14.10.1 General

23.2053GPPBearer-independent circuit-switched core networkRelease 17Stage 2TS

Customised Alerting Tone may be provided from a "CAT Server" switched into the call at appropriate trigger points. The "CAT Server" is out of the scope of the 3GPP architecture and therefore the signalling procedures towards the CAT Server (if not collocated) are not described.

The Forward CAT indicators and Backward CAT indicators information elements are specified in 3GPP TS 29.205 [22].

The following clauses describe extensions to basic call handling and unless explicit exceptions are made all basic call protocol handling support is unchanged.

14.10.2 Audio CAT

14.10.2.1 Audio CAT Activated by the Calling Party

If the calling party subscriber has activated the CAT service (CAT-A) for audio and the subscriber is in its Home PLMN and the calling party subscriber gives priority to CAT-A, the originating MSC Server supporting the audio CAT-A service shall connect that calling party UE towards the CAT Server during alerting phase. The MSC Server is responsible for switching the user plane towards the CAT Server during alerting phase and then on receipt of answer message from the called party the MSC Server shall switch the user plane towards the called party and releasing the connection to the CAT Server.

The originating MSC may indicate the CAT priority to the terminating network by setting the CAT Priority indicator of the Forward CAT indicators information element when sending an IAM to the succeeding node.

If the calling party subscriber supports CAT-A but gives priority to CAT-B and the originating MSC receives an ACM or CPG from the succeeding network with the CAT content Indicator in the Backward CAT indicators information element set to "inband media content available" then it shall not switch the user plane towards the CAT Server. If however the originating MSC does not receive an indication that CAT-B is present then it shall switch the user plane towards the CAT server as described above for the CAT-A priority case.

NOTE: Providing alerting tone at the calling party rather than from the called party network can cause voice clipping if the called party answers and speaks while the calling party network has not disconnected the inband alerting tone.

14.10.2.2 Audio CAT Activated by the Called Party

If the called party subscriber has activated the CAT service (CAT-B) for audio, the GMSC Server supporting the audio CAT-B service shall check the CAT Priority indicator if the Forward CAT indicators information element is received. If CAT-A priority is requested the GMSC Server shall handle the call in accordance with normal call handling not supporting CAT-B service. If no CAT-A priority is received the GMSC Server shall connect the calling party network to the CAT Server during alerting phase. The GMSC Server should indicate in ACM or CPG message if CAT-B is inserted by setting the CAT content Indicator in the Backward CAT indicators information element. The GMSC Server is responsible for switching the user plane towards the CAT Server during alerting phase and then on receipt of answer message from the called party the GMSC Server shall switch the user plane towards the called party and releasing the connection to the CAT Server.

14.10.3 Multimedia CAT

14.10.3.1 Introduction

The following clauses specify the mobile originated and mobile terminated multimedia call procedures to generate a multimedia CAT to a UE supporting the multimedia CAT capability.

A UE supporting multimedia CAT will indicate support of this capability to the MSC server as specified in 3GPP TS 24.008 [4].

To play a multimedia CAT to such a UE, upon receiving the indication that the called party is being alerted and that a multimedia CAT is available, the MSC Server supporting multimedia CAT shall request the MGW to both-way through-connect the bearer towards the source of the multimedia CAT using the Change Through-Connection procedure, and then request the calling UE to attach to the user connection for multimedia as soon as an appropriate channel in multimedia mode is available and to set up an H.324 call by indicating "inband multimedia CAT available" within an ALERTING message or a PROGRESS message as specified in 3GPP TS 24.008 [4] clause 5.3.6.4.

14.10.3.2 Multimedia CAT Activated by the Calling Party

If the calling party subscriber has activated CAT service (CAT-A) for multimedia and the subscriber is in its Home PLMN and the calling party subscriber gives priority to CAT-A, the originating MSC Server supporting multimedia CAT-A service for multimedia it shall connect that calling UE towards the CAT Server during alerting phase. The MSC Server is responsible for switching the user plane towards the CAT Server during alerting phase and then on receipt of answer message from the called party the MSC Server shall switch the user plane towards the called party and release the connection to the CAT Server.

The originating MSC may indicate CAT-A has priority to the terminating network by setting the CAT Priority indicator of the Forward CAT indicators information element to ‘Priority given to Calling party (CAT-A)’ when sending an IAM to the succeeding node.

NOTE: As the Forward CAT indicators information element is required to be sent by the originating MSC with the Multimedia CAT capability Indicator set to to "MCAT supported" if it permits multimedia CAT-B, the inclusion of the Forward CAT indicators information element is not strictly required since if no CAT parameters are included it has the same effect as indicate CAT-A has priority however it does provide explicit indication that the calling party supports CAT-A.

If CAT-B is given priority then the Multimedia CAT capability Indicator of the Forward CAT indicators information element shall be set as described in clause 14.10.3.3.1.

If the calling party subscriber supports CAT-A but gives priority to CAT-B and the originating MSC receives an ACM or CPG from the succeeding network with the CAT content Indicator in the Backward CAT indicators information element set to "inband media content available" then it shall not switch the user plane towards the CAT Server and shall follow the procedures defined in clause 14.10.3.3.1. If however the originating MSC does not receive an indication that CAT-B is present then it shall switch the user plane towards the CAT server as described above for the CAT-A priority case.

14.10.3.3 Multimedia CAT Activated by the Called Party

14.10.3.3.1 Mobile originated multimedia call

If the calling UE supports the multimedia CAT service and no CAT-A priority exists, the originating MSC supporting the multimedia CAT service shall inform the succeeding node that multimedia CAT is supported by setting the Multimedia CAT capability Indicator of the Forward CAT indicators information element to "MCAT supported" in the IAM message sent to the succeeding node.

The MSC Server supporting the multimedia CAT service shall consider that a multimedia CAT is available if it receives an ACM or CPG message including the Multimedia CAT content Indicator in the Backward CAT indicators information element set to "inband media content available".

14.10.3.3.2 Mobile terminated multimedia call

If the called party subscriber supports the "CAT-B" service for multimedia and if the GMSC Server supporting the optional CAT-B service for multimedia receives the Multimedia CAT capability Indicator in the Forward CAT indicators information element set to "MCAT supported" in the IAM from the preceding node, the GMSC Server shall connect the calling party to the CAT Server during the alerting phase.

To play a multimedia CAT to the calling subscriber, upon receiving the indication that the called party is being alerted, the GMSC supporting multimedia CAT shall request the MGW to establish, and both-way through-connect, a bearer connection towards a CAT Server.

The GMSC shall inform the originating MSC that a multimedia CAT is available (if any) by setting the Multimedia CAT content Indicator of the Backward CAT indicators information element to "inband media content available" in the ACM or CPG message.

14.10.3.3.3 Example

Figure 14.10.3.3.3.1 shows the message sequence chart example for providing multimedia CAT activated by the called party during a basic multimedia call establishment. In the example, the calling UE, the originating MSC and GMSC support the multimedia CAT capability.

The calling UE initiates a multimedia call establishment during which it indicates that it supports multimedia CAT capability (bullet 1). The originating MSC sends an Initial Address message to the GMSC of the called party indicating that Multimedia CAT is supported.

The GMSC server determines that the called party has a multimedia CAT subscription (bullet 6) e.g. based on OSS code retrieved from the HLR.

Upon receiving the indication that the called party is being alerted, the GMSC sets up a UDI call towards the CAT Server. Upon reception of an Address Complete message from the CAT server, it returns to the originating MSC an Address Complete or Call Progress message indicating that multimedia CAT is available (bullet 13). This leads the originating MSC to request the calling UE (supporting multimedia CAT) to attach to the user connection and to set up a H.324 call to the CAT server (bullet 14).

When the called party accepts the call, the GMSC releases the call towards the CAT server (bullet 18), waits for the Release Complete message from the CAT Server, and then through-connects the calling and called legs and returns an answer indication to the originating MSC.

Upon reception of the CONNECT message, the calling UE releases any on-going H.324 call and establishes a new H.324 call towards the called party.

Figure 14.10.3.3.3.1: Multimedia CAT during a multimedia mobile originated call (message sequence chart)