S.1 General

26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS

This annex describes an extension to MTSI, which is optional to implement for an MTSI client. It contains descriptions of both mandatory and optional functionalities for the particular type of MTSI client that is called MSMTSI client, which is better suited for group audio/video communication than a regular MTSI client. The specifications in this Annex apply in addition to the rest of this specification, except when it is explicitly stated that the text here replaces other parts of this specification.

The intention with this extension is to avoid transcoding as far as possible in multiparty calls. For example, without this extension, video conferencing would need transcoding in the MRF to compose the different videos received from different senders into a single video, containing a main video and a number of thumbnails, which is then sent to each receiver. It may happen that the MRF needs to compose different video compositions to different receivers, which multiplies the need for transcoding and thus scales poorly with the size of the group. With this extension, the MRF can forward the necessary streams, without transcoding, to each MSMTSI client in terminal, which then composes the final image that is presented to the user.

Avoiding transcoding reduces the complexity in the MRF and reduces the end-to-end delay. Quality degradations caused by the transcoding can often also be avoided. The benefits with this extension are further described in TR 26.980 [152].