How ADSL Works

ADSL is a passband system, meaning that it carries its information in one or more channels above the baseband region of the frequency spectrum. This frees the 4kHz baseband section for use by standard voice transmission. In order to transmit data at high rates over a relatively lossy medium, the hardware must implement similar techniques to those employed by standard modems. This involves sending "symbols" rather than individual bits, a process known as quadrature amplitude modulation (QAM). ADSL has two competing standards for implementing this.

The accepted standard, Discrete Multi-Tone (DMT), divides the spectrum into 256 4kHz channels called bins. Some of these channels are used bidirectionally and require echo cancelling to prevent corruption, and the rest of the channels are used for downstream data only. The baseband voice channel is well separated from these, and can be accessed by use of a simple splitter device. The effect is similar to a standard voice line and a bank of standard modems operating in parallel to deliver the high bandwidth service, but over a single twisted pair. This is depicted by figure.

Since many impairments of the twisted pair cable access are frequency sensitive, the hardware can optimise transfer rates by monitoring the performance of each individual channel. If the quality degrades, then the bit rate can be reduced on that channel, and possible reassigned to another channel with a better signal to noise characteristic. Bandwidth within individual channels can be optimally used with modulation techniques similar to those employed by traditional modems.