6.14.2 Codec Modification/ Mid-Call Codec Negotiation after Inter-MSC Handover

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

6.14.2.1 Codec Modification Initiated by the Far End Side

Modification of Available Codec List

If after inter-MSC handover in GERAN AoIP mode the anchor MSC (MSC-S-A) receives a "Modification of Available Codec List" procedure from the far end side as described in clause 5.8.2, i.e. the Available Codecs List (BICC) is reduced, the anchor MSC may terminate the procedure at that point or forward the "Modification of Available Codec List" procedure to the serving MSC (MSC-S-A’). I.e. no MAP signalling is used.

Modification of Selected Codec

If after inter-MSC handover in GERAN AoIP mode the anchor MSC (MSC-S-A) receives a "Modification of Selected Codec" procedure from the far end side as described in clause 5.8.1, and both the old and the new Selected Codec (BICC) are speech codecs, the anchor MSC may terminate the codec modification procedure, inserting a transcoder if required. Alternatively, the anchor MSC may forward the request to modify the codec to the serving MSC (MSC-S-A’), using the procedures as described in clause 5.8.1. I.e. no MAP signalling is used.

NOTE: The anchor MSC may decide to forward the request to the serving MSC (MSC-S-A’), e.g. when the new Selected Codec (BICC) for the call leg between the anchor MSC and the far end side is TrFO compatible to a codec that was included in the AoIP-Available Codecs List (MAP) previously received from the serving MSC, and it is possible to (re‑)establish TrFO end-to-end from the far end side up to the serving MSC.

If the serving MSC (MSC-S-A’) receives a request to modify the codec, both the old and the new Selected Codec (BICC) are speech codecs, and the new Selected Codec (BICC) is TrFO compatible to a codec sent by the serving BSC‑A’ in the latest Supported Codecs List (BSSMAP) – "BSC-SCL", then the serving MSC (MSC-S-A’) may initiate an internal handover procedure towards the serving BSC to modify the A interface used codec as specified in 3GPP TS 23.009 [11], clause 6.3, and 3GPP TS 23.205 [8], clause 8.4.6. Upon completion of the internal handover procedure, the serving MSC (MSC-S-A’) shall proceed with the codec modification procedure towards the anchor MSC (MSC-S-A).

If the anchor MSC (MSC-S-A) receives a "Modification of Selected Codec" procedure from the far end side as described in clause 5.8.1, and the new Selected Codec (BICC) is the multimedia dummy codec, i.e. the far end side requests a service change from speech to multimedia then the anchor MSC shall reject the request for codec modification towards the far end party.

6.14.2.2 Mid-Call Codec Negotiation Initiated by the Far End Side

Support of this procedure is FFS.

6.14.2.3 Modification Procedure after Codec Change in the Serving MSC

According to 3GPP TS 23.009 [11], after a subsequent intra-MSC handover, the serving MSC (MSC-S-A’) informs the anchor MSC (MSC-S-A) by sending a MAP Process Access Signalling request including a BSSMAP Handover Performed message. As specified in 3GPP TS 23.009 [11], clause 6.1 and 6.3.2, the serving MSC (MSC-S-A’) may inform the anchor MSC (MSC‑S-A) about a change of the AoIP-Selected codec Target (MAP) during the subsequent intra-MSC handover by including the new AoIP-Selected codec Target (MAP) in the MAP Process Access Signalling request. If the AoIP-Available Codecs List (MAP) was changed during the handover, the serving MSC may insert the new AoIP-Available Codecs List (MAP) into MAP Process Access Signalling request.

NOTE 1: If the codec type or codec configuration used at the AoIP interface was changed and TrFO was established between the serving MSC/MGW and the anchor MSC/MGW before the handover, the serving MSC will insert a transcoder to maintain an end-to-end voice connection before informing the anchor MSC about the handover.

On reception of the MAP Process Access Signalling request the anchor MSC may initiate one of the modification procedures as described in clauses 5.8.1, 5.8.2, and 5.8.3 towards the serving MSC and/or towards the far end side. I.e. towards the serving MSC no MAP signalling is used.

NOTE 2: The anchor MSC can initiate the codec modification procedure as a result of the change of the codec type or codec configuration used at the AoIP interface, if MSC-A knows by means of configuration information that all nodes of the network support TrFO/TFO interworking and TFO, including codec mismatch resolution. The intention of that codec modification procedure is to remove the transcoder inserted by the serving MSC according to NOTE 1 and not to trigger a new change of the radio access bearer.

Alternatively the serving MSC may initiate one of the modification procedures as described in clauses 5.8.1, 5.8.2, and 5.8.3 towards the anchor MSC.

In the event of a collision of codec modification/mid-call codec negotiation procedures initiated by the anchor MSC and the serving MSC, the procedures as specified in clause 6.11.2.2 apply.