G.992.1

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.^[1]

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.