rssi and snr

In telecommunications, received signal strength indicator (RSSI) is a measurement of the power present in a received radio signal. RSSI is generic radio receiver technology metric, which is usually invisible to the user of device containing the receiver, but is directly known to users of wireless networking of IEEE 802.11 protocol family. RSSI is often done in the intermediate frequency (IF) stage before the IF amplifier. In zero-IF systems, it is done in the baseband signal chain, before the baseband amplifier. RSSI output is often a DC analog level. It can also be sampled by an internal ADC and the resulting codes available directly or via peripheral or internal processor bus.

"RSSI" values have unspecified unit, ie. their meaning depends on manufacturer. Some report as dBm, some as a percent, some even in milliwatts. There is no standard. Also, the proper way to find SNR *is* to subtract dB values, as dB are logarithmic and hence the difference of logs is the same as a ratio without log. In other words, if a signal has amplitude 1 and noise is 0.5, the SNR is (1/0.5) = 2, but if the signal is measured as being at (say) -12 dB and noise floor is -16 dB, the SNR is (-12-(-16)) = 4 dB.

Moreover, link quality depends on many other factors and the signal-to-noise ratio (computed from rssi and noise floor) is only one such factor.

And finally, finding SNR of a wifi signal by subtracting noise floor from the rssi is of course only applicable if both values are reported in a log/dB scale. Otherwise if they are linear you'd need to use ratio (as you said) and if they are some other sort of scale then who knows what needs to be used!