What Is...DSL and xDSL (Digital Subscriber Line) (a definition)

DSL and xDSL (Digital Subscriber Line and its variations)

DSL (Digital Subscriber Line) is a technology for bringing high-bandwidth information to homes and small businesses over ordinary copper telephone lines. xDSL refers to different variations of DSL, such as ADSL, HDSL, and RADSL. Assuming your home or small business is close enough to a telephone company central office that offers DSL service, you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second (of a theoretical 8.448 megabits per second), enabling continuous transmission of motion video, audio, and even 3-D effects. More typically, individual connections will provide from 1.544 Mbps to 512 Kbps downstream and about 128 Kbps upstream. A DSL line can carry both data and voice signals and the data part of the line is continuously connected. DSL installations began in 1998 and will continue at a greatly increased pace through the next decade in a number of communities in the U.S. and elsewhere. Compaq, Intel, and Microsoft working with telephone companies have developed a standard and easier-to-install form of ADSL called G.Lite that is accelerating deployment. DSL is expected to replace ISDN in many areas and to compete with the cable modem in bringing multimedia and 3-D to homes and small businesses.

How It Works

Traditional phone service (sometimes called "plain old telephone service" or POTS) connects your home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair. Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's signalling is already set up for this analog wave transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. That's why your computer has to have a modem - so that it can demodulate the analog signal and turn its values into the string of 0 and 1 values that is called digital information.

Because analog transmission only uses a small portion of the available amount of information that could be transmitted over copper wires, the maximum amount of data that you can receive using ordinary modems is about 56 Kbps (thousands of bits per second). (With ISDN, which one might think of as a limited precursor to DSL, you can receive up to 128 Kbps.) The ability of your computer to receive information is constrained by the fact that the telephone company filters information that arrives as digital data, puts it into analog form for your telephone line, and requires your modem to change it back into digital. In other words, the analog transmission between your home or business and the phone company is a bandwidth bottleneck.

Digital Subscriber Line is a technology that assumes digital data does not require change into analog form and back. Digital data is transmitted to your computer directly as digital data and this allows the phone company to use a much wider bandwidth for transmitting it to you. Meanwhile, if you choose, the signal can be separated so that some of the bandwidth is used to transmit an analog signal so that you can use your telephone and computer on the same line and at the same time.

Splitter-based vs. Splitterless DSL

Most DSL technologies require that a signal splitter be installed at a home or business, requiring the expense of a phone company visit and installation. However, it is possible to manage the splitting remotely from the central office. This is known as splitterless DSL, "DSL Lite," G.Lite, or Universal ADSL and has recently been made a standard.

Modulation Technologies

Several modulation technologies are used by various kinds of DSL, although these are being standardized by the International Telecommunication Union (ITU). Different DSL modem makers are using either Discrete Multitone Technology (DMT) or Carrierless Amplitude Modulation (CAP). A third technology, known as Multiple Virtual Line (MVL), is another possibility.

Factors Affecting the Experienced Data Rate

DSL modems follow the data rate multiples established by North American and European standards. In general, the maximum range for DSL without repeaters is 5.5 km (18,000 feet). As distance decreases toward the telephone company office, the data rate increases. Another factor is the gauge of the copper wire. The heavier 24 gauge wire carries the same data rate farther than 26 gauge wire. If you live beyond the 5.5 kilometer range, you may still be able to have DSL if your phone company has extended the local loop with optical fiber cable.

The Digital Subscriber Line Access Multiplexer (DSLAM)

To interconnect multiple DSL users to a high-speed backbone network, the telephone company uses a Digital Subscriber Line Access Multiplexer (DSLAM). Typically, the DSLAM connects to an asynchronous transfer mode (ATM) network that can aggregate data transmission at gigabit data rates. At the other end of each transmission, a DSLAM demultiplexes the signals and forwards them to appropriate individual DSL connections.

Who's Offering It When

Here is a sample of current and planned offerings in the U.S. DSL is also being offered in the UK and elsewhere.

According to Flashcom, it is now the largest and fastest growing provider of DSL service in the United States. Service is now available in all major US cities. Pricing starts at $49.95 monthly and includes Internet access. In most locations, installation and customer premise equipment is free with a two-year agreement.
Beginning in April 2000, FreeDSL will offer free hardware and setup and no monthly charge in a number of markets. For the service, users must agree to provide personal information for demographic use and to have a small navigational bar containing advertising always visible while connected. To get the free DSL modem, you need to refer 10 people to the FreeDSL site. There may be other requirements.
In the Midwest United States, Primary Network is offering DSL service to St. Louis, Missouri-area residents and businesses. Primary Network plans to become the largest Midwest provider of DSL service. Primary's DSL Accelerator service offers download maximums between 384 Kbps and 1.54 Mbps. Upload maximums are between 128 Kbps and 384 Kbps. Prices start at $49.95 monthly and include Internet Access. For more information, please visit http://www.primary.net/dsl/.
Bluestar Communications is currently offering DSL in Nashville and Memphis, Tennessee, and in Louisville and Lexington, Kentucky, and plans to offer service in 25 other cities by early 2000.
SBC Communications plans to bring ADSL to over 8 million homes in California, Missouri, and Texas by the beginning of 2000. In California, over 255 telephone company central offices will provide service to 5 million homes and 900,000 businesses. In Missouri and Texas, SBC's Southwestern Bell company will upgrade 271 central offices for 3.2 million homes and 440,000 businesses. Customers will need a $198 "ADSL modem" and will pay a basic $39 a month on yearly basis for unlimited service, or $49 with access to the Internet. Business or high-demand users can pay more and get faster download and upload speeds. For the basic rate, users are guaranteed 384 Kbps downstream and 128 Kbps upstream. Power users can get up to 6 Mbps downstream and 384 Kbps upstream.
Bell Atlantic has announced plans for a wide deployment of ASDL in the Northeastern U.S. to both home and corporate customers. The service is currently offered in the Boston, Philadelphia, Pittsburgh, Washington DC, and Northern New Jersey metropolitan areas. The New York City metropolitan area launched in mid-July, 1999. Additional markets will be announced in the future. Bell Atlantic offers what it calls Personal Infospeed DSL at speeds of 640 Kbps downstream and 90 Kbps upstream for $39.95 a month, or $59.95 a month including Internet access. Professional Infospeed offers speeds of 1.6 Mbps downstream and 90 Kbps upstream at $59.95 per month, or $109.95 per month with Internet access. Power Infospeed provides up to 7.1 Mbps downstream and 680 Kbps upstream for $109.95 per month, or $189.95 per month with Internet access. Network equipment providers are Alcatel, Globespan, and Westell. Among PC manufacturers that will support Infospeed technology are Apple Computer, Compaq, and Dell Computer.
BellSouth is offering a splitter-based ADSL service in 30 markets through Network Service Provider (NSP) channels. BellSouth provides access to all DSL-qualified loops through a single asynchronous transfer mode (ATM) port in each of 13 LATAs in eight Southeastern states. Access One, BellSouth's service partner, has committed to deploy a minimum of 10,000 DSL lines to its customers over the next two years.
US West plans to offer DSL service in 40 cities in the western part of the U.S. Currently, DSL is offered in Portland, Oregon, and Seattle, Washington. US West uses CAP modulation but says they are equipped to support DMT if that becomes a standard.
GTE Corporation has offered ADSL to 1,000 living units in Marina del Rey, California since November, 1997. Downstream data rates are up to 1.5 Mbps and upstream up to 384 Kbps. Residences are charged $99 a month.
NETinc, a Canadian company, is deploying ADSL in Hamilton, Ontario, using Paradyne technology. Dowstream data rates will be up to 7 Mbps and upstream up to 1 Mbps. Service to residences will be about $50 a month, to corporations $200 a month.
@tlas Internet in Miami, Florida, was one of the first DSL providers in the United States. They report converting between 10 to 20 businesses per week from traditional T-1 and ISDN connections to "new way" DSL.
Optimum Communications furnishes both ADSL and HDSL in the Florida West Coast/Tampa area. Downstream data rates are up to 3.2 Mbps and upstream up to 1.2 Mbps. Monthly rates are about $99 a month.

The ADSL Forum offers a much more complete List of ADSL Trials and Deployments.

Hardware Offerings

Rockwell's Consumer DSL chipset is used in telecommunications equipment made by Nortel. Nortel sells the equipment to carriers and to Internet service providers and Rockwell sells the modems through the usual retail channels. The equipment offers a 1 Mbps data rate.
3Com and Texas Instruments offer a hybrid modem supporting both dial-up 56 Kbps and rate adaptive ADSL (RADSL).

Types of DSL

ADSL

The variation called ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. However, most Internet and especially graphics- or multi-media intensive Web data need lots of downstream bandwidth, but user requests and responses are small and require little upstream bandwidth. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or hooked-in TV set. In addition, a small portion of the downstream bandwidth can be devoted to voice rather data, and you can hold phone conversations without requiring a separate line.

Unlike a similar service over your cable TV line, using ADSL, you won't be competing for bandwidth with neighbors in your area. In many cases, your existing telephone lines will work with ADSL. In some areas, they may need upgrading.

CDSL

CDSL (Consumer DSL) is a trademarked version of DSL that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) but has the advantage that a "splitter" does not need to be installed at the user's end. Rockwell, which owns the technology and makes a chipset for it, believes that phone companies should be able to deliver it in the $40-45 a month price range. CDSL uses its own carrier technology rather than DMT or CAP ADSL technology.

FreeDSL

A service offering and not a technology, FreeDSL is a company offering free ADSL hardware and setup with no monthly charge for service. For the service, users must agree to provide personal information for demographic use and to have a small navigational bar containing advertising always visible while connected. To get the free DSL modem, you need to refer 10 people to the FreeDSL site. There may be other requirements. FreeDSL reportedly plans to offer optional premium services at a future time.

G.Lite or DSL Lite

G.Lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially a slower ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company. This saves the cost of what the phone companies call "the truck roll." G.Lite, officially ITU-T standard G-992.2, provides a data rate from 1.544 Mbps to 6 Mpbs downstream and from 128 Kbps to 384 Kbps upstream. G.Lite is expected to become the most widely installed form of DSL.

HDSL

The earliest variation of DSL to be widely used has been HDSL (High bit-rate DSL) which is used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions. For this reason, the maximum data rate is lower than for ADSL. HDSL can carry as much on a single wire of twisted-pair as can be carried on a T1 line in North America or an E1 line in Europe (2,320 Kbps).

IDSL

IDSL (ISDN DSL) is somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL.

RADSL

RADSL (Rate-Adaptive DSL) is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line.

SDSL

SDSL (Symmetric DSL) is similar to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It's symmetric because the data rate is the same in both directions.

UDSL

UDSL (Unidirectional DSL) is a proposal from a European company. It's a unidirectional version of HDSL.

VDSL

VDSL (Very high data rate DSL) is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission technology (CAP, DMT, or other) and its effectiveness in some environments is not yet determined. A number of standards organizations are working on it.

x2/DSL

x2/DSL is a modem from 3Com that supports 56 Kbps modem communication but is upgradeable through new software installation to ADSL when it becomes available in the user's area. 3Com calls it "the last modem you will ever need."

A DSL Summary Table

DSL Type	Description	Data Rate Downstream; Upstream	Distance Limit	Application
IDSL	ISDN Digital Subscriber Line	128 Kbps	18,000 feet on 24 gauge wire	Similar to the ISDN BRI service but data only (no voice on the same line)
CDSL	Consumer DSL from Rockwell	1 Mbps downstream; less upstream	18,000 feet on 24 gauge wire	Splitterless home and small business service; similar to DSL Lite
DSL Lite (same as G.Lite)	"Splitterless" DSL without the "truck roll"	From 1.544 Mbps to 6 Mbps downstream, depending on the subscribed service	18,000 feet on 24 gauge wire	The standard ADSL; sacrifices speed for not having to install a splitter at the user's home or business
G.Lite (same as DSL Lite)	"Splitterless" DSL without the "truck roll"	From 1.544 Mbps to 6 Mbps , depending on the subscribed service	18,000 feet on 24 gauge wire	The standard ADSL; sacrifices speed for not having to install a splitter at the user's home or business
HDSL	High bit-rate Digital Subscriber Line	1.544 Mbps duplex on two twisted-pair lines; 2.048 Mbps duplex on three twisted-pair lines	12,000 feet on 24 gauge wire	T1/E1 service between server and phone company or within a company; WAN, LAN, server access
SDSL	Symmetric DSL	1.544 Mbps duplex (U.S. and Canada); 2.048 Mbps (Europe) on a single duplex line downstream and upstream	12,000 feet on 24 gauge wire	Same as for HDSL but requiring only one line of twisted-pair
ADSL	Asymmetric Digital Subscriber Line	1.544 to 6.1 Mbps downstream; 16 to 640 Kbps upstream	1.544 Mbps at 18,000 feet; 2.048 Mbps at 16,000 feet; 6.312 Mpbs at 12,000 feet; 8.448 Mbps at 9,000 feet	Used for Internet and Web access, motion video, video on demand, remote LAN access
RADSL	Rate-Adaptive DSL from Westell	Adapted to the line, 640 Kbps to 2.2 Mbps downstream; 272 Kbps to 1.088 Mbps upstream	Not provided	Similar to ADSL
UDSL	Unidirectional DSL proposed by a company in Europe	Not known	Not known	Similar to HDSL
VDSL	Very high Digital Subscriber Line	12.9 to 52.8 Mbps downstream; 1.5 to 2.3 Mbps upstream; 1.6 Mbps to 2.3 Mbps downstream	4,500 feet at 12.96 Mbps; 3,000 feet at 25.82 Mbps; 1,000 feet at 51.84 Mbps	ATM networks; Fiber to the Neighborhood

Selected Links

2Wire offers a DSL Tutorial that includes a Bandwidth Simulator.

Randy Day's summary of xDSL Technology provides a practical overview, with special information for US West users.

OpNet, an Internet service provider in Philadelphia, answers these questions for their own users: What equipment do I need and how does it all go together?. (Note: Your own situation may be somewhat different.)

ZDNet's How DSL Works provides several good illustrations.

FreeDSL's home page explains how it it works.

Get to Know the xDSL Family from Ericsson's Fast-Lane Magazine describes how DSL has evolved.

DSL Reports is a non-vendor-affliated site that includes information about consumer experiences as well as other information. Some parts require registration.

Netspeed offers a good xDSL Overview.

The ADSL Forum, an industry group, offers an illustrated ADSL Tutorial.

For more on DSL Lite, see the home page of Aware, a small DSL company in Bedford, Massachusetts.

The ADSL Forum provides a developing VDSL Tutorial.

Another major manufacturer of DSL infrastructure and modems is Westell.

Keep up with US West's subscriber plans at the US West home page.

Selected Books

Walter J. Goralski's ADSL and DSL Technologies surveys all the kinds of DSL and how they work, and provides practical information as well.

This term was suggested by Ann R., Misa Radujko, Alex Shnitman, and Johan Thureson. Assistance was provided by Fred Calm, Stefan Holmstr, and Christine Hohenberger.
Last update: June 16, 2000