How long does my frame occupy the air?
A LoRa frame is not instantaneous: between 50 milliseconds and more than two seconds depending on the settings. That time decides your battery, how many messages the law allows you per hour, and how much room you leave to your neighbours. Sounds familiar? Let’s look at it together.
A quick reminder before we start
LoRa speaks more or less slowly depending on the spreading factor (SF7 to SF12). The higher it is, the further the signal goes… and the longer each symbol lasts: one SF step almost doubles the airtime. In Europe, the 868 MHz band imposes a 1 % duty cycle: over one hour you may transmit 36 seconds per sub-band, not one more. The community network TTN is stricter still: 30 seconds of airtime per day.
Settings
Result
Airtime
Allowed maximum (1 %) — frames per hour, at this setting, on one sub-band
At your rate — of airtime per day
EU868 duty cycle —
TTN fair use (30 s/day) —
The same frame, from SF7 to SF12 — Airtime for your payload, at each spreading factor.
The trap
A sensor that “works in SF7 on the desk” goes to the field, loses sight of the gateway, and ADR pushes it to SF12. Same frame, same rate: the airtime is multiplied by 30 to 50, battery life collapses and the duty cycle is exceeded without anyone noticing, until the day the network refuses the frames.
The right setting
Do the maths at the worst SF you accept in the field, not at the desk SF. Shrink the payload (a 2-byte integer rather than text), group measurements in one frame, and leave ADR on: it brings the SF back down as soon as the link allows.
In short
- One SF step ≈ airtime × 2; SF12 ≈ 30 to 50 times SF7.
- EU868: 1 % duty cycle = 36 s of transmission per hour and per sub-band.
- TTN: 30 s of airtime per day, on top of the 1 %.
- Size at the worst SF, not the most comfortable one.
- Small payload + ADR = a battery that lasts.
So, which SF does your sensor really speak once it is in the field?
Carry this time (plus the radio wake-up) as the transmission duration into the battery-life calculator.
Formulas: Semtech, LoRa Modem Designer’s Guide (AN1200.13); payload limits: LoRaWAN Regional Parameters RP002, EU863-870.
Missing a tool?
Tell us what you are trying to do: if we already have it on the bench, we will put it online.