Bluetooth is a short-range wireless technology standard that enables connection and communication between devices over distances up to 100 meters. The key to Bluetooth lies in its protocol stack which defines the entire Bluetooth system from the radio layer to high-level application.
The Bluetooth protocol stack consists of 5 layers – the Radio Layer, Baseband Layer, Link Layer, Logical Link Control and Adaptation Protocol (L2CAP) Layer, and Application Layer.
The Radio Layer handles modulation, transmission, and reception of raw Bluetooth radio signals in the 2.4 GHz ISM band. It defines requirements for the radio transceiver circuits. Different versions of Bluetooth operate in different frequency ranges and use different modulation schemes. For example, Bluetooth Low Energy uses Gaussian Frequency Shift Keying modulation while Classic Bluetooth uses Gaussian Frequency Shift Keying initially and switches to Phase Shift Keying modulation later.
The Baseband Layer lies over the Radio Layer and manages physical RF links between Bluetooth devices. It handles channel setup, packet timing, power control, ARQ, and security features like authentication and encryption. The Baseband protocol defines inquiry and paging procedures that allow devices to discover and connect with each other. It also defines logic for master-slave roles, clock distribution, and hopping sequences.
The Link Layer provides data packet segmentation, reassembly, and flow control. It ensures data integrity over the air interface through CRC checks. The link layer uses slave latency management to reduce power consumption in slave devices. It defines the structure of a Bluetooth piconet which has 1 master connected to up to 7 active slaves.
L2CAP provides protocol multiplexing to support higher level protocols. It segments and reassembles application layer data into baseband packets. L2CAP also handles quality of service by providing prioritized channels with guaranteed data rates. It defines protocol state machines and procedures for channel configuration and connection establishment between two devices.
The topmost Application Layer defines adopted protocols and profiles that allow creation of Bluetooth applications. Example adopted protocols include PAN for Bluetooth networking, object exchange for data synchronization, and telephony control protocol for Bluetooth headsets. Standard profiles define specifications for Bluetooth services like file transfer, cordless telephony, human interface devices and many more.
In summary, the well-defined architecture of the Bluetooth protocol stack is key to enabling robust wireless connections. Each layer has specific responsibilities related to lower-level hardware, radio transmission, connections, or high-level applications. The modular stack design allowed Bluetooth to grow from its initial focus on cable replacement to support Internet of Things sensor devices and entire wireless networks today.