7.5.13 Customized Alerting Tones (CAT)

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

7.5.13.1 General

The Customized Alerting Tones supplementary service is described in 3GPP TS 24.182 [132].

The interworking procedures in this clause enable the transport of customized alerting tones provided by an AS in the IM CN subsystem towards a caller in a CS network.

Based on operator options, one of three different models described in 3GPP TS 24.182 [132] is used in an IM CN subsystem to provide the CAT service. The CAT interworking procedures at the O-MGCF differ between these models and are described separately for each model in the subsequent clauses. Support of the interworking procedures for each of those models is optional.

I-MGCF procedures are not affected.

7.5.13.2 Early session model

7.5.13.2.1 Interworking at the O-MGCF

When CAT service is provided by using early session model as specificed in 3GPP TS 24.182 [132], and if the O-MGCF supports early-sesssion as a network option, then it shall include the option tag "early-session", as defined in IETF RFC 3959 [131], in Supported header field in each outgoing initial SIP INVITE request.

After receiving the first SIP 18x response including an early-session SDP, then the O-MGCF shall send the address complete message (ACM) or Call Progress message (CPG) with the Optional backward call indicators parameter set to "in-band information or an appropriate pattern is now available". If the SIP 18x response is the SIP 180 Ringing response, then additionally the Called party’s status indicator in the ACM message shall be set to "subscriber free" or the Event information in the CPG message shall be set to "ALERTING".

If the O-MGCF applies codec interworking procedures according to Annex B, it shall interwork the "session" disposition SDP (rather than the "early session" disposition SDP).

7.5.13.2.2 MGCF – IM-MGW Interaction for CAT service
7.5.13.2.2.1 General

When the IMS CAT is provided by using early session model as described in 3GPP TS 24.182 [132], the procedures described in clauses 9.2.3.1 to 9.2.3.3 shall be applied with the following additions.

7.5.13.2.2.2 IM CN subsystem side Early Session establishment

If the MGCF supports early-session as a network option, upon receipt of a SIP 18x response with an early-session SDP, the MGCF shall use the Configure IMS Resources procedure (signals 10 and 11 in figure 7.5.13.2.2.5-1, signals 7 and 8 in figure 7.5.13.2.2.5-2) to provide configuration data (derived from early session SDP received in signal 8 in figure 7.5.13.2.2.5-1 and local configuration data) to the IM-MGW as detailed below:

– The MGCF shall indicate the remote IP address and UDP port, i.e. the destination IP address and UDP port for data sent in the user plane towards the IM CN subsystem,

– The MGCF shall indicate the remote codec(s), i.e. the speech codec(s) for data sent in the user plane towards the IM CN subsystem.

– The MGCF should select the same codec(s) for early-session SDP with the codec(s) used for normal session SDP.

– The MGCF may indicate the local codec(s) and the local IP address and UDP port. The MGCF shall indicate the local codec(s) if a change is required.

– The MGCF may indicate that IP interface type is for MboIP.

The IM-MGW shall reply with the selected remote codec(s) and reserve resources for these codec(s). If local codec(s) were received, the IM-MGW shall also reply with the selected local codec(s) and reserve the corresponding resources.

The MCGF shall send the local codec(s), UDP port and IP address in the early-session SDP to the IMS in the PRACK messages (signal 12 in figure 7.5.13.2.2.5-1 and 7.5.13.2.2.5-2).

7.5.13.2.2.3 IM CN subsystem side normal Session establishment

When the MGCF receives a SIP 200(OK) response to the SIP INVITE request (signal 14 in figure 7.5.13.2.2.5-1, signal 15 in figure 7.5.13.2.2.5-2), it shall request the IM-MGW use the Configure IMS Resources procedure for normal session SDP (signals 15 and 16 in figure 7.5.13.2.2.5-1, signal 16 and 17 in figure 7.5.13.2.2.5-2) to provide configuration data (derived from SDP received in signal 8 in figure 7.5.13.2.2.5-1 and local configuration data) to the IM-MGW as detailed below:

– The MGCF shall indicate the remote IP address and UDP port, i.e. the destination IP address and UDP port for data sent in the user plane towards the IM CN subsystem,

– The MGCF shall indicate the remote codec(s), i.e. the speech codec(s) for data sent in the user plane towards the IM CN subsystem.

– The MGCF may indicate the local codec(s) and the local IP address and UDP port. The MGCF shall indicate the local codec(s) if a change is required.

– The MGCF may indicate that IP interface type is for MboIP.

The IM-MGW shall reply with the selected remote codec(s) and reserve resources for these codec(s). If local codec(s) were received, the IM-MGW shall also reply with the selected local codec(s) and reserve the corresponding resources.

7.5.13.2.2.4 Called party alerting

If the MGCF supports early-session as a network option, upon receipt of a SIP 18x provisional response with an early-session SDP, the MGCF shall not request the IM-MGW to provide an awaiting answer indication (ringing tone) to the calling party.

7.5.13.2.2.5 Message sequence chart

Figure 7.5.13.2.2.5-1 shows an example message sequence chart for an CS network originating Session Setup with ISUP, where IMS CAT is provided by early session model.

Figure 7.5.13.2.2.5-1: CS Network Originating Session with Early Session SDP, ISUP (message sequence chart)

Figure 7.5.13.2.2.5-2 shows an example message sequence chart for an CS network originating Session Setup with BICC, where IMS CAT is provided by early session model.

Figure 7.5.13.2.2.5-2: CS Network Originating Session with Early Session SDP, BICC (message sequence chart)

7.5.13.3 Forking model

Interworking at O-MGCF follows the basic call interworking procedures using the P-Early-Media header field and related procedures as described in clause 7.2.3.2.1, 7.2.3.2.2, 7.2.3.2.4, 7.2.3.2.5, 7.2.3.2.6, 7.2.3.2.7, 7.2.3.2.8, and 7.2.3.2.9.

The O-MGCF shall apply the procedures in clause 9.2.3.4 and shall use the P-Early-Media header field that authorizes early media as trigger to configure the IM-MGW as described in Clause 9.2.3.4.2.

7.5.13.4 Gateway model

Interworking at O-MGCF follow the basic call interworking procedures using the P-Early-Media header field and related procedures as described in clause 7.2.3.2.1, 7.2.3.2.2, 7.2.3.2.4, 7.2.3.2.5, 7.2.3.2.6, 7.2.3.2.7, 7.2.3.2.8, and 7.2.3.2.9.

Example callflows are shown in annex J