adsl broadband technology
...idth 3,200 Hz and needs a very good connection, one with a high signal-to-noise ratio. Modems operating at 56 kbps achieve their rates by taking advantage of digital connections that circumvent some sources of noise in transmissions toward the end user. But these bit rates are far from the maximum possible on a twisted pair alone. One process that limits bandwidth and signal strength is the steady attenuation of the signal as it travels down the line, with the higher frequencies being affected more severely. Greater capacity is therefore available if the lines are kept short. Originally, the Discrete Multitone approach was intended for sending entertainment video over telephone wires. Because such use relies principally on one-way transmission, most of the subchannels were devoted to the downstream signal, carrying about 6 Mbps, with about 0.6 Mbps available in the other direction. This asymmetric form of DSL has become known as ADSL, and the signal coding is now a worldwide standard. Although the video application has not yet borne fruit, asymmetric transmission fortuitously lends itself to browsing on the World Wide Web. Over the past year ADSL has begun to be widely installed in telephone networks for always-on Internet access, typically operating at several hundreds of kbps or higher over phone wires up to about 5.5 kilometers in length. The beauty of ADSL, unlike the multilevel coding used in HDSL, is that the data can use channels The backbone networks for ADSL carry composite signals for a few hundred consumers at 155 Mbps and up...