General Discussion
  >> Fibre Broadband


Register (or login) on our website and you will not see this ad.


Pages in this thread: 1 | 2 | (show all)   Print Thread
Standard User WWWombat
(fountain of knowledge) Tue 28-May-13 17:50:18
Print Post

Attainable Bandwidth vs Crosstalk


[link to this post]
 
Hi all,

Over time, I've been reporting how my "attainable bandwidth" has been decreasing. As this has happened, my SNRM figure has been dropping. And I have believed this to be caused by increasing crosstalk as usage increases.

My history is, roughly, as follows:
Date      Attain Actual SNRM
01/2012   90     40
02/2012   91     40
02/2012   85     80     7.3dB   * 80/20 package turned on
03/2012   84     80     7.1dB
06/2012   83     80     6.9dB
07/2012   82     80     6.6dB
09/2012   82     80     6.5dB   * Voice fault occurs around here
11/2012   81     80     6.3dB
12/2012   78     80     5.4dB
01/2013   76     80     4.6dB
05/2013   72     80     3.0dB


It has stayed synchronised since the beginning of December, perfectly stable, running at 80Mbps, even as the SNRM has dropped to 3dB.

Last week, I finally bit the bullet, and decided to re-sync my line, even though I expected my sync speed to drop to below 72Mbps.

Imagine my surprise, then, when it resync'ed at 80Mbps with an attainable value of 83Mbps. Over the week since, this attainable figure has dropped some, but is still higher than it was earlier this year.

Date      Attain Actual SNRM
05/2013   83     80     6.7dB
05/2013   79     80     5.6dB


I finally managed to turn some of my graphs into animations to highlight the line conditions. It looks like there has been hardly any change whatsoever.

Bitloading Animation
Hlog Animation
QLN animation
SNR animation

So - is a dropping "max attainable" speed really a reflection on the crosstalk on the line? What else would cause it to drop over time?

Confused...
Standard User RobertoS
(sensei) Tue 28-May-13 22:04:31
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: WWWombat] [link to this post]
 
In reply to a post by WWWombat:
So - is a dropping "max attainable" speed really a reflection on the crosstalk on the line? What else would cause it to drop over time?

Confused...
It's probably very similar to ADSLx DLM in what happens with bit-swapping.

(Broad brush description). If a frequency/bin gets to empty due to bit-swapping away from it, (possibly near empty), due to repeated instances of noise on the line, that frequency gets removed from the available and isn't used again. As and when several frequencies suffer this fate the "Attainable" is bound to decrease, and usually the actual as well.

A re-sync starts from scratch, with all the frequencies that qualify at that time being taken into consideration for attainable and actual use. What happened previously is forgotten.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet UnLim Fibre (FTTC). Sync ~ 53.0/16.7Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User Chrysalis
(legend) Wed 29-May-13 16:28:54
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: WWWombat] [link to this post]
 
I cant explain what you seeing but I can assure you the attainable reported my my HG does also reflect actual sync speeds aside from the fact it tends to overestimate a bit by 1-2mbit, so if eg it drops and I resync then the next sync will be lower.

edit, just read Robertos reply and that has logic to it, so is possible your line is disabling tones due to bursts of noise or something.

BT Infinity 2 Since Dec 2012

Edited by Chrysalis (Wed 29-May-13 16:30:20)


Register (or login) on our website and you will not see this ad.

Standard User WWWombat
(fountain of knowledge) Wed 29-May-13 17:46:35
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: RobertoS] [link to this post]
 
Ah. That might be something... I'll see if I can establish anything from the accumulated stats.
Standard User WWWombat
(fountain of knowledge) Wed 29-May-13 17:47:56
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: Chrysalis] [link to this post]
 
That is my experience too, and what I was expecting - something just under the max.

I'll have to check onRoberto's idea.
Standard User Ignitionnet
(knowledge is power) Wed 29-May-13 18:57:44
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: RobertoS] [link to this post]
 
In reply to a post by RobertoS:
As and when several frequencies suffer this fate the "Attainable" is bound to decrease, and usually the actual as well.


Sure? For the actual to drop you either need SRA to be in place or a full resync, nothing to do with bit swapping?
Standard User RobertoS
(sensei) Wed 29-May-13 22:34:54
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: Ignitionnet] [link to this post]
 
Pity I didn't read that 25 minutes earlier. I can't edit it now. Of course, you are right. Slip of the brain!

I stand by my thoughts about the change in the attainable though smile.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet UnLim Fibre (FTTC). Sync ~ 53.4/16.8Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.

Edited by RobertoS (Wed 29-May-13 22:35:20)

Standard User WWWombat
(fountain of knowledge) Wed 29-May-13 23:01:27
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: RobertoS] [link to this post]
 
I just used the graphing scripts to strip the "bit loading" data for downstream only, and checked your idea out for the 6 month period that the modem stayed in sync (Nov 2012 to May 2013).

It originally had 22 tones where zero bits were allocated, and this became 36 tones at the end of the 6 month period.

Unfortunately, there was a single tone that started with zero bits, that was then allocated some bits part-way through.

The 15 "newly-zero" tones all dropped from around 4 or 5 bits to zero. Interestingly, most of the tones with zero bits, and all of the "new" ones, were located in tones 212-314, firmly in the region where the PSD mask was most strictly reducing my power for ADSL2+ crosstalk purposes.

15 tones aren't enough to explain the scale of the change I saw (a drop of 9Mbps in attainable, followed by instantaneous increase of 11Mbps).

If you look at the bit-loading animation, the period I checked is covered by frames 9-14. You can see some of the zero's being put into place. However, you also see a big increase in the number of bits in the rest of D1 (tones 350-850) that far outweighs the impact of just 15 tones.

So, I don't think the problem is the number of tones becoming zero.

But I did see something else, which is best left to a separate reply.
Standard User WWWombat
(fountain of knowledge) Wed 29-May-13 23:18:50
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: Ignitionnet] [link to this post]
 
In reply to a post by Ignitionnet:
For the actual to drop you either need SRA to be in place or a full resync, nothing to do with bit swapping?

I just did some analysis on the bit-loading data (as part of investigating RobertoS' idea.

Before that analysis, I would have agreed with you 100% on this - in theory and (I'd have said) in practice. Bit-swapping really does only move bits around.

If bit-swapping does indeed work like this, then you'd expect the sum total of bits to never change. If you total all the downstream bits (zero, or between 2 and 15 on each tone), and the number should be unchanged on every single sample.

However, my analysis of the bit-loading data for the 6 month period that sync was maintained showed some interesting changes to the number of bits (and, when multiplying by 4000, the equivalent data transfer rate including the framing overheads)
Date      Attain Actual SNRM    #Bits   Data Rate
11/2012   81     80     6.3dB   20882   83.5Mbps
11/2012   81     80     6.3dB   20883   83.5Mbps
12/2012   78     80     5.4dB   21100   84.4Mbps
01/2013   76     80     4.6dB   21336   85.3Mbps
05/2013   72     80     3.0dB   21686   86.7Mbps
05/2013   72     80     3.0dB   21718   86.9Mbps


So, even though the published sync speed never changed (it was always 79,999Kbps), the bit loading suggests that it was transferring more data down.

I'm pretty sure that the VDSL2 framing overheads are not variable, and there was no change in the FEC/interleaving settings through this time (which would have required a re-sync anyway). What the data was used for is a mystery.

Kinda baffling.
Standard User Chrysalis
(legend) Thu 30-May-13 14:12:26
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: Ignitionnet] [link to this post]
 
ignition snrm does move around without SRA. Bitswapping can turn tones off without SRA.

The modem estimates attainable based on snrm.

So if bitswapping reduces tones available for use then existing tones will reduce snrm to maintain sync.

BT Infinity 2 Since Dec 2012

Edited by Chrysalis (Thu 30-May-13 14:12:52)

Standard User Ignitionnet
(knowledge is power) Thu 30-May-13 15:46:51
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: Chrysalis] [link to this post]
 
In reply to a post by Chrysalis:
ignition snrm does move around without SRA. Bitswapping can turn tones off without SRA.

The modem estimates attainable based on snrm.

So if bitswapping reduces tones available for use then existing tones will reduce snrm to maintain sync.


I didn't say anything about the attainable rate or the SNRM.
Standard User asbokid
(member) Thu 30-May-13 16:14:11
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: WWWombat] [link to this post]
 
On your line, the SNR per subcarrier has noticeably dropped over those months of scrutiny. Yet QLN has remained virtually unchanged (except in frame 5 of your excellent animation - what happened there? The noise floor dropped up to -20dBm. Just as if other subscribers in your cable bundle were disconnected while your line stayed connected.)

Putting those two factors together - the static noise floor and yet falling SNR across the subcarriers - suggests that (downstream) transmit power was cut. Would you have kept a record of transmit power? The xdslcmd command only reports an aggregate - the ACTATP (Actual Aggregate Transmit Power) in each direction, but it should still indicate a cut in power (if there was one).

Max Attainable Rate a.k.a. ATTNDR is calculated in basic form using the following equation:

http://huaweihg612hacking.files.wordpress.com/2013/0...

According to G.993.2 (see page 143 in [1]) to obtain a Bit Error Rate (BER) of 10^-7 - the accepted minimum error rate- the SNRGAP in that equation is set to 9.75dB.

Since the Target SNRM on your line has dropped from 6dB to around 3dB, the basic ATTNDR equation illustrates how and why, with a lowered TARSNRM, the bit-loading still edged higher.

With max data rates of 80/20 in the channel profile, this has resulted in a significant number of surplus bits per frame.

That's my reading any way!

cheers, a

[1] http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-R...

Edited by asbokid (Thu 30-May-13 19:38:48)

Standard User Chrysalis
(legend) Thu 30-May-13 18:36:32
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: asbokid] [link to this post]
 
yeah could be that also.

BT Infinity 2 Since Dec 2012
Standard User WWWombat
(fountain of knowledge) Thu 30-May-13 23:51:17
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: asbokid] [link to this post]
 
In reply to a post by asbokid:
On your line, the SNR per subcarrier has noticeably dropped over those months of scrutiny. Yet QLN has remained virtually unchanged

For SNR, are you talking about over the entire period, or just within the single 6 month period without a resync?

In either case, I'm not really seeing the same thing that you are - except for the first 2 or 3 frames, which do seem a little higher. Perhaps because these were on a 40/10 package (before 80/20 became available, but after the 17a bandplan was turned on)? Maybe that's a little unfair.

I've done another animation of just 4 frames: the very first SNR frame of an 80/20 sync, then the start and end frames of that 6 month single-sync period, then the first sync after that 6 month period. It really doesn't seem that much different to me. At least not compared to the attainable values of 84, 81, 72, 83.

4-Frame SNR animation

(except in frame 5 of your excellent animation - what happened there? The noise floor dropped up to -20dBm. Just as if other subscribers in your cable bundle were disconnected while your line stayed connected.)

Certainly a drop to -140dBm is an indication that there is *no* noise whatsoever.

Actually, I don't know what happened there. I just looked back, and the modem must have resync'ed a good 2 months before I captured that set of stats - and I'm pretty sure we were on holiday on the date that resync would have happened.

The QLN figures are only gathered at the time of sync, aren't they? Perhaps the cabinet was reset in some way, and every line (including mine) needed to resync. Note that there isn't a change in tones 0-350, suggesting that the ADSL modems (exchange-fed) are still out there.

Putting those two factors together - the static noise floor and yet falling SNR across the subcarriers - suggests that (downstream) transmit power was cut. Would you have kept a record of transmit power? The xdslcmd command only reports an aggregate - the ACTATP (Actual Aggregate Transmit Power) in each direction, but it should still indicate a cut in power (if there was one).

I do indeed have the power values recorded in the Plink files, but they don't vary much... although I have seen the aggregate value vary by as much as 0.2dB between consecutive xdslcmd attempts (one --show, and one --stats).

These figures are from the --pbParams output:
Date      TX Power (dBm)
Date      Aggr     D1     D2     D3
02/2012   13.0     9.7    7.7    6.8
03/2012   13.0     9.6    7.7    6.8
06/2012   12.9     9.7    7.8    7.0
07/2012   13.0     9.7    7.7    7.0
09/2012   13.0     9.4    7.9    7.0
11/2012   13.1     9.7    7.8    6.9
12/2012   13.1     9.7    7.8    6.9
01/2013   13.1     9.7    7.8    6.9
05/2013   13.1     9.7    7.8    6.9


Every indication of the aggregate power was either 12.9, 13.0 or 13.1dBm.
The 3 power values for D1, D2 and D3 haven't changed since November.

Max Attainable Rate a.k.a. ATTNDR is calculated in basic form using the following equation:

http://huaweihg612hacking.files.wordpress.com/2013/0...

According to G.993.2 (see page 143 in [1]) to obtain a Bit Error Rate (BER) of 10^-7 - the accepted minimum error rate- the SNRGAP in that equation is set to 9.75dB.

Since the Target SNRM on your line has dropped from 6dB to around 3dB, the basic ATTNDR equation illustrates how and why, with a lowered TARSNRM, the bit-loading still edged higher.

With max data rates of 80/20 in the channel profile, this has resulted in a significant number of surplus bits per frame.

That's my reading any way!

This bit I'll have to look into tomorrow or Saturday, but note that it is my actual SNRM that dropped from 7dB down to 3dB - my target SNRM is still 6dB, I think. Doesn't the target stay at 6dB on Openreach cabinets?

Ta!
Standard User RobertoS
(sensei) Fri 31-May-13 00:05:41
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: WWWombat] [link to this post]
 
4-frame animation link doesn't work for me.

My broadband basic info/help site - www.robertos.me.uk | Domains,website and mail hosting - Tsohost.
Connection - Plusnet UnLim Fibre (FTTC). Sync ~ 53.4/16.8Mbps @ 600m. - BQM

"Where talent is a dwarf, self-esteem is a giant." - Jean-Antoine Petit-Senn.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.
Standard User WWWombat
(fountain of knowledge) Fri 31-May-13 01:18:03
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: RobertoS] [link to this post]
 
Hmm. Worked here agains.

How about this one?
Standard User asbokid
(member) Fri 31-May-13 18:44:38
Print Post

Re: Attainable Bandwidth vs Crosstalk


[re: WWWombat] [link to this post]
 
In reply to a post by WWWombat:
I'm not really seeing the same thing that you are - except for the first 2 or 3 frames, which do seem a little higher. Perhaps because these were on a 40/10 package (before 80/20 became available, but after the 17a bandplan was turned on)? Maybe that's a little unfair.


Aha! So we should only look at graphs from roughly the beginning of the year?

I've done another animation of just 4 frames: the very first SNR frame of an 80/20 sync, then the start and end frames of that 6 month single-sync period, then the first sync after that 6 month period. It really doesn't seem that much different to me. At least not compared to the attainable values of 84, 81, 72, 83.

4-Frame SNR animation


You're right, the SNR drop couldn't account for such a variation in ATTNDR.

I just looked back, and the modem must have resync'ed a good 2 months before I captured that set of stats - and I'm pretty sure we were on holiday on the date that resync would have happened.. The QLN figures are only gathered at the time of sync, aren't they?


Yup, during initialisation.

Perhaps the cabinet was reset in some way, and every line (including mine) needed to resync. Note that there isn't a change in tones 0-350, suggesting that the ADSL modems (exchange-fed) are still out there.


Sure. Your line must have been first to sync! Or at least it beat the disturbers to sync.

I do indeed have the power values recorded in the Plink files, but they don't vary much... although I have seen the aggregate value vary by as much as 0.2dB between consecutive xdslcmd attempts (one --show, and one --stats)...

Every indication of the aggregate power was either 12.9, 13.0 or 13.1dBm.
The 3 power values for D1, D2 and D3 haven't changed since November.


Well tx power is not the answer then!

This bit I'll have to look into tomorrow or Saturday, but note that it is my actual SNRM that dropped from 7dB down to 3dB - my target SNRM is still 6dB, I think. Doesn't the target stay at 6dB on Openreach cabinets?


Yeah, the line-profile sets a target SNRM of 6dB for all lines. You're right viz actual SNRM vs target SNRM. I was trying to shoehorn your SNR figures into the ATTNDR equation. Although the outcome to subcarrier bit-loading and ultimately ATTNDR is the same, whether the margin is forced or has found its own level.

cheers, a

Edited by asbokid (Fri 31-May-13 20:10:05)

Pages in this thread: 1 | 2 | (show all)   Print Thread

Jump to