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A couple weeks ago, I ran user-oriented tests to determine the impacts of SNR (signal-to-noise ratio) values on the ability to associate with an 802.11b/g access point and what level of performance that you experience. SNR is what I recommend using to define range boundaries when performing an RF site survey. For various SNRs, here's what I found for the signal strength on a wireless laptop (found in the Windows radio status), association status, and performance when loading the http://wireless-nets.com/staff.htm webpage...
> 40dB SNR = Excellent signal (5 bars), always associated, lightening fast.
25dB to 40dB SNR = Very good signal (3 - 4 bars), always associated, very fast.
15dB to 25dB SNR = Low signal (2 bars), always associated, usually fast.
10dB - 15dB SNR = very low signal (1 bar), mostly associated, mostly slow.
5dB to 10dB SNR = no signal, not associated, no go.
As a result, I recommend using around 20dB as the minimum SNR for defining the range boundary.
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