5G SRVCC (Single Radio Voice Call Continuity) is a solution specified by 3GPP in Release 16 which allows ongoing IMS call to be seamlessly switched between 5G NG-RAN and 3G UTRAN, i.e. It allows a PS to CS domain switch of an ongoing IMS call. To support this network feature, an MME_SRVCC (i.e. MME Supporting 5G-SRVCC) sits between the AMF(5G Core) and the MSC Server (3G CS Core).
5G-SRVCC UE includes the 5G-SRVCC capability indication as part of the “UE Network Capability” in the Registration Request to the 5G core (AMF).
When a 5G-SRVCC UE moves outside the 5G NR coverage:
- If there are suitable 4G cells in the area, the ongoing VoNR/IMS call can be handed over to the 4G cells using PSHO(PS handover), and will continue as an IMS call over the PS domain.
- If the 5G device can not find suitable 4G cells but there are suitable 3G UTRA cells in the area, the ongoing IMS call can be handed over to the 3G cells, using SRVCC procedures, and will continue as an IMS call over the CS domain.
Functions of ICS (IMS Centralized Services) and ISC (IMS Service Continuity) are collocated in a single SCC AS (Service Centralization and Continuity Application Server). When 5G-SRVCC is used, additional functions of ISC are provided by the ATCF/ATGW (Access Transfer Function/Access Transfer Gateway) in the serving (visited if roaming) network. ICS specifies functions and procedures for use of CS bearer for the media of the IMS sessions.
If both UE and network supports the ICS UE capabilities , these capabilities are used for communication of required information if needed for enablement of PS-CS Access Transfer of IMS multimedia sessions.
Here I am trying to depict 5G-SRVCC architecture for NG-RAN to 3GPP UTRAN and the SRVCC procedure as well.
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