BLE System Architecture

Bluetooth Low Energy (BLE) stands as a wireless Personal Area Network (PAN) technology meticulously crafted and overseen by the Bluetooth Special Interest Group (SIG). Bluetooth technology comes in several iterations, with BLE referring to versions 4.2 and onwards. The most recent advancements in this series are marked as v5.0 and v5.1. BLE specifications are strategically designed to curtail power consumption and device costs while upholding an efficient coverage range. It's recognized under the moniker "Bluetooth Smart," in contrast to its predecessor, known as "Bluetooth Classic."

Here are some key attributes of BLE:

1. Backward Compatibility: BLE does not support backward compatibility with BR/EDR (Bluetooth Radio/Enhanced Data Rate) protocols.
2. Frequency Band: BLE operates within the 2.4 GHz ISM (Industrial, Scientific, and Medical) frequency band, which can function in either dual mode or single mode. The dual mode accommodates both Bluetooth Classic and low-energy peripherals.
3. Generic Attribute Profile (GATT): All BLE devices utilize the GATT profile (Generic Attribute Profile). GATT provides a series of commands enabling the client to unearth information about the BLE server.
4. Protocol Stack Architecture: The BLE protocol stack architecture is bifurcated into two primary components: the controller and the host. These components are interconnected via the Host to Controller Interface (HCI).
5. Profiles and Applications: Any profiles and applications operate atop the foundational layers of GAP (Generic Access Profile) and GATT.

A) Physical Layer:

1. The transmitter employs GFSK modulation and functions within the unlicensed 2.4 GHz frequency band.
2. With this PHY layer, BLE provides data rates of 1 Mbps (Bluetooth v4.2) or 2 Mbps (Bluetooth v5.0).
3. It utilizes a frequency-hopping transceiver.
4. There are two defined modulation schemes capable of delivering 1 Msym/s and 2 Msym/s, respectively.
5. Two variants of the PHY layer are specified: uncoded and coded.
6. Both PHY modes employ a Time Division Duplex (TDD) topology.

B) Link Layer:

Positioned above the Physical layer, this layer assumes the crucial responsibilities of advertising, scanning, and establishing/maintaining connections. The behavior of BLE devices can shift between peer-to-peer (Unicast) or broadcast modes, where typical roles encompass Advertiser/Scanner (Initiator), Slave/Master, or Broadcaster/Observer. The various states of the Link layer are illustrated in Figure 1, depicting BLE Device States.

A BLE device can exist in any of these states, including Standby state, Advertising state, Scanning state, Initiating state, Connection State, and Synchronization state.

• HCI (Host-Controller Interface): This layer facilitates communication between the controller and the host using standard interface types. The HCI layer can be realized either through the utilization of APIs or by employing interfaces like UART, SPI, or USB. The Bluetooth specifications define a set of standard HCI commands and events.

• L2CAP (Logical Link Control and Adaptation Protocol): This layer provides data encapsulation services to upper layers, enabling logical end-to-end data communication.

• SMP (Security Manager Protocol): The SMP layer manages device pairing and key distribution, offering secure connectivity and data exchange services to other layers within the BLE protocol stack.

• GAP (Generic Access Profile): The GAP layer directly interfaces with the application layer and/or profiles above it. It handles tasks related to device discovery and connection services for BLE devices, including the initiation of security features.

• GATT (Generic Attribute Profile): GATT serves as a service framework that specifies sub-procedures for using ATT (Attribute Protocol). Data communication between two BLE devices is managed through these sub-procedures, with applications and profiles interacting directly with GATT.

• ATT (Attribute Protocol): The ATT layer enables BLE devices to expose specific pieces of data or attributes.

C) Application Layer:

1. The BLE protocol stack's interaction with applications and profiles is customizable to meet specific needs. Bluetooth system achieves application interoperability through Bluetooth profiles.
2. These profiles outline both the vertical interactions between the layers and the peer-to-peer interactions of individual layers between devices.
3. A profile is formed by combining one or more services, each tailored to address a particular use case. Each service comprises characteristics or references to other services.
4. All profiles and applications run on the upper layers of the BLE protocol stack, specifically the GAP/GATT layers, which are responsible for managing device discovery and connection-related services for BLE devices.