Asymmetric Digital Subscriber Line (ADSL) modems are in widespread use in homes and small businesses for high-speed access to the Internet. Asymmetric means that the data rates are different in different directions. In ADSL, the downstream direction is from the service provider to the customer, and the upstream direction is from the customer to the service provider. In the ADSL standards, the downstream data rate is higher than the upstream data rate.

ADSL modems rely on discrete multitone modulation (DMT). DMT divides a broadband channel into many narrowband subchannels and modulates encoded signals onto the narrowband subchannels by using the fast Fourier transform (FFT). In ADSL standards, the lowest subchannels are not used for data transmission so as not to interfere with voice and ISDN signals, and one subchannel is often reserved for a pilot tone. Each of the other subchannels either 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.

Before a bit allocation table can be created, the ADSL receiver has two different equalization tasks to perform:

  1. time-domain equalization (TEQ) to shorten the effective duration of the channel impulse response, and
  2. frequency-domain equalization (FEQ) to compensate for magnitude and phase distortion.
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.

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