When a wireless client joins a network, it first issues a probe request management frame, which 
notifies access points in the area that a client wishes to join the named network. If a wireless network which matches the SSID responds, the client connects to that access point.
Many client wireless management systems, both those provided by wireless card vendors and those built into the operating system, maintain a list of preferred networks: Networks which the user has connected to in the past, and networks which have been pre-configured by the IT department. When the client is not connected to a network, it will begin probing for networks to join by looping through the preferred network list. Defending your clients against wireless hackers with sniffers - Network World
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