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Why the sudden resync and high SNR

pacem
Grafter
Posts: 175
Registered: 07-09-2007

Why the sudden resync and high SNR

Hi all,
I've had a stable connection for the last 10 days (2720Mbs @ 9dB). When my connection does re-sync it's usually by just a few Mbs.

In the last 40 minutes my connection has just jumped to 1760Mbs @ 16.5dB.

I can see no reason for this sudden change (just 2 CRC errors since the re-sync). And I'm concerned this is going to get me stuck on an even lower BRAS profile.

Re-booting the router made no difference.

Paul.<><
30 REPLIES
Lorian
Grafter
Posts: 699
Registered: 31-07-2007

Why the sudden resync and high SNR

I had this too, good luck in trying to get it fixed :shock:
Community Veteran
Posts: 1,139
Thanks: 4
Registered: 20-07-2007

Why the sudden resync and high SNR

Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

Why the sudden resync and high SNR

Hi,

There could be a lot of reasons for this, because of the time of year the first thing I'm going to say is Christmas lights.

Every year we see an increasing number of faults reported to us where Christmas lights are causing problems, in the past they've mainly been intermittant sync faults, but with Max I'm suspecting we'll see more faults like this where the sync speed decreases. The lights cause interference, the connection drops and the BT Dynamic line management increases the target SNR to compensate.

Could be something else, if so first thing to do is have a read through:

http://usertools.plus.net/tutorials/id/13

to see if anything there helps.
pacem
Grafter
Posts: 175
Registered: 07-09-2007

Why the sudden resync and high SNR

Anyone know how I raise a ticket to get my SNR capped at 9dB?

I can't find a way through the help system. It insists I do a bt speed test but that breaks at 96% every time. The the PN help page says its unavailable and to please try later. ARGH.


Paul.<><
James
Grafter
Posts: 21,036
Registered: 04-04-2007

Why the sudden resync and high SNR

It would be technically possible, but owing to the distance you are from the exchange, I'm not so sure it would be a good idea as it would possibly result in intermittency.
pacem
Grafter
Posts: 175
Registered: 07-09-2007

Why the sudden resync and high SNR

If you do the maths my SNR going up and sync going down suggests that the level of noise on my line hasn't changed at all. Having the same number of CRC errors per hour as I've had since being max'd 6 months ago would also suggest there is no more noise.

I was online when my sync changed. My downstream line/router is very stable: re-syncs happen typically only once every week or two, and re-connects happen about once every other month if I don't force one sooner. It has remained on 1760kbs/16dB without fluctuation for 48 hours now.

16dB is just crazy, I never had any problems when the service started on 6dB (after stabilisation period) I would ask for it to be put back to 6dB were it not for all the warnings. (aside: there is still no interleaving option on my account yet.) If BT would stop messing with my SNR I might have a sensible line speed.

As for light: I live in an area where people only have static white lights, but if it were interference wouldn't I notice multiple re-syncs and changes in speeds?

Paul.<><
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

Why the sudden resync and high SNR

Generally I'd say yes you would probably expect to see multiple resyncs, however the increase in the SNR may be high enough now so that it doesn't resync if the interference re-occurs.

It is possible to decrease the SNR, BT have said that if the line remains stable then the Dynamic Line Management kit at the exchange will automatically decrease the target SNR, although how long it needs to be stable for seems a bit vague.

The other way is to raise a fault, if you go through the fault checker on the portal you shouldn't need to add the speedtest results so long as you explain in the extra comments box that it is a sync speed problem and that you want the target SNR decreasing.

Do bear in mind that should the target SNR be decreased and whatever caused it to go up in the first place re-occur then the SNR may just go back up again, as such I would recommend trying to find the cause, if your router/modem can graph the SNR over time then it would be useful to do this for a couple of days to see what it does.
pacem
Grafter
Posts: 175
Registered: 07-09-2007

Why the sudden resync and high SNR

I meant I didn't see any re-syncs before the SNR change other than the 1 sudden unexpected change about which I am complaining.

Thanks guys, I'll raise a ticket and hope it was a one-off and my SNR stays on 9db this time (I can't see why it shouldn't; it's not fluctuating from 16). I don't want a repeat of the thread posted by Boulby.

Paul.<><
pacem
Grafter
Posts: 175
Registered: 07-09-2007

Why the sudden resync and high SNR

Hmm, looks like I am in store for a repeat of Boulby's thread. Why would BT send out an Engineer when I have said I don't have a line fault just an incorrect SNR? Is PN just trying to stall here?

Transcript so far...(starting with me):
Quote

Curiously my comments from the fault reporting system don't show. Have you read my original comments?

Please set my target SNR to 9dB. It should not be at 16dB

Paul.<><
...

Dear Mr Gibbs,
We are unable to change the SNR on the line as these are the stats that are picked up from the physical line.
Do you have any noise on the line when making calls?

Regards,
XYZ
...

Don't lie to me, you can change the target SNR, but if there are problems with my line it will not stay at that target.

There is no noise on my line. There was no noise on my line when the SNR changed. My line hadn't needed to re-sync for 10 days before the change (not even to adjust by a few kbs/dB.)

Please set my SNR target to 9dB.

Paul.<><
...

Dear Mr Gibbs,
The only way to alter the SNR is by raising this as a fault through to BT wholesale. If they arrange an engineers visit and find that there is not actually a fault on the service they can charge a callout fee of £54.04.

Please let us know how you wish to proceed.

Regards,
XYZ



Any ideas anyone?
Paul.<><

PS I publically apollogise for the "Don't lie to me", right or wrong I shouldn't be so direct and accusative.
shellsong
Grafter
Posts: 2,191
Registered: 03-08-2007

Why the sudden resync and high SNR

Whoever it is probably isn't a liar-- just ignorant of the difference between noise level and SNR target!

Presumably you need to persevere until you get someone who knows what you and they are talikng about-- one of the Comms Team would know but obviously not all CS staff are of the same quality
Community Veteran
Posts: 1,139
Thanks: 4
Registered: 20-07-2007

Why the sudden resync and high SNR

Hi, I just thought I'd give you an update on my situation.

A few days ago, I decided to try leaving my PC switched on for a couple of days, (remember I have a modem which disconnects every time I switch my PC off), and when I rebooted I was back up to a sync rate of 7,616, SNR ratio 12dB. Eureka I thought, I've cracked it but last night my SNR was back up to 16dB with a sync rate of <7,000. This morning it's 13dB with a sync rate of 7,456.

I'm not aware of having had any disconnects during this whole period but could my modem be re-syncing without my knowledge - how cold I tell?

I've done a quiet test on my line and it sounds fine.

In the grand scheme of things I guess I shouldn't loose any sleep over this. Even at my lowest sync rate I'm still getting off-peak download speed of close to 5Mbps and I know there are many worse off than myself.

I'm just curious to understand what's going on.

Bob
pacem
Grafter
Posts: 175
Registered: 07-09-2007

Why the sudden resync and high SNR

AAaarrrgh!

I've been away for a couple of weeks. I return and check my router to find that it last re-synced 11 and a half days ago (so atleast it's stable) but....

... wait for it...

..it had re-synced...


... to 1312Kbs at a SNR of 19.5dB!

This is ridiculous, my attenuation remains the same at 59dB and my upstream speeds and ratios also remain unchanged at 448Kbs, SNR 16dB with Attenuation of 30dB.

So why is BT setting my downstream SNR so stupidly high? In the last 11.5 days I've had just 565 downstream CRC errors.

Oh, and in my absence my ticket has expired. grumph.

At 9dB (SNR) my line could sustain 2.5Mbs, so 1Mbs now seems painfully sloooow.

Paul.<><
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

Why the sudden resync and high SNR

To be honest I don't think the line's going sync much faster. 59dB attenuation is a very high figure amd indicates a very long line (the limit for a fixed rate 1Mbps connection is for the attenuation to be below 60dB).

Whilst in theory with the stats your line has it could sync at 2-2.5Mbps in reality that's probably just a bit too high. The longer the line the higher the chance of errors and "noise" causing problems, so the more likely the requirement for an increased SNR.

19.5dB does sound very high though, and it's likely that it the SNR was lower when it sync'd last and that it's increased in the last few days. Rebooting the router now and causing a resync would likely see it increase in sync speed as it trains to the SNR that BT have set, but as I say, to be honest I don't think you're going to see much more.
sinewave
Grafter
Posts: 297
Registered: 17-08-2007

Why the sudden resync and high SNR

I feel there is a bit of misunderstanding here.

The value of SNR you see when you check your router stats is a ~measurement~. It effectively tells you how much noise there is on the line at the moment the measurement was taken. This "moment" will be when you request the line-stats info from your router.

If you see a SNR measurement of greater than 6dB then you can (generally) conclude that your line is capable of syncing at a higher speed than it currently is. If you switch your router off and back on, to cause a re-sync, then you will probably see your router syncing at a higher speed.

That value of 6dB is the "target Signal to Noise Ratio". This is the SNR the exchange tells your router to aim to sync at (the speed of sync directly affects the SNR). Sometimes that target value is changed to a higher value for lines that have high fluctuations of noise. This is to reduce the number of times your router re-syncs, to provide a more stable connection.

The problem is that the noise on your line changes throughout the day and night. My line often seems to get more noisy in the early hours of the morning. When the noise gets worse than about 6dB (ie that figure goes ~less~ than 6dB) your router will re-sync. It will always try to sync so that the SNR is about 6dB (for most people). The frustrating thing is when your line suddenly goes noisy, for whatever reason (including interference from Christmas tree lights), for just a small period of time, causing a re-sync to a slower speed, when the line gets better again and noise reduces (SNR goes back ~up~) you will not see a re-sync and the slow sync speed will remain in place. Though now you will see a high SNR. In this case your router will not re-sync unless its "told" to do so. ~You~ can tell it to re-sync by power cycling.

If you just leave things alone you ~will~ see the router re-sync after a while as the exchange gear will give it a prod after a while - but it might take a day or three. The problem is that you only need to have another burst of noise for it all to slow down again. The approach that BT and router designers take is to limit the number of re-syncs the router goes through. They believe its better to sync at a low speed than to have a couple of re-syncs a day, so if you have a line with "bursty noise" you'll find you'll sync at a slow speed at all times.

You can get around this by power cycling the router to re-sync whenever you see slow sync speeds but if you do this often (a few times a day) the BT gear will interpret the interruptions as a bad line and slow you down even more!

BT were certainly not lying when they replied to you Pacem.

It sounds very likely that your problem is from some short living "bursty" noise. This could be due to interference that is beyond practical control of BT or you (you have to prove where the interference is coming from before you do anything about it - and its usually not straightforward). It could, however be due to a line fault, some dodgy connection, in which case its an engineers job. I'd celebrate the fact they've taken your complaint seriously with an engineer and not just brushed you aside as "another christmas tree light victim".

I hope this helps.