Bluetooth LE Protocol Stack

The Bluetooth LE Protocol Stack (Bluetooth LE Stack) architecture is as shown in the figure below.

The Bluetooth LE Stack consists of the Controller, the Host Controller Interface (HCI), and the Host.

Controller

• Physical Layer (PHY): Supports 1-Mbps and 2-Mbps adaptive frequency hopping and Gaussian Frequency Shift Keying (GFSK).
• Link Layer (LL): Controls the RF state of devices. Devices are in one of the following five states, and can switch between the states on demand: Standby, Advertising, Scanning, Initiating, and Connection.

HCI

• HCI: Enables communication between Host and Controller, supported by software interfaces or standard hardware interfaces, for example, UART, Secure Digital (SD), or USB. HCI commands and events are transferred between Host and Controller through HCI.

Host

• Logical Link Control and Adaptation Protocol (L2CAP): Provides channel multiplexing and data segmentation and reassembly services for upper layers. It also supports logic end-to-end data communication.
• Security Manager (SM): Defines pairing and key distribution methods, providing upper-layer protocol stacks and applications with end-to-end secure connection and data exchange functionalities.
• Generic Access Profile (GAP): Provides upper-layer applications and profiles with interfaces to communicate and interact with protocol stacks, fulfilling functionalities such as advertising, scanning, connection initiation, service discovery, connection parameter update, secure process initiation, and response.
• Attribute Protocol (ATT): Defines service data interaction protocols between a server and a client.
• Generic Attribute Profile (GATT): Based on the top of ATT, it defines a series of communication procedures for upper-layer applications, profiles, and services to exchange service data between GATT Client and GATT Server.