H3C Technologies H3C WX3000E Series Wireless Switches - Manual (Page 352)

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1. Active scanning
A wireless client periodically sends probe request frames and obtains wireless network
information from received probe response frames. Active scanning includes the following modes:
Active scanning without an SSID—The client periodically sends a probe request frame without
an SSID on each of its supported channels. APs that receive the probe request send a probe
response, which includes the available wireless network information. The client associates with
the AP with the strongest signal. This mode enables the client to find the optimal wireless
network.
Figure 321 Active scanning without an SSID
Active scanning with an SSID—If the wireless client is configured to access a wireless network
or has associated with a wireless network, the client periodically sends a probe request that
carries the SSID of that wireless network. When the target AP receives the probe request, it
sends a probe response. This mode enables the client to access a specified wireless network.
Figure 322 Active scanning with an SSID
2. Passive scanning
A wireless client listens to the beacon frames periodically sent by APs to discover surrounding
wireless networks. Passive scanning is used when a client wants to save battery power. Typically,
VoIP clients adopt passive scanning.
AP 2
Client
AP 1
Prob
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(wit
h no
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Probe r
equest
(with no
SSID) Probe r
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AC 1
AC 2
Prob
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