(Blog Updated as of May 1, 2014)
The SCC-AS, (Service Centralization and Continuity Application Server), is the entity in the VoLTE capable IMS network that determines how to terminate the call to the UE by applying either a Network Based T-ADS or a UE T-ADS selection process. In this process, the network based T-ADS process is discussed.
A UE might not always be in an IMS Voice over PS capable cell, i.e. E-UTRAN. According to clause 7.4b of TS 23.221, one of the T-ADS functions that is performed by the SCC-AS is to determine if the call is to be delivered to a PS or CS domain.
There are several factors that play a role in the T-ADS procedure of terminating a session as stated in clause 7.2b of TS 23.221. Some of which are the UE’s and the access network capabilities, the user’s preferences and the operator’s policy. Clause 5.3.1 of TS 23.292 states: “SCC AS shall retrieve from the serving PLMN via the HSS the most recent access network capabilities ("IMS voice over PS Session Supported Indication", current RAT type)”. Two important pieces of information are returned to the SCC AS by the HSS, the IMS voice over PS Session Supported Indication and the Radio Access Type. Per clause 7.6.18 TS 29.238 the possible values of IMS voice over PS session support returned by the HSS might be one of the following:
The RAT type per clause 5.3.31 of 29.212 “identifies the radio access technology that is serving the UE”, such as GERAN, EUTRAN, HRPD, EHRPD, etc. The above information enables the SCC AS in terminating the call to the proper PS or CS domain.