Bluetooth mesh explained: nodes, relays and uses

Bluetooth mesh is a way for Bluetooth Low Energy devices to form a network in which every message can reach every device, even ones far outside the range of a single radio. The Bluetooth SIG, the body behind Bluetooth, published the first version in 2017 and a major update in 2023. It has become a standard choice for lighting control in offices, warehouses and shops, and it turns up at home in some bulbs, switches and sensors. This guide explains how it works, where it fits and how it compares with the other home radios. It belongs with the smart home guides, next to the Zigbee vs Z-Wave vs Wi-Fi comparison and the Thread border router guide.

How Bluetooth mesh differs from normal Bluetooth

Ordinary Bluetooth connects two devices: a phone to earbuds, a phone to a fitness band. Each connection is one to one, and range is limited to what one radio can reach.

Bluetooth mesh uses the same Low Energy radio but works differently. Devices do not pair one to one. Instead, they broadcast short messages using Bluetooth advertising packets, and other devices in the network pick them up and pass them on. A message from a wall switch can reach a lamp at the far end of a building by hopping through the lamps in between. A network can hold thousands of devices, and adding more devices tends to improve coverage rather than weaken it.

How messages travel: managed flooding

Bluetooth mesh moves messages by managed flooding. When a device sends a message, every device in range that acts as a relay repeats it, and the devices that hear those repeats relay it again, until the message has spread across the network. The flooding is “managed” by three rules that stop it running out of control:

• Time to live (TTL). Each message carries a hop count that drops by one at every relay. When it reaches its limit, nobody repeats it.
• Message cache. Each device remembers messages it has recently seen and ignores copies, so the same message is not relayed twice by one device.
• Only some devices relay. Usually mains-powered devices, such as lamps, take on the relay job. Battery devices do not.

Flooding sounds wasteful, but it has a useful property: there are no routes to set up or repair. If a lamp is switched off at the wall, messages simply take another path. Mesh 1.1 added directed forwarding for large networks, where messages follow chosen paths instead of flooding everywhere, which cuts radio traffic in big installations.

The four node features

Every device in a mesh is a node, and a node can take on extra jobs:

• Relay. Repeats messages to extend the network. Typically lamps and other mains-powered devices.
• Proxy. Lets a device that only speaks ordinary Bluetooth, such as a phone, talk to the mesh. The phone connects to one proxy node, and that node passes its messages in and out. This is how a phone app controls a mesh of bulbs.
• Low power node. A battery device, such as a motion sensor or a remote, that sleeps most of the time and cannot listen for messages constantly.
• Friend. A mains-powered device that holds messages for a sleeping low power node. When the low power node wakes, it asks its friend for anything that arrived while it slept.

Publish, subscribe and models

Devices in a Bluetooth mesh do not usually address each other one by one. A switch publishes its messages to an address, often a group address such as “living room lights”, and every lamp that subscribes to that address acts on them. Moving a lamp to a different room means changing its subscription, not reprogramming the switch.

What a message means is defined by models: standard building blocks such as Generic OnOff, Light Lightness, Light CTL (color temperature) and Sensor. Two devices from different makers that use the same SIG models can, in principle, work together. In practice, many consumer products use their own models or a vendor’s own mesh on top of Bluetooth Low Energy, which is why mixing brands at home is often harder than the standard suggests.

Security and provisioning

Security is built in and cannot be switched off. Adding a device to a network is called provisioning: a phone app or a controller exchanges keys with the new device, after which it holds:

• a network key, which lets it relay and decode network traffic for that mesh;
• one or more application keys, which separate traffic for different uses, so a lighting key cannot be used to open a door;
• a device key, used only between the device and the provisioner to configure it.

Messages are encrypted and authenticated at both levels, and include protections against replayed messages. Mesh 1.1 added certificate-based provisioning, so a device can prove it is a genuine product before it is admitted, and remote provisioning, so devices can be added through the mesh rather than within range of the phone.

When a device is removed or lost, a well-designed app runs a key refresh so the old device can no longer read the network. It is worth checking that your app does this. The general rules in IoT device security basics apply here as for any connected device.

What Mesh 1.1 added

The Bluetooth SIG announced Mesh 1.1 in September 2023. Besides directed forwarding and the provisioning changes above, it added device firmware update over the mesh, private beacons that make devices harder to track, and subnet bridging for large sites split into separate networks. The Bluetooth SIG’s mesh page lists the full set. Devices built on 1.0 still work with 1.1 networks, without the new features.

Bluetooth mesh compared with Zigbee, Thread and Wi-Fi at home

Against Zigbee. Both are low-power meshes on 2.4 GHz. Zigbee has a far larger range of home devices and works well with local hubs. Bluetooth mesh has the advantage that a phone can control devices directly, with no hub, which suits a few bulbs in a flat. For a whole house with sensors and automations, Zigbee is the more practical base; the Zigbee vs Z-Wave vs Wi-Fi guide covers why.

Against Thread. Thread is IP-based and is one of the two networks Matter runs on, which is why most new low-power home devices are moving to it. Bluetooth mesh is not part of Matter: Matter uses Bluetooth only to set a device up, and then hands it over to Wi-Fi or Thread. See how Matter works for that step.

Against Wi-Fi. Wi-Fi needs no hub and has plenty of bandwidth, but battery devices struggle on it. Bluetooth mesh runs battery switches and sensors for a long time and lamps relay for each other, so coverage grows with the number of devices.

Where Bluetooth mesh is used

• Commercial lighting. This is its main market: networked lighting in offices, schools and warehouses, where every fitting contains a sensor and a mesh node and the building can be controlled without separate control wiring.
• Smart bulbs and switches at home. Some mainstream bulbs, plugs and switches use Bluetooth mesh, or a maker’s own mesh built on Bluetooth Low Energy, so they can be set up from a phone without a hub.
• Asset tracking and sensors in buildings, using the mesh to carry readings and beacon sightings back to a gateway.

Before you buy Bluetooth mesh devices for a home

• Check what the app needs. Control from a phone in the room usually works with no hub. Control from outside the house, voice assistants and schedules usually need a bridge or a hub that stays on.
• Check for local control. If everything goes through the maker’s cloud, the lights may stop responding to automations when the internet is down. The local-only smart home guide explains how to check.
• Do not expect brands to mix. Unless both products say they support the same SIG mesh models, assume they will only work with their own app.
• Consider Matter instead for new purchases that need to work with several ecosystems. Matter over Thread gives the low-power mesh benefits with wide support.

For the other radios a smart home uses, read the Zigbee vs Z-Wave vs Wi-Fi comparison, the Thread border router guide and the rest of the smart home guides.