C.2 ATCF enhancements SRVCC Architecture
23.2373GPPIP Multimedia Subsystem (IMS) Service ContinuityRelease 17Stage 2TS
Figure C.2-1 provides the reference architecture for SRVCC using the ATCF enhancements. The figure only depicts the specific reference points for the ATCF. For other reference points of the general architecture, refer to the reference architecture in TS 23.292 [5], clause 5.2.
Figure C.2-1: IMS Service Centralization and Continuity Reference Architecture when using ATCF enhancements in the HPLMN
If the ATCF is co-located with P-CSCF, the interface between ATCF and ATGW is Iq, according to TS 23.334 [30].
If the ATCF is co-located with IBCF, the interface between ATCF and ATGW is Ix, according to TS 29.162 [31].
For CS to PS SRVCC scenarios, the interface between MSC Server and ATCF is Mx. This interface does not require roaming level agreements.
SIP signalling related to session transfer is transported from the MSC server in the VLPMN via the Mx reference point and via the IBCF in both VPLMN and the HPLMN to the ATCF in HPLMN. Since Mx is used a s a roaming interface in this case, the support of mid-call features depends on the ability to successfully transport the SIP headers associated with these features between the VPLMN, and the HPLMN and the support in the MSC server and the ATCF.