23.1723GPPRelease 17Stage 2Technical realization of Circuit Switched (CS) multimedia service UDI/RDI fallback and service modificationTS
The Service Change and UDI Fallback (SCUDIF) is a function which applies to UDI/RDI multimedia calls (see 3GPP TS 22.101 , clause 7.2.1), and shall support the following:
a) Fallback to speech during call setup: allow a user to attempt to set up a multimedia call, and try a speech connection if the former doesn’t succeed;
b) Fallback to the less preferred service (speech or multimedia) during call setup: allow the terminating side via specific settings for this service in the terminal to accept or reject a multimedia call, without interrupting the call setup;
c) Fallback to the preferred service (speech or multimedia) or speech during call setup: allow the call setup to proceed with a single service if the transit network does not support the signalling of this functionality;
d) BC negotiation at the terminating side: allow the terminating side via specific settings for this service in the terminal to turn a speech call (with service change) into a multimedia call and vice-versa;
e) Service change: allow a speech call to be turned to multimedia by either of parties, and back to speech, through a successful in call modification procedure;
f) Allow any of the users to reject a multimedia request from the other party while in speech mode.
g) Network-initiated service change: The network shall initiate a sevice-change from multimedia to speech during the active call if a multimedia call can no longer be supported. If a multimedia call can again be supported at a later point in time, the network may initiate a service change from speech to multimedia.
– service request signalling between the UE and the MSC;
– service request signalling across the Core Network.
This functionality is not supported for multimedia with Fixed Network User Rate set to 32 kbit/s. In this case, the MSC shall revert to a multimedia only call.