Wlan Survey Signal Strength, SNR and DBM explained

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Feb 26th, 2009
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Hi,


Does anyone know of a simple explanation of what signal strengths are required for wlan, SNR values and Dbm etc etc that doesnt look like a Harvard professor wrote it.


Thanks

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George Stefanick Thu, 02/26/2009 - 10:16
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I might also suggest www.cwnp.com as a source of information.

jeff.kish Thu, 02/26/2009 - 11:17
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You're looking for specific values? Or are you looking for explanations for terminology?


Values:

-67dBm (SNR 30) for voice at 54Mbps

-72dBm (SNR 25) for data at 54Mbps


These are 2.4GHz values. I believe the SNR needs to be a bit better for 5GHz implementations.

terminalvelocity777 Thu, 02/26/2009 - 11:53
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Jeff,


I must be incredibly stupid I constantly get the values mixed uop and cant remember if its a high SNR or Signal strength thats better.


I was looking at my Ekahau site survey real time network results and thought the values looked odd, then I realised i couldnt remember which one is meant to be high for good and low for bad.


Basic stuff I know


Mick

George Stefanick Thu, 02/26/2009 - 12:09
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An easy way to remember:


Signal - The lower the dB value the louder the signal

-30 Is right on top of the AP

-50 Is 20 feet or so

-70 Is 50 feet or so


Noise - You want on the other end

-100 Is no noise

-95 means there is -5 dB of noise

-90 means there is -10 dB of noise


SNR

Is signal minus noise. The HIGHER the value the better. Most voip is designed around -25.


Hope this helps...

Johannes Luther Thu, 02/26/2009 - 12:41
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The higher the SNR the better.

You can derive the data-rate from the SNR.

On an IOS AP you can issue the

"show controller dot11radio<0|1>" command.

There you will see, what data-rate the AP will use for what SNR. With this information you can conduct a good site-survey.


For example:

A Cisco AP will use 54 MBit/s if the client is seen better than 31dBm SNR.


With an SNR of 19dBM - 18MBit/s are possible.


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