A Big Hurdle: Network Latency
There's a fundamental difference between transmitting a videoconference over ISDN and over an IP network. ISDN connections are clean and steady, while IP is bursting with variable delays. Outside the LAN, IP packets travel through gateways and routers on the Internet, which introduce transit delays. Delay variation (jitter) is much worse than slow (but steady) transmission on the perceived quality of a videoconference.
People are somewhat tolerant of video degradation. Audio is another story. Human factor studies have shown that people start to get annoyed when end-to-end audio delays approach 400 milliseconds (ms), and 700 ms to 800 ms delay is beyond human tolerance. Ideal quality of service (QoS) is considered to be less than 45 ms delay, plus variance in a one-way audio stream. Video can be up to 95 ms delay, plus variance, before frame-freezing sets in.
W
hile testing the Intel and PictureTel systems, we ping-tested in the San Mateo, Calif., lab and measured only 8 ms to 10 ms delays. When testing between San Mateo and the University of Wisconsin lab on our frame relay network, delays varied between 85 ms and 120 ms. Audio and video quality remained acceptable.
However, in the home office tests when we connected to San Mateo over the public Internet (a tester's home to ISP connection running at 128-Kbps ISDN), delays were measured in the 130 ms to 140 ms range. To obtain barely acceptable audio, we had to throttle the Intel Business Video Conferencing System back to 64 Kbps. In this configuration, video lip sync was a full one to two seconds ahead of the audio. We couldn't hold down PictureTel's LiveLAN's video to run effectively over a 128-Kbps network connection, so we only tested audio across this link. --Dave Brown
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