Error recovery for Packet Audio and Video Henning Schulzrinne Columbia University New York, NY

Options for reliability retransmission redundancy interleaving forward error correction

Redundancy packet redundant coding offset in time but: lose coding state RTP packet header hi-bw content (e.g. MPEG audio) lo-bw content

Retransmission receiver requests missing packets by sequence number scalable multicast under development used by RTP H.261 codec usually, delay too high useful for passive listeners, recording devices

Forward Error Correction exclusive OR (XOR) or Reed-Solomon N > 1 redundant packets within block XOR: parity of other packets in block can recover if <= N lost XOR: low complexity media independent great for multicast

Interleaving typically, several frames per packet (header overhead!) split samples or frames across packets  packet loss affects smaller amount of voice data  increases delay, header overhead