Bluetooth Mesh Protocol Stack Layers and Their Functions

This article covers the Bluetooth Mesh Protocol Stack Layers and their functions.

Introduction

The Bluetooth mesh profile and mesh model specifications were officially published on July 18, 2017, by Bluetooth SIG. This enables the use of BLE (Bluetooth Low Energy) for many-to-many communications between nodes. It’s used for home automation, lighting, sensor-based networks, and other applications. Previous Bluetooth versions supported one-to-one and one-to-many topologies.

Bluetooth mesh has been developed to support many-to-many topology and runs on the existing Bluetooth stack. Devices that have joined the Bluetooth mesh network are known as “nodes.” Devices that haven’t joined the network are known as “unprovisioned” devices.

Without any connection, these nodes relay messages to each other using “advertising” and “scanning” states.

The different nodes with their functionalities are relay node, low power node, friend node and proxy node.

• Relay Node: This node type can retransmit the messages broadcast by other BLE nodes. This allows messages to extend their reach and traverse further in the network.
• Low Power Node (LPN): These power-constrained nodes stay asleep for most of the time. They are mainly used for message transmissions.
• Friend Node: These nodes cache messages so that they can deliver them to power-constrained LPN nodes when they wake up. Friend nodes are always ON and listen to all broadcast messages.
• Proxy node: It implements BLE and mesh stack. It uses GATT, which makes communication between non-mesh devices and mesh devices possible.

The messages are used by BLE mesh nodes for communication with each other. These messages are used for various jobs, such as controlling nodes, relaying information, and reporting status. These messages are categorized into acknowledged and unacknowledged types. The addresses are used to identify the source node and destination node of the message. The different types of addresses are unicast, group address, and virtual address.

Protocol Layers of Bluetooth Mesh Stack

The figure above depicts the layers of the Bluetooth mesh stack. Let’s understand the functions of each of these protocol layers.

• Model Layer: This layer addresses model implementations, their behaviors, messages, states, and their bindings.
• Foundation Model Layer: This layer is concerned with network configuration and management models.
• Access Layer: It defines the format of application data and how higher-layer applications can use the upper transport layer.
• Upper Transport Layer: This layer is responsible for encryption, decryption, authentication, etc. This layer also manages transport control messages such as heartbeat, friendship, etc.
• Lower Transport Layer: This layer handles the segmentation of packets from the upper transport layer and the reassembly of packets from the bearer layer.
• Network Layer: It defines the addressing mechanism used for transport layer messages. It takes care of message addressing towards one or more elements.
• Bearer Layer: It defines how network messages are transported between nodes and how mesh packets (i.e., PDUs) are handled inside the mesh. There are two types of bearers: advertising and GATT bearer.
• BLE Core Specification: It defines building blocks required by developers to create interoperable devices which make up the bluetooth ecosystem. It is developed, updated and managed by Bluetooth SIG (Special Interest Group) working groups. The latest release is BLE version 5.2 published on 31 st Dec. 2019. This BLE layer uses advertising and scanning features. It also uses GATT bearer for proxy nodes. These bluetooth core specifications can be downloaded from bluetooth.com website.