The 802.11 State Machine: Joining Your Friends
A station will search for a BSS to join by either active, passive scanning or both. In passive scanning a STA listens to Beacons and uses the information contained in the Beacon frame to decide which BSS to join. In active scanning the STA sends a probe request. Any AP that hears the Probe request will send a Probe Response (although with some traffic management technologies the response may be delayed. For example, if band steering is in use Probe Response can be suppresse