`
`::. Kilz - ADSL Technology & DMT - Bit Allocation + Bit Swapping.::
`
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`
`ADSL Technology and DMT
`
`This tutorial is aimed at those that wish to know more about the technology behind ADSL and how it is
`possible to transmit digital data over the telephone line between our home and the exchange.
`Unlike cable broadband which uses specially laid optical fibre to the premises and forms part of a
`larger network, ADSL utilises existing telephone cable to bring broadband to the home.
`
`How does adsl work — on the line.
`What is modulation?
`What is DMT?
`How does DMT work?
`What frequencies are used?
`VDSL2 tones in use.
`Tones which aren't in use.
`What are Tones, carriers, buckets, bins?
`Initialisation and synchronisation
`Bit Loading
`Bit Allocation
`Bit Allocation Table
`What is Bit Swapping?
`What is Bit Error Rate (BER)?
`Bit Allocation and Power Cut Back
`Bit Allocation and Low SNR
`Bit Allocation and Long Lines
`What is Seamless Rate Adaption (SRA)?
`
`~ How does adsl work - ‘on the line‘.
`
`Telephone lines can be used to convey analogue signals and the copper pair has the ability to carry a
`range of frequencies. DSL makes use of the fact that voice signals for telephone devices are all under
`4kH2, and utilises the previously unused higher frequencies to transmit data.
`
`An adsl splitter is used in the home to separate the telephony and adsl signals and to help
`--
`ensure that telephony devices don't use any of the frequencies used by ADSL. W
`- ADSL1 and ADSL2 utilise frequencies up to 1.1 MHz
`- ADSL2+ utilises frequencies up to 2.2 MHz
`- VDSL2 utilises frequencies up to 17.664 MHz
`
`DSL uses a technology called Frequency Division Multiplexing (FDM) which means it
`combines multiple frequency signals onto a single carrier wave over a shared medium
`such as cable, wire or fibre.
`
`Several years ago it was not unusual to hear BT engineers refer to DSLAMs at the
`exchange as a "Mux" or Multiplexor — DSLAM stands for Digital Subscriber Line Access
`Multiplexer.
`
`Top I
`
`~ What is Modulation?
`
`Because our computers use digital data a MODEM (Modulator-DEModulator) is needed to
`code and decode between digital and analogue signals. The method of converting digital
`data into analogue signals which can be carried over the copper wire is known as
`Modulation.
`
`-----.=@----:.
`N’--'
`'
`
`Our modem/routers and the DSLAM at the exchange are the hardware equipment that perform
`modulation.
`
`There are different methods of modulation, but the common standard used for ADSL is called DMT.
`Variations on the basic DMT modulation may be used depending upon the adsl type, but the basic
`
`http:/lwww.kitz.co.uk/adsl/adsl_technology.htrn
`
`Dish
`Exhibit 1033, Page 1
`
`
`
`7/2/2016
`
`::. Kitz - ADSL Technology & DMT - Bit Allocation + Bit Swapping.::
`
`theory remains the same.
`
`Standard
`
`G.992.1
`ADSL2 G.992.3
`
`g.DMT.bis
`
`DMT standard for adsll
`Improved modulation method with flexible framing and optimised use of RS
`coding gain within the frame structure. Enhanced channel overhead
`configuration.
`
`Doubling channels available for use.
`G.992.5
`G.993.2 ‘ Extensions: G.INP = G.998.4 Vectoring = G.993.5
`
`Top I
`
`~ What is DMT?
`
`DMT (Discrete Multi Tone) is a method of converting digital data into tones or frequencies that can be
`carried over telephone wire. Called ‘Multi-tone‘ because it splits the available frequencies into a defined
`number of smaller sub-channels or tones and 'Discrete' from the mathematical term meaning distinct
`nr cnnaral-cu
`
`Cookies help us deliver our services. By using our services, you agree to our use of cookies. Learn mor
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`Close messacl
`
`-VDSL2 (profile 17a) has 4-(-)-9V6-‘sub-channels
`-
`DMT deploys many "virtual modems" which are responsible for the control of each sub-channel. These
`virtual modems all work in tandem to carry the data bitstream.
`
`DMT is a modulation method for Frequency Division Multiplexing (FDM) which is when multiple signals
`are combined and carried over the same medium.
`
`Top I
`
`~ How does DMT work?
`
`DMT makes use of the available frequencies that can be transmitted on the telephone line and splits
`them into 256/512 equal sized frequency bins of 4.3125 kHz each.
`
`Sub-channels (or carrier bins) are where data bits are transmitted to and from our modem. Each sub-
`channel within a specific frequency range will be responsible for either upstream or downstream data.
`
`PSTI‘-l
`
`Upstream
`
`ADSL Frequencies
`Downstream
`
`lll ii
`3|2|
`4
`25
`133142
`Hz
`ltHz
`ltHz
`ltHz
`ltHz
`
`In-
`1104
`ltHz
`
`- Each carrier bin of 4.3125 kHz is the tone that you may see recorded on some router stats or in
`DMTtoo|.
`- You may also see sub-channels referred to as carriers, bins or buckets because this is what
`carries the data bits within each frequency range.
`- The full frequency range is split - regardless if you can make use of those frequencies or not.
`
`Not all channels are actually usable for the transmission of data. Some tones are not used such as the
`pilot tone, whilst some tones are reserved for voice or to prevent overlap of the different signal types.
`
`- Some tones, particularly the higher frequency ones may not be not be in use on longer lines due
`to the signal strength at that frequency being too weak.
`
`It may help you visualise how DMT works if you imagine this scenario:
`Imagine an old 56k modem that worked on the voice band frequency of 0-4kHz.
`Now imagine lots of ‘virtual‘ 56k modems each working on their own frequency each giving you 56kbps. As well
`as splitting the available frequencies, DMT is responsible for ‘binding' all these ‘virtual modems’ together.
`ADSL(1&2) has a maximum available 223 downstream subchannels. 223 x 56kbps = 12Mbps.
`ADSL2+ doubles the amount of available frequencies (subchannels) and we get 24Mbps.
`
`DMT is the technology which divides the whole bandwidth on the telephone line into lots of sub-
`channels and then controlling these ‘virtual modems‘ as one together in order to get higher speeds.
`
`http:/lwww.kitz.co.uk/adsl/adsl_technology.htrn
`
`Dish
`Exhibit 1033, Page 2
`
`
`
`7/2/2016
`
`::. Kitz - ADSL Technology & DMT - Bit Allocation + Bit Swapping.::
`
`Top I
`
`~ What frequencies are used?
`
`Below is a chart showing the available frequencies and corresponding frequency tones for various adsl
`standards.
`
`BINs
`Tones
`Start
`Tones
`Start
`Frequency
`Up to
`Standard
`Type
`22-22:‘
`
`E
`
`The calculator on the right will help convert tones into the
`frequency that it is centered at.
`
`DSL Frequency Bins & Tones
`
`Tone
`an
`
`'
`
`Frequency
`....
`.
`
`nu-
`
`Cookies help us deliver our services. By using our services, you agree to our use of cookies. Learn mor
`
`Close messaq
`
`Top I
`
`~ VDSL2 Tones in use.
`
`I
`
`Convert
`
`I
`
`The following tones are in use for VDSL2 F'|TC in the UK. The ECI and Huawei cabs have slightly
`different band plans:
`
`one Set
`
`VDSL2 Band Plans in the UK
`
`Bj
`
`jjjjjjijjijjij
`1111
`laflfiljjjjjjjjjjjj
`
`KZZKKKZZZZZZ
`-33$
`jjjmjjjjjj
`
`Top I
`
`~ Tones which aren't in use.
`
`Certain sub carrier channels are not used. Some of these are laid down in the g.DMT standard, whilst
`some others may depend upon the DSLAM/MSAN manufacturer and vary slightly.
`Common tones not in use are:-
`
`DC (First Tone). Tone O.
`Guard Band (Tones 1 to 5 < 25.875 kHz). Tone 1 POTS. Tones 2-5 prevents cross talk between POTS + adsl.
`Guard Band (Tone 32 - 138kHz). Prevents cross talk between upstream and downstream data.
`Nyquist frequency (Final frequency tone)
`Upstream Pilot Tone. (Tone 16 - 69kHz)
`Downstream Pilot Tone. (Tone 64 - 276kHz).
`Adaptive Pilot Tone. (Some ads|2+ systems select best channel conditions for pilot. Often tone 105, 110 etc)
`Annex_M Stop Band. (Tone 59 - 2S4kHz). ADSL2+ Annex_M only.
`DSLAM Specific Tones (eg Tones 476 - 499 2053kHz-2156kHz on Be*/O2 MSANs only).
`
`Your router will also mark any sub-channels where the SNR is too low to carry data as unusable.
`
`Top I
`
`~ Tones, carriers, buckets, frequency bins
`
`Generally all of the above terms may be used interchangeably, but to recap and clarify a bit further:
`
`http:/lwww.kitz.co.uk/adsl/adsl_technology.htrn
`
`Dish
`Exhibit 1033, Page 3
`
`
`
`7/2/2016
`
`::. Kitz - ADSL Technology & DMT - Bit Allocation + Bit Swapping.::
`The whole available frequency band is split into a
`distinct number of sub-channels.
`With adsl over POTs, the sub-channels form 3
`distinct separate channels:
`N Voice
`~ Upstream
`~ Downstream
`
`kHz
`_
`Bin lElucltet
`Sggagflrg
`Tone #
`
`Frequency
`
`3ll.‘| E75
`
`Sub channels may also be referred to as carriers, bins or buckets - so called because they are
`used to carry data bits.
`Each sub-channel has the same amount of bandwidth (4.3125 kHz) but transmits on different
`frequencies.
`The tone relates to the frequency on which the signal is transmitted.
`Each tone is centered on a specific frequency at which it transmits.
`The amount of data bits that can be carried in the bin can vary depending upon the quality of the
`signal at the particular frequency range for that particular bin.
`
`Ton ‘I
`
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`Close messaa
`
`exchange and sync speed is the rate at which it is agreed that your line can
`
`U1 ‘U mpg
`
`sustain.
`
`On the rate adaptive products before your router is said to be in sync it must
`go through a process called initialisation to set up the sync speed. Initialisation
`.
`consists of four key stages:
`
`i % U
`|
`% .
`443 an
`
`"5
`
`Handshake - Basically saying ‘hello can you hear me‘ and giving details of which technology is
`to be used (adsl 1, adsl 2 etc) and which protocols are to be used.
`Depending upon the technology the number of available subchannels are determined as per the
`defined standard (G.992.1 G.992.3 etc) The DSLAM will define which sub-channels may be used -
`for example certain tones are deliberately not used.
`Transceiver Training - Preliminary estimation of loop attenuation, test datastream, reporting of
`upstream power levels, power level adjustment (cut-back) if needed for spectral masks. Some
`sub-channels may have masks applied which limit the maximum power level at a particular
`frequency in order to reduce the risk of cross-talk.
`Channel Analysis - The modem will respond and the condition of each sub-channel is analysed.
`Power levels are reported and SNR and attenuation is calculated. Depending upon the condition
`(Noise/power level) of each channel, this determines the amount of data bits that can be carried
`in each channel. See Bit Loading for more information about this stage.
`Exchange - Setting the sync speed. The amount of overall bits that can be carried across all the
`sub-channels will determine your sync speed. (See Bit Allocation). The dslam will check that the
`modem can receive data at that speed ok and the router should respond and synchronisation is
`attained. If not the initialisation process is repeated until sync is achieved.
`
`Ton ‘I
`
`~ Bit Loading
`
`The amount of bits that can be carried per channel depends upon the SNR at that particular frequency,
`lower SNR levels may need more power to transmit data and since each frequency is subject to an
`overall power limit, those frequencies are able to carry less bits than a channel with a better SNR.
`
`- The better the SNR at that frequencies in the sub-channel range, then the more bits that can be
`allocated to that particular carrier bin.
`If the signal is good then 15 bits (maximum) can be allocated to that tone.
`If the SNR is weak/weaker at a particular frequency range, then not as many bits can be carried
`by the tone.
`Each 3dB of SNR equates to 1 bit (of data),
`A minimum of 2 bits per bin is needed for the tone to be usable for ADSL1 (6dB)
`ADSL2 and ADSL 2+ support single bit tones (3dB).
`If there's insufficient SNR in the channel, then the carrier bin is marked by the router as
`unusable.
`
`Bits are encoded as a constellation QAM (Quadrature Amplitude Modulation) which transmit the data by
`modulating the amplitude of 2 carrier waves. Called Quadrature since the 2 waveforms are out of
`phase by 90 degrees.
`
`The higher frequencies tend to carry less bits purely because the SNR isn't as good for those channels.
`Higher frequencies are more likely to be attenuated, therefore the SNR isn't as good and
`consequentially the carrier bins for those tones cant carry as many data bits.
`
`http:/lwww.kitz.co.uk/adsl/adsl_technology.htrn
`
`Dish
`Exhibit 1033, Page 4
`
`
`
`7/2/2016
`
`::. Kitz - ADSL Technology & DMT - Bit Allocation + Bit Swapping.::
`With rate adaptive dsl, its the SNR of the sub-channels which will determine your sync speed not the
`frequency of the tone. As long as the SNR at that particular frequency is good then modulation will
`load x no of bits to the bin regardless if its a high or low frequency.
`Lines which are more attenuated will see SNR decrease more rapidly at the higher frequencies hence
`less bit allocation overall and a lower sync speed.
`
`Twp 1
`
`DMT lit Allocation
`
`‘194
`536.6
`
`195
`4|:|.9
`
`'19?
`349.5
`E llilz ZIIIIIIB
`
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`Close messao
`
`As an example:
`
`Say you have a bin at tone 192 @ 828kHz that has an SNR of say 50dB, then that's more
`than enough SNR to allocate the full 15 bits to the carrier.
`Now say at tone 194 @ 836kHz, there's some noise broadcasting at that same frequency
`which takes the SNR down to 25dB, then only 5 bits may be able to be encoded on that
`channel.
`But frequency bands slightly further up may be fine and back up at 50dB SNR so 15 data
`bits are encoded on that tone.
`
`Bit Allocation is not actually quite as straight forward as in the above example and there's more to it
`during the sync negotiation period which has to cover an allowance for errors as defined by the Bit
`Error Rate (BER) and involves a fairly complicated process called Quadrature Amplitude Modulation
`(QAM) which is beyond the scope of this tutorial, and this is what determines the final sync speed.
`Somewhere in that process is the required overhead for Interleaving and/or more correctly Error
`Correction, and of course the Target SNR which sets some sort of base line, but...
`
`- The QAM rate is said to be 4,000 symbols per second, therefore each 3dB of SNR available in the
`sub channel over the base line will give approx 4kbps of sync speed, subject to a maximum of
`60kbps (15 x 4kbps) per carrier.
`- Sync rate depends on the number of bits loaded. With adsl each 3db of SNR is worth anywhere
`between 400 to 1200kbps of speed.
`- VDSL uses higher frequencies and has more available tones for loading. A rough guide is :
`~ Sync speeds of around 20Mbps - each 3dB is worth 3Mbps
`~ Sync speeds of around 40Mbps - each 3dB is worth 6Mbps
`~ Sync speeds of 60Mbps or more - each 3dB is worth 11Mbps.
`
`Ten 1
`
`~ Bit Allocation Table
`
`Once a line has sync'd a Bit Allocation Table (BAT) is defined which is what specifies how many bits are
`used/can be used within the sub-carrier channel.
`All modem routers will maintain a Bit Allocation Table and some routers allow you to view the Bit
`Allocation Table which may be useful for diagnostics.
`
`Top ‘I
`
`~ Bit Swapping
`
`Bit Swapping is a way of keeping the line more stable by constantly monitoring the frequency bins
`(carriers) in use and reusing them if possible.
`
`The bit swap process enables the connection to either change the
`number of bits assigned to each individual subchannel or if
`necessary increase the power level (gain) whilst still maintaining
`the data flow.
`
`http:/lwww.kitz.co.uk/adsl/adsl_technology.htrn
`
`Dish
`Exhibit 1033, Page 5
`
`
`
`7/2/201 6
`
`2:. Kilz - ADSL Technology & DMT - Bit Allocation + Bit Swapping.::
`
`If after sync the SNR within a specific tone falls too low to transmit
`x no of bits, then bit swapping allows any 'spare'
`in other sub
`channels to be used, whilst still maintaining the same number of
`total bits in the Bit Allocation Table.
`
`There's a minimum amount of bits in a channel.. and if the bits fall
`too low (say if you had a quick noise burst at a certain frequency)
`then the router could mark that channel as unusable which may
`cause the line to lose connection. A carrier bin needs a minimum
`of 2 bits = 6dB of SNR to be usable for adsll or 3dB for ads|2/2+.
`
`With bit swapping the router can swap the bits around and
`redistribute the bits to other channels by using any spare SNR at
`other frequencies or increasing the gain.
`
`Without bit-swapping, your connection would lose sync every time
`there was a noise burst that meant any sub-channel wasn't able to
`transmit its allocated number of bits.
`
`Too 1'
`
`lit Swapping Notes:
`_
`_
`1. Bit Allocation
`
`IEI
`
`11
`
`I2 13 II 1515 ‘IT ‘IE ‘IQ
`
`“"“‘
`
`Oltllz 21103
`
`~ Bit Error Rate (BER)
`
`The Bit Error Rate is calculated as the percentage of transmitted bits which contain errors. Many
`routers have the ability to conduct a Bit Error Rate Test (BERT). For the duration of the test the router
`will count the number of bits transmitted which is divided by the number of erroneous bits.
`
`The result may be displayed as a figure with 10 to a negative power for example 10'7 which is
`0.0000001 or 1e-7. To put it simply a BER of 10'5 is one errored bit in every 1,000,000 bits
`transmitted.
`
`The standard allowance for Bit Error Rate in adsl is a BER of 10'7. The SNR margin is calculated as
`SNR minus an allowance for a 10'7 BER.
`
`During a BER test, if the router is measuring CRCs in the FAST path bit stream , then 1 CRC is the
`equivalent of 20 bit errors and 50 bits for interleaved path.
`
`Top I
`
`~ Bit Allocation and Power Cut Back
`
`Below is an example of a healthy line showing the Bit Allocation Table and SNR statistics from a router
`with the aid of DMT tool.
`
`88
`
`104
`
`120
`
`136
`
`152
`
`168
`
`1&4
`
`2|!)
`
`216
`
`104
`
`8
`
`24
`
`56
`
`72
`
`88
`
`104
`
`121]
`
`136
`
`152
`
`168
`
`181
`
`21]]
`
`216
`
`232 Tote
`
`- Notice how the shape of the Bit Allocation (blue lines) correlate to the SNR (grey lines) in the
`lower table. The yellow lines show the SNR Margin.
`o Also note the smooth curves at on the upstream causing a inverted 'U' and also at the beginning
`of the downstream. This is likely due to spectral masks applied at the DSLAM which ensures that
`power levels are cut back to reduce the likely hood of cross-talk on those frequencies.
`- Another interesting observation about this particular line, is that power levels have been cut back
`across all bins (From a max of 20dBm down to 11.9dBm).
`
`Cutting back on power reduces the signal strength, which in turn reduces the SNR. The above line is
`very good with little noise, therefore without this cut back the above line would likely ‘drown out‘
`neighbouring lines at the DSLAM causing cross-talk for other lines.
`
`Note also how it can easily sync at the full 8128 kbps without having to assign the full 15 bits to any
`one particular tone.
`
`http://www.kilz.co.uk/adsl/ads|_techno|ogy.htm
`
`Top I
`
`Dish
`Exhibit 1033, Page 6
`
`
`
`7/2/2016
`
`2:. Kitz - ADSL Technology & DMT - Bit Allocation + Bit Swapping.::
`
`~ Bit Allocation Table and Low SNR
`
`Below is an example of a line that is more susceptible to noise.
`10-I
`T20
`‘.36
`152
`
`168
`
`‘.5-1
`
`
`
`n‘_'aP§1".¢il'3E5ur_n¢-J:
`
`-13
`
`EE
`
`‘:2
`
`S5
`
`104
`
`ta}
`
`-.35
`
`:52
`
`‘.
`
`2iE
`
`Despite having an attenuation of 30dB, the line was slightly under-performing at around 5.8
`mbps.
`Note the gaps and decline in SNR (yellow lines) at around tones 203 — 211.
`This is indicative of additional noise/interference in the frequency ranges 875kHz - 914kHz
`The decline in SNR at these frequencies has had a corresponding effect on the bit loading so that
`either side only 2 bits per bin have been able to load.
`
`ATEEF llltefnal Fe'Wll'|flg anu cnanglng TIIEEFS FIOISE was FECIUCECI OH UTE onenaing cnanneis ana Ule line IS
`now able to sync at the full 8Mb.
`
`Top I
`
`~ Bit Allocation and Long Lines
`
`Below is a graph showing the bit loading on a long (59dB) line. Although the line is quite long it is able
`to sync at speeds up to around 2Mbps
`
`24
`
`-ID
`
`72
`
`83
`
`1|}!
`
`121]
`
`136
`
`152
`
`158
`
`181
`
`21]]
`
`215
`
`THE
`
`1DI1?22-I-231DElIlIJl-IB51B588Il68?2-I?!)-I932
`
`NUIl5=:
`
`B
`
`24
`
`-ID
`
`56
`
`72
`
`83
`
`1|}!
`
`121]
`
`136
`
`152
`
`158
`
`181
`
`21]]
`
`asses
`
`215 1 Tu!
`
`14
`11
`
`B52
`
`asses
`
`Long lines are naturally higher attenuated therefore SNR across all channels is lower.
`Lower SNR means less bits can be loaded per bin = lower sync speed
`Notice natural tail off of SNR at the higher frequencies
`Note where the SNR + SNR Margin at Tones 132-137 is too low to allocate any bits
`A few tones at 140+ have just sufficient SNR (grey lines) to allocate 2 bits (blue lines).
`
`What is interesting about the above line is notice the corresponding high peak in the SNR Margin, with
`the lower bit loading at tone 74 and tone 101. This particular line had been up for a couple of days
`when the graph was loaded. The yellow peaks indicate that that if a resync was performed 'now' then
`those bins would likely load more bits to gain a higher sync speed. This could be typical of a line that
`each evening receives a small amount of background noise‘ on those channels. On this particular line,
`the amount of additional bits available probably wont make that much difference, but the graph is a
`good indication what to look for and shows how lines can vary over the course of the day.
`
`Top I
`
`~ Seamless Rate Adaption (SRA)
`
`Seamless Rate Adaption (SRA) is a method which dynamically adapts your line rate /sync speed on the
`fly depending upon the current condition of your line without having to perform a full retrain or resync.
`
`Normally when your SNR Margin falls too low for your router to ‘hear’ the signal from the exchange
`your connection will drop and you lose synchronisation with the exchange. The router then has to re-
`negotiate a lower sync speed causing a small period of time without connectivity.
`
`With SRA, line conditions are constantly monitored and any increases/decreases in SNR result in an
`increase/decrease in the line connection speed without having to go through the initialisation process.
`
`Line speed will always be at the highest possible rate depending upon your set target SNR, which is
`particularly useful if you have had a low synch and line conditions later improve, as SRA will ensure
`that your line speed will increase in line with the better conditions.
`
`Both the router and DSLAM need to be able to support SRA.
`
`http://www.kitz.co.uk/adsl/ads|_techno|ogy.htm
`
`Dish
`Exhibit 1033, Page 7
`
`
`
`7/2/2016
`
`2:. Kitz - ADSL Technology & DMT - Bit Allocation + Bit Swapping.::
`
`Presently the only ISP in the UK that supports SRA is UKOn|ine on their adsl 2+ LLU exchanges.
`
`Copyright © Kitz 2003- 2016
`All rights reserved
`Unauthorised reproduction
`prohibited
`
`'i_
`'
`
`,
`
`'
`
`ll Broadband || ISPs || Tech || Routers || Site || Wiki
`| About | Privacy Policy |
`
`|| Forum ||
`
`http://www.kitz.co.uk/adsl/ads|_techno|ogy.htm
`
`Dish
`Exhibit 1033, Page 8