How does Android's scan-only mode work?

Question

Android 4.3 added new 'software' feature that keeps Wifi get scan results even if Wifi is off. It says that this mode consumes very low power.

My question is how this technically done, and how much power saved compared to just enabling Wifi?

Answer 1

The logic is simple: driver and wpa_supplicant is never unloaded from the memory, unless scan-only mode is disabled in WiFi advanced menu via Scanning always available option.

In terms of power consumption - the numbers will differ for every WiFi solution, but according to my experience (being an WiFi engineer for one of the Android OEM's for last 4 years), the difference is minor. Below is an example of some average consumption numbers:

• Phone is awake, WiFi disabled: ~220mA (depends on many factors, just an example)
• Phone sleeps, WiFi disabled: ~5mA
• Phone sleeps, WiFi enabled (sure, applies to Scan-Only mode as well): +5mA

So, if you are an active user, turning Scan-Only off will not give you huge power savings. On the other hand, if the phone usage is somewhat passive, turning this option off could give some extra battery life.

Personally, I keep this option on, since I prefer immediate WiFi availability when I enable it over that minor battery win.