H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 257)

31-3
Figure 31-2 Active scanning (the SSID of the probe request is null, that is, no SSID information is
carried)
Prob
e req
uest
(wit
h no
SSI
D)
Probe r
equest (
with no
SSID) Probe r
espons
e
Prob
e res
pons
e
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 31-3 .
Figure 31-3 Active scanning (the probe request carries the specified SSID)
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. APs providing wireless services periodically broadcast
beacon frames so that the clients can get the information of the surrounding wireless networks by
listening to beacons on each of the channels supported. 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 31-4 .