Bradner's column: Hope for Digital Subscriber Line technology?

As a columnist for a technical publication, I am presented with a surfeit of opportunities to speak to marketing droids from companies that claim to have answers to questions I didn't even know I had. (A disclosure: I do try to arrange some of the visits for lunch time so I can at least get a few free meals out of the encounters.)From time to time, these meetings actually result in useful information. One such meeting took place recently when Rick Gilbert, CEO of digital subscriber line (DSL) equipment vendor Copper Mountain, came by. (Unfortunately, the only time that worked was early morning, so no free lunch.)I've been hearing horror stories for the past few years about the problems encountered when ISPs and other companies attempt to deploy various types of DSL connections to their customers. Particular problems have included difficulty in getting wires from the phone company that are clean enough to provide good performance and potentially severe cross talk that results from having more than one DSL link in the same cable bundle running down the street. I've also heard tell of significant distance limitations.

Of course, the most severe challenge to DSL's future is that in general it is a technology empowered by the aggressive, innovative environment common among telephone companies. Not!

DSL is seen by these companies as a technology for providing mixed voice and data services over the same line. As a result, the version of DSL they are working on uses ATM (asynchronous transfer mode) to multiplex the services and is tightly tied to the voice world. In addition, the same people in the telephone companies who brought you ISDN are involved in bringing you DSL.

Now there's a potentially fatal burden if there ever was one.

There seem to be dozens of DSL flavours, and not all of them have the same set of issues. ISDN DSL (IDSL) and Symmetric DSL (SDSL), the types of DSL that Copper Mountain and others are promoting, use the same on-the-wire technology (2B1Q) that ISDN uses.

This means, among other things, that the cross talk problems that plague some types of DSL are not a significant issue. The ability to run at a reasonable speed - 128Kbit/sec over 22,000 feet of wire and faster over shorter cable runs - means far better coverage than DSL versions that have shorter limits.

More than 99 per cent of all customers are within 22,000 feet of a phone central office, and this percentage drops to less than 50 per cent for 12,000 feet.

But the thing that seems most promising about the Copper Mountain approach is that the company sees this as a data service and not some mixed-media service. Copper Mountain is dealing with ISPs, not telephone companies, and uses frame-based transport rather than ATM. Frame-based is less expensive and less complex to deal with than ATM.

DSL deployment numbers are still small, far smaller than cable modem deployment numbers, for example. But developments such as those taking place at Copper Mountain and perhaps the DSL-lite technology under development by Microsoft and others, may mean that DSL will have a better future than it has a present.

Disclaimer: Harvard has a long history of dealing with the future but has no opinion on DSL technology.

Scott Bradner is a consultant with Harvard University's University Information Systems. He can be reached at sob@harvard.edu.

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