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[chris87]

i dont really understand it

i just signed up to DSLmax 8meg on karoo everytime my internet dissconnects my SNR goes back to 6db but why? lol

so my SNR target is at 6db?

[mr_chris]

As speed increases, it uses more of the available frequencies on the phone line, and so SNR decreases, since higher frequencies are more susceptible to interference, they have a lower tolerance.

On a DSLMax system, your modem and the DSLAM in the exchange negotiate a speed that gives you the maximum speed possible whilst still keeping the SNR margin at least equal to the target. In this case, your target is 6dB.

After that, it doesn't matter what happens to your SNR. It will naturally fluctuate during the day, but won't cause a resync unless it gets too low and the line drops.

You can see what your target SNR is by looking at it straight away after a resync. In your case, it's 6dB because as you've observed, that's what it goes back to after every resync.

So hopefully that explains why!

[chris87]

ahh thanks

but howcome some people have higher SNR targets?

is it because my line must be pretty good at handling low SNR's?

[guest]

Generally because their lines have more electrical noise on them. Some lines will work absolutely fine with a margin of 3dB or less - most won't.

6dB SNR means that the power of the ADSL signal is 4 times the power of the noise on the line. The noise level is an average though so some frequencies may have more noise than others. 6dB Signal to Noise Ratio is generally the accepted minimum for all sorts of data transmission systems, hence it being the default here.

[setecio]

You can see what your target SNR is by looking at it straight away after a resync.

Where do you find your target SNR .... I thought modems reported the SNR margin .... surely looking at this after a resync would only show the increase in SNR margin after a resync.

If my ISP set a target SNR of 6 for me and I have a modem reported SNR margin of 3 ... then my actual SNR is 9. If noise increases on the line and my SNR margin drops to zero so that my actual SNR hits the target set by my ISP (6) and causes a resync at a lower speed , then when I look now at the SNR margin, surely it only indicates the SNR increase due to the resync and cannot tell me what my actual target SNR set by the ISP is ?

[kitz]

>> Where do you find your target SNR

The best indication is by looking at your SNR Margin immediately after your router has sync'd to the exchange.
If your SNRM is 6dB, then your Target SNR is likely 6dB.  Target SNR is set by BT's DLM system and increases in 3dB stages ie 6,9,12,15 dB.

Theres a bit more info on How maxdsl works

[setecio]

mmm, I'm still not getting it, but I think I've got that the title should be Target SNR margin, the lack of margin confused me.

(If I'm wrong and the ISPs set a target SNR   (not margin) then my first post above still stands)

OK so got that, but I would have thought that to see what Target SNR margin you are set at, you would have to look at the SNR margin reading in the modem immediately before a resync and not immediately after ...... surely after a resync then the SNR margin should then have a bit of extra leeway over the target SNR and should be reading higher than your SNR margin target.

eg . you have an actual SNR margin of 10 which slowly decreases to your target SNR margin of 6. Just before a resync, your SNR margin reading in the modem is 6, however just after you have resynced at a lower speed my SNR increases, which also means my SNR margin increases above 6 again.

Do you see what I'm saying ?

[kitz]

Whilst related - SNR Margin and Target SNR arent the same and are two entirely different things. - Both relate to the SNR (Signal to Noise Ratio).

The ISP doesnt set either the SNR Margin nor the Target SNR. 
Target SNR is set by the BTw DLM system depending on what it thinks is best for your line. Once set it will normally* stay at this figure.
SNR Margin fluctuates depending upon the amount of noise on your line at various times of the day.

Target SNR is how much SNR Margin your line should have when the line first sync's up. Hence why its the only time you can see what it is. 
SNR Margin is then free to fluctuate as much as it wants.. but the line will drop once the SNR Margin gets too low for the line to "hear" the adsl signal. 
Then it will resync again at a new speed as appropriate to your Target SNR.

eg

Target SNR is set at 6dB

Line syncs at 6000.. as the night progresses the SNR gets worse and is reflected by the SNR Margin fluctuating...  and then goes down.  When it gets to 1dB it cant "hear" the adsl signal so the line drops out.
So your router has to resync - but because the SNR is worse using a Target SNR of 6dB now only allows the line to sync up at 5000.
The line will come back up and your SNR Margin is now 6dB again.

For more reading see An explanation of SNR and SNR Margin and Eric's excellent explanation of Signal-to-noise ratio in this post.

Hope that helps to clarify

ps - ive coloured the relevant bits in the hope that it makes them easier to identify.

[setecio]

TYVM Kitz, excellent explanation which I now understand. 

[kitz]

yw