Notes for Team 1

Remarks

• Use UDP sockets for data transfer. Use RTP (Real Time Protocol) for your transport protocol.
• The main job of this team is to capture voice samples from the microphone, encode it (into PCM or the like) or not, get the bits across the net. See "vat", free voice-terminal software from LBL (ftp.ee.lbl.gov, in conferencing/).
• Allow for multicast.
• Playout at the other end turns the bits into voice samples. One thing to decide is how to handle pauses in the conversation. When no packets are being sent, you should record silence or white noise.
• You should buffer incoming packets in order to smooth the playout. This requires a histogram of delay values, the tail of which corresponds to discarded packets. That is, a few packets take such a long time to arrive that they are useless for playback.
• You don't have the histogram yet; try to experiment to find out what it is. Or program your stuff to dynamically adjust to a dynamically changing histogram.
• Refer to the Columbia website for some RTP tools.
• Playout has to handle bursts and dropouts. A trick involves storing hi-res and low-res samples in the same packet, one for sample i-1 and the other for sample i. If you missed sample i-1, you can find a low-res copy of it in the packet for sample i. A couple of these schemes have been implemented at inria.fr
• Prof. Keshav might set up a long-distance reflector to help you experiment with long-haul conversations. Or you could use the simulator.

Questions

1. Should we use the simulator or the real internet? Start coding on the simulator where you can introduce arbitrary delay and distance. The final project implementation will be on the real internet.
2. How can we use the real internet when we can't ship UDP packets through the firewall? Prof. Keshav will put up an audio machine outside the firewall if necessary. Do most of your work on the simulator. Or, run entirely within the firewall.
3. What timestamps does RTP use? It uses those of the sender. You can use RTSP to sync timers and clocks. These days PC's do not gain or lose time very much, so syncing up time stamps is not likely to be a big problem.
4. How bad is dropping packets to the audio? Dropped packets cause a pause, which is not audible if less than every 10 seconds or so.