Bluetooth LE Basics

SimpleBLE abstracts away much of the complexity of Bluetooth Low Energy (BLE), but having a basic understanding of how BLE works will help you use the library more effectively.

Device Roles

In BLE, there are two primary roles when two devices are communicating:

• Central: Typically a smartphone or computer (like the one running SimpleBLE). The Central scans for advertisements and initiates connections.
• Peripheral: Typically a small, low-power device (like a heart rate monitor or a smart bulb). The Peripheral advertises its presence and waits for a Central to connect.

Advertising and Scanning

Peripherals send out small packets of data called Advertisements to announce their presence. These packets can contain:

• The device name.
• List of supported services.
• Manufacturer-specific data.
• TX Power level.

Centrals perform Scanning to listen for these advertisements. Once a Central finds a Peripheral it's interested in, it can initiate a Connection.

Services and Characteristics (GATT)

Once connected, devices communicate using the Generic Attribute Profile (GATT). Data is organized into a hierarchy:

Services

A Service is a collection of data and associated behaviors to accomplish a particular function. For example, a "Heart Rate Service" would group together all the features related to heart rate monitoring.

Characteristics

A Characteristic is the smallest logical unit of data within a service. It contains a single value and any number of Descriptors that describe the characteristic's value. Characteristics can have different properties, such as:

• Read: The Central can read the value from the Peripheral.
• Write: The Central can write a new value to the Peripheral.
• Notify/Indicate: The Peripheral can push updates to the Central when the value changes.

UUIDs

Every Service and Characteristic is identified by a Universally Unique Identifier (UUID).

• Standardized UUIDs (defined by the Bluetooth SIG) are 16-bit (e.g., 0x180D for Heart Rate Service).
• Custom UUIDs are 128-bit (e.g., 00001234-0000-1000-8000-00805f9b34fb).

Connections and Pairing

• Connection: A state where two devices are synchronized and can exchange data.
• Pairing: The process of exchanging security keys to encrypt the connection.
• Bonding: The process of storing those keys for future use, so the devices don't have to pair again next time they connect.