H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 18)

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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 .
Figure 3 Active scanning (the probe request carries the specified SSID AP 1)
2. Passive scanning
Passive scanning discovers surrounding wireless networks by listening to the beacon frames
periodically sent by APs. 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
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. 802.11 links define two
authentication mechanisms: open system authentication and shared key authentication.
• Open system authentication
• Shared key authentication
For more information about the two authentication mechanisms, see "Configuring WLAN security."
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.