RSSI=Received Signal Strength Indication which generally appears as "bars" in our software. It indicates how much power is in the radio signal your router/AP is sending you. The RSSI number also determines whether CTS (clear to send) is set to on or off. An RSSI of 0 means no signal and CTS if set to off. RTS of 1 or above generally set CTS to on but that is very poor signal strength. The RSSI number can go as high as 255(?) but different companies use different numbers. Cisco goes from 0-100, Netgear, I think, only goes up to 60 for a top signal strength. It varies.

RSSI is a sampled number so I‘d say it‘s not a 100% accurate figure, just my opinion, more of a close approximation.

Netstumbler is simply one (or the) most popular wireless network monitoring tools. There are other software packages but RSSI is pretty much measured the same way. Netstumbler is probably the most widely used freeware product. I‘ve never bothered with any other program to compare it to.

RSSI is being replaced by RCPI (received channel power indicator). You might want to consider dropping your research on RSSI and instead switch to RCPI or combine both in your research.

Addition: the RSSI is part of the preamble in the signal from a router/AP. That‘s how a client gets and measures the RSSI or RCPI. This makes sense since if the RSSI is zero, you aren‘t going to get a preamble anyway. If the RSSI is 1 or above, the client needs to know some data about how much to amplify the signal. RSSI is read before signal amplification, another reason why I think it‘s more of an approximation rather than an exact number.

RSSI is estimated, in consumer WiFi as either a marketing-speak percentage or as a received power level in dBm. On the latter (dBm), the numbers are very approximate and are not actually calibrated to the standard where 0dBm = 1mWatt.

Most WiFi products kind of "cheat" by estimating power based on detected energy after demodulation. The (baseband) signal level, in the absence of a received signal, is approximately the noise floor of the receiver. For a certain receiver bandwidth (design parameter) the noise power can be a constant, to which one adds the noise figure of the receiver. The sum is roughly the noise floor - if there is no signal present, to include non-WiFi and adjacent channels.

So the reported RSSI is derived from the noise floor in ratio to a different time when there is a WiFi signal present. But there may be two or more signals present, and adjacent channel power present.

So this scheme for reporting dBm RSSI is just an approximation- good enough for its purpose, if there is no strong adjacent or same channel interference. WiFi radios may make this measurement during one of the low speed bits in the frame preamble. This is also when the radio may decide to try the other (switched diversity) antenna.