How Much Bandwidth To Buy

The PictureTel Concorde videoconferencing system that we set up for our tests performed admirably, but any modern H.320 standards-compatible system capable of 30 frames per second (fps) operation would likely perform just as well over a 384-Kbps channel. We used 396-Kbps CIR (committed information rate) to allow for frame relay packet overhead.

The value differences in room-based videoconferencing systems depend largely on the camera(s), monitor(s) and microphone system(s) placed ahead of the codecs. Manufacturers distinguish their products with features such as intelligent audio noise cancellation or automatic "camera follows voice." Some offer supplemental collaboration services: whiteboarding software, fax channels and data file transfers over the conference connection. But all of this is multiplexed within the videoconferencing system.

The network video framing device gets a constant bit rate stream, which is clocked at a preset speed.

A videoconferencing application can afford little to no loss. Beyond compressed audio and video frames, there may be T.120-standard information for collaboration and remote camera control. In the H.320-related signaling scheme, audio gets top priority, any T.120 traffic comes next and video gets whatever is left over.

When you negotiate your network service-level agreement with your carrier, you must look at everything likely to be carried within the CBR stream and sent to the network video framing device.

In our tests, we sent only audio and video. The ACT AVI 2000 framer with SDM 9300 FRAD, which had a speed limit of 256 Kbps, constrained the video only slightly--down to 26 fps. The Memotec Video Framer, running on a 384-Kbps connection to the CX900e FRAD, supported 30 fps. Modern H.320 codecs producing 30 fps need all of this bandwidth. If you want to ensure that video won't freeze momentarily when camera controls or collaboration traffic is also on the line, consider a 496-Kbps payload and buy a 512-Kbps CIR.

RemotEVU: High Quality At THE low end

In addition to the four products in our main test, we investigated Motorola's RemoteVU, a low-cost, low-speed video-over-frame-relay system that shows what an excellent medium frame relay can be for low-bandwidth applications, such as remote surveillance.

The RemoteVU is a codec (coder/decoder) packaged on a daughterboard that plugs directly into any entry-level or midrange FRAD (frame relay access device) in Motorola's Vanguard Family. The Vanguard 320 FRAD is used in typical retail applications where many surveillance cameras feed into a video switch at one location. Each RemoteVU daughterboard can support four cameras, with RS-422 control of pan-tilt-zoom.

Functionally, the RemoteVU daughterboard is similar to ABL's VideoExpress device, except that the RemoteVU's onboard codec uses a proprietary heuristic algorithm that is incompatible with industrywide H.320, H.323 and H.324 standards. But that's not a problem if you have compatible Motorola server software in your home office.

Motorola's RemoteVU technology is optimized for security applications. It uses fundamental HDLC framing over the network. The codec scales resolution and sets a constant frame rate appropriate to the committed bandwidth, which can be set as low as 2.4 Kbps or as high as 115 Kbps. A field motion-sensing option causes video transmission only when activity is detected.

Server software in the home office can record and display security video in black-and-white or color, at up to 320 x 240 pixel resolution (or 640 x 480 with bit-fattening, a process that displays each pixel at double-size). Motion histograms also can be developed and analyzed to provide a nonlinear method for quickly reviewing recorded video.

The RemoteVU daughtercard sells for $900. For more information, contact Motorola at (508) 261-4000; fax (508) 261-4545. www.mot.com/ing