I noticed that many of the airtime calculators available over the internet (including the one offered by TTN) gives the same time-on-air for packets with slight variations in payload size.
Is there any good explanation to this? Maybe someone has a good source about it (I am writing about it in my masters thesis )
In which case maybe we should leave you to work this out for yourself from the LoRaWAN specs and 1st principles vs telling you the secrets (also GIYF!) spoilers: LoRaWAN Headers, Payload format and structures, Forward error correct/CRC check/Coding scheme etc. yields a SF specific staircase/quantised increase in total payload/on airtime
At the lowest level of LoRa modulation, data is transferred in symbols, which may consist of between 6 to 12 bits (corresponding to the spreading factor). This is after channel coding, like bit shuffling, whitening, 4/5 error coding. So the 6…12 bits that are encoded on the air for each symbol do not align with a 8-bit input byte. It might happen that the last symbol only contains only one “useful” bit, but still a full symbol needs to be transmitted.