M.1 P‑CSCF located in visited network

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

M.1.1 Description

M.1.1.0 General

The architectures and flows in this clause are only showing EPS and 5GS. The principles shown are also applicable for GPRS Core Network.

For 5GS, there is no support for roaming interface between vPCF and hPCF in this Release of the specification.

M.1.1.1 Architecture

The architecture for this scenario is shown in figure M.1.1.1.

Figure M.1.1.1: EPS/5GS architecture for IMS Local Breakout with P‑CSCF located in visited network

Optionally IBCF and TrGW may be present in the HPLMN and VPLMN according to II‑NNI reference architecture (see Annex K), and thus there will be an Ici reference point between the IBCFs and an Izi reference point between the TrGWs.

M.1.1.2 Flow for originating session

The information flows for originating session for this scenario is illustrated in figure M.1.1.2.

Figure M.1.1.2: Example scenario with P‑CSCF located in visited network and IBCF and TrGW in home network

1. The UE obtains an IP address from the EPS/5GS in the visited network according to the procedures specified by TS 23.401 [70] / TS 23.502 [94].

2. The EPS/5GS obtains default PCC rules and associates it with this IP‑CAN. The V‑PCRF/vPCF and H‑PCRF (in the case of S9 in EPS) provides these rules according to TS 23.203 [54] / TS 23.503 [95].

3. Using the IP address obtained in step 1, the UE performs IMS registration. This SIP message is routed by the EPS/5GS in the visited network through the P‑CSCF in the visited network, which was discovered according to the procedures in Annex E/Annex Y, to the S‑CSCF in the home network, via IBCFs also in the visited and home network if deployed.

4. Using the IP address obtained in step 1 in the SDP, the UE initiates a SIP session. The INVITE request is routed by the EPS/5GS in the visited network through the P‑CSCF to the IBCF in the home network.

5. If the IBCF decides to route media to home based on operator policy, it then allocates resources in TrGW and alters the offered SDP accordingly.

NOTE 1: Per operator policy, the IBCF may have other reasons than only address translation to route media home.

6. IBCF sends the INVITE further to the S‑CSCF, and S‑CSCF continues the session towards the far-end.

7 The 200 OK received from the far-end is sent by the S‑CSCF to the IBCF. If a TrGW was allocated in step 5, then IBCF changes the SDP answer accordingly.

NOTE 2: Step 7a) If the IBCF decides to anchor the call when it has received SDP answer (e.g. because the MRFP needs to be involved in the user plane or because of other reasons), then step 5 in the procedure starts again, and it re-INVITEs the far-end.

8. The 200 OK is sent further on to the P‑CSCF and via EPS/5GS in the visited network towards the UE.

9. The P‑CSCF in the visited network also provides the session information to the V‑PCRF/vPCF in the visited network.

10. The H‑PCRF in the home network provides PCC rules to the V‑PCRF in the visited network when S9 is supported. The V‑PCRF/vPCF in the visited network provisions PCC rules in the EPS/5GS in the visited network

11. Media exchanged between the UE and the far end is now routed either between the 5GS in the visited network and the far end, thus achieving local breakout mode of operation; or between the EPS/5GS in the visited network via the TrGW in the home network if IBCF was deployed.

NOTE 3: Per operator policy, the IBCF can route media home due to other reasons than stated in this specification, thus also giving the possibility to get home routed mode of operation.