B.1 Basic call with UE busy with T-AS involvement (NDUB condition check)

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

This subclause describes the signalling flow for the case when the user is busy but the network does not consider the user to be busy.

Figure B.1 shows the signalling flow for the scenario.

NOTE: The signalling flow is simplified for readability reasons.

Figure B.1: Basic call with UE busy with T-AS involvement (NDUB condition check)

This signalling flow assumes the following:

– the user in the terminating network needs the involvement of an AS for NDUB or other busy condition activated services like CCBS or CFBS; and

– the filter criteria are set for basic communication accordingly.

NOTE: The same scenario applies also for other error responses e.g. for the 403 (Service Denied) response, the 480 (Temporarily Unavailable) response.

The steps of the flow are as follows:

1) The S-CSCF serving the terminating user receives an INVITE request from the originating network. The originating network can be an IMS network, a PSTN/ISDN Emulation network, another SIP based network or a MGCF interworking with PSTN/ISDN.

2) The S-CSCF checks the IFC and finds that a trigger fires and sends the INVITE request to the AS. The address to the AS is obtained from the IFC.

3) The AS checks the busy condition and it is not NDUB and sends the INVITE request to the S‑CSCF.

4) The S-CSCF sends the INVITE request according to the P-CSCF.

5) The P-CSCF sends the INVITE request according to the UE#2.

6) The UE#2 is e.g. involved in another communication and determines itself to be busy and sends a 486 (Busy here) response to the P-CSCF.

7) The 486 (Busy here) response to originating network via the S-CSCF and the AS.