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

2. Options for reliability
retransmission
redundancy
interleaving
forward error correction

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

4. 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

5. 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

6. 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