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Figure 1 Establish a client access
Client
Active/Passive scanning
Authentication response
Scanning
A wireless client can get the surrounding wireless network information in two ways: passive scanning or
active scanning. With passive scanning, a wireless client gets wireless network information through listening
to Beacon frames sent by surrounding APs. With active scanning, a wireless actively sends a probe request
frame during scanning, and gets network signals by received probe response frames.
Actually, when a wireless client operates, typically it uses both passive scanning and active scanning to get
information about surrounding wireless networks.
Active scanning
1.
When a wireless client operates, it periodically searches for (scans) surrounding wireless networks. Active
scanning falls into two modes according to whether a specified SSID is carried in a probe request.
A client sends a probe request (with the SSID null, or, the SSID IE length is 0): The client periodically
sends a probe request frame on each of its supported channels to scan wireless networks. APs that
receive the probe request send a probe response, which carries the available wireless network
information. The client associates with the AP with the strongest signal. This active scanning mode
enables a client to actively get acquainted with the available wireless services and select to access the
proper wireless network as needed. The active scanning process of a wireless client is as shown in
Figure
2.
Figure 2 Active scanning (the SSID of the probe request is null, or, no SSID information is carried)
Client
Authentication request
Association request
Association response
AP 1
AP 2
AP
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