Joining BSS with AP

Scanning

A station willing to join a BSS must get in contact with the AP. This can happen through:

• Passive scanning.

  • The station scans the channels for a Beacon frame that is sent periodically from an AP to announce its presence and provide the SSID, and other parameters for WNICs within range.

• Active scanning (the station tries to find an AP).

  • The station sends a Probe Request frame - Sent from the station when it requires information from another station.

  • All APs within reach reply with a Probe Response frame - Sent from an AP containing capability information, supported data rates, etc., after receiving a probe request frame.

Beacon frame

Probe request

Probe response

Association

Once a station is authenticated, it starts the association process, i.e., information exchange about the AP/station capabilities and roaming.

• STA → AP: Associate Request frame.

  • Enables the AP to allocate resources and synchronize. The frame carries information about the WNIC, including supported data rates and the SSID of the network the station wishes to associate with.

• AP → STA: Association Response frame.

  • Acceptance or rejection of an association request. If it is an acceptance, the frame will contain information such as association ID and supported data rates.

• New AP informs old AP (if it is a handover).

Only after the association is completed, a station can transmit and receive data frames.

Association request - from Station

Association response - from AP

Data frame

Station “IntelCor*” sending data to station “D-LinkIn*” (via AP).

Frame captured between station “IntelCor*” and AP (“Cisco*”).