F.1 General

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

An incoming call for a subscriber with a service provided by the IMS (e.g. ICS, SC, SR-VCC) can be received either through the CS domain or IMS. For such a subscriber, certain calls that are received through the CS domain need to be routed to the IMS for anchoring at the SCC AS, prior to the onward routing of the call to the subscriber. Solutions for this are discussed in clause F.2.

NOTE: Requirements and preconditions for anchoring of CS calls for a subscriber with a service provided by the IMS are documented in clause 7.4 of the present document.

When an incoming CS call is anchored at the SCC AS using certain implementation options described in clause F.2, then as a result of domain selection, the call can be routed back to the CS domain to determine the CSRN for CS termination. In this case, the call can be treated as a new incoming call from the CS network, and then be re-routed to the IMS for anchoring again, thus creating a circular loop. Solutions for this are discussed in clause F.3.