If ISDN service were cheap and ubiquitous, it would be the unquestioned favorite for connecting VT systems. Two-way videoconferencing was the killer app that telephone carriers expected to sell ISDN, and manufacturers designed legacy videoconferencing systems to fit ISDN's bandwidth-on-demand call placement technique over switched digital networks.

These days, ISDN service is getting cheaper, but it's still not available everywhere. And in most cases, it's still hard for ISDN to compete with the cost-effectiveness of scaling up your enterprise's existing network versus buying discrete chunks of bandwidth from a public carrier.

But ISDN did have its influence on VT system evolution. H.261 codec-based room systems were designed to operate in a sweet spot (384 Kbps) where nearly full screen, full motion video could be delivered over three (relatively cost-effective at that time) inverse multiplexed BRI lines. Early desktop and small-group systems were aimed at the cost-effectiveness of 128-Kbps single-BRI operations. These speed selections (multiples of 64) still appear in H.323 LAN/WAN network designs because ISDN often is used for supplementary circuits.

ISDN's beauty is that it provides steady, unvarying bandwidth. Establish a 384-Kbps circuit, and it's like a smooth pipe. A CBR (constant bit rate) stream arrives at the destination with no jitter (i.e., rate variations). The ISDN circuit itself causes negligible delay in audio transmission. In fact, a Receive Path Delay loop must be calibrated in the codec to make audio lip-synch with the arrival of video frames, which may be delayed by the amount of time required for the video encoder to perform compression.

There is an important difference between H.320 and H.323 regarding bandwidth planning. Under H.320, everything-audio, video and T.120 collaboration and control information-is multiplexed into the guaranteed pipe. In an H.320 call, audio always is set up first and gets the guaranteed bit rate required by the algorithm in use. Video gets whatever is left over, unless T.120 services are being provided. Under H.320, whiteboarding and file transfer activities steal from the video bandwidth. If the collaboration activity requires updating a complicated or picture-filled whiteboard, video frame rates may slow or freeze momentarily.

Because present videoconferencing system technology evolved to perform satisfactorily with transmission speeds that range from 128 Kbps to 384 Kbps, you can safely constrain individual H.323 VT system calls to use between 200 Kbps and 400 Kbps for combined audio/video. In some extreme applications, you might get by with as little as 64 Kbps. Anything more than 400 Kbps is overkill. Video might look slightly better, but the result hardly will be worth the cost of extra bandwidth.