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{{Infobox technology standard
{{ADSL standards}}
|title=G.992.1
In [[telecommunications]], '''ITU G.992.1''' (better known as '''G.DMT''') is an [[International Telecommunication Union|ITU]] standard for [[Asymmetric Digital Subscriber Line|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.
|preview_date=
|long_name = Asymmetric digital subscriber line (ADSL) transceivers
|preview =
|version_date = December 2003
|version = (12/03)
|year_started = 1999
|status = In force
|caption=
|image=
|organization = [[ITU-T]]
|committee = [[ITU-T Study Group 15]]
|license= Freely available
|domain= [[Telecommunications]]
|abbreviation=
|related_standards= [[G.992.2]], [[G.992.3]]
|base_standards=
|website= https://www.itu.int/rec/T-REC-G.992.1
}}


In [[telecommunications]], '''ITU-T G.992.1''' (better known as '''G.dmt''') is an [[International Telecommunication Union|ITU]] standard for [[Asymmetric Digital Subscriber Line|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&nbsp;Mbit/s downstream and 1.3&nbsp;Mbit/s upstream.<ref>{{Cite web|title=G.992.1: Asymmetric digital subscriber line (ADSL) transceivers|url=https://www.itu.int/rec/T-REC-G.992.1|url-status=live|archive-url=https://web.archive.org/web/20210412123513/https://www.itu.int/rec/T-REC-G.992.1|archive-date=2021-04-12|access-date=2021-04-12|website=www.itu.int}}</ref>
==DMT history and line rates==


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.
[[image:ADSL_Line_Rate_Reach.gif|frame|Line rate obtainable (Mbit/s) against corresponding line '''length''' (Km) for '''[[Asymmetric Digital Subscriber Line|ADSL]]''', '''[[ITU_G.992.5|ADSL2+]]''' and '''[[VDSL]]''']]
[[image:ADSL_Line_Rate_Attenuation.gif|thumb|418px|Line rate obtainable (Mbit/s) against corresponding line '''[[attenuation]]''' ([[Decibel|dB]]) for '''[[Asymmetric Digital Subscriber Line|ADSL]]''', '''[[ITU_G.992.3/4|ADSL2]]''' and '''[[ITU_G.992.5|ADSL2+]]''']]


There are 2 competing standards for DMT ADSL - ANSI and G.dmt; [[ANSI T1.413 Issue 2|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 error correction|Reed–Solomon]] encoding and further protection can be used if [[Trellis modulation|Trellis]] encoding is used at both ends. [[Forward error correction#Interleaving|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 [[Asymmetric Digital Subscriber Line|ADSL]] signal, known as [[discrete multitone modulation|Discrete Multi-Tone]] (DMT) and [[Carrierless Amplitude Phase Modulation|Carrierless Amplitude Phase]] (CAP). CAP was the original technology used for DSL deployments, but the most widely used method now is DMT.


==DMT history and line rates==
The graphs on the right summarise the speeds obtainable for each [[Asymmetric Digital Subscriber Line|ADSL]] standard based on line '''length''' and '''attenuation'''. The [[media:ADSL_Line_Rate_Attenuation.gif|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. The [[media:ADSL_Line_Rate_Attenuation.gif|second graph]] clearly shows that for longer lines exceeding around 50 dB attenuation, [[ITU_G.992.3/4|ADSL2]] and [[ITU_G.992.5|ADSL2+]] brings no benefit in terms of speed. However, [[ITU_G.992.3/4|ADSL2]] is able to extend the reach of extremely long lines that have around 90 dB attenuation. Standard [[Asymmetric Digital Subscriber Line|ADSL]] is only able to provide a service on lines with an attenuation no greater than about 75 dB.
[[File:ADSL Line Rate Reach.gif|frame|Line rate obtainable (Mbit/s) against corresponding line '''length''' (km) for '''ADSL''', '''[[G.992.5|ADSL2+]]''' and '''[[VDSL]]''']]
[[File:ADSL Line Rate Attenuation.gif|thumb|418px|Line rate obtainable (Mbit/s) against corresponding line '''[[attenuation]]''' ([[Decibel|dB]]) for '''ADSL''', '''[[G.992.3|ADSL2]]''' and '''[[G.992.5|ADSL2+]]''']]


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 Modulation|Carrierless Amplitude Phase]] (CAP). CAP was the original technology used for DSL deployments, but the most widely used method now is DMT.
[[VDSL]] (very high bit-rate DSL), which is shown in the first graph, is not discussed here but further reading is available at:

*http://computer.howstuffworks.com/vdsl.htm
The graphs on the right summarise the speeds obtainable for each ADSL standard based on line '''length''' and '''[[attenuation]]'''. The [[Media:ADSL Line Rate Attenuation.gif|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. [[G.992.3|ADSL2]] is able to extend the reach of extremely long lines that have around 90&nbsp;dB attenuation. Standard ADSL is only able to provide a service on lines with an attenuation no greater than about 75&nbsp;dB.
*http://www.dslforum.org/aboutdsl/vdsl_tutorial.html


==DMT technical details==
==DMT technical details==
===BINS (carrier channels)===
Discrete Multi-Tone (DMT), the most widely used modulation method, separates the DSL signal into 256 channels (bins) of 4.3125 kHz each. DMT has 224 downstream frequency bins (or carriers) and 32 upstream frequency bins. Up to 15 bits per signal can be encoded on each frequency bin on a good quality line. A guard band (an unused frequency range that sits between the first bin and the voice channel of the line) exists to prevent interference between the voice channel (narrowband) and the bins operating at higher frequencies (broadband). The frequency layout can be summarised as:
*0-4 kHz, voice.
*4-25 kHz, unused guard band.
*25-160 kHz, 32 upstream bins.
*200 kHz-1.1 MHz, 224 downstream bins.


===Bins (carrier channels)===
===Coded Orthogonal Frequency Division Multiplexing (COFDM)===
Discrete Multi-Tone (DMT), the most widely used modulation method, separates the ADSL signal into 255 carriers (bins) centred on multiples of 4.3125&nbsp;kHz. DMT has 224 downstream frequency bins and up to 31 upstream bins. Bin 0 is at DC and is not used. When voice ([[Plain old telephone service|POTS]]) is used on the same line, then bin 7 is the lowest bin used for ADSL.
The use of bins produces a transmission system which exhibits a form of [[frequency-division_multiplexing|Frequency Division Multiplexing]] (FDM) known as [[COFDM|Coded Orthogonal FDM]] (COFDM). This 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 (line quality can change radically due to temperature and other natural causes). With [[COFDM]], a combined signal containing many frequencies (for each bin) is transmitted down the line. Band pass filters in the equipment at the exchange (DSLAM) are used to split this single signal back into the original frequency components relating to each bin.


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&nbsp;kHz wide channel. The orthogonality of [[Coded orthogonal frequency-division multiplexing|COFDM]] makes this possible without interference.
===Reducing Bit Errors with QAM & PSK===
<!-- (not sure about the next 2 paragraphs - basing assumptions from my network comms course so please correct if wrong) -->
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.


Up to 15 bits per symbol can be encoded in a bin on a good quality line.
===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. Many factors affect this including:
*'''[[Electrical_resistance|resistance]]''' which reduces the [[Current (electricity)|current]] so increases signal loss.
*'''[[capacitance]]''' which effectively short circuits the line more as frequencies increase resulting in signal loss.
*'''[[inductance]]''' which resists current flow more as frequencies increase resulting in signal loss.
*'''[[interference]]''' from radio waves, transmitters or electrical equipment that is within close proximity to the line, [[Crosstalk (telecommunication)|cross-talk]] and [[inductive coupling]]. All have potential to corrupt the signal.
*'''[[Impedance|impedance]]''' which is effectively a special case of [[Electrical_resistance|resistance]] but applied to [[AC current]] (alternating current that may contain [[information]]) rather than [[DC current]] (static current that contains no [[information]] at all). This is particular of importance considering the way DMT utilises BINs (carrier channels at certain frequencies) because [[Impedance|impedance]] is related to [[frequency]] (similar to [[Loudspeaker|loud speakers]]) and so is another factor which governs how well a BIN will perform.
All of the above result in line '''[[attenuation]]''' and '''[[Signal_noise|noise]]'''. The attenuation and '''[[SNR|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.
<!-- Can someone please confirm if the 6 dB per bit assumption is correct? This is taken from digital audio where each bit gives 6 dB of dynamic range. -->


The frequency layout can be summarised as:
===Echo cancellation===
* 30&nbsp;Hz-4&nbsp;kHz, voice.
Echo cancellation can be used so the downstream channel overlaps the upstream channel meaning simultaneous upstream and downstream signals are sent on the lower frequency bins. Given this information, it is possible to calculate where the 8 Mbit/s (downstream) and 1 Mbit/s (upstream) figures originate from. Remember, a maximum of 15 bits can be encoded on a single bin and each bin has a bandwidth of 4.3125 kHz. Therefore:
* 4–25&nbsp;kHz, unused guard band.
*'''Upstream''' -> ''(32*15*4.3125*1000)/2 ='' '''1.035 Mbits'''.
* 25–138&nbsp;kHz, 25 upstream bins (7-31).
*'''Downstream''' -> ''(224*15*4.3125*1000)/2 = 7.245 Mbits'' or ''(256*15*4.3125*1000)/2 ='' '''8.28 Mbits''' with echo cancellation.
* 138–1104&nbsp;kHz, 224 downstream bins (32-255).
Multiplication by 1000 is required to convert from kHz to Hz and division by 2 is required since analysis shows that the transmission bandwidth of AM is twice the signal's original (baseband) bandwidth.


Typically, a few bins around 31-32 are not used in order to prevent interference between upstream and downstream bins either side of 138&nbsp;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.
===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.


===Coded orthogonal frequency-division multiplexing (COFDM)===
==DMT bits per BIN examples==
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.

===Reducing bit errors with QAM or PSK===
<!-- (not sure about the next 2 paragraphs - basing assumptions from my network comms course so please correct if wrong) -->
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&nbsp;dB and these small amounts make a vast difference when sending signals over long distance copper lines of 6&nbsp;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 line]]s, it depends on the [[attenuator (electronics)|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&nbsp;dB of available dynamic range above the noise floor within a transmission medium so, for example, a bin with an SNR of 18&nbsp;dB would be able to accommodate 6 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.


==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. This confirms that the encoding of each bit requires 6 dB of dynamic range since 40/6 is approximately 6.5 which is exactly half way between 6 and 7.
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&nbsp;dB with the bits per bin being around 6 or 7.


===Textual===
===Textual===
Line 66: Line 83:
24 0.0 1.3 3 * 25 0.0 1.0 2 * 26 0.0 0.7 0 * 27 0.0 0.7 0 <- upstream [END]
24 0.0 1.3 3 * 25 0.0 1.0 2 * 26 0.0 0.7 0 * 27 0.0 0.7 0 <- upstream [END]
28 0.0 0.7 0 * 29 0.0 0.0 0 * 30 0.0 0.0 0 * 31 39.9 0.9 6 <- downstream [BEGIN]
28 0.0 0.7 0 * 29 0.0 0.0 0 * 30 0.0 0.0 0 * 31 39.9 0.9 6 <- downstream [BEGIN]
32 38.4 0.9 6 * 33 39.9 1.1 7 * 34 256.0 1.0 0 * 35 39.8 1.2 7 <- downstream (1 unused BIN - interference?)
32 38.4 0.9 6 * 33 39.9 1.1 7 * 34 256.0 1.0 0 * 35 39.8 1.2 7 <- downstream (1 unused bin - interference?)
36 39.8 1.1 7 * 37 35.3 1.1 6 * 38 39.5 0.9 6 * 39 37.5 1.0 6 <- downstream
36 39.8 1.1 7 * 37 35.3 1.1 6 * 38 39.5 0.9 6 * 39 37.5 1.0 6 <- downstream
40 36.4 0.8 5 * 41 37.5 0.9 5 * 42 32.3 1.0 4 * 43 34.8 1.1 5 <- downstream
40 36.4 0.8 5 * 41 37.5 0.9 5 * 42 32.3 1.0 4 * 43 34.8 1.1 5 <- downstream
Line 74: Line 91:
56 34.3 1.1 5 * 57 31.9 0.9 4 * 58 33.7 0.9 4 * 59 31.5 1.2 4 <- downstream
56 34.3 1.1 5 * 57 31.9 0.9 4 * 58 33.7 0.9 4 * 59 31.5 1.2 4 <- downstream
60 30.6 1.1 5 * 61 30.2 1.1 4 * 62 17.3 1.1 3 * 63 25.7 1.1 3 <- downstream
60 30.6 1.1 5 * 61 30.2 1.1 4 * 62 17.3 1.1 3 * 63 25.7 1.1 3 <- downstream
64 21.9 0.8 2 * 65 22.8 0.8 2 * 66 256.0 1.0 0 * 67 255.9 1.0 0 <- downstream (2 unused BINs - interference?)
64 21.9 0.8 2 * 65 22.8 0.8 2 * 66 256.0 1.0 0 * 67 255.9 1.0 0 <- downstream (2 unused bins - interference?)
68 255.9 1.0 0 * 69 19.5 1.1 3 * 70 25.8 0.9 3 * 71 23.1 1.0 3 <- downstream (1 unused BIN - interference?)
68 255.9 1.0 0 * 69 19.5 1.1 3 * 70 25.8 0.9 3 * 71 23.1 1.0 3 <- downstream (1 unused bin - interference?)
72 23.3 1.0 3 * 73 16.9 1.2 4 * 74 21.7 0.8 2 * 75 23.2 0.7 2 <- downstream
72 23.3 1.0 3 * 73 16.9 1.2 4 * 74 21.7 0.8 2 * 75 23.2 0.7 2 <- downstream
76 22.0 1.0 3 * 77 25.3 0.7 2 * 78 24.7 0.7 2 * 79 20.8 0.9 2 <- downstream
76 22.0 1.0 3 * 77 25.3 0.7 2 * 78 24.7 0.7 2 * 79 20.8 0.9 2 <- downstream
Line 127: Line 144:


===Graphical with SNR===
===Graphical with SNR===
[[Image:Draytek_Vigor2600_BIN_Graphs.gif]]
[[File:Draytek Vigor2600 BIN Graphs.gif]]


==Summary and Key Points==
==Summary==
*DMT uses '''COFDM''' to create '''256 bins''' (carrier channels) using frequencies above voice on the line.
* DMT uses ''COFDM'' to create ''256 bins'' (carrier channels) using frequencies above voice on the line.
*The frequency layout can be summarised as:
* The frequency layout can be summarised as:
**0-4 kHz, voice.
** 0–4&nbsp;kHz, voice.
**4-25 kHz, unused guard band.
** 4–25&nbsp;kHz, unused guard band.
**25-160 kHz, 32 upstream bins.
** 25–138&nbsp;kHz, 25 upstream bins (7–31).
**200 kHz-1.1 MHz, 224 downstream bins.
** 138–1104&nbsp;kHz, 224 downstream bins (32–255).
*Each bin has a bandwidth of '''4.3125''' kHz.
* Bin N is centered on a frequency of ''N × 4.3125'' kHz.
* The bandwidth used by each bin overlaps neighbouring bins.
*The number of '''bits encoded''' on each bin is '''between 2 and 15''', depending on the '''attenuation''' and '''signal to noise ratio''' for that bin.
* The number of ''bits encoded'' on each bin is ''between 2 and 15'', depending on the ''signal to noise ratio'' (SNR) for that bin.
*For each '''6 dB of dynamic range''' above the noise floor within a bin, '''1 bit can be encoded''' reliably. Based on this, and the fact that only a '''minimum of 2 bits''' are encoded per bin, the SNR of any one single bin must '''not drop below 12 dB''' and should really be '''at least 15 dB''' to avoid bit errors occuring. Such errors would lead to the end user modem/router losing sync with the remote exchange (DSLAM).
* For each 3&nbsp;dB of SNR within a bin, 1 bit can be encoded reliably.
*'''Echo cancellation''' can be used on the lower frequency (upstream) bins to allow '''all 256 bins''' to be used for downstream.
* Too many errors that cannot be corrected by the built in error correction would lead to the end user modem/router losing sync with the remote exchange (DSLAM or MSAN).
*The '''1 and 8 Mbit''' figures originate from ''(32*15*4.3125*1000)/2 ='' '''1.035 Mbits''' and ''(256*15*4.3125*1000)/2 ='' '''8.28 Mbits''' respectively (see previous section for explanation).
* ''Echo cancellation'' can be used on the lower frequency (upstream) bins to allow ''all 256 bins'' to be used for downstream.


== ADSL statistics==
==ADSL statistics==
Figures in brackets have been shown to provide a stable service in practice.
Figures in brackets have been shown to provide a stable service in practice.
* Attenuation - How much signal is lost on the line (should be <56&nbsp;dB downstream, <37&nbsp;dB upstream)

*Attenuation - How much signal is lost on the line (should be <56 dB downstream, 37 dB upstream)
* [[Noise margin]] - 12&nbsp;dB or higher, for both downstream and upstream
* Attainable bit rates - Maximum speed line is capable of supporting
*Noise Margin - How loud the signal is compared to the line noise (should be >15 dB)
* DMT bits per bin - Shows which channels are in use
*Attainable bit rates - Maximum speed line is capable of supporting
* CV - Coding violations
*DMT bits per bin - Shows which channels are in use
* ES - [[Errored second|Errored Seconds]] - number of seconds that have had [[Cyclic redundancy check|CRC]] errors
*CV - Coding violations - line voltage should go +-+-+- but has gone +++ or ---
* Relative capacity occupation (RCO) - Percentage of the attainable line bit rate that is in use. This takes into account interference on the line and the target noise margin at the remote DSLAM.
*ES - Errored Seconds - number of seconds that have had CVs
*SES - Severely Errored Seconds - after 10 seconds of ES we start counting SES
* SES - Severely Errored Seconds - after 10 seconds of ES we start counting SES
*UAS - Unavailable Seconds - Seconds where we had no sync
* UAS - Unavailable Seconds - Seconds where we had no sync
*LOS - Loss of Sync
* LOS - Loss of Sync
* LPR - Loss of [[Customer-premises equipment|CPE]]<!--correct?--> power
*LPR - Loss of CPE power (only works about 50% of time if CPE supports it)
*LOF - Loss of Framing - DSL frames don't line up (usually ANSI/DMT mismatch)
* LOF - Loss of Framing - DSL frames don't line up


==External links==
==External links==
* [http://www.itu.int/rec/T-REC-G.992.1/en ITU-T Recommendation G.992.1: Asymmetric digital subscriber line (ADSL) transceivers]

== References ==
{{reflist}}


{{DSL technologies}}
*[http://www.alliedtelesyn.com/corporate/media/whitepapers/dsl_wp_c.pdf ADSL White Paper] ([[Portable Document Format|PDF]]) (Allied TeleSyn)
*[http://people.freenet.de/michael.schlegel/diplomarbeit.pdf Advanced reading covering the maths and science behind GMT, QAM and Trellis Constellation Coding] ([[Portable Document Format|PDF]])


[[Category:Broadband]]
[[Category:Digital subscriber line]]
[[Category:ITU-T recommendations]]
[[Category:ITU-T G Series Recommendations]]
[[Category:Telecommunications-related introductions in 1999]]

Latest revision as of 02:32, 17 December 2024

G.992.1
Asymmetric digital subscriber line (ADSL) transceivers
StatusIn force
Year started1999
Latest version(12/03)
December 2003
OrganizationITU-T
CommitteeITU-T Study Group 15
Related standardsG.992.2, G.992.3
DomainTelecommunications
LicenseFreely available
Websitehttps://www.itu.int/rec/T-REC-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.

DMT history and line rates

[edit]
Line rate obtainable (Mbit/s) against corresponding line length (km) for ADSL, ADSL2+ and VDSL
Line rate obtainable (Mbit/s) against corresponding line attenuation (dB) for ADSL, ADSL2 and ADSL2+

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.

DMT technical details

[edit]

Bins (carrier channels)

[edit]

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 in a bin on a good quality line.

The frequency layout can be summarised as:

  • 30 Hz-4 kHz, voice.
  • 4–25 kHz, unused guard band.
  • 25–138 kHz, 25 upstream bins (7-31).
  • 138–1104 kHz, 224 downstream bins (32-255).

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)

[edit]

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.

Reducing bit errors with QAM or PSK

[edit]

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.

Bin quality and bit rate

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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

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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.

DMT bits-per-bin examples

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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.

Textual

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-----------------------------------------------------------------------------
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 *   1   0.0  0.0  0 *   2   0.0  0.0  0 *   3   0.0  0.0  0  <- unused
  4   0.0  0.0  0 *   5   0.0  0.0  0 *   6   0.0  0.7  0 *   7   0.0  0.7  0  <- unused
  8   0.0  0.9  2 *   9   0.0  1.2  4 *  10   0.0  1.0  5 *  11   0.0  0.8  5  <- upstream   [BEGIN]
 12   0.0  1.0  6 *  13   0.0  0.9  6 *  14   0.0  0.9  6 *  15   0.0  1.1  7  <- upstream
 16   0.0  1.1  7 *  17   0.0  1.0  7 *  18   0.0  0.9  7 *  19   0.0  0.7  7  <- upstream
 20   0.0  1.0  6 *  21   0.0  0.9  5 *  22   0.0  0.9  4 *  23   0.0  1.2  4  <- upstream
 24   0.0  1.3  3 *  25   0.0  1.0  2 *  26   0.0  0.7  0 *  27   0.0  0.7  0  <- upstream   [END]
 28   0.0  0.7  0 *  29   0.0  0.0  0 *  30   0.0  0.0  0 *  31  39.9  0.9  6  <- downstream [BEGIN]
 32  38.4  0.9  6 *  33  39.9  1.1  7 *  34 256.0  1.0  0 *  35  39.8  1.2  7  <- downstream (1 unused bin  - interference?)
 36  39.8  1.1  7 *  37  35.3  1.1  6 *  38  39.5  0.9  6 *  39  37.5  1.0  6  <- downstream
 40  36.4  0.8  5 *  41  37.5  0.9  5 *  42  32.3  1.0  4 *  43  34.8  1.1  5  <- downstream
 44  31.6  1.0  4 *  45  37.7  0.9  5 *  46  35.7  1.1  6 *  47  34.3  1.2  5  <- downstream
 48  37.8  1.1  6 *  49  36.9  0.9  5 *  50  36.1  1.0  5 *  51  34.5  1.2  5  <- downstream
 52  32.3  1.0  4 *  53  31.6  1.0  4 *  54  33.6  0.9  4 *  55  31.6  1.1  4  <- downstream
 56  34.3  1.1  5 *  57  31.9  0.9  4 *  58  33.7  0.9  4 *  59  31.5  1.2  4  <- downstream
 60  30.6  1.1  5 *  61  30.2  1.1  4 *  62  17.3  1.1  3 *  63  25.7  1.1  3  <- downstream
 64  21.9  0.8  2 *  65  22.8  0.8  2 *  66 256.0  1.0  0 *  67 255.9  1.0  0  <- downstream (2 unused bins - interference?)
 68 255.9  1.0  0 *  69  19.5  1.1  3 *  70  25.8  0.9  3 *  71  23.1  1.0  3  <- downstream (1 unused bin  - interference?)
 72  23.3  1.0  3 *  73  16.9  1.2  4 *  74  21.7  0.8  2 *  75  23.2  0.7  2  <- downstream
 76  22.0  1.0  3 *  77  25.3  0.7  2 *  78  24.7  0.7  2 *  79  20.8  0.9  2  <- downstream
 80  19.1  1.0  2 *  81 255.9  1.0  0 *  82 256.0  1.0  0 *  83 255.9  1.0  0  <- downstream [END]
 84   0.1  1.0  0 *  85 255.8  1.0  0 *  86 255.8  1.0  0 *  87 255.9  1.0  0  <- unused
 88 256.0  1.0  0 *  89 256.0  1.0  0 *  90 255.9  1.0  0 *  91 255.9  1.0  0  <- unused
 92 256.0  1.0  0 *  93 255.9  1.0  0 *  94 255.8  1.0  0 *  95 255.3  1.0  0
 96   0.1  1.0  0 *  97 255.6  1.0  0 *  98 255.8  1.0  0 *  99 255.9  1.0  0     higher frequencies suffer greater
100 255.9  1.0  0 * 101 255.8  1.0  0 * 102 255.8  1.0  0 * 103   0.0  1.0  0     loss rates over longer lines
104 255.8  1.0  0 * 105 255.7  1.0  0 * 106 255.2  1.0  0 * 107 255.6  1.0  0
108 255.6  1.0  0 * 109 254.6  1.0  0 * 110 255.9  1.0  0 * 111 254.6  1.0  0
112 254.7  1.0  0 * 113 255.4  1.0  0 * 114 254.7  1.0  0 * 115 255.2  1.0  0
116 256.0  1.0  0 * 117 256.0  1.0  0 * 118 256.0  1.0  0 * 119 256.0  1.0  0
120 256.0  1.0  0 * 121 256.0  1.0  0 * 122 256.0  1.0  0 * 123 256.0  1.0  0
124 256.0  1.0  0 * 125 256.0  1.0  0 * 126 256.0  1.0  0 * 127 256.0  1.0  0
128 256.0  1.0  0 * 129 256.0  1.0  0 * 130 256.0  1.0  0 * 131 256.0  1.0  0
132 256.0  1.0  0 * 133 256.0  1.0  0 * 134 256.0  1.0  0 * 135 256.0  1.0  0
136 256.0  1.0  0 * 137 256.0  1.0  0 * 138 256.0  1.0  0 * 139 256.0  1.0  0
140 256.0  1.0  0 * 141 256.0  1.0  0 * 142 256.0  1.0  0 * 143 256.0  1.0  0
144 256.0  1.0  0 * 145 256.0  1.0  0 * 146 256.0  1.0  0 * 147 256.0  1.0  0
148 256.0  1.0  0 * 149 256.0  1.0  0 * 150 256.0  1.0  0 * 151 256.0  1.0  0
152 256.0  1.0  0 * 153 256.0  1.0  0 * 154 256.0  1.0  0 * 155 256.0  1.0  0
156 256.0  1.0  0 * 157 256.0  1.0  0 * 158 256.0  1.0  0 * 159 256.0  1.0  0
160 256.0  1.0  0 * 161 256.0  1.0  0 * 162 256.0  1.0  0 * 163 256.0  1.0  0
164 256.0  1.0  0 * 165 256.0  1.0  0 * 166 256.0  1.0  0 * 167 256.0  1.0  0
168 256.0  1.0  0 * 169 256.0  1.0  0 * 170 256.0  1.0  0 * 171 256.0  1.0  0
172 256.0  1.0  0 * 173 256.0  1.0  0 * 174 256.0  1.0  0 * 175 256.0  1.0  0
176 256.0  1.0  0 * 177 256.0  1.0  0 * 178 256.0  1.0  0 * 179 256.0  1.0  0
180 256.0  1.0  0 * 181 256.0  1.0  0 * 182 256.0  1.0  0 * 183 256.0  1.0  0
184 256.0  1.0  0 * 185 256.0  1.0  0 * 186 256.0  1.0  0 * 187 256.0  1.0  0
188 256.0  1.0  0 * 189 256.0  1.0  0 * 190 256.0  1.0  0 * 191 256.0  1.0  0
192 256.0  1.0  0 * 193 256.0  1.0  0 * 194 256.0  1.0  0 * 195 256.0  1.0  0
196 256.0  1.0  0 * 197 256.0  1.0  0 * 198 256.0  1.0  0 * 199 256.0  1.0  0
200 256.0  1.0  0 * 201 256.0  1.0  0 * 202 256.0  1.0  0 * 203 256.0  1.0  0
204 256.0  1.0  0 * 205 256.0  1.0  0 * 206 256.0  1.0  0 * 207 256.0  1.0  0
208 256.0  1.0  0 * 209 256.0  1.0  0 * 210 256.0  1.0  0 * 211 256.0  1.0  0
212 256.0  1.0  0 * 213 256.0  1.0  0 * 214 256.0  1.0  0 * 215 256.0  1.0  0
216 256.0  1.0  0 * 217 256.0  1.0  0 * 218 256.0  1.0  0 * 219 256.0  1.0  0
220 256.0  1.0  0 * 221 256.0  1.0  0 * 222 256.0  1.0  0 * 223 256.0  1.0  0
224 256.0  1.0  0 * 225 256.0  1.0  0 * 226 256.0  1.0  0 * 227 256.0  1.0  0
228 256.0  1.0  0 * 229 256.0  1.0  0 * 230 256.0  1.0  0 * 231 256.0  1.0  0
232 256.0  1.0  0 * 233 256.0  1.0  0 * 234 256.0  1.0  0 * 235 256.0  1.0  0
236 256.0  1.0  0 * 237 256.0  1.0  0 * 238 256.0  1.0  0 * 239 256.0  1.0  0
240 256.0  1.0  0 * 241 256.0  1.0  0 * 242 256.0  1.0  0 * 243 256.0  1.0  0
244 256.0  1.0  0 * 245 256.0  1.0  0 * 246 256.0  1.0  0 * 247 256.0  1.0  0
248 256.0  1.0  0 * 249 256.0  1.0  0 * 250 256.0  1.0  0 * 251 256.0  1.0  0
252 256.0  1.0  0 * 253 256.0  1.0  0 * 254 256.0  1.0  0 * 255 256.0  1.0  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

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Summary

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  • DMT uses COFDM to create 256 bins (carrier channels) using frequencies above voice on the line.
  • The frequency layout can be summarised as:
    • 0–4 kHz, voice.
    • 4–25 kHz, unused guard band.
    • 25–138 kHz, 25 upstream bins (7–31).
    • 138–1104 kHz, 224 downstream bins (32–255).
  • Bin N is centered on a frequency of N × 4.3125 kHz.
  • The bandwidth used by each bin overlaps neighbouring bins.
  • The number of bits encoded on each bin is between 2 and 15, depending on the signal to noise ratio (SNR) for that bin.
  • For each 3 dB of SNR within a bin, 1 bit can be encoded reliably.
  • Too many errors that cannot be corrected by the built in error correction would lead to the end user modem/router losing sync with the remote exchange (DSLAM or MSAN).
  • Echo cancellation can be used on the lower frequency (upstream) bins to allow all 256 bins to be used for downstream.

ADSL statistics

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Figures in brackets have been shown to provide a stable service in practice.

  • Attenuation - How much signal is lost on the line (should be <56 dB downstream, <37 dB upstream)
  • Noise margin - 12 dB or higher, for both downstream and upstream
  • Attainable bit rates - Maximum speed line is capable of supporting
  • DMT bits per bin - Shows which channels are in use
  • CV - Coding violations
  • ES - Errored Seconds - number of seconds that have had CRC errors
  • Relative capacity occupation (RCO) - Percentage of the attainable line bit rate that is in use. This takes into account interference on the line and the target noise margin at the remote DSLAM.
  • SES - Severely Errored Seconds - after 10 seconds of ES we start counting SES
  • UAS - Unavailable Seconds - Seconds where we had no sync
  • LOS - Loss of Sync
  • LPR - Loss of CPE power
  • LOF - Loss of Framing - DSL frames don't line up
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References

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  1. ^ "G.992.1: Asymmetric digital subscriber line (ADSL) transceivers". www.itu.int. Archived from the original on 2021-04-12. Retrieved 2021-04-12.