Wlan Survey Signal Strength, SNR and DBM explained

[terminalvelocity777]

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

[mike-greene]

Hi,

This is a good post I was reading this morning.

http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&topicID=.ee6e8c2&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.2cd23e06

Hope that helps.

[George Stefanick]

I might also suggest www.cwnp.com as a source of information.

[terminalvelocity777]

Thanks

I will check them both out

[jeff.kish]

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]

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]

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...

[terminalvelocity777]

It does thats exactly what i wanted clear and precise

many thanks

mick

[Johannes Luther]

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.