An ADSL circuit connects an ADSL modem
on each end of a twisted-pair telephone line, creating three
information channels--a high-speed downstream channel, a
medium-speed duplex channel, and a basic telephone service channel
.
The basic telephone service channel is split off from the digital
modem by filters, thus guaranteeing uninterrupted basic telephone
service, even if ADSL fails. The high-speed channel ranges from 1.5
to 6.1 Mbps, and duplex rates range from 16 to 640 Kbps. Each
channel can be submultiplexed to form multiple lower-rate channels.
ADSL modems provide data rates
consistent with North American T1 1.544-Mbps and European E1
2.048-Mbps digital hierarchies and can be purchased with various
speed ranges and capabilities. The minimum configuration provides
1.5 or 2.0 Mbps downstream and a 16-Kbps duplex channel; others
provide rates of 6.1 Mbps and 64-Kbps duplex. Products with
downstream rates up to 8 Mbps and duplex rates up to 640 Kbps are
available today as well. ADSL modems accommodate Asynchronous Transfer Mode
(ATM) transport with variable rates and compensation for ATM
overhead, as well as IP protocols.
Downstream data rates depend on many
factors, including the length of the copper line, its wire gauge,
presence of bridged taps, and cross-coupled interference. Line
attenuation increases with line length and frequency and decreases
as wire diameter increases. Ignoring bridged taps, ADSL performs as
shown in the Table
.
Although the measure varies from
telco to telco, these capabilities can cover up to 95 percent of a
loop plant, depending on the desired data rate. Customers beyond
these distances can be reached with fiber-based digital loop carrier
(DLC) systems. As these DLC systems become commercially available,
telephone companies can offer virtually ubiquitous access in a
relatively short time.
Many applications envisioned for ADSL
involve digital compressed video. As a real-time signal, digital
video cannot use link- or network-level error control procedures
commonly found in data communications systems. ADSL modems,
therefore, incorporate FEC (Forward Error Correction) that
dramatically reduces errors caused by impulse noise. Error
correction on a symbol-by-symbol basis also reduces errors caused by
continuous noise coupled into a line.