Error recovery for Packet Audio and Video

Henning SchulzrinneColumbia UniversityNew York, NY

Options for reliability

retransmissionredundancyinterleavingforward error correction

Redundancy

packet

redundant coding offset in timebut: lose coding state

hi-bw content

(e.g. MPEG audio) lo-bwcontent

RTP packet header

Retransmission

receiver requests missing packets by sequence number

scalable multicast under developmentused by RTP H.261 codecusually, delay too highuseful for passive listeners, recording

devices

Forward Error Correction

exclusive OR (XOR) or Reed-SolomonN > 1 redundant packets within blockXOR: parity of other packets in blockcan recover if <= N lostXOR: low complexitymedia independentgreat 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