Test: Zigbee and Thread (meshes on 802.15.4)
Zigbee and Thread: Meshes on 802.15.4
Why a mesh? The glasshouse is too big for one hop
The greenhouse has grown into a glasshouse 120 m long, with battery sensors everywhere. Wi-Fi would flatten their batteries, and BLE is built for one phone at a time. IEEE 802.15.4 is a third 2.4 GHz radio: 16 channels of 2 MHz (11 to 26), 250 kbit/s, frames of at most 127 bytes, and microamps when idle. Indoors among wet plants one hop reaches about 25 m, so most sensors cannot reach the Pi at the end of the house. Zigbee and Thread both solve this the same way: the mains-powered devices (fans, pumps, lights) become routers that relay for everyone else. Turn relaying off to see the alternative.
Challenge: With relaying off, find how many sensors can still reach the Pi; then turn it back on and get a reading from S10, the farthest.
The radio: IEEE 802.15.4
16 channels of 2 MHz (11 to 26) at 2.4 GHz, 250 kbit/s, frames of at most 127 bytes.
about 25 m a hop here
Routers
Mains powered, radio always on, relay frames for anyone. Zigbee calls them routers; so does Thread.
End devices
On batteries, asleep almost all the time, talk only to one parent router.
11 sensors and actuators
Sensors that reach the Pi
11 of 11
Readings delivered
0
Readings lost
0
Every sensor also reports by itself every 8 s; the route of the latest reading is drawn in orange. Readings from S9 and S10 cross six hops, and each hop is acknowledged at the MAC layer before the next begins.
Radio log, newest first
Nothing on the wire yet.
© 2026 Derek Molloy, Dublin City University. All rights reserved.
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