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 carrier less amplitude and phase (CAP) modulation alternative to the discrete multi tone (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., Tel trend, 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.


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