30-04-2014, 02:43 PM
Asymmetric Digital Subscriber Line (ADSL)
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Definition
Asymmetric digital subscriber line (ADSL) is a new modem technology that
converts existing twisted-pair telephone lines into access paths for high-speed
communications of various sorts.
Overview
ADSL can transmit more than 6 Mbps to a subscriber—enough to provide
Internet access, video-on-demand, and LAN access. In interactive mode it can
transmit more than 640 kbps in both directions. This increases the existing
access capacity by more than fifty-fold enabling the transformation of the existing
public network. No longer is it limited to voice, text, and low-resolution graphics.
It promises to be nothing less than an ubiquitous system that can provide
multimedia (including full-motion video) to the entire country. ADSL can
perform as indicated in Table 1.
A Short History of Analog Modems
The term modem is actually an acronym which stands for
MOdulation/DEModulation. A modem enables two computers to communicate
by using the public switched telephone network. This network can only carry
sounds so modems need to translate the computer's digital information into a
series of high-pitched sounds which can be transported over the phone lines.
When the sounds arrive at their destination, they are demodulated—turned back
into digital information for the receiving computer (see Figure 1).
Digital Subscriber Line (DSL)
Despite its name, DSL does not refer to a physical line but to a modem—or rather
a pair of modems. A DSL modem pair creates a digital subscriber line, but the
network does not purchase the lines when it buys ADSL—it already owns those—
it purchases modems.
A DSL modem transmits duplex (i.e., data in both directions simultaneously) at
160 kbps over copper lines of up to 18,000 feet. DSL modems use twisted-pair
bandwidth from 0 to approximately 80 kHz which precludes the simultaneous
use of analog telephone service in most cases (see Figure 2).