ADSL Data Transfer

The dominant form of ADSL technology at the time of writing is Discrete Multi-Tone (DMT).

There are two other forms of ADSL enabling technology, such as Carrierless Amplitude Phase Modulation (CAP) and Quadrature Amplitude Modulation (QAM).

For the time being, we will concentrate on DMT, but we will look at CAP and QAM in due course.

DMT ADSL Systems

The maximum bandwidth supported by a copper twisted pair is approximately 1MHz.

This bandwidth is split into three distinct sections:

(A)Normal speech telephony

(B)Low bit rate ADSL return channel

(C) High bit rate ADSL forward channel

As can be seen from the diagram below

DMT functions by dividing the available bandwidth into a multitude of sub-channels.

These sub-channels are allocated a number of bits (0-8) per hertz in each 4kHz sub-channel band, depending upon the signal-to-noise ratio experienced in the sub-channel.

For example, if a particular sub-channel experiences noise from an external source, the adaptive rate mechanism built into the latest chipsets will automatically allocate the channel fewer bits of the data stream until the noise signal has subsided.

NOTE: In this context, noise can be derived from transients such as AM radio interference or the inherent attenuation of the data stream by the effects of capacitance on the copper loop as line length increases.

This also explains the reduction in data rate as line length is increased.