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Can I get my Target SNR margin reduced?

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Can I get my Target SNR margin reduced?

Hi all, I've been on DSL Max now for about a year. Recently, my router decided that it was time to die, so I bought a Netgear DG834GT, I had some problems setting it up at first, which caused it to constantly resync.

As a result, my target SNR margin has now been set to the highest setting (15dB), and as a result of that, my sync rate, and BRAS profile have dropped dramatically. Cry

Now I know my line is perfectly stable on a 6dB target margin, as it was stable for a year, until the new router resynced 10 times in about 15 mins, which made the DSLAM think there was a problem with the line (which there isnt)

What I'd like to know is, if theres any chance PN can reset my SNR margin to the default of 6dB, or failing that, request that BT do so?

Peace
12 REPLIES
Community Veteran
Posts: 14,469
Registered: 30-07-2007

Can I get my Target SNR margin reduced?

You are not alone with this problem and like others you are going to find it very difficult to get BT to reset the limit. There is very little PN can do as it is BT and the exchange that decides what is best for your line, even when they are wrong.

You need to raise a ticket and ask PN to report it to BT but as I said, very few have managed to get it reset.
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Can I get my Target SNR margin reduced?

Some people use the DMT tool. It depends on the router, only some work with this program.
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Can I get my Target SNR margin reduced?

Yea im already using DMT to override the SNR margin, but it will only override it by so much, ive got it from 15dB down to 11, which helps me a bit at least.

I've heard that if your line is stable for 14 days, the automated system will lower your target by 3dB, if thats the case, ill need to wait 6 weeks to get my line back to normal.

I hope thats not the case Cry
Soapy
Grafter
Posts: 38
Registered: 01-10-2007

Can I get my Target SNR margin reduced?

I was in the same position for the last few months with my target SNR margin at 15. Then last week I changed my router, and immediately my sync speed increased from 2400 to 4000.

The old router showed an SNR value of 15, whilst the new router reports a noise margin of 6. I assume they are different names for the same thing.

This made me think the problem was not with BT , but with my old router.

My new router is a XyXEL P-660HW-T1.

Stephen.
Community Veteran
Posts: 4,729
Registered: 04-04-2007

Can I get my Target SNR margin reduced?

Its also worth checking what your SNR is overnight.

I have the same router and daytime my SNR is about 12dB, over night (23:30 to 06:00) it falls to 6dB.

Chilly
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Can I get my Target SNR margin reduced?

I think sometimes people get mixed up between SNR and noise margin.
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Can I get my Target SNR margin reduced?

Yea they do,just to clarify the figure im relating to is the Noise Margin, not SNR (Signal to Noise Ratio).

On fixed speed lines, the higher your Noise margin the better. However on Max DSL lines, the higher your noise margin, the lower you will sync with the exchange, but with more stability. And the lower your noise margin, the higher you will sync, but with more line errors.

On MAX DSL noise margins are generally 6, 9, 12 or 15dB, you start off on 6dB as is the default on new DSL Max lines and if BT's equipment sense a fault on your line, they will increase this setting to add stability, at the cost of sync speed.

This is whats happened to me.
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Can I get my Target SNR margin reduced?

I'd like to try a few different routers on my line which is long to see how it affects all these figures.

Luckily I never seem to have congestion at my little country tel exchange in the middle of nowhere it's always green.
Soapy
Grafter
Posts: 38
Registered: 01-10-2007

Can I get my Target SNR margin reduced?

One question though. If BT had set my target noise margin at 15, is this a contraint on the line that is independent of the router used?

Since my speed doubled when i got a new router, just assumed that the target set by BT was being ignored by the new router. How is the target by BT used by the routers when they are syncing?
Nocturne
Grafter
Posts: 140
Registered: 01-08-2007

Can I get my Target SNR margin reduced?

Following a BT fault and repair my Target SNM increased from 6 dB to 15 db.

PN CS assured me BT’s systems automatically monitor the situation every few days and the Margin would reduce if the line conditions warranted it. I didn’t believe them but the options open to me were limited since as a customer I could not raise this directly with BT.

After five months of very stable connection I raised this again with PN.
Five days later, including a weekend, BT lowered the Target SNM to 6 dB and I continue to have a stable connection.

I don’t know what criteria, if any, BT uses to determine whether to lower your Margin or not.
A period of stability i.e. no or few losses of sync could well be one of them.

It does however depend on PN being prepared to ask, which for whatever reasons they seem reluctant to do.


Quote
One question though. If BT had set my target noise margin at 15, is this a contraint on the line that is independent of the router used


Your choice of Router has no effect on the Target SNM BT impose - 'it is a constraint on the line'.

Edit: When you connect, your Router determines your SNR and BT apply the current Target SNM which leaves the remaining SNR available for speed. The higher your SNR the higher your speed.
Some Routers allow you to tweak the Margin up or down.
If you tweak it down you have more SNR remaining and your sync rate will increase.
If you have problems of stability associated with deteriorating SNR i.e. loss of sync, this will tend to make the stability worse.
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Can I get my Target SNR margin reduced?

So do you want a high SNR and a low noise margin of around 6db ?

Is that the ideal ?
Nocturne
Grafter
Posts: 140
Registered: 01-08-2007

Can I get my Target SNR margin reduced?

The higher your SNR the higher speed you can achieve.
BT imposes a safety margin, Target SNM, to allow for fluctuation in the SNR.
Deduct the Target SNM from your SNR and the speed you can achieve is dependent on the SNR remaining. The higher the Target SNM the less SNR available for speed.
The higher your SNR therefore the better.

The lower your Target SNM the better provided the Margin is sufficient for whatever fluctuation in SNR you typically experience i.e. you do not suffer ‘frequent’ loss of sync. If loss of sync becomes too frequent BT will increase the Margin and this will reduce the SNR remaining for speed. You will typically find your maximum potential speed reduced unless you are close to the exchange and enjoy a very high SNR.