4.16 Support of multimedia telephony

23.2283GPPIP Multimedia Subsystem (IMS)Release 18Stage 2TS

4.16.1 Telephony Application Server

The Telephony Application Server is a SIP AS providing the network support for the multimedia telephony service, TS 22.173 [53]. If specific procedures and message flows include or require media interaction, the TAS and MRFC may be collocated.

NOTE: The support of multimedia telephony services may be allocated to one or more Application Servers.

4.16.2 Identification of multimedia telephony

The multimedia telephony communication service shall be associated with a communication service identifier to allow easy identification of the service.

When multimedia telephony is supported in a network, Voice/video calls originating from the PSTN/CS domain shall be marked with the communication service identifier associated with multimedia telephony communication service.

4.16.3 Session setup principles

When establishment of UE initiated IP‑CAN bearer(s) for the media is required it is recommended to reserve IP‑CAN bearer(s) at the reception of the SDP answer. If the UE has been made aware of the operator policies with regards to allowed media for the multimedia telephony service, then the UE may reserve IP‑CAN bearer(s) at the sending of the SIP INVITE request. For multimedia telephony, the UE should only mark resource reservation as required for voice and video.

When there are no requirements for resource reservation or when required resources are available on the originating side, the P‑CSCF on the terminating side may send available session information to the PCRF/PCF at the reception of the SDP offer, as in such cases the UE can attempt resource reservation before sending the SDP answer.

If configured through policies, the telephony AS, or any other AS, may perform for originating requests attestation of the identity of the originating subscriber.

If configured through policies, the telephony AS, or any other AS, may perform for originating IMS priority sessions, assertion of authorization for the Resource-Priority information.

If configured through operator policies, the telephony AS may perform for diverted sessions attestation of the identity of the diverting subscriber initiating the diversion,

In addition, and if configured through policies, the telephony AS, or any other AS, may perform for terminating requests signature verifications, if one or more signatures is included.

NOTE: Only one network element performs attestation for an originating subscriber in the originating network.