5G Basic Call Flows.pdf

5G Call flow

1. Koorosh Hoveyda
5GC / EPC Consultant , Summer 2021

3. A.TheUEsendsanNASRegistrationRequestmessagetoAMFvia
the(R)AN.
B.IncaseanewAMFisselected(e.g.,becausetheUEregistersinan
areanotservedbytheoldAMF),andtheUEprovidedaGUAMI
containingtheidentityoftheoldAMF,thenewAMFretrievesthe
UEcontextfromtheoldAMF
C.Authenticationiscarriedout,eitherusing5GAKAorEAP-AKA
D.IncaseanewAMFhasbeenselected,thenewAMFindicatesto
theoldAMFthatitisnowtakingoverasservingAMFfortheUE.
E.TheAMFregistersasservingAMFfortheUEinthespecificaccess
technologyusingNudm_UECMservice.TheAMFalsorequests
subscriptiondataandsubscribestosubscriptiondataupdatesusing
Nudm_SDMservice.
5G Call Flow:
Registration

4. F.theAMFinitiatesestablishmentoftheAMpolicyassociationwith
PCFandretrievestheAMpolicies
G.IftheUEhasindicatedthatitwantstoactivateUserPlane
connectionforexistingPDUSessions,theAMFinvokesthe
Nsmf_PDUSession_UpdateSMContextserviceoperationforthose
PDUSessions.IfthereisamismatchintheUEandAMFPDUSession
state,theAMFinvokestheNsmf_PDUSession_ReleaseSMContext
serviceoperationtonotifytheaffectedSMFsaboutthosePDU
Sessions.
H.IftheRegistrationprocedureissuccessfulsofar,theAMF
providesaNASRegistrationAccepttotheUE.Insomecases,theUE
sendsaNASRegistrationCompletemessagetotheAMF.
I.IncaseUEpolicies(ANDSPand/orURSP)aredeployed,theAMF
initiatesestablishmentoftheUEpolicyassociationwithPCF.This
allowsthePCFtoprovideUEpoliciestotheUE
5G Call Flow:
Registration

5. Note:
TheAMFmayalsochecktheME
identitywithanEquipmentIdentity
Register(EIR).typicallybetweensteps
DandE.TheEIRcanbeusedto
blacklist,forexample,stolenUEs.
Dependingontheresponsefromthe
EIR,theAMFmaycontinuethe
attachmentprocedureorrejecttheUE
5G Call Flow:
Registration

6. 5G Call Flow:
Deregistration
A.TheUEsendsanNASDeregistrationRequestmessagetoAMF
viathe(R)AN.
B.TheAMFnotifieseachSMFwithactiveSMcontextthatthe
correspondingSMcontextisreleased.TheSMFtheninturn
notifiesotherNFsaboutthereleaseofthe
C.TheAMFreleasestheAMpolicyassociationwithPCF(ifany).
D.TheAMFreleasestheUEpolicyassociationwithPCF(ifany).
E.TheAMFsendsaDeregistrationAcceptmessagetotheUE,
unlesstheUEindicatedswitch-off,i.e.,theUEinthatcasedoes
notwaitforanyacceptancefromthenetwork.
F.Finally,theAMFinstructs(R)ANtoreleasetheN2UEcontext.If
thereisstilla(R)AN-levelassociationbetweenUEand(R)AN,the
(R)ANmayrequesttheUEtoreleaseit.

7. A.WhentheUEwantstoinitiatetheServiceRequest
procedure,itsendsaNASServiceRequestmessagetoAMF
viathe(R)AN.IftheUEwantstoestablishUserPlane
connectionsforoneormoreexistingPDUSessions,theUE
includesinformationtoAMFaboutthePDUSessionIDsfor
thesePDUSessions.
B.ThenetworkmayoptionallyreauthenticatetheUE
duringthisprocedure.
C.IftheUEindicatedinstepAthatitwantstoestablish
UserPlaneconnectionforoneormorePDUSessions,the
AMFnotifieseachSMFservingthecorrespondingPDU
Session.InthesimplestcasetheSMFcanreplytothis
messagewiththeUPFtunnelendpointidentifiers.Inother
cases,e.g.,iftheUElocationisoutsidetheservingareaof
thecurrentUPF,theSMFmayhavetoselectanewI-UPF
andthenthisstepCbecomesmoreinvolved,with
signalingtowardstheoldI-UPF(ifany),newI-UPFandthe
anchor(PSA)UPF.Forsimplicitywehavenotshownthis
additionalN4signaling.
5G Call Flow:
Service Request
UE Triggered

8. D.IftheprocedurewastriggeredbytheUE,theAMFsends
aNASServiceAccepttotheUE.IfUserPlaneistobe
established,theAMFalsoforwardsthePDUSession
informationtothe(R)AN,includingtheUPFtunnel
endpointidentifiers.The(R)ANconfigurestheUserPlane
connectiontowardstheUE.Theexactdetailsofhowthisis
donedependsontheANtechnology.For3GPPRAN,this
may,e.g.,bedoneviaRRCreconfiguration.Oncethishas
beendone,up-linktrafficcanstarttobesent.The(R)AN
thenreplieswiththe(R)ANtunnelendpointidentifier(s)to
theAMF.
E.TheAMFnowneedtonotifyeachSMFagain,toprovide
theresultoftheUserPlaneestablishmentandthe(R)AN
tunnelendpointidentifier(s).IfthePCFhassubscribedto
UElocationinformation,theSMFthennotifiesthePCF
aboutthenewUElocation.TheSMFalsoprovidesthe
(R)ANtunnelendpointidentifier(s)totheUPFsothat
down-linktrafficcanbesenttowards(R)AN.
5G Call Flow:
Service Request
UE Triggered

9. 5G Call Flow:
Service Request
Network Triggered
A.IftheUPFreceivesdown-linkdataandhasbeeninstructedbySMFto
bufferthepackets,theUPFdoesthebufferingandnotifiesSMFaboutthe
receiveddata.
B.TheSMFsendsPDUSessioninformation(UPFtunnelendpoint
informationandQoSinformation)toAMFforforwardingto(R)AN.
C.IftheUEisinCM-IDLEstate,theAMFneedstopagetheUE.TheAMF
storesthePDUSessioninformationandsendsapagingrequesttoNG-RAN.
TheNG-RANthenpagestheUE.WhentheUEreceivesthepage,theUE
sendsaServiceRequestmessagetothenetwork.Thatmessageandthe
restoftheprocedurethenfollowstheUE-triggeredServiceRequest
describedinSection15.3.2.
D.IftheUEisinCM-CONNECTEDstate,thereisnoneedtopagetheUE.
InsteadtheAMFsimplyforwardsthePDUSessioninformationreceived
fromSMFtothe(R)ANsothat(R)ANcanproceedwithsettinguptheUser
Plane.Thisstep(A)(B)(C)(D)Fig.15.6Network-triggeredServiceRequest.
Selectedcallflows403andtherestoftheprocedurealsofollowstheUE-
triggeredServiceRequestdescribedinSection15.3.2,buthereonlythe
steps(C)and(D)areneededintheUE-triggeredServiceRequest.

10. A.TheUEsendsa5GSMNASPDUSessionEstablishmentmessage
totheAMF,includingPDUSessionId,DNN-requestedS-NSSAI,
PDUSessiontype,etc.TheAMFprocessestheNASsecurity.Ifthe
PDUSessionEstablishmentisarequestforanewPDUSession,the
AMFselectsanewSMF.TheAMFmayusetheNRFtodiscover
availableSMFsservingthespecificDNNandS-NSSAI.IfthePDU
SessionEstablishmentisarequestforhandingoveranexisting
PDUSession,theAMFusesitsUEcontexttodeterminewhatSMF
isservingthePDUSessionId.
B.TheAMFthenforwardsthe5GSMcontainer(containingthe
PDUSessionEstablishmentmessage)totheSMF.TheSMF
retrievestheSessionManagementrelatedUEsubscriptiondata
fromUDMandalsosubscribestosubscriptiondataupdatesfrom
UDM
C.Incasesecondaryauthenticationisapplied,itisperformednow
D.TheSMFthenselectsaPCFandinitiatesSMpolicysession
establishmenttoretrievetheinitialsetofPCCrules.TheSMFalso
selectsaUEIPaddressandUPFandinitiatesN4session
establishmenttowardsthatUPF
5G Call Flow:
PDU Session
Establishment

11. E. The SMF then sends a 5GSM NAS PDU Session
Establishment accept message towards the UE as well as the
UPF GTP-U tunneling endpoint information and QoS
information towards (R)AN. This message is sent via the AMF
G. The AMF forwards the PDU Session information received
from (R)AN to the SMF so that SMF can provide the (R)AN
GTP-U tunnel endpoint to the UPF for downlink forwarding.
Now also the down-link data path is ready for use.
H. If all works well, the SMF then registers itself in UDM, as
serving the PDU Session Id in UDM.
F.TheAMFcreatesandsendstheN2messagecontainingthe
NASmessage(PDUSessionEstablishmentaccept)aswellas
thePDUSessioninformationfor(R)AN,i.e.,GTP-Utunneling
informationandQoSinformation.The(R)ANestablishesthe
requiredresourcestowardstheUEandrepliestotheAMF
withinformationaboutthe(R)ANGTP-Utunnelendpoint.
Whenthisisdone,theup-linkdatapathisreadyforuse
5G Call Flow:
PDU Session
Establishment

12. 5G Call Flow:
UE Configuration
Update
A.TheAMFdecides,e.g.,basedonupdatedsubscriptiondata,that
UEconfigurationneedstobechangedorare-registrationis
needed.
B.TheAMFsendsaUEConfigurationUpdatecommandthatmay
includetheupdatedconfiguration.Itmayalsoincludeindications
whetherornotUEshallsendanacknowledgement(UE
ConfigurationUpdateComplete)orwhetherornotRegistration
procedureshallbeperformed.
C.TheUEthenappliesthenewparameters.Dependingon
updatedparameters,theAMFalsomayneedtonotifyother
entities.Forexample,if5G-GUTIischanged,AMFneedstoinform
(R)ANaboutthenewtemporaryidentity.Iftheupdatewasdueto
aNetworkSlicingSubscriptionChangeIndication,thenUDMis
informedthatUEhasreceivedtheupdate
D.Dependingontheupdatedparameters,theAMFmayrelease
theANassociation(e.g.,AllowedNSSAIorConfiguredNSSAIare
updatedinawaythataffectsexistingconnectivitytoNetwork
Slices).TheUEmayalsoinitiateare-registration,e.g.,toallowthe
UEtobeconnectedtoanewsetofNetworkSlices.

13. 5G Call Flow:
Handover from
5GS to EPS
A.TheNG-RANdecidestotriggerahandovertoEPSand
sendsaHandoverRequiredmessagetoAMF.Basedonthe
target“RAN”identifiertheAMFdeterminesitisa
handovertoE-UTRAN,andtheAMFthenrequestPGW-
C+SMFtoprovideSMContextthatisneededforAMFto
transferSMcontexttoMME.
B.TheAMFthenselectsatargetMME(basedon
informationabouttargetE-UTRANprovidedbyNG-RAN).
TheAMFthenactsasasourceMMEandsendsa
RelocationRequesttotheMME.TheMMEwillinitiate
sessionsetupandhandoverintargetE-UTRANaccess,
basicallyfollowingtheprocedureforS1-basedhandover.
Ifindirectforwardingapplies(i.e.,userdataforwarding
fromsourceNG-RANtotargetE-UTRANviathecore
network),theMMEinitiatessetupofforwardingtunnels
ontheEPCside.
C.Ifindirectforwardingapplies,theAMFtriggersSMFto
setupforwardingtunnelsalsoon5GCside.

14. 5G Call Flow:
Handover from
5GS to EPS
D.ThesourcesidenowissuestheHandoverCommand
totheUE,andUEmovestotargetcell.
E.Thedatapathforindirectforwarding(ifapplicable)is
establishedanddown-linkdatacanbeforwarded.Also,
theup-linkdatapathisavailableoverthetargetaccess.
F.ThetargetE-UTRANthensendsaHandoverNotifyto
targetMME,andbythisthetargetE-UTRANconsiders
thehandoversuccessful.TheMMEnownotifiesthe
sourceside(AMF)thatthehandoveriscomplete.
G.Finally,theUEinitiatesaTrackingAreaUpdate
procedure(asdoneaccordingtonormalS1-based
handover).Thenetworkalsotriggersestablishmentof
dedicatedbearersthatwerenotestablishedduringthe
actualhandoverprocedure.Thenetworkalsoreleases
theindirectforwardingtunnelswhentheyareno
longerneeded.

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