7.4.3 Terminating sessions over Gm rejected by the UE

23.2923GPPIP Multimedia Subsystem (IMS) centralized servicesRelease 17Stage 2TS

Figure 7.4.3-1 provides an example flow for a call destined to a UE-2 and where the incoming session is delivered over the Gm reference point offering either PS speech media, CS media, or PS speech media with alternative CS media, because the SCC AS is unable to accurately determine that the CS domain should be used, e.g. when ISR according to TS 23.401 [34] is active and if the UE hasn’t re-registered in the IMS. In this example call flow the UE-2 rejects the incoming session and T-ADS in the SCC AS selects CS domain for termination. The flow applies to both ICS UE and non ICS UE.

Figure 7.4.3-1: Terminating session rejected by the UE

1. The request is received at S‑CSCF following normal IMS session set up procedures.

2~3. The service logic with iFC causes the request to be forwarded to the SCC AS.

4. The SCC AS anchors the session. T-ADS is executed and the PS access is selected for termination.

5~6. The SIP INVITE is forwarded to the UE via the selected PS access. Paging is triggered if the UE is in Idle state.

7~8. The UE determines that PS access cannot be used for a session with the offered media and it returns a response to the IMS indicating the error condition. The S‑CSCF forwards the message to the SCC AS.

9. On receiving the response, the SCC AS reattempts the terminating call establishment directly towards the same UE over CS domain if possible.

10~11. The SIP INVITE is forwarded to UE-2 via the MSC Server enhanced for ICS (or via MGCF and MSC if the MSC is not enhanced for ICS).

12. Terminating CS call establishment.

NOTE: If the UE determines that PS access cannot be used for the media flows it might be worth providing the caller party with a courtesy tone generated by the IMS while the termination is being re-attempted.