Network Computingwww.networkcomputing.com

	Issue TOC Print full article Print this page Download as PDF E-Mail this URL Flame the author     In this article Introduction Bandwidth Requirements Content Delivery Companies to Watch Standards Enterprise Speed Limits Web Links

Even as enterprises adopt gigabit per second switch technology, T1 leased lines--a technology holdover from the 1960s--continue to be the most common bandwidth enterprises use to connect LANs and tap the Internet. T1 lines carry both voice and data over 24 channels of 64 Kbps with TDM (time-division multiplexing).

Enterprises that need more bandwidth than T1's 1.54 Mbps can install additional T1 lines or lease fractional T1s in two-channel, 128-Kbps increments. But at approximately $800 per Mbps, speed can kill. There has been movement from private leased lines to less expensive alternatives in shared networks, such as frame relay and ATM. But frame relay is a poor choice for voice traffic because it lacks the guaranteed quality of service found in ATM networks.

ATM remains the only way to deliver data, video and voice over one network with proven QoS (quality of service). As the demand for converged network applications increases, the U.S. market for this technology is projected to grow from $4.7 billion in 2001 to $9.9 billion in 2005, according to the 2002 Telecommunications Market Review and Forecast.

By breaking down information into 53-byte cells, ATM gives voice traffic a high priority without forcing it to wait behind long data packets, the way Ethernet does. A 5-byte header atop a 48-byte payload establishes priority, routing information, packet-sequencing and error-checking. Unlike Ethernet, ATM is switched in hardware without table lookups and therefore reduces latency, another crucial factor in dealing with time-sensitive video and voice traffic. Various media types, such as twisted pair, coaxial and fiber optic cable, support ATM, which scales from 56 Kbps to OC-12 (622 Mbps).

Because installing ATM is more expensive and complex than installing Ethernet, ATM is most commonly used in carrier backbone networks. But 12 percent of the U.S. ATM equipment market consisted of enterprise WAN switches in 2001 (2002 Telecommunications Market Review and Forecast). Enterprises use ATM for multimedia transmissions and videoconferencing. For example, ATM interconnects large health care organizations to transmit X-rays and images and to engage in videoconferencing.