The following text is copyright 1998 by Network World, permission is hearby given for reproduction, as long as attribution is given and this notice is included.

Is there hope for digital subscriber line
technology?

By Scott Bradner

Network World, 11/09/98

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 two weeks ago 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 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 flavors, 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 - 128K bit/sec over 22,000 feet of wire and
faster over shorter cable runs - means far better coverage than DSL
versions that have shorter limits.

Over 99% of all customers are within 22,000 feet of a phone central
office, and this percentage drops to less than 50% 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.