Overview
Asymmetrical Digital Subscriber Line
(ADSL) uses the plain twisted pair wiring already carrying phone service
to subscribers’ homes to transmit video signals and high-speed data
to the home. ADSL uses adaptive digital filtering to overcome noise and
other problems on the line. Initially, the telephone companies hoped to
use ADSL to provide Video on Demand service in competition with cable
pay-per-view and neighborhood video rental stores. But ADSL can also offer
a wide range of other applications, including Internet service, work-at-home
access to corporations, and interactive services, such as home shopping
and home banking. In addition, ADSL could make at-home educational access
affordable for consumers.
Early Development
As early as 1991, Bellcore, the
research company associated with the seven regional Bell operating companies,
began touting ADSL to expand the transmission capacity of the copper-based
telephone networks. This was originally seen as the telephone companies’
answer to CATV’s encroachment into telephone service and their entree
into providing video on demand to telephone customers. Both the cable
companies and the telephone companies were itching to get into each other’s
businesses, but their networks were totally different, each with its own
strong points and shortcomings. The telephone companies had greater access
to homes in the United States (more than 90 percent), but the cable companies
had more bandwidth capacity going into homes. The telephone companies
were set up for two-way communication, but lost video quality over distance.
The cable companies had better quality but limited upstream capacity.
Both industries knew that their ultimate solution would be fiber-optic
networks connecting everyone, but realistically this was not possible.
Although fiber has been run by both cable companies and telcos over the
vast majority of their network, taking it from the curb to the customer’s
residence or business was the problem. The cost alone would run into the
billions, and nobody could afford to keep laying fiber in hopes that the
home they went to would use all of the capabilities fiber had to offer.
Enter ADSL. ADSL allows a standard copper telephone line to carry a high-speed
digital signal while simultaneously transmitting a voice conversation.
The asymmetrical part of the service refers to the fact that the high-speed
transmission of data is one-way, from the central office to the home or
business. Since most homes or small businesses only need the speed to
receive information, not transmit it, this works very well. And, initially,
ADSL permitted transmission at 1.5 megabits per second (Mbps) over copper
wire for up to 18,000 feet. The vast majority of small businesses and
residences easily fell within 18,000 feet of a telephone switching office.
Because of the poor initial success of the last great plan to use the
telephone companies’ copper wire, Integrated Services Digital Networks
(ISDN), ADSL was met with skepticism. Many telcos, as well as manufacturers,
originally developed a wait-and-see attitude before investing in the new
service. But, in 1993, a tiny California company called Amati
teamed up with Northern Telecom
to prove that ADSL could be used to send 6 Mbps of full-motion video down
a conventional telephone line. Suddenly the telephone companies had a
weapon, albeit an interim one, that could be used against the cable companies.
Big companies, like Bell Atlantic,
realized that ADSL could be used immediately to stay in the game, removing
the pressure to replace the copper wire with fiber. Instead of spending
time and money to bring hybrid fiber/coax (HFC) or fiber-to-the-curb (FTTC)
into a large area for an unknown number of users, the telcos could now
target specific users who were willing to pay for the equipment necessary
to make the service work.
ADSL Today
By 1994, ADSL development provided for 7 Mbps of downstream bandwidth
and up to 576 kbps of return bandwidth. This enabled the telcos to use
the copper wire paths to offer basic telephone service, ISDN, full-motion
video, and videoconferencing. AT&T
began its own development process using a carrierless amplitude and
phase (CAP) modulation alternative to the discrete multitone (DMT) developed
by Amati for ADSL. Although DMT appeared to be the best of the two alternatives,
CAP was available more quickly. Eventually, it was clear that either alternative
could be used by an operating company, they just couldn’t’t be mixed
in the same system.
The first trials of ADSL were relatively simple. There was a box at each
end of a conventional telephone line, that is, one in the customer’s
home/business and one in the phone company’s switching office. The
box divided the phone line into multiple paths, one to carry compressed
video signals to the customer, a second to carry questions and commands
back to the signal provider, and a third for normal telephone service.
Additional paths could be added to support services such as videoconferencing.
The major drawback was the cost of the boxes, up to $1000 each.
As interest in ADSL continued to alternate back and forth in the United
States, companies in other parts of the world quickly snapped it up. Developing
countries attempting to compete in the new world couldn’t afford
high-tech fiber pathways. Established cities, such as Rome and London,
faced almost insurmountable problems if they wanted to dig up the streets
and replace copper with fiber. ADSL quickly gained supporters around the
world.
By early 1995, ADSL could provide high-speed data over a single twisted
copper pair at the rate of 1.544 to 6.144 Mbps downstream (central office
to customer) and 16 to 640 kbps upstream (customer to central office)
for up to 18,000 feet. By shortening the distance to 9000 feet, ADSL could
provide four compressed video channels to the user. These channels could
then be used for video on demand, instant replay, broadcast TV, interactive
games, home shopping, and educational data bases. But the cost of the
boxes to provide the service was still high.
In mid-1995, a new version of ADSL, called V-ADSL, was introduced. V-ADSL
was designed to work in conjunction with FTTC network architecture. As
telcos brought fiber closer to the homes, the telcos could use V-ADSL
as the last connection to the home or business. With shorter distances
being covered by copper wire, V-ADSL could provide higher bit rates, 51
Mbps for distances of about 1000 feet and 25.6 Mbps for distances of 3000
to 4000 feet.
By early 1996, the benefits of using ADSL for Internet access were being
explored. GTE Corporation began
a test in the Dallas-Ft. Worth area in February using residential customers,
high-traffic public areas, and small businesses. In the third quarter,
US West began a trial in Denver and
Boulder, Colorado and Minneapolis-St. Paul, Minnesota. US West’s
trial was designed to link users to the Internet and corporate intranets.
New Competitors Line Up
With the success of these trials and the potential cost savings being
demonstrated, several additional vendors began developing the transceiver
boxes necessary to support ADSL. In May, 1996, Motorola
announced plans to release a single-chip device designed to enable video-on-demand
and Internet access by early 1997. Motorola’s ADSL chip would be
capable of speeds up to 8 Mbps.
In July of 1996, Bell Canada announced
that it would be offering widespread ADSL service by early 1997 and Amati
Communications Corporation previewed its Ethernet-compatible ADSL modem.
Amati’s modem provided bit rates of 8 Mbps at distances up to 12,000
feet and 1.5 Mbps up to 15,000 feet. In addition, Amati announced the
development of a very-high-speed digital subscriber line modem which could
provide bit rates of up to 60 Mbps at 1000 feet and 12 Mbps at 6000 feet,
which it planned to release in the first quarter of 1997. Unfortunately
for most subscribers, the cost is still prohibitive, with Amati’s
device coming in at $2500. Other manufacturers, such as Northern Telecom,
Motorola, Ericsson Inc., Teltrend,
Aware, Inc., Analog
Devices, Inc., and Alcatel Data
Networks, have also announced products for delivery in 1997, which
should lower costs.
ADSL service, which was originally delegated to the background and frequently
ignored, has suddenly become the means for the telephone companies to
compete in the information delivery business. Faced with an outmoded network
of noisy copper lines, the telephone companies appeared to be in a losing
battle with the cable television industry and its coaxial network and
cable modems. Now ADSL has allowed the telephone companies to use their
existing networks to provide expanded interactive and video services to
their subscribers. Ultimately, both industries want to replace their networks
with fiber or fiber/coax, but realistically, this could take 15 to 20
years. In the meantime, ADSL will provide subscribers with another option
for high-speed data service.
Bibliography
Newton, Harry
"Newton’s Telecom Dictionary" Flatiron Publishing Inc.,
New York, 1994
www.prnewswire.com,
August 22, 1996
www.prnewswire.com,
August 27, 1996
www.prnewswire.com,
August 28, 1996
Schroeder,
Erica "ADSL Answering Threat of Cable: Vendors Speed Field Trials"
PC Week, July 8, 1996
McCarthy,
Shira "Vendors Exploit New Interest in ADSL" Telephony, June
24, 1996
Dvorak, John
C. "It’s the Internet, Stupid" PC Magazine, June 11, 1996
Surkan, Michael
"A New Twist for Old Telephone Wire" PC Week, June 3, 1996
Woods, Bob
"Cable Modem Market to Almost Double" Newsbytes News Network,
May 29, 1996
Woods, Bob
"High Speed Multimedia Via Copper Wiring Undergoes Trials" Newsbytes
News Network, May 22, 1996
Hardie, Crista
"Motorola Readies ADSL Video IC for ’97 Entry" Electronic
News (1991), May 6, 1996
Schroeder,
Erica "US West, GTE Kick Off ADSL Technology Trials" PC Week,
April 8, 1996
Kopf, David
"Internet Makes Case for ADSL" America’s Network, April
1, 1996
Schroeder,
Erica "ADSL is Gathering Telco Adherents" PC Week, March 25,
1996
McCullough,
Don "FTTC the Right Choice in the Long Term" America’s
Network, February 15, 1996
Vigoda, Arlene
"Speedier Access: Cable and Phone Companies Compete, Internet Future
Rests on Rival Technologies as Users Express Frustrations with Delays"
USA Today, 1996
Machrone,
Bill "ADSL Delivers High-speed Hope" PC Week, September 11,
1995
O’Shea,
Dan "Aware/ADI Raise Curtain on DMT Technology First" Telephony,
August 28, 1995
Krause, Reinhardt
"Improved ADSL Compression Lures Semiconductor Firms" Electronic
News (1991), April 10, 1995
Krapf, Eric
"Bell Atlantic Video Trial Gives ADSL First Big Test" America’s
Network, March 1, 1995
Lefkowitz,
Mike "The Last Mile: Linking Fiber and Copper" Communication
News, January 1995
Nak-Hieon,
Kim "Goldstar Develops World’s Second ADSL" Electronics,
November 28, 1994
Steward, Alan
"Telcos Finding Virtues of Video Over Copper" America’s
Network, October 1, 1994
Vittore, Vince
"ADSL: Overhyped or Well-kept Secret?" America’s Network,
August 15, 1994
Huthseesing,
Nikhil "Copper Highway?" Forbes, June 6, 1994
Kastre, Michael
"Fast Lane on the Information Superhighway" Hispanic Engineer,
March 31, 1994
Stewart, Alan
and Stoffels, Bob "Can ADSL Rescue Copper from Oblivion?" America’s
Network, March 15, 1994
Karpinski,
Richard "ADSL: Alive and (Seemingly) Well" Telephony, March
14, 1994
Gilder, George
"The Death of Telephony" The Economist, September 11, 1993
Brody, Herb
"Information Highway: The Home Front" Technology Review, August
1, 1993
Blankenhorn,
Dana "Bellcore Says TV, Voice Can Share a Phone Line" Newsbytes
News Network, June 6, 1991