A.2 Providing announcements to a user during an established communication

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

The way an announcement is sent to a user during an established communication depends on the scenario and the importance of the announcement.

The following scenarios exist:

– scenario 1: two users are communicating with (at least) one AS in the signalling path (UE AS -UE); or

– scenario 2: two (or more) users communicating with (at least) one AS in the signalling and media path
(UE-AS/MRFC-UE); or

– scenario 3: two users communicate and one of the users is connected to PSTN/ISDN (UE-MGCF). This scenario can be seen as part of basic communication and requires no SIP signalling; or

– scenario 4: two users communicate directly with each other without involving an AS in the signalling path and without involving an AS in the media path (UE-UE). This scenario is out of scope of the present document.

A.2.1 Scenario 1: UE – AS – UE

Two users are communicating with (at least) one AS in the signalling path. In this scenario the AS is connected to the S-CSCF over the ISC interface acting as a SIP proxy or an AS performing 3rd party call control.

IETF RFC 3261 [4] specifies the Call-Info header field as a means to indicate a source of media to be played by the receiving endpoint.

Figure A.6 shows an example of the use of this mechanism in the INVITE request.

NOTE: Some signalling details are left out of the figure for simplicity.

Figure A.6: Call-Info header field in a re-INVITE request to indicate media

A user wants to place a communication session on hold and sends a re‑INVITE request towards the remote user involved in the communication.

The steps of the flow are as follows:

1) S-CSCF receives a reINVITE request from a user. The user can be a user served by this S-CSCF, a user served by another S-CSCF or a user connected to the PSTN via MGCF.

2) S-CSCF sends the reINVITE request along the signalling path to the AS using the route set received in the re-INVITE request.

3) Service logic in the AS decides to include a reference to a wav file with an announcement or music.

4) The AS sends the reINVITE request to the S-CSCF. Including in the Call-Info header a URL to a media file containing the appropriate announcement or music, for example http://operator.net/announcement.wav (in the picture abbreviated to http://url.wav).

5) The S-CSCF sends the reINVITE request along the signalling path towards the remote user. The remote user can be a user served by this S-CSCF, a user served by another S-CSCF or a user connected to the PSTN via MGCF.

6-9) The 200 (OK) response from the remote user is forwarded via the S-CSCF and the AS towards the originating user.

10-13) The ACK request from the originating user is forwarded via the S-CSCF and the AS towards the remote user.

14) The http://url.wav file is retrieved and played to the user. In the case the user is connected to the PSTN via a MGCF, the T-MGF retrieves and plays the announcement towards the user. In case the user is connected to IMS the UE retrieves and plays the announcement.

A.2.2 Scenario 2: UE – AS/MRFC/MRFP – UE

This subclause describes the scenario when two (or more) users are communicating with (at least) one AS controlling the media path. The MRFP is in the media path. In this scenario the AS acts as a B2BUA.

Figure A.7 shows the signalling flow for the scenario.

Figure A.7: In-band announcement during an established communication

An AS, acting as a B2BUA, is involved in a communication session. The AS controls the media path via a co-located MRFC controlling a MRFP.

The steps of the flow are as follows:

1) Service logic in the AS decides to start an in‑band announcement towards a user e.g. "Music on hold".

2) The MRFC co-located with the AS interacts with the MRFP in order to start the announcement.

3) The MRFP sends the announcement towards the remote user.

4) The MRFC co-located with the AS interacts with the MRFP to stop the announcement.

5) MRFP stops the announcement provided to UE-2.