Imagine being at a large, crowded concert. Everyone is screaming and singing along to the music. You turn to your significant other and say how awesome the concert is. Even though you’re right next to each other, you can barely hear one another.
The people around you and the band playing is called the noise floor. That is what would be the ambient noise for the duration of the concert.
You would have to speak above the noise floor for your partner to hear and understand what you’re saying. The difference between your voice and the noise floor is what SNR is.
It’s important to know that the receiving person not only HEAR you but also UNDERSTAND. This is comparable to saying a wireless radio has signal, but can’t demodulate a signal from an access point. The device radio must be able to demodulate in order to understand the bits.
SNR stands for Signal-to-Noise Ratio. SNR is a comparison of the signal to the background noise or otherwise known as noise floor. In fact, it’s not actually a ratio at all.
If the signal level is near the noise floor level then we witness data corruption. Not only is this bad for the sending and receiving radio, but also for surrounding radios (clients). The sender, such as an access point, will need to retransmit the frames back to the receiver because of data corruption. This is more known as retry rates.
A large retry rate decreases throughput. Everyone gets less airtime because of those retransmissions. Thus the wireless network appears slow, if not unusable. Frame retry rates as high as or above 15% should be looked into. And this could be a result of interference from neighboring access points, large number of clients, microwave ovens, bluetooth headsets, and other interferers.
Ideally, you want to aim for a higher SNR. I’d say 20 dB or greater is good SNR. Greater than 40 dB is even better!
Recommended minimum SNR for data is 18 dB and for voice over wifi it is 25 dB. As more interference is introduced, the SNR decreases because it raises the noise floor.
Let’s take an example of a laptop and an access point. If my RSSI is -52 dBm and the noise floor (could be called background noise or just noise) is -95 dBm, then the SNR is 43 dB.
The closer RSSI is to zero the better. If my RSSI was -45 dBm and the noise remained the same, then my SNR would be 51 dB. You subtract the dBm value of noise from the RSSI value to get your SNR value.
Why is SNR in dB while your RSSI and noise is in dBm?
That’s because SNR is a comparison between two absolute values. dBm is an absolute value, whereas, dB is a comparison value. More accurately described, SNR is a unit of comparison.
How is the noise floor determined?
Noise is all the RF around the radio in the spectrum from all RF sources. It can come from a microwave, bluetooth headsets, wireless video cameras, etc.
Our wireless adapters can generally determine what the noise floor is but you really should use a spectrum analyzer such as Chanalyzer or a network scanner such as Wifi Explorer.
On-board wireless NICs for laptops, tablets, etc aren’t capable of determining accurate SNR. These wifi NICs are unable to understand unmodulated signals coming from non-802.11 sources. There are other ambient RF sources which an 802.11 NIC cannot see.
An example is when a microwave is turned on. The built-in NIC will not be able to see the RF signal because the microwave is sending unmodulated bits. Thus the built-in NIC believes there is no noise.
So in short, the noise floor determined by the built-in wifi NIC is of noise it can see, leaving out the unmodulated signals coming from non-802.11 sources. Additionally, many vendors implement their own algorithms to determine SNR.
The only way to get SNR values is to use a spectrum analyzer such as Wi-Spy DBx which is used with Chanalyzer or TamoSoft Spectrum Analyzer.
Thanks to Michael Berg @tamosoft, Henry Stukenborg @henrystukenborg, Omar Vazquez @omrvazriv, Keith Parsons @keithrparsons, for providing feedback on this post and correcting me on the errors as well.
How does SNR impact wifi?
A low SNR value (under 20 dB) will degrade a wireless network in a number of ways:
- Decreased throughput
- Decreased data rate
- Increased retransmissions
- Data corruption
Throughput is decreased because of the amount of retransmissions. For every frame unacknowledged, the sender has to resend the original frame. This takes up precious airtime and prolongs other wifi transmissions.
The reason for retransmissions usually can be attributed to corruption of a wireless frame being received by a client radio. SNR may be too low for a client radio to demodulate the signal.
Data rates are decreased because MCS is susceptible to noise. The modulation methods used are dropped to less complex modulation methods when SNR is lower.
How to determine SNR
There are a number of ways to get SNR values. I will outline some methods below. The methods using the built-in wireless NIC will provide inaccurate SNR calculations due to the lack of capabilities in hearing unmodulated signals.
Mac OS X
Option-Click the wifi indicator on the top menu bar.
Take the “noise” value and subtract it from “RSSI” to get your SNR value.
Enable the SNR column:
The value taken from Chanalyzer will show you the SNR between an SSID and the noise floor.
Get the signal level for your SSID from the networks tab. Then get the noise floor value from the Channels tab. Then subtract the signal level from the noise floor.
View Metageek’s post about SNR.
Tamosoft Site Survey
Tamosoft has a spectrum analysis feature. To determine SNR, it is a similar process to finding it in Chanalyzer. You take the signal strength and subject from the noise floor.