Bluetooth® Low Energy (LE) is one of the world’s most power-efficient, short-range wireless communications technologies. Its low power consumption is widely praised by developers and consumers. With the release of Bluetooth Mesh networking, developers may be wondering whether Bluetooth Mesh has also been designed with low power consumption in mind. Does it inherit the beauty of the low power consumption of Bluetooth LE?
The answer is yes, definitely! Bluetooth Mesh networking includes various measures to optimize power consumption and, in particular, a feature called friendship.
The application of the friendship feature in Bluetooth Mesh networking is likely to be very diverse. Some products, like lights, will be connected to the main electricity power source (the national power grid), and the power consumption of the Bluetooth Mesh module, compared to the power consumption of the light itself, will be negligible. But other products, like smart sensors or locks, will be power constrained, which means they need to be powered by a small battery or energy harvesting technique. It’s products like these that are most likely to leverage the friendship concept of Bluetooth® Mesh.
If you’ve read earlier articles in our Bluetooth Mesh Networking Series, you already know that a node is a device which has been provisioned and is a member of the mesh network. Nodes have functionality related to the product type, but may also have functionality concerning the operation of the network itself and can take on special roles. This is determined by the mesh features they support. All nodes can transmit and receive mesh messages in the network. In addition, nodes can also optionally support one or more additional network features as listed below:
- Relay feature: The ability to receive and retransmit mesh messages over the advertising bearer to enable larger networks.
- Proxy feature: The ability to receive and retransmit mesh messages between GATT and advertising bearers.
- Low-power feature: The ability to operate within a mesh network at significantly reduced receiver duty cycles. Minimizing the time the radio receiver is on leads to lower power consumption with the node only enabling the receiver when strictly necessary. Low-Power nodes (LPN) achieve this through establishing a friendship with a Friend node.
- Friend feature: The ability to help an LPN operate by storing messages destined for the LPN and only forwarding them to it when the LPN explicitly requests messages from the Friend node.
To understand how friendship enables LPNs to reduce their power consumption, consider sensors. Sensors are a good example of a type of node likely to exploit friendship and act as LPNs. They usually spend the most significant proportion of their time transmitting data and only rarely need to receive it. Maybe a sensor only transmits a temperature reading whenever it falls outside of a set of configured limits, and perhaps this only happens twice a day. This infrequent transmission of data keeps the energy use for this type of device low.
But what if those temperature limits need to be modified to use different values according to the season and modification of these limits is achieved by sending a configuration message to the sensors? For a sensor to receive such messages directly, it needs to switch the radio on and listen. Most of the time it is listening it receives nothing, but energy is expended nevertheless.
So, working with a friend allows the LPN to schedule its use of the radio to receive messages to whatever frequency makes sense for that device, and at a much lower frequency than it would otherwise need if it had to listen for messages all the time. LPNs poll their friends for new messages, which the friend stores only occasionally. This is how power is saved.
Friends and LPNs
An LPN must establish a friendship relationship with another node supporting the Friend feature in order to reduce its receiver duty cycles and save energy. Figure 1 is taken from the Bluetooth Mesh Profile Specification. Amongst other things, it illustrates the relationship between LPNs and Friend Nodes. In particular it shows:
- Light blue: LPNs
- Dark grey: Friend nodes associated with and service specific LPNs
- Light grey: Friend nodes which do not have a relationship with an LPN