23.1533GPPOut of band transcoder controlRelease 17Stage 2TS
Figure 6.4/1: Multi-party Call
The operation of the MGW for conference calls is implementation dependent. The sequence in Figure 6.4/1 shows three connections to the MGW, where two were configured TrFO and have matching codecs but the third connection could not be made with the same codec type.
The Iu Framing connections for each multi-party call leg shall be terminated in the MGW where the multi-party call is controlled. The MGW shall control each connection independently during the multi-party call.
When the multi-party call is released, if two parties remain in the connection it shall be possible to either revert directly to a TrFO connection if both codecs match or OoBTC procedures could be performed to modify one or both of the codec types to achieve a TrFO connection. However, if the Server does not perform this then the MGW shall continue to resolve the difference in codecs by internal transcoding procedures.
Codec modification procedures may be employed (see clause 5.8.1) if a common codec exists, this is shown in Figure 6.4/2, where codec v is common to all parties.
Figure 6.4/2: Multi-party Call, with codec modification