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MaxDSL SNR tuning - PN waffle or fact

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MaxDSL SNR tuning - PN waffle or fact

Hi,
On 1 January I did some internal rewiring. This increased my sync rates by around 700kbps and my Brass profile has been upped to 3000kbps (from 2500). However my SNR ratio has not improved. It is syncing at 15db SNR - (the lowest I have seen this fallen at night is to 12.5db - before it used to be down to around 9db).
PN have said that MaxDSL will tune the SNR level down if the line is quieter. This seems to be opposed to what other websites say - indicating that BTw have to restart the MSR profiling (the initial 10 days used to set SNR and MaxSyncRates) to pick up improved line conditions.
As MSR starts with a 6db SNR - I would anticipate the SNR could reduce from 15db to 9db giving me faster sync rates.

Does anybody know if what PN say is true or just plain waffle ? I have asked to switch to non-interleaved hoping that MSR will be restarted by BT. Anybody know if this will work ?

Thanks
Richard
14 REPLIES
Community Veteran
Posts: 14,469
Registered: 30-07-2007

MaxDSL SNR tuning - PN waffle or fact

The PN comment is true as that is what BT state should happen, however PN have no control over it, BT do, and from experience it does not happen. PN cannot force BT to lower the limit and BT appear to not care when questioned, and only y state the automated systems will adjust to the better line, which they don't.

This is one of the fundamental flaws in BTs MAX implementation that many people get frustrated with. The automated systems don't appear to work very well, once it has detected a worse line condition it never goes back.

I had a similar issue months ago where my SNR increased from 6db to 15dB and no amount of tickets could get it to reduce. I could not get to the bottom of why it ever increased in the first place or why after many weeks, it never went any lower, because I was moved to LLU.
N/A

MaxDSL SNR tuning - PN waffle or fact

Mine at home is also around the 15Db mark with a sync rate of 3776 IIRC

I've powered down my router for a few minutes every few days to see if my sync rate increases, but no - it remains more or less where it is now.

I'd like to see it higher, although TBH I'm not gonna lose sleep or get upset about it.
Community Veteran
Posts: 38,208
Thanks: 898
Fixes: 54
Registered: 15-06-2007

MaxDSL SNR tuning - PN waffle or fact

From other posts it would appear that there are two possibilities. If the default snr has changed automatically then the system will (should) reduce it after about 14 days of no problems but apparently there is an alternative scenario where the default snr is hard coded and then it never alters. This is the case with my line when PlusNet requested that interleaving be applied (as part of the investigation as to why the reports were saying it wasn't when it actually was). The effect of this request was to push up my default snr to 15db where it has remained ever since. Fortunately I have a Speedtouch router and I can force the default snr down by 5dB which got me out of the problem.
N/A

MaxDSL SNR tuning - PN waffle or fact

people get confused over the snr as some routers report it in a different way normally the higher the number the better things are so 15 db is better than 6db.It depends if the router is reporting snr or snr margin.

The snr is effected by the speed so if you connect at 2 meg you may get little noise and have a snr of 25 connect at 4 meg and it may drop to 9.

It can also be effected by weather as this may cause more pops and crackles on the line. Some find speed goes up on wet days and some find dry days best
pacem
Grafter
Posts: 175
Registered: 07-09-2007

MaxDSL SNR tuning - PN waffle or fact

All of a sudden a whole host of users come out of the woodwork that have had or still have stuck SNR targets.

Well I'll join the club, but mine means my stable rate is only 1000kbs when it should 2000kbs.
See http://ccgi.pacem.plus.com/test.php for real time start on my router.

(You'll need svg for it to work. FF2.0 is great or use this http://www.adobe.com/svg/viewer/install/main.html for IE)

So we really need to get PN to get BT to set our lines to re-train rather than try to get the SNR target dropped by a few dB?

Paul.<><
Soapy
Grafter
Posts: 38
Registered: 01-10-2007

MaxDSL SNR tuning - PN waffle or fact

I was having the same issue with my SNR aiming for 15 db when the line sync'd. That was causing me to have a normal stable rate of 1500 Kbs, and occasionally 2000 Kbs. When I was first max'd it was 2500 or 3000 Kbs.

However, on Friday I got a new router, a ZyXEL Prestige 660HW-T1, and straight away my sync rate increased to 4000 (had been 2400 with old router). Also on the new router the Noise Margin (SNR?) is now showing 6 db.

I know some routers are better that others, but that seems a big jump in sync speed.

Also, if BT does have a 15db target on the line, how does that work with the new router that seems to be sync'ing at a value of 6 db

Since it looks like my new router is better ( and possibly more stable ), would I be able to get the line MSR profiling process re-initiate for the line?

Stephen
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MaxDSL SNR tuning - PN waffle or fact

If your snr is down to 6 that is bad and you are lucky to hold the line but maxdls its a bit more tolerant than normal for those who are still confused about snr or snr margin take a look at this

An explanation of SNR and SNR Margin * from

http://www.kitz.co.uk/adsl/linestats.htm


The SNR Margin is not the same as the Signal to Noise Ratio (SNR), although it is related.

The SNR is the signal-to-noise ratio. It is the ratio between the strength of the signal and the background noise on the line and is universally expressed as a logarithmic ratio (decibels or dB).
For a given line the SNR remains essentially fixed whatever the speed you are running.

ADSL routers and modems actually report the SNR Margin, and the term used to describe this varies but is generally “SNR Margin”, “Noise Margin”, “Margin”, “Receive Margin”.

The SNR Margin is the difference between the actual SNR and the SNR required to run at a given speed.
For example, if you need 20dB of SNR to run at 512Kbps, and the actual line SNR is 45dB, then the SNR Margin is 25dB.

The main reason why SNR margins fall as the line speed increases is that the required SNR increases.
For example, if increasing the speed from 512Kbps to 2MBps raises the required SNR from 20db to 30dB then the margin left will fall from 25dB to 15dB (as 45dB-30dB = 15dB.)
The one refinement is that if it is necessary for a faster speed to use some higher frequencies then these will have inherently lower SNRs and that will also contribute to the reduced SNR margin somewhat.
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MaxDSL SNR tuning - PN waffle or fact

Does Plusnet recommend one particular type of router or are the main popular brands usually OK ?
Soapy
Grafter
Posts: 38
Registered: 01-10-2007

MaxDSL SNR tuning - PN waffle or fact

Quote
If your snr is down to 6 that is bad and you are lucky to hold the line but maxdls its a bit more tolerant than normal


I assume that the old router was reporting a SNR margin (even although it only said SNR), and certainly the new router reports a noise margin.

But, why does the new router look likes it targets a noise margin of 6, whilst the old one targeted a margin of 15. Can BT influence the target margin on the router by setting a target on the line?
pepsi_max2k
Grafter
Posts: 108
Registered: 01-08-2007

MaxDSL SNR tuning - PN waffle or fact

On the subject of SNR, how does changing between interleaved and fast mode change it? I just got my connection moved from interleaved to fast and, after hovering between 2 and 6db for the first day, the SNR went up to 15db (almost halfing my speed profile). On interleaved it always stayed around 9db. I guess this is to be expected with the higher error rates on fast mode, but if I move back to interleaved mode should the snr drop back to 9db, or is that gonna be down to BT's not-so-automated systems seeing that my line is more stable and slowly taking the snr margin down?
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

MaxDSL SNR tuning - PN waffle or fact

Quote
Does Plusnet recommend one particular type of router or are the main popular brands usually OK ?


We don't recommend any particular brand of router, we supply a range of different routers (BT Voyager via signup and Netgear, Belkin, Zyxel in the shop). Most should work fine although some routers work better on longer lines that others.
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MaxDSL SNR tuning - PN waffle or fact

OK thanks for the reply.
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

MaxDSL SNR tuning - PN waffle or fact

Quote
But, why does the new router look likes it targets a noise margin of 6, whilst the old one targeted a margin of 15. Can BT influence the target margin on the router by setting a target on the line?


The target SNR from BT is by default 6dB on Max, if the line becomes unstable then they can increase it to 9, 12 or 15dB.

Similarly if it's increased too high and the line remains stable for a certain amount of time then BT can decrease it back down again.

On the other hand some routers allow you to set the target SNR to be higher than the BT SNR. So it could be that the line's been stable and the target SNR was decreased by BT or that the old router was set to sync at a higher SNR than the new.
Plusnet Staff
Plusnet Staff
Posts: 12,169
Thanks: 18
Fixes: 1
Registered: 04-04-2007

MaxDSL SNR tuning - PN waffle or fact

Quote
On the subject of SNR, how does changing between interleaved and fast mode change it? I just got my connection moved from interleaved to fast and, after hovering between 2 and 6db for the first day, the SNR went up to 15db (almost halfing my speed profile). On interleaved it always stayed around 9db. I guess this is to be expected with the higher error rates on fast mode, but if I move back to interleaved mode should the snr drop back to 9db, or is that gonna be down to BT's not-so-automated systems seeing that my line is more stable and slowly taking the snr margin down?


It depends on the stability of the line. On an interleaved line small amounts of noise and small noise spikes won't have any effect on the line because the error correction from the interleaving will take care of it.

However without interleaving on the line any random noise spikes will cause packet loss and more errors and can easily cause more disconnections. The automated systems in order to prevent this if it can't use interleaving will be to increase the target SNR and thus decrease the speed to increase the stability.

Switching interleaving back on probably won't cause the SNR to drop automatically, the line will need to remain stable long enough for the systems to automatically decrease it.