"Sub-clinical" sync losses

My parents are on the end of a long (3+km) rural telephone line that comes all the way from the exchange overhead on poles. Consequently it acts as a brilliant MW aerial and their SNR margin drops steeply at night when MW interference is at its worst.

Their router is a Netgear DG834G (v2). It syncs at around 3200 kb/s and never seems to vary. However, looking at their router log it shows a loss of sync about 6 - 8 times a night. It does not drop the ISP connection however (there are no LCP or CHAP authentication events logged). It also occasionally shows a stupid 10 digit number in the downstream SN margin field which suggests a negative number at that point. During the day their SN margin varies between 3db and 6db.

When asked, they say they have noticed the odd pause in web pages appearing when browsing in the evening but are otherwise content. Their usage is around 1Gb a month so they do not download anything much - just browsing and e-mailing.

Couple of questions:
If the system is resyncing so regularly, why hasn't the BT exchange equipment set a higher SN Margin?

Would BT set a higher SN Margin for this kind of situation if asked?

If the system is resyncing so regularly, why hasn't the BT exchange equipment set a higher SN Margin?

I doubt 6 - 8 times a night is enough to trigger the "rubbish line" threshold that BT have, which sets a higher SNR/noise margin.

Would BT set a higher SN Margin for this kind of situation if asked?

Unlikely, unless you have an ISP that's able to get through to BT and get them to understand what is required.
BTW, BT seem to either flatly refuse to lower SNR/margin from 15db (when line is stable at 6db), or manage to mis-underdstand the request each time. So I see no reason to hold out any hope BT would be capable of manually increasing the SNR/noise margin, as they sure as heck aren't capable of lowering it.

I had a very similar experience with a DG834, and I came to the conclusion that half a dozen resyncs a night wasn't enough to trigger the system into raising the target noise margin. I would be inclined to leave it as it is if your parents are not unhappy with the situation. Getting BT to raise it manually would mean persuading the ISP to persuade BT... a lot of effort for not much return.

Eric

Obviously you have had some experience with this
Clearly then this is not a straightforward job and I don't want to create unecessary agro for the folks so will probably not bother to contact their ISP.

I might try rebooting their router in the middle fo the night to force a lower sync. That might help some.

When I asked my ISP to remove interleaving (at my last house) my SN margin went up 3db (to 9db) simultaneously. Could have just been coincidence but it might be worth a bash if it gets any worse. Ironically it made my line a lot more stable than the interleaving. It barely dropped at all after that.

Thanks for the reply. Couple of follow-up questions:
Did you notice any symptoms when it dropped sync?
What router do you have now?
Is it used on the same line as your old DG834?
Does it hold sync any better?

OK, make that four follow-up questions

Ta.

Obviously you have had some experience with this

Absolutely, almost three months and counting, with repeated requests to two ISPs being forwarded to BTw, with no action (or the incorrect action) - The last one saw BTw getting it completely wrong and messing around with line cards at the exchange (instead of simply changing the SNR/margin, like they were asked).

Clearly then this is not a straightforward job and I don't want to create unecessary agro for the folks so will probably not bother to contact their ISP.

It is by no means, a straightforward job. ISPs have little effect on BTw (BT, as usual, doing what it wants, when it wants, if it can be bothered).
My current request to ISP to initiate a retraining of the line (in the hope this would start it, from scratch, with no data and a 6db target margin), was passed to BT in Mumbai (yes, you read correctly, even BTw tech support for ISPs has been moved to India), with a response that line retraining would occur either yesterday or today.
So far, nothing (no big surprise there), as I suspect my ISP had just as much joy in getting the Mumbai call centre to understand, as I had last time I tried to call my insurance company to see if a dealer loan/courtesy car was covered on my insurance (the response being, "No Sir, we do not insure taxis"????).

When I asked my ISP to remove interleaving (at my last house) my SN margin went up 3db (to 9db) simultaneously. Could have just been coincidence but it might be worth a bash if it gets any worse. Ironically it made my line a lot more stable than the interleaving. It barely dropped at all after that.

Always a risk. Interleaving tends to lower the sync a little (maybe putting them down a speed level). It does aid error correction, but shouldn't automatically increase the target margin.

A better plan, as we may only be talking about a 3db increase, and certainly easier than convincing BT to get off its ass, would be to go for a Speedtouch router, and tweak up the noise margin a little.

In reply to:

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Interleaving tends to lower the sync a little (maybe putting them down a speed level). It does aid error correction, but shouldn't automatically increase the target margin.

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You misunderstand. They already have interleaving enabled. I am suggesting removing interleaving. My experience of this is that removing interleaving drops the sync rate a level. Interleaving seems to help a connection maintain a speed that otherwise would not be stable.

Anyway, thanks for the advice on BTw - will avoid going down that route. Sounds a nightmare! Hope you get it fixed soon.

My parents don't notice the problem anyway. I migrated them from Wannadoo / Orange to Newnet so they could save

In reply to:

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Did you notice any symptoms when it dropped sync?
What router do you have now?
Is it used on the same line as your old DG834?
Does it hold sync any better?

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I didn't notice any particular symptoms. The DG834 series is well known for its capability to hold sync down to noise margins of zero or even less, and the resyncs seemed to be related to short-term noise problems.
Since then I've swapped between the DG834 and a Speedtouch 546 and back again a few times (on the same line). The ST546 is more stable on my line, but syncs at a lower speed, with a resulting lower bRAS profile, so currently I'm back on the DG834.

Eric

My parents have a similar issue with a line that is at least 6.5km from the exchange. They are only able to get fixed 512kb
Removing the ringwire helped quite a bit. As it the drops mostly happen at night, it doesn't bother them that much anyway.
Also found that the dishwasher on a timer was causing problems, but as that only goes on and off in the early hours of the morning, they weren't too bothered about doing anything to fix it.

If removing the ring wire helped, you could consider installing a faceplate filter, which would improve the SNR even more. It would mean that you'd need to run the ADSL modem from either the faceplate, or a different wire running off it.