CWSP-205 – 802.11 Association

I have previously discussed machine state and 4 Way Authentication briefly, here I want to discuss Open System Authentication specifically 802.11 Association. Before an RSNA is established between two devices they have to first go through Open System Authentication. STA must be Associated and Authenticated to the device.

As shown in the picture below following events happen when a client starts the connection process in Open System Authentication:

Probe Request – Client sends out its capabilities, uses the lowest data rate that the protocol allows, sends it at highest power.
Probe Response – This comes from the access point advertising its parameters for a given SSID. It uses the lowest mandatory basic data rate as shown, 12 Mbps in this case. Figure above also shows the RSN IE parameters that the device must support for the 802.11 association to complete successfully
Association Request – This frame comes from the client device and in this frame it actually syncs its WLAN NIC with the parameters sent by the access point in Probe Response. You can see that this time it is sent out at mandatory lowest basic data rate that was in the Probe Response and all supported rates, RSN IE etc are now synced with the Probe Response parameters.
Association Response – This frame is from the access point back to the client, if all parameter match then the Status Code is Successful. One key IE in here is the Association ID that gets assigned to each successfully associated device and is included in the TIM (Traffic Indication Map) of a Beacon Frame. PS-Poll uses this ID for frame delivery and NDPA also uses AID for beam forming.

Finally the station is associated and Open System Authentication is complete. If there is an issue with any capabilities not matching then this association would not complete. Quick example would be clients that only support WPA/TKIP but the access point parameters are WPA2/AES. In this case RSN IE capabilities will not match and the Association will not complete successfully.

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