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 convert it to 'bars' of signal strength as seen on phones, tablets and laptops, the mapping might be roughly this:
|below -87||zero (still connected, but barely)|
|-87 to -82||1|
|-82 to -77||2|
|-77 to -72||3|
|-72 to -67||4|
There are many more signal strength levels above -67 - values of up to -11 have been reported - but that's not important: 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.