4.15a Roaming Architecture for Voice over IMS with Local Breakout

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

The Transit and Roaming Function is the combined functionalities of the transit network functions as defined in clause 4.15 and the roaming specific functions defined in this clause.

The following architectural requirements apply:

– The P-CSCF, S-CSCF, the Transit and Roaming Function, and other nodes performing routing procedures in different networks may control the application of OMR procedures by indicating in the signalling whether an IBCF/TrGW should not apply OMR.

– In order to allow scenarios where the media is not routed through the originating HPLMN, IBCFs handling incoming requests to the network should support OMR and allow bypass of TrGWs. Anchoring of media may be controlled via outgoing IBCFs.

– The HPLMN shall decide whether to perform the loopback procedure based on local policy and on knowledge of the support of the procedure in the VPLMN.

– The VPLMN shall be provided by the HPLMN with enough information to determine whether home routing has been applied (or has not been applied):

– The HPLMN shall send an indication to the VPLMN that this session set-up is a loopback to allow differentiation from any other incoming call. By this means the VPLMN is able to apply the correct treatment for this looped incoming leg incl. charging and routing decisions.

– If local policy requires access to BGCF routing data to make the loopback decision for a particular originating INVITE request, then the loopback decision should be performed in the BGCF. Else it should be performed in the S‑CSCF.

– The Transit and Roaming Function shall perform call routing towards the terminating network by selecting appropriate egress point (e.g., MGCF for CS/PSTN, IBCF to other IMS networks, or I-CSCF for termination in own network). The Transit and Roaming Function may use information such as originating UE location information to select a nearby egress point for media anchoring.

– The VPLMN may provide the HPLMN with a reference to the preferred Transit and Roaming Function to steer the selection of the Transit and Roaming Function. If the VPLMN does not provide the Transit and Roaming Function address then the HPLMN shall use the default derived address for the VPLMN.

– When the HPLMN operator does not use loopback to the Transit and Roaming Function in VPLMN, the HPLMN shall be able to anchor the media. This ensures that the signalling and media are routed together.

An overview of the principles and flows of the Roaming Architecture for Voice over IMS with Local Breakout are depicted in Annex M, clause M.3.