A.5 Providing announcements to a terminating user just after the call is answered and before establishing direct communication session between end users

24.6283GPPCommon Basic Communication procedures using IP Multimedia (IM) Core Network (CN) subsystemProtocol specificationRelease 17TS

A.5.1 Switch media path from (UE-AS/MRFC/MRFP) to (UE-UE) within the same dialog

In order to provide announcement to the terminating UE when the called user answers the call, the AS associated with MRFC providing announcement, acting as a B2BUA, will intercept all SIP messages exchanged between the end UEs prior to establishing media stream between end UEs. The AS will replace SDP indicated by the originating UE to establish media stream with MRFC and provide announcement from MRFC once the call is answered, which later on can be replaced with SDP indicated originally by the originating UE.

Figure A.13 shows the message flow for the scenario.

Figure A.13: Announcement provided to the terminating UE when the called user answers the call and prior to establishing media stream between users

The steps of the flow are as follows:

1-2) The originating UE sends INVITE request to the intermediate IM CN subsystem.

3-4) The AS modifies the SDP in the received INVITE request so that the media stream will be established to the MRFP associated with the AS.

5-8) The called party is alerted. The terminating UE sends SIP 183 (Session Progress) provisional response for the INVITE request.

9-12) PRACK request is sent towards the terminating UE.

13-14) The called party answers the call. The terminating UE sends SIP 200 (OK) response for the INVITE request.
The AS providing the announcement terminates this response.

15-16) The AS sends ACK request to the terminating UE. The MRFP associated with the AS starts the announcement to the terminating UE, and the called user will receive the announcement.

17-18) When the announcement finishes, the AS sends UPDATE request containing SDP received in the INVITE request from the originating UE to the terminating UE.

19-20) The terminating UE updates the media stream based on the information received in the UPDATE request. The terminating UE sends SIP 200 (OK) response to the UPDATE request.
The AS providing the announcement terminates this response.

NOTE: SDP answer conveyed in this 200 (OK) response from the terminating UE is assumed to be the same as the SDP answer conveyed in 183 (Session Progress) response in step 5-8.

21-22) The AS sends 200 OK response to the originating UE.

23-24) The originating UE sends ACK request. The AS providing the announcement terminates this request. The media between the two UEs are established.

A.5.2 Using 180 (Ringing) response towards originating UE

This subclause shows an example signalling flow of how an AS can send an announcement to the called user just after the call is answered and before establishing direct communication session to the calling user.

Separate early dialogs are established between the originating UE and the AS controlling the announcement, and the originating UE and the terminating UE. The dialog between the originating UE and AS is in this example only used for sending a 180 (Ringing) response.

Figure A.14 shows the message flow for the scenario.

Figure A.14: Announcement provided to the terminating UE when the called user answers the call and prior to establishing media stream between end users

NOTE: For clarity, the SIP 100 (Trying) responses are not shown in the signalling flow.

The steps of the flow are as follows:

1-2) The originating UE sends INVITE request to the intermediate IM CN subsystem. The originating UE does not have resources available.

3-4) The AS modifies the SDP in the received INVITE request so that the media stream will be established to the MRFP associated with the AS. The media parameters may be different than those received in the offer. The AS indicates that the MRF resources are available.

5-6) Since the MRF resources are available, the terminating UE starts reservation of the resources and when the resources are available the called party is alerted. The terminating UE sends SIP 180 (Ringing) provisional response for the INVITE request.

7-8) PRACK request is sent towards the terminating UE.

9-10) terminating UE confirms the PRACK with 200 (OK) response to PRACK.

11-12) The AS sends a 180 (Ringing) response without pre-condition towards the calling party.

13-14) The called party answers the call. The terminating UE sends SIP 200 (OK) response for the INVITE request.
The AS providing the announcement terminates this response.

15-16) The AS sends ACK request to the terminating UE. The MRFP associated with the AS starts the announcement to the terminating UE, and the called user will receive the announcement.

17-18) When the announcement finishes, the AS sends a re-INVITE request containing the SDP offer received in the INVITE request from the originating UE to the terminating UE. All media offered for providing the announcement is removed (i.e. media lines are set to port "0") if not included in the original offer from the calling party. The remaining media are indicated without having resources at the originating side.

19-20) Since the originating UE does not have resources, the terminating UE sends the SDP answer in 183 (Session Progress) response to the re-INVITE request based on the information received in the re-INVITE request.

21-22) The AS sends a 183 (Session Progress) response towards the calling party with the SDP received in message 20. Note that the response is a different dialog than the dialog created by the 180 (Ringing) response in step 11-12.

23-30) The calling party sends the PRACK request which the AS forwards to terminating UE and which the terminating UE confirms by 200 (OK) for PRACK.

31-38) When resources are reserved by the originating UE the originating UE sends an UPDATE request which the AS forwards to the terminating UE and which the terminating UE confirms by 200 (OK) for UDPATE.

39-40) When the terminating UE has all the resources available, the terminating UE sends 200 (OK) response to the re-INVITE request.

41-42) The AS sends the 200 (OK) response to the original INVITE request.

43-46) The originating UE sends ACK request. The AS providing the announcement forwards the ACK to the terminating UE.

The media between the two UEs are established.

Annex B (informative):
Signalling flows for Network Determined User Busy (NDUB)