SNR, I'm confused now

Can someone please clarify the snr value for me. I was under the impression from what I've read, that the higher this value the better in that it meant that more signal was getting through than noise. However, I have now been told twice that this is wrong and the lower the snr the better.

Thanks

Yeah you're right, it just represents the difference between signal and noise level, so the higher the value the greater the difference.

Confusion occurs because there are two different measurements made by modems and routers.

There's SNR, which is the ratio between signal and noise, usually expressed in non-linear measurement units (dB's).

There's also SNR margin which is usually the number of dB's above a minimum working level that a device needs to work (0dB).

So, your phone line can have a SNR of 42dB, and your router can have a SNR margin of 7dB at the same time (very confusing !).

I think your friends are getting confused with line attenuation, which is simply a case of "the lower the better".

Edited by deleted (Sat 29-Mar-08 13:11:13)

Strange, the 'twice' that I was told this came from Zen CS when I was querying my download speed with what my snr is.

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I think your friends are getting confused with line attenuation, which is simply a case of "the lower the better".

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It may well be that I didn't explain myself clearly enough to CS when I called. I was pleased to find that my attenuation, when I moved, had dropped from 54 to 21 and the SNR had gone up, perhaps I was expecting too much.

This SNR certainly can be confusing as you say.

Well, I'm only guessing, but this is what I suspect......

Your line has a lower attenuation, but for some reason it is picking up more noise at certain times of the day, so you have a higher noise margin, in order to keep the connection stable. You should, however, be connected at a much higher speed, if your line attenuation is now 21dB, when it was 54dB at your previous house.

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So, your phone line can have a SNR of 42dB, and your router can have a SNR margin of 7dB at the same time (very confusing !)

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As I understand it, in your above example 42 dB is the minimum SNR that you need to maintain a connection, but in reality it is 49 dB but you don't see this SNR figure but see the 7 dB SNR margin figure instead.

I also believe that if the SNR margin dropped to 0 db (which is bad) then it would in reality be 42 dB and if the SNR margin went into negative figures like -1 dB then this would be 41 dB and the connection would probably drop because the line noise is 'louder' than the signal ?

Edited by deleted (Sat 29-Mar-08 15:53:27)

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I was under the impression from what I've read, that the higher this value the better in that it meant that more signal was getting through than noise. However, I have now been told twice that this is wrong and the lower the snr the better

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If (on Max), you can sync at the maximum 8128 then having more SNR margin is better because it insulates your line from excess noise making it more stable in adverse conditions.

If you can't sync at the full 8128 then you sacrifice as much SNR margin as possible to get faster speeds but not so much that your line becomes unstable.

BT's default value is 6 dB

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Your line has a lower attenuation, but for some reason it is picking up more noise at certain times of the day, so you have a higher noise margin, in order to keep the connection stable.

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This may well be the reason. The property is in the middle of an industrial estate with (if it matters) a radio station just over the road.

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You should, however, be connected at a much higher speed, if your line attenuation is now 21dB, when it was 54dB at your previous house.

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I certainly have a higher speed than at the old place. If I reboot the router it will sync at 8124 but will drop fairly quickly to 6xxx which gives me 5 - 5.5 download speed as apposed to the 3.5 I was getting. As I said, perhaps I was expecting too much in that I was hoping to get 6+ out of this line. Oh well..

You might be able to improve it if you have extension wiring which can be tidied up. Take a look in the technical forums for advice on how to reduce noise pickup by extension wiring.