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AESTECHNO
Semtech AN1200.13

LoRaWAN time on air calculator

A LoRa packet's time on air is ToA = Tpreamble + Tpayload, where Tsym = 2^SF / BW. Set SF, BW, CR, preamble, payload, header, CRC and LDRO: this calculator returns the time on air in milliseconds plus the effective payload bitrate.

Inputs

Higher SF gives more range and roughly doubles time on air per step.

Not all SF/BW combos are legal in every region.

4/5 is the LoRaWAN value. Stronger coding (4/8) adds robustness and time on air.

Bytes at the PHY level. In LoRaWAN, add about 13 bytes of frame overhead to your application payload.

8 symbols is the standard LoRaWAN value.

Header

Explicit (LoRaWAN default) adds a PHY header. Implicit omits it when both ends know the format.

CRC

On for LoRaWAN uplinks, off for downlinks.

Low data rate optimize (LDRO)

Auto follows the LoRaWAN rule: on when Tsym exceeds 16 ms (SF11 and SF12 at 125 kHz), off otherwise.

Preamble n + 4.25 symbols Header Payload CRC Tsym = 2^SF / BW Time on air (ms)

LoRa frame in transmission order (segment widths indicative).

Result

41.216 ms

Breakdown

2329 bit/s · Tsym 1.024 ms · 28 sym · DE 0 · <= 873/h @1% (PHY)

Semtech LoRa airtime. Respect the regional duty cycle (EU868 is about 1 percent) and the max airtime; time on air grows fast with the spreading factor.

Frequently asked questions

FAQ

Which airtime formula does this calculator use?
The one from the Semtech LoRa Modem Designer's Guide (AN1200.13). Tsym = 2^SF / BW (BW in Hz), payloadSymbNb = 8 + max(ceil((8*PL - 4*SF + 28 + 16*CRC - 20*H) / (4*(SF - 2*DE))) * (CR+4), 0), Tpreamble = (Npreamble + 4.25) * Tsym, then ToA = Tpreamble + payloadSymbNb * Tsym. The CRC term (0/1) generalises the formula; the brocaar reference assumes CRC is always on.
When should low data rate optimize (LDRO) be on?
The LoRaWAN rule turns LDRO on (DE = 1) when the symbol time exceeds 16 ms, which in practice means SF11 and SF12 at 125 kHz. Auto mode applies exactly that rule; you can force On or Off to match a stack that behaves differently.
Does the ToA include LoRaWAN overhead or only the PHY layer?
Only the raw PHY time on air, computed on the PL byte count you enter. In LoRaWAN the frame adds MHDR + FHDR + FPort + MIC, roughly 13 bytes, on top of your application payload. Add those to PL to get the real ToA of an uplink.
How many messages per hour can I send under a 1 percent duty cycle?
A 1 percent duty-cycle budget allows about 36 s of transmission per hour (1 percent of 3600 s), per ETSI EN 300 220 on the EU 868 MHz ISM bands. Divide 36000 ms by one packet's ToA to get the send count; the calculator shows it directly.
Why does going from SF7 to SF12 multiply the time on air?
Symbol time Tsym = 2^SF / BW doubles with each SF step. Because the preamble and payload are counted in symbols, the ToA follows: raising the spreading factor by one roughly doubles airtime, buys range and sensitivity, but cuts the number of transmissions allowed under duty cycle.
Go further
LORAWAN // TIME ON AIR

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