Understanding Signal Strength
The most accurate way to express it is with milliwatts (mW), but you end up with tons of decimal places due to WiFi's super-low transmit power, making it difficult to read. For example, -40 dBm is 0.0001 mW, and the zeros just get more intense the more the signal strength drops.
Ultimately, the easiest and most consistent way to express signal strength is with dBm, which stands for decibels relative to a milliwatt.
You can convert between mW and dBm using the following formulas:
P(dBm) = 10 * log10(P(mW))
For example, a power of 2.5 mW in dBm is:
dBm = 10log2.5 = 3.979
dBm is that we're working in negatives. -30 is a higher (stronger) signal than -80.
Signal Strength
Rating
Required for
-30 dBm
Amazing
Max achievable signal strength. The client can only be a few feet from the AP to achieve this. Not typical or desirable in the real world.
N/A
-67 dBm
Very Good
Minimum signal strength for applications that require very reliable, timely delivery of data packets.
VoIP/VoWiFi, streaming video
-70 dBm
Okay
Minimum signal strength for reliable packet delivery.
Email, web
-80 dBm
Not Good
Minimum signal strength for basic connectivity. Packet delivery may be unreliable.
N/A
-90 dBm
Unusable
Approaching or drowning in the noise floor. Any functionality is highly unlikely.
N/A
3 dB of gain = +3 dB = doubles signal strength (Let's say, the base is P.
So 10*log10(P/P)= 0 dB and 10*log10(2P/P) = 10*log10(2) = 3dB -> double signal)
3 dB of loss = -3 dB = halves signal strength strength (10*log(1/2) = -3.0103)
10 dB of loss = -10 dB = 10 times less signal strength (0.1 mW = -10 dBm, 0.01 mW = -20 dBm, etc.)
10 dB of gain = +10 dB = 10 times more signal strength (0.00001 mW = -50 dBm, 0.0001 mW = -40 dBm, etc.) Reference:
Simple rule of thumb:
When working with power, 3 dB means double (twice) the factor and 10 dB means 10-fold.