Zigbee, Thread and Wi-Fi explained
Every smart light, plug and sensor needs a way to hear instructions. Which radio it uses decides whether your home keeps working when the internet, or the maker, goes away.
Three radios, one job
A smart device is a switch, sensor or motor with a small radio attached. The radio carries two things: instructions to the device, such as turn on, and reports from it, such as the door opened. Three radios do most of this work in homes and small buildings: Zigbee, Thread and Wi-Fi. All three work inside your building. None of them needs the internet to carry a message from a light switch to a light.
The difference is in what each is good at, and in where the messages go once they leave the device.
Zigbee
Zigbee is a low-power radio designed for exactly this job. A battery sensor spends almost all its time asleep, wakes to send a few bytes, and sleeps again, so it runs for years on a couple of AA cells or a coin battery. Mains-powered devices, such as plugs and bulbs, also pass messages on for their neighbours. That makes a mesh: the more devices you fit, the more routes a message has, and the network gets more reliable as it grows rather than less.
Zigbee devices talk to a coordinator, a small radio stick in the computer that runs your home. From there the computer does the rest. Nothing goes outside the building unless you choose to send it. Zigbee has been in homes for over a decade, so there are devices from hundreds of makers at every price.
Thread
Thread is the newer of the two low-power radios, and it works on the same kind of mesh as Zigbee: battery devices sleep, mains devices relay. The difference is that Thread speaks the same language as the rest of your network, so a Thread device can be addressed like any other computer in your building. Several border routers can join the mesh to the network, and if one fails another takes over.
Thread is the radio behind Matter, the standard that Apple, Google, Amazon and Samsung agreed so that devices work across their systems. It is the direction the industry is going, and the hardware is arriving at reasonable prices. Home Assistant supports both Zigbee and Thread, so you can mix them and buy whatever is cheapest for the job.
Wi-Fi
Wi-Fi is the network you already have. It carries far more data than Zigbee or Thread, so it is right for cameras, speakers and screens, where there is a lot to send and a mains socket to power it. It is wrong for a battery sensor: a Wi-Fi radio drinks power, so the battery lasts weeks rather than years. It also leans on your router. Every device takes a slot, and a cheap router gets slow with sixty smart plugs on it.
Wi-Fi itself is not the problem. The problem is what many Wi-Fi devices do with it.
The device that only works through its maker
A great many cheap smart plugs, bulbs and sensors use Wi-Fi to reach their maker's servers, and only those servers. When you tap the app, the tap goes to a data centre, often on another continent, and the instruction comes back the same way. The light turns on a second later, if everything along the way is up.
If the maker's service goes down, the device stops. If your internet drops, the device stops. If the company closes, the device stops for good. In April 2022 a smart home maker called Insteon switched its servers off with no notice, and its customers found their apps dead and their automations gone overnight. Makers also change their minds: in 2024 one platform revoked the access it had granted to other systems, and homes that had relied on it went dark.
There is a privacy cost too. The app knows when you come home, when you go to bed and when the house is empty, and that record sits with a company you have never heard of. The National Cyber Security Centre's advice on smart devices is to ask what the device does with your data and what happens when support ends. Most of these devices do not have a good answer.
Many of them are the same device inside. A handful of platforms supply the radio module, the cloud and the app to anyone who wants to put their own name on the box, so hundreds of brands share one set of servers. The name on the box tells you nothing about who you are depending on.
Local first
The alternative is a device that answers to a computer in your building. Zigbee and Thread devices do this by design. Many Wi-Fi devices can do it too if you choose makers that support local control, and I check before I buy.
With local control the light turns on in the time it takes to flick a switch. It keeps working when the broadband is out, and it keeps working if the maker is bought, closes or changes its terms. Devices from different makers mix freely, so you choose on price and on what they do. Nothing about your home leaves your home unless you ask it to. And when you move on from me, or from the computer that runs it, the devices stay yours.
That is what I fit. It costs no more than the cloud alternative and often less, because you are not paying for a service that someone has to keep running.
You do not have to choose
The computer that runs your home speaks all three radios at once. Home Assistant has a Zigbee stick on one side, a Thread border router on another and your Wi-Fi on the third, and treats a light as a light whichever way it is connected. So you never pick a camp. Buy Zigbee where it is cheapest, Thread where the new device is better, Wi-Fi where there is data to carry, and run them together as one system.
It also means the devices you already own are not waste. If you have a drawer of smart plugs, a brand's bulbs in every room or a hub that came with the house, I start by seeing what can be kept. Many Wi-Fi devices can be switched to local control. Where one cannot, Home Assistant can usually still drive it through its maker's service, so it keeps working until it wears out and you replace it with something local when the time comes. Nothing goes in the bin because it was the wrong brand.
The computer itself is the cheap part. A Raspberry Pi is enough for most homes, and if you do not have one I will help you order the right one. It is yours, not mine, and it is a general computer: when it is not running your home, it can run whatever else you fancy.
That is the real argument for an open system. It is not that one radio wins. It is that your home can use what it has, add what is cheapest, and outlast every one of the makers.