Why is the RSSI negative?

See: http://en.wikipedia.org/wiki/RSSI

I would have thought it would be between 0 and 255 - an unsigned short. Could it be that the AirPort menu is displaying it as a signed short, rather than an unsigned short?

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I'm not an expert but my theory is Apple is using dBm for measurement. The greater the negative number, the worse off your signal is. The equation is something like db=10log(p2/p1) where P1 is your original signal in decibels and P2 is the received signal level in decibels.

So, if you have P1=P2 (no signal loss) the signal loss is 10log1 which is 0. If P1=100 and P2=10, the signal loss is 10log.1 which is -10. For P1=1000 and P2=10, the loss is 10log.01 which is -20.

Someone correct me if I'm wrong, but that's the gist of it.

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Not sure about 802.11x but in the cell phone world (GSM, CDMA, UMTS, etc.) the RSSI is usually expressed as negative number (0 being the optimal signal with no loss at all). My base station is sitting about six feet from my laptop and I get a RSSI of -48. All the neighbors are coming in at about -90 so that seems about right.

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