SNR check possible?

[#1]

I've recently moved to PN (from TT) and am getting a very healthy (for rural) 5900+ sync rate. Checking my router shows an SNR of 14dB (does not vary much). When with TT the line was very poor quality (since fixed by BT replacing a drop cable) - is it possible that the SNR target has gone up as a result of historical faults? Does this return to 6dB automagically, or can it be 'prompted' from PN end?
Many thanks,
Gill

[#2]

You also need to have a look at the error rate. With that margin I'd expect to see very, very few errors logged. If errors are being logged it probably means you need the high margin.
If it's clean PN can initiate a retrain which if you are on 20CN will set it back to 9db.

[#3]

On 20CN I have heard anedotally that if your line remains stable and on 24/7 for a very long time the exchange may drop the SNR target - we are talking weeks here not hours.
Uncorrectable CRC Errors/15 minute period are was is usually looked at.  Unfortunately there seems no publically available document/guidance as to what sort of error rate is acceptable.  Quite possibly the programing algorithm of the DSLAM is rather complex in working out what do do in what circumstance!
Personally I've never waited. 
On about 3 occassions since 2008  I've had to request a SNR reset from the Plusnet team.  The cause for the target SNRmargin being raised on two occassions was an intermittant fault on the line - I made my request after the problem was sorted by my phone provider BT and after I had satisfied myself the fault really had gone -so I waited a week or so. The third occassion was almost certainly BT messing about in the horror that is my line's street cabinet trying to fix someone else's problem and accidentally disturbing/breaking/mending my circuit into the bargain.
I once talked to an Openreach employee acting in a training/supervisory role and we discussed the problem of lines which are accidentally mucked about with while mending someone else's voice fault on the line and the unfortunate problem that may arise if the line being mucked about has the broadband on at that time as the exchange DSLAM will see this as disconnections and/or sudden massive noise on the line and may take action to increase the target SNR - quite uncessarily.
I think ISP's are able to do a SNR reset now easier and as a standalone action rather than as part of a total line retrain, though they will want some sort of reason to do it as some longer lines are naturally prone to errors and doing a SNR reset on these  is pointless as the exchange will merely stick it straight back up the next day or so. I believe the target steps on 20CN are 6,9,12,15 and possibly 25.

[#4]

Quote from: jelv

PN can initiate a retrain which if you are on 20CN will set it back to 9db.

6dB actually :).

[#5]

I know that 21CN is different so I thought it was 9 on 20CN. Does 21CN go down to 3?

[#6]

Quote from: jelv

Does 21CN go down to 3?

Yes.

[#7]

Over on this thread is a post with some threshold values in DLM. That suggests that when DLM makes a change, it does so while monitoring both CRC errors (the MTBE) and the resyncs (the MTBR), where MTB = "Mean Time Between".

[#8]

Interesting finds indeed
I always suspected it was quite complicated.
On 20CN the minimum SNR target for the exchange to aim for is 6dB - I'm on this setting
On 21CN the min possible is 3dB as stated in above posts
The difference is something to do with the way the overheads of the packets are transmitted, and even if I did sort out in my own mind the whys and wherefor I doubt anyone else would care or be interested  - even if I could explain it simply!

[#9]

Hi gillianfuller,
I'd be happy to set this back to 6dB for you, although I'd rather have it in the master/test socket before I do.
Jojo

[#10]

Many thanks Jo,
we only have the master socket, it's one of those with a single phone and a single ADSL port side by side and was installed by a BT engineer years ago. We recently had a BT  (Openreach) engineer call due to a noisy line and he checked our house wiring before locating a fault outside the property.
I'll check the router stats/fails when I'm next at home and post here if they're interesting.
Gill
dick:quote

[#11]

OK GIll, let me know how you get on

[#12]

Jo,
the Netgear shows no errors and has been up & running for 150+ hours.
The dB was 13.6 when I checked earlier, still syncing happily at 5.9Mbps.
Can you request a 'kick' back to 6db, or a 'retrain' of some sort?
Many thanks,
Gill

[#13]

Hi Jo,
quick bump as this has dropped off first page.
Do I need to raise a ticket for this request?
Gill

[#14]

Hi Gill,
I've requested that now. I'll check later to see if it's taken effect.
Jojo

[#15]

Jo,
certainly has taken effect, Netgear is now sync'd at just over 8100K !
Well done, and thank you.
Gill