General DataComm (GDC) sent us the most expensive encoder/decoder we tested. The ATM version of the MAC 500 is worth the price if you want the ultimate in quality and you've invested in ATM technology, in which latency ceases to be an issue. Most of the other units we tested -- the exception being Cisco's VN 2900 -- are based solely on IP and Ethernet. We put the MAC 500's Ethernet and ATM interfaces through their paces and found the ATM interface offers the best quality. That's not surprising because circuit-based networks like ATM do not have problems with latency deviation. Our test results, however, are based on the Ethernet interface to keep an even playing field. GDC ships the IP-capable and ATM versions separately because they require different firmware. GDC says, however, it will release by the end of the summer one set of firmware that will let the MAC 500 transmit on IP and ATM-based networks at the same time.

This big boy is designed for the most discriminating of video customers, such as television studios and others in the video industry. The MAC 500 has three slots for encoder/decoders and one slot for the network interface. The unit we tested had a DS-3 ATM interface, but GDC also makes an OC-3 interface for customers with high-bandwidth needs. An IMA (inverse multiplexed ATM) interface that will accept up to eight DS-1s is available if you need less bandwidth. The Ethernet interface is built into the control card. The MAC 500 is similar to the Minerva VNP-201 since each card contains both an encoder and a decoder.

The encoder/decoder accepts both S-Video and composite connections for input and output. Audio input is unbalanced, provided by a DB-9 connector that breaks into four RCA plugs: one each for right and left input and output. MPEG-2 encoding is supported in three resolutions: SIF, half D-1 and full D-1. Compression quality is broken down into four categories, depending on which frames are encoded. Intracoded (I) compression uses the highest bandwidth by encoding every frame; IBBP compresses frames at a higher ratio to create a lower bandwidth need (see "Gobs of GOPs").

Transporting our video over IP took less than five minutes to configure. By default, the units operate bidirectionally, but we chose to have one unit transmit only and the other receive only. The one thing that might cause some users headaches in the configuration is having to know not only the destination's IP address, but also which decoder/encoder to use. It is necessary to tell the MAC 500 which slot is used at both ends. This was easy for us because both units were sitting side by side. Deploying devices miles apart and then trying to remember which slot is connected to the correct output source could require a little coordination.

On the ATM network side, video can be transported over PVCs (permanent virtual circuits) or SVCs (switched VCs). Although SVCs provide a more efficient use of bandwidth -- they are torn down when not in use -- they can cause problems if the network goes down during transmission. To eliminate the wait while the call is being re-established, GDC has implemented PSVCs (persistent SVCs), which reconnect when the network comes back up.

ATM networks handle multicast traffic -- data that travels from one source to many receivers -- differently from IP networks. Because ATM is circuit switched, a path from transmitter to end point must be built. By comparison, IP relies on broadcast traffic to find receivers or transmitters. GDC makes sure your video data will get there no matter which ATM switches your network uses. Network multicasting lets the ATM switches route the traffic to its multicast destinations. If simple ATM switches are in the network, the MAC 500 supports logical multicasting, which does the decision-making itself.

The MAC 500's configuration interface resembles that of the Amnis NAC 3000 and 4000's telnet session. The MAC 500 uses a rather difficult text-based hierarchical interface. We prefer a nonhierarchical, browser-based interface, but you shouldn't be spending much time using the interface after you configure the device anyway.

The GDC unit has one more unique feature up its sleeve. It can sit on the edge of your network and transmit your LAN traffic via the Ethernet connection while it transmits video data over your WAN. Frame relay networks can also be connected to the MAC 500 and routed to the LAN or ATM interfaces. Although the MAC 500 is expensive, its feature set is sure to appeal to the most demanding segments of this market.

MAC 500, starts at $31,000. General DataComm, (514) 335-3015; fax (514) 335-1614. www.gdcmultimedia.com