23.1723GPPRelease 17Stage 2Technical realization of Circuit Switched (CS) multimedia service UDI/RDI fallback and service modificationTS
If the 3G-324.M codec is included in the list of supported codec types received by a Gateway MSC, and the external network does not support BICC or does not support Codec Negotiation, the Gateway MSC shall terminate the codec negotiation and fallback to a single service.
NOTE 1: If the route is known not to support the SCUDIF functionality, the Gateway MSC may decide by configuration to terminate the codec negotiation and follow the procedure described in this clause.
In the case where the 3G-324.M codec is the first in the list, the network decides by configuration to fallback either to a UDI multimedia-only call or to speech. In the case where the 3G-324.M codec is not the first on the list, the call shall fallback to speech only.
If fallback to multimedia occurs, the call control parameters sent towards the external network shall be set according to the setting for multimedia calls, and TMR is set to "64 kbit/s unrestricted". The 3G-324.M codec shall be returned to the originating MSC server as the selected codec and be the only member of the available codec list.
NOTE 2: For multimedia calls, 3GPP TS 27.001 , annex B, and 3GPP TS 29.007 , table 7A, describe the setting and validity of the PLMN BC-IE as well as the comparable settings of parameters in the PLMN and ISDN BC-IEs. As the ISDN BC-IE parameter values used for UDI/RDI multimedia calls are identical to the BICC USI IE parameter values (see 3GPP TS 29.205 ), the setting of call control parameters sent towards the external network in case of fallback to multimedia can be derived straightforward.
If fallback to speech occurs, the call control parameters shall be set according to the setting for speech calls, and TMR is set to "speech". The 3G-324.M codec shall be removed from the available codec list. Speech codec selection shall be made according to normal OoBTC procedures for interworking to external networks, and the selected codec and available codec list returned to the originating MSC server.