23.1723GPPRelease 17Stage 2Technical realization of Circuit Switched (CS) multimedia service UDI/RDI fallback and service modificationTS
The codec negotiation procedures transmit an ordered list of preferred codecs from the originating to the terminating MSC. A node that requires interaction with the user plane will remove the codecs it does not support. The terminating MSC shall select the codec to use ("selected codec") from the list of available codecs for the call. This selection shall be based on the received list of codecs and on the information given by the terminating UE in the CALL CONFIRMED message.
A dummy codec (defined in 3GPP TS 26.103  for a BICC based CS core network and in 3GPP TS 29.007  as dedicated RTP payload types for a SIP-I based CS core network) is included in the codec list to indicate that a multimedia call is requested. Two dummy codecs are defined:
– Dummy codec 1 shall be used for a terminal-initiated service change from speech to multimedia. Based on the operator policy the dummy codec 1 may be used for the network-initiated service change from speech to multimedia, if one of the MSCs has not included the dummy codec 2 to the available codec list during the call setup phase.This dummy codec 1 is in the present document referred to as the 3G-324.M codec. It shall be supported by any SCUDIF compliant MSC.
– Dummy codec 2 shall be used for a network-initiated service change from speech to multimedia, if all the MSCs in the call have indicated the dummy codec 2 in the available codec list during the call setup phase. This codec is in the present document referred to as the 3G-324.M2 codec. The 3G-324.M2 codec is identical to the 3G-324.M codec except for the CoID. It shall be supported by any SCUDIF compliant MSC that supports a network- initiated service change from speech to multimedia.
NOTE: An Rel-5 MSC does not support the 3G-324.M2 codec. A Rel-6 MSC not supporting the optional network-initiated service change procedures could also not support the 3G-324.M2 codec.
These codec are in the present document referred to as the 3G-324.M codec.
This codec is only used by the Core Network and shall not be sent from the terminal in the Supported Codec List IE.
For a BICC based CS core network, the 3G-324.M codec shall be indicated to the MGW as codec media stream property in accordance with the3GPP TS 29.232. The 3G-324.M codec shall be indicated to the MGW also if the 3G-324.M2 codec is selected in Out-of-Band Transcoder Control procedures. The MGW shall treat the User Plane configuration (SDU Format) of the 3G-324.M codec as for PCM, as defined in 3GPP TS 26.102 .
For a SIP-I based CS core network, instead of the 3G-324.M or 3G-324.M2 codecs, the CLEARMODE RTP payload type (see IETF RFC 4040 ) shall be indicated to the MGW as media type in accordance with the3GPP TS 29.232. The MGW will treat the user plane for the Clearmode RTP payload type as defined in 3GPP TS 29.007 .