J.2 IMS CAT provided by the terminating IMS user using gateway model towards user served in CS networks

29.1633GPPInterworking between the IP Multimedia (IM) Core Network (CN) subsystem and Circuit Switched (CS) networksTS

Callflows for providing IMS CAT provided by the terminating IMS user towards user served in CS networks are based on procedures specified in 3GPP TS 24.182 [132], with procedures for MGCF as shown in the following clauses.

J.2.1 Callflow without using precondition mechanism

The following callflow shows the case when precondition mechanism is not used, and 100rel extension is used.

Figure J.2.1: CAT gateway model callflow without using precondition mechanism

The content of the SDP message body described in figure J.2.1 is meant to be as follows:

– SDP_O in step 2 indicates the SDP offer inserted by the MGCF;

– SDP_A_cat in step 3 indicates the SDP answer received by the MGCF, which in general is the SDP answer generated by the CAT AS based on the content for CAT media;

– SDP_O_Regular in step 7 indicates the SDP offer received by the MGCF, which in general is the SDP offer sent by the CAT AS based on the SDP answer received by the terminating UE; and

– SDP_A in step 8 indicates the SDP answer generated by the MGCF.

Upon receiving IAM (see figure J.2.1 step 1), MGCF sends SIP INVITE request containing the SDP offer to terminating IMS (see figure J.2.1 step 2).

Upon receiving 183 (Session Progress) response to the SIP INVITE request containing SDP answer (see figure J.2.1 step 3), MGCF sends ACM to the originating CS network (see figure J.2.1 step 4). Also, MGCF sends a PRACK request towards terminating IMS (see figure J.2.1 step 5). A 200 (OK) response will be received later as a result of the PRACK request (see figure J.2.1 step 6).

Upon receiving SIP UPDATE request from terminating IMS with a SDP offer (see figure J.2.1 step 7), MGCF sends a 200 (OK) response to the terminating IMS with an SDP answer (see figure J.2.1 step 8).

Upon receiving 200 (OK) response to the INVITE request sent in figure J.2.1 step 2 (see figure J.2.1 step 9), MGCF sends an ANM to the originating CS network (see figure J.2.1 step 10).

Annex K (normative):
T.38 interworking between the IP Multimedia Core Network (CN) Subsystem (IMS) and Circuit Switched (CS) networks