4 Overview of multimedia telephony communication service and associated supplementary services in the IP Multimedia (IM) Core Network (CN) subsystem

24.1733GPPIMS Multimedia telephony communication service and supplementary servicesRelease 18Stage 3TS

4.1 General

In accordance with the service definition and requirements in 3GPP TS 22.173 [2], the IMS multimedia telephony communication service specified herein allows multimedia conversational communication between two or more end points. An end point is typically located in a UE, but can also be located in a network entity.

As for traditional circuit-switched telephony, the protocols for the IMS multimedia Telephony communication service allow a user to connect to any other user, regardless of operator and access technology.

The IMS multimedia Telephony communication service consists of two principal parts: a basic communication part, and an optional supplementary services part.

4.1A Roles

4.1A.1 Multimedia telephony participant

A UE shall implement the role of a multimedia telephony participant.

4.1A.2 Multimedia telephony application server

An application server shall implement the role of a multimedia telephony application server. Various application server usages are called out by references to the various supplementary services, see clause 4.3. It is an implementation decision on how to allocate the functionality to one or more application servers.

4.2 Overview of basic communication part

The basic communication part of an IMS multimedia telephony communication service session is realised by a single SIP session. It utilises media capabilities and flexibility provided by the SIP protocol and the 3GPP IMS specifications. In accordance with the service definition in 3GPP TS 22.173 [2], media capabilities include RTP-based transfer of voice, real-time video, real-time text and data, and UDPTL-based transfer of fax (IETF RFC 3362 [28]), as well as TCP/MSRP-based transfer of text, arbitrary files and sharing of media files with predefined formats.

To ensure interoperability, media handling (including codecs and formats) is fully specified for RTP-based and MSRP-based transfer in:

– 3GPP TS 26.114 [12] for 3GPP systems;

– 3GPP2 C.S0055-A [21] for 3GPP2 systems; and

– ETSI TS 181 005 [22] (codecs) and 3GPP TS 26.114 [12] (formats and other media handling) for fixed-broadband accesses.

The service is highly dynamic in terms of media component usage: the protocols allow a communication session to start with one or more media components, and components can then be added and/or removed during the communication session. The protocols allow both one-way and two ways transfer between end points. Full duplex speech, and speech combined with other media components, are typical media cases but the protocols do not mandate the use of speech in all sessions.

4.3 Overview of supplementary services part

The supplementary services part of the IMS multimedia telephony communication service consists of a number of specified supplementary services. These are fully standardized to ensure interoperability between multiple end points, and between end points and network control entities. The behaviour of supplementary services is similar to supplementary services specified for CS speech (TS 11). Supplementary services uses SIP as enabling protocol. Configuration of supplementary services by the user should:

– take place over the Ut interface using XCAP as enabling protocol as described in 3GPP TS 24.623 [11]; or

– use SIP based user configuration as described in 3GPP TS 24.238 [20] ;

NOTE: Other possibilities for user configuration, such as web-based provisioning or pre-provisioning by the operator are outside the scope of the present document, but are not precluded.

The "SS_domain_setting" leaf in 3GPP TS 24.167 [32] provides a mechanism to:

a) instruct the UE to only use the PS domain for SS setting control;

b) instruct the UE to only use the CS domain for SS setting control; or

c) instruct the UE to use the PS domain for SS setting control when the PS domain is being used by the UE for voice services, and to use the CS domain for SS setting control when the CS domain is being used by the UE for voice services.

If one of the restrictions described in the bullets a) or c) applies, then the "PS_domain_IMS_SS_control_preference" leaf in 3GPP TS 24.167 [32] provides a mechanism to:

– restrict the UE to use the Ut interface with XCAP as enabling protocol as described in 3GPP TS 24.623 [11]; or

– restrict the UE to use SIP based user configuration as described in 3GPP TS 24.238 [20].