# Forum

## Need Some Explaining

7 posts by 4 authors in: Forums > CWTS - Enterprise Wi-Fi Fundamentals
Last Post: May 14, 2014:
• So with my newly acquired wireless job I have seen a lot in the field so far and have questions regarding the things I am witnessing. One in particular thing would be noise floor & SNR. We deploy a lot of Ubiquiti products and I am constantly viewing signal strengths, link quality ratings, & noise floor ratings. So, my questions are as followed.

1.) Noise floor. Is it better to have a lower neg number? ie. -93 compared to a -71? I've been told by a co-worker that an ideal noise floor is -100 or so.

2.) SNR. Can the SNR be calculated by taking the signal strength and subtracting that number from the noise floor? Example, noise floor of -91 and a signal strength of -37 = SNR 54? I've read where an ideal SNR is 15-20 dBm. Correct me if I'm wrong.

Thanks in advance guys & gals.

• Hey Charlie,

I normally see a noise floor of around -90 or so. The lower the better, as in -100 is better than -90.

You won't much see a -37  signal unless you are right by the AP but the calculations are correct. A fair SNR is 10 for data. Voice I think should be around 25 or so.

Did you get a new job or is this the same one you were at a week or so ago?

• Same job. I did an install at the Hampton Inn downtown this AM with 2 Ubiquiti radios that were roughly 400ft apart so the signal was -37 to -39. I figured the lower would be better, -100 compared to -89 etc.

So, on this particular install taking the signal strength of -37 and subtracting -89 for the noise floor, the SNR would be 52?

• -37 at 400ft? That sounds unusual to me. If you have some time around 8pm tonight I will give you a call. Going to shoot you an email as well to get the numbers exchanged.

Cheers

• By Howard - edited: January 22, 2014

As I recall, Ubiquiti does make some higher powered routers.   That does sound very high though, even for Ubiquiti.

As far as required SNR levels go, the farther you go along the b/g/a/n/ac path  the higher the SNR needs to be.

Also remember that Noise Floor is usually reported by the chipset manufacturer, not by the radio mfg.   Hopefully they've been told the whole scoop.   Remember noise floor is a "calcualted number", and not a directly measured value.    It will not show microwave oven interference for example.

Regardless of the outcome, please let us all know what you find out.

• By Charlie - edited: January 22, 2014

Yeah, this particular install was 2 Nanobridge M5s with a link of <400 ft and output power set to 15dB on each side. I was just curious of the noise floor rating and the SNR etc.

• Bear in mind that most 802.11 chip sets do not have the capability to measure spectral noise, it's why vendors like AirMagnet (Fluke) and Ekahau sell dedicated hardware with their spectrum analyzer programs .  Regardless, plenty of vendors have solutions that guess at the actual noise floor based on measurements from the 802.11 frames, and most of the time that will be close enough.

Also, for a link with parabolic dishes, an SNR of 54 at 400' seems fine to me.

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