Content for  TS 23.292  Word version:  17.0.0

All ICS User incoming sessions are directed to IMS for delivery to the ICS User. Non ICS UEs which have been successfully registered in IMS by the MSC Server will have a registration binding at the S-CSCF containing the MSC Server as the contact address. The SCC AS shall be inserted in the IMS session path using the terminating iFC. The SCC AS performs T-ADS for selection of an access and returns information to assist with S-CSCF selection of a registered contact address. When the T-ADS function selects the MSC Server enhanced for ICS, the SCC AS directs the IMS terminating session towards the contact address of the MSC Server. On receipt of the session initiation message, the MSC Server enhanced for ICS shall perform the necessary interworking between the I2 reference point and CS signalling (e.g. as described in TS 24.008). The MSC Server shall also control a CS-MGW using the Mc reference point to perform the necessary interworking between RTP bearers on the Mb reference point and CS access bearers and adds the User Location Information (e.g. CGI or SAI) and/or UE Time Zone Information to the response to the session initiation. The SCC AS may select to breakout an incoming session to the CS domain in following cases:

• A non ICS UE is not registered in IMS but can be attached to the CS network at an MSC Server. In this case, terminating iFC forwards the call to the SCC AS.
• A non ICS UE is registered in IMS, the access network does not support the media type (as defined in clause 22.4 of TS 22.101), but the UE still can be attached to the CS network at an MSC Server. In this case, terminating iFC forwards the call to the SCC AS.

For both scenarios the SCC AS shall retrieve from the HSS the C-MSISDN(s) associated with all the IMPIs that are associated with the IMPU that is the target of the session. The SCC AS shall select a single C-MSISDN and shall use it to fetch a CSRN for routing the call to the CS domain. The INVITE shall be sent to the S-CSCF which then performs CS breakout according to standard IMS procedures. The INVITE may include an indication that this is an ICS session, which can be used by the MGCF to handle this session in a specific manner, for example to make a different cause mapping between ISUP/BICC and SIP than a normal session breakout would do. The SCC AS may also retrieve the User Location Information (e.g. CGI or SAI) and/or UE Time Zone Information from the HSS at this time or at the reception of the response, to include it into the response to the session initiation. For video call terminating sessions that use CS media, the MSC Server enhanced for ICS shall also, after the multimedia connection is established, perform the video codec negotiation for the non ICS UE and set up the video media bearer based upon the procedures defined in TS 29.163 for 3GPP systems and based on procedures defined in 3GPP2 C.S0042 [38] for 3GPP2 systems. If the UE does not support video, or the access network is not capable of video, then the video component of the session shall not be accepted by the MSC Server enhanced for ICS. For voice call terminating sessions that use CS media via I2,, re-routing from the terminating MSC Server enhanced for ICS to another MSC (ICS enhanced or not) may be triggered (i.e, due to Mobile Terminating Roaming Retry Call procedure as defined in TS 23.018, Roaming Retry for CS fallback or Roaming Forwarding for CS fallback as defined in TS 23.272 is triggered). If the SCC AS receives from the new MSC (enhanced for ISC- see clause 7.2.1.2) a new IMS registration for the UE for which the SCC AS just delivered a session, then:

• S-CSCF releases the current terminating session setup toward SCC AS/T-ADS as defined in TS 24.229; and
• T-ADS redirects the session to the new MSC.

If the old MSC (terminating MSC) performs IMS deregistration over I2 (see clause 7.2.1.3), then:

• S-CSCF releases the current terminating session setup toward SCC AS/T-ADS as defined in TS 24.229;
• Based on operator policy, T-ADS may start a timer. The timer is stopped when new registration is received for the UE;
• If the T-ADS stopped the timer itself, T-ADS redirects the session to the new MSC;
• If the timer expires, T-ADS may invoke other procedure to redeliver the call (e.g. routing the call to the CS domain).

Figure 7.4.2.1.2-1 describes how IMS terminations are performed via CS access for non ICS UE registered in IMS. This call flow also applies for an ICS UE attached to an MSC Server enhanced for ICS where the incoming session is delivered to the ICS UE using standard CS terminating procedures, as specified in clause 7.4.2.2.5.

Figure 7.4.2.1.3-1 describes how IMS terminations are performed via CS access for non ICS UE not registered in IMS. This call flow also applies for an ICS UE attached to an MSC Server where the incoming session is delivered to the ICS UE using standard CS terminating procedures, as specified in clause 7.4.2.2.5.