Реферат: ADSL Essay Research Paper Overview

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