A station will search for a BSS to join by either active, passive scanning or both. In passive scanning a STA listens to Beacons and uses the information contained in the Beacon frame to decide which BSS to join. In active scanning the STA sends a probe request. Any AP that hears the Probe request will send a Probe Response (although with some traffic management technologies the response may be delayed. For example, if band steering is in use Probe Response can be suppressed to steer the STA to the preferred band. The STA then decides which BSS to join. The STA is still unauthenticated and unassociated. Probe Requests and Beacons are not unicast. Probe Response are unicast and are therefore ACK’d by the STA.
The STA now send an Authentication Request frame to the AP. The AP ACK’s that frame. The AP then sends an Authentication Response frame. The STA sends an ACK frame to the AP. If the AP responded in the affirmative the STA is Authenticated but, not yet Associated.
The next step is for the STA to send an Association Request. The AP sends an ACK. Then the AP send an Association Response frame. The STA sends an ACK. The STA now has layer 2 connectivity and is a member of the BSS.
The process stops here unless the BSS is using RSN authentication. RSN authentication will be covered in an upcoming post.