5G SRVCC Architecture for 5G NG-RAN to 3G UTRAN

:white_check_mark: 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).

:white_check_mark: 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).

:white_check_mark: 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.

:white_check_mark: 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.

:white_check_mark: 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.

:white_check_mark: Here I am trying to depict 5G-SRVCC architecture for NG-RAN to 3GPP UTRAN and the SRVCC procedure as well.

LinkedIn: :point_down:

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Thanks for the info. Do you know about some network that has implemented this HO from 5G (PS) to 3G (CS)? Some months ago, another expert told me that this HO was not allowed and common HO path was 5G → 4G → 3G

According to the 3GPP specification (23.216), if the UE reports Event B2 (3G/UTRA cell) to the gNB when it has got VoNR service, gNB should send “Handover required” message to the AMF with 5G-SRVCC Ho Indication, but when you refer to the NGAP specification (38.413, Rel.16&17) , there is not any Information Element related to “5G-SRVCC Ho Indication”, i.e. to run the 5G SRVCC from 5G to 3G the mentioned IE in the Handover Required message from the gNB to the AMF should be included similar to what we see in SRVCC from 4G to 3G (eNB to MME).

To run the 5G-SRVCC, the AMF sends “Forward Relocation Request” message with 5G-SRVCC HO Indication to the MME, and then the MME sends “SRVCC” PS to CS Request" message to the MSC Server.

Thanks for the info. So, for 5G SRVCC from 5G to 3G. the AMF (5G) sends the relocation message to MME (4G) and MME sends the message to MSC (3G): 5G → 4G → 3G, good to know about it, thanks. And do you know about some country that has implemented this?

As I mentioned in my first reply, I have not seen “5G-SRVCC HO Indication” Information Element in Handover Required message (3GPP Rel.17, 38.413 version 3) from the gNB to the AMF which is a vital IE in the request.To run SRVCC procedure, the mentioned IE should be specified by 3GPP.
To run the SRVCC procedure SRVCC capable MME sits between the AMF and MSC server.

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Thanks for the info, and sorry for the misunderstanding

You’re welcome