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NTTDOCOMOTechnicalJournalCoreNetwork(EPC)forLTE

LTECoreNetworkControlMethodEPC

SpecialArticlesonXi(Crossy)LTEServiceTowardSmartInnovation

CoreNetwork(EPC)forLTE

InparallelwiththedeploymentofLTEradioaccesstechnol-ogytocopewithdramaticincreasesintraffic,NTTDOCOMOisintroducingEPCasacorenetworkforaccommodatingLTEandotherradioaccesssystems.TheEPCconsistsofMME,S-GW,P-GWandPCRFfunctionsforperforming

CoreNetworkDevelopmentDepartment

KeisukeSuzukiTakashiMoritaShinNarahaZhenMiao

10000

authentication,mobilitymanagement,bearercontrol,charg-ingandQoScontrol.TheEPCandSGSNsupportmobilitymanagementbetweentheLTEand3Gaccesssystems,andHSSsupportsthemanagementofLTEsubscriberinforma-tion.

PacketDataNetworkGateway(P-

TheEPCprovidesavarietyoffunc-

GW),andPolicyandChargingRulesFunction(PCRF).Itadoptsanarchi-forthePacketDataNetwork(PDN),control,andmobilitymanagement.1.IntroductionTherecentjumpindatacommuni-cationstrafficisexpectedtocontinueintothefutureasthepenetrationofsmartphonesescalatesandtheprovisionofrichcontentexpands.NTTDOCOMOisrespondingtothisdemandbydeploy-

*3*4

tecturethatseparatestheMMEandPCRFforcontrollingtheControlPlane(C-Plane),andtheS-GWandP-GWforcontrollingtheuserplane(U-Plane),tomakeiteasiertocopewithgrowing

tionsincludinguserauthentication,user-contractanalysis,authentication*7

settingoftransferpathsforuserdatapackets,QualityofService(QoS)*8

ItadoptstheAlways-ONcontinu-

ingLTEradioaccessfeaturinghigher

trafficontheU-Plane[2][3].TheEPC

ous-connectionconceptasabearer

*9

Node(SGSN)andtheHomeSub-scriberServer(HSS)tocontrolinter-(MME),ServingGateway(S-GW),26

transmissionspeeds,shorterdelaysandlargercapacities.ItisalsointroducingEvolvedPacketCore(EPC),whichwasspecifiedatthesametimeasLTE(3GPPRelease8),asthecorenetworktoaccommodateLTEandotherradioaccesssystems[1].TheEPCconsistsoftheMobilityManagementEntity*1*2

alsocooperateswiththeServingGPRS(GeneralPacketRadioService)Support*5*6

workingwith3Gandsubscriberscon-tractinformation.Inthisarticle,thepor-tionofthenetworkconsistingoftheEPC,SGSNandHSSisdenotedastheEPCnetwork.

*1MME:AlogicalnodeaccommodatingeNodeBbasestationsandprovidingmobilitymanagementandotherfunctions.*2S-GW:Theareapacketgatewayaccommodat-ingthe3GPPaccesssystem.*3P-GW:Agatewayactingasapointofconnec-

controlmethodinwhichthemobileter-minalisallocatedanIPaddresswhenturnedONsothatIPcommunicationscanbeperformedthesameasinthefixednetwork.TheEPCalsosupportspolicyandchargingrulestocontrolbillingandQoSandmobilitymanage-mentbetweendifferentradioaccesssystems.

tiontoaPDN(see*7),allocatingIPaddressesandtransportingpacketstotheS-GW.*4PCRF:AlogicalnodecontrollingQoS(see*8)andcharginginuserdatatransfers.

NTTDOCOMOTechnicalJournalVol.13No.1

NTTDOCOMOTechnicalJournalThisarticleprovidesanoverviewofEPCnetworkconfigurationandequip-mentanddescribesthebasicEPCfunc-tionsandsignalingflows.

2.OverviewofEPCNetworkNodeConfigurationThenodeconfigurationoftheEPCnetworkisshowninFigure1.ThisnewlydevelopedEPCconsistsoftheMMEandPCRFforprocessingonlycontrolsignalsandtheEPCServing

accordingtothatvalue.TheS-GW,meanwhile,relaysuserdatapacketsreceivedfromtheeNodeBtotheP-GWorviceversa,andfunc-tionsasananchorpointforuserdatapacketsontheoccasionofahandoverbetweenLTEand3Gradioaccess.Additionally,intheeventthatuserdatapacketshappentobereceivedfromtheP-GWforamobileterminalforwhicharadiobearerisnotcurrentlyestablished,S-GWinstructstheMMEtoperformpagingtoreestablisharadiobearer.

registration,pagingandthetransferofuserdatapacketson3Gasinthepast,theSGSNalsoperformsmobilityman-agementwithLTEandestablishesandreleasesbearerswiththeS-GW.2)AdditionalHSSfunctionsTheHSSmanagessubscriberpro-filesandauthenticationinformationforLTEusersandpassesthatinformationtotheMMEatthetimeoflocationreg-istration.Italsohasafunctionforlimit-ingtheuseofanLTEterminaltoonlytheLTEsubscriberbasedonthetypeof

andPDNGateWay(ESPGW)

*10

for

TheP-GWconnectsthemobileter-

userterminalandLTEsubscriberpro-

pagingprocessingthetransferofuserdata.TheMMEandPCRFarethesameastheonesin3GPPstandardarchitectureandESPGWcorrespondstoS-GW/P-GW.

2.1OverviewofNewlyDevelopedEquipmentTheMMEperformsmobilityman-agementsuchaslocationregistration,*11andhandoveraswellasbear-

minalwiththePDN.ItisalsoequippedwithfunctionsforallocatinganIPaddresstothemobileterminalandcol-lectingchargingdata.

2.2ExtensionsinExistingEquipment1)AdditionalSGSNFunctionsInadditiontoperforminglocation

file.

3.BasicFunctionsandSignalingFlows3.1Always-ONTheLTEsystemandEPCadopttheAlways-ONconceptwherebyabear-erisestablishedandanIPaddressisallocatedtothemobileterminalwhen

RNC/erestablishmentandrelease.Italsoper-formsuserauthenticationbasedonauthenticationinformationreceivedfromtheHSS,anditprovidesafunc-

S16IuNodeBSGSN

S3

Gr

HSS

S4

ControlsignalinterfaceUserdatatransferandcontrolsignalinterface

tionforencryptingthesignalbetween

mobileterminals.ThePCRF,inturn,providesfunc-tionsfordeterminingtheQoSand

S1-MME

MME

S10

S6a

S11

MME

PCRF

GxcGx

PCRF

chargingpolicytobeappliedtodatapacketssentandreceivedbytheuser.

eNodeB

EPC

S1-U

S-GWP-GWS5ESPGW

SGi

PDN(mopera,etc.)

OndeterminingaQoSvalue,thePCRF

passesittotheP-GW,S-GWandbasestation(eNodeB),eachofwhichper-formsQoScontrolonuserdatapackets

*AlphanumericcharacterssuchasS5arestandardinterfacenamesRNC:RadioNetworkControllerFigure1EPCnetworkarchitecture

*5SGSN:Alogicalnodehavingpacketcommu-nicationfunctionsspecifiedby3GPPstan-dards.*6HSS:Asubscriberinformationdatabaseina3GPPmobilecommunicationnetwork;it

workvisitinginformation.*7PDN:AnexternalnetworkconnectedtobyEPC.*8QoScontrol:Atechnologyforcontrollingthequalityofcommunicationsbyvarious

transmissions.*9Bearer:Inthisarticle,thepathtakenbyuserdatapackets.*10ESPGW:EquipmenthavingthecapabilitiesofS-GWandP-GW.

managesauthenticationinformationandnet-


meanssuchasspecifyingthepriorityofpacket

27


(APN),andapplicationinuse.Thisprocessisdividedintopolicycontrolofradioresources.Thismeansthat,areestablishedinattach,thePCRFthepowertotheterminalisturnedON.ThisIPaddressisalwaysallocatedtothemobileterminalsothatserversontheservice-provisionsidecanprovideIPservicesthesameasinafixedcom-municationsnetworkwithouthavingtobeawarethattheaccessnetworkisactuallyamobilecommunicationsnet-work.Thealways-onbearerissituatedinsidetheEPC(betweentheS-GWandP-GW),butifanon-communicatingstatecontinues,theradiobearer(mobileterminaleNodeBS-GW)isreleasedtomakemoreefficientuse*12

whenanattempttocommunicatereoc-curs,onlytheradiobearerhastobereestablishedtoreopenthecommunica-tionslink,whichshortenstheconnec-

tiondelay(Figure2).

3.2PCCInEPC,aprocesscalledPolicyandChargingControl(PCC)achievesflexi-blecontrolofQoSandchargingwithrespecttopacketdataaccordingtotheuserscontract,AccessPointName*13*14

andchargingcontrol.InadditiontoQoSmanagement,policycontrolpro-videsgatecontroltoallowtheoutflowandinflowofpacketstoandfromthenetwork[4].ThecurrentEPCdevelopmentintroducedhereprovideschargingcon-trolinpart.Whenpacketconnections*15

referencesthechargingpolicyitstoresforeachAPN.Inthecaseofno-charge,thePCRFinstructsfortheP-GWtoperformno-chargecontrolandtosuppressthegenerationofdetailedbillingdataforthepacketsofthatAPN.

3.3AttachProcedureTheattachprocedureperformedwhenturningpowertothemobileter-minalONisshowninFigure3.Steps(1)-(4):ThemobileterminalsendsanattachrequestmessagetotheMME,whichperformsuserauthen-ticationbasedonauthenticationinformationreceivedfromtheHSSandobtainsandmanagessub-scriberscontractinformationfrom

PoweronAttach

eNodeBS-GWP-GW

Afterattach,theradiolinkisreleasedifnocommunicationsoccurforacertaintimeperiod

PDN

eNodeBS-GWP-GWBegincommunications

PDNeNodeBS-GWP-GWAfixedtimeperiodelapsesaftercommunicationsterminate

Afterafixedtimeperiod,theradiobearerisreleased

PDN

Detach

eNodeB

S-GWP-GW

Poweroff

eNodeBS-GWP-GW

Figure2Always-ON

28

*11Paging:Aprocessthatcallsallmobiletermi-nalsinunisonatthetimeofanincomingcall.*12Radioresources:Generaltermforresourcesneededtoallocateradiochannels(frequen-cies).

*13APN:Thenameofaconnectionpoint;thenameofanetworkconnectionpointpreparedbyacorporateuserasaconnectiondestination.*14Policycontrol:TechnologyforcontrollingcommunicationsasinQoSortheenabling/dis-ablingofpackettransfersbasedonnetworkor

subscriberinformation.*15Attach:Aprocessforregisteringamobileter-minalwiththenetworksuchaswhenturningterminalpoweron.


NTTDOCOMOTechnicalJournalMobileterminaleNodeBMMEHSSS-GWPCRFP-GWPDN(1)Attachrequest

(2)Authentication,security,integritycontrol(3)Updatelocationrequest

C-PlaneU-Plane

(4)Updatelocationresponse(5)Createsessionrequest

(6)Establishbearerprocessing

(8)Initialcontext

(7)Createsessionresponse

(9)RRCconnectionreconfiguration

setuprequest(attachaccept)

(attachaccept)(10)RRCconnectionreconfigurationcomplete(11)Initialcontextsetupresponse(12)AttachcompleteFirstuplinkuserdatapackets(13)Modifybearerrequest

(14)Modifybearerprocessing(15)ModifybearerresponseFirstdownlinkuserdatapackets

Figure3LTEattachprocedure

DomainNameSystem(DNS),theHSStoestablishabearer.Steps(5)-(11):BasedontheAPNreceivedfromthemobileterminal,theMMEselectstheS-GWandP-GWtobeusedasdestinationswhenestablish-ingabearerinaccordancewiththe*16

andsendsacreatesessionrequestmessagetotheselectedS-GW.TheS-GWnowperformsestab-lishbearerprocessingwithrespecttotheP-GWspecifiedinthecre-atesessionrequestmessage.TheP-GWinterfaceswiththePCRFtogetinformationonwhatchargingneedstobeappliedandalsoper-

*16DNS:Asystemforassigningacorrespon-dencebetweenahostnameandanIPaddressonanIPnetwork.


formsconnectionprocessingwithaPDN.OncompletingbearersetupbetweentheS-GWandP-GW,theS-GWsendstotheMMEinforma-tiononpropagationconditionsfortheeNodeB.TheMMEsendsthispropaga-tioninformationreceivedfromtheS-GWtotheeNodeBasaninitialcontextsetuprequest,whichincludesanattachacceptmessageforthemobileterminal.TheeNodeBnowestablishesaradiobearerwiththemobileterminalandsendsittheattachacceptmessage,andthenreceivesanRRCconnec-tionreconfigurationcompletemes-

sagefromthemobileterminalandpassespropagationinformationfortheS-GWtotheMME.Steps(12)-(15):Onreceivinganattachcompletemessagefromthemobileterminal,theMMEsendsthepropagationinformationreceivedfromtheeNodeBtotheS-GW.Finally,basedonthepropaga-tioninformationsoreceived,theS-GWcompletestheestablishmentofabearerbetweentheeNodeBandS-GW.Thiscompletestheestab-lishmentofabearerinthemannerofmobileterminaleNodeBS-GWP-GW.

29


3.4MobilityManagementbetweenDifferentRadioAccessSystemsTherearetwomethodsformanag-ingmobilitybetweendifferentradioaccesssystems:thehandovermethod,whichestablishesabearerforthemobileterminalatitsnewlocationbeforehand,andtheReleasewithRedi-rectionmethod,whichcutsoffthebear-eratthemobileterminalsoldlocationandswitchesthecommunicationspathonregisteringthemobileterminalsnewlocation.NTTDOCOMOisadopt-ingthelattermethodforitsLTEservicelaunch,sincethemethodisamandatoryfunctioninthe3GPPstandard.

TheprocedureofReleasewithRedirectionfromLTEto3GisshowninFigure4.Steps(1)-(2):ThemobileterminalsendsaMeasurementreporttotheeNodeB,whichusesthatinformationasabasisforreleasingtheRRCconnec-tionandinstructingaswitchto3G.Steps(3)-(4):Basedontheswitchinginstruc-tionsreceivedfromtheeNodeB,themobileterminalselects3GandsendsaroutingareaupdaterequestthatincludesMMEidentificationinformationtotheSGSN.

Steps(5)-(8):TheSGSNspecifiesaMMEbasedontheMMEidentificationinformationreceivedfromthemobileterminalandobtainsbearercontext(S-GWIPaddress,bearerinformation,etc.).Steps(9)-(11):Basedonthebearercontextsoreceived,theSGSNsendstheS-GWaModifyBearerRequestmes-sagethatincludesbearerinforma-tion(SGSNIPaddress,etc.).TheS-GW,inturn,interfaceswiththeP-GWandPCRFbasedonthebearerinformationreceivedandperformsmodifybearerprocessing.Itthen

MobileterminalRNCeNodeBSGSNMMES-GWP-GWPCRFHSS(1)MeasurementreportUserdatapackets(2)RRCconnectionrelease

(3)Reselectcellandestablishradiolink(4)Routingareaupdaterequest(5)Contextrequest(6)Contextresponse(7)Authentication/security(8)Contextacknowledge(9)Modifybearerrequest

(10)Modifybearerprocessing(11)Modifybearerresponse(12)Updatelocationrequest/answer(13)Routingareaupdateaccept(14)Routingareaupdatecomplete

(15)Sequenceforreestablishingradiobearer

Figure4ReleasewithRedirectionprocedure

C-PlaneU-Plane

30


NTTDOCOMOTechnicalJournalswitchesthecommunicationspathtotheSGSNandreturnsamodifybearerresponsemessagetotheSGSN.Steps(12)-(14):TheSGSNperformsaupdatelocationrequest/answerprocesswithHSSandsendsaroutingareaupdateaccepttothemobiletermi-nal.Thiscompletestheswitchingoftheuser-data-packettransferpathinsidethecorenetworktothe3Gside.Step(15):Finally,aprocedureforreestab-lishingtheradiobearerisper-formedtoinitiate3Gcommunica-

tions.

3.5RadioBearerReestablish-mentProcedureIntheeventthatthemobileterminalsendsorreceivesuserdatapacketswhenaradio-bearerisreleased,itwillreestablishtheradiobearerfollowingtheproceduredescribedbelow(Figure5).Steps(1)-(3):Thisprocedurewillbeper-formedonlywhenthePDNsendsuserdatapacketstothemobileter-minal.IftheS-GWreceivesuserdatapacketsfromtheP-GWandthe

radiobearerhasbeencutoff,ittem-porarilyholdsthepacketsandsendsadownlinkdatanotificationtotheMME.TheMMEnowpagesthemobileterminal.Subsequentpro-cessingbeginswithstep(4)oftheprocedure.Steps(4)-(8):Therestoftheprocedureisper-formedwhenthemobileterminalsendsuserdatapacketstothePDNorwhenthePDNsendsuserdatapacketstothemobileterminal.Tobeginwith,themobileter-minalsendsaservicerequesttotheMME.Afterperformingauthentica-

MobileTerminaleNodeBMMES-GWP-GWPCRFHSS

RadiobearerreleasedstateAlways-ONstate

(1)DownlinkdataPerformedonlywhenuserdatapackets

(4)Servicerequest

(2)Downlinkdatanotification(3)Paging

Bufferedpackets

destinedforthemobileterminalemergeinthenetwork

C-Plane

U-Plane(5)Authentication/security(6)Establishradiobearerrequest(7)EstablishradiobearerFirstuplinkuserdatapackets(8)Establishradiobearerresponse(9)Modifybearerrequest

(10)Modifybearerprocessing(11)ModifybearerresponseFirstdownlinkuserdatapackets

Figure5Radiobearerreestablishmentprocedure


31


tionandsecurity-relatedproce-

naltotheS-GW.Atthispoint,thepro-

bysupportingCSFallback

*17

[5]to

32

dures,theMMEsendsanestablishradiobearerrequestincludingtheS-GWIPaddressandbearerinfor-mationtotheeNodeB.TheeNodeBnowestablishesaradiobearerwiththemobiletermi-nalandcreatesapathuptotheS-GW.ItalsoreturnsanestablishradiobearerresponsethatincludestheeNodeBIPaddressandbearerinformationtotheMME.Steps(9)-(11):TheMMEsendsamodifybear-errequesttotheS-GWtransferringtheeNodeBIPaddresstoit.TheS-GWestablishesapathtotheeNodeBandreturnsamodifybear-erresponsetotheMME.

Theaboveprocedurereestablishestheradiobearerfromthemobiletermi-

cedureforestablishingabearerinsidethecorenetworkcanbeomittedthere-byshorteningconnectiondelaywhenreinitiatingcommunications.

4.ConclusionNTTDOCOMOhasintroducedEPCasacorenetworkincombinationwithitsdeploymentofLTEradioaccess.Thisarticledescribedthecon-figurationoftheEPCnetwork,overviewedtheequipmentused,andexplainedthebasicfunctionsandsig-nalingflows.TheEPCnetworkconsistsoftheMME,PCRF,S-GWandP-GWfunctionalnodesandprovidesAlways-Onbearercontrol,chargingcontrolandmobilitymanagementbetweendifferentradioaccesssystems.Lookingforward,ourplanistofur-therenhancetheEPCnetworksuchas

enableanLTE-compatibleterminaltomakevoicecallsvia3Gradioaccess.

References[1]K.Nishidaetal.:BasicSAEManagementTechnologyforRealizingAll-IPNetwork,NTTDOCOMOTechnicalJournal,Vol.11,No.3,pp.4-12,Dec.2009.[2]3GPPTS23.401V8.12.0:GeneralPack-etRadioService(GPRS)enhancementsforEvolvedUniversalTerrestrialRadioAccessNetwork(E-UTRAN)access,Dec.2010.[3]3GPPTS23.402V8.9.0:Architectureenhancementsfornon-3GPPaccesses,Jun.2010.[4]3GPPTS23.203V8.11.0:Policyandchargingcontrolarchitecture,Sep.2010.[5]I.Tanakaetal.:CSFallbackFunctionforCombinedLTEand3GCircuitSwitchedServices,NTTDOCOMOTechnicalJour-nal,Vol.11,No.3,pp.13-19,Sep.2009.

*17CSFallback:TheprocedureforswitchingtoaradioaccesstechnologythatsupportsCSdomainwhenoriginatingorterminatingcir-cuit-switchedservicessuchasvoicecallswhileinLTE.


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