WiFi: Android Band Preference / Roaming

[tim....@devoteam.com]

Hey all

Can anyone tell me how exactly Android chooses which BSSID to connect to upon initially turning on the WiFi / or when roaming (all BSSIDs share the same SSID)?

I know that a 5 GHz BSSID RSSI value is compared with the 2.4GHz BSSID RSSI value, and 5GHz is favored in some conditions.

It is explained here in the source code:

/* Set G-SCAN roam parameters */

 

/**

 * Firmware roaming is implemented with two modes:

 *   1- "Alert" mode roaming, (Note: alert roaming is the pre-L roaming, whereas firmware is

 *      "urgently" hunting for another BSSID because the RSSI is low, or because many successive

 *      beacons have been lost or other bad link conditions).

 *   2- "Lazy" mode, where firmware is hunting for a better BSSID or white listed SSID even though

 *      the RSSI of the link is good.

 *      Lazy mode is configured thru G-scan, that is, the results of G-scans are compared to the

 *      current RSSI and fed thru the roaming engine.

 *      Lazy scan will be enabled (and or throttled down by reducing the number of G-scans) by

 *      framework only in certain conditions, such as:

 *          - no real time (VO/VI) traffic at the interface

 *          - low packet rate for BE/BK packets a the interface

 *          - system conditions (screen lit/dark) etc...

 *

 * For consistency, the roam parameters will always be configured by framework such that:

 *

 * condition 1- A_band_boost_threshold >= (alert_roam_rssi_trigger + 10)

 * This condition ensures that Lazy roam doesn't cause the device to roam to a 5GHz BSSID whose RSSI

 * is lower than the alert threshold, which would consequently trigger a roam to a low RSSI BSSID,

 * hence triggering alert mode roaming.

 * In other words, in alert mode, the A_band parameters may safely be ignored by WiFi chipset.

 *

 * condition 2- A_band_boost_threshold > A_band_penalty_factor

 *

 */

/**

 * Example:

 * A_band_boost_threshold = -65

 * A_band_penalty_threshold = -75

 * A_band_boost_factor = 4

 * A_band_penalty_factor = 2

 * A_band_max_boost = 50

 *

 * a 5GHz RSSI value is transformed as below:

 * -20 -> -20+ 50 = 30

 * -60 -> -60 + 4 * (-60 - A_band_boost_threshold) = -60 + 16 = -44

 * -70 -> -70

 * -80 -> -80 - 2 * (A_band_penalty_threshold - (-80)) = -80 - 10 = -90

 */

I've tested this with a few devices and it just doesn't seem to be consistent, sometimes 2.4GHz is chosen when it should have been 5GHz if I follow the above rules. 

So my question is, do the manufacturers change how this selection process works, or is this how it works in every Android device?