Possibly 'stuck' SNR margin?

SSK · Oct 19, 2010, 10:30:43

On 27th Sept the exchange disconnected me and SNR margin went from 6dB to 9dB for no apparent reason - there had been more than three days of continuous synch and there were only a tiny percentage of RX errors.

After that there was continuous uptime until 10th Oct with only about 8200 CRC RX errors out of 1260246900 RX cells and still with 9dB SNR margin. Then there was a synch but no access to the internet. I later learned it was a problem caused by BT backhaul, but at the time there was no way to know that so I tried rebooting the router.

Now (19th Oct) there has been about 190 hours of continuous connection with only about 14300 CRX errors out of about 127083000 RX cells. That would seem to be a stable line with a low error rate, but still the SNR margin seems to be 'stuck' at 9dB.

Questions:

Bearing in mind the abouve informatioon, is it likely now that the BT equipment will ever automatically bring the SNR margin down to 6dB?

If it is likely that the SNR margin will come down automatically, how much longer might it take?

Will rebooting the router help to 'unstick' the SNR margin or will it make it worse?

Gary · Oct 19, 2010, 11:00:26

I think its two weeks without a drop, and a manual reboot after that for it to lower, you could always call IDNet and ask if they can lower it for you, they will advise you further on your line specs as well.

Odos · Oct 19, 2010, 11:03:49

I had a similar problem and after much searching I never found an "Official" answer but the general consensus was that if you have no errors at all and a continuous unbroken connection for 14 days it should drop. If you have any errors then it takes 30 days continuous connection. So don't reboot the router.

This however did not apply for me. In my case the SNR rose to 15db. No matter how long I had a stable connection ( almost a year ) it never changed. I contacted support, they had BT reset it. The reset lasted three days then shot up to 15db again in one jump. After this I gave up.

A month or two ago There was work done at my exchange ( not by BT ) and suddenly my SNR dropped as it should. It's now back to 6db. So make of that what you will

BTW When mine started to work as it should I didn't need to manual reboot, it did that itself

karvala · Oct 20, 2010, 14:11:00

Check out my previous threads on the topic; mine behaved in EXACTLY the same way. At the time, IDNet tried to tell me that the problem was with my equipment, when it was already plainly obvious to me for a variety of reasons (all of which I vainly attempted to explain to IDNet customer service) that the problem was faulty exchange equipment. Lo and behold, after exchange work, SNR adaptation started to work correctly. I've seen several other instances of this elsewhere as well. I don't know what to suggest to you really; you can contact IDNet and ask them to get BT to look into it, but the chances of BT acknowledging that their exchange might be anything less than perfect is pretty minimal, which leaves you with few options. Don't have them send a BT engineer to you, and if you've done all the usual tests on your equipment, don't be fooled into thinking that it's your fault either. It's most likely BT's fault.

SSK · Nov 20, 2010, 11:42:14

Quote from: Odos on Oct 19, 2010, 11:03:49
I had a similar problem and after much searching I never found an "Official" answer but the general consensus was that if you have no errors at all and a continuous unbroken connection for 14 days it should drop. If you have any errors then it takes 30 days continuous connection. So don't reboot the router.

To follow upon this...
The connection has now been stable and unbroken for 736 hours (which my arithmetic gives as more than 30 days) and the situation is still the same since the unexplained jump from 6dB to 9dB SNR margin.

In that 736 hours there have been 970418270 RX cells and only 34879 RX errors.
Loop Att has been 23 dB and SNR Margin has been 8 or 9 dB every time I've looked at it (at least twice per day).
Sync rate (17,656,303) has also always been the same.

Is there any chance now that the BT equipment will automatically return the SNR margin to the 6dB it was originally?
Is there anything I can do to have that done? (AIUI IDNet with ADSL2+ has no direct control over it).
Would rebooting the router be likely to make things better or worse?

Thanks for any advice.

Sean

Rik · Nov 20, 2010, 11:45:45

Reboot the router, but only once, Sean. If that doesn't help, have a word with support on Monday.

SSK · Nov 23, 2010, 11:30:00

This morning, when an auto-reset of SNR Margin did not happen after 33 days I powered down my router for over 30 mins. After turning power back on there was a slightly improved sync rare of 17784247. However, SNR Margin is still 9dB with Loop Att still 23 dB.

I appreciate that this is a relatively trivial matter and most people wouldn't even consider it a problem at all. I'm not particularly concerned about the situation, and am mostly trying to understand what might be going on. Although I've contacted support, it wouldn't be fair to expect them to take time away from customers with real problems just to educate me in this matter.

Apparently after 14 days error free the SNR Margin should come down. But I was also told that with just a very small number of errors it would come down in 30 days.

The line was totally stable for 33 days, with only 38238 RX errors out of 974339549 RX cells.
That's only 0.0039 percent errors - if my sums are correct, that's less than 4 errors out of every 100,000 cells. Maybe that doesn't count as a 'small number of errors'.

Is it ever possible to have a completely error-free Iine for 14 days?

At the moment, my interleaving is turned off.
Do I understand correctly that turning on interleaving will decrease the number of errors?
If so, might the SNR margin go down if I get interleaving turned on?

Sean

Rik · Nov 23, 2010, 11:37:03

Hi Sean

BT tend to set very tight limits pn error counts, but they don't tell anyone what they are. Ask support if they can get your target margin reset to 6db. If they can, and it goes back to 9db quite quickly, then BT has decided your line needs that noise margin to remain stable. Interleaving might help, but it will cost you in terms of latency, so if pings are important to you, you probably don't want to go there.

SSK · Nov 23, 2010, 12:09:53

Thanks for the quick response, Rik.

Do you happen to know if the BT limit is number of errors or percentage of errors?

If it's the former then presumably if I don't download much then there won't be so many errors and BT algorithms might decide that there is a low enough error count to bring the SNR Margin down.

On a slight tangent - Apart from online gaming, which I don't do, is there any advantage to a low ping?

Thanks again,

Sean

Steve · Nov 23, 2010, 12:16:43

I thinking always dangerous whether interleave may help to reduce the error load, the total amount of errors with interleave on will be the same but forward error correction (which is enabled by interleave on) will reduce the total CRC and thus reduce the amount of data that has to be re requested. So it might work???

Rik · Nov 23, 2010, 12:18:25

I don't know, Sean, it's a sort of black box system, no-one knows just what counts and what doesn't.

There's no great advantage to a low ping if you're not a gamer - one of my lines gives 24ms average to IDNet, the other 14ms. In use, I can't tell the difference.