STA selects AP using one of the two ways: active scanning and passive scanning. In active scanning, an STA sends a probe request frame, and the AP replies with a probe response frame. This frame exchange allows the STA to obtain basic information about the AP cell such as signal strength, available transmission modes, encryption, etc. The STA repeats this frame exchange for all APs in its vicinity. Alternatively, in passive scanning, an STA listens to beacon frames, which are periodically transmitted by APs. The STA measures the RSSI of beacon frames or probe response frames transmitted by APs. The STA then selects the AP from which it received highest RSSI frame. Afterwards, the STA stays associated with that AP until the RSSI falls under a predefined threshold.
