W.3 Adaptation to Packet Losses using Application Layer Redundancy

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

An MTSI terminal supporting the CHEM feature supporting use of application layer redundancy to improve robustness 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 ALR parameter in 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 ALR parameter in the PLR attribute in the media description for that media type in the SDP answer only if the ALR parameter in the PLR_adapt attribute was received in an SDP offer

In addition to requesting the media sender to adapt codec mode/configuration as described in clause W.2, the MTSI terminal may request the media sender to use non-zero application layer redundancy to improve robustness to packet loss. Clause X.2.3 provides examples of how the PLR_adapt attribute and ALR parameter (i.e., with application layer redundancy) is used in SDP.

When using CMR to request application layer redundancy the following CMR code points shall be used only if the MTSI client receives the ALR parameter in the PLR_adapt attribute in either an SDP offer or SDP answer:

Table W.3.1: Code points for AMR

CMR code	Application layer redundancy AMR request
9	RED 2×4.75
10	RED 2×5.15
11	RED 2×5.9
12	Not used
13	Not used
14	Not used

Table W.3.2: Code points for AMR-WB

CMR code	Application layer redundancy AMR-WB request
9	RED 2×6.6
10	RED 2×8.85
11	RED 2×12.65
12	Not used
13	Not used
14	Not used

Table W.3.3: Code points for EVS

CMR code	Application layer redundancy EVS request
111 0000	RED 2×7.2-NB
111 0001	RED 2×8-NB
111 0010	RED 2×9.6-NB
111 0011	RED 2×13.2-NB
111 0100	RED 2×7.2-WB
111 0101	RED 2×8-WB
111 0110	RED 2×9.6-WB
111 0111	RED 2×13.2-WB
111 1000	RED 2×13.2 CAM WB
111 1001	RED 2×13.2 CAM SWB
111 1010	RED 2×9.6-SWB
111 1011	RED 2×13.2-SWB
111 1100	RED 2×6.6-IO
111 1101	RED 2×8.85-IO
111 1110	RED 2×12.65-IO

To enable use of application layer redundancy the max-red SDP parameter should be selected in the SDP offer and answer to give at least some room for application layer redundancy adaptation and in accordance with clauses 6.2.2.2 and 6.2.2.3. The recommended redundancy offset for application layer redundancy should be 3 frames.