GATT Bluetooth Low Energy

GATT

GATT is an acronym for the Generic Attribute Profile, and it defines the way that two Bluetooth Low Energy devices transfer data back and forth using concepts called Services and Characteristics. It makes use of a generic data protocol called the Attribute Protocol (ATT), which is used to store Services, Characteristics and related data in a simple lookup table using 16-bit IDs for each entry in the table.

GATT comes into play once a dedicated connection is established between two devices, meaning that you have already gone through the advertising process governed by GAP.

The most important thing to keep in mind with GATT and connections is that connections are exclusive. What is meant by that is that a BLE peripheral can only be connected to one central device (a mobile phone, etc.) at a time! As soon as a peripheral connects to a central device, it will stop advertising itself and other devices will no longer be able to see it or connect to it until the existing connection is broken.

Establishing a connection is also the only way to allow two way communication, where the central device can send meaningful data to the peripheral and vice versa.

Connected Network Topology

The following diagram should explain the way that Bluetooth Low Energy devices work in a connected environment. A peripheral can only be connected to one central device (such as a mobile phone) at a time, but the central device can be connected to multiple peripherals.

If data needs to be exchanged between two peripherals, a custom mailbox system will need to be implemented where all messages pass through the central device.

Once a connection is established between a peripherals and central device, however, communication can take place in both directions, which is different than the one-way broadcasting approach using only advertising data and GAP.