cancel
Showing results for 
Search instead for 
Did you mean: 

SNR Noise margin

Soapy
Grafter
Posts: 38
Registered: 01-10-2007

SNR Noise margin

Since I was maxed a couple of months ago, I have consistantly sync'd at 3400 and above. Normally my stable rate was 2.5Mb, and frequently 3Mb. Then last week I lost my connection, and when I checked my router my sync rate had dropped. Now I struggle to sync at 2500, with stable rate now 1.5Mb.

Before, my SNR margin was always <10, now its always 15 or more. Has BT changes the way maxDSL works, as I thought it tried to minimise the SNR margin to 8 or so.

Results from my router are below. (I have to different routers and both give similar results).


Downstream Upstream
SNR Margin 15.3 22.0 dB
Line Attenuation 53.5 31.0 dB
Data Rate 2528 448 kbps
Latency INTERLEAVED INTERLEAVED

One thing that might be connected is, last week (28th July) I came back after a few days holiday and switched on my router that had been turned off. My router would sync (at its normal 3400+) but I could not connect to plusnet, my router reported me as offline. I tried a different router, but I still could not connect to Plusnet, even althogh my router had sync'd. So I kept rebooting my router to try to force a connection. Finally I got connected and everything was fine.

It was after that (I can't remember if it was the same day or next) that I noticed my sync rate had dropped, and my SNR margin was up. I have read that unstable connections can cause the target noise margin to be raised to make the line more stable. Could the repeated reboots of the router have caused that. And if that has happened, can I get the target noise margin reduced to where it was, as it was not a problem with the line sync'ing that cause the repeated disconnections. My connection was rock solid at 3400+ sync rate previously.

Stephen
32 REPLIES
Community Veteran
Posts: 14,469
Registered: 30-07-2007

SNR Noise margin

Same thinghappened to me - origibally syncing fine @ 2500ish, rebooted my router, it failed to connect, rebooted a few more times and eventually it did connect but at a lower sync rate. This was due to the min SNR now set at 9dB instead of 6. Over the next few weeks the min SNR went up to 15dB with a corresponding reduction of sync rate.

This is one of the problems with Max. The exchange thinks your line is bad so ups the SNR and there is now way I found to get it reduced again, even after 3 weeks of ticket tennis with PN.

You need to raie a fault and see if PN can be convined to get BT to set a lower SNR.

I did not follow this through as I get LLUed after 3 weeks of tickets!
craiglay
Grafter
Posts: 138
Registered: 27-06-2007

SNR Noise margin

Hmmmm.. It seems the SNR margin can never be just right...

Too low, disconnections, unreliable connection, constant problems.

Too high, speed dies

Can we not have an easier way of changing it? Please?Huh
ScoUK
Grafter
Posts: 61
Registered: 04-08-2007

SNR Noise margin

Hi all,

This sounds familiar. My SNR seemed to jump up to around 15db several weeks ago.. Have got an ongoing ticket tennis match with plusNet at the moment and it's starting to go round in circles. Strangely after the last update to the ticket 2 days ago my noise margin has dropped to around 10db and sync speeds have increased to between 1.5Mbit and 2Mbit..

Never had any problems with line stability.. SNR jumped up to 15db when interleaving was clamped on my line. My fault there was messing about with wiring and forced many disconnection in a short space of time. Sadly line sync speed dropped to under 1Mbit. have since got interleaving removed however the high noise margin still exists...


Just waiting for the stable rate to catch up with the new sync speed and take things from there. *sigh*

. o O (I really hope the SNR will go back to the normal 5-6db automatically...)
N/A

SNR Noise margin

You all seem to have this the wrong way around snr margin isnt the same a snr. The higher the snr margin the betterso 6 is ba 15 is good.

The snr margin changes with sync speed so if you connect at 2 meg it may be 15 but connect at 4 meg it may drop to 8. The time of day and the weather and the ice cream man all effect it.

The problem can be you have a high snr margin so the line says its a very good line connect at 8 meg which it does but it cant keep that speed and the snt margin dops below 6 and it disconnects and trys again.
I have a very good filter if I put that on I connect at 6.5 meg but it cant hold the line. So I have to put a rubbish filter on my snt margin is 8 so it connects at 4.5 meg and stays on ok. If while I am connected I put the good filter back on my snr jumps up to 15.

ao its a game of chick and egg to get te best speed but stable.

You could look at your router as a lot have a setting called target snr margin which means you can set it on the router to not connect unless it gets a snr of say 10 which will redice the speed but give you a stable line.

If you play about you can find anything can efect the snr even moving the wire a foot so its not near electric or a radio. FM radio uses a simliar frequancy to adsl so putting the radio can give problems
Community Veteran
Posts: 14,469
Registered: 30-07-2007

SNR Noise margin

I'm not sure you understand how MAX works in relation to SNR.

Yes the sync speed is related to the SNR you have but the problem here is BT can set the min SNR that they will sync at - i.e they will try to get the best sync speed they can but keep the SNR value at or above a minimum that they set in the exchange.

When Max is first installed, BT set the min SNR to 6dB (as we have all had), but if the exchange equipment finds a bad line or thinks the line is bad due to resyncs / disconnections, it will autmatically raise the min SNR by 3dB to 9dB and try to resync with that value. Sometimes the exchange gets confused and decides to up the min to 12dB or to 15dB (the highest min the exchange can set) and as a result the max sync rate possible will decrease.

The problem experienced by those posting here is the line had been working perfectly fine at a 6dB min SNR level but due to reasons unknown to any of us, the exchange decides to up the min for no reason and once it has been increased, it is impossible (so far) to get BT to reduce it. The reason they often quote is the line must have been bad for the automated checking to determin a 15dB min SNR is required for reliable operation. This is proving to be wrong on many cases (you have 3 listed here) where the line was working perfectly at 6dB until some event change (like swithing the router off in my case) causes the exchange to think the line is bad and up the min SNR to compensate.

This is one of the most frustrating problems with Max which has affected many people )and why there are so many people complaining abou bad speeds after upgrading to max). The lines are capable of much faster sync speeds but the exchange will not allow those speeds to be used due to decisions it has taken on the line quality which have clearly been shown to be wrong on many occations.
ScoUK
Grafter
Posts: 61
Registered: 04-08-2007

SNR Noise margin

Yeah peter.. That's exactly what I'm finding with my connection..

Noise margin level was always fine and stable at 5-6db but somewhere the minimum noise margin has been increased to 15db. Which in turn means sync speeds dropping to just over 576kbps for me. not nice..


As mwright suggests some routers can let you fiddle with the set minimum noise margin. the Netgear DG834 range in particular have a hidden option somewhere where you can set a percentage ratio (roughly speaking) of what to adjust the noise margin to either to Increase or decrease it.

I've fiddled with it in the mean time to get my reported noise margin lowered to around 8-10db which is allowing the sync speed to be sitting between 1.5Mbit and 2Mbit... At the moment it's the best 'fix' I can think of until there is a system in place to have the minimum noise margin levels decreased at a the BT exchange.

Further bit of info... With my sync speed now floating at he 1800kbps mark the reported stable_rate had jumped up to 1000kbps from 500kbps as of this morning. So hopefully I should start seeing increased speeds soon (just did benchmark still getting 0.5Mbit speeds even after re-sync.)

I guess is just a waiting game until the system sorts itself out while making sure the noise margin doesn't jump up to 15db again.. Evil
Community Veteran
Posts: 14,469
Registered: 30-07-2007

SNR Noise margin

What is this hidden page on the Netgear. I have a DG834v2 and as far as I know it had no way to adjust the SNR used?
ScoUK
Grafter
Posts: 61
Registered: 04-08-2007

SNR Noise margin

The V2 is different AFAIK.

On the DG834GT load the page:
http://<ip-address>/setup.cgi?todo=debug

This enables a telnet service to be started on the router so you can telnet onto the router and play with all sorts of settings. Most of which is available via the standard interface.

The key command for the DG834GT is:
adslctl configure --snr XX

Where XX is a percentage of the snr you want. e.g. set it to 60 if you are previously getting a noise margin of 10db then it will try and force it to 6db similarly you can put in 200 (as in 200% to increase it to 20db for stability. (Min value of 1 but that doesn’t mean a noise margin of 0.1 it’s not a guaranteed percentage really)

NOTE: Dont play with this option more than say twice ever few hours or you may trigger interleaving and high noise margin on your line!

I'm fairly sure the same option is available on the V2 but the command is different..
craiglay
Grafter
Posts: 138
Registered: 27-06-2007

SNR Noise margin

I have a DG834v2 router too...

I've seen somewhere that telnetting into the router and changing some files on it can meddle with the SNR margin. That requires a hidden page to enable telnet connections.

I haven't been Maxed yet but it would be nice to know the tricks before it happens. Shouldn't have many problems with an Att of 4db and SNR margin of about 50 i think. Cheesy The exchange is across the street from me.
Rooster
Dabbler
Posts: 18
Registered: 30-07-2007

SNR Noise margin

Quote
Same thinghappened to me - origibally syncing fine @ 2500ish, rebooted my router, it failed to connect, rebooted a few more times and eventually it did connect but at a lower sync rate. This was due to the min SNR now set at 9dB instead of 6. Over the next few weeks the min SNR went up to 15dB with a corresponding reduction of sync rate.


This is exactly the problem I'm suffering.

Has anyone else who is in this boat had the problem resolved by raising a ticketHuh
Soapy
Grafter
Posts: 38
Registered: 01-10-2007

SNR Noise margin

I understand that BT's system will automatically change settings to give a stable line but is does seem strange, if not stupid, that the changes are only ever made in one direction. Surely any properly designed system would every so often try to push the limits in the opposite direction to try an achieve an optimum set of settings.
ScoUK
Grafter
Posts: 61
Registered: 04-08-2007

SNR Noise margin

Hi all,

Still haven't come across any details for the v2 version of the netgear router.. Sorry!


However just thought I'd give an update on what the situation is with my connection so far.

Playing with the SNR target on the router I''ve found that a "percentage" of 35 seems to work best (sounds about right.. 35% of 15db is around 5db) However the reported noise margin only seems to go down to about 8.3db

Have tried lower percentages but that just seems to drop the sync speed so no use what so ever.

Anyhow, keeping the noise margin set I’ve been able to get my speeds increased. Keeping sync rate consistently between 1.5Mbit and 2Mbit. Disconnect a few times each day for 2-3 days (leave a few hours between reconnections) then leave the connection up and running for another 2-3 days without disconnecting. I understand this forces the BRAS profile to re assess itself and update the stable rate. I did this ar first and found my reported stable rate to increase to 1000 (from 500) so next few days I did the same and it increased to 1500 Great! But despite this benchmark speeds and download speeds remained at 500kbit speeds Sad.

About to reply to a plusNET ticket when I decided to try and connect the router using the BT speed test account. Connected ok but couldn’t get the speed test page up. Bit odd. So reconnected with plusNET account and somehow the connection burst into life giving accurate speeds of a 1.5Mbit connection! (Think someone else on these forums mentioned a trying a bt speed test can trigger this sort of thing.. Strange...)



Hopefully this information can help a few folk. It is frustrating that there appears no system in place for BT to reduce the target SNR once it’s increased. However there is one problem I thought of with applying the SNR fix on the router. If BT ever do come up with a system and set the target noise margin back to 5db. If you still have the percentage fix on the router set to say 35% you may find an unstable connection. In which case if BT detect this before you do. You could well see yourself back to square one with BT increasing the target SNR to 15db once again thinking your line is unstable. Oh well.. I’ll cross that hurdle when I get to it Smiley

Final Note: Just got a sync speed of 2368 should 'just' be enough to see 2Mbit.. Woohoo!
N/A

SNR Noise margin

Hmm....

I think some people are getting the concept of signal to noise ratio wrong..
The thread seems to indicate that a lower SNR figure is good.. WRONG... The higher the SNR the better the quality of the line.
example,
a SNR of 3db means that the required signal (data) is 2 times "stronger" than the noise on the line, 6db = 4 times, 10 db = 10 times, 13 db = 12 times, 60db = 1,000,000 times etc etc etc..... So what you are looking for is the HIGHEST signal to noise ratio you can get. That's why it is called the SIGNAL to NOISE ratio, its the difference in the required signal against the amount of noise on the line..

take a mooch here :
http://en.wikipedia.org/wiki/Signal-to-noise_ratio
ScoUK
Grafter
Posts: 61
Registered: 04-08-2007

SNR Noise margin

True.

The larger the SNR the more margin there is for errors which gives you a better line quality.

But that's quality in terms of stability. the SNR does have an effect on sync speed. And the lower the SNR (more unstable your likely to make your line) the more likely you are to get a higher sync rate.


So if you are looking for the highest possible sync speed then a low SNR is good!

If you are looking for a reliable connection low SNR is bad.


The problem some of us are seeing is that our lines were perfectly stable when the BT system allowed the target SNR to be 5-6db.. However for whatever reason the BT system has thought "ooh that lien is unstable I need to adjust the target SNR so it's more reliable".. At which point users are finding their SNR getting thrown up to 15db (approx). Which has a knock on effect to the sync speed. because there is less margin for bandwidth and more margin for noise. If the system worked fine then the target SNR would drop down again for those of us whose lines were perfectly stable at the 5-6db target. But sadly the BT system at the moment only seems to be able to increase the target SNR for reliability and not decrease it for when detection was incorrect.



This is how I see it anyway...