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Author Topic: William Grimsley's Line - After DLM Reset  (Read 35560 times)

Dray

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Re: William Grimsley's Line - After DLM Reset
« Reply #180 on: April 28, 2016, 03:58:27 PM »

I wonder if customers are informed of that before sale?
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WWWombat

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Re: William Grimsley's Line - After DLM Reset
« Reply #181 on: April 28, 2016, 05:16:56 PM »

BTW's FTTC Handbook says this:
Quote
When customers order Self Install they should understand that there is a higher risk of the line being impacted with wiring issues and End Users getting lower line and throughput rates. We recommend that CPs use Range B Broadband Availability Checker (BBAC) values when providing estimated speeds to their customers.

Customers might not be informed about the reason why range B is being used, but the estimates provided by the ISP at point of sale (they all do this, right) should indeed come from range B.
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mlmclaren

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Re: William Grimsley's Line - After DLM Reset
« Reply #182 on: April 28, 2016, 11:15:31 PM »

BTW's FTTC Handbook says this:
Quote
When customers order Self Install they should understand that there is a higher risk of the line being impacted with wiring issues and End Users getting lower line and throughput rates. We recommend that CPs use Range B Broadband Availability Checker (BBAC) values when providing estimated speeds to their customers.

Customers might not be informed about the reason why range B is being used, but the estimates provided by the ISP at point of sale (they all do this, right) should indeed come from range B.

That would explain why some ISP's range the speed from the worst of B to the best of A....

So due to my VDSL being activated remotely instead of an engineer visit I could well be demoted to range B... however I did have an engineer visit to activate and test the phone... he was supposed to do VDSL too but BT hadn't ordered it and it had to be expedited to the next morning under a complaint....

I get the feeling that with Openreach's movedment's regarding the removal of CP equipment and pushing self installs could lead to the BTw checker removing the 2 ranges and resulting in one range for all.  ::)
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kitz

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Re: William Grimsley's Line - After DLM Reset
« Reply #183 on: April 29, 2016, 02:53:40 PM »

Quote from: wwwombat
I was always of the opinion that under 1dB was a good line, and that up to 3dB could be expected. From this, I thought the reasoning for a 6dB target was to allow the swing to go no lower than 3dB ... because the 3dB margin was where the errors really started to bite.

Thats always been the case of how I've understood things were supposed to work and why I keep saying 3dB is considered normal - going right back to the days when rate adaptive DSL was first introduced.  Its not an Openreach thing, its world wide DSL technology thing.
Prior to that on fixed rate lines (512, 1Mb 2Mb) it was 6dB that was required.

We seem to have lost sight of the fact that swings are considered normal over copper and its why I dont want anyone going off on a wild goose chase for something which is highly likely to come back 'no fault found'.

Quote
On the technology of "impulse noise protection":
My weakest point on understanding the aspects of DSL is indeed "coding gain" - especially when you include the hidden parts, such as trellis coding, and then start combining them as inner/outer pairs.

I know :(  When I see all the complicated formulas which show how it works,  my eyes just glaze over Im afraid.

Quote
I noted @ejs made a comment about whether a G.INP-enabled modem could make predictions about the improvements in coding gain, and attempt to make an allowance for extra speed. That is an interesting avenue to explore, alongside the relative efficiency of different methods.

I was about to respond to ejs about the Broadcom white paper (which is why I re-opened this thread) and talk a little bit about coding gain and 'double booked' a couple of days ago.    I'd typed a long reply, but had to go out and left it unfinished... only to have FF crash and I lost the post.  :'(  I shall go back to that post again and try recreate what I was originally going to say.
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kitz

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Re: William Grimsley's Line - After DLM Reset
« Reply #184 on: April 29, 2016, 03:16:21 PM »

@ejs

I think when it says doubled booked its talking about the overheads required for FEC... and also when using INP that can further reduce the line rate.
INP is about applying sufficient protection against noise to ensure that 'x' DMT symbols can be recovered in the event of noise burst.
 
AIUI when it starts going on about the BER, I think its trying to say, that when you adjust the Bit Error Rate to increase error protection, then this 'takes away' SNRm.  I think coding gain is what makes the differential between the true SNR and the SNR margin.

ie this is what makes the SNRm increase/decrease at each stage of INP.   

eg. 

INP = 3 may take an additional 3bB of SNRm from the actual SNR value
INP = 4 may take an additional 6dB of SNRm from the actual SNR value

[I made up those SNRm figures - they could vary depending on various factors such as line rate and Im not about to start playing with complicated formulae]

So when its saying 'double booked' I'm assuming they mean FEC overheads and any adjustments to the BER which will adjust the SNRm further.   I could be wrong, that's the way I read it.
Quote
I think what the Broadcom whitepaper is saying, is that if you switch on interleaving to fix errors, and then what interleaving does is also add coding gain,

Interleaving itself doesnt fix errors, it chops up and spreads the data, so that if there is a noise burst, FEC has an increased chance of being able to correctly re-assemble the data.
Its FEC/INP adds coding gain.  Coding gain is the amount of SNR used by Error Protection to reach the desired BER rate compared against a line without RS encoding.

Although you can have FEC without interleaving, interleaving on its own without is useless.  But unfortunately it is interleaving that adds latency and delay.   This is a brief and crude example why delay occurs

Original data packets:
     abcd efgh ijkl mnop

Interleaved packets:
     aeim bfjn cgko dhlp

So not only the time taken to chop up and reassemble the data,  but it has to wait longer for data packet containing 'abcd' to arrive... because it cant re-assemble it until packet containing 'mnop' also arrives.

If we increase the depth we get:
     abcde fghij klmno pqrst uvwxy

Interleaved:
     afkpu bglqv chmrw dinsx ejoty

So now we have to wait for a 5 packet spread which takes longer to receive before data can be re-assembled.. and more delay.
The above are just examples to show in a very simplified way. Packets dont really contain only 4 or 5 chars but by using the alphabet its easier to see what is happening to the data stream.

Quote
I know my ADSL2 line works better with interleaving on, and I get a little more downstream bandwidth,

I'm not disputing that you may do.     
But the DSL theory is that FEC will always reduce line rate because of overheads.   There was some advances in RS coding algorithms ie (s=1/2 mode) which actually made for more efficient overheads, but once FEC is on, then those overheads are going to be there.
When on ADSL2+, FEC reduced my line rate of 24Mbps down to about 18.5Mbps :(

Quote
rather than using interleaving to optimise the line towards more bandwidth rather than lower latency, interleaving only gets used to try to correct errors, but it's not really powerful enough at that.

Interleaving cant correct any errors at all.  Its spreads data to help protect against burst errors.   Its FEC which does the all the error correcting.   
When they say Interleaving improves FEC, its because FEC has more chance of being able to recover from a single noise burst.

eg if the noise burst was
     abcd e--- ijkl mnop

on interleaved data, that same noise burst would be
     aeim b--- cgko dhlp

after its been de-interleaved then the actual damage of the noise burst would be
     a-cd e-gh i-kl m-op


FEC stands little chance of recovering packet e---, but it can easily correct a-cd e-gh i-kl m-op and therefore no data loss.
   
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kitz

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Re: William Grimsley's Line - After DLM Reset
« Reply #185 on: April 29, 2016, 03:24:56 PM »

Quote from: wwwombat
I tend to think that we are seeing the "allowance for extra speed" come out in a different way; and that BT are recognising this added efficiency through this different way: By allowing the target margin to drop below 6dB. By allowing the target margin to drop below 6dB, they are inviting more (raw) bit errors into the stream, under the belief that the new system can cope with, and fix, more errors.

Wondering if they could lower BER probability, in the confidence that retransmission is so effective.  Whilst true SNR will always remain the same, changing parameters could affect the differential between SNR and SNRm.    I don't know enough about this though and its why I dont even begin to touch it on the main site because it is so complicated.    :(
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ejs

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Re: William Grimsley's Line - After DLM Reset
« Reply #186 on: April 29, 2016, 06:58:33 PM »

@ejs

I think when it says doubled booked its talking about the overheads required for FEC... and also when using INP that can further reduce the line rate.
INP is about applying sufficient protection against noise to ensure that 'x' DMT symbols can be recovered in the event of noise burst.
 
AIUI when it starts going on about the BER, I think its trying to say, that when you adjust the Bit Error Rate to increase error protection, then this 'takes away' SNRm.  I think coding gain is what makes the differential between the true SNR and the SNR margin.
I'm sorry but I don't think that's correct. If you adjusted the Bit Error Rate, you would get a lower speed, with Interleaving+FEC or without. It's talking about the BER because it's saying interleaving+FEC can't achieve a BER suitable for IPTV, not without increasing the target SNRM which would reduce the bandwidth further. And then it talks about how PhyR can achieve a much better BER at a 0dB margin than the interleaver+FEC can at 3dB.

ie this is what makes the SNRm increase/decrease at each stage of INP.   

eg. 

INP = 3 may take an additional 3bB of SNRm from the actual SNR value
INP = 4 may take an additional 6dB of SNRm from the actual SNR value

[I made up those SNRm figures - they could vary depending on various factors such as line rate and Im not about to start playing with complicated formulae]
I think considering increasing INP levels as "taking away from the actual SNRM" is an unhelpful way of looking at it. There may have been a "sync speed" (net data rate) reduction comparable to changing the target SNRM, but I think the entire net data rate reduction comes from the amount of bandwidth used to carry the FEC data. Providing higher INP levels would require a greater portion of the line's bandwidth to be allocated for carrying the FEC data. Perhaps it would be useful to calculate the "gross" line rate, adding the bandwidth used for FEC data to the net data rate, to see if that shows that the total bandwidth is all still there.

Regarding the "inflated" attainable net data rate, that may be because the original definition for the attainable rate was poorly defined, or defined to be the highest possible rate, taking into account some parameters, but ignoring others. G.993.2 mentions two attainable rate methods, 11.4.1.1.7.1 is the basic method, which can take into account "all available coding gains" within the maximum latency, but it does not mention it must meet the required minimum INP level. Following that it describes an improved method for the attainable rate, which can take into account far more parameters including the minimum INP level, but support for the improved method is optional. Then G.998.4 section C.1.4.2 makes further additions to the improved attainable rate method. I do not know of any VDSL2 modems which mention which attainable rate method they use.

So when its saying 'double booked' I'm assuming they mean FEC overheads and any adjustments to the BER which will adjust the SNRm further.   I could be wrong, that's the way I read it.
The Broadcom whitepaper usually said over-booked rather than double-booked. It was happy enough to book the RS coding gain for use with PhyR.
I don't think there are any adjustments to the BER being considered. Technically speaking, I think the SNRM is defined as the maximum increase in noise power for which the BER would be maintained. The BER is another parameter that would affect the line speed achieved, but I don't think it should be somehow "converted" into an equivalent change in the SNRM.

Quote
I think what the Broadcom whitepaper is saying, is that if you switch on interleaving to fix errors, and then what interleaving does is also add coding gain,
That was me being lazy and saying interleaving to mean "interleaving+FEC" or even "FEC (Interleaving + RS Coding)".

Quote
I know my ADSL2 line works better with interleaving on, and I get a little more downstream bandwidth,
Ditto. Thinking of "switching on interleaving" as in setting the DLM to have interleaving+FEC as "opt in".

Quote
rather than using interleaving to optimise the line towards more bandwidth rather than lower latency, interleaving only gets used to try to correct errors, but it's not really powerful enough at that.
Yes, and again, interleaving to mean interleaving+FEC.

Quote from: That Broadcom Whitepaper
DSL technologies, including ADSL1/2/2+ and VDSL2, define a Forward Error Correction (FEC) scheme
based on a combination of RS coding and convolutional interleaving to provide extra coding gain in first
instance and, by extension, protection against impulsive noise (SHINE or REIN).

But the DSL theory is that FEC will always reduce line rate because of overheads.
That doesn't take into account the effect of this coding gain. If the benefit of the coding gain is greater than the overheads, the net data rate can be higher.
The other aspect of the theory is the effect of the constraints of the minimum INP level and maximum delay, which could greatly reduce the maximum rate on a line that might otherwise be short enough to get the full 24Mb on ADSL2+.

Coding gain is a positive, an advantage, it improves your speed. Even that Broadcom whitepaper mentioned that with PhyR, RS encoding (FEC) can be used to increase bandwidth.
Quote from: That Broadcom Whitepaper
As retransmission is also complementary to RS encoding, the RS overhead can now be selected solely
to optimize the coding gain. Thus, RS encoding becomes a benefit rather than an overhead.

If you are not using RS coding (FEC) to provide INP, then the RS coding can be used only for increasing the bandwidth. Obviously the coding gain would need to increase the total bandwidth by more than what the FEC data uses for it to be of overall benefit and increase the net data rate. Even when the FEC+interleaving is being used to provide INP, the coding gain (benefit) can still be taken into account, even though in most cases, the bandwidth used by the FEC data takes up more bandwidth than what's added by the coding gain, and so the net data rate is reduced.

Quote from: http://www.cs.cmu.edu/~guyb/realworld/reedsolomon/reed_solomon_codes.html
Coding Gain

The advantage of using Reed-Solomon codes is that the probability of an error remaining in the decoded data is (usually) much lower than the probability of an error if Reed-Solomon is not used. This is often described as coding gain.

Example: A digital communication system is designed to operate at a Bit Error Ratio (BER) of 10-9, i.e. no more than 1 in 109 bits are received in error. This can be achieved by boosting the power of the transmitter or by adding Reed-Solomon (or another type of Forward Error Correction). Reed-Solomon allows the system to achieve this target BER with a lower transmitter output power. The power "saving" given by Reed-Solomon (in decibels) is the coding gain.
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ejs

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Re: William Grimsley's Line - After DLM Reset
« Reply #187 on: April 30, 2016, 08:01:30 AM »

I did a bit more reading of the ITU DSL documents about the BER.

On ADSL2/2+ the bit error ratio is an adjustable parameter, error_max can be set for an error ratio of 10-3, 10-5 or 10-7.

On VDSL2 the bit error ratio is a constant, to not exceed 10-7.

I can't quite work out how the Broadcom whitepaper got:
Quote
With the typical error patterns observed in DSL modems, a BER of 10–7 at 30 Mbps translates into one error every 13 seconds!
It may have something to do with those "typical error patterns observed in DSL modems" rather than a simplistic one bit error every 107 = 10,000,000 bits.

If e.g. Openreach wanted to achieve fewer errors than the nominal maximum BER of 10–7, they'd have to work out what combination of target SNRM and INP level would be expected to achieve that, while trying to minimize the bandwidth reduction (from the increased SNRM and FEC overheads) and latency increase (from interleaving). They'd probably have to do that with consideration to the typical noise characteristics found on their particular network.
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kitz

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Re: William Grimsley's Line - After DLM Reset
« Reply #188 on: April 30, 2016, 10:22:41 AM »

@ ejs

Thanks for that info. 
I was aware of adjustable params for BER, but wasnt aware that on vdsl2 it was a constant. 

There is still something about coding gain that niggles, but I need to do further research to explain it properly..  and unfortunately I dont appear to have time to do any of that these days.   
I seem to be constantly chasing my tail to keep up with admin which is still backlogged despite me being sat here until gone 2am last night doing stuff no-ones sees, and I still havent caught up on PMs and emails. So its not really a good time for me to go reading white paper type stuff.  In a way its ironic that I never seem to get chance to do the stuff I love doing these days, because I get so tied up doing the more mundane things.   
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