23.1723GPPRelease 17Stage 2Technical realization of Circuit Switched (CS) multimedia service UDI/RDI fallback and service modificationTS
Whenever an In-Call Modification procedure is invoked by a terminal, unless it is not allowed as determined at call setup, the following shall take place:
– if the current mode is the speech mode and the MODIFY message contains a multimedia BC-IE, the normal Out‑of-Band Transcoder Control procedures shall be invoked in order to change the Selected Codec to the 3G‑324.M/3G-324.M2 codec;
– if the current mode is the multimedia mode and the MODIFY message contains a speech BC-IE, the normal Out‑of-Band Transcoder Control procedures shall be invoked in order to change the Selected Codec to the preferred speech codec.
When a visited MSC invokes Network-initiated Service change in the active state (see Clause 4.2.5), this visited MSC shall also invoke the normal Out‑of-Band Transcoder Control procedures in order to change the Selected Codec to speech or to the 3G‑324.M/3G-324.M2 codec, respectively.
For a User-initiated service change from speech to multimedia, the visited MSC shall use the 3G-324.M codec as Selected Codec.
If the visited MSC received the 3G-324.M2 codec in the available codec list, the visited MSC shall use the 3G-324.M2 codec as Selected Codec for a Network-initiated Service change procedure from speech to multimedia.
If the visited MSC did not receive the 3G-324.M2 codec in the available codec list, depending on operator policy, the visited MSC may either use the 3G-324.M codec as Selected Codec for a Network-initiated Service change procedure from speech to multimedia, or the visited MSC may not perform a Network-initiated Service change procedure from speech to multimedia.
NOTE: The the 3G-324.M2 codec in the available codec list indicates that all nodes in the call support collecting appropriate charging records for the network initiated service change from speech to multimedia.
If the 3G-324.M2 codec is used as Selected Codec in BICC signalling, the 3G-324.M codec shall be signalled instead to the MGW.
NOTE: This avoids that MGWs need to support the 3G-324.M2 codec.
The Codec Modification procedure shall be supported for service change. The use of mid-call codec negotiation procedure is optional for service change.
When a MSC detects through an Out-of-Band Transcoder Control procedure that the selected codec has changed from a speech codec to the 3G-324.M codec, or vice-versa, it shall initiate an In-Call Modification procedure towards the UE with a MODIFY message containing the multimedia BC-IE (or the speech BC-IE), unless the new mode has been denied at call setup (see clause 4.2.4).
22.214.171.124 Mid-Call Codec Modification Procedure For Service Change
The Codec Modification procedures as defined in 3GPP TS 23.153  shall be applied with the following specific additional rules for the Service Change procedure.
In order to prevent the MGW generating an error or seizing resources during the interim period, when its terminations are being altered and it may have a speech codec on one side of the context and the 3G-324M codec on the other side the Server shall modify the Stream–mode of the affected terminations to inactive during the Service change and shall restore the stream mode to active – (send/receive – bothway) on completion of the service change procedure. In order to restore the stream mode to active, the MSC servers shall use the “Modify Bearer Characteristics” procedure for Iu terminations and for Nb terminations towards the succeeding node with respect to the “Modify Codec” message. The MSC servers shall use the “Confirm Bearer Characteristics” procedure for Nb terminations towards the preceding node with respect to the “Modify Codec” message.
If the affected termination’s stream mode is inactive a MGW shall not reject a “Modify Bearer Characteristics” or a “Reserve Bearer Characteristics” procedure because the multimedia codec and a speech codec are interconnected simultaneously in the same context.
For a service change where the CN shall initiate the IuUP on the Nb interface, the MSC server terminating the service change shall trigger the IuUP initialisation towards the core network by setting the IuUP initialisation direction to “out” in the “Confirm Bearer Characteristics” procedure for the corresponding termination towards the core network.
Example call flows are shown in Figure 126.96.36.199/1 to 188.8.131.52/10.
Figure 184.108.40.206/1: Service change speech to MuMe
Figure 220.127.116.11/2: Service change speech to MuMe (continued)
Figure 18.104.22.168/3: Service change MuMe to AMR
Figure 22.214.171.124/4: Service change MuMe to AMR (continued)
Figure 126.96.36.199/5: Service change MuMe to PCM(G.711)
Figure 188.8.131.52/6: Service change MuMe to PCM(G.711) (continued)
Figure 184.108.40.206/7: Network-initiated service change from MuMe to speech
Figure 220.127.116.11/8: Network-initiated service change from MuMe to speech (continue)
Figure 18.104.22.168/9: Network-initiated service change speech to MuMe
NOTE: The RAB Assign Modify sent towards RNC-A may also be sent by MSC-A before receiving the Successful Codec Modification Message.
Figure 22.214.171.124/10: Network-initiated service change speech to MuMe (continued)
126.96.36.199 Unsuccessful Service Change
In the case the service change is denied by the UE at the terminating side, the procedures for the unsuccessful Codec Modification as defined in  shall be applied to revert to the old medium (speech or multimedia).
The through-connection of terminations shall be performed as described in Subclause 188.8.131.52. The "Codec Modification Failure" message shall interact with the "Modify Bearer Characteristics" procedure and the "Confirm Bearer Characteristics" procedure in the same way as the “Successful Codec Modification” message.
An example sequence is shown in Figure 184.108.40.206/1 to 220.127.116.11/5.
Figure 18.104.22.168/1:Service Change Rejected