Passive Scan and Active scan by client

Clients gather information about the APs by scanning the channels one by one either through passive scanning or active scanning.

Passive Scanning:

• In passive scanning mode, the client station moves the radio into each channel and waits to listen for beacons frame on the channel. 
• The client station listens for beacons containing SSID that it may have already connected to before.
• If the client receives beacons from multiple APs for the same SSID, it attempts to connect to the AP with the best RSSI (receiver signal strength indicator). 
• This passive scanning will save battery power as it does not need to transmit.

Active Scanning:

• Client stations send out probe request frames on each channel. 
• These probe requests may contain SSID of a specific WLAN that the client is looking for or the probe requests can also look for “any” SSID to find out all the SSIDs in the proximity of the client. 
• These are requests for APs to send out information about themselves. 
• APs respond to Probe Requests with probe response frames, the contents of which are similar to Beacon frames.
• The APs operating on a particular channel responds back to probe request with a probe response with its SSID, supported rates, and security rates. 
• If a client station receives probe responses from multiple APs (and/or multiple SSIDs), the client station uses RSSI of the AP as a judge to connect to an AP with best signal strength.