheEPSbearers,whichareacollectionofradio,S1andS5/S8bearers.AnEPSbeareridentityuniquelyidentifiesanEPSbearerforoneUEaccessingviaE-UTRAN.TheEPSBearerIdentityisallocatedbytheMMEandistheonethatcarriestheinformation;usuallyitcarriestheuserdata.

TherearethreekindsofbearersinLTE:RadioBearers,S1BearersandEPSBearersasdepictedintheFigure3.IntheUE,theuplinkTFTmapsatrafficflowaggregatetoanEPSbearerintheuplinkdirectionandinPGWthedownlinkTFTmapsatrafficflowaggregatetoanEPS

bearerinthedownlinkdirection.Thereismuchmore

complexitythanwhatissummarizedhere,butforFigure3.LTEBearerArchitecturebrevity’ssakeitsufficestosaythattrafficoff-loaders

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classifytrafficstreamsusingDPIandthen,basedontheoperator’spolicies,offloadpartofthetrafficdirectlyontotheInternetwhilesendingtheremainingtraffictothecorenetwork.Trafficoff-loaderstypicallywillbedeployedas“bump-in-the-wire”boxesbetweentheradioaccessnetwork(RAN)andthecorenetwork(CN);inthefuture,someRadioNetworkControllers(RNCs)mightincludethisfunctionalityinsidethebox.

RadioBearer:AradiobearertransportsthepacketsofanEPSbearerbetweentheUEandaneNodeB.

S1Bearer:AnS1bearertransportsthepacketsofanEPSbearerbetweenaneNodeBandaS-GW.

S5/S8Bearer:AnS5/S8bearertransportsthepacketsofanEPSbearerbetweentheS-GWandthePDNGW.

Thereisaone-to-onemappingbetweenradio,

S1andS5/S8bearers;thisend-to-endEPSbearerrealizesthenegotiatedQoSfortheservice.

SystemInformationBroadcasting

Togetservicefromthenetwork,aUEhastoselectthenetworkandcamponacell.Forthistohappen,theUEhastosynchronizeitselfwiththenetworkattheframeandslotlevel.Afterward,itrequirestheinformationlikeNetworkID(PLMNID),TrackingAreaID,CellIDandtheRadioandCoreNetworkcapabilitiesforitsnetworkselection.ThenetworkbroadcaststhisinformationtohelptheUEsintheirselectionprocess.

TheLTEnetworksupportsbroadcastingofSystemInformationintheformofMIBsandSIBs;Figure4outlinesthesysteminformationbroadcastprocedure.OncetheUEissynchronizedwiththenetworkattheframeandslotlevel,itreadsthebroadcastinformationandselectsit(PLMNandcellselection).

Figure4.SystemInformationBroadcast

Figure5.MMEConfigurationUpdateProcedure

MIBcontainscellbandwidthandinformationaboutPHICHandtheSFN.

SIBcontainslistofPLMNIDs,TAC,CellId,CSGIdentity(optional),neighborcellinformation,etc.

eNodeBwillgetmostoftheelementsfromtheMMEinanS1APMMEConfigurationUpdate

MessageontheS1interfaceasshowninFigure5.

ThishappensaftertheS1interfaceestablishmentbetweeneNodeBandMME.

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RandomAccessProcedure—SystemAccess

TheUEcanutilizetheservicesofthenetworkonce

itissynchronizedinboththedownlinkaswellasthe

uplinkdirection.AfterthePLMNandCellselection,

theUEissynchronizedwiththenetworkinthe

downlinkdirectionandnowitneedstosynchronize

withthenetworkintheUplinkdirection.TheRandom

AccessProcedure(RAP)overPRACHisperformedby

theUEforthispurpose;RAPischaracterizedasone

procedureindependentofcellsizeandiscommon

forbothFDD&TDD.TherearetwotypesofRAPs:

contention-basedandnon-contention-based.

Figure6.Contention-BasedRandomAccessProcedure

Contention-basedRandomAccessProcedure

Inthismode,multipleUEsmayattempttoaccessthenetworkatthesametime,therebyresultingincollisions.ThecontentionisresolvedamongthecontendingUEsinthefollowing4-stepprocess(Figure6):

Step1:RandomAccessPreambleonRACHinuplink

PreamblesaregroupedbasedonthesizeoftheL3messagethe,UEwouldliketotransmit;thisprovidesanindicationofresourcerequirementsfortheUEtothenetwork.

Step2:RandomAccessResponsegeneratedbyMAConDL-SCH

ThisisanindicationtotheUEthattheeNodeBreceiveditsPreambleandconveystheresourcesreservedforthisUE.

Step3:FirstscheduledULtransmissiononUL-SCHTheUEsendstheRRCConnectionRequestusingtheresourcesgivenbytheeNodeBinStep2.IntheRRCConnectionrequestmessage,theUEsendsitsidentifiertothenetworkanditisusedbytheeNodeBtoresolvethecontentioninthenextstep.

Step4:ContentionResolutiononDL

TheeNodeBechoesbackthereceivedUEidentifiertoresolvethecontention,andatthispointtheUEwhichhasreceiveditsIDcontinueswiththetransmissionwhileotherswillbackoffandtryagain.

Figure7.Non-Contention-BasedRandomAccessProcedure

Non-Contention-Based

RandomAccessProcedure

ThenetworkinitiatesthisprocedureincaseofahandoverofaUEfromoneeNodeBtoanotherinordertokeephandoverlatencyundercontrol.ItusuallyreservesasetofRACHpreamblesforthispurposeandwilltransmitonefromthatgrouptotheUE.TherearenocollisionswithotherUEsbecausetheeNodeBcontrolstheprocedure,whichisshowninFigure7.

OncetheUEreceivestheassignedRAPreamble,itsendsittotheeNodeBwhichrespondsback.Steps3and4giveninthecontention-basedRAParenotrequiredhere.

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RRCConnectionandInitialAttachProcedure

AftertheRandomAccessprocedure,iftheUEisnotalreadyattachedtothenetworkithastodosobyinitiatingtheattachprocedure.Otherwise,theUEinitiatesthetrackingareaupdateifitchangeditstrackingareasincethelastupdate.ForinitiatinganyNASprocedure,theUEisrequiredtoestablishanRRCconnectionwiththeeNodeB.Thepurposeofthisprocedure,depictedinFigure8,istorequesttheresourcesfromthenetworkforitsserviceneeds.

RRCconnectionestablishmentinvolvesSignalingRadioBearer1(SRB1)establishment.TheprocedureisalsousedtotransfertheinitialNASmessagefromtheUEtotheMMEviatheeNodeB,thelatterofwhichdoesnotinterprettheNASmessage.

NASAttachProcedure

TogetNAS-levelservices(forexample,internetconnectivity)fromthenetwork,NASnodesinthenetworkhavetoknowabouttheUE.Tofacilitatethis;theUEhastoinitiatetheAttachProcedure,whichismandatoryfortheUEatpoweronandalsoduringtheinitialaccessofthenetwork.

Oncetheattachproceduresucceeds,acontextisestablishedfortheUEintheMME,andadefaultbearerisestablishedbetweentheUEandthePDNGWandanIPaddressisallocatedtoit.NowthattheUEhasIPconnectivity,itcanstartusingIP-basedinternetservicesorIMSservicesiftheIMSnetworkisavailableandiftheUEhasasubscriptionforthesame.TheNASAttachprocedureisdepictedintheFigure9andthestepsaregivenbelow:

UEestablishestheRRCConnectionwiththeeNodeB.

TheUEsendstheATTACHREQUESTmessagetogetherwithaPDNCONNECTIVITYREQUESTforthePDN(IP)connectivityontheestablishedRRCConnection.Aspartofthis,theeNodeBestablishestheS1logicalconnectionwiththeMMEforthisUE.

IftheNetworkisnotabletoidentifytheUEwiththeIdentitygivenintheAttachRequestmessage,itinitiatestheidentificationfollowedbyAuthenticationandSecurityModeproceduresasgiveninFigure10.

Figure8.RRCConnectionSetup

Figure9.AttachProcedure

Figure10.NASCommonProcedures

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TheMMEupdatetheHSSwiththelocationoftheUEusingtheUpdateLocationrequestmessageusingtheDiameterprotocol;italsorequeststhesubscriberprofilefromtheHSSusingthismessage.

TheHSSupdatesitsdatabasewiththecurrentlocationoftheUEandsendsthesubscriberprofileinformationtotheMMEintheDiameterUpdateLocationAcknowledgemessage.

TheMMEnowestablishesaneGTPUserTunneltoestablishthedefaultbearerattheSGW;itsendsaCreateSessionRequest(eGTP-Cprotocol)towardtheSGW.

TheSGWcreatesthedefaultbearerforthisUEandrequeststhePGWtocreateabearerforthisUEbetweentheSGWandthePGWtoprovideend-to-endbearerconnectivity.ThePDN-GWthencreatesthebearerandallocatesanIPAddressfortheUE.

OncetheSGWreceivestheresponsefromthePGW,itrespondswithaCreateSessionResponsetoMME.

TheMMEnowhastoestablishthebearerbetweentheeNodeBandSGW.ItsendstheS1APInitialContextSetupRequesttotheeNodeBtocreate

acontextforthisUE,whichincludesthebearerContextandthesecurityContext.

AfterreceivingtheInitialContextSetupRequest,theeNodeBnowestablishesthesecurityparameterswiththeUEbyinitiatingtheASSecurityModeCommandProcedure.

TheUEestablishesthesecurityparameters(parametersrequiredforcipheringtheIntegrityprotection)andsendstheSecurityModeCompleteMessagetotheeNodeB.Fromnowon,allthemessagesexchangedbetweentheUEandeNodeBontheradiointerfacearecipheredaswellasintegrity-protected.

TheeNodeBreconfigurestheresourcestotheUEbysendinganRRCConnectionReconfigRequesttotheUE.Inthismessage,theeNodeBpiggy-backstheActivateDefaultEPSBearerContextRequestNASmessagetotheUE.

TheUEupdatesitsRRCConnectionconfigurationandrespondsbackwithanRRCConnection

ReconfigComplete.

TheeNodeBnowsendstheInitialContextSetupResponsetotheMME.

TheMMEsendstheeGTP-CModifyBearerRequesttotheSGWtoupdatetheeNBTunnelIdforthedefaultbearer.

Afterupdatingtheinformation,theSGWrespondswithaModifyBearerResponsetotheMME.

TheMMEnowsendstheAttachAcceptandActivateDefaultBearerContextRequestNASmessagetotheUE.

IftheMMEhasallocatedaGUTIwhilesendingtheAttachAccept,theUEneedstoprocessitandgivebacktheAttachCompleteasaresponsetoit.TheUEpiggy-backstheActivateDefaultEPSbearercontextAcceptNASmessagetotheMME.

NASCommonProcedures

TheMMEcaninitiatetheNAScommonproceduresduringtheinitialAttach,TrackingAreaUpdateandotherdedicatedNASprocedures.Theseprocedures,depictedinFigure10,areusedtoidentifyandauthenticatetheUE.

TheMMEsendstheidentityRequesttotheUEifitisunabletoretrievetheUEprofilefromtheHSS.

TheUErespondsbackwithitsIMSIintheIdentityResponsemessagetoMME.

AftergettingthevalidIMSIfromtheUE,thenetworknowauthenticatestheUEtoascertainwhethertheUEisgenuineornot.ThenetworkandUEshareasecretkey(theAuthenticationCentrestoresitinthenetworkandtheSIMcardstoresitinUE).Usingthissecretkeyandarandomnumber,thenetwork(AuC)computesaresultandexpectstheUEtogivebackthesameresultaspartofthisprocedure.

Afterreceivingtheparameters(RANDandA

UTNfromthenetwork),theUEcomputestheresultandgivesitbacktothenetworkintheAuthenticationResponse.

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Aftersuccessfulauthentication,thenetworkinitiatestheSecuritymodecommandtoencrypttheNASmessagesbetweentheUEandMME;thisistoprotecttheprivacyofthesubscriber.

TheUEagreestotheSecuritymodecommandandsendstheresponsebacktotheNetwork.Afterthisstep,boththeUEandthenetworkwillencrypttheNASmessageswhilesendinganddecryptthemwhilereceiving.Similarly,NASmessagesareintegrity-protectedfromnowonwards.

UEInitiatedDetachProcedure

Figure11.UEInitiatedDetach

IftheUEdoesnotrequireservicesfromthenetwork,itneedstoderegisteritselfwiththenetworkbyinitiatingthedetachprocedure.OneexampleforthisiswhentheUEisswitchedoff.

ThisdetachprocedureisinitiatedbytheUEbysendingaDETACHREQUESTmessage.TheDetachtypeIEincludedinthemessageindicateswhetherdetachisduetoa“switchoff”ornot.

ThenetworkandtheUEdeactivatetheEPSbearercontext(s)forthisUElocallywithoutpeer-to-peersignalingbetweentheUEandMME.Ifthedetachtypeisa“switchoff”thentheMMEdoesnotsendtheDetachAccept,otherwiseitdoessendittoUE.WhileprocessingtheDetachfromtheUE,theMMEinitiatesthereleaseoftheEPSbearersinthenetworkanditalsoclearstheUEContextheldattheeNodeB(notshowninFigure11).

TrackingAreaUpdate

Afterasuccessfulattachtothenetwork,theUEcanroamfreelyinthecurrentTrackingArea.Ifitdetectsadifferenttrackingarea,itneedstoupdatethenetworkwiththisnewtrackingarea.Similarly,thenetworkcanrequesttheUEtoupdateitstrackingareaperiodically,whichhelpsthenetworkinquicklylocatingtheUEwheneveramobile-terminatedcallisreceivedbythenetworkforthisUEsinceitcanjustpagetheUEinthelastreportedarea.

Figure12.TrackingAreaUpdatewithoutAuthentication

Figure13.TrackingAreaUpdatewithAuthentication

Duringthetrackingareaupdatingprocedure,theMMEmayinitiateanauthenticationprocedureandsetupasecuritycontext.TheTrackingAreaUpdateProcedurewithouttheAuthenticationstepisgiveninFigure12,andthestepsWithAuthenticationandSecuritymodecommand(NASCommonProcedures)includedaregiveninFigure13.

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Mobile-OriginatedDataCall

Aftersuccessfullyattachingtothenetwork,theUEcanrequesttheservicesfromtheNetworkusingtheservicerequestprocedure.OneexamplescenarioiswhentheUErequestsresourcesfromtheNetworktoinitiateadatacall;theUEcanutilizetheNASservicerequestprocedureforthispurpose.AnotherexampleofthisprocedureistoinvokeMO/MTCSfallbackproceduresiftheyaresupportedbythenetwork;thesignalingmessagesinvolvedinthisprocedurearegiveninFigure14.

TheUEestablishestheRRCConnectionwiththeeNodeB.

TheUEsendstheServiceRequesttotheMMEandrequests(dedicated)bearerresourcesbyincludingtheBearerResourceAllocationReq.Aspartofthis,theeNodeBestablishestheS1logicalconnectionwiththeMMEforthisUE.NotethattheUEmayalsosendtheBearerResourceAllocationReqtotheMMEasastandalonemessageatalaterpointintimetoo.

AtthispointthenetworkcaninitiateanoptionalidentificationfollowedbyAuthenticationand

SecurityModeproceduresasgiveninFigure10.

AfterthecompletionoftheAuthenticationand

SecurityModecontrolprocedures,theMMEinitiatestheactivationofdefaultbearerwiththeSGW/PGWbyinitiatingtheeGTP-CModifyBearerReqmessagetowardtheSGW.

AfterreceivingtheModifyBearerReq,theSGWactivatestherequiredresourcesandforwardstheModifyBearerReqtowardthePDNGW.

ThePGWprocessestheModifyBearerReqandactivatesrequiredresources.NotethattheIPAddressisallocatedduringtheAttachprocedure,soitwillnothappennow.ItrespondsbackwiththeModifyBearerResponsetotheSGWandtheSGWforwardsittotheMME.

TheMMEnowinitiatestheDedicatedBearerestablishmentbysendingtheeGTP-CBearerResourceCommandtotheSGW.

TheSGWprocesstheBearerResourceCommandandforwardsittothePGW.

Figure14.Mobile-OriginatedDataCall

ThePGWrespondswithaCreateBearerRequesttowardtheSGWafterallocatingthededicatedbearerresource(TFTinitialization,etc).

TheSGWprocesstheCreateBearerRequestandforwardstotheMMEforfurtherprocessing.

TheMMEnowsendstheE-RABSetupReqtotheeNodeBtoallocatethebearerbetweentheeNodeBandtheSGW;itpiggy-backstheNASActivateDedicatedEPSBearerContextReqtotheUE.

TheeNodeBallocatestheresourcesfortheRadio

BearersusinganRRCConnReconfigReqmessagetotheUE.TheeNodeBincludesthereceivedNASmessageinit.

TheUEestablishestheRadiobearersandrespondsbackwithanRRCConnection

ReconfigurationCompleteMsgtotheeNodeB.

RadioBearersareestablishedbetweentheeNodeBandtheUEbynow,sotheeNodeBsendstheE-RABSetupresponsetotheMME.

TheUEnowsendstheActivateDedicatedEPSBearerContextAcceptNASmessagetotheMMEviatheeNodeB.

TheMMEsendsaCreateBearerResponsetotheSGWtocompletetheDedicatedBearerActivation.TheSGWforwardsittothePGW.

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Mobile-InitiatedDataCallTermination

OncetheUEfinishestheDatacallitcantriggerthereleaseofthededicatedbearersbysendingtheBearerResourceModificationReqmessagetotheMME,whichcanthentakecareofreleasingthededicatedbearerwiththeSGWandPGW.

TheUEtriggersthededicatedbearerreleasebysendingtheBearerResourceModification

RequesttotheMME.

TheMMEinitiatesanEPSbearercontextdeactivationprocedurebysendingtheeGTP-CBearerResourceCommandMsgtotheSGW.

TheSGWprocesstheBearerResourceCommandandforwardsittothePGW.

ThePGWinitiatestheDeleteBearerReqmsgtowardtheSGW/MMEtocleartherequestedbearerresources.TheSGWforwardsthesametotheMME.

TheMMEinitiatestheE-RABReleaseCommandtotheeNBtoclearthebearerresources.ItincludestheNASMsg:DeactivateEPSBearerContextReqmessagefortheUE.

Figure15.Mobile-InitiatedDataCallRelease

Figure16.PagingProcedure

TheUEreceivestheNASmessagefromtheeNBintheDLNASprocedure.ItclearsthebearerresourcesandsendsaDeactivateEPSBearerContextAccepttotheMME.

TheeNBnowsendstheE-RABReleaseResponsetotheMME.

TheMMEsendstheDeleteBearerResponsetotheSGWandtheSGWforwardsittothePGWafterclearingthebearerresources.

ThePGWclearstherequestedBearerresources.

IfitisthereleaseofthelastdedicatedbearerforthisUE,theMMEshallreleasetheContextassociatedwiththisUEbysendinganS1APUEContextReleaseCommand.

TheeNodeBclearstheRadioresourcesallocatedtothisUEbysendingtheRRCConnectionReleasemessagetotheUE.

TheeNodeBsendstheUEContextReleasecompletemessagetotheMME.

Paging

ThepagingprocedureisusedbythenetworktorequesttheestablishmentofaNASsignalingconnectiontotheUE.IfthereisanIPpacketthatcomesforaUEfromtheexternalnetworktothePGWandifthereisnodedicatedbearerexistingfortheUE,itwillforwardtheIPpackettotheSGWonthedefaultbearer.OncethepackethasreachedtheSGWonthedefaultbearer,theSGWdetectstheneedtocreate

adedicatedbearerandsendstheDownlinkDataNotificationmessagetotheMMEinorderthepagetheUEandcreatethededicatedbearers.

NowtheMMEhastoensurethattheUEestablishesanRRCconnection,sotheMMEsendsaPagingRequestmessagetoalleNodeBsassociatedwiththelastknownTrackingArea.

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Figure17.MobileTerminatedDataCall

Mobile-TerminatedDataCall

ForaUE-terminatedcall,thenetworksendsthepagingrequesttoalltheeNodeBsassociatedwiththelastknowntrackingareaasdiscussedabove.WhenreceivingthePagingRequestmessagefromtheMME,theeNodeBsendsthepagingmessageovertheradiointerfaceinthecellswhicharecontainedwithinoneofthetrackingareasprovidedinthatmessage.

TheUEisnormallypagedusingitsS-TMSIorIMSI.ThePagingmessagealsocontainsaUEidentityindexvalueinorderfortheeNodeBtocalculatethepagingoccasionsatwhichtheUElistensforpagingmessage.TheprocedureisdepictedinFigure17andexplainedbelow:

ThePG