April 1998

xDSL Technology and The Internet (Part I)

Copper Applications in Electronic & Communications

By Bill Black


xDSL is a technology which enables ordinary voice-grade copper telephone wires, used to service more than 600 million customers worldwide, to also carry high-speed data traffic at the same time, thus obviating the need for expensive system upgrades.

Not long ago those small copper twisted-pair wires run by the local phone company to virtually every home and business in the USA, and throughout the developed world, were thought of as yesterday's technology. One major telephone operating company was even organized into departments essentially described as old technology-copper-and new technology-fiber. It was common knowledge that copper was "noisy" and could only carry voice, and perhaps faxes. The error-free transmission of high-speed data was going to require fiber optics technology, no question.

Those making predictions about the need to invest tens of billions of dollars to upgrade to all-fiber phone networks were, to put it politely, misinformed. Copper telecommunications cabling is in fact today's telephone, Internet, data, security and entertainment wiring-and tomorrow's as well.

This article discusses developments using the newly appreciated capabilities of copper twisted pairs in the outside plant of the telephone network, i.e. outside the customer's facility, be it residential or non-residential. The technology to be examined is DSL, or digital subscriber line, technology. DSL can have several permutations, collectively known as xDSL. For information on inside, or premises wiring for the modern residence, see a companion article in Innovations .

This overview article will be followed in subsequent issues of Innovations by more-detailed articles concentrating on various aspects of xDSL, such as the technology itself and its various permutations; xDSL's deployment by telephone operating companies; and the evolution of various applications other than simple hookup to the Internet. As always it is the intent and purpose of Innovations to describe these and other subjects in simple, understandable terms. If you the reader would like to have some specific subject addressed, please let us know.


The fundamental work leading to Digital Subscriber Line (DSL) technology was done in the early 1980s by AT&T's Bell Labs. Later, after the AT&T divestiture, the work was assumed and brought to a practical level about 1989 by Bellcore, at the time the research facility owned by the RBOCs. (See the Glossary for an explanation of the acronyms used in this article.) At present the main focus of xDSL development and marketing, other than within the microchip, equipment and telephone companies themselves, is the ADSL Forum (ADSLF), a group to which virtually everyone in the business belongs.

Without going into technical detail, xDSL encodes a stream of digital data so as to maximize the use of the available bandwidth and continuously monitors the transmission, even "cleaning it up" by detecting and correcting errors. This continuous encoding, sampling and adjusting of the data stream requires modern VLSI high-speed chip technology. Such features as equalization and echo cancellation are required to identify and correct errors that occur during transmission.

Two main encoding schemes are being used for xDSL: discrete multi-tone (DMT) and carrier-less amplitude/phase modulation (CAP). A later article will explore these schemes, but suffice it to say that DMT appears to be preferred at this time, with its preliminary acceptance by key standards bodies and its adoption by most RBOCs in their early rollouts of xDSL technology. DMT also has the advantage of chopping the available spectrum up into multiple channels and, when electromagnetic noise or other impediments to the free flow of digital bits are detected, changing channels "on the fly" to go around the trouble.

Encoding enables the available spectrum, expressed in kilohertz or megahertz, to be used more efficiently. Basically these schemes permit multiple bits of information to be transmitted by each hertz (Hz) of bandwidth.

Several variations of xDSL technology are becoming commercially important. Early deployments to the residential market will be either ADSL or RADSL, as described below.

ADSL (see Glossary for descriptions) devotes a larger part of the bandwidth to downstream applications, since most residential Internet users will need it to download graphics-heavy images. The speed of the downstream channel can vary widely, up to about 6 Mbps, depending on the vendor and the specific service. The upstream bit rate is always less, hence the name asymmetric DSL. At the lower end of the spectrum is a 4 kHz channel devoted to voice, and functionally independent of the data stream. In fact, if the local electric-utility power goes down, the voice channel will remain active, since it is still powered (as now) by 48 volt batteries, while the DSL channels will become disabled.

RADSL is a variation of ADSL in which the equipment can sense how high a bit rate the line can handle and adjust the speed downward accordingly. This "safe speed" can either be determined by testing the line during installation and locking it in at a set speed, or it can continuously adjust itself during operation.

HDSL has been used for several years in commercial applications, and typically transmits 1.5 megabits per second (Mbps) each way. Since it requires two wire pairs, it is not considered applicable to the residential market.

SDSL is a symmetrical form of xDSL, but at roughly half the speed of HDSL and using one wire pair instead of two.

VDSL is a high-speed version of xDSL, but only for short distances, perhaps a mile or less. For very short distances, speeds as high as 50 Mbps can be attained. A basic law of physics states that the shorter the distance the higher the speed that can be attained.

Main Features

Perhaps the biggest advantage of xDSL is its speed-typically from one to two orders of magnitude faster than dial-up modems. The apparent difference is fairly minor for the transmission of text, but for graphics is dramatic.

Always On
One primary advantage of xDSL technology is that it is always on for the customer-24 hours per day. There is no need to dial it up each time the user wants to log into the Internet or access the company network. Surprisingly, this is also an advantage for the phone company, since the xDSL signals passing through its central office bypass the switches used to connect phone calls, thus relieving congestion in the plain old telephone service (POTS) network, commonly known as the public switched telephone network, or PSTN. With dial-up modems, congestion is becoming a major problem for local phone companies, since typical Internet users stay connected for long periods of time.

Simultaneous Voice and Data
Another advantage is that xDSL uses different parts of the available spectrum to carry voice and data, and each can be used simultaneously, independent of the other. Thus, even though the computer is being used, phone calls can be made, and vice-versa. Most early systems use a piece of hardware at the customer premises known as a splitter to separate the two streams of information. However, splitterless xDSL, known at present as "ADSL Lite", is rapidly being developed and deployed, primarily at the behest of a powerful new group known as the ADSL Universal Working Group (UAWG), made up of Intel, Microsoft, Compaq, the large local telephone companies, equipment manufacturers and others. Not requiring a splitter will eliminate one piece of equipment and, perhaps even more important, may eliminate the need to dispatch a truck to the customer's location for installation.

Only Installed Where Needed
For the phone companies, xDSL technology can be added incrementally. There is no need for a large investment in plant and equipment, since the main addition is a black box at each user's location and a larger one, known as a digital subscriber line access multplexer (DSLAM) at the telephone central office (CO). Of course, other investments in digital data networking equipment, such as asynchronous transfer mode (ATM) or its alternatives, must be made, but this is already occurring. High "take rates" are advantageous for the phone companies, but can be built over time in a controllable manner.


In the early days of xDSL technology, video-on-demand (VOD) was visualized to be the application that would drive the technology. Today VOD is all but forgotten as the Internet has emerged as the "killer app." xDSL represents a quantum leap forward in high-speed access to the Internet, where high-bandwidth graphics play an increasingly important role. Although many downstream (Internet-to-user) speeds are being deployed by various telephone companies, many are focusing on 1.5 megabits per second, which is 27 times as fast as a 56 kilobit per second modem. Thus, a five minute download is reduced to 11 seconds, provided some other link in the Internet system doesn't slow down the transmission.

The second most common xDSL application is remote LAN connection. This enables telecommuters to talk to the office network at speeds essentially equal to what they would have available at the office itself. Video conferencing becomes possible if speeds higher than about 400 kbps are available. For the healthcare industry xDSL technology enables high-resolution images to be transmitted, perhaps for study and diagnosis by a specialist in real time. Educational applications will enable students to learn in a remote setting. Many other applications will be added as the technology becomes widely adopted.


After many years of promise, deployment of xDSL by all the large US telephone operating companies is proceeding at a brisk pace in early 1998. Many smaller companies also have plans to deploy the technology and many competitive local exchange carriers (CLECs) hope to beat the incumbent local exchange carriers (ILECs) to the punch. Some have strategies aimed particularly at business customers. A later article will detail the varied deployment strategies by company.

The Competition

The telephone companies don't have the field to themselves in providing Internet service. One competitor is here now, cable modems, while the other, wireless systems, is just over the horizon.

Cable Modems
An estimated 70% of US homes are cable TV subscribers. (This compares to 95%-plus who have a telephone.) In those cable-based Internet systems that have been announced to date, one or more cable TV channels are devoted to these data services.

Upstream communication can be a problem since coax works much better as a one-way system. The cable TV system uses coaxial cable into the home, with a mixture of coax and optical fiber in the outside plant. A given length of coax has considerably more capacity than the copper twisted pairs of the phone system. However, this capacity is shared among many users as it loops around a neighborhood, so available bandwidth can decrease dramatically as more users come online. Security can also be more of a concern than with the "home runs" of copper twisted pair loops. Whether there will be a clear winner in this battle is very much up in the air, but many natural advantages lie with the phone companies and xDSL.

Wireless Systems
Serious attention is being given to Internet and other data streams coming into the residence on selective channels of low-earth-orbiting satellite systems, at which point the signals can be moved onto twisted-pair or coaxial lines, or even be moved throughout the building on wireless signals. Such systems may have particular applicability in rural areas where cable TV is not available and where the customer may be so remote from his central telephone office that xDSL cannot be deployed.


xDSL technology is rapidly coming to market in the USA, and is expected to revolutionize the use of the Internet among home users and telecommuters. One version, HDSL, has been successfully used in commercial applications for several years already, but is not applicable to the residential market. Two versions are being deployed to the home-ADSL, with higher speeds downstream than upstream, and a variation of ADSL known as RADSL, which allows the equipment to sense the speed that can be delivered and then throttles down the transmission to match. (RADSL allows more distant users to be served by the technology.) Cable modems and wireless systems both will provide vigorous competition to xDSL systems deployed by local phone companies.

xDSL technology allows the use of the copper twisted pair network already in place to virtually every home in the USA for high-speed data traffic, piggybacked on the voice network already in place. Besides the Internet, applications include remote LAN access, video conferencing, medical diagnosis, distance learning, video on demand and others yet to be imagined.

xDSL Glossary

Also in this Issue:


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