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RFC 2198 on RTP Payload for Redundant Audio

The RTP Payload for Redundant Audio Data, RFC 2198, was published today. This is an IETF proposed standard RFC that describes the redundant packet transmission forward error correction scheme developed in RAT.

Packet loss in an IP based network, such as the Internet or the Mbone, can result in broken up audio, which rapidly becomes unintelligible as the loss rate increases.

The mechanism described in this RFC sends a (more) heavily compressed copy of a packet is piggy-backed onto the following packet. If the original packet is lost, the redundant copy can be used in its place. Because the redundant packet is very heavily compressed, sound quality suffers, but is still better than having no audio to play out in the place of the lost packet. Clearly, there exists a trade-off between the amount of compression used for the redundant packet (and hence stream bandwidth/overhead), and the quality of the resultant audio.

The underlying ideas, and their evaluation, are described more in the following papers: