Low-power radio
A door sensor runs for years on one coin-cell battery. It sleeps almost all the time, wakes for a moment to send a few bytes, then sleeps again. It needs low power, not speed.
Zigbee and Thread both use the IEEE 802.15.4 radio: 2.4 GHz, 250 kb/s, frames of at most 127 bytes. A 50-byte frame is on air for just 1.8 ms. The catch: a weak radio only reaches a room or two.
Mesh
The hub is out of reach, but a smart bulb is close. Bulbs are wired to mains power, so they can keep their radio on and listen. The sensor sends its message to the kitchen bulb.
The bulb passes the message on to the hub. Mains-powered devices relay for battery ones, so many short hops cover the whole house. That web of hops is called a mesh. Path: door sensor → kitchen bulb → hub.
Zigbee and Thread
In Zigbee the hub is the coordinator: it starts the network and lets devices join. The bulbs are routers that relay. The sensor is an end device: it sleeps, and its parent bulb holds messages for it.
Thread uses the same radio, but every device gets its own IPv6 address. An IPv6 header is 40 bytes, a third of a 127-byte frame, so 6LoWPAN squeezes it, to as little as 2 bytes.
The hub is a Thread border router: it joins the radio mesh to your home Wi-Fi and Ethernet. A phone can then reach a bulb by its IPv6 address. Matter, the shared smart-home language, runs on top.
Self-healing
Someone switches the kitchen bulb off at the wall. With no power its radio is silent, so it cannot relay. When the sensor next sends to it, no acknowledgement comes back.
The sensor looks for another neighbour that can hear it and finds the hall bulb. It attaches to the hall bulb as its new parent and sends the message there instead.
The hall bulb relays the message to the hub. Messages now go door sensor → hall bulb → hub. Nobody set this up by hand: the mesh routed around the gap.
Long range: LoRa
A soil sensor in a field 2 km away uses LoRa instead, on 868 MHz in Europe or 915 MHz in the US. Its signal is a slow chirp, a tone sliding up in pitch, that reaches kilometres: one hop to a gateway is enough.
The price is speed: in Europe, 250 bit/s to about 5.5 kb/s. At the slowest, this 15-byte packet is on air for 1.2 s, and in Europe a device may only send 1% of the time. Fine for a few readings an hour.
On the air, in numbers
;; step 1/12 · tiny-power · local ;; at door sensor door sensor (battery, sleepy) message "door opened" + battery level: 2 bytes of data radio on only while sending or briefly listening for a reply radio off the rest of the time (that is where the battery life comes from) Wi-Fi is built for speed: its radio draws far more power, so it suits laptops and phones, not a sensor that must last for years on a coin cell
The 1% limit applies to the 868.0-868.6 MHz sub-band that holds the default EU868 channels (ETSI EN 300 220). US915 has no duty cycle but limits how long one transmission may last. Range depends on walls, antennas and interference. "A room or two" for 802.15.4 and "kilometres" for LoRa are typical, not guaranteed.