Bad times. SNR through the roof

adamb · Aug 25, 2012, 19:16:56

My normal stats are around:

3dB SNR
24.5 Attenuation
Downstream of between 18500 - 19500 kbit/s
Loop length around 1100m

For a day or two I had noticed a drop in sync speed after a couple of random disconnections. It turned out the my microfilter had kicked the bucket. I had lots of CRC errors etc (Interleaved). I replaced it today with a new one (ADSL2+ compatible) and my sync rate went back up to normal.

This evening we had had quite a bad thunderstorm which caused my connection to drop a few times. Now my SNR has shot up to 12.9dB and my sync speed is around 12000 kbit/s. Even with the SNR at 12+dB I am still getting lots of CRC errors. This could simply be down to the storm still being in the area.

I have tweaked my SNR down to about 6.3dB (Netgear 834GT with DGTeam firmware) and now I am sync'd at 15588 kbit/s. I know that tweaking is probably not the best idea considering that there are errors on the line but to be honest, the error rate is about the same.

I have read some not so encouraging things about BT's DSLAM on this subject but I wondered if anyone has any experience or knowledge with regards to SNR recovery times?

Cheers, Adam

Before:

After:

EDIT: Turning the SNR tweak off now, terrible speed, too many errors and overheads lol

Steve · Aug 25, 2012, 20:19:47

Just a thought on BT adsl2+ connections I didnt think that you could over ride the margin. I know you can with IDNet LLU. I might be wrong of course.

adamb · Aug 25, 2012, 20:29:27

Quote from: Steve on Aug 25, 2012, 20:19:47
Just a thought on BT adsl2+ connections I didnt think that you could over ride the margin. I know you can with IDNet LLU. I might be wrong of course.

Yeah, you definitely can. I tweaked from 12+dB SNR and got 12,000 kbit/s, down to 6.3dB SNR and got 15,588 kbit/s. You can do it via Telnet or DMT (http://lop.im/m68eU)

The storm seems to be passing and errors are not as bad. The RCO has risen to 18,364 kbit/s so things are looking good. SNR is still high though. Hopefully it will come down.

Fingers crossed

Steve · Aug 25, 2012, 20:47:39

Are you definitely with BT DSLAM. The only way I know to tell the difference on a Be* enabled exchange is failure to perform a BT speedtest (old version which gives IP profile or via support.

adamb · Aug 25, 2012, 21:18:13

Yes, I'm on BT DSLAM. The only LLU round here is TalkTalk. The BT speedtest works for me. Just wish it was quicker

Steve · Aug 25, 2012, 21:27:55

Thanks for that.

adamb · Aug 28, 2012, 18:57:00

Hmmm, SNR has now gone down to 6dB. No CRC errors at all but my profile seems to be stuck. I am stuck on 14333kb/s (used to get 19000+kb/s) Maybe the cap will be removed once it's been stable a while. We'll see.

Steve · Aug 28, 2012, 19:23:59

BT uses banded profiles on adsl2+ to limit speed on unstable connections ie high error rates, noise etc. The highest banding ends at 14336 which is very similar to your current sync. As far as I am aware they can only be removed via request.