The ultimate Signal-to-Noise Ratio (SNR) discussion

**MetaGeek** · 09-01-2010, 12:47 PM

I'm interested in hearing your thoughts about Signal-to Noise Ratio. How many of you use it daily? What do you use to calculate it? Does it fulfill your need for a spectrum analyzer?

For a refresher on signal-to-noise (snr), I'm providing some links:

"Although not technically correct, many Wi-Fi vendors define signal quality as Signal-to-Noise Ratio (SNR). The signal-to-noise ratio is simply the difference in deccibles between the received signal and the background (noise floor, or most importantly the receiver's threshold)."
Signal-to-Noise Ratio defined by CWNP

When performing a RF site survey, it’s important to define the range boundary of an access point based on signal-to-noise (SNR) ratio, which is the signal level (in dBm) minus the noise level (in dBm).
Wi-Fi: Define Minimum SNR Values for Signal Coverage - Enterprise Networking

To visually explain the Signal-to-Noise ratio, I will include this modified image of a Wi-Spy and Chanalyzer. It is a very simple calculation, subtract the noise level from the signal. As you can see in the image (and contrary to popular belief), the SNR does little to indicate interference from non-wi-fi devices co-existing in the same spectrum.

There have been numerous Signal-to-Noise feature requests for both inSSIDer and Chanalyzer. I believe there are a lot of professionals who actively use it daily. It can be a very useful calculation for environments with varying noise floors. Because many Wi-Fi vendors don't report SNR and the ones that do calculate it do so inaccurately I'm curious to know more about what our customers are doing with SNR.

If you use a Wi-Fi card to detect SNR, does it have a fixed noise floor at -100dBm or does it actually detect the noise floor? If so, which card do you use?

How has calculating SNR improved your WLAN installations?

Do you use SNR as a quick and easy replacement for spectrum analysis and site survey?

**Novarum** · 11-18-2010, 10:30 PM

SNR is mission critical, as is a good definition of noise floor. Sadly, the algorithm most WiFi chipsets use to pick modulation rate is proprietary based on some combination of RSSI, SNR, retry rate and black magic and vary widely.

In large venues, interference from too many transmitters (particularly too many APs) can raise the noise floor to begin deliver false positives on CCA.

How does Chanalyzer measure noise floor and how does that relate to the noise floor reported by VisiWave and CACE Pilot?

**MetaGeek** · 11-22-2010, 03:04 PM

Originally Posted by Novarum

SNR is mission critical, as is a good definition of noise floor. Sadly, the algorithm most WiFi chipsets use to pick modulation rate is proprietary based on some combination of RSSI, SNR, retry rate and black magic and vary widely.

In large venues, interference from too many transmitters (particularly too many APs) can raise the noise floor to begin deliver false positives on CCA.

How does Chanalyzer measure noise floor and how does that relate to the noise floor reported by VisiWave and CACE Pilot?

I will agree that the algorithm most WiFi chipsets vendors use vary too much. Which makes me wonder why it is mission critical if no one can agree on how to calculate it?

Because we can't rely on the User's wireless NIC to calculate the noise floor, or even RSSI accurately, can chanalyzer give an honest SNR calculation?

I see the noise floor vary a lot, especially when there is an analog transmitter. Chanalyzer calculates the noise floor by looking at the average activity in the Wi-Fi channel and time range. It is not calculated as average power. I don't konw how VisiWave and CACE Pilot calculate it so I can't speak for them, I'd be happy to inquire how they do it though

**mwalker** · 11-22-2010, 06:22 PM

The way i see it, what is important is that the signal is above the noise floor. This should mean the signal will be strong enough to be heard. The actual strength of the signal is not important as long as it can be heard be the receiver without errors.

Im sure there will be some guidelines on how big a gap (SNR) there should be, but based on what I have seen, base it on the docs from the vendor of the product you use.