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Please help me understand Noise Margin.

paulo
Grafter
Posts: 157
Registered: ‎02-01-2008

Please help me understand Noise Margin.

I can understand signal to noise ratio and basic noise on a cable but cannot get my head around Signal Noise Margin!
I have been useing the excellent 'RouterStats Lite' program to monitor my SNM and sync speed. 
I understand, rightly or wrongly, that the Margin is set by the 'provider' to act as a safety-net to optimise the go and no-go characteristic of the particular line, compromising between speed and reliability.  If this is vaguely correct then it throws up the following questions.
Is the SNM set by provider and held in the router's memory?
If the Margin is a fixed operating characteristic, ie a target figure, why do I see it varying with the basic noise?
If the SNM is held on the router, how often is it 'reveiwed' by the provider?
Nearly every morning when I switch on my PC (router on and connected permanently) is my SNM showing 16.5dB and sync speed 230 kbps and stays like that all day unless I reboot the router and straight away get 9dB and 800kbps?
There are quite a few other questions but lets try to get to grips with it bit at a time.  My router is Thomson TG585 v7.  I am on a very long rural cable.  My daytime SNM is 8.5 to 9.5dB and night time is 10.5 to 14dB.
Any help with explanations would be much appreciated.
I can resist everything except temptation.
10 REPLIES 10
Oldjim
Resting Legend
Posts: 38,460
Thanks: 787
Fixes: 63
Registered: ‎15-06-2007

Re: Please help me understand Noise Margin.

Something is badly wrong.
Can you post the full stats from the router - nioise margin, attenuation and speeds.
Did you really mean 230 kbps or 2,300 kbps
paulo
Grafter
Posts: 157
Registered: ‎02-01-2008

Re: Please help me understand Noise Margin.

Thank you Father Christmas Jim.    Could not reply before because, you'll never guess, our local farmer ripped my 10 pair phone cable out of the ground just 10 minutes after I made posting.    BT, bless them, have just made a temp repair, good service there.
Yes speed often 230kbps (confirm not 2300kbps)  Best ever 864kbps.  Line attn 63.5dB which is I think off the scale.  I did say it was a long rural line.  BT tell me I am very lucky boy to get BB at all.
I can resist everything except temptation.
Oldjim
Resting Legend
Posts: 38,460
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Registered: ‎15-06-2007

Re: Please help me understand Noise Margin.

I will try to cover the basics.
Default downstream noise margin - this is set by the equipment in the exchange and is the noise margin you see immediately after a resync.
Once connected the reported downstream noise margin varies dynamically according to the state of the line. Typically it reduces after dark and rises during the day. This is usually because of radio interference being worse at night.
What I suspect happens ias that the router loses sync overnight and reconnects at a speed of 230kbps with a noise margin of 9dB. In the morning the noise margin increases up to 16.5dB but with the same speed. A resync at that point will change back to the default noise margin of 9dB with the higher sync speed of 800kbps.
I suggest you install Routerstats http://www.vwlowen.co.uk/internet/files.htm as this will allow you to monitor and log the changes in noise margin and speed continuously if you leave the computer on with Routerstats running.
The reason I suggested that something was badly wrong was the big change in noise margin overnight and the fact that it won't apparently hold sync.
The first thing to do, if you can, is to plug the router and phone  into the test socket via a filter which will isolate all your internal wiring. If this doesn't improve the variation in noise margin and stability then you are stuck with it although some routers are better than the 585v7 for a long line
Have a read here http://community.plus.net/forum/index.php/topic,81129.0.html
mal0z
Grafter
Posts: 3,486
Registered: ‎02-10-2008

Re: Please help me understand Noise Margin.

Look for :
What is SNR Margin?
on this site:
http://www.kitz.co.uk/adsl/linestats.htm
paulo
Grafter
Posts: 157
Registered: ‎02-01-2008

Re: Please help me understand Noise Margin.

Thanks mal0z for that.
Oldjim, thanks for taking time to explain that, think I'm getting the hang of it,  High Signal to Noise Ratio is good, high Signal to Noise Margin is bad!  ie. SNM 6dB is good 15dB is bad.
Think you must be right about loosing sync late at night and seeing the re-sync at a lower speed in the morning when PC is powered up.  I have RouterStats Lite so will try leaving it running overnight to see what is happening.    I do not usually leave PC on at night (router is left on) because I do not have mains electricity here (told you I was very rural) so the house is on batteries and inverter during the night.    And, before you make the suggestion, the inverter does not seem to affect to noise figures.
Have checked house wiring, filters etc and see no improvement.
May join the router discussion as I have several routers to try.
I can resist everything except temptation.
mal0z
Grafter
Posts: 3,486
Registered: ‎02-10-2008

Re: Please help me understand Noise Margin.

Not quite  Wink
In any telecommunication system you need
a) a strong enough signal for the receiver ( router in this case ) to detect the intelligence / data
b) enough signal level level above the noise to be able to retreive the intelligence reliably
now ADSL has a large volume of intelligence / data - much more so that say a simple voice
so it needs a higher signal to noise ratio - (SNR ) so ADSL2+ may need a higher SNR as there is a higher throughput of data.
Not sure of the exact figures without looking them up - but just as an example lets say a router needs 35dB SNR to give a throughput of 6M
Now the SNR Margin is the level above this that allows normal operation, so if you have a 6dB SNR margin, there is room for the SNT to fall 6dB before any degredation starts.
If the SNR Margin is 12dB - you have an ever bigger margin for error . So the higher the SNR Margin the better.
The same sort of thing happens with for example a radio link between a cellular base station and the backbone site that transmits the signals to the switches / main exchange.
It will be designed to give the receiver a SNR of lets say 40dB to work as designed. But You design in a "fade margin" to allow room for the signal to drop because of rain, snow etc etc. This margin might be 30dB - so there is a margin in which the signal can move up and down in level without effecting performance.
HTH and isn't too techie.
And i hope I've remembered correctly - it's been some time since I was at college or pracising it professionally. Grin Grin



paulo
Grafter
Posts: 157
Registered: ‎02-01-2008

Re: Please help me understand Noise Margin.

Thanks for your detailed reply.  I do understand the margin is there to make the circuit more reliable, a safety net so to speak.  I think my remark 'SNM of 6dB is good and a SNM of 15dB is bad' may have been a bit confusing.  What I meant was a circuit that has a margin of 6dB must be a better quallity circuit than one that has a target of 15dB.
Anyway back to my case.  Over the last 24 hours or so the SNM recorded by RouterStats Lite varies from 4dB to 11.5dB with no loss of sync.  Does this mean that my profile would be set at 12dB ?
I can resist everything except temptation.
HPsauce
Pro
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Registered: ‎02-02-2008

Re: Please help me understand Noise Margin.

Quote from: paulo
 What I meant was a circuit that has a margin of 6dB must be a better quallity circuit than one that has a target of 15dB.
Anyway back to my case.  Over the last 24 hours or so the SNM recorded by RouterStats Lite varies from 4dB to 11.5dB with no loss of sync.   Does this mean that my profile would be set at 12dB ?

1. No, because there are many other constraints, it's just not that simple. In fact a 15dB margin may represent a very good verey quiet line.
2. Your margin IMMEDIATELY after a resync will indicate your TARGET SNRM. Noise varies through the day and is typically worse in the evening shortly after sunset. If you don't resync this is when your SNRM will be at it's LOWEST.
Without graphs and timings the figures you've quoted sadly mean almost nothing.
paulo
Grafter
Posts: 157
Registered: ‎02-01-2008

Re: Please help me understand Noise Margin.

Here are two typical SNM graphs, one for day time and one for night.  Hope they come over ok as not tried this before.
Any comments about them?
I can resist everything except temptation.
zubel
Community Veteran
Posts: 3,793
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Registered: ‎08-06-2007

Re: Please help me understand Noise Margin.