Zigbee, Thread and LoRa: low-power radios
Smart-home sensors and IoT devices must run for years on a small battery, so they trade speed for power. Follow one message from a door sensor through a mesh of smart bulbs to a hub, watch the mesh heal when a bulb goes off, then send a reading 2 km over LoRa.
5 quick questions
Pick an answer to see why. Your score saves with your progress.
01Why can a smart bulb relay messages for a door sensor, but the door sensor does not relay for others?
Relaying means listening all the time. A mains-powered bulb can afford that; a coin-cell sensor would drain its battery, so it is an end device that sleeps.
02What do Zigbee and Thread have in common?
They share the same small, low-power radio. They differ above it: Thread carries IPv6 (compressed by 6LoWPAN), Zigbee uses its own network layer.
03The kitchen bulb is switched off at the wall. What happens to the door sensor's messages?
A mesh has more than one path. The sensor attaches to another router that can hear it, and messages go sensor → hall bulb → hub.
04What does a Thread border router do?
Thread devices are on IPv6, so a router at the edge of the mesh can pass packets to and from the rest of your network. Matter runs on top of that.
05A LoRa sensor 2 km away sends 15 bytes at the slowest setting (SF12). About how long is it on the air?
Range costs speed: at 250 bit/s the packet takes 1155 ms. With a 1% duty cycle in Europe, the device then has to stay quiet for about 114 s.
Glossary
- IEEE 802.15.4
- A low-power, short-range radio standard: at 2.4 GHz it sends 250 kb/s in frames of at most 127 bytes. Zigbee and Thread both use it.
- Mesh network
- A network where devices pass messages on for each other, so many short hops can cover a whole building and there is more than one path.
- Coordinator / router / end device
- Zigbee's three jobs: the coordinator starts the network, routers (mains-powered) relay, end devices never relay, and battery-powered ones sleep most of the time.
- Thread
- A mesh protocol on 802.15.4 that gives every device an IPv6 address, with no single coordinator.
- 6LoWPAN
- The adaptation layer that squeezes IPv6 headers (40 bytes) down to as little as 2 bytes and splits big packets so they fit 802.15.4 frames.
- Border router
- A router at the edge of a Thread mesh that forwards IPv6 packets to and from the home Wi-Fi or Ethernet network.
- Matter
- A shared smart-home application standard that runs over IPv6 on Thread, Wi-Fi and Ethernet.
- LoRa / LoRaWAN
- A long-range, very-low-rate radio on sub-GHz bands (868 MHz in Europe, 915 MHz in the US); LoRaWAN is the network built on it, with devices talking straight to gateways. Its signal is a chirp, a tone sliding up in pitch; the spreading factor (SF7 to SF12) sets how stretched each chirp is: higher is slower but reaches further.
Notes and sources
- IEEE 802.15.4 at 2.4 GHz: O-QPSK, 250 kb/s, channels 11-26 (2405-2480 MHz), at most 127 bytes per frame. The 50-byte frame size is a round example; real Zigbee and Thread frames vary.
- IPv6 header 40 bytes and minimum MTU 1280 bytes (RFC 8200); 6LoWPAN fragmentation (RFC 4944) and header compression down to 2 bytes in the best case (RFC 6282). Addresses use the 2001:db8::/32 documentation prefix (RFC 3849).
- LoRaWAN EU868 data rates from LoRaWAN Regional Parameters RP002-1.0.4: DR0 (SF12, 125 kHz) 250 bit/s, DR5 (SF7, 125 kHz) 5470 bit/s. Time on air uses Semtech's formula with an explicit header, CRC on and 8-symbol preamble.
- 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.
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