What is voice-over-ip?

Conversation between parties over ip.

Need to maintain conversation, min-delay.

Rule of thumb:

<150ms: good, >400 ms: bad

Implementation details

IP address, port no., encoding algorithms


Value-added services: call forwarding, screening, recording.

emergency services:

Problems, solutions

Packet loss

Maximize: low playout delay, low late loss rate

Choosing the right timeout

Determined by d, where d is packet timeout delay.

If long, we have less packet loss, less interactivity.

If short, we have more interactivity, higher loss.

Adaptive solution, minimize both packet delay and loss.

di = (1-ɑ) di-1 + ɑ (ri-ti)


Each ACK/NAK takes ~ one RTT.

alternative: Forward Error Correction (FEC)

  • Send enough bits to allow recovery w/o retransmission (2-d parity in ch.5)

  • Simple way to do so:

    Create redundant chunk by XORing n chunks.

    send n+redundant chunk.

    Reconstruct original n chunk, if 1 lost chunk.

Send 2 streams

one higher, one lower quality stream, interleave to conceal loss.

If packet lost, still have most of every original chunks.