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.