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Do noise margins matter?

philarv
Newbie
Posts: 8
Registered: ‎01-02-2012

Do noise margins matter?

Our broadband service has always seemed a bit unstable with disconnections from time to time, but as speeds have increased the issue has got worse.
Every few weeks or sometimes days the router will still appear to be connected, but wont be talking any more.  Sometimes it will do this every 10 minutes.  Sometimes pressing the reconnect button on the connection status screen will reconnect it, sometimes the router has to be rebooted.
I started to monitor with Router Stats and sometimes the upstream noise margin varies from 20 down to 9dB with drops to zero, at which point the connection drops completely.  See attachment from 281111.
At other times the noise margin is rock solid. Today I have had router stats running for the last 2 hours, and the upstream noise margin is rock solid between 23.8 and 24.5 dB.
Its difficult to be definitive, but I am also 99% sure that when the noise margin is poor an incoming call will FIX the line!  See router stats trace from 300112.
1) Is there anything else I can do to monitor ?
2) What does the incoming call fixing the noise margin mean ?
Thanks,
Phil
10 REPLIES 10
x47c
Grafter
Posts: 881
Thanks: 3
Registered: ‎14-08-2009

Re: Do noise margins matter?

I think we can certainly say there there is some sort of fault on the line with those traces!
You need to monitor downstream SNR rather than upstream - though if you have total disconnections rather than just raised noise levels its not really an issue.
It is possible you have a high resistance fault (HR fault)
Somewhere on the line there is corrosion in a joint - causing a bad connection - possibly caused by water/damp.
The ringing signal/voltage applied to the line to make your phone ring "bridges the fault" by quite simply "blasting though the corrosion" and makes is better - for a while.
A ring signal uses much more power than an ADSL signal.
A simple line test may not show the fault.
Ideally you need a proper TDR test (time domain reflectometer) which would very accurately pin point the location of the dodgy connection.
Edit
Sometimes HR faults manifest themselves when the broadband is switched off for a period.  Upon switch on you get bad speeds which gradually get better over the next week or so.  People quite often misdiagnose this and it leads them to think that turning off the router overnight leads to bad speeds in all cases for everybody and you must not do this, when actually its the fault on their line which is causing this.
Over to plusnet Staff for their comments.....
adamwalker
Plusnet Help Team
Plusnet Help Team
Posts: 16,871
Thanks: 882
Fixes: 221
Registered: ‎27-04-2007

Re: Do noise margins matter?

If the noise margin does get too high then disconnections will most likely become apparent so I would agree that this for whatever reason seems to be happening in your case so I do think a fault is at play here.
I'd advise raising one via the troubleshooter as usual but feel free to reply back when you've done so and I'll have a look.
If this post resolved your issue please click the 'This fixed my problem' button
 Adam Walker
 Plusnet Help Team
philarv
Newbie
Posts: 8
Registered: ‎01-02-2012

Re: Do noise margins matter?

Quote from: x47c
I think we can certainly say there there is some sort of fault on the line with those traces!
You need to monitor downstream SNR rather than upstream - though if you have total disconnections rather than just raised noise levels its not really an issue.

The phenomenon is more clearly visible in the upstream SNR.  The same pattern occurs in the downstream but not so extreme.  However the upstream SNR fluctuation is associated with the router locking up and needing to be reconnected or restarted even though sync has not been lost.  Perhaps if the noise threshold varies a lot due to HR fluctuation, and upstream is mainly affected, then the router may get to the point where it cant maintain the connection and gives up?  On the previous router in the log it looked like it was dropping external connections in the NAT map, and reforming them until the router showed an error like "too many simultaneous sessions" or something like that.  The router failure appears to be unmanaged, in that the router doesnt try to reconnect even though the internet connection is no longer functioning.  Is that normal ?
Quote from: x47c
It is possible you have a high resistance fault (HR fault)
Somewhere on the line there is corrosion in a joint - causing a bad connection - possibly caused by water/damp.
The ringing signal/voltage applied to the line to make your phone ring "bridges the fault" by quite simply "blasting though the corrosion" and makes is better - for a while.
A ring signal uses much more power than an ADSL signal.
A simple line test may not show the fault.
Ideally you need a proper TDR test (time domain reflectometer) which would very accurately pin point the location of the dodgy connection.
Edit
Sometimes HR faults manifest themselves when the broadband is switched off for a period.  Upon switch on you get bad speeds which gradually get better over the next week or so.  People quite often misdiagnose this and it leads them to think that turning off the router overnight leads to bad speeds in all cases for everybody and you must not do this, when actually its the fault on their line which is causing this.
Over to plusnet Staff for their comments.....

I'm very happy to hear that assessment.  As a former Marconi electronics tech it looked like a bad joint to me!  Especially the way ringing in fixes it.  Surprising to see that broadband connection can keep a line alive.  You would have thought the low voltage / current wouldnt be enough.  Thanks.
philarv
Newbie
Posts: 8
Registered: ‎01-02-2012

Re: Do noise margins matter?

Quote from: _Adam_Walker_
If the noise margin does get too high then disconnections will most likely become apparent so I would agree that this for whatever reason seems to be happening in your case so I do think a fault is at play here.
I'd advise raising one via the troubleshooter as usual but feel free to reply back when you've done so and I'll have a look.

I have a question open: 51556849
There is also an old question discussing the same issue from December: 49303085
The support team have been commenting that basic line test doesnt show a problem, the disconnect history isnt enough to justify a line intervention and that an engineer wont find anything if the line is okay when he visits.  If there is a TDR type investigation that can be done, I would appreciate that.  I having been putting up with intermittent periods of connection loss for more than a year.
Thanks, Phil
x47c
Grafter
Posts: 881
Thanks: 3
Registered: ‎14-08-2009

Re: Do noise margins matter?

Not sure I can add anymore to your questions about the router 'locking up' I've no experience of this sort of thing - hopefully others may be able to advise.
With your constant disconnections the exchange will have raised the downstream target noise margin from its standard of 6dB Downstream to something much higher
(either 9,12,15) in an attempt to provide it with enough margin at all time to work with.
Once your line is fixed and stable you will need to request a SNR reset to set it back, this will allow the line to sync at the max speed possible for the line and then your speed will come back up after 72 hours connected or thereabouts.
Anyway, your graphs shows quite clearly that the levels of disconnections are such that any attempt to pass it off as  "its not abnormal" is no longer sustainable.
since your ex-marconi you probably know more about this than me - my field was power transmission not data transmission.
philarv
Newbie
Posts: 8
Registered: ‎01-02-2012

Re: Do noise margins matter?

Quote from: x47c
since your ex-marconi you probably know more about this than me - my field was power transmission not data transmission.

My training was radio and microwave, and a long time ago.  I know something about transmission lines but very little about how the data transmission devices behave.
Thanks for your comments
Phil
adamwalker
Plusnet Help Team
Plusnet Help Team
Posts: 16,871
Thanks: 882
Fixes: 221
Registered: ‎27-04-2007

Re: Do noise margins matter?

I'm just talking to a member of our faults team now who is going to pick up the open fault ticket. Once resolved I'll be able to help you make sure an SNR reset is pushed through.
If this post resolved your issue please click the 'This fixed my problem' button
 Adam Walker
 Plusnet Help Team
philarv
Newbie
Posts: 8
Registered: ‎01-02-2012

Re: Do noise margins matter?

Have agreed with the support team that we will book an engineer the next time the SNR is bad, in the hope it will stay bad till they get here, and give them a better chance of identifying the location of the fault.  I do understand that intermittent faults are really hard to address if they arent happening, believe me I have had enough experience of that !
As the support team guy said to me, broadband is still quite arcane.  Really the problem lies with the network specifiers.  There is an intelligent box at the exchange end and another at my end.  If the design engineers were asked to build fault tracing into the hardware, the system could tell you where the problem is.  How much would it cost to but time domain reflectometry into the hardware these days?  Yes it will add something to every unit price, but its the only way the copper loop will be made robust enough to stop each user from getting a variable service.  It will be a very long time before everyone has fibre at the house, so surely its the next step?
Will start another thread about the type of fault that can be fixed by incoming call.
Thanks for you help
Phil
orbrey
Plusnet Alumni (retired)
Plusnet Alumni (retired)
Posts: 10,540
Registered: ‎18-07-2007

Re: Do noise margins matter?

Hi philarv,
Whilst I'm not sure if it's the same kit as you describe there is equipment that will measure a line's length and resistance, these tests are run prior to us raising each fault and I believe the information is viewable by BT once the fault is raised. As far as testing each section goes I believe engineers are able to do this - though again I'm afraid I'm not sure of the specific equipment used. Hopefully we'll be able to get an engineer to you when the fault is apparent, you should be able to have a chat and make some suggestions (always worth having tea and biscuits in for such an eventuality Smiley )
philarv
Newbie
Posts: 8
Registered: ‎01-02-2012

Re: Do noise margins matter?

Engineers always get tea and biscuits if they come to help me out !  The upstrream SNR is now sitting obstinately in a dead flat line at 26dB.  Thought I heard some crackle on the line the other day though, when I have a moment I'll listen again.......
Phil