3 September 1997
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