N.2 Termination procedures

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

N.2.1 (FT#1) Fixed xDSL termination, home

NOTE: The below flows are example flows. The detailed stage 2 description of the RACS information flows can be found in ETSI ES 282 003 [78].

This termination procedure applies to users located in their home service area. As in clause 5.7.2, the UE is located in the home network, but has registered to the IM CN Subsystem via an xDSL IP‑CAN.

Figure N.2.1: Fixed xDSL terminating – home (example flow)

Procedure FT#1 is as follows:

1. UE#1 sends the SIP INVITE request, containing an initial SDP, via one of the origination procedures and the S‑S procedures, to the S‑CSCF for the terminating UE.

2. S‑CSCF validates the service profile, and invokes any termination service logic required. This includes authorization of the requested SDP based on the user’s subscription for multi-media services.

3. S‑CSCF remembers (from the registration procedure) the P‑CSCF address for this UE. The S‑CSCF forwards the INVITE to the P‑CSCF, which in this case is located in the home network.

4. The P‑CSCF triggers RACS which reserves a connection in C‑BGF with optional NAT binding retrieval.

5. P‑CSCF remembers (from the registration procedure) the UE address, and forwards the INVITE to the UE.

6. UE determines the subset of the media flows proposed by the originating endpoint that it is capable and willing to support, and responds with an Offer Response message back to the originator. The SDP may represent one or more media for a multi-media session. This response is sent to the P‑CSCF.

7. P‑CSCF triggers RACS to perform admission control based on Offer and Answer parameters.
RACS configures the connection in the C‑BGF based on SDP answer with optional NAT binding retrieval.

8. P‑CSCF forwards the Offer Response message to S‑CSCF.

9. S‑CSCF forwards the Offer Response message to the originator, per the S‑S procedure.

10-15. The originating endpoint sends a Response Confirmation via the S‑S procedure, to the terminating S‑CSCF. The Response Confirmation may also contain SDP. This may be the same SDP as in the Offer Response sent in step 19 or a subset. If new media are defined by this SDP, a new interaction with the RACS (as in steps 4‑8) will be done by the P‑CSCF. The originating UE is free to continue to offer new media in this request or in a subsequent request using the Update method. Each offer/answer exchange will cause the P‑CSCF to repeat the RACS interactions (steps 4-8) again.

16-18. UE may alert the user and wait for an indication from the user before completing the session. If so, it indicates this to the originating party by a provisional response indicating Ringing. This message is sent to P‑CSCF and along the signalling path to the originating endpoint.

19. When the destination party answers, UE sends a SIP 200 OK final response to P‑CSCF.

20. P‑CSCF indicates that the resources reserved for this session should now be committed.

21-22. P‑CSCF forwards the 200 OK to S‑CSCF, following the signalling path.

23-25. The session originator responds to the 200 OK by sending the ACK message to S‑CSCF via the S‑S procedure and it is forwarded to the terminating end along the signalling path.