The other way relies on energy-constrained devices calling a "friend". Temperature sensors, for example, might not be required to send out regular readings, but only to report when the temperature drifts outside a set range. That saves energy on the transmission side, as the sensors only have to signal rare exceptions -- but constantly listening for updates to the set temperature range can soon flatten batteries. By designating a device with plentiful power as their friend, such sensors need only turn on their radio receiver once a day (or whatever interval has been agreed), at which point their friend hands over all the messages sent to them while they've been offline. It's a little like having someone pick up your mail for you while you're on vacation, rather than running home every day to check your mailbox.
Choosing which mesh network to join, or which devices to be friends with, will be a little more complicated than pairing two Bluetooth devices to make a point-to-point connection.
The process, called provisioning, will require an app running on a Bluetooth-enabled smartphone, tablet or computer. New mesh devices, be they lightbulbs, industrial sensors or security cameras, will arrive unprovisioned, and will need to download encryption keys from the app to join the network as a node.
Those keys can be revoked using the same app, making it possible to safely dispose of a broken smart lightbulb, say, without the risk that a determined dumpster diver might recover the keys from the discarded bulb and use them to subvert the entire network.
The spec's developers have also taken care to protect Bluetooth mesh networks against replay attacks by ensuring each message has a unique sequence number.
The spec also provides different encryption layers for the network infrastructure and the applications that run over it. This means that a hotel's smart lighting network could carry a message to a hotel room door telling it to unlock -- but only the hotel's smartphone app, say, would hold the encryption keys necessary to generate the message in the first place.
The introduction of mesh networking will help Bluetooth overcome some of its existing limitations on range and network size, according to Andrew Zignani, a senior analyst at ABI Research.
"Mesh networking represents a new phase in Bluetooth's evolution and will be a critical enabler of its transition from a personal area network and pairing technology towards a more scalable, robust, low-power IoT connectivity solution with the ability to connect to the things around us," he said via email.
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