Downstream SNR keeps dropping

Ever since I've been using a Billion Bipac 7800N router at home, I've noticed that my ADSL line's downstream SNR decreases over a number of days from 4 down to 1, then drops the ADSL connection and spends quite a while re-connecting. I noticed the dropped connection on Tuesday night when I had no internet access at around midnight - a quick check of the ADSL status on the router's status page at that point confirmed that the router was indeed reconnecting to ADSL and taking it's time about it.

I checked the SNR last night and it's now 3 and ADSL is connected.

Can anyone explain the above behaviour? Is it normal?

I'd say not normal, a drop overnight and a rise in the morning is typical.

Run RouterStats http://www.vwlowen.co.uk/internet/files.htm if possible. A graph may help in diagnosis.

In reply to a post by john2007:

I'd say not normal, a drop overnight and a rise in the morning is typical.

Run RouterStats http://www.vwlowen.co.uk/internet/files.htm if possible. A graph may help in diagnosis.

Thanks for that, I'll give it a go

Well I'm running Routerstats since 22:05 tonight and at about 22:14 the graph drops to zero for noise margins and sync speeds momentarily before recovering back to the previous level around a minute later. I didn't notice any drop in the ADSL connection though i.e. I was browsing normally

Is this expected?

Most people reboot in the quiet hours to get a high sync / low SNR. If you reboot in the busy times (7.30pm ish) you might get a more stable sync.

Uh?

If you really mean SNR then fine. But do you, or do you mean SNR Margin? That's what 90% of readers will think you mean, and of course that isn't true.

When's the "quiet time" anyway?

SNR - Signal to noise ratio.

Quiet time for me is 4am to 6.30am if I were being pedantic, but generally 2am to 8am will do.

Wierd - that's the opposite of what most people experience. So I think those times would give most people a low sync and high day-time noise margin, particularly in the winter. I.e. stability rather than speed.

I seem to remember this being discussed before about your line?

Here's my experience. The target SNR is 3.0dB. In the busy times it drops to as low as 2.5dB, but in quiet times rises to 3.6 dB. If I reboot at 3.6dB it resets at 3.0 / 3.1dB and the sync increases. However, if the busy time is very busy the SNR drops to below 2.5dB and might trigger a disconnection. Then it resets to 3.0 / 3.1dB but at a lower sync (more stable). Then in the quiet times it pops back up to 3.6 or more.

I did mention to the Plusnet handler that it was a bad time to switch to ADSL2+ and train the line (Jubilee, Euro 2012, Olympics). I got a really decent sync on Friday morning, only to get a disconnection when the country decided to go online around the time of the England match. No idea why, since it's on FTV telly, but those are the trends I'm seeing.

Edited by camieabz (Sun 17-Jun-12 13:23:42)

Mine wanders around by 3-4 db and there doesn't seem to be any rhyme or reason for it. Had BT on the fault for 8 weeks last year and every section of my copper was changed, all joints checked, lift and shift, tie pair modify and no change.

So I just live with it I guess

Here's my Friday data:

My Broadband Ping

See how I rebooted at 2am and then the sync was 500k or so higher, then at 8pm the disconnection forced it back down by about the same amount.

The ping changes are incidental as far as I know. Plusnet gateway random factor.

Getting back to the original post, I have a bipac billion 7700n and have exactly the same problem.
If left on overnight, the snr drops from 6 to 4, but only recovers slightly the following morning.
This causes a re-sync after two or three days. It looks like it could possibly be a router problem.

I switch the router off at bed time and re-boot in the morning giving me the best possible sync.

That sounds like the noise is wiping out some frequencies. Bit-swapping will initially increase the usage on others to compensate, but it comes to the point where that can't be done any more so the sync becomes unsustainable.

A drop of 3 or 4dB overnight is common. What is less common is for it not to recover at dawn. However, even when it does recover, as I understand it no reverse bit-swapping takes place. (Maybe it does with some kit, and that's why they hold sync better? I dunno - just wondering).

Thank you Roberto.
Problem solved. I have disabled bitswapping. The SNR drops by around 2 at night and now fully recovers next morning.
My line is now stable with practically no re-syncs.

Great .

First time I've heard of that, but I'll bear it in mind for others. Then see whether it is repeatable on other lines.