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Scanning – H3C Technologies H3C WX3000E Series Wireless Switches User Manual

Page 17

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Figure 1 Establishing a client access

Scanning

A wireless client gets surrounding wireless network information in two ways, passive scanning and active
scanning. With active scanning, a wireless actively sends a probe request frame, and gets network

signals from received probe response frames. With passive scanning, a wireless client gets wireless

network information by listening to Beacon frames sent by surrounding APs.
Actually, when a wireless client operates, it usually uses both passive scanning and active scanning to
get information about surrounding wireless networks.

1.

Active scanning
When a wireless client operates, it periodically searches for (scans) surrounding wireless networks.
Active scanning falls into two modes according to whether a specified SSID is carried in a probe

request.

{

A client sends a probe request with no SSID (the SSID IE length is 0): The client periodically
sends a probe request frame on each of its supported channels to scan wireless networks. APs

that receive the probe request send a probe response, which carries the available wireless

network information. The client associates with the AP with the strongest signal. This active
scanning mode enables a client to actively get acquainted with the available wireless services

and select to access the proper wireless network as needed. The active scanning process of a

wireless client is as shown in

Figure 2

.

Figure 2 Active scanning (the SSID of the probe request is null, or, no SSID information is carried)

AP 2

Client

AP 1

Prob

e req

uest

(wit

h no

SSI

D)

Probe r

equest (

with no

SSID)

Probe R

espons

e

Prob

e Re

spon

se