26.1143GPPIP Multimedia Subsystem (IMS)Media handling and interactionMultimedia telephonyRelease 18TS
An MSMTSI client in the terminal should know which RTP streams that share a common channel resource (such as a radio bearer), and shall take this into account when performing per-stream rate adaptation. Per-stream adaptation is specified in clause 10. An MSMTSI MRF should, unless explicitly configured otherwise, assume during rate adaptation that all media streams of the same SDP media type (audio, video, etc) from a single MSMTSI client share a common channel resource. The sum of bitrates used for streams sharing a common channel resource shall be controlled such that they jointly do not exceed the estimated available channel resource. This applies to all MSMTSI clients both in the send direction and in the receive direction, providing rate adaptation information feedback (e.g. TMMBR or any rate control mechanism natively available to the used codec) to the sending party.
In situations where a MSMTSI client or MRF media sender that has the ability to send multiple media streams is faced with scarce network resources, insufficient to support maximum bitrate for all streams the sender is capable to send, it can be necessary for the sender to choose which streams to either entirely omit, temporarily stop, or degrade more than others. Degrading all streams evenly may not be the best approach, but consistently degrading one stream at a time may also not be preferable. The individual media receiver of those multiple streams is likely the one best equipped to decide which streams are most interesting to receive in a specific communication scenario, but some general guidance can be given. It is recommended that the main audio stream is considered most important, except for special use cases that do not require use of audio. Screenshare video is likely next most important, after which comes main video and BFCP. When using simulcast, main video simulcast using different codecs is likely more important than video thumbnail simulcast. Video thumbnails are likely least important in most use cases.
Annex T (informative):
SDP examples for Multi-party Multimedia Conference Media Handling