Frequency Spectrum

Bluetooth operates in the 2.4 GHz ISM (Industrial, Scientific, and Medical) band, specifically between 2400-2483.5 MHz. This globally unlicensed frequency band allows devices to work across international borders without requiring specific permits.

Range and Classes

Bluetooth devices are categorized into three power classes:

• Class 1: Up to 100 meters (330 feet) range with 100 mW (20 dBm) power
• Class 2: Up to 10 meters (33 feet) range with 2.5 mW (4 dBm) power
• Class 3: Up to 1 meter (3.3 feet) range with 1 mW (0 dBm) power

Most mobile devices and peripherals use Class 2, balancing range and power consumption.

Power Consumption

Bluetooth technology is designed to be power-efficient, especially with the introduction of Bluetooth Low Energy (BLE) in version 4.0. The power usage scales with the data transfer requirements and connection frequency:

• Classic Bluetooth: ~30 mA during data transfer
• Bluetooth Low Energy: as low as 15 μA in sleep mode

Frequency Hopping Spread Spectrum (FHSS)

Bluetooth uses FHSS technology to combat interference and enhance security. The signal rapidly switches among 79 different frequency channels at 1,600 hops per second, following a pseudo-random pattern known to both connected devices.

This frequent channel switching provides several benefits:

• Reduced interference from other devices using the 2.4 GHz band
• Enhanced security as the hopping pattern is difficult to intercept
• Improved reliability in crowded RF environments

Piconet and Scatternet Architecture

Bluetooth devices organize themselves into small networks called piconets:

• A piconet consists of one master device and up to 7 active slave devices
• The master device controls timing and frequency hopping patterns
• All slave devices synchronize their clocks to the master

Multiple piconets can overlap to form a scatternet, where devices can participate in multiple piconets simultaneously, serving as a bridge between different networks.

Connection Process

Establishing a Bluetooth connection involves several steps:

1. Inquiry: Device discovery phase where the master scans for nearby Bluetooth devices
2. Paging: The master initiates connection with a specific device
3. Pairing: Authentication process using PIN codes or keys
4. Connection: Establishment of a communication channel
5. Service Discovery: Devices exchange information about supported services and profiles

Once connected, devices maintain synchronization by adjusting their clocks to compensate for drift.

Protocol Stack

The Bluetooth protocol stack consists of multiple layers:

• Radio Layer: Physical radio frequency (RF) transceiver
• Baseband Layer: Handles physical channel management
• Link Manager Protocol (LMP): Manages link setup and security
• Host Controller Interface (HCI): Interface between hardware and software
• Logical Link Control and Adaptation Protocol (L2CAP): Multiplexes data between higher level protocols
• RFCOMM: Serial port emulation for legacy applications
• Service Discovery Protocol (SDP): Allows devices to discover available services
• Application Layers: User applications and profiles that define how devices use Bluetooth