H3C Technologies H3C WA2600 Series WLAN Access Points - Manual (Page 71)

10-3
Figure 10-2 Active scanning (the SSID of the probe request is null, that is, no SSID information is
carried)
z When the wireless client is configured to access a specific wireless network or has already been
connected to a wireless network, the client periodically unicasts 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 10-3 .
Figure 10-3 Active scanning (the probe request carries the specified SSID AP 1)
2) Passive scanning
Passive scanning is used by clients to discover surrounding wireless networks through listening to the
beacon frames periodically sent by an AP. All APs providing wireless services periodically send
beacons frames, so that wireless clients can periodically listen to beacon frames 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 10-4 .
Figure 10-4 Passive scanning