No Blues with Bluetooth! GATT Explained - Part 5 - Understanding Higher Protocol Layers Provides Interoperability Help

After exploring low-level layers of Bluetooth Low Energy in previous editions, and establishing connections in Part 4, Robert examines Attribute Protocol and Generic Attribute Profile or GATT in Part 5. These higher layer protocols provide developers with greater opportunity for interoperability with third-party applications or products.

- Robert Lacoste

Welcome back to the world of Bluetooth. In my previous articles, I presented an overall introduction to Bluetooth and then explained the low-level layers of Bluetooth Low Energy (BLE). Last month we established a BLE connection between two devices ("No Blues with Bluetooth part 4 / Let's connect with BLE", Circuit Cellar 409, August 2024).

In this part, we will examine what information can be exchanged through such a connection and how the developer can make the BLE experience fully plug-and-play for the end user. Another protocol called the BLE application layer is required, often called by its nickname GATT.

BLE RECAP

Let's start with a small refresher on BLE. For simplicity, I stick with its basic version introduced in Bluetooth 4.0. Have a look again at the BLE protocol stack in Figure 1. As you probably remember, its physical layer uses 40 frequency channels in the ubiquitous 2.4GHz frequency band. Three of these channels (numbered 37, 38, and 39) are dedicated to BLE device discovery and connection establishment. This is done through the Generic Access Profile protocol (GAP, top right on the figure). A BLE peripheral broadcasts periodically an advertising frame to indicate its presence, as detailed in part 3 of this series.