Media handling and Quality aspects of SRVCC
Enhanced Single Radio - Voice Call Continuity (eSRVCC) is an existing standard ([3], [4]) 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 [7] on the CS accesses. Since Rel-13, the specifications for CS networks include the Codec Type UMTS_EVS with several Configurations, called UMTS_EVS (Set 0) to UMTS_EVS (Set 3).
In the IMS Core Network, the voice call is typically anchored in the ATCF/ATGW (Access Transfer Control Function/ Access Transfer Gate Way).
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.
The present Technical Report has the following detailed objectives:
- Identify relevant eSRVCC scenarios, especially with Codec Mode Control;
from AMR-WB and/or EVS in VoLTE to AMR-WB and/or EVS in CS;
but include also other important Codecs, such as AMR and G.722.
- Analyse Speech Quality Aspects and Media Handling Aspects, based on these scenarios.
- Analyse Codec Mode Control before, during and after eSRVCC;
recently SA4 has clarified some essential details on Rate Control for AMR and AMR-WB;
Rate Control and Audio Bandwidth Control for EVS are still under discussion to some extent.
- Analyse the existing SDP Offer - Answer protocol between Target MSC and Anchor-ATCF during eSRVCC,
as specified in 3GPP TS 23.216 [3], Stage 2;
This analysis will include the whole eSRVCC procedure for at least one essential scenario
(e.g. eSRVCC to GERAN) and will identify the potential reasons for transcoding and too long speech path interruptions.
- Clarify the existing Codec Compatibility aspects for eSRVCC;
especially the interworking between CS and IMS for AMR, AMR-WB and EVS needs to be documented.
- Propose enhancements for media and quality aspects of eSRVCC with the aims:
a) to avoid transcoding cases as much as possible;
b) to minimize the speech path interruption time during eSRVCC;
- Support the SA2 SETA work by SA4 expertise in speech quality and media handling.