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[JohnRYoung]

It couldn't last. Following resolution of my problems described in "Low Profile", I had about two months of stable connection from BT broadband with downsteam sync rates 1700-2000 kbps and SNr margin of 9-15 dB. Now, suddenly, once or twice a day I notice sudden drastic sluggishness. Inspection with DMT.exe (thank you, thank you to the providers of this!!!) reveals that the connection has resynched at >2000 kbps and SNr margin of 6-8 dB, and the crc error rate has gone off scale. Using DMT to resynch at higher SNr margin (>10) restores the effective connection speed and drops the crc error rate. So it appears that someting has been changed in the exchange that has lowered the target SNr too far. Kitz said something about imminent changes by BT, and I wondered if this was one of them. The connection is currently non-interleaved, which may contribute to nthe crc error rate at low SNr?  This was the "solution" to the earlier low profile speed problem. My router/modem is Netgear DG834GT. The current profile setting is 1500 kbps. Present connection is 1952 kbps attenuation 58 bD, SNr 11.4, crc error rate low; 60 mins ago it was 2400 kbps with crc error rate off scale, before manual resynch. Is there a way of telling the modem to always ask for the larger SNr margin. or of getting BT to change the target SNr at the exchange? It seems they have responded to many problems with the slow response of the  rate adaptive system by now making it vtoo aggressive in speeding up!

[roseway]

The effects you are seeing are not likely to be due to anything BT have done. They are the typical result of an intermittent noise problem. There's a lot of helpful information about noise problems at http://www.kitz.co.uk/adsl/lowSNR.htm . If there's any audible crackling on the telephone then there's probably a voice fault and you can report it to BT Retail (dial 150). All sorts of electrical equipment can produce interference, so you might consider whether there's any new equipment in your home or nearby.

I don't want to depress you, but with an attenuation of 58 dB you're always going to be vulnerable to this sort of happening.

[JohnRYoung]

Well .... I picked up the phone to see if I could hear a crackle as you suggested, and I heard radio 5 live on it !!!??? Turns out the extension in my bedroom, which I never use, had fallen off its hook.....I left the radio on after I got up this morning .... I rarely use the landline phone, so this may have been the cause all along ..... I am a complete idiot!

[roseway]

 

Thanks for letting us know.

[Achilles Last Stand]

..... I am a complete idiot!

Don't worry, you are amongst friends 

[mr_chris]

LOL

Nice one - and all credit for admitting it!!

However..... (serious hat on) - even if the extension in your bedroom had fallen off its hook, it shouldn't have interfered with your broadband! Have you got an ADSL filter on that phone, even if you never use it?

If not, might be time to stick one on there, and on any other telephone connected devices, including sky boxes (they are notoriously bad for causing problems), alarm systems, ALL telephones etc.

Thanks for giving me a laugh this morning though

[JohnRYoung]

Filter on everything, but the trend continues. This morning the sync speeed was up at highest yet 2900 kbps, SNRM=6.6, erors off scale, no functional connection. Without DMT I would have no connection at all!

[roseway]

What ought to happen automatically is that the exchange system will increase your target noise margin to make the connection more stable. It looks as though you're still at the default 6 dB target margin, and I guess it's possible that, by using the DMT tool to stabilise your connection, you're fooling the exchange system into thinking that it doesn't need to do anything. If you can stand a couple of days of hopeless connections, it might be worth resetting the default with DMT (set the value in the modemoptions 00 box to 00) and leaving the automatic process to do its work.

Of course, it could still be worth doing some more investigation to see if you can trace the source of the interference.

[JohnRYoung]

That makes sense. I am trying to see if there is a temporal pattern now. next week I am going to spend a couple of days in hospital, so then I can try that. It has now started dropping the connection completely with huge error rates after being stable for many hours. I guess that is consistent with an intermittent noise source of some kind. Have to look at the radio five schedule ;-)

[JohnRYoung]

Now I am getting really confused. This behaviour seems reproducible - After resynching the router, I get good connection with low error rates for hours, even if I leave it with 6 dB snr margin. Then suddenly get huge error rates and either the snr drops to less than 6 or the router drops the connection completely. But resynching the router at this point always restores a good connection immediately. This morning, it was hanging on at 2300 kbps with snrm=2.3 ... error rates off scale ... then I resynched and got >3000 kbps at snrm=6.3 and low error rate. What gradually accumulation effect, or randomly occurring event, can lead to degradation of the connection that is invariably recovered after a resynch even when the latter is at an increased speed?

[roseway]

That is rather odd. I suspect that what's happening is that something sets off a short burst of interference which causes the router to go into an unstable mode, even after the interference has stopped. You might consider upgrading the router firmware if a more recent version is available. There's also an unofficial firmware version available from http://dragonslight.altervista.org/ubergt/index.html which is worth considering. When I was using a DG834GT I used this firmware and it was noticeably more stable than the official firmware.

A very short burst of interference like this suggests a dodgy freezer thermostat or something like that, or some fairly powerful electrical equipment starting up.

[JohnRYoung]

Thanks again.
At last I found a way to make the target snrm go up. I use routerstat's alarms to force
up to 10 reboots per hour when snrm went below 6 dB. During the night there were
two occasions with ten successive reboots....then 15 minutes after the second there
was a resynch to give a 9dB snrm, and rebooting now starts at 9dB. So now we will
see if this solves the problem! Synch is now at 2528 kbps and profile up to 2000 

[roseway]

That could be a good result. Here's hoping!

[kitz]

>> At last I found a way to make the target snrm go up. I use routerstat's alarms to force up to 10 reboots per hour when snrm went below 6 dB.

Brilliant 
Fingers crossed for you that it now stays stable.

[mr_chris]

Gawd, I wish my SNR margin was that insensitive - it seems 2 reboots was enough to push it up from 9 to 12dB