BLE Stack

The architecture of BLE Stack is shown in Figure 1.

The BLE Stack consists of the Controller, the Isochronous Adaptation Layer (ISOAL), 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 modes, and can be switched between the modes on demand: Standby, Advertising, Scanning, Initiating, or Connection.

ISOAL

• The Isochronous Adoption Layer (ISOAL) enables adaptation of isochronous data between the Host and the Controller by assembling segmented data frames into data streams for the Application layer, or by segmenting data streams from the Application layer into data frames and transmitting the data frames through air interfaces.

HCI

• The Host Controller Interface (HCI) enables communications 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 Adaption Protocol (L2CAP) provides channel multiplexing, data segmentation, and reassembly services for upper layers. It also supports logic end-to-end data communications.
• 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, which fulfills 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) is based on the top of ATT. It defines a series of communications procedures for upper-layer applications, profiles, and services to exchange service data between GATT Client and GATT Server.

For more information about Bluetooth LE technologies and protocols, visit Bluetooth SIG official website.

Specifications of GAP, SM, L2CAP, and GATT are provided in Bluetooth Core Spec. Specifications of other profiles/services at the Bluetooth LE application layer are available on the GATT Specs page. Assigned numbers, IDs, and code which may be used by BLE applications are listed on the Assigned Numbers page.