The AP (access point) advertises its presence by transmitting short wireless message at regular interval, typicall 10 times per second. The short message is called beacon which allow the STA (station) to discover the identify of the AP.
A STA will start to search fo an AP after start up initialization phase. There are a number of radio frequencies (called channels) that could be used and so the STA must tune in each channel in turn to discover the beacons. This process is called scanning. Alternatively, the STA may send a probe request message for in-ranged AP to response. Probing is also used for roaming.
When the STA is ready to connect to the AP, it first sends an authenticatae request message to the AP. The AP will reply with an authenticate response indicating acceptance. This give permission for the STA to connect.
The STA then issues an association request to connect. AP will response with an association response to complete the connection.
Durig roaming scenario, the STA choose to move from one AP to another. The STA will first send the old AP a disassocation message and reconnect to the new AP using a reassociation message. The reassociation message has information about the old AP to allow the new AP to talk to the old AP to confirm roam has taken place.
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