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High downstream SNR

jab1
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High downstream SNR

DSL.PNG

As can be seen from the above data, I have a very high D/S SNR. While this doesn't really worry me (I'm no gamer or suchlike), I was told by the last BT engineer who came out to resolve another problem, that he had switched me to a 'good' pair in the DP and I should get 11-13Mbps with no problem.

I did get around 12 for while, then dropped to 9-9.5 (still 'good enough' for my needs). Ever since the drop, occasionally DLM will reset my speed to around 13.1, but with an SNR of 3, which, considering my distance from the exchange is too low. It will hold this for 24-36 hours, then kick me back to around 9 and stay there for a while, until the DLM resets it - again.

The other thing, which IS causing me some concern, are the very high error figures.

Anyone care to investigate or offer suggestions?

Edited to add: I go away for 3 weeks 3 or 4 times a year, and disconnect the router/turn off all but essential electrics, and when I get back and restart, it is usually at the higher speed/lower SNR.

John
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summers
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Re: High downstream SNR

My line is doing the same Wink

Don't worry about the high FEC count - that is corrected errors. ADSL is designed such that it has a certain error rate, but that the encoding is such that they can be corrected. This is what FEC are - me I get a FEC second maybe every couple of minutes, but one second typically has a count of 20. Bad seconds can have a count of 2000 for me!

The CRC errors are more important. It means that the error correction did not work, so there is a known error. When this happens in a second, this creates an Error Second, and that is what is said to be used to set your line speed. Now you CRC errors are very low, so that shouldn't be what is causing the problem, but you have a huge number of error seconds. So why your Error Seconds and CRC are so different is odd, you would expect CRC >> ES. Anyway yes that high Error Second could explain the speed drop, question though what is causing it ...

For me what seems to give the dropping SNR (mine is at 15dB right now!) is that I occasionally get a few seconds of extreme noise, that takes the line down. Doesn't look like you have this from your statistics though ...

jab1
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Re: High downstream SNR

Thanks for the quick reply, @summers - appreciated Smiley

I'm not  concerned about the FEC count, but it could be lower. I am aware of the technology - I spent about 20 years in IT support as one of the facets of my job (no offence), but I don't understand the DLM behaviour.

The ES/CRC disparity is one of the things I would like an explanation for, too.

My SNR is fairly steady, and as I'm running Router-Stats, I can monitor that constantly. And no, I don't get extreme noise - again RS  would alert me to that.

 

John
JayG
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Re: High downstream SNR

@jab1 IIRC (Roll_eyes) that's a TG582n router, which again, IIRC remember correctly, doesn't reset the error second count until rebooted, so assuming it's more than 4 days since the last reboot the error seconds might not be as bad as they look at first glance.

Having said, that, your line is clearly far from perfect and the sync speed consdierably lower than you might hope for - you might not get far trying to raise a fault if it's not actually dropping the connection on a frequent basis, although I'd be surprised if there isn't some audible noise on the line, even if intermittent, which could provide a route to raising a phone fault.

jab1
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Re: High downstream SNR

@JayG Correct - it is a TG582n -I'd forgotten about the ES not resetting until a reboot.Embarrassed. Yup, I'm aware the line is pants, but unfortunately the noisy line had crossed my mind, so it is checked on a regular basis, so that doesn't seem like an option.

Last router reboot was early August on my return from my holiday.

John
summers
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Re: High downstream SNR

I should have explained more carefully! My SNR is also steady, varying about 1dB between day and night.

The noise that takes my line down, seems to be about 3s where the FEC/CRC error rate goes huge - so say 7000FEC in 3 seconds. That takes the line down, and it resyncs at a slower speed. I'm not clear if it is my ADSL router or the DSLAM taking the line down, but one side of the other doesn't like it. I've yet to see a problem in the SNR (even looking at SNR per bit).

Alas for me to get this far has been a fair bit of work on my side. The behavior of the DLM doesn't seem like like described on https://kitz.co.uk/adsl/DLM.htm.

So it would be interesting if you have a similar effect, but difficulty is it will be hard for you to diagnose ...

jab1
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Re: High downstream SNR

I don't think my problem is quite the same as yours, @summers - once it has dropped the speed/upped the SNRM, it stays happy until DLM forces the SNR down to 3 - too low for the length of my line, and the speed up to c13Mbps - slightly too high, from what my very knowledgeable engineer told me.

John
jab1
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Re: High downstream SNR

@Gandalf Any chance you could cast your eye over this. Sorry to pick on you, but you have been very helpful in the past.

If you want to do any intrusive testing, I won't be inconvenienced this afternoon - going out for 2 or 3 hours.Smiley

 

TIA

John
MatthewWheeler
Plusnet Help Team
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Re: High downstream SNR

Unfortunately @Gandalf isn't in the office now but I've tested your line.

Looking at the graph attached below it backs up what you're seeing

What's interesting is that your target SNR has been set and is still at 3db which implies something on the line is making it sync at the higher SNR

With regards to stopping your line being moved onto the 3db target there are settings we can change which essentially makes DLM think the line can't do any better.

If you want us to go ahead with that let us know

If this post resolved your issue please click the 'This fixed my problem' button
 Matthew Wheeler
 Plusnet Help Team
jab1
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Re: High downstream SNR

@MatthewWheeler   Thanks for your reply, Matthew - much appreciated. I only tagged Anoush because I was trying to resolve this problem with him last year (I think), and we agreed a 6dB target was needed. He did do me a pinned 6dB, but at some point earlier this year, something (DLM again?) decided to destroy what he did and drop it back to an unusable 3dB. I lived with the situation, because it doesn't impact me too much, but it would be nice to have that extra 2-3Mb/s when I do the occasional biggish download.

So, yes please, apply whatever you need to, but if it is likely to drop my connection, please give me a bit of advance warning.

 

 

John
ejs
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Re: High downstream SNR

It appears that it isn't the DLM causing the high downstream SNRM. It looks more like there's an occasional short burst of noise, which knocks out the connection, and causes it to reconnect at a lower speed. Then shortly after it has reconnected at the lower speed, the noise which knocked out the connection is gone, and so it remains connected at a lower speed and high SNRM. You could probably just reboot your modem or otherwise make your modem re-connect to get back to a high speed and lower SNRM.

If not staying connected forever is a problem, then yes, starting with a higher target SNRM might help.

The idea that any particular target SNRM is appropriate for a particular line length is at best oversimplified, but basically it's just rubbish.

jab1
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Re: High downstream SNR

@ejs I understand what you say, but believe me please when I say 3dB appears (to humour you) too low, but 6dB leaves me with a higher stable speed - been there, got the tee-shirt (& shortsSmiley)

John
Gandalf
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Re: High downstream SNR

Hi @jab1

Our tests are showing a high amount of errors on the line now.

 

xDSL Status Check
Circuit ID: CBUK00000000 Service ID: BBEU00000000
Telephone NO.: N.A. Test Executed On: 2018-11-22T08:51:43.714Z
xDSL Status Test Summary
Sync Status: Circuit In Sync
General Information
NTE Status:   NTE Power Status: Unknown Bypass Status:  
 
  Upstream DSL Link Information Downstream DSL Link Information
Loop Loss: 16.3 33.0
SNR Margin: 5.8 3.3
Errored Seconds: 0 355
HEC Errors: 0  
Cell Count: 9936 48997
Speed: 804 13456
 
Maximum Stable Rate (KBPS): 11360 Fault Threshold Rate (KBPS): 9088
Mean Time Between Retrains (Seconds): 17301 Mean Time Between Errors Upstream (Seconds): 7689
Indicative Line Quality: A Mean Time Between Errors Downstream (Seconds): 3
Custom Thresholds
MTBR_RED: 0 MTBE_RED: 0
MTBR_GREEN: 86400 MTBE_GREEN: 86400

I've nudged the target SNR back up to 6dB and asked our suppliers again to pin it at that level.

Discussing this with Matt, what he said about making DLM think the line can't do any better is where we'd place a modify order to set a custom threshold that DLM works on try to stop it making positive changes like moving the target SNR down.

I did that around this time last year. Sad 

Testing the line doesn't show the custom threshold set anymore and going through the modify order process again, the custom threshold I set is still showing, but I've submitted the order once more, which should complete tomorrow.

Let us know how things go over the next 24 hours.

 

[edit]

Coming back in to edit my response to paste the test results in to my post, I've checked the results in the supplier database of the test I ran, and the custom threshold is actually still set. Hopefully our suppliers have more sticky glue to pin the SNR!

 

Failing that if it drops down again, we could attempt to switch DLM off, though that may cause further issues if it's at all possible.

From 31st October 2022, I no longer have a regular presence here as I’ve moved on to a new role.
Anoush Mortazavi
Plusnet
jab1
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Re: High downstream SNR

@Gandalf Looks like your nudge has taken effect - now showing a steady 6.3dB in RouterStats. Lets hope they get the pinning done quick.Smiley

Thanks - as always.

John
Gandalf
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Re: High downstream SNR

Glad to hear that! 

As I type this I've just received email confirmation advising the SNR should now be pinned at 6dB.

Fingers crossed that enough glue has been used.

From 31st October 2022, I no longer have a regular presence here as I’ve moved on to a new role.
Anoush Mortazavi
Plusnet