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Also, holding down option while clicking on the airport symbol in the menu bar will give you a bit more information about your current connection.
 
If you hold down the option key and click on the airport symbol in the menu bar you will get an indication of the "strength" of the signal (RSSI), plus other informations on the connected access point. While holding down that key you can also hover the mouse on the other available access points and see the strength of them.
 
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