The UE informs the MSC Server about the MME/SGSN the UE had last contact with using P-TMSI+RAI+P-TMSI signature or GUTI upon MSC request. For CS to PS SRVCC, MSC Server first receives the handover request from UTRAN/GERAN with the indication that this is for CS to PS SRVCC handling and then triggers the CS to PS SRVCC procedure with the target MME/SGSN enhanced with CS to PS SRVCC via the enhanced Sv reference point. The MME enhanced with CS to PS SRVCC triggers PS HO in target E-UTRAN/UTRAN (HSPA). If the target MME/SGSN does not have the UE context then the target MME/SGSN retrieves the UE context from the source SGSN/MME.
MSC Server then initiates the session transfer procedure to IMS and coordinates it with the PS handover procedure to the target cell. MME then sends CS-PS handover Response to MSC Server, which includes the necessary CS to PS HO command information for the UE to access the E-UTRAN/UTRAN (HSPA).
Service continuity to UTRAN is supported only for the voice component of an IMS Session or IMS Emergency Session and only when both the UE and the network support 5G-SRVCC.
For facilitating session transfer (SRVCC) of the voice component to the CS domain, the IMS multimedia telephony sessions needs to be anchored in the IMS.
For 5G-SRVCC from NG-RAN to UTRAN, AMF first receives the handover request from NG-RAN with the indication that this is for 5G-SRVCC handling, and then AMF sends the forward relocation request with 5G-SRVCC HO Indication to MME_SRVCC together with STN-SR and C-MSISDN. This MME_SRVCC then triggers the SRVCC procedure with the MSC Server enhanced for SRVCC via the Sv reference point. AMF is aware of which PDU session is used for IMS session based on the DNN. The MSC Server enhanced for SRVCC then initiates the session transfer procedure to IMS and coordinates it with the CS handover procedure to the target cell (as in other SRVCC cases). MSC Server enhanced for SRVCC then sends PS-CS handover Response to MME_SRVCC, which includes the necessary CS HO command information for the UE to access the UTRAN. MME_SRVCC sends the forward relocation response message including this information to AMF. AMF sends the Handover Command to NG-RAN and NG-RAN sends the Handover Command to UE.
After UE moves to UTRAN and AMF receives the forward relocation complete, AMF requests PDU session release for all the PDU session.
This specification introduces an additional function to those defined in the 5GC architecture in TS 23.501 for 5G-SRVCC. This additional function is provided by the MSC Server (i.e., MSC Server enhanced for SRVCC) and the MME_SRVCC.
UE initiates the IMS emergency session as specified in TS 23.167 and TS 23.501. For facilitating session transfer (SRVCC) of the IMS emergency session to the CS domain, the IMS emergency session needs to be anchored in the serving IMS (i.e., in visited PLMN when roaming) as specified in TS 23.237.
The NG-RAN initiates the SRVCC procedure as specified for regular Voice over IMS session.
If the UE has an Emergency PDU Session established, the AMF shall send the Emergency Indication and Equipment Identifier to the MSC Server enhanced for SRVCC via the MME_SRVCC. MSC Server then initiates the IMS service continuity procedure with the locally configured E-STN-SR to the serving IMS. When handover of the emergency session has been completed, the MSC Server may initiate location continuity procedures for the UE as defined in TS 23.271.