3.15.1.2 Roaming Candidates List

When a station is connected and the roaming mechanism is enabled, it starts to scan periodically; this process is called a background scan. The scanning interval is defined during connection, and depends on a customer's needs. The characteristic value is 10 seconds. During each scan instance, the station only scans one channel. The purposes of the scan are to detect APs with SSID, like the currently connected SSID of the AP, and to reveal its RSSI. Once an AP with the same SSID is detected, its channel is scanned during each scan period, in addition to the regular one-by-one channel scan to get the updated RSSI level information. The scan may be active, using probe requests and receiving probe responses, or passive, by listening to AP beacons, depending on driver configuration and other aspects. All channels are scanned according to a regulatory domain configuration.

If the RSSI level of the current connected AP decreases below the defined RSSI level threshold, the station invokes a one-time scan instance on all channels within one scan interval. The purpose is to detect a roaming candidate AP with a higher RSSI level, to avoid a performance degradation and a potential disconnect. Then, it continues the one-by-one channel scanning.