W.2 Adaptation to Packet Losses without application layer redundancy

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

An MTSI terminal supporting the CHEM feature without using application layer redundancy for a media type (e.g., speech or video) shall support the following procedures:

– when sending an SDP offer, the MTSI client shall include the PLR_adapt attribute in the media description for that media type in the SDP offer

– when sending an SDP answer, the MTSI client shall include the PLR_adapt attribute in the media description for that media type in the SDP answer regardless of whether the PLR_adapt attribute was received in an SDP offer

– if, and only if, the MTSI client receives the PLR_adapt attribute in either an SDP offer or SDP answer then,

– when the MTSI client receiving media detects packet losses higher than tolerable by the current codec mode/configuration in use and a more robust codec mode/configuration is available for the same codec, the MTSI client shall send a CMR request to the media sender to use a more robust codec mode/configuration of the same codec

– when the MTSI client receiving media detects a packet loss rate low enough to support a codec mode/configuration of the same codec that provides better media quality than the current codec mode/configuration, and switching to the new codec mode/configuration will not cause oscillating between more robust and less robust codec modes/configurations, then the MTSI client should send a CMR request to the media sender to use the codec mode/configuration of the same codec that provides better media quality

Clause X.2.2 provides examples of how the PLR_adapt attribute without application layer redundancy is used in SDP.

Robustness adaptation without application layer redundancy can involve switching to a codec mode/configuration with partial-redundancy (e.g., EVS Channel Aware mode for speech), better error correction, or lower bit rate. In the last scenario this can benefit the resulting media quality if the terminal is near the edge of coverage and the link can better support transport of lower rate codec modes/configurations.