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2. ADSL
Asymmetric Digital Subscriber Line (ADSL) is a standard for digital communications between client and
remote network host. It is asymmetric because the transmission rates for downstream and upstream are
different with the downstream rate higher than the upstream rate. The downstream direction is from the
service provider to the customer, and the upstream direction is from the customer to the service provider
ADSL generally offers downstream rates of about 512Kbps to 6Mbps. ADSL modems are in widespread use in
homes and small businesses for high-speed access to the Internet.
The need for speed at a lower cost is the basic requirement for web connections today due to which ADSL is
the way to go. ADSL meets both these requirements offering amazing speed increase at a relatively low cost,
to both provider and consumer. ADSL has bought future entertainment to the computer at the same quality
and speed as your television and it has paved the way for digital video conferencing and other such
applications. However, ADSL does suffer from one major weakness; signal attenuation in the copper, which
increases with increasing line length, increasing signal frequency, and decreasing line gauge.
ADSL modems rely on discrete multitone modulation (DMT). DMT divides a broadband channel into many
narrowband sub channels and modulates encoded signals onto the narrowband sub channels by using the
fast Fourier transform (FFT). In ADSL standards, the lowest sub channels are not used for data transmission so
as not to interfere with voice and ISDN signals, and one sub channel is often reserved for a pilot tone. Either
each of the other sub-channels carry a QAM signal, or no message, depending on the bit allocation
determined by the receiver and sent back to the transmitter. A bit allocation is determined during modem
initialization. During data transmission, the receiver can request small changes to the bit allocation table, e.g.
as a response to changes in the channel.
ADSL receiver does the following two equalizations:
Time-domain equalization (TEQ) to shorten the effective duration of the channel impulse response
Frequency-domain equalization (FEQ) to compensate for magnitude and phase distortion.
The above equalizations are done before a bit allocation table is created.
The various ADSL standards support training sequences during initialization for the purpose of training an
equalizer and transmission of synchronization symbols during data transmission which can be used to adapt
the equalizer. The bit allocation table can be derived from the equalizer. Equalizer design is the key to
maximizing bit rate in an ADSL modem.
The new generation of DMT-based wireline modems has been defined by three standards: ADSL+, ADSL2,
and Very High Speed DSL (VDSL) standards. These three standards were approved in 2002 and 2003. VDSL is
essentially a higher speed version of ADSL. VDSL provides training sequences to train the equalizer, and uses
a cyclic prefix length that is 1/16th the symbol length. VDSL experiences interference from AM radio stations
and amateur radio. A VDSL tutorial is available under tutorials. As in ADSL modems, equalizer design is the
key to maximizing bit rate in an VDSL modem.
History:
ADSL was started up by telephone company’s as a way to compete with cable TV by delivering both TV and
phone service on the plain old copper phone line. ADSL was originally designed so that it could still be used as
a regular phone line if the power happened to go out. At the beginning only a tiny uplink of 16 or 64kbps was
supported now ADSL can support up to ten times that much. The idea for ADSL was spawned by Joe
Leichleder in 1987, he was a Bellcore researcher and the first idea for it was analog to digital conversion at
the subscriber end of an advanced transmission technology over a twisted pair copper line. ADSL was
designed to provide megabit access for interactive video (video on demand, video games, delayed TV
segments, etc.) and high speed data communications (Internet access, remote LAN access, other network
access) but it is also perfect for surfing the web.
In the 1990s, ADSL emerged as a winner for web surfing at home. The only thing missing was the telephone
line, so frequency-division multiplexing (FDM) was added to ADSL to reserve a separate 4-kHz POTS (plain old
telephone system) channel below the higher frequency data band. ADSL uses a single wire pair. Speeds on
