Y.9 Support of IMS Services for roaming users

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

Y.9.1 General

This clause describes support for IMS services for roaming users using IMS level roaming interfaces or without IMS level roaming interfaces.

Y.9.2 Architecture without IMS-level roaming interfaces

This clause describes the functions that are used to support IMS services for roaming users in deployments without IMS-level roaming interfaces. In this roaming model the UPF holding the UE’s IP point of presence is located in the home PLMN and therefore UE IMS signalling and user plane are routed to home PLMN.

The architecture to support IMS services for roaming users, including Voice over IMS in deployments without IMS-level roaming interfaces is shown in figure Y.9.2-1.

The following architecture requirements apply:

– P-CSCF (in HPLMN) identifies the serving network (VPLMN) where the UE is located using the procedure defined in clause Y.9.3.

Figure Y.9.2-1: IMS traffic home routed

Y.9.3 Architecture with IMS-level roaming interfaces

For IMS services with roaming level interfaces the P-CSCF and UPF are located in VPLMN (local breakout), see clauses 4.2.3 and M.1. For roaming architecture for voice over IMS with local breakout see clause 4.15a. For information on how to apply the loopback possibility in clause 4.15a, see clause M.3.

Y.9.4 Subscription to changes in PLMN ID at IMS Initial Registration

In IMS local breakout where P-CSCF is located in VPLMN (see Annex M.1 and Annex M.3), the home network determines the serving PLMN of the UE from the location of the P-CSCF during initial IMS Registration, using the P‑CSCF network identifier.

In deployments without IMS-level roaming interfaces, the home network determines the serving PLMN of the UE using procedure defined in TS 23.503 [95], where P-CSCF requests the PCF to report the PLMN identifier where the UE is currently located. The received PLMN ID information is then forwarded in the SIP REGISTER request.

This procedure shall be applied by the P-CSCF at initial UE IMS registration.

Figure Y.9.4-1: Subscription by P-CSCF to changes in PLMN ID during initial IMS Registration

1. The UE sends a SIP REGISTER request to the P-CSCF.

2. If this is initial IMS registration then the P-CSCF subscribes to the PCF to be notified of the PLMN ID where the UE is currently attached. The subscription to PLMN changes is active as long as the UE is IMS registered.

NOTE: P-CSCF cancels the subscription to PLMN changes at the PCF when the UE is IMS de-registered.

3. The PCF forwards the PLMN ID to the P-CSCF. The P-CSCF stores the PLMN ID.

4. The P-CSCF includes the received PLMN ID in the SIP REGISTER request before forwarding the request to the I-CSCF.

5. Normal IMS registration procedure is then completed.