5.8.3 Mid-call Codec negotiation

23.1533GPPOut of band transcoder controlRelease 17Stage 2TS

The Selected Codec (BICC) and the Available Codecs List (BICC) can be (re-) negotiated during the call using the "Mid Call Codec Negotiation" mechanism. The Mid-Call Codec Negotiation mechanism results in a new Available Codecs List (BICC), where new codec types or modes not within the previous Available Codecs List (BICC) may be included. The codec negotiation procedure is performed as for call set-up.

The procedures described in Q.1902.4, clauses 10.4.4 to 10.4.6 [6] shall apply.

The sequence is shown in Figure 5.8.3/1. Starting with the Modify to Selected Codec message, the remaining sequence is the same as for the Codec Modification in clause 5.8.1 except that the message name for the modify request is "Modify To Selected Codec" (instead of "Modify Codec") in order to allow collisions between the two procedures to be resolved. Everything stated in clause 5.8.1 shall also apply for the Mid-Call Codec Negotiation procedure.

The node initiating the "Mid Call Codec Negotiation" mechanism (MSC Server A in Figure 5.8.3/1) shall select a Preferred Codec and a Supported Codecs List (BICC), which may contain new codecs and also may not contain codecs from the previous Available Codecs List (BICC). If the list no longer contains the previous Selected Codec (BICC), then a new codec shall be selected as Preferred Codec. If the previous Selected Codec (BICC) exists within the Supported Codecs List (BICC), this codec should be selected as the Preferred Codec.

If a server node removes the Preferred Codec, from the Supported Codecs List (BICC), the node shall select a new Preferred Codec.

Figure 5.8.3/1: Mid Call Codec Negotiation