Cisco Support Community
Showing results for 
Search instead for 
Did you mean: 

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. If you'd prefer to explore, try our test area to get started. And see here for current known issues.

New Member

How is noise floor determined for SNR calculations?

Hi all, long time lurker, first time poster here.

Quick background: I am a network administrator at a small college where I maintain our wireless network (Cisco 1142 LAPs with 4404 WLCs). I am currently studying for the CWNA certification and am reading about SNR and have come across the following statement in my study guide, "... an 802.11 NIC is not a spectrum analyzer... it cannot see raw ambient RF energy... [so] it cannot truthfully measure the noise floor". It goes on to say that the only way the noise floor can properly be determined is with a spectrum analyzer.

So my question is: how can a non-CleanAir AP (such as the Cisco LAP1142) determine the noise floor in order to calculate the SNR value that I see in the 4404 WLC under the client statistics page?

Is it only measuring the 802.11 signals that it can see? Such as signals from neighboring or rogue APs? And thus ignoring all non-802.11 energy that it cannot decode? Does this mean that the SNR value reported in the WLC is not completely accurate since it doesn't take non-802.11 RF energy into account?

I've searched these forums and done a bit of Googling but haven't found anything explaining this.



  • Security and Network Management
New Member

How is noise floor determined for SNR calculations?

Hi Erin, the APs Input Filter qualifies 802.11 Frames on the Basis of "has the incoming Signal an 802.11 Preamble, then its WLAN, everything that has not, is Noise". Noise qualifying is the Business of Spectrum Analyzers, like SAGE, the chip behind CleanAir.

Regards, Michael

This widget could not be displayed.