B.2 Minimize the transcoding for SRVCC

23.2373GPPIP Multimedia Subsystem (IMS) Service ContinuityRelease 17Stage 2TS

B.2.1 Codec re-negotiation after session transfer (SRVCC)

After SRVCC has been successfully performed (see clauses 6.3.2.1.9.1 and 6.3.2.1.9.2), MSC Sever may initiates a SIP REINVITE to modify the Selected Codec towards the remote end in order to minimize the transcoding points in the voice path.

Figure B.2-1 below illustrates this procedure with the assumption that the remote end supports the selected target RAN codec (B) in the Re-INVITE.

Figure B.2.1-1: Re-negotiation method towards the remote end

1. SRVCC is performed. MSC Server has included all supported codecs into the session transfer request to the ATCF. In this flow the codec list may include the codec that is currently used in the ongoing IMS session and ATCF has selected this codec, therefore there is no transcoding in ATGW but there may be transcoding in CS-MGW. The session between UE and CS-MGW uses the codec-B. The session between CS-MGW, ATGW and remote end uses the codec-A.

NOTE 1: If the codec list from MSC Server does not include the codec that is currently used in the ongoing session, ATCF initiates transcoding.

2. The MSC server sends a Re-INVITE to remote end with list of supported codecs in MSC server to ATCF, codec B is the most preferred codec in the list.

NOTE 2: It may be that the CS MGW or ATGW supports the audio Codec that is compatible or equal to the audio Codec used for the PS session but a change of the codec mode (such as bitrate, audio bandwidth, EVS Primary/AMR-WB IO modes) of the audio Codec used for the PS session is required. In this case the MSC server or ATCF may initiates a signalling towards to remote end to modify the codec mode of the audio Codec used for the PS session. The signalling to modify the codec mode is specific to the audio Codec used for the PS session as discussed in TS 26.114 [35], i.e. this procedure may be taken place by SIP invite, but may also be taken place by RTCP-APP or CMR (Codec Mode Request).

3. ATCF passes the Re-INVITE towards the SCC AS with the codec list.

4. SCC AS performs a remote leg update towards the remote end.

5-7. The remote end accepts the offer and selects the most preferred codec it can support, in this case codec B was selected. From now on the codec B is used e2e in TrFO manner.

B.2.2 Codec inquiry prior to session transfer (SRVCC)

B.2.2.1 Procedure for codec inquiry prior to session transfer

During SRVCC with ATCF procedure and prior to the session transfer and CS radio reservation procedure, MSC Server request from ATCF for the codec choice to be used for CS Codec on the target RAN.

Figure B.2.2.1-1 below illustrates this procedure.

Figure B.2.2.1-1: codec inquiry prior to session transfer

1. PS-CS handover procedure is triggered from MME with SRVCC PS to CS Request (see TS 23.216 [10]).

2a. MSC Server, optionally prior to executing the normal SRVCC procedure as defined in TS 23.216 [10], MSC Sever sends a PS-to-CS-Preparation-Request message (STN-SR, C-MSISDN, and MSC Supported Codec List) to the ATCF. The call-specific MSC Supported Codec List contains all Codecs (Codec Types plus Codec Configurations) that are supported and offered commonly by the UE, the target RAN and the target MGW.

2b. The ATCF compares the IMS Selected Codec with the Codecs of the MSC Supported Codec List and generates the call-specific IMS Preferred Codec List for the PS-to-CS-Preparation-Response message. The Payload Type Number of the IMS Selected Codec (Codec Type plus Codec Configuration) is included.

3. MSC Server takes the information of the PS-to-CS-Preparation-Response message into account when continuing the SRVCC procedure.

During the Session Transfer procedure, ATCF takes the MSC Preferred Codec List (indicated by MSC Server in the Initiation of Session Transfer as defined in TS 23.216 [10]) into account to determine the optimal Codec for the MSC-IMS interface. The downlink flow of VoIP packets is switched towards the CS access leg at this point. The ATCF responds to the Initiation of Session Transfer with the chosen Codec for the MSC-IMS interface back to the MSC Server.

NOTE 1: In case the CS Codec (used in target RAN) is still not compatible to the IMS Selected Codec and Transcoding is required, it is up to the ATCF to initiate a SIP REINVITE to modify the IMS Selected Codec towards the remote end. This modification may take place any time and is independent of the rest of the SRVCC procedure. Typically it is performed after the SRVCC was successful.

NOTE 2: It may be that the target RAN Codec is compatible or equal to the IMS Selected Codec, but a change of the codec mode (such as bitrate, audio bandwidth, EVS Primary/AMR-WB IO modes) of the IMS Selected Codec is required. In this case the ATCF initiates a signalling towards the remote end to modify the codec mode of IMS Selected Codec for the direction from the remote end towards the target RAN The signalling to modify the codec mode is specific to the IMS Selected Codec. This modification may take place any time and is independent of the rest of the SRVCC procedure. This procedure may be out-of-band, i.e. by SIP invite, but may also be in-band by RTCP-APP or CMR (Codec Mode Request). In many cases, it is beneficial and possible to start this signalling as soon as the ATCF got the MSC Supported Codec List as discussed in TS 26.114 [35].