Everything's Coming up Packets
Public Switched Systems Are Becoming the Leading Edge in Wide-Area Networks
By Bruce Schneier
May 17, 1993
For many years, the only way for distant computers to communicate over the public telephone system was via a voice-quality link, either a dialup line or a point-to-point leased line. Big companies needed better connections, and several data communications standards, such as X.25, were developed to provide them on these lines. As networks expanded and applications required speedier transmission rates, time-division multiplexing (TDM) technologies stepped in to provide cheaper and faster data transfers on large-bandwidth circuits, often making it cheaper to lease a dedicated T1 line than to run several low-speed lines.
In recent years, increased transmission rates, moreintelligent network equipment and cleaner transmission lines have spurred the growth of packet-switching technologies. Unlike TDM networks, which give each user a data pipeline regardless of whether it is needed or used, packet-switching technologies make much more efficient use of communications channels by giving users access to an entire communications channel when they need it, for only as long as they need it.
Three packet-switching standards - frame relay, Switched Multimegabit Data Service (SMDS) and Asynchronous Transfer Mode (ATM) - have emerged to provide high-bandwidth communications while taking advantage of the economies of on-demand packet networks.
The Same, Only Different
Each of the three standards divides the local data stream into packets, which contain chunks of data, before sending it over the wide-area network. The packet acts as a data envelope, specifying a recipient and a return address. Most similarities among the three types of packet-switching networks end there.
Strictly speaking, frame relay and SMDS are not technologies but services that you buy from your local telephone company. The two services are based on different underlying technologies and suit different needs. Frame relay is based on fast packet-switching technology and ISDN (Integrated Services Digital Network) protocols, and it is best suited for distributed applications running among a relatively few fixed sites. SMDS is based on IEEE 802.6 metropolitanarea network technology and is geared toward more-flexible networks, where many different sites must be connected at different times.
ATM, on the other hand, is an emerging technology whose primary benefit is carrying time-sensitive data, such as video or sound, without interruption. ATM has attracted a lot of attention because it is intended for both WANs and LANs. Future Broadband-ISDN (B-ISDN) services will be based on ATM. B-ISDN is a fiber-optic public network service that will accommodate voice, data and video at once.
The three standards are different in other respects as well. Frame relay uses variable-length packets called, appropriately enough, frames. SMDS and ATM use short, fixed-length packets called cells. SMDS is "connectionless": Like correspondence sent through the mail, each cell of data can take a different path to the recipient. Frame relay and ATM are "connection-oriented": The network creates a fixed path between two endpoints for the duration of the communication, like a voice telephone call.
Compatibility, Obsolescence Not Problems
These WAN technologies work with AppleTalk networks, assuming the presence of an intelligent WAN router. The router encapsulates relatively high-level networking protocols in a packet network's low-level link protocol. With properly configured routers, AppleTalk networks sending data over packet-based WANS should experience no compatibility problems.
"Frame relay, SMDS and ATM don't care what the LAN protocol looks like; it's just a bunch of packets to them," said Dean Gahlon, systems programmer at Network Systems of Minneapolis.
Frame relay and SMDS services are available and expanding, and ATM is approaching fast, but network designers who want the largest bandwidth and best performance for the least amount of money can choose between the two current offerings without fear of buying into obsolete equipment.
"Carriers will install switches that can support SMDS, frame relay and native ATM interfaces," said Charles Gallucci, product manager at Netrix Corp., a network product manufacturer in Herndon, Va.
In fact, some equipment vendors are covering all their bets: designing ATM switches that can accommodate input and output in X.25, frame relay and SMDS.
As with any technology, waiting for the perfect solution to come along can be self-defeating.
"If you need networking, get what's available," said Jim Lane, president of TRAC Associates, a networking-consulting company in Sagamore, Mass. "It's going to be a while before ATM is common. Frame relay and SMDS are available now."
While observers say the regional Bell operating companies had been pushing SMDS at the expense of frame relay, now the companies are offering both.
"A year ago they changed their tune," Gallucci said. "They found that some customers weren't ready for SMDS."
SMDS' access speeds are bound to increase in the coming years as it migrates to the Synchronous Optical Network (Sonet) standard, which supports speeds in excess of 155 Mbps. Because of this and other technical similarities, many expect SMDS will migrate gracefully to ATM when the latter is available more widely. Most of the SMDS data service units being built today should be upgradable to ATM.
The App's the Thing
A typical technology rule of thumb applies well to the new breed of packet-switched WANs: Let your application be your guide.
Brian Fairness, network analyst at the University of Oklahoma in Oklahoma City, chose frame relay for the Oklahoma Medical Imaging Network, which uses Macintosh computers to transfer imaging data among rural hospitals.
"We have dedicated T1 lines running frame relay. When we start doing interactive video, we'll have to switch to ATM," he said.
Dave Stacey, president of Schism, a multimedia company in Los Gatos, Calif., is holding out. "ATM offers superior capability to transmit video and image traffic. Apple is trying to build more and more capabilities for multimedia, and that will translate to networking needs that require ATM."
For many applications, frame relay will remain the best datatransmission choice for years to come. Frame relay's current limitations in circuit allocation should be eliminated as the technology continues to evolve. Switched virtual circuits, which allow users to set up connections only when they are needed, will largely supplant the permanent virtual circuits in use today.
Gripes about Frame relay's speed limitations will likely disappear as transmission rates higher than T1 become cheaper and more available.
Even if ATM provides an all-encompassing voice and data-communications technology, organizations can continue to use frame relay where appropriate.
"Frame relay will continue to be the low-cost option that will become the front end for ATM," said Paul Callahan, senior analyst for network strategies at Forrester Research of Cambridge, Mass.
Photo of Bruce Schneier by Per Ervland.
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