Field Guide to Wireless LANs for Administrators and Power Users, A

What Causes a Probe Response?

The simple answer is that a Probe Request MMPDU does. Under what circumstances is such a frame sent?

A STA seeking to join a WLAN has a choice between listening on successive channels, waiting on each channel to hear a Beacon from an AP (this is known as a passive scan), or performing an “active scan” of the network, in which the STA transmits Probe Request MMPDUs on all available channels until the STA finds an AP, a timer expires and the user gives up, or until the device's battery runs out.

One disadvantage of a passive scan is that the STA does not know a priori which channel(s) are in use in a given physical location, so it must listen for a potential Beacon on each channel for some amount of time before moving ...