Time On Air calculation

do you know where i can find an explanation for the calculation of Time On Air?


See Spreadsheet for LoRa airtime calculation and Semtech’s documentation.

thanks lot.

According to Semtech formula:

Tsym = 2^SF / BW

Why is SF in the formula?
Does SF (spreding factor) represent the number of changing frequency happening to send the symbol?
Honestly i thought SF = 7,… 12 were just number agreed by the standard to distinguish the modes.