4.3.2 Originating side – initiation of call setup

23.1723GPPRelease 17Stage 2Technical realization of Circuit Switched (CS) multimedia service UDI/RDI fallback and service modificationTS

4.3.2.1 Originating MSC Handling

The originating MSC has a list of supported codec types, listed in order of preference.

If the SETUP message received from the UE contains a Repeat Indicator with a value of "service change and fallback", in addition to a multimedia BC-IE and a speech BC-IE, the MSC shall include a 3G-324.M codec in the list of supported codec types according to the following rule:

– If the multimedia BC-IE is listed first, then the 3G-324.M codec shall be the first codec in the list (see figure 4.15) If the originating MSC supports a network-initiated service change from speech to multimedia, it shall include the 3G-324.M2 codec as second codec in the list.

– If the speech BC-IE is listed first, then the 3G-324.M codec shall be the last codec in the list (see figure 4.16) sent by a originating MSC not supporting a network-initiated service change from speech to multimedia. If the originating MSC supports a network-initiated service change from speech to multimedia, the 3G-324.M codec shall be the last but one codec in the list and the 3G-324.M2 codec shall be the last codec in the list.. In the case that the maximum number of codecs is already reached before insertion of the 3G-324M and possibly 3G-324.M2 codec(s), the optional speech codec(s) with the least preference shall be replaced by those codec(s).

The list shall then be sent according to the Out-of-Band Transcoder Control codec negotiation procedures. The TMR field, although mandatory BICC/ISUP parameter, has no meaning when using OoBTC/BICC codec negotiation (the link characteristics and QoS are determined from the codec type and signalled to any intermediate switches via the bearer control protocol) and thus shall be set arbitrarily to "speech". A transit node that requires interaction with the user plane will remove from the list the codecs it does not support. If the 3G-324.M codec is not supported, and thus removed, the call shall be turned into a normal speech call (fallback to speech) and follow the normal call setup procedures.

x, y, z: speech codecs.

mm: dummy multimedia codec 3G-324.M.

NOTE: If the originating MSC supports incoming requests for a network-initiated service change from speech to multimedia, it shall include the 3G-324.M2 codec as second codec in the list after 3G-324.M. This is not depicted in the figure.

Figure 4.15: Multimedia BC as first BC

NOTE: If the originating MSC supports incoming requests for a network-initiated service change from speech to multimedia, it shall include the 3G-324.M2 codec as last codec in the list after 3G-324.M. This is not depicted in the figure.

Figure 4.16: Speech BC as first BC

4.3.2.2 VMSC Handling at Originating Side

Depending on operator policy, the VMSC may remove the 3G-324.M2 codec from the list of supported codec types if the call is routed to selected other PLMNs.

NOTE: This enables the operator to block a network initiated upgrade if inter-operator accounting agreements do not allow this functionality, e.g. because billing schemes are not compatible for the network-initiated upgrade.