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LoRaWAN explained

What a radio technology looks like when it is built for range and battery life — and what it is no good for.

Almost every radio technology is built for speed. LoRaWAN is built for the opposite: very little data, very far, on very little power.

Sensors Gateways in range Network server Application Meter pulses, reading Probe temperature, humidity Button three keys Gateway A hears all in range Gateway B hears the same packet radio · 868 MHz Network server merges duplicates picks the way back internet Application sees the readings reply · via ONE gateway A sensor knows no gateway. It transmits blind — whoever is in range hears it. Two gateways, one packet: the network server turns that into one message — and replies through the best of them.
Schematic. The payload stays encrypted the whole way — readable only where the key is.

The trade behind it

A radio signal becomes easier to understand the more slowly you send it — the same message spread over more time stands out better from the noise. That is exactly what LoRa does, and the dial for it is called the spreading factor.

At SF7 a packet is through in milliseconds but only travels so far. At SF12 the same packet takes over a second — and is still readable where SF7 has long vanished into the noise. Every step costs roughly double the airtime and buys a piece of range.

A gateway listens on all spreading factors at once. So a device can pick whatever step suits it, without coordinating with anybody.

Spreading factor Airtime for the same packet Range SF7 ≈ 0,06 s SF8 ≈ 0,10 s SF9 ≈ 0,19 s SF10 ≈ 0,37 s SF11 ≈ 0,74 s SF12 ≈ 1,48 s Every step doubles the airtime. The 1 % duty cycle still applies — at SF12 that is fewer than 25 packets an hour.
Values for a small payload in the 868 MHz band, rounded. The range is a ratio, not a distance in kilometres.

What that means for a device

A LoRaWAN sensor sleeps almost all the time. It wakes up, measures, sends a few dozen bytes and goes back to sleep. A radio device that transmits for minutes a year gets years out of one battery.

The price: there is no downlink whenever you feel like it. In the usual mode (class A) a device opens two short receive windows, and only after transmitting itself. If you want to send something down, you wait until the device speaks up. An actuator that has to react immediately is in the wrong place here.

The limit you have to know

The 868 MHz band is free to use but not without rules: a duty cycle applies, typically 1 % of the time per channel. So a device may transmit around 36 seconds per hour — and that rule applies to the gateway too when it answers.

Almost everything else follows from that. Keep the payload small — not out of thrift, but because a large packet at a high spreading factor eats the budget. No downlink every second. And: a network does not get crowded by the number of devices but by their airtime.

What the gateway does — and what it does not

It listens and passes on. It knows none of the devices, decides nothing, and cannot read the payload: that is encrypted from the device all the way to the application (AES-128), and the key for it never sits on the gateway. So somebody else's device that my gateway hears gets passed through without me seeing its readings — that is deliberate, and the reason an open community network can work at all.

Where a packet belongs is decided by the network server behind it. If the same packet is heard by several gateways, it arrives there several times and is merged into one — with the gateway that received it best as the route for a possible reply.

So a sensor does not register with any gateway. It transmits, and whoever listens, listens. That is why a device does not fail when a gateway fails — as long as another one is in range.

What it pays off for

For everything that says little, rarely, and should live long: fill levels, meter readings, soil moisture, door contacts, environmental values in places without a socket. A weather station in the garden, a sensor in the cellar, a letterbox at the end of the drive.

And for the fact that you can run it yourself. A gateway on your own roof, a network server in your own house — the data need not see anybody else's data centre. That is the difference from mobile-network solutions, where range always runs through somebody else's contract.

What it is not for

Anything that needs bandwidth: images, voice, firmware updates over the air. Anything that has to react instantly. And anything reporting every second — that is what the airtime budget is for, and it is used up faster than you think.