BLE Introduction

Bluetooth SIG

Formed in 1998, the Bluetooth SIG is the not-for-profit trade association that oversees Bluetooth® technology. Bluetooth SIG members created and maintain the specifications and drive global interoperability. The Bluetooth SIG is a global community of over 36,000 companies.

There is a question about BLE naming: While commonly referred to as BLE, the Bluetooth SIG discourages this name because BLE isn't a trademark they own. Bluetooth LE is the proper way to refer to it.

Bluetooth Classic (BR/EDR) and Bluetooth LE

The Bluetooth Classic is ideal for use in-car infotainment systems, cable replacement, and audio stream applications. It is not optimized for low consumption but for high transfer speed.

Bluetooth LE (BLE) devices are used in applications where power consumption is crucial. The two wireless protocols are incompatible and BLE devices do not communicate with BR/EDR.

BLE Architecture

The BLE protocol stack architecture consists of two parts: Controller and Host. Both are interfaced using HCI (Host to Controller Interface). The HCI layer provides communication between controller and host through standard interface types. Standard HCI commands and events are defined in the Bluetooth specifications.

Figure 795: BLE protocol stack architecture

Host Layer

The Host layer contains the following layers:

Generic Access Profile (GAP)
Generic Attribute Profile (GATT)
Attribute Protocol (ATT)
Security Manager (SM)
Logical Link Control and Adaptation Protocol (L2CAP)
Host Controller Interface (HCI) - host part

Controller Layer

The controller contains the following layers:

Physical Layer (PHY)
Link Layer
Direct Test Mode
Host Controller Interface (HCI) - controller part

Physical Layer (PHY)

The BLE transmitter uses FHSS Frequency Hopping Spread Spectrum transceiver and GFSK modulation. It operates at an unlicensed 2.4 GHz frequency band. BLE offers data rates of 1 Mbps (Bluetooth 4.2) or 2 Mbps (Bluetooth 5.0).

The Bluetooth LE specification divides the band into 40 channels of 1 MHz spaced 2 MHz apart. Channels 37 to 39 are Advertisement Channels used by BLE devices exclusively to send advertising and beacon packets. The advertising channel frequencies were chosen to minimize interference from Wi-Fi and other sources.

Figure 796: BLE channels

Link Layer

This layer sits above the Physical Layer. It is responsible for advertising, scanning, and creating/maintaining connections. The role of BLE devices changes in peer-to-peer or broadcast modes. The GAP (Generic Access Profile) controls the RF state of the device, with the device in one of five states:

Standby
Advertising
Scanning
Initiating
Connected

Figure 797: BLE link layer states

BLE Roles

Bluetooth LE specification defines two roles: The Peripheral and Central. The real difference between the two is who initiates a connection. Central devices are the ones that initiate a connection, while Peripheral accept connections. In this sense, the Central acts as a "Master", with many Peripheral "Slaves" attached.

Application Layer

The application layer is use-case dependent and refers to the implementation on top of the GAP Generic Access Profile and GATT Generic Attribute Profile. This layer is generally not part of the BLE stack.

Broadcast-Only Devices (Beacons)

The beacon devices broadcast data so other devices may discover them and read their data but do not establish a connection.

The Apple company created a custom version of the beacon: iBeacon.
Eddystone is an open beacon format from Google: Eddystone specification.

BLE Indication and Notification

The BLE standard defines two ways to transfer data from the Peripheral to the Central: Notification and Indication. Notification and Indication are initiated by the Peripheral. The Indication can be enabled or disabled by the Central.