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Adoption has been slow for both ISND and xDSL. Yet faster access on the last mile is a priority for remote workers and branch offices. Two experts take a look at how we got here, and ask whether it's going to get better.

The copper loop has been viewed as a barrier to high-speed services since divestiture. Judge Greene called it a major competitive bottleneck. It was designed to carry analog voice signals. Dial-up modems enable data to be transmitted at speeds up to 56 Kbps. Basic rate ISDN provides 144 Kbps. The asymmetrical digital subscriber line (ADSL) pushed the speed limit to 1.5 Mbps. Today, a number of new digital lines known as xDSL have been announced in an attempt to open up the bottleneck.



The most successful member of the DSL family, high-bit-rate digital subscriber line (HDSL), uses a bit-coding technique known as 2B1Q to simplify deployment of private T1 circuits for business customers.

Since its inception in the late 1980s, HDSL has garnered wide support among carriers. Local exchange carriers (LECs), who were under pressure from competitive access providers in the T1 marketplace, adopted it because it reduced their costs and made them more competitive.

HDSL can now deliver full T1 service over a single pair of wires instead of two. It has been adapted to provide data services to small business customers by adding a router at either end. There are more than a million lines of HDSL in service, and deployment is likely to grow for several more years.


The inventors of asymmetrical digital subscriber line (ADSL) adapted dial-up modem techniques to provide high-speed digital transmission over single-line copper telephone loops. These techniques, which included carrierless amplitude and phase (CAP), quadrature amplitude modulation (QAM), and discrete multitone (DMT), became the focus of a budding standards controversy that still dogs the xDSL industry today.

The problem for ADSL is that it never took on a shape that end users could identify. It started life as a means of delivering digitally compressed movies on demand. Then it was promoted as high-speed Internet access for single-line subscribers. Today it has many potential applications, but is still struggling to establish itself in the marketplace.

Another major issue, still not entirely laid to rest, is incompatibility between ADSL equipment and services based on different technologies. In 1994, the ANSI T1E1.4 committee decided to standardize on DMT even though most telephone companies were basing their ADSL evaluation trials on CAP, introduced in 1992 by AT&T Paradyne. However, even without this problem, it is unlikely that ADSL would have been widely deployed.

By far the biggest problem until the mid-1990s was that the industry focus was on fiber optics, not copper. Despite efforts on the part of the author and others to interest major vendors such as AT&T Network Systems in ADSL, little enthusiasm was apparent. Although the ADSL Forum struggled hard from 1995 on to interest the telephone companies in the new technology, only lukewarm support was generated. Everyone tried ADSL, but no one deployed it widely.


In 1996 several other digital-line solutions were added to broaden the DSL marketplace. These included symmetrical digital subscriber line (SDSL), rate-adaptive-DSL (RADSL), very-high-bit-rate DSL (VDSL), and something called IDSL. No one seemed to know whether the "I" stood for "integrated," "ISDN," or some other word. Recently, a stripped-down solution called ADSL Lite has surfaced at the urging of computer companies.

Along with the different service options, the number of vendors also increased. The original ADSL technology licensees were AT&T Paradyne and Amati, with Westell and Performance Telecommunications as equipment manufacturers. Since that time AT&T divested itself of Paradyne, and Amati has been acquired by TI. Today there are dozens of vendors, each of which makes a slightly different flavor of ADSL.

The net result of these alternative technologies, products, and companies is that both carriers and end users are confused. The letter "x" was added to DSL, to denote the existence of a family of digital-line solutions. Despite this broader based service offering, not one of the xDSL offerings yet appears to have found a substantial marketplace.


From 1992 until 1995, Bell Atlantic was the only regional Bell company to announce interest in ADSL. In 1996, US WEST announced that it would deploy ADSL as part of its "Interprise" network, using various options for business and residential customers. Around the same time, GTE launched an ADSL demonstration in Irving, Texas, to provide service to a technical bookstore, a public library, and other kinds of business customers. Most other telephone companies soon followed.

Today, it is politically correct to support copper. With nearly 150 million copper access lines out there, it could hardly be otherwise. The new regime is reflected in the fact that every major carrier is engaged in xDSL trials, and most have announced ambitious plans for deployment in the future. Several regional Bells placed a major order with Alcatel a year ago, while others have decided to work with Westell, Paradyne, and other vendors.

Yet all these plans are hedged with restrictions. It is not known how the xDSL market will play out, so no major carrier wishes to fully commit itself to xDSL. Consequently, vendors have to protect themselves by inflating the potential marketplace, continuously coming up with new xDSL iterations and qualifying their production plans.

Telephone company subscribers are not scrambling for new digital-line solutions, which have always faced a difficult marketplace. Business users have other alternatives, and residential customers often do not need the additional speed. As for its major application, no one knows whether today's Internet can support high-speed access by tens of millions of customers.

Will ADSL Lite rejuvenate the xDSL marketplace? The computer companies are banking on the fact that a simple, low-cost, easy-to-use-and-install version will enable PC and workstation users to connect to the network at high speed. They say that removing the splitter and reducing the speed will lead to widespread user acceptance. If so, ADSL has one more chance to open up the access bottleneck.


Two transceivers are required, one at the CO and one at the subscriber's premises, connected over a single telephone line. Conventional ADSL requires a box called a splitter, which separates the telephone handset from the computer. A new version, called ADSL Lite, being pushed by a Microsoft-led alliance, does not need a splitter.

Services include downsteam speeds from 1.5 to 8.0 Mbpps according to the type of equipment purchased. Upstream speeds range from 64 Kbps to 500 Kbps. Subscribers who live more than three miles from the exchange are unlikely to receive ADSL. Higher-speed ADSL suffers even more from distance restrictions.

Customers must buy their ADSL products--under the guidance of the service provider--from a number of vendors, some of which use incompatible technologies. Building wiring must often be upgraded to support ADSL signals. Service costs range from $50 to $200 per month, with an up-front cost ranging from $200 to $500 according to the speed required.

Not all telcos offer ADSL service today. Only a few thousand lines have been installed, mainly for market trial purposes. Although most carriers have announced plans to deploy ADSL, rollout is only in selected portions of their service areas. Potential users should contact their local provider for more information.
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Title Annotation:Technology Information; includes related article on user's guide to ADSL; Will Digital Copper Catch On?
Author:Stewart, Alan
Publication:Communications News
Date:May 1, 1998
Previous Article:Will digital copper catch on? ISDN.
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