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A client sends a probe request (with a specified SSID): When the wireless client is configured to access
a specific wireless network or has already successfully accessed a wireless network, the client
periodically sends a probe request carrying the specified SSID of the configured or connected wireless
network. When an AP that can provide the wireless service with the specified SSID receives the probe
request, it sends a probe response. This active scanning mode enables a client to access a specified
wireless network. The active scanning process is as shown in
Figure 3 Active scanning (the probe request carries the specified SSID AP 1)
Client
Probe Request
Probe Response
Passive scanning
2.
Passive scanning is used by clients to discover surrounding wireless networks by listening to the beacon
frames periodically sent by an AP. All APs providing wireless services periodically send beacons frames, so
that wireless clients can listen to beacon frames periodically on the supported channels to get information
about surrounding wireless networks. Passive scanning is used by a client when it wants to save battery
power. Typically, VoIP clients adopt the passive scanning mode. The passive scanning process is as shown
in
Figure
4.
Figure 4 Passive scanning
Client
Client
Authentication
To secure wireless links, the wireless clients must be authenticated before accessing the AP, and only wireless
clients passing the authentication can be associated with the AP. 802.1 1 links define two authentication
mechanisms: open system authentication and shared key authentication.
For more information about the two authentication mechanisms, see the chapter ―WLAN security
configuration."
Association
A client that wants to access a wireless network via an AP must be associated with that AP. Once the client
chooses a compatible network with a specified SSID and passes the link authentication to an AP, it sends an
association request frame to the AP. The AP detects the capability information carried in the association
request frame, determines the capability supported by the wireless client, and sends an association response
to the client to notify the client of the association result. Usually, a client can associate with only one AP at a
time, and an association process is always initiated by the client.
AP 1
(SSID=AP1)
(SSID=AP1)
AP
Figure
11
3.
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