ADSL Modems
Does the humble telephone lines play a major role in shaping the
third
millennium? Can a mere pair of thin copper wires twisted around each
other
transmit Internet data reliably and securely at blazing fast speed,
making it
possible to view high-quality moving images, sound and vast amounts
of data on
your personal computer screen or television? The answer is yes, as
the growing
success of DSL (digital subscriber line) technology abundantly
demonstrates. The
capacity of a communications channel depends on its
bandwidth and its
signal-to-noise ratio. A voice connection through a
conventional phone network
uses a bandwidth of about 3,000 hertz (Hz): from
about 300 Hz to 3,300 Hz. An
analog modem operating at 33.6 kilobits per
second (kbps) requires a slightly
wider bandwidth 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
operating above the voice
frequency band, so a single phone line can
simultaneously transmit voice and
high-speed data. The newest standard of ADSL
is G lite which is just for home
users, a global standard that limits the data
rates to 1.5 Mbps downstream to
the consumer and about 0.5 Mbps upstream. By
limiting the speed G.lite is
able to operate reliably on more than 70 percent of
unaltered phone lines and
lowers costs and power usage. Home computers
containing G.lite-ready
circuitry are already being sold. ADSL has a number of
advantages over
systems that use a cable television network. With ADSL the
signal 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 receiving
data,
someone could be listening in on your data signal. Where as telephone
wires,
on the other hand, are physically secure. The backbone networks for
ADSL
carry composite signals for a few hundred consumers at 155 Mbps and up.
A
television channel has an effective throughput of only about 24 Mbps,
greatly
limiting its effectiveness under heavy use by hundreds of cable
modems. The ADSL
traffic also benefits from a statistical economy of
scale--for example, 1,550
people sharing a backbone of 155 Mbps will
experience better performance than
240 sharing 24 Mbps. Although cable
networks cover 90 percent of the homes in
the U.S., they do not serve many
businesses. Telephone networks are ubiquitous.
Moreover, for effective
use of cable modems the cable operator must invest
billions to upgrade the
cable network with fiber optics and two-way transmission
equipment; ADSL, on
the other hand, takes advantage of the same kind of
telephone pairs that
Alexander Graham Bell used in the 19th
century.
Bibliography
http://www.xdslresource.com/xDSLFAQ.shtm
http://webopedia.internet.com/TERM/x/xDSL.html
http://www.everythingdsl.com/