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ADSL - Causes of Sudden Large Changes of SNR???

Posts: 5
Registered: ‎03-12-2013

ADSL - Causes of Sudden Large Changes of SNR???

This is bog-standard BT-supplied ADSL max over a 3 mile rural phone line. It works well - except for sudden large changes (4 - 6dB steps up or step down) of the downstream SNR. The steps in SNR started around a year ago (about the time FTTC equipment began to be installed at the exchange). Steps take place perhaps once a day on average at seemingly random times, but with a bias towards late evening. Sometimes there is a second step reversing the initial step change after around an hour. The attached RouterStats plot shows a typical "step event" that occurred last night.
I have done the usual diagnostics and remedies:
- tried two modems/routers (an old Netgear and a Billion 8800nl),
- checked line for noise,
- filtered mains incoming,
- switched off houshold mains powered stuff,
- connected modem direct to to incoming phone socket
- etc.
It's not a show-stopper as the lower SNR state is good enough for a 2.5 or 3Mb/s profile, but I would very much like to get to the bottom of the problem. Anyone suggestions as to the cause? Have you encountered something similar (and how did you fix it)?
Rising Star
Posts: 392
Thanks: 4
Fixes: 2
Registered: ‎06-09-2007

Re: ADSL - Causes of Sudden Changes of SNR???

Probably crosstalk...the jump from 6 to 12 may be a neighbour switching off their router for the night, you are loosing about 2 Mbps synch speed.
Do the times match up with that?
Posts: 5
Registered: ‎03-12-2013

Re: ADSL - Causes of Sudden Large Changes of SNR???

I don't think the pattern fits a router being switched on and off as it is very irregular for a daily routine - sometimes dips for half an hour, sometimes for a day or more.
Yes, the SNR change is equivalent to a loss of around 2Mb/s. I have no experience of router-to router crosstalk - would it really be that large an effect unless there was some fault condition?