This method returns the current WiFi signal strength in dBm. The result is negative, and the higher (closer to zero) the number the greater the signal strength.
This is the raw value reported by the WiFi hardware: to think of it in terms of “bars” of signal strength, as seen on phones etc., the mapping might be roughly this:
|below -87||zero (still connected, but barely)|
|-87 .. -82||1|
|-82 .. -77||2|
|-77 .. -72||3|
|-72 .. -67||4|
There's a lot more range above -67 (it's been seen at up to about -11), but that's not important, as anything above -67 is good enough for everything the imp needs to do.
If the WiFi is not active at all, or during an outage, the result will be zero.