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In telecommunications, ITU G.992.1 (better known as G.DMT) is an ITU standard for ADSL using discrete multitone modulation. 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 Mbit/s upstream. DMT history and line rates
BINS (carrier channels) 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. DMT has 224 downstream frequency bins and up to 31 upstream bins. Bin 0 is at DC and is not used. When voice (POTS) is used on the same line, then bin 7 is the lowest bin used for ADSL. The centre frequency of bin N is (N x 4.3125) kHz. The spectrum of each bin overlaps that of its neighbours: it is not confined to a 4.3125 kHz wide channel. The orthogonality of COFDM makes this possible without interference. Up to 15 bits per symbol can be encoded on each bin on a good quality line. The frequency layout can be summarised as: Typically, a few bins around 31-32 are not used in order to prevent interference between upstream and downstream bins either side of 138 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. Coded Orthogonal Frequency Division Multiplexing (COFDM) The use of bins produces a transmission system which exhibits a form of Frequency Division Multiplexing (FDM) known as Coded Orthogonal FDM (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. Reducing Bit Errors with QAM & PSK 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 beating 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 0.5-1.5 dB and these small amounts make a vast difference when sending signals over long distance copper lines of 6 km or more. BIN quality and bit rate 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 6 dB of available dynamic range above the noise floor within a transmission medium so, for example, a bin with a SNR of 18 dB would be able to accommodate 3 bits. Echo cancellation 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. Other information In DMT, up to 15 bits may be assigned to each channel (BIN). The default is 2, but as channels suffer interference and attenuation, those bits are swapped onto other channels. If bit swapping is disabled then this does not happen and the line rate suffers. There are 2 competing standards for DMT ADSL - ANSI & DMT; ANSI T1.413 is the North American standard G.992.1 (DMT) is the ITU (United Nations Telecom committee) standard. 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. DMT bits per BIN examples Below are examples of how the bin layout may look on various ADSL modems/routers. 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. Textual ----------------------------------------------------------------------------- Bin SNR Gain Bi - Bin SNR Gain Bi - Bin SNR Gain Bi - Bin SNR Gain Bi dB dB ts dB dB ts dB dB ts dB dB ts --- ----- ---- -- - --- ----- ---- -- - --- ----- ---- -- - --- ----- ---- -- 0 0.0 0.0 0 Bin SNR Gain Bi - Bin SNR Gain Bi - Bin SNR Gain Bi - Bin SNR Gain Bi dB dB ts dB dB ts dB dB ts dB dB ts Graphical with SNR Summary and Key Points ADSL statistics Figures in brackets have been shown to provide a stable service in practice. | |||||||||
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