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ADSL Tutorial
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Twisted Pair
Access to the Information Highway |
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| Asymmetric Digital Subscriber Line
(ADSL), a new modem technology, converts existing twisted-pair
telephone lines into access paths for multimedia and high speed
data communications. ADSL transmits more than 6 Mbps to a subscriber,
and as much as 640 kbps more in both directions. Such rates
expand existing access capacity by a factor of 50 or more without
new cabling. ADSL can literally transform the existing public
information network from one limited to voice, text and low
resolution graphics to a powerful, ubiquitous system capable
of bringing multimedia, including full motion video, to everyone's
home this century. |
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| ADSL will play a crucial role over
the next ten or more years as telephone companies enter new
markets for delivering information in video and multimedia formats.
New broadband cabling will take decades to reach all prospective
subscribers. But success of these new services will depend upon
reaching as many subscribers as possible during the first few
years. By bringing movies, television, video catalogs, remote
CD-ROMs, corporate LANs, and the Internet into homes and small
businesses, ADSL will make these markets viable, and profitable,
for telephone companies and application suppliers alike. |
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| Technology
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| ADSL depends upon advanced digital
signal processing and creative algorithms to squeeze so much
information through twisted-pair telephone lines. In addition,
many advances have been required in transformers, analog filters,
and A/D converters. Long telephone lines may attenuate signals
at one megahertz (the outer edge of the band used by ADSL) by
as much as 90 dB, forcing analog sections of ADSL modems to
work very hard to realize large dynamic ranges, separate channels,
and maintain low noise figures. On the outside, ADSL looks simple
-- transparent synchronous data pipes at various data rates
over ordinary telephone lines. On the inside, where all the
transistors work, there is a miracle of modern technology. |
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| To create multiple channels, ADSL
modems divide the available bandwidth of a telephone line in
one of two ways -- Frequency Division Multiplexing (FDM) or
Echo Cancellation. FDM assigns one band for upstream data and
another band for downstream data. The downstream path is then
divided by time division multiplexing into one or more high
speed channels and one or more low speed channels. The upstream
path is also multiplexed into corresponding low speed channels.
Echo Cancellation assigns the upstream band to over-lap the
downstream, and separates the two by means of local echo cancellation,
a technique well know in V.32 and V.34 modems. With either technique,
ADSL splits off a 4 kHz region for POTS at the DC end of the
band. |
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| Source: http://www.yellowhead.com/adsl.htm
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