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Content for  TS 23.632  Word version:  17.0.0

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5.4  Scenarios of Interworking with IMSWord‑p. 21

5.4.1  T-ADS

Figure 5.4.1-1 shows the scenario where the HSS receives a T-ADS request from the AS for a subscriber who has a 5GC subscription.
Reproduction of 3GPP TS 23.632, Figure 5.4.1-1: T-ADS request for 5G subscriber
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Step 1.
The HSS receives a T-ADS request from the AS. The request includes the user's IMPU or MSISDN and optionally the user's IMPI.
Step 2.
If the EPS-UDR is used, the HSS reads subscription data from the EPS-UDR. The HSS, retrieves the user's IMSI associated with the user's identifiers received in step 1. The subscription data may contain an indication that the user has a 5GC subscription.
Step 3.
The HSS performs T-ADS info retrieval in the EPC. This may include contacting the MME and/or SGSN.
Step 4.
If the HSS detects in step 2 that the user has a 5GC subscription and unless the user is known not to be registered in 5GC (e.g. in case the user is registered in EPC and the HPLMN is working in single registration mode with N26), the HSS uses the Nudm_MT_ProvideDomainSelection service operation to retrieve T-ADS information from the UDM. The HSS includes the user's IMSI associated with the user's identifiers received in step 1 as the SUPI in this request.
Step 5.
The UDM reads data from the 5GS-UDR to see whether the registered AMF(s) support IMS voice over PS.
Step 6-7.
If support/non-support of IMS voice is not homogeneous for the User in the AMF for 3GPP access, the UDM retrieves domain selection information from the AMF.
Step 8.
The UDM provides the 5G domain selection info to the HSS.
Step 9.
The HSS combines the 5G domain selection info with the EPS domain selection info from step 3 and sends the result to the AS.
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5.4.2  P-CSCF RestorationWord‑p. 23

Figure 5.4.2-1 shows the scenario where the HSS receives a P-CSCF restoration indication from the S-CSCF for a subscriber who has a 5GC subscription.
Reproduction of 3GPP TS 23.632, Figure 5.4.2-1: P-CSCF Restoration for 5G subscriber
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Step 1.
The HSS receives a P-CSCF Restoration indication from the S-CSCF. The request includes the user's IMPU and IMPI.
Step 2.
If the EPS-UDR is used, the HSS reads subscription data from the EPS-UDR. The HSS, retrieves the user's IMSI associated with the user's identifiers received in step 1. The subscription data may contain an indication that the user has a 5GC subscription.
Step 3-4.
The HSS acknowledges receipt of the request and performs P-CSCF restoration in the EPC. For details see TS 23.380, clause 5.4.2.1.
Step 5.
If the HSS detects in step 2 that the user has a 5GC subscription and unless the user is known not to be registered in 5GC (e.g. in case the user is registered in EPC and the HPLMN is working in single registration mode with N26), the HSS uses the Nudm_UECM_P-CscfRestorationTrigger service operation to trigger P-CSCF restoration in 5GC. See also TS 23.380, clause 5.8.4. The HSS includes the user's IMSI associated with the user's identifiers received in step 1 as the SUPI in this request
Step 6.
The UDM reads data from the 5GS-UDR to see which NFs have subscribed to P-CSCF Restoration Notifications (implicitly during registration).
Step 7.
The UDM acknowledges receipt of Nudm_UECM_PCscfRestorationTrigger to the HSS.
Step 8-9.
The UDM sends a notification to one of the subscribed NFs (either the AMF or the SMF).
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5.4.3  Network Provided Location Information RequestWord‑p. 24

Figure 5.4.3-1 shows the scenario where the HSS receives a Location Information retrieval request from the IMS-AS for a subscriber who has a 5GC subscription.
Reproduction of 3GPP TS 23.632, Figure 5.4.3-1: Location Information retrieval request for 5G subscriber
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Step 1.
The HSS receives a Location Information retrieval request (requested nodes includes AMF) from the IMS-AS.
Step 2.
The HSS reads subscription data from the EPS-UDR.
Step 3.
The HSS performs Location Information Retrieval from other nodes if so requested. This may include contacting the MME and/or SGSN.
Step 4.
If the HSS detects in step 2 that the user has a 5GC subscription and unless the user is known not to be registered in 5GC, the HSS uses the Nudm_MT_ProvideLocationInfo service operation to retrieve 5GS Location Information. The service request includes indicators for Current Location, Serving Node Indication, Local Time Zone Indication, and RAT-Type requested as has been received in step 1.
Step 5.
The UDM reads data from the 5GS-UDR to get the AMF and SMSF for 3GPP Access Registration Information. If the Serving Node Indication was received in step 4, steps 6-7 shall be skipped.
Step 6-7.
The UDM retrieves location information from the AMF (for 3GPP access) by means of the Namf_Location_ProvideLocationInfo service operation .
Step 8.
The UDM provides the 5GS location information info to the HSS.
Step 9.
The HSS provides the requested location information to the IMS-AS.
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5.4.4  User State RetrievalWord‑p. 25

Figure 5.4.4-1 shows the scenario where the HSS receives a PS User State retrieval request from the IMS-AS for a subscriber who has a 5GC subscription.
Reproduction of 3GPP TS 23.632, Figure 5.4.4-1: PS User State retrieval request for 5G subscriber
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Step 1.
The HSS receives a PS User State retrieval request (requested nodes includes AMF) from the IMS-AS.
Step 2.
The HSS reads subscription data from the EPS-UDR.
Step 3.
The HSS performs PS User State Retrieval from other nodes if so requested. This may include contacting the MME and/or SGSN.
Step 4.
If the request includes AMF as requested node and if the HSS detects in step 2 that the user has a 5GC subscription and unless the user is known not to be registered in 5GC, the HSS uses the Nudm_MT_ProvideUserState service operation to retrieve the 5GS User State. Otherwise, continue with step 9.
Step 5.
The UDM reads data from the 5GS-UDR to get the AMF for 3GPP Access Registration Information.
Step 6-7.
The UDM retrieves the user state from the AMF (for 3GPP access) by means of the Namf_EventExposure Subscribe service operation (one time immediate report requested).
Step 8.
The UDM provides the 5GS PS User State to the HSS.
Step 9.
The HSS provides the requested user state to the IMS-AS.
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5.4.5  UE ReachabilityWord‑p. 27

Figure 5.4.5-1 shows the scenario where the HSS receives a Subscription to notification request for UE-reachability from the IMS-AS for a subscriber who has a 5GC subscription.
Reproduction of 3GPP TS 23.632, Figure 5.4.5-1: Subscription to UE reachability for 5G subscriber
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Step 1.
The HSS receives a Subscribe request for UE reachability for IP from the IMS-AS.
Step 2.
The HSS reads subscription data from the EPS-UDR.
Step 3.
The HSS sets the UE Reachability flags for EPC and if the UE is registered in EPC contacts the registered MME and SGSN to get notified when the UE becomes reachable.
Step 4.
If the HSS detects in step 2 that the user has a 5GC subscription, the HSS uses the Nudm_EventExposure_Subscribe service operation (one time immediate report requested) to get notified when the UDM detects UE reachability for SMS over IP. Otherwise, continue with step 10.
Step 5.
The UDM sets the URRP-AMF flag in the 5GS-UDR and reads data from the 5GS-UDR to get the AMF for 3GPP Access Registration Information and the AMF for non-3GPP Access Registration Information if any.
Step 6-7.
[Conditional] If an AMF is registered in UDM for the target UE and the UDM has not already subscribed in AMF due to a previous subscription from a different NF, the UDM subscribes to UE reachability notifications at the registered AMFs by means of the Namf_EventExposure_Subscribe service operation (see TS 23.502).
Step 8.
The UDM updates the 5GS-UDR with the EE-Subscription for the HSS.
Step 9.
The UDM acknowledges the EE Subscription to the HSS.
Step 10.
The HSS acknowledges the Sh subscription to the IMS-AS.
Step 11.
The HSS updates the EPS-UDR with the Sh subscription for the IMS-AS.
Figure 5.4.5-2 shows the scenario where the UDM detects UE reachability for SMS over IP and notifies the HSS that has previously subscribed.
Reproduction of 3GPP TS 23.632, Figure 5.4.5-2: UE reachability notification for 5G subscriber
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Step 1.
The UDM receives a Notification or Registration from the AMF.
Step 2.
The UDM reads subscription data from the 5GS-UDR.
Step 3.
The UDM acknowledges step 1 towards the AMF. If an old AMF is registered in the UDM, the UDM sends a Nudm_UECM_DeregistrationNotification service operation to the old AMF.
Step 4.
The UDM notifies the HSS (and any other NF that has subscribed) about the reachability of the UE.
Step 5.
The HSS reads data from the EPS-UDR to see whether an IMS-AS has subscribed do reachability notification.
Step 6.
The HSS acknowledges step 4.
Step 7.
The UDM updates the 5GS-UDR to delete the EE-Subscription(s).
Step 8.
The HSS notifies the IMS-AS about UE reachability for IP.
Step 9.
The IMS-AS acknowledges step 8.
Step 10.
The HSS updates the EPS-UDR to delete the IMS-AS's subscription.
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5.4.6  IMEI RetrievalWord‑p. 29

A pre-requisite for the retrieval of the IMEI when requested by an IMS-AS is that the IMEI for a given UE stored in HSS and UDM is always synchronized. This is, when the HSS detects that the IMEI for a UE changes (e.g. during an Update Location in EPS), the HSS informs the UDM about the IMEI change which stores the new IMEI accordingly. Similarly, when UDM detects that the IMEI changes, the UDM informs the HSS about the IMEI change which stores the new IMEI accordingly.
This allows that the retrieval of the IMEI for a given UE requested by an IMS-AS from HSS via Sh can be executed locally by HSS without the need of additional interworking between the HSS and the UDM
Figure 5.4.6-1 shows the scenario where the HSS receives an IMEI retrieval request from the IMS-AS for a subscriber who has a 5GC subscription.
Reproduction of 3GPP TS 23.632, Figure 5.4.6-1: IMEI retrieval request for 5G subscriber
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Steps 1 to 4 are executed when the HSS detects a change in the IMEI for a given UE (IMSI).
Step 1.
The HSS receives a request from the MME including an IMEI for the UE (e.g. Update Location Request or Notify Request).
Step 2.
The HSS detects that the IMEI received in the request is different from the previously stored in the EPS-UDR. The HSS stores the new IMEI in the EPS-UDR.
Step 3.
The HSS informs the UDM about the new PEI (IMEI) using the Nudm_UECM_Update service operation.
4. The UDM stores the new PEI for the UE.
Steps 5 to 8 are executed when the UDM detects a change in the PEI for a given UE (SUPI).
Step 5.
The UDM receives a request from the AMF including a PEI for the UE (e.g. Nudm_UECM_Registration/Update).
Step 6.
The UDM detects that the PEI received in the request is different from the previously stored in the 5GS-UDR. The UDM stores the new IMEI in the 5GS-UDR.
Step 7.
The UDM informs the HSS about the new IMEI using the Nhss_UECM_Update service operation.
Step 8.
The HSS stores the new IMEI for the UE.
Steps 9 to 11 are executed when the HSS receives an IMEI retrieval request from the IMS-AS for a subscriber who has a 5GC subscription.
Step 9.
The HSS receives a request from IMS-AS to retrieve the IMEI for a UE.
Step 10.
The HSS reads the IMEI stored in the EPS-UDR.
Step 11.
The HSS replies to the IMS-AS with the users IMEI. Since the IMEI for the UE has been sinchronized between HSS and UDM at every IMEI/PEI change event as in steps 1 to 4 or 5 to 8, the HSS can reply to the IMS-AS without any additional interworking with the UDM.
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5.4.7  SRVCC: IMS AS obtaining SRVCC dataWord‑p. 30

Figure 5.4.7-1 shows the scenario where the HSS communicates with the IMS-AS in support of (5G) SRVCC for a subscriber who has a 5GC subscription. An operator shall be able to control whether the interaction between HSS and UDM is performed based on local policy.
Reproduction of 3GPP TS 23.632, Figure 5.4.7-1: IMS AS obtaining data related to (5G) SRVCC and updating STN-SR
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Step 1.
As described in TS 23.237, the IMS-AS sends Sh-Pull message to the UDM/HSS in order to know the SRVCC capability indicated by the UE in the EPS, and to retrieve the STN-SR stored in the UDM/HSS.
Step 2.
The HSS reads STN-SR and UE SRVCC Capability data from the EPS-UDR. The HSS reads the C MSISDN bound to the IMS Private User Identity.
Step 3.
The HSS, based on operator policy, may query UDM to retrieve SRVCC parameters. The HSS uses the Nudm_MT_Provide5GSRVCCInfo service operation to retrieve the STN-SR and the UE SRVCC Capability indicated by the UE in the 5GS (see TS 23.008).
Step 4.
The UDM reads the STN-SR and this UE SRVCC Capability data from the 5GS-UDR.
Step 5.
The UDM provides the C-MSISDN, STN-SR and UE SRVCC Capability data to the HSS.
Step 6.
The HSS provides a single C MSISDN, a single STN-SR and UE SRVCC Capability data to the IMS-AS, as follows:
  • if the C MSISDN from the HSS and the C MSISDN from the UDM are empty, the HSS indicates an empty C MSISDN to the SCC AS. The HSS indicates a non-empty C MSISDN, otherwise;
  • if the STN-SR from the HSS or the STN-SR from the UDM is empty, it indicates the user is not subscribed to (5G) SRVCC. If the HSS and the UDM indicate the user is not subscribed to (5G) SRVCC, the HSS indicates an empty STN-SR to the SCC AS. The HSS indicates a non-empty STN-SR, otherwise.
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5.4.8  SRVCC: IMS AS obtaining SRVCC data - HSS using Nudr

When the HSS receives an S6a-AIR from the MME, it may check (by means of an EPC-UDR query) whether the subscriber has a 5G subscription. If so, the HSS can use of the Nudr_DM_Query Get service operation to retrieve the 5G SRVCC subscription data from the 5GS UDR.
Figure 5.4.8-1 shows the scenario where the HSS communicates with the IMS-AS in support of (5G) SRVCC for a subscriber who has a 5GC subscription. An operator shall be able to control whether the interaction between HSS and 5GS-UDR is performed based on local policy.
Reproduction of 3GPP TS 23.632, Figure 5.4.8-1: IMS AS obtaining data related to (5G) SRVCC - HSS using the Nudr
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Step 1.
As described in TS 23.237, the IMS-AS sends Sh-Pull message to the UDM/HSS in order to know the SRVCC capability indicated by the UE in the EPS, and to retrieve the STN-SR stored in the UDM/HSS.
Step 2.
The HSS reads STN-SR and this UE SRVCC Capability data from the EPS-UDR. The HSS reads the C MSISDN bound to the IMS Private User Identity.
Step 3.
The HSS, based on operator policy, may query 5G-UDR to retrieve SRVCC parameters. The HSS uses the Nudr_DM_Query service operation to retrieve the STN-SR and the UE SRVCC Capability indicated by the UE in the 5GS (see TS 23.008) from the 5GS-UDR.
Step 4.
The HSS provides a single C MSISDN, a single STN-SR and UE SRVCC Capability data to the IMS-AS, as follows:
  • if the C MSISDN from the HSS and the C MSISDN from the UDM are empty, the HSS indicates an empty C MSISDN to the SCC AS. The HSS indicates a non-empty C MSISDN, otherwise;
  • if the STN-SR from the HSS or the STN-SR from the UDM is empty, it indicates the user is not subscribed to (5G) SRVCC. If the HSS and the UDM indicate the user is not subscribed to (5G) SRVCC, the HSS indicates an empty STN-SR to the SCC AS. The HSS indicates a non-empty STN-SR, otherwise.
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5.4.9  SRVCC: IMS AS updating STN-SRWord‑p. 31

Figure 5.4.9-1 shows the scenario where the IMS-AS updates the STN-SR at the HSS. An operator shall be able to control whether the interaction between HSS and UDM is performed based on local policy.
Reproduction of 3GPP TS 23.632, Figure 5.4.9-1: Updating STN-SR
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Step 1.
As described in TS 23.237, the IMS-AS can use Sh-Update to provide the STN-SR received from the ATCF to the UDM/HSS in order to replace the STN-SR at the UDM/HSS.
Step 2.
While this updated STN-SR is a transient value and subject to the user being subscribed to SRVCC, the HSS can update the STN-SR at the EPS-UDR. If the user is subscribed to SRVCC, the HSS updates the STN-SR at the EPS-UDR and at the MME/SGSN (not shown).
Step 3.
The HSS, based on operator policy, may update UDM: the HSS updates the STN-SR at the UDM.
Step 4.
While this updated STN-SR is a transient value, the UDM can update the STN-SR at the 5GS-UDR using Nudr_DM_Update. The UDM updates the STN-SR at the AMF using Nudm_SDM_Notification service operation (not shown).
Step 5.
The UDM responds to the request to update the STN-SR.
Step 6.
The HSS responds by sending Sh-Update Response message to the IMS AS.
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