Bluetooth  mesh introduction

After the Bluetooth 5 specification was released the SIG focused on formalizing mesh networking in Bluetooth. The Bluetooth SIG published the mesh profile, device, and model specification 1.0 on July 13, 2017. This comes six months after the Bluetooth 5.0 specification was released. Before the official specification with Bluetooth 5 by the Bluetooth SIG, there had existed proprietary and ad hoc schemes using older Bluetooth versions to build mesh fabrics. The three specifications published by the Bluetooth SIG are as follows:

• Mesh profile specification 1.0: Defines the fundamental requirements to enable an interoperable mesh networking solution
• Mesh model specification 1.0: Basic functionality of nodes on the mesh network
• Mesh device properties 1.0: Defines the device properties required for the mesh model specification

It is not yet known if there is any limit to the size of the mesh network. There are some limits built into the specification. As of the 1.0 specification, there can be up to 32,767 nodes in a Bluetooth mesh and 16,384 physical groups. The maximum time-to-live, which is indicative of the depth of the mesh, is 127.

Bluetooth mesh is based on BLE and sits on the  BLE physical and link layer described earlier. On top of that layer is a stack of mesh-specific layers:

• Models: Implements behaviors, states, and bindings on one or more model specifications.
• Foundation models: Configuration and management of the mesh network.
• Access layer: Defines format of application data, encryption process, and data verification.
• Upper transport layer: Manages the authentication, encryption, and decryption of data passing to and from the access layer. Transports control messages such as friendships and heartbeats.
• Lower transport layer: Performs segmentation and reassembly (SAR) of fragmented PDUs if necessary. 
• Network layer: Determines which network interface to output messages on. Manages the various address types and supports many bearers. 
• Bearer layer: Defines how mesh PDUs are handled. Two types of PDUs are supported: advertising bearer and GATT bearer. The advertising bearer handles the transmission and reception of mesh PDUs, while the GATT bearer provides a proxy for devices that don't support the advertising bearer.
• BLE: The complete Bluetooth LE specification.

Bluetooth mesh can incorporate mesh networking or BLE functions. A device that is capable of mesh and BLE support can communicate to other devices like smartphones or have beacon functions. Shown below is the Bluetooth mesh stack. What is important to realize is the replacement of the stack above the link layer.