ABL doesn't provide any data routing interfaces. You'll need an external FRAD or router to multiplex video and data traffic to the WAN. ABL claims to interoperate with both the ACT and Memotec FRADs we examined, but we did not attempt to verify this in our tests.

RADVision VIU-323 Video Interface Unit
The RADVision VIU-323 lets you take the Layer 3 approach to delivering video over a frame relay WAN, and it's worth considering if--and only if--you have a well-managed IP network with bandwidth to spare. The VIU-323 doesn't hook directly into the frame relay network; rather, it connects H.320-based systems to an IP network, and translates multimedia traffic into the H.323 protocol, the emerging standard for video and audio communications over packet networks. If and when your network is ready for it, this particular approach could prove to be the wisest investment.

The VIU-323 is not a video framer comparable to Science Dynamics' VFX-250S or ABL's VT2C, since it uses IP traffic (Layer 3). But few IP networks deliver the QoS guarantees that frame relay supports, so you'll be taking a big chance if your internetwork is not large enough and delivers packets inconsistently.

On the plus side, H.323 can introduce a new level of multivendor interoperability and enable you to use WAN bandwidth much more efficiently than native frame relay systems can muster.

Using the RADVision VIU-323, your H.320 videoconferencing system can participate as an H.323 terminal. If you have an H.323 gateway and other H.323-compliant desktop videoconferencing systems (DVCs), your H.320 room system can interoperate with one or more other DVCs in the enterprise.

To keep our testing of these disparate products as comparable as possible, we did not use an H.323 gateway device. At each end of our frame relay network, we had V.35 RS449 adapter cables to connect a VIU-323 to a PictureTel room videoconferencing system. A second RS366 connection allowed the PictureTel to control "dialing" through the VIU-323 to the other end. The LAN output side of each VIU-323 was connected to our Ethernet subnet. In the configuration of each VIU-323, we defined its "telephone number" (e.g., "1234" or "5689") and associated it with the box's IP number.

During testing of the PictureTel/VIU-323 connections, the Memotec CX900e FRAD directed traffic from the Ethernet over the frame relay network. As in all the tests, the FRAD multiplexed all traffic onto a single 396-Kbps CIR PVC on a 768-Kbps access circuit. The video stream's traffic was set to the highest priority possible on the CX900e.

Over the RADVision VIU-323, the PictureTel units ran at 30 frames per second without measurable degradation. This didn't surprise us; H.320-H.323 transcoding provides significant compression efficiency over a LAN. You'd have to run tests in your own enterprise LAN/WAN environment to observe how your competing traffic and bridge/ router delays will affect videoconferencing quality using this approach. However, we suspect that a connection such as our PictureTel/VIU-323 will maintain at least 15-fps performance using between 200- and 400-Kbps LAN bandwidth (see "Making H.323-to-H.320 Connections With Two Videoconferencing Solutions," www.networkcomputing.com/908/908r2.html).

Dave Brown is a videoconferencing consultant. Send your comments on this article to him at dave@dbec.com or to David Willis at dwillis@nwc.com.