G.dmt: Стандарт ADSL с дискретным мультитоновым кодированием

In telecommunications, ITU T G.992.1 (better known as G. dmt) is an ITU standard for ADSL using discrete multitone modulation (DMT). 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. DMT allocates from 2 to 15 bits per channel (bin). As line conditions change, bit swapping allows the modem to swap bits around different channels, without retraining, as each channel becomes more or less capable. If bit swapping is disabled then this does not happen and the modem needs to retrain in order to adapt to changing line conditions. There are 2 competing standards for DMT ADSL ANSI and G. dmt; ANSI T1.413 is a North American standard, G.992.1 (G. dmt) is an ITU (United Nations Telecom committee) standard. G. dmt is used most commonly today, throughout the world, but the ANSI standard was formerly popular in North America. There is a difference in framing between the two, and selecting the wrong standard can cause frame alignment errors every 5 or so minutes. Error correction is done using Reed–Solomon encoding and further protection can be used if Trellis encoding is used at both ends. Interleaving can also increase the robustness of the line but increases latency.

Modulation is the overlaying of information (or the signal) onto an electronic or optical carrier waveform. There are two competing and incompatible standards for modulating the ADSL signal, known as discrete multitone modulation (DMT) and Carrierless Amplitude Phase (CAP). CAP was the original technology used for DSL deployments, but the most widely used method now is DMT. The graphs on the right summarise the speeds obtainable for each ADSL standard based on line length and attenuation. The second graph is of more importance since it is attenuation which is the governing factor for line speed because attenuation rate over distance can vary significantly between various copper lines due to their quality and other factors. ADSL2 is able to extend the reach of extremely long lines that have around 90 dB attenuation. Standard ADSL is only able to provide a service on lines with an attenuation no greater than about 75 dB.

The use of bins produces a transmission system known as coded orthogonal frequency division multiplexing (COFDM). In the context of G.992.1, the term "Discrete Multi Tone" (DMT) is used instead, hence the alternative name of the standard, G. dmt. Using DMT is useful since it allows the communications equipment (user modem/router and exchange/DSLAM) to select only bins which are usable on the line thus effectively obtaining the best overall bit rate from the line at any given moment in time. With COFDM, a combined signal containing many frequencies (for each bin) is transmitted down the line. 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. This is a complex and mathematical subject and will not be discussed further here. However, much research has been done on these modulation techniques and they are used for transmission because they allow the SNR to be improved, thus lowering the noise floor and enabling more reliable transmission of a signal with fewer errors. 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.

The quality of the line (how well it performs) at the frequency of the bin in question determines how many bits can be encoded within that bin. As with all transmission lines, it depends on the attenuation and signal to noise ratio. SNR may differ for each bin and this plays an important factor for deciding how many bits can be encoded reliably on it. Generally speaking, 1 bit can be encoded reliably for each 3 dB of available dynamic range above the noise floor within a transmission medium so, for example, a bin with an SNR of 18 dB would be able to accommodate 6 bits.

Echo cancellation can be used so the downstream channel overlaps the upstream channel, or vice versa, meaning simultaneous upstream and downstream signals are sent. Echo cancellation is optional and is typically not used.

Below are examples of how the bin layout may look on various ADSL modems. Both show similar information and in each example there are 256 bins with a varied number of bits being encoded on each one. We can see that at around the frequency range of bin 33, the SNR is 40 dB with the bits per bin being around 6 or 7.