What units are being reported by Apple for AirPort signal strength?
If you run:
You'll get some good info about your wireless setup. I'm guessing that avgSignalLevel and avgNoiseLevel (-I option) are reported in dBm, but when you use the -s option, you'll get a column labeled "Sig" with a different value (this time, it's positive).
So what units are used in the "Sig" column?
And can someone help me understand the meaning of negative dBm? I get that 0 dBm = 1 mW, but I can't really imagine that the received power is less than one milliWatt. So what does a negative value mean, and if it is just a measurement relative to some other baseline value, then what is the baseline?
The negative sign does not mean that you have less power. The 2 values are using two different measure units, dBm and Watt. In WiFi, 0dBm means that you have the maximum signal strength in your Antenna (RSSI - Received Signal Strength Indication).
If you are using Leoaprd, hit the Option bottom and keep pressed + Click on the AirPort Icon, a menu will appear telling you your RSSI value.