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How to fix downstream SNR stuck at 12dB?

albany
Grafter
Posts: 94
Thanks: 6
Registered: ‎18-08-2007

Re: How to fix downstream SNR stuck at 12dB?

I'm afraid it's still stuck at between 11 and 11.5dB and has been since I fired up Routerstats this morning (hasn't budged by more than 0.5dB all day).
Dip in trace is me re-booting the router to see if anything has changed.
jojopillo
Plusnet Alumni (retired)
Plusnet Alumni (retired)
Posts: 9,786
Registered: ‎16-06-2010

Re: How to fix downstream SNR stuck at 12dB?

HI albany,
I've just ran some checks and the target is definitely set at 6dB. I've no idea why you keep getting that 11dB all the time though. Have you another router you can try? or maybe another RJ11 cable?
Jojo Smiley
albany
Grafter
Posts: 94
Thanks: 6
Registered: ‎18-08-2007

Re: How to fix downstream SNR stuck at 12dB?

Hi JoJo,
I tried it all before I raised a ticket and posted here (3 different routers, different cables, different filters, test socket - all the same result).
In my first post  on this thread I mentioned that the line has never responded properly to configuration changes and explained some of the weird behaviour.
I understand that you need to go trhough the tests, but to my knowledge, this line has never suffered a hardware fault but  has always had very weird operation from a control systems point of view.
I think you're finding what others before you have found, the controls on this line are broken.  I think it really does need excalating through BT to have the record wiped and rebuit from scratch (not just a BRAS profile reset) and confirmed to be mapped to the correct physical pair.  If possible I'd like the target SNR margin locked to 6dB and Interleaving locked to FAST mode.  The line will comfortably support that configuration.
My suspicion is that orders put through on this line are actually being applied to a completely different line.
If the line won't respond to an SNR margin target reset, I don't see there is much more you can do without escalation.  
I've now monitored the SNR margin for 24 hours and it is rock-solid at between 11 and 11.5dB.  When I've had problems before, I've monitored the line for months at a  time and also seen an absolutley rock-solid noise margin and good clean spectrum except for narrowband interfence from a powerful BBC radio transmiitter in North London at 909kHz (Radio 5 Live) and 1089kHz (TalkSport) - which reduces the SNR specifically in those bands.  In hardware terms this is a perfect line!
EDIT
One further thing which may just be relevant.  The DSLAM chipset reports back as "unknown" which perhaps points to an incorrect/incomplete configuration at the exchange.
Andy
jelv
Seasoned Hero
Posts: 26,785
Thanks: 971
Fixes: 10
Registered: ‎10-04-2007

Re: How to fix downstream SNR stuck at 12dB?

What speed are you currently synced at?
jelv (a.k.a Spoon Whittler)
   Why I have left Plusnet (warning: long post!)   
Broadband: Andrews & Arnold Home::1 (FTTC 80/20)
Line rental: Pulse 8 Home Line Rental (£14.40/month)
Mobile: iD mobile (£4/month)
jojopillo
Plusnet Alumni (retired)
Plusnet Alumni (retired)
Posts: 9,786
Registered: ‎16-06-2010

Re: How to fix downstream SNR stuck at 12dB?

Hi Andy,
I have picked up your ticket and passed your comments on to our faults team for further investigation.
Jojo Smiley
albany
Grafter
Posts: 94
Thanks: 6
Registered: ‎18-08-2007

Re: How to fix downstream SNR stuck at 12dB?

Quote from: jelv
What speed are you currently synced at?

Just over 4Mbit/s, INTERLEAVED.  Line will happily support 8128, FAST when it gets the chance.
albany
Grafter
Posts: 94
Thanks: 6
Registered: ‎18-08-2007

Re: How to fix downstream SNR stuck at 12dB?

Quote from: Joanne
Hi Andy,
I have picked up your ticket and passed your comments on to our faults team for further investigation.
Jojo Smiley

Many thanks.  I see your internal ticket mentions that BT systems report FAST mode is operating.  I confirm my modems are all reporting INTERLEAVED mode (and ping times support the interleaved mode reading).
That inconsitency has happened before (http://community.plus.net/forum/index.php/topic,75467.msg603667.html#msg603667) - FAST mode was eventually enabled on the line by putting an order through to turn FAST mode off!
Andy
albany
Grafter
Posts: 94
Thanks: 6
Registered: ‎18-08-2007

Re: How to fix downstream SNR stuck at 12dB?

Now this seems to be getting really strange.
At about 6.56 yesterday morning the SNR margin finally dropped to 6dB on a resync. Grin (see attached Routerstats output)
I didn't notice it until late in  the evening when I got home and swapped in my just-received DG834 v.4.  DMT was reporting fast mode - all was well with the world.
And then as I watched, the modem re-synced and the margin jumped to 15dB Sad with a consequent huge drop in speed. (Second routerstats output).  I then had a bit of a play with the SNR tweaking feature (couldn't make it any worse...) and then left it at 100% overnight.
This morning still running at what looks like a 15dB target, slow sync speed and yet (see DMT plot) between 19 and 36dB SNR available across practically the whole band i.e. massive headroom to go faster.
It is almost as if DLM is measuring another line and applying fixes to my line for noise which isn't there (my monitoring has yet to capture a noise event).
Still no response to the ticket JoJo referred on Wednesday morning Cry
jojopillo
Plusnet Alumni (retired)
Plusnet Alumni (retired)
Posts: 9,786
Registered: ‎16-06-2010

Re: How to fix downstream SNR stuck at 12dB?

HI albany,
I see we have now raised this to our supplier as of yesterday.
Jojo Smiley
albany
Grafter
Posts: 94
Thanks: 6
Registered: ‎18-08-2007

Re: How to fix downstream SNR stuck at 12dB?

OK.  Egg on face here  Embarrassed
It turns out there was a line fault.  My phone line had become trapped in the door of my nearest green cabinet.  Just discovered after a very speedy response and visit from BT.
My line attentuation has now dropped from around 30dB to 21dB (which I now see from old records is it's normal level - hence embarrassment since I perhaps should have spotted this before) and the line is syncing around 6.5Mbit/s despite running a 15dB SNR target.
JoJo, any chance of another order to set SNR target to 6dB and FAST mode?  Hopefully this time it will take and stick.
Thanks, Andy
PS I've managed to get sync up to 8000 by using the DGTeam Router firmware SNR tweak (by forcing sync at 11.5dB SNR) to get the BRAS profile re-training.  This is the best I can do with the firmware GUI.  To get up to 8128 will require me using the firmware CLI so I haven't bothered for now.  Would be good to get the proper sync speeds without my "bodge" though.
jojopillo
Plusnet Alumni (retired)
Plusnet Alumni (retired)
Posts: 9,786
Registered: ‎16-06-2010

Re: How to fix downstream SNR stuck at 12dB?

Hi Andy,
No worries, I'll request that for you. I'll let you know when it's completed.
Jojo Smiley
jojopillo
Plusnet Alumni (retired)
Plusnet Alumni (retired)
Posts: 9,786
Registered: ‎16-06-2010

Re: How to fix downstream SNR stuck at 12dB?

Hi Andy,
That's complete now, just need to wait for it to take effect. Once it has I'll be able to change it on your account too. I'll check before I go home to see if it's changed.
Jojo Smiley
jojopillo
Plusnet Alumni (retired)
Plusnet Alumni (retired)
Posts: 9,786
Registered: ‎16-06-2010

Re: How to fix downstream SNR stuck at 12dB?

HI Andy,
Still not changed yet. I'll check again tonight after the soaps if I get a chance.
Jojo Smiley
jojopillo
Plusnet Alumni (retired)
Plusnet Alumni (retired)
Posts: 9,786
Registered: ‎16-06-2010

Re: How to fix downstream SNR stuck at 12dB?

Still not changed I'm afraid, I'll check again in the morning.
Jojo Smiley
albany
Grafter
Posts: 94
Thanks: 6
Registered: ‎18-08-2007

Re: How to fix downstream SNR stuck at 12dB?

OK,  Thanks