· Unfailing QoS (quality of service). The single most important factor in the delivery of acceptable videoconferencing over frame relay is protection of the video stream from frame drops.

· Adequate bandwidth. You'll need 384 Kbps or more for room-based systems, 128 to 256 Kbps for desktop systems and up to 56 Kbps for surveillance systems. This is comparable to the requirements for circuit-switched connections such as ISDN.

· CIR in the frame relay WAN. It must be 1 percent to 3 percent higher than selected bandwidth; you'll need additional transmission space to carry frame relay packet overhead without impeding delivery of the payload.

We began our testing with equipment from the two manufacturers that claim to support existing room videoconferencing systems: ACT and Memotec. Both solutions couple their FRAD with a framer--in fact, each vendor chose the same framer. In this category, our Editor's Choice award goes to the Memotec CX900e, an outstanding general-purpose WAN device that smoked through our testing.

If you don't already own a videoconferencing system, ABL Canada's VT2C Apollo system--an integrated codec (coder/decoder) in a box that outputs directly to a frame switch--offers an interesting alternative. Finally, we were impressed by RADVision's VIU-323 transcoder, a more standards-based alternative that encapsulates H.320 video into IP packets for transmission with other data over any network, including frame relay.

Watching the FRADs
Videoconferencing applications are like the little girl of nursery-rhyme fame: When FRADs are good, they can be very, very good--but when they are bad, they are horrid. After proper configuration, the Memotec CX900e combo performed flawlessly. Even when we flooded the concurrently running Ethernet with so much traffic that 98 percent of data packets were thrown away, the video stream rolled merrily along at 30 frames per second with no evidence of tiling faults or frame drops.

On the other hand, ACT's SDM-9300 FRAD pair had a bad test week, even though we extensively reviewed every configuration parameter with the company's engineers. Despite the setbacks, we don't believe there is a fundamental design flaw in ACT's FRAD hardware, or in most other FRAD hardware on the market today; software improvements arrive on the scene frequently. We think its showing is more a reflection of how vitally important it is that the FRAD offer the proper level of prioritization and buffer management. Unfortunately, in our testing, this release of ACT's software couldn't make the grade.

Framing the Picture
For our tests, we used a PictureTel Concorde videoconferencing system. Preparing video for the frame network requires a framer that packetizes output from the codec: We obtained a video framer from Memotec to work in front of its CX900e, and an AVI-2000 from ACT to complement the ACT SDM-9300 FRAD. To our surprise, these were actually identical boxes--only the cover paint and logos were different. We discovered that rather than build a video framer of their own, both FRAD manufacturers use the Science Dynamics VFX-250S to become "video-capable."

The video framer interacts with the RS449 output of data terminal equipment (DTE)--in our case, the PictureTel system. The VFX-250S provides clock to the DTE at 64, 96, 128, 192, 256, 384, 512, 768, 1,024 or 1,536 Kbps. It supports continuous full-duplex data transfers at up to 2,048 Kbps.

Normally, the VFX-250S's selected clock is locked to a submultiple of the network clock. We used 384 Kbps when running with the Memotec CX900e, but had to drop to 256 Kbps to accommodate ACT's SDM-9300 FRAD. Another option, which we did not choose, allows the VFX-250S automatically to vary the output clock rate to the DTE by slight increments as required to maintain slip-free data transfer.

The VFX-250S creates standard frame relay frames with 6-byte headers and user-selected packet lengths. Short packets (for example, 256 bytes) add frame overhead but have less impact on the audio/video stream if they get lost. Long packets (up to 1,580 bytes) are more efficient but increase delay. We ran ACT's AVI-2000 (VFX-250S) framer both at the ACT-recommended setting of 256 bytes and at 1,500 bytes (which is in line with the recommendations of other framer users). It made no difference in ACT's overall performance.

You can connect either the VFX-250S or the ABL VT2C directly to a DSU/CSU on a frame relay network. The two products have another common trait: If you intend to share the video stream with other types of traffic, you'll need to connect the VFX-250S or ABL VT2C to one of the inputs on a FRAD or router acting as a frame relay switch.