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Content for  TS 23.632  Word version:  17.0.0

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5.3  5GC-EPC Mobility ScenariosWord‑p. 14

5.3.1  General

As defined in TS 23.501, when interworking procedures with N26 are used, the UE operates in single-registration mode. For the 3GPP access, either the AMF or the MME is registered in the HSS+UDM. This implies that:
  • The registration of an MME in HSS for a UE that is capable to access the 5GC, triggers the cancellation of the AMF address for 3GPP access registered in the UDM, if any.
  • The registration of an AMF in UDM for a UE that is capable to access the EPC, triggers the cancellation of the MME address registered in the HSS, if any.
Interworking procedures without N26 do not require that the HSS+UDM cancels the location of the old serving node in the old access domain. However, the UDM may request from the HSS the cancelation of the SGSN.
Additionally, upon mobility from GERAN/UTRAN to 5GS, the previously registered SGSN needs to be cancelled from HSS/HLR, otherwise the incoming session can fail.
Additionally, IP address preservation for PDU sessions that support interworking is provided to UEs during inter-system mobility procedures by storing and fetching the PGW-C+SMF FQDN for S5/S8 interface per APN/DNN via the HSS+UDM.
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5.3.2  Mobility from 5GC to EPC

Figure 5.3.2-1 shows the interaction between the HSS and UDM in an interworking scenario when the UE attaches to the EPC.
Reproduction of 3GPP TS 23.632, Figure 5.3.2-1: Mobility from 5GC to EPC
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Step 1.
The HSS receives an S6a ULR request containing the IMSI of the subscriber.
The HSS stores the new MME address. If the EPC UDR is used, the HSS updates the EPS-UDR with the new MME address and reads the subscription information related to the IMSI from the EPS-UDR.
The UE's subscription may include restriction for Core Network Type (5GC). If restriction for Core Network Type indicates that the UE can access to 5GC, it implies that the UE has 5G subscription data.
Step 2.
The HSS responds to the MME with an S6a-ULA.
Step 3.
If the dual registration 5GS indicator is not set in the S6a ULR request and the subscription information related to the IMSI does not indicate that 5GC is restricted, then the HSS uses the Nudm_UECM_AMFDeregistration service operation to request the UDM to cancel the registration of the user in 5GC for 3GPP access, if any. The HSS includes the user's IMSI received in step 1 as the user's SUPI in this request.
If the S6a ULR request includes the initial attach indicator set, the HSS indicates to the UDM that the deregistration reason is due to "Initial Registration". When the initial attach indicator is not set, the HSS indicates to the UDM that the deregistration reason is due to 5GS to EPS mobility.
Step 4.
The UDM responds to the HSS.
The UDM checks if the user is registered in 5GC over 3GPP access. The UDM may use the 5GS-UDR to retrieve the address of the AMF for 3GPP access registered in UDM for that user, if any. If the user is registered at an AMF for 3GPP access, step 5 is executed.
Step 5.
If there is an AMF address for 3GPP access found for the user, the UDM cancels the user registration in 5GC by sending a Nudm_UECM_DeregistrationNotification to the AMF.
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5.3.3  Mobility from EPC to 5GCWord‑p. 15

Figure 5.3.3-1 shows the interaction between the UDM and the HSS in an interworking scenario when the UE attaches to the 5GC.
Reproduction of 3GPP TS 23.632, Figure 5.3.3-1: Mobility from EPC to 5GC
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Step 1.
The UDM receives a Nudm_UECM_Registration request for 3GPP access containing the SUPI of the subscriber. In this case the SUPI shall be based on an IMSI.
The UDM stores the new AMF address for 3GPP access. If the 5GS UDR is used, the UDM updates the 5GS-UDR with the new AMF address for 3GPP access and reads the subscription information related to the SUPI from the 5GS-UDR.
The UE's subscription may include restriction for Core Network Type (EPC). If restriction for Core Network Type indicates that the UE can access to EPC, it implies that the UE has EPC subscription data and that the UE may also be able to access via GERAN/UTRAN.
Step 2.
The UDM responds to the AMF with a Nudm_UECM_Registration Response.
Step 3.
If the dual registration and the initial registration flags are not set in the Nudm_UECM_Registration request and the subscription information related to the SUPI does not indicate that EPC is restricted, then the UDM uses the Nhss_UECM_SNDeregistration service operation indicating that the deregistration reason is due to EPS to 5GS mobility to request the HSS to cancel the MME/SGSN registration of the user in EPC, if any. The UDM includes the IMSI based on SUPI received in step 1 as the user's IMSI in this request.
If the dual registration flags is set in the Nudm_UECM_Registration request and the subscription information related to the SUPI does not indicate that EPC is restricted, then the UDM uses the Nhss_UECM_SNDeregistration service operation indicating that the deregistration reason is due to Initial and Dual Registration to request the HSS to cancel the SGSN registration of the user in EPC, if any. The UDM includes the IMSI based on SUPI received in step 1 as the user's IMSI in this request.
If the dual registration flag is not set and the initial registration flag is set in the Nudm_UECM_Registration request and the subscription information related to the SUPI does not indicate that EPC is restricted, then the UDM uses the Nhss_UECM_SNDeregistration service operation indicating that the deregistration reason is due to Initial and Single Registration to request the HSS to cancel the MME/SGSN registrations of the user in EPC, if any. The UDM includes the IMSI based on SUPI received in step 1 as the user's IMSI in this request.
Step 4.
The HSS responds to the UDM.
The HSS checks if the user is registered in EPC. The HSS may use the EPS-UDR to retrieve the address of the MME or SGSN registered in the HSS for that user, if any. If the user is registered at an MME or SGSN, step 5 is executed.
Step 5.
If there is an MME address found for the user and the deregistration reason in the Nhss_UECM_SNDeregistration service operation indicated "Initial and Single Registration" or "EPS to 5GS mobility", the HSS cancels the attachment of the IMSI in the MME by sending a CLR via the S6a interface setting the cancellation type to MME Update Procedure, and if a VLR number was found in the HSS for the user, the HSS also cancels attachment of the IMSI in this MSC/VLR by sending the MAP Cancel Location (IMSI) to the MSC/VLR via the D interface.
If there is an SGSN address found for the user and the deregistration reason in the Nhss_UECM_SNDeregistration service operation indicated "Initial and Single Registration", the HSS cancels the attachment of the IMSI in the SGSN by sending a CLR via the S6d interface or by sending the MAP Cancel Loc via the Gr interface setting the cancellation type to SGSN Update Procedure. When deregistration reason indicated "Initial and Dual Registration" or "EPS to 5GS Mobility", the HSS cancels the attachment of the IMSI in the SGSN by sending a CLR via the S6d interface setting the cancellation type to SGSN Update Procedure.
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5.3.4  Support for PDU session continuity during intersystem mobility proceduresWord‑p. 16

When the UE is connected to 5GS or 5GC-N3IWF, the PGW-C+SMF may store in UDM the PGW-C+SMF FQDN for S5/S8/S2b interface, within the UE context in SMF data, when the PWG-C+SMF registers in UDM. The AMF serving 3GPP access may also notify the UDM to store the association between DNN and PGW-C+SMF FQDN which supports EPS interworking as Intersystem continuity context.
During mobility from 5GC to EPC or EPC/ePDG, or mobility from 5GC-N3IWF to EPC, or mobility from EPC/ePDG to EPC, the HSS uses the Nudm_SDM_Get service operation to retrieve the PGW-C+SMF FQDN for S5/S8/S2b interface from UDM, and subscribes to be notified using Nudm_SDM_Subscribe when the Intersystem continuity context data or the UE context in SMF data are modified. The HSS retrieves both the UE context in SMF data and the Intersystem continuity context data from UDM. The HSS checks the Intersystem continuity context first. If no PGW-C+SMF FQDN associated with the APN exists in Intersystem continuity context, the HSS selects one of the PGW-C+SMF FQDN for the APN from the UE context in SMF data based on operator's policy.
During UE registration for the scenario when ePDG is connected to SMF+PGW-C and S6b in not used, the HSS subscribes to be notified from the UDM using Nudm_SDM_Subscribe when the UE context in SMF data is modified as new PDU sessions established in the ePDG are not updated in the HSS via the SWx interface.
Figure 5.3.4-1 shows the interaction between the HSS and the UDM in an interworking scenario when the UE attaches to the EPC or EPC/ePDG.
Reproduction of 3GPP TS 23.632, Figure 5.3.4-1: PDU session continuity during mobility from 5GC to EPC
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Step 0.
While the UE is connected to the 5GC, the AMF and/or SMF may store in UDM the association between DNN and PGW-C+SMF FQDN which supports EPS interworking.
If the 5GS UDR is used, the UDM updates the 5GS-UDR with the DNN and PGW-C+SMF FQDN association.
Step 1.
The HSS receives an S6a ULR or SWx SAR request containing the IMSI of the subscriber.
The HSS reads the subscription information related to the IMSI. If the EPC-UDR is used, the HSS retrieves the subscription information from the EPS-UDR. The UE's subscription may include subscribed APNs that support interworking with 5GS.
Step 2.
If the subscription information related to the IMSI includes subscribed APNs that support interworking with 5GS, then the HSS uses the Nudm_SDM_Get service operation to fetch the UE context in SMF data and the Intersystem continuity context data for the user in AMF data, if any. The HSS includes the user's IMSI received in step 1 as the user's SUPI in this request.
Step 3.
The UDM responds to the HSS.
The HSS checks the Intersystem continuity context first. If no PGW-C+SMF FQDN associated with an DNN exists in Intersystem continuity context, the HSS selects one of the PGW-C+SMF FQDN for the APN from the UE context in SMF data based on operator's policy.
Step 4-5.
The HSS subscribes to be notified using Nudm_SDM_Subscribe service operation when the Intersystem continuity context data or the UE context in SMF data are modified.
Step 6.
The HSS responds to the MME with an S6a-ULA response or to the AAA with an SWx-SAA response including the PGW-C+SMF FQDN associated with subscribed APNs that support interworking with 5GS, if any. This step may happen at any time after step 3, not necessarily after steps 4-5.
If an S6a ULR is received, the HSS performs steps 3 and 4 of clause 5.3.2.
Figure 5.3.4-2 shows the interaction between the HSS and the UDM when the HSS receives notification from the UDM when the data requested is modified.
Reproduction of 3GPP TS 23.632, Figure 5.3.4-2: Notification of PGW-C+SMF FQDN from UDM to HSS
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Step 0.
The AMF and/or SMF may add or remove or update the association between DNN and PGW-C+SMF FQDN stored in the UDM.
If the 5GS UDR is used, the UDM updates the 5GS-UDR with the DNN and PGW-C+SMF FQDN association.
Step 1.
If the UDM finds the UE context in SMF data or the Intersystem continuity context is changed, and the active subscription from the HSS exists for the UE to be notified on the change of the UE context in SMF data or the Intersystem continuity context, then the UDM uses the Nudm_SDM_Notification service operation to notify the HSS. As part of this notification the UDM informs the HSS whether the SMF indicated that the access to 5GC was via an ePDG.
Step 2.
The HSS responds to the UDM.
The HSS checks the change of the association between APN and PGW-C+SMF FQDN. If multiple PGW-C+SMF FQDN exists in the UE context in SMF data, the HSS selects one of the PGW-C+SMF FQDN for the APN based on operator's policy.
Step 3.
The HSS may further trigger PDN GW Identify Notification procedure, as defined in clause 12.1.4 and clause 12.1.5 in TS 23.402.
Figure 5.3.4-3 shows the interaction between the HSS and the UDM when the HSS unsubscribes from the UDM to be notified when the data requested is modified.
Reproduction of 3GPP TS 23.632, Figure 5.3.4-3: Unsubscribe of PGW-C+SMF FQDN Notification from HSS to UDM
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Step 0.
The HSS checks the conditions to unsubscribe the notification of the UE context in SMF data or the Intersystem continuity context from the UDM, when the UE's 3GPP access or Non-3GPP access registration in EPC is not valid. For example, when the MME performs purge UE procedure, or the AAA performs UE deregistration towards the HSS, or the registration of the user in EPC is cancelled due to mobility from EPC to 5GC.
Step 1.
The HSS unsubscribes to be notified using Nudm_SDM_Unsubscribe service operation towards the UDM.
Step 2.
The UDM responds to the HSS.
When the UE is connected to EPC, the MME stores in HSS the PGW-C+SMF FQDN for S5/S8 interface via S6a reference point. When the UE is connected to EPC via an ePDG, and the S6b reference point is deployed between the PGW-C+SMF and AAA, the AAA stores in HSS the PGW-C+SMF FQDN for S5/S8 interface via SWx reference point.
During mobility from EPC or EPC/ePDG to 5GC, or mobility from EPC to 5GC-N3IWF, the UDM uses the Nhss_SDM_Get service operation to retrieve the PGW-C+SMF FQDN for S5/S8 interface from HSS and subscribes to be notified using Nhss_SDM_Subscribe when the data requested is modified.
Figure 5.3.4-4 shows the interaction between the HSS and the UDM in an interworking scenario when the UE moves from EPC or EPC/ePDG to 5GC.
Reproduction of 3GPP TS 23.632, Figure 5.3.4-4: PDU session continuity during mobility from EPC/ePDG to 5GC
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Step 0.
While the UE is connected to the EPC or EPC/ePDG, the MME or the AAA informs the HSS of the PGW-C+SMF address and associated APN.
Step 1.
UE moves to 5G coverage, the AMF registers with the UDM using Nudm_UECM_Registration for 3GPP access.
The UDM performs steps 3 and 4 in clause 5.3.3.
Step 2.
The AMF retrieves the Access and Mobility Subscription data, SMF Selection Subscription data and UE context in SMF data using Nudm_SDM_Get.
Step 3.
If the subscription information related to the SUPI includes subscribed DNNs that support interworking with EPS, then the UDM uses the Nhss_SDM_Get service operation to fetch the PGW-C+SMF information. The UDM includes the user's IMSI derived from SUPI.
Step 4.
The HSS responds to the UDM.
Step 5.
The UDM includes the PGW-C+SMF address associated with subscribed DNNs that support interworking with EPS, if any, as UE context in SMF data in Nudm_SDM_Get_Response.
Step 6.
After a successful response is received, the AMF subscribes to be notified when the data requested is modified using Nudm_SDM_Subscribe service operation.
Step 7.
If the UE context in SMF data is subscribed to be notified and the subscription information related to the SUPI includes subscribed DNNs that support interworking with EPS, the UDM subscribes towards the HSS (if not already subscribed, for the same UE) to be notified when the PGW-C+SMF information is modified using Nhss_SDM_Subscribe service operation.
Step 8.
The HSS responds to the UDM.
Step 9.
The UDM responds to the AMF.
Figure 5.3.4-5 shows the interaction between the HSS and the UDM when the UDM receives notification from the HSS when the data requested is modified.
Reproduction of 3GPP TS 23.632, Figure 5.3.4-5: Notification of PGW-C+SMF FQDN from HSS to UDM
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Step 0.
The MME or the AAA may add or update the association between APN and PGW-C+SMF FQDN stored in the HSS.
Step 1.
If the HSS finds the association between APN and PGW-C+SMF FQDN is changed, and the active subscription from the UDM exists for the UE to be notified on the change of the data requested, then the HSS uses the Nhss_SDM_Notification service operation to notify the UDM.
Step 2.
The UDM responds to the HSS.
Step 3 - 4.
If the UDM finds the association between DNN and PGW-C+SMF FQDN is changed, and the active subscription from the AMF exists for the UE to be notified on the change of the UE context in SMF data, then the UDM notifies the AMF using the Nudm_SDM_Notification service operation.
Figure 5.3.4-6 shows the interaction between the HSS and the UDM when the UDM unsubscribes from the HSS to be notified when the data requested is modified.
Reproduction of 3GPP TS 23.632, Figure 5.3.4-6: Unsubscribe of PGW-C+SMF FQDN Notification from UDM to HSS
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Step 1 - 2.
The AMF unsubscribes to be notified using Nudm_SDM_Unsubscribe service operation.
Step 3.
If the UE context in SMF data is unsubscribed, and no other AMF has subscribed to the same data (UE context in SMF) for the same UE, the UDM unsubscribes towards the HSS to be notified when the PGW-C+SMF information is modified using Nhss_SDM_Unsubscribe service operation.
Step 4.
The HSS responds to the UDM.

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