Enhanced Single Radio - Voice Call Continuity (eSRVCC) is an existing standard specifying the handover of a Voice or Video call from LTE access to CS-radio access, either to GERAN (2G) or to UTRAN (3G) or other CS networks. The present document considers only enhanced SRVCC for voice calls between 3GPP accesses.
This study assumes that the Codecs defined in TS 26.114 are used on the LTE access and the Codecs defined in 3GPP TS 26.103 on the CS accesses.
In the IMS Core Network, the voice call is typically anchored in the ATCF/ATGW (Access Transfer Control Function/ Access Transfer GateWay).
The eSRVCC procedure, as specified, may cause additional transcoding between the target radio leg and the ATGW, even though in theory it would be possible to avoid it. As a result, the eSRVCC procedures may add one or more unnecessary transcoding point(s) for the call and thereby degrade the quality of the ongoing call unnecessarily.
The main objectives of this study are to analyse example call scenarios and find potential solutions to minimize the number of transcoding cases. Another objective is to optimize the interworking and the transition between EVS and AMR-WB during eSRVCC. The study should also show the reasons and potential solutions for too long speech path interruptions during eSRVCC.