QuickStudy: Asynchronous Transfer Mode (ATM)

For many Fortune 1,000 companies, the choice of Asynchronous Transfer Mode (ATM) as the underlying transport service for their wide-area networks (WAN) is akin to the choice of cable service over rabbit ears on a television set. ATM handles many different types of data, including voice and video, on a single network. Thus, large companies generally use ATM for their WANs, says Lisa Pierce, an analyst at Giga Information Group Inc. in Cambridge, Mass.

One reason ATM works well with disparate kinds of data is that it's connection-oriented. A sender and receiver on a network set up a fixed path between each other before sending data, and the information arrives in the order it was sent.

Other protocols, such as TCP/IP, are connectionless. That is, they don't have fixed connections, so individual data packets may go to different destinations and they may be delayed or arrive in the wrong order.

Fixed-route transmission is what makes ATM optimal for real-time communication like voice and video, says Lawrence Orans, an analyst at Gartner Group Inc., in Stamford, Conn. With proper tuning, the quality of voice traffic will equal telephone quality, and video traffic will mirror cable television, says Orans. It's also easier to track and bill network usage.

An ATM network transfers data in 53-byte cells. Cell size never varies, and cells with the same source, destination and class of service parameters always follow the same path, as long as that path meets performance criteria. To deal with congestion or network failure, there are also pre-established secondary routes.

Real-time data takes precedence over non-real-time data on an ATM network. For example, if voice and e-mail traffic are sent simultaneously, the switch grabs the voice traffic before grabbing the e-mail.

In addition, the small, constant cell size enables data to be forwarded through the network more efficiently. Frame relay and other connection-oriented protocols transfer data in packets of varying lengths, which makes for variable delays between packet transfers, Orans explains. With ATM, those delays are eliminated, because the switch is "always looking for 53-byte chunks," Orans says.

But while ATM seems to be the perfect answer for WANs, many large companies are more likely to use a hybrid approach, such as Gigabit Ethernet over ATM, on LANs.

Piling another protocol on top of ATM may slow it down, but there's an advantage: Information technology staff is typically already trained and comfortable with Ethernet, says Esmeralda Silva, an analyst at International Data Corp. in Framingham, Mass. "It's easier to continue using a familiar technology than to learn a new one," she adds. ATM is also less necessary in LANs, which typically don't carry much real-time data, she adds.

ATM is also expensive. On networks where users don't do much videoconferencing, a cheaper approach makes sense, says Pierce.

Catalano is a freelance writer in Holliston, Mass.

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