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Client access, Scanning – H3C Technologies H3C WX6000 Series Access Controllers User Manual

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31-2

Client Access

A client access process involves three steps: active/passive scanning, authentication and association.

Figure 31-1 Establish a client access

Scanning

A wireless client can get the surrounding wireless network information in two ways, passive scanning or

active scanning. With passive scanning, a wireless client gets wireless network information through

listening to Beacon frames sent by surrounding APs; with active scanning, a wireless client actively

sends a probe request frame during scanning, and gets network signals by received probe response

frames.

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 (that is, 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 the SSID null, that is, the length of the SSID is 0.): The client

broadcasts a probe request frame on each of the channels supported by the NIC to scan wireless

networks. APs that receive the probe request frame send a probe response frame. The client

associates with the AP with the strongest signal. This active scanning mode enables a client to

know the available wireless services and then access the target wireless network.