SNR (Signal-to-Noise Ratio)
For the client, the most important indicator to evaluate its performance is the signal-to-noise ratio (SNR).
Following SNR, we can focus on the received signal strength and retransmission rate.
The signal-to-noise ratio is the difference between the signal and the noise floor. The larger the difference
is, the better the signal quality will be. For example, if a client receives a signal strength of -65dBm, while
the noise floor is -92dBm, the signal-to-noise ratio is -65dBm-(-92dBm)=27dB.
If the received signal strength of the client is -60dBm, the noise floor is -80dBm, and the signal-to-noise
ratio is -60dBm-(-80dBm)=20dB. Although the received signal strength of the second example is stronger,
its signal quality is poor.
We often encounter a misunderstanding that the stronger the signal, the better the signal quality, which is
a wrong impression. The stronger signal does not necessarily mean a better performance, and at the same
the noise is often stronger. The result is poor signal-to-noise ratio and poor signal quality.
It’s SNR that determines what channel bandwidth can be used, how many spatial streams, and what
modulation method is going to be used. All these will determine the data rate of your connection. That is,
the use of 80MHz channel width requires a much higher signal-to-noise ratio than the 20MHz channel width.
Establishing three-space stream communication requires a higher signal-to-noise ratio than establishing a
single spatial stream. And the same applies to the modulation method upon SNR requirement.
Furthermore, to establish a high data connection rate for traditional 802.11a/g clients and APs, 20 dB of
SNR is generally required; 25 dB is required for 802.11n; and 30 dB or more is required for 802.11ac.
In general, you can follow the following guidelines to determine your SNR number readings:
1. SNR > 40 dB, signal quality is excellent, client can always associate with AP, and the highest data
connection rate can be established;
2. SNR between 25-40 dB, signal quality is good, client can always associate with AP to establish a
high data connection rate.
3. SNR between 15-25 dB, signal quality is OK, client can always be associated with AP, and a good
data connection rate can be established;
4. SNR between 10-15 dB, signal quality is fair, client can be associated with AP but with a low data
connection rate;
5. SNR between 0-10 dB, signal quality is poor, client and AP can hardly associate each other.
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