Understanding Noise Margin, Line Attenuation & Signal Attenuation

[tom_uk]

My apologies if this is a silly question, but I've only just started getitng my head around these topics.

I've read the technical glossary on these topics and that's told me broadly what they are, but of course I don't have any comparison, and I'd welcome that. I have Infinity 2 (?) at 76Mbps (recent free upgrade), with my cabinet maybe 250 yards away.

First, Line (and Signal) Attenuation. The 'Information for Helpdesk Agents' page from my HH5 tells me that the figures for Line Attenuation are 22.4/17.2, and 22.5/17.3 for Signal Attenuation. Is that OK? I'm also running the RouterStats5A application, and according to that those values haven't changed since I opened the app, nearly 36 hours ago.

Now for the noise. Upload noise is stable at around 6dB. Download noise is obviously set to a target (?) of 3dB, but it does vary. Attached are a couple of graphs. The first is from readings overnight, showing mainly steady levels, but the second is from this morning showing steadily dropping levels - down to 0dB or even below 0. How is that possible? And MaxDataRate has also slowly declined over the same period, from 68Mbps to 66Mbps. Are the two (Noise margin and Max Data Rate) conncted?

[imjolly]

have you read the aricles on your subjests which are on here  http://www.kitz.co.uk/

[tom_uk]

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@imjolly wrote:

have you read the aricles on your subjests which are on here  http://www.kitz.co.uk/

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I have now, or at least a few of them, although I did find that site hard to understand. Nontheless, thanks for the link.

I gained at least one insight from it concerning the Noise Margn reading. It's the amount by which the actual SNR exceeds the minimum specified SNR required for the connection to work. Example: if the minimum acceptable SNR was set at 5dB and the measured SNR was 9dB, the current Noise Margin would be 4dB (9 minus 5).

As chance would have it, my HH5 reset the connection while I was reading all that background stuff, and on going back to the RouterStatsHub5a readings, I saw that the Noise Margin readings had been bouncing along just above or just below 0dB for quite a while. Given the explanation of Noise Margin above, this would suggest that the HH5 could no longer maintain the link with such a low noise margin - the SNR (whatever value it actually was) had dropped below the minimum required. Here's my graph - you can see the moment when the connection is re-set.

That all begs a few questions:

a) what might be making the downstream Noise Margin steadily reduce during the morning?

b) will a replacement router make any difference?

c) Does anyone know of some software similar to RouterStatsHub5a that will work on other routers/later Home Hubs? - the author of this software says (on his website) that Home Hub 5 Type A is the most recent HH is works with;

d) and is it worth going back to BT with this info?

[imjolly]

your normal noise margin should be about 6db

try quiet line test and check for line noise  dial 17070 oprtion 2  should be quiet and bets with corded phone

are you using the test socket to elimiante any problems caused by your internal wiring?

[tom_uk]

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@imjolly wrote:

your normal noise margin should be about 6db

try quiet line test and check for line noise  dial 17070 oprtion 2  should be quiet and bets with corded phone

are you using the test socket to elimiante any problems caused by your internal wiring?

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Thanks for your reply. Answers:-

a) I thought the downward noise margin had been reduced to 3dB in about March? That's why I wasn't surprised to see different values for upload (about 6dB) and download (ideally around 3dB, which is what it jumped back up to after the reset);

b) I've down that (though not today) and it was silent as the tomb. I'll leave the software running and wait unti the Noise Margin drops again and do it then;

c) not yet - I only read about that option this morning. In fact I haven't even confirmed yet that I can get to it.....

[imjolly]

they were trialing the 3db noise margin and you may well have a 3db noise margin but majority are still on 6db

[tom_uk]

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@imjolly wrote:

they were trialing the 3db noise margin and you may well have a 3db noise margin but majority are still on 6db

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Ah! That actually fits with another factor that I haven't mentioned (on this thread). All these problems started about the time we got the free upgrade from 54Mbps to 76Mbps. Do you think it'd possible that our connection switched to one on which the 3dB noise margin is being trialled at that time? i.e. we had a 6dB margin connection up until then and there was enough space on that to handle any noise margin fluctuations that might have been occuring?

On other issues - I've had the main plate on my OpenReach dual socket box off, and under it I can see just one phone jack socket. Presumably that's the test socket? And presumably in order to use it I need to add a VDSL splitter to that socket and then connect the phone and HH5 appropriately?

[imjolly]

it would seem an unlikely coincidence that at time you got free boost that 'normal' noise margin was dropped to 3db

yes the test socket is where you describe and you do need a filter - filter same for adsl/vdsl - to conenct to test socket hen hub and phone to filter

[barnpiece]

My noise margin for download was changed from 6 to 3 at the same time as I got the speed boost.

[pippincp]

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@barnpiece wrote:

My noise margin for download was changed from 6 to 3 at the same time as I got the speed boost.

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Coincidence, nothing more.