Does the humble telephone lines play a major role in shaping the thirdmillennium? Can a mere pair of thin copper wires twisted around each othertransmit Internet data reliably and securely at blazing fast speed, making itpossible to view high-quality moving images, sound and vast amounts of data onyour personal computer screen or television? The answer is yes, as the growingsuccess of DSL (digital subscriber line) technology abundantly demonstrates. Thecapacity of a communications channel depends on its bandwidth and itssignal-to-noise ratio. A voice connection through a conventional phone networkuses a bandwidth of about 3,000 hertz (Hz): from about 300 Hz to 3,300 Hz.
Ananalog modem operating at 33.6 kilobits per second (kbps) requires a slightlywider bandwidth 3,200 Hz and needs a very good connection, one with a highsignal-to-noise ratio. Modems operating at 56 kbps achieve their rates by takingadvantage of digital connections that circumvent some sources of noise intransmissions toward the end user. But these bit rates are far from the maximumpossible on a twisted pair alone. One process that limits bandwidth and signalstrength is the steady attenuation of the signal as it travels down the line,with the higher frequencies being affected more severely. Greater capacity istherefore available if the lines are kept short.
Originally, the DiscreteMultitone approach was intended for sending entertainment video over telephonewires. Because such use relies principally on one-way transmission, most of thesubchannels were devoted to the "downstream" signal, carrying about 6Mbps, with about 0.6 Mbps available in the other direction. This asymmetric formof DSL has become known as ADSL, and the signal coding is now a worldwidestandard. Although the video application has not yet borne fruit, asymmetrictransmission fortuitously lends itself to browsing on the World Wide Web. Overthe past year ADSL has begun to be widely installed in telephone networks foralways-on Internet access, typically operating at several hundreds of kbps orhigher 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 channelsoperating above the voice frequency band, so a single phone line cansimultaneously transmit voice and high-speed data. The newest standard of ADSLis G lite which is just for home users, a global standard that limits the datarates to 1.5 Mbps downstream to the consumer and about 0.5 Mbps upstream. Bylimiting the speed G.
lite is able to operate reliably on more than 70 percent ofunaltered phone lines and lowers costs and power usage. Home computerscontaining G.lite-ready circuitry are already being sold. ADSL has a number ofadvantages over systems that use a cable television network. With ADSL thesignal on your line is not shared with other users.
Where as cable modems are,which work over a giant network (party line) when someone else is receivingdata, someone could be listening in on your data signal. Where as telephonewires, on the other hand, are physically secure. The backbone networks for ADSLcarry composite signals for a few hundred consumers at 155 Mbps and up. Atelevision channel has an effective throughput of only about 24 Mbps, greatlylimiting its effectiveness under heavy use by hundreds of cable modems.
The ADSLtraffic also benefits from a statistical economy of scale--for example, 1,550people sharing a backbone of 155 Mbps will experience better performance than240 sharing 24 Mbps. Although cable networks cover 90 percent of the homes inthe U.S., they do not serve many businesses.
Telephone networks are ubiquitous.Moreover, for effective use of cable modems the cable operator must investbillions to upgrade the cable network with fiber optics and two-way transmissionequipment; ADSL, on the other hand, takes advantage of the same kind oftelephone pairs that Alexander Graham Bell used in the 19th century.Bibliographyhttp://www.xdslresource.com/xDSLFAQ.shtm http://webopedia.internet.com/TERM/x/xDSL.htmlhttp://www.everythingdsl.com/
