6.5 5G-SRVCC from NG-RAN to 3GPP UTRAN

23.2163GPPRelease 16Single Radio Voice Call Continuity (SRVCC)Stage 2TS

6.5.1 NG-RAN Registration procedure for SRVCC

5GS Registration procedure for 3GPP 5G-SRVCC UE is performed as defined in TS 23.502 [45] with the following additions:

– 5G-SRVCC UE includes the 5G-SRVCC capability indication as part of the "UE Network Capability" in the Registration Request.

– 5G-SRVCC UE includes the MS Classmark 2 and Supported Codecs IE in the Initial Registration message and in the non-periodic Registration messages.

– UDM/HSS includes STN-SR and C-MSISDN as part of the subscription data sent to the AMF. If the STN-SR is present, it indicates the UE is 5G-SRVCC subscribed.

NOTE: For an Emergency Registration in which the UE was not successfully authenticated, this step does not occur.

– AMF includes a "5G-SRVCC operation possible" indication in the N2 Request, meaning that both UE and AMF are SRVCC-capable.

6.5.2 Service Request procedures for SRVCC

Service Request procedures for 3GPP 5G-SRVCC UE are performed as defined in TS 23.502 [45] with the following additions:

– AMF includes a "5G-SRVCC operation possible" indication in the N2 Request, meaning that both UE and AMF are 5G-SRVCC-capable.

6.5.3 Intra-5GS PS Handover procedure for SRVCC

Intra-5GS handover procedures for 3GPP 5G-SRVCC UE are performed as defined in TS 23.502 [45] with the following additions:

– MS Classmark 2, STN-SR, C-MSISDN, and the Supported Codec IE shall be sent from the source AMF to the target AMF if available.

– The target AMF includes a "5G-SRVCC operation possible" indication in the N2 Handover Request, meaning that both UE and the target AMF are 5G-SRVCC-capable.

For Xn-based handover, the source NG-RAN node includes a "5G-SRVCC operation possible" indication in the Xn-AP Handover Request message to the target NG-RAN node.

6.5.4 5G-SRVCC from NG-RAN to 3GPP UTRAN procedure

Depicted in Figure 6.5.4-1 is a call flow for 5G-SRVCC from NG-RAN to 3GPP UTRAN.

Figure 6.5.4-1: 5G-SRVCC from NG-RAN to 3GPP UTRAN call flow

1. UE establishes the PDU session for IMS.

2. 5G-SRVCC HO is triggered by NG-RAN.

3. NG-RAN sends a Handover Required (Target ID, generic Source to Target Transparent Container, 5G-SRVCC HO indication) message to the source AMF. The Target ID is the UTRAN RNC-ID. 5G-SRVCC HO Indication indicates to AMF that this if for 5G-SRVCC. The Generic Source to Target Transparent Container is the Source RNC to Target RNC Transparent container.

4. AMF determines the HO is used for 5G-SRVCC by the 5G-SRVCC HO Indication. AMF selects an MME_SRVCC that can have Sv connection to the MSC SERVER/MSC according to the target RNC ID which is included in the Target ID.

NOTE 1: The MME_SRVCC selection can be realised through operator’s configuration.

5. AMF sends the forward relocation request (IMSI, Target ID, STN-SR, C-MSISDN, MM Context, Generic Source to Target Transparent Container, 5G-SRVCC HO Indication, Supported Codec IE, MS ClassMark 2, Emergency Indication, Equipment Identifier) to MME_SRVCC. The Emergency Indication and the equipment identifier are included if the ongoing session is emergency session. Authenticated IMSI and C MSISDN are also included in this case, if available.

6. MME_SRVCC initiates the PS-CS handover procedure towards MSC Server. Steps 5 to 13 as specified in Figure 6.2.2.1-1 (referenced by clause 6.2.2.1A SRVCC from E-UTRAN to UTRAN without PS HO) are performed for the PS-CS handover procedure.

7. MME_SRVCC receives the response message from MSC server after HO preparation is completed. MME_SRVCC sends the Forward Relocation Response message (Target to Source Transparent Container, MSC Server Identity) to AMF.

8. AMF sends the HO command to NG-RAN.

9. NG-RAN sends a HO command to the UE. UE detects the 5G-SRVCC HO.

10. UE tunes to the target UTRAN cell.

11. Handover Detection at the target RNS occurs, then the target RNS sends Handover Detection message to the target MSC SERVER/MSC.

12. The UE sends a Handover Complete message via the target RNS to the target MSC SERVER. Steps 19 to 21as specified in Figure 6.2.2.1-1 (referenced by clause 6.2.2.1A SRVCC from E-UTRAN to UTRAN without PS HO) are performed for this handover complete procedure. At this stage, the target MSC SERVER/MSC can send/receive voice data.

13. MSC SERVER sends MME_SRVCC the SRVCC PS to CS completion.

14. MME_SRVCC forwards the Forward Relocation completion message which includes the information received in step 13 to AMF.

15. AMF forwards the Forward Relocation Complete ACK message to MME_SRVCC. AMF releases the UE context related to MME_SRVCC.

16. MME_SRVCC forwards the PS to CS Complete ACK message to MSC server. MME_SRVCC removes stored UE context. After receives the message, MSC server removes the UE context related to the MME_SRVCC.

17. AMF performs the PDU session release procedure for all the PDU session(s) which is described in TS 23.502 [45] indicating that the release is due to PS to CS handover for 5G SRVCC.

18. For an emergency services session after handover is complete, the source AMF or the MSC Server may respectively invoke the Namf_Location_EventNotify service operation or send a Subscriber Location Report, towards a GMLC associated with the source or target side, respectively, carrying the identity of the MSC Server, as defined respectively in TS 23.273 [50], TS 23.502 [45] or TS 23.271 [29], to support location continuity.

NOTE 2: Some configuration of the choice between a source AMF versus MSC Server update to GMLC needs to ensure that a single update occurs from one of these entities.

19. The MSC server executes steps 17f and 17 g of Figure 6.2.2.2-1

The IMS service control is described in TS 23.292 [13] when UE accesses in UTRAN cell due to 5G-SRVCC.

6.5.5 Emergency PDU session establishment and release procedure for SRVCC

SRVCC for IMS emergency session can be supported regardless of the subscription data from the HPLMN. The Emergency PDU session is established as defined in TS 23.502 [45] with the following additions:

– AMF includes a "5G-SRVCC operation possible" indication in the N2 Request, if needed.

NOTE: During the Emergency PDU session establishment, if AMF has not sent the "5G-SRVCC operation possible" before, then the AMF send the "5G-SRVCC operation possible" indication to NG-RAN, otherwise AMF can skip this step.

The Emergency PDU session is released as defined in TS 23.502 [45] with the following additions:

– AMF shall restore the SRVCC indication to the same status as prior to the emergency PDU session was established.