Section: 5.3.6.1 Service description
Content: The 3GPP circuit-switched multimedia call is based on the 3G-324M (see 3GPP TS 26.111[ Codec for circuit switched multimedia telephony service; Modifications to H.324 ]  [80]), which is a 3GPP-variant of the ITU-T Recommendation H.324 [146]. CS Multimedia telephony is a Bearer Service, which utilizes the Synchronous Transparent Data service (BS30), see 3GPP TS 22.002[ Circuit Bearer Services (BS) supported by a Public Land Mobile Network (PLMN) ]  [3]. At the multimedia call setup the required call type, 3G-324M, is indicated, for the network to be able to invoke appropriate interworking functionality. In the peer end the H.324 information is used to invoke the terminal application. In addition to H.324 indication the terminal must select Information Transfer Capability (ITC) for the multimedia call. The 'correct' ITC depends on the peer end and the transporting networks; an all-ISDN call is a UDI/RDI call, and a call, which involves PSTN, is an analog "3.1 kHz audio" call. For the case when the setup of a multimedia call is not successful, fallback to speech is specified. Users may also request a service change between UDI/RDI multimedia and speech modes during a call (see 3GPP TS 23.172[ Technical realization of Circuit Switched (CS) multimedia service UDI/RDI fallback and service modification; Stage 2 ]  [97]).