Technical Article

Bluetooth Mesh Network Basics—Nodes, Elements, and Node Features

April 24, 2022 by Dr. Steve Arar

Learn about basic Bluetooth mesh concepts such as nodes and elements, as well as the different requirements for nodes in a Bluetooth mesh network.

In this article, we’ll get familiar with some basic Bluetooth mesh concepts such as nodes and elements. We’ll also see that nodes in a Bluetooth mesh network have different requirements. In order to address these requirements, the Bluetooth mesh protocol allows each node to have some additional optional features namely the relay, friend, low power, and proxy features.

 

Understanding Nodes in a Bluetooth Mesh Network

To begin with, let’s consider a hypothetical smart home example to explain these concepts. Consider the floor plan shown in Figure 1.

 

An example of nodes within a smart home layout

Figure 1. An example of nodes within a smart home layout

 

In this example, there are one or more lamps in each room along with a thermostat to control the room temperature. Additionally, there is an occupancy sensor in the corridor (Node 15 in the figure). Each of these devices incorporates a Bluetooth radio to be able to communicate with other parts of the system. In Bluetooth mesh terminology, each of these devices is called a node.

Said another way, a node is a basic unit of the Bluetooth mesh network that performs a particular function, such as lighting or sensing a physical parameter, and includes a Bluetooth radio to be able to communicate with the rest of the network.

A given node can directly communicate with one or more nearby nodes but normally it cannot directly communicate with every other node in the network. The nodes that are not within direct radio range of each other should communicate through intermediary nodes. For example, in the above diagram, Node 11 can communicate with Node 14 through Node 9.

 

Bluetooth Mesh Node Elements

A node in a Bluetooth mesh network might have multiple constituent parts. In the above example, a group of three lightbulbs is abstracted as a single node, Node 6, in the network.In this case, communications of these three lightbulbs occur through a single Bluetooth radio. In the context of Bluetooth mesh networking, the constituent parts of a node are called elements.

Every element within a node has a unique address, known as a unicast address. Through these unicast addresses, we are able to control the elements of a node independently. Per Bluetooth mesh specification, an element is an addressable entity within a node.

A node can have different types of elements. For example, consider a switch that also includes an occupancy sensor. In this case, the switching function and the occupancy sensing feature are two different elements of the node. Similarly, an occupancy sensor that also has a light-level sensor forms a node with two elements. Each node of the network has at least one element called the “primary element”. The additional functionalities of the node (if there are any) are called “secondary elements”.

 

Bluetooth Node Features: Nodes Are Heterogeneous!

In a Bluetooth mesh network, the nodes have different requirements. For example, a lightbulb node is mains powered whereas a sensor node might be battery powered. The power requirements of these two nodes are significantly different. While the lightbulb can be always on, the sensor should spend most of its time in a sleep mode to save power.

The processing power of the nodes is not identical as well. For example, a node that routes the information through the network needs more processing power than a node at the edge of the network that doesn’t act as a router. In addition, some nodes in the network might need to be able to communicate with devices, such as smartphones and tablets, that don’t directly support Bluetooth mesh.

To address all these requirements, the Bluetooth mesh protocol allows each node to have some additional optional features. There are four predefined node features (Figure 2) namely the relay, friend, low power, and proxy features.

 

Figure 2. A typical Bluetooth mesh network consists of nodes with different features. Image used courtesy of Infineon

 

Let’s take a look at these node features.

 

Relay Node Features: Differences From Zigbee and Thread

A relay node retransmits the information it receives. This is a fundamental requirement of mesh networking, enabling extended coverage range and enhanced reliability. However, it should be noted that this is an optional feature and not all nodes need to act as a repeater. For example, a node that is at the edge of the network is not required to be a relay node.

Since a relay node needs to continuously listen to the network and retransmit the received information, it has a higher power consumption and usually needs to be mains powered.

There is a fundamental difference between Bluetooth mesh and protocols such as Zigbee and Thread. Zigbee and Thread use routing tables, meaning that a specific path is chosen for the messages going from node A to node B. However, with Bluetooth mesh, there is no specific path and data packets are simply broadcast to all nearby nodes. The relay nodes forward the messages they receive until it reaches their eventual destination.

Due to this routing method, which is referred to as the “flooding transmission” model, the number and location of the relay nodes must be chosen carefully to provide enough transmission paths without unnecessarily increasing the network traffic. A large number of relay nodes can increase the packet collision rate, delay, and energy consumption of the network.

 

Low Power Node and Friend Features

As the name suggests, a low power node (LPN) attempts to minimize power consumption. This is useful for devices that have limited power supply such as battery-powered sensor nodes in home automation and price tags in a retail outlet. To minimize power consumption, an LPN spends most of its time in a sleep mode where the node radio hardware is switched off.

Overall, an LPN wakes up only briefly to communicate with the network and cannot receive its messages in sleep mode. Therefore, another node that doesn’t have power limitations and can be always on is required to collect the messages that should be delivered to the LPN. This associated node, through which the LPN communicates with the network, is called a friend node.

A friend node always listens to the network and collects the incoming messages for its associated LPN. When the LPN wakes up, it pings its friend node to check for any new messages. The friend node also relays the messages coming from the LPN into the network, meaning a friend node should also be a relay node.

One example, a light bulb, is a common device type that can act as a friend node for its nearby power-constrained devices. As shown below in Figure3, a friend node can have a friendship relationship with multiple LPNs.

 

An example showing the relationships of a friend node and LPNs

Figure 3. An example showing the relationships of a friend node and LPNs

 

The friendship feature allows us to add power-constrained devices to a Bluetooth mesh network. To learn more about friendship in Bluetooth mesh, please refer to this article.

 

Node Proxy Feature

The proxy feature allows a node to act as the interface between the mesh network and devices such as smartphones that don’t directly support Bluetooth mesh but support Bluetooth low energy (BLE). In a Bluetooth mesh network, there are two different methods for transporting messages: ADV (or advertising) bearer and GATT (or General ATTribute) bearer, shown in Figure 4.

 

Figure 4. ADV bearer (left) and GATT bearer (right) overviews. Image used courtesy of Renesas.

 

The ADV bearer uses the advertising state of BLE devices to send and receive messages. When a BLE device advertises, it periodically sends data packets that can be detected and processed by its nearby devices. Messages sent by the ADV bearer can be received by many nodes simultaneously. This method is used for communications inside the mesh network but it is not supported by all available smartphones. The ADV bearer is based on BLE’s GAP (or Generic Access Profile) and uses a special type of advertising referred to as non-connectable and non-scannable undirected advertising.

On the other hand, the GATT bearer sends and receives messages in a point-to-point topology. In this case, establishing a connection is required. All BLE-supporting smartphones can communicate through the GATT bearer. Proxy nodes of a Bluetooth mesh network support communications using both the GATT bearer and the ADV bearer. In other words, proxy nodes have the ability to relay messages between the GATT and ADV bearers, and as a result, can act as the mesh network interface for devices such as smartphones that don’t support Bluetooth mesh but have BLE capability.

The proxy feature can also be used to connect an old sensor that supports BLE to a Bluetooth mesh network. For example, an old sensor based on Bluetooth 4.0 can communicate with a light bulb that has the proxy feature. In this case, the light bulb translates the messages from one bearer to the other so that the old sensor can communicate with the network.

 


 

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