A typical Bluetooth mesh network consist of a mix of relay and edge nodes. It uses a flooding approach to increase the range of Bluetooth communication through relay nodes that re-broadcast received Bluetooth mesh messages. The nodes that are actively participating in the network typically employ continuous scanning for incoming advertising packets. However, continuous scanning requires a significant amount of power and hence such nodes are typically wall-powered (for example, light bulbs).
There are certain use cases where battery-based operation is desired, for example, battery-operated light switches or sensor nodes. In such cases, the node may not be able to do continuous scanning, but still would want to participate in the Bluetooth mesh network to control and communicate with other Bluetooth mesh nodes. To enable such use cases, the Bluetooth mesh specification defines a Low Power feature. A node that uses the Low Power feature is called a Low Power node (LPN). The LPN makes use of a special node, called Friend node, to participate in the Bluetooth mesh network with significantly shorter scanning duty cycles.
The following figure shows an example of network topology with LPNs and other Bluetooth mesh nodes.
The LPN establishes a special relationship, called friendship, with a neighboring Friend node. This node starts storing all Bluetooth mesh messages (security updates and regular messages) destined for the befriended LPN in a cache known as the Friend Queue. One Friend node can establish friendship with one or more LPNs. The LPN wakes up periodically and polls the Friend node for any new messages. The Friend node then delivers stored security updates and Bluetooth mesh messages to this LPN, one by one.
The following image shows a typical life cycle of the LPN.
When LPN boots for the first time, it sends out unprovisioned node beacons and waits to be provisioned.
After the LPN is provisioned, the application firmware can finish its house keeping tasks (if any) as part of the normal operation and start the friendship establishment procedure. As a result, the continuous scanning will be turned off, which will reduce the power consumption of the device.
The Bluetooth mesh stack wakes up the device periodically to send Friend Poll messages to the Friend node and fetch any messages that the Friend may have received.
If the device fails to get any response from the Friend node, the Bluetooth mesh stack will terminate the friendship.
If required, the application firmware can also explicitly terminate the friendship.
Read the following pages for more information about the Low Power node feature support in the nRF5 SDK for Mesh:
For information about high level LPN feature APIs offered by the nRF5 SDK for Mesh, check the Mesh Low Power node (LPN) page.