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Step 1: Registration Request UE –> RAN
Step 2: AMF selection
If 5G-S-TMSI or GUAMI is not included or it does not indicate a valid AMF, then RAN will send to the default AMF.
3. Registration Request AN –> AMF:
-> For gNB, the N2 parameters include the Selected PLMN ID, Location Information and Cell Identity related to the cell in which the UE is camping, UE Context Request includes the security information, Establishment cause .
-> If the Registration type indicated by the UE is Periodic Registration Update, then steps 4 to 19 may be omitted.
-> When the Establishment cause is associated with priority services (e.g. MPS, MCS), the AMF includes a Message Priority header to indicate priority information.
4. [conditional] new AMF -> old AMF
-> The new AMF determines the old AMF using the UE’s 5G-GUTI.
-> If there was a change in AMF since the registration and both new AMF and old AMF are in same AMF set, then the new AMF retrieves the stored UE’s SUPI and UE context directly from the UDSF using Nudsf_UnstructuredDataManagement_Query, if UDSF is deployed.
-> If UDSF is not deployed, then the new AMF may invoke the Namf_Communication_UEContextTransfer service operation on the old AMF including the complete Registration Request NAS message to request the UE’s SUPI and UE Context.
5. [Conditional] old AMF to new AMF:
-> If the UDSF was queried in step 4, the UDSF responds to the new AMF for the Nudsf_Unstructured Data Management_Query invocation with the related contexts including established PDU Sessions, the old AMF includes SMF information DNN, S-NSSAI(s) and PDU Session ID, active NGAP UE-TNLA bindings to N3IWF/TNGF/W-AGF, the old AMF includes information about the NGAP UE-TNLA bindings.
-> If the Old AMF was queried in step 4, Old AMF responds to the new AMF for the Namf_Communication_UEContextTransfer invocation by including the UE’s SUPI and UE Context.
6. [Conditional] new AMF to UE: Identity Request ().
-> If the SUCI is not provided by the UE, then the Identity Request procedure is initiated by AMF sending an Identity Request message to the UE requesting the SUCI.
7. [Conditional] UE to new AMF: Identity Response ().
-> he UE responds with an Identity Response message including the SUCI.
8. AUSF selection
If AMF needs to initiate UE authentication, then AMF selects an suitable AUSF based on SUPI or SUCI.
9. Authentication/Security
9a. AMF requests authentication from the AUSF
9b. If NAS security context does not exist, the NAS security initiation is performed.
9c. The AMF initiates NGAP procedure to provide the 5G-AN with security context.
9d. The 5G-AN stores the security context and acknowledges to the AMF. The 5G-AN uses the security context to protect the messages exchanged with the UE.
10. [Conditional] new AMF to old AMF: Namf_Communication_RegistrationStatusUpdate
-> If the AMF has changed the new AMF informs the old AMF that the registration of the UE in the new AMF is completed by invoking the Namf_Communication_RegistrationStatusUpdate service operation.
11. [Conditional] new AMF to UE: Identity Request/Response (PEI).
-> If the PEI was not provided by the UE the Identity Request procedure is initiated by AMF sending an Identity Request message to the UE to retrieve the PEI.
12. [Optional] New AMF initiates ME identity check by invoking the N5g-eir_EquipmentIdentityCheck_Get service operation
13. UDM selection
-> If step 14 is to be performed, the new AMF, based on the SUPI, selects a UDM, then UDM may select a UDR instance.
14 a-c.
a:
-> If the AMF has changed since the last Registration procedure, or,
-> If the UE provides a SUPI which doesn’t refer to a valid context in the AMF
-> Then the new AMF registers with the UDM using Nudm_UECM_Registration for the access to be registered.
b: If the AMF does not have subscription data for the UE, the AMF retrieves the Access and Mobility Subscription data, SMF Selection Subscription data, UE context in SMF data and LCS mobile origination using Nudm_SDM_Get.
c: The AMF subscribes to be notified using Nudm_SDM_Subscribe when the data requested is modified.
14d: Nudm_UECM_DeregistrationNotify
-> When the UDM stores the associated Access Type (e.g. 3GPP) together with the serving AMF as indicated in step 14a, it will cause the UDM to initiate a Nudm_UECM_DeregistrationNotification (see clause 5.2.3.2.2) to the old AMF corresponding to the same (e.g. 3GPP) access, if one exists.
-> If the serving NF removal reason indicated by the UDM is Initial Registration, then, as described in clause 4.2.2.3.2, the old AMF invokes the Nsmf_PDUSession_ReleaseSMContext (SM Context ID) service operation towards all the associated SMF(s) of the UE to notify that the UE is deregistered from old AMF for the same Access Type.
14e: [Conditional] If old AMF does not have UE context for another access type, the Old AMF unsubscribes with the UDM for subscription data using Nudm_SDM_unsubscribe.
15. PCF Selection
-> If the AMF decides to initiate PCF communication, the AMF acts as follows.
-> If the new AMF decides to use the (V-)PCF identified by the (V-)PCF ID included in UE context from the old AMF in step 5, the AMF contacts the (V-)PCF identified by the (V-)PCF ID to obtain policy.
16. [Optional] AM Policy Association Establishment/Modification
-> New AMF performs an AM Policy Association Establishment/Modification.
-> If the new AMF selects a new (V-)PCF in step 15, the new AMF performs AM Policy Association Establishment with the selected (V-)PCF.
17. [Conditional] AMF – SMF Nsmf_PDUSession_UpdateSMContext
-> The AMF invokes the Nsmf_PDUSession_UpdateSMContext in the following scenario(s):
-> If the List Of PDU Sessions To Be Activated is included in the Registration Request in step 1, the AMF sends Nsmf_PDUSession_UpdateSMContext Request to SMF(s) associated with the PDU Session(s) in order to activate User Plane connections of these PDU Session(s).
-> If the AMF has determined in step 3 that the UE is performing Inter-RAT mobility to or from NB-IoT, the AMF sends Nsmf_PDUSession_UpdateSMContext Request to SMF(s) associated with the UEs PDU Session(s), so the SMF(s) can update them according to the “PDU Session continuity at inter RAT mobility” subscription data.
18. [Conditional] N2 AMF Mobility Request
-> If the new AMF and the old AMF are in the same PLMN, the new AMF sends a UE Context Modification Request to N3IWF/TNGF/W-AGF as specified in TS 29.413.
19. N2 AMF Mobility Response
-> N3IWF/TNGF/W-AGF sends a UE Context Modification Response to the new AMF.
19a.
[Conditional] After the new AMF receives the response message from the N3IWF, W-AGF or TNGF in step 19, the new AMF registers with the UDM using Nudm_UECM_Registration as step 14a, but with the Access Type set to “non-3GPP access”.
19b.
-> [Conditional] When the UDM stores the associated Access Type together with the serving AMF as indicated in step 19a, it will cause the UDM to initiate a Nudm_UECM_DeregistrationNotification to the old AMF corresponding to the same access.
19c.
-> The Old AMF unsubscribes with the UDM for subscription data using Nudm_SDM_unsubscribe.
20. void
21. Registration Accept AMF -> UE
New AMF to UE: Registration Accept (
5G-GUTI,
Registration Area,
[Mobility restrictions],
[PDU Session status],
[Allowed NSSAI],
[Mapping Of Allowed NSSAI],
[Configured NSSAI for the Serving PLMN],
[Mapping Of Configured NSSAI],
[NSSRG Information],
[rejected S-NSSAIs],
[Pending NSSAI],
[Mapping Of Pending NSSAI],
[Periodic Registration Update timer],
[Active Time],
[Strictly Periodic Registration Timer Indication],
[LADN Information],
[accepted MICO mode],
[IMS Voice over PS session supported Indication],
[Emergency Service Support indicator],
[Accepted DRX parameters for E-UTRA and NR],
[Accepted DRX parameters for NB-IoT],
[extended idle mode DRX parameters],
[Paging Time Window],
[Network support of Interworking without N26],
[Access Stratum Connection Establishment NSSAI Inclusion Mode],
[Network Slicing Subscription Change Indication],
[Operator-defined access category definitions],
[List of equivalent PLMNs],
[Enhanced Coverage Restricted information],
[Supported Network Behaviour],
[Service Gap Time],
[PLMN-assigned UE Radio Capability ID],
[PLMN-assigned UE Radio Capability ID deletion],
[WUS Assistance Information],
[AMF PEIPS Assistance Information],
[Truncated 5G-S-TMSI Configuration],
[Connection Release Supported],
[Paging Cause Indication for Voice Service Supported],
[Paging Restriction Supported],
[Reject Paging Request Supported],
[Paging Restriction Information acceptance / rejection],
[“List of PLMN(s) to be used in Disaster Condition”],
[Disaster Roaming wait range information],
[Disaster Return wait range information]
).
22:
[Conditional] UE to new AMF: Registration Complete ().
23: [Conditional] Nudm_SDM_Info AMF->UDM
-> If the Access and Mobility Subscription data provided by UDM to AMF in 14b includes Steering of Roaming information with an indication that the UDM requests an acknowledgement of the reception of this information from the UE, the AMF provides the UE acknowledgement to UDM using Nudm_SDM_Info.
24: [Conditional] Nudm_UECM_Update AMF -> UDM
25: [Conditional] Network Slice-Specific Authentication and Authorization
-> If the UE indicates in the registration request that it supports the network slice authentication process in the UE MM core network
Source: TS 23.502