In a previous LTE University blog, the network based T-ADS was discussed. Clause 5.3.1 of TS 23.292 discusses the UE T-ADS in which the network provides the UE the option to select the terminating access network domain which will be used for the delivery of a VoLTE call to the UE.

Clause of TS 23.292 discusses how the SCC AS, (Service Centralization and Continuity Application Server), determines the access network for call delivery, (please refer to the figure below):

- There are 2 alternatives for the SCC AS to choose from when selecting a T-ADS:

  • Alternative 1 (Network Based T-ADS Selection):
    • Please see the previous blog on Network Based T-ADS Selection.
  • Alternative 2 (UE based T-ADS Selection):
    • The SCC AS initially selects a network based T-ADS and selects the IMS PS domain for signaling, (over the Gm interface between the UE and the SCC AS), to deliver terminating call INVITE requests.
    • The SCC AS gives the UE an option as part of the INVITE request to select the terminating access for the VoLTE session.
    • The INVITE request also includes a dynamic SCC AS PSI, (Public Service Identifier), field which allows the SCC AS to correlate the outgoing service control signaling with the incoming signaling that is being received from the CS side.
    • The UE selects the proper terminating access, PS or CS, and informs the SCC AS in the response to the INVITE request.

The following figure subset is copied from TS 23.292.

"Release 10 41 V10.5.0 (2012-03)

Figure ICS UE Terminations with CS media information flows using Gm reference point when using an MSC Server enhanced for ICS”