Introduction
Airtime is the time it takes to transmit or receive a LoRa® packet. Because data packets are broadcasted through the air every LoRa® device is competing for the same shared medium. The longer it takes for a packet to transfer over the air the larger chunck of time it takes from available time. This makes airtime a good and fair measurement and used by ThingsIX to calculate data costs.
Background
Airtime is a function of the size of a packet and how fast it can be transferred. The size of a packet is pretty self explanatory, the larger the packet the longer it takes to transmit.
How fast a packet can be transferred requires understanding of several concepts. This is by no means a definite explanation of these core LoRa® concepts but an introduction that should be enough to get an understanding how airtime is calculated.
Spreading factor
The first important factor to consider is the Spreading Factor (SF). Before data can be transferred it needs to be converted into a signal that can be send through the air. This is done by modulating data into a Radio Frequency (RF) signal and demodulating it on the receiver side.
LoRa® uses the Chirp Spread Spectrum (CSS) for modulating data. Data is encoded into individual chirps (also called symbols) that are transmitted as a stream. How long it takes and how much data a single chirp can contain is determined by the SP. LoRa® supports 6 spreading factors, named SF7, SF8, ..., SF12. To be complete, LoRa® also supports SP5 and SP6 but both are not used for LoRaWAN packets that ThingsIX routes.
The most important factor is how long it takes to send a chirp. SF12 takes twice
as long to send a single chirp as SF11. SF11 takes twice as long as SP10, etc...
SF7 can therefore send 2**5 times more symbols in the same amount of time as
SF12.
The next thing to take into consideration is the amount of data each chirp contains. On SF7 each chirp contains 7 bits of data, on SF8 each chirp contains 8 bits of data, ..., SF12 chirps contain 12 bits of data. Therefore SF7 can transfer slightly less than twice the amount of data than SF8 in the same time.
A lower SP has the benefit that data can be transferred faster and therefore reduces power usage and airtime. The downside is that a lower SP is more susceptible to noise and as a result has lower range in comparison with a higher SP.
Bandwidth
The second important factor is bandwidth. LoRa® supports 3 different bands, 125kHz, 250kHz and 500 kHz. Which combinations of spreading factors and bands are available is described in a frequency plan. You can find the frequency details here.
With the same spreading factor a chirp send on 125kHz takes approximately 1/2 the time when it was send on 250kHz and 1/4 the time when it was send on 500kHz. This is an approximation because LoRa® uses CSS to modulate data into chirps and the frequency range per band results in that each chirp is not send exactly in a frequency band that are a multiplies of each other.
Code rate
While both the spreading factor and bandwidth control physical parameters the code rate indicates how much error correction (FEC) is used for each data transmission. LoRa® packets are transmitted over the air which is a shared medium. That makes each transmission susceptible to interference. LoRa® adds extra data (FEC) to each transmission that the receiver can use to detect and correct corrupt data.
The code rate indicates how much extra data is added. In environments with low interference a low code rate can be choosen while in environments with high interference more FEC data can be picked. It therefore can make data transmissions more reliable in an environment with high interference at the expense of larger packets.
Calculating airtime
Calculating the airtime is a complex task but there are several online calculators available that make this job easy. You should now have a good enough understanding about the factors that influence airtime and can use a calculator to see the effect when changing settings.
We just described key factors for LoRa® transmissions. End devices typically use LoRaWAN. LoRaWAN is a protocol that is build on top of LoRa® and describes several types of messages and their format. This calculator uses LoRaWAN packets to calculate airtime and shows the time it takes to transmit the payload and the total packet.