G.992.1

G.dmt full-rate ADSL expands the usable bandwidth of existing copper telephone lines, delivering high-speed data communications at rates up to 8 Mbit/s downstream and 1.3 Mbit/s upstream.

If bit swapping is disabled then this does not happen and the modem needs to retrain in order to adapt to changing line conditions.

The graphs on the right summarise the speeds obtainable for each ADSL standard based on line length and attenuation.

Discrete Multi-Tone (DMT), the most widely used modulation method, separates the ADSL signal into 255 carriers (bins) centred on multiples of 4.3125 kHz.

These unused bins constitute a guard band to be chosen by each DSLAM manufacturer - it is not defined by the G.992.1 specification.

Fast Fourier Transform (and the inverse iFFT) is used to convert the signal on the line into the individual bins.

A type of quadrature amplitude modulation (QAM) or phase-shift keying (PSK) is used to encode the bits within each bin.

The gain obtainable above the noise floor can be anything from 0.5 to 1.5 dB and these small amounts make a vast difference when sending signals over long distance copper lines of 6 km or more.

SNR may differ for each bin and this plays an important factor for deciding how many bits can be encoded reliably on it.