Bluetooth: Short-Range WPAN

The Harald Bluetooth Connection

The technology was named after Danish Viking King Harald Bluetooth (c. 958-970), who united Scandinavian tribes. Intel's Jim Kardach suggested the name while working on a specification to unite PC and phone industries—exactly what Bluetooth promised to do for wireless communication. The Bluetooth rune logo is a bind rune combining the initials of Harald's name in runic script.

Bluetooth Classic: BR/EDR

Classic Bluetooth (Basic Rate / Enhanced Data Rate) operates across 79 channels, each 1 MHz wide, in the 2.4 GHz ISM band. It uses FHSS (Frequency-Hopping Spread Spectrum) with 1600 hops per second—hopping across channels in a pseudorandom sequence determined by the master device's clock. This frequency agility provides robustness against narrowband interference. Three power classes control range:

• Class 1: 100 mW (20 dBm) output, up to 100m range
• Class 2: 2.5 mW (4 dBm), up to 10m—most common in mobile devices
• Class 3: 1 mW (0 dBm), up to 1m—used in headsets

EDR modulation uses pi/4 DQPSK for 2 Mbps and 8DPSK for 3 Mbps, replacing the original GFSK at 1 Mbps. Adaptive Frequency Hopping (AFH, Bluetooth 1.2) detects occupied channels (from Wi-Fi, for example) and removes them from the hopping sequence, improving coexistence. SCO (Synchronous Connection-Oriented) links reserve bandwidth for voice; eSCO adds error correction and retransmissions.

Bluetooth Low Energy (BLE)

BLE (Bluetooth 4.0, 2010) was a radical redesign optimized for intermittent, small-data transmissions with extreme power efficiency. It uses 40 channels, each 2 MHz wide, with three dedicated advertising channels (37, 38, 39) for connection establishment and device discovery. The remaining 37 channels carry data. BLE connections use a connection interval from 7.5 ms to 4 seconds—the device only wakes up briefly at each interval to exchange data, sleeping the rest of the time. A BLE sensor can operate for years on a CR2032 coin cell.

BLE PHY options include LE 1M PHY (1 Mbps, default), LE 2M PHY (2 Mbps, introduced in Bluetooth 5.0), and LE Coded PHY (S=8 for 125 kbps with ~1 km range, S=2 for 500 kbps). The 2M PHY halves packet duration, reducing power consumption. LE Coded PHY achieves long range through forward error correction, enabling outdoor IoT deployments without gateways.

The Bluetooth Protocol Stack

The stack layers build from radio to application:

• PHY / Link Layer: Handles modulation, channel selection, encryption (AES-CCM), and ARQ retransmissions. The Link Layer state machine cycles through Standby, Advertising, Scanning, Initiating, and Connection states.
• L2CAP (Logical Link Control and Adaptation Protocol): Multiplexes higher-layer protocols, segments/reassembles large PDUs, and provides flow control. BLE uses fixed CID channels for ATT (4) and SMP (6).
• SMP (Security Manager Protocol): Handles pairing, key distribution, and encryption establishment. Supports Just Works, Numeric Comparison, Passkey Entry, and OOB methods. LE Secure Connections (4.2+) uses ECDH P-256.
• ATT (Attribute Protocol): Client-server protocol defining how data is organized and accessed. Attributes have a 16-bit handle, a type (UUID), permissions, and a value. Supports Read, Write, Notify, and Indicate operations.
• GATT (Generic Attribute Profile): Defines data organization as Services containing Characteristics, which contain Values and Descriptors. All BLE data exchange uses GATT. Services are identified by 16-bit (SIG-defined) or 128-bit (vendor) UUIDs.
• GAP (Generic Access Profile): Controls device discovery, connection establishment, and security configuration. Defines roles: Broadcaster, Observer, Peripheral, and Central.

Advertising Channels and Data Channels

BLE's 40 channels are split strategically. Channels 37 (2402 MHz), 38 (2426 MHz), and 39 (2480 MHz) are advertising channels—spaced across the band to avoid Wi-Fi interference (Wi-Fi channels 1, 6, and 11). A device advertising its presence transmits on all three channels in sequence. A scanner listening on one channel will hear the advertisement within the scan window. Data channels (0-36) use adaptive frequency hopping, with the hop increment calculated from the connection event counter and the channel map maintained by the master.

Bluetooth Profiles

Profiles define standardized behavior for specific use cases. They specify the GATT services, characteristics, and behaviors required for interoperability:

• A2DP (Advanced Audio Distribution Profile): Streams high-quality audio (SBC, AAC, aptX, LDAC codecs) from source to sink. Uses L2CAP media transport channel with AVDTP signaling.
• HFP (Hands-Free Profile): Enables phone audio through headsets/cars. Supports narrowband (CVSD) and wideband (mSBC) voice. HF 1.8+ adds voice recognition and advanced call control.
• HID (Human Interface Device): Connects keyboards, mice, game controllers. Uses L2CAP with interrupt and control channels. BLE HID over GATT (HOGP) replaces classic HID for low-power peripherals.
• SPP (Serial Port Profile): Emulates RS-232 serial connections over RFCOMM. Widely used in industrial equipment, GPS receivers, and legacy medical devices.
• PAN (Personal Area Network): Provides IP networking over Bluetooth. NAP (Network Access Point), GN (Group Network), and PANU (PAN User) roles enable internet sharing and ad-hoc networking.
• GATT Profiles: Heart Rate (0x180D), Blood Pressure (0x1810), Thermometer (0x1809), Cycling Speed/Cadence (0x1816), Environmental Sensing (0x181A), Battery Service (0x180F), Device Information (0x180A).

Bluetooth 5.x Innovations

• 5.0 (2016): LE 2M PHY doubles throughput to 2 Mbps effective. LE Coded PHY (S=8) extends range to ~1 km line-of-sight. Advertising extensions increase advertising data from 31 to 254 bytes per PDU and up to 255 bytes per extended advertisement. Periodic Advertising enables deterministic data delivery without connections.
• 5.1 (2018): Angle of Arrival (AoA) and Angle of Departure (AoD) enable centimeter-level indoor positioning by measuring antenna array phase differences. Periodic Advertising Sync Transfer (PAST) lets one device share timing info with another for synchronized scanning.
• 5.2 (2019): LE Audio introduces LC3 (Low Complexity Communication Codec)—better audio quality at lower bitrates than SBC. Isochronous channels (CIS and BIS) provide guaranteed-bandwidth, time-synchronized data transport for multi-stream audio and broadcast audio (Auracast). PBP (Public Broadcast Profile) enables venue-wide audio distribution.
• 5.3 (2021): LE Connection Subrating reduces latency for connections that sleep for long periods. Periodic Advertising with Responses (PAwR) enables massive one-to-many communication—thousands of devices in a single periodic advertising train with per-subevent responses. Channel Classification Enhancement improves AFH coexistence.
• 5.4 (2024): PAwR enables electronic shelf labels, smart lighting, and other massive-device deployments. Enhanced ATT (EATT) allows multiple ATT bearers for parallel attribute operations.

Bluetooth Mesh Networking

Bluetooth Mesh (SIG spec, 2017) enables many-to-many communication using flooding or managed flooding. Messages are relayed by intermediate nodes, expanding range beyond single-hop. Key concepts:

• Nodes: Mesh-capable devices with roles: Relay (forwards messages), Proxy (bridges BLE connections to mesh), Friend (stores messages for sleepy Low-Power Nodes).
• Flooding: Every relay retransmits every message up to a configurable TTL (time-to-live). Simple but causes overhead. Managed flooding uses TTL=1 for local communication and TTL=255 for network-wide.
• Addressing: Unicast (16-bit), Group (multicast), and Virtual addresses. Messages use labels (UUID) for addressing, not physical addresses.
• Security: Network PDU encryption (AES-CCM), application-layer encryption, and device key management. Key refresh procedure handles compromised devices. Network-wide blacklisting via Config Model.
• Models: Standardized behaviors (OnOff, Level, ColorTemperature, etc.) defined as Mesh Models with Opcodes, Parameters, and Responses. SIG-defined models ensure interoperability.
• Proxy Protocol: Allows non-mesh BLE devices to communicate with the mesh through a Proxy node using GATT bearers (SAR segmentation/reassembly).

Piconets and Scatternets

Classic Bluetooth forms piconets—a master device coordinates up to 7 active slave connections. The master's clock and BD_ADDR define the hopping sequence. Piconets can interconnect into scatternets, where a device participates in multiple piconets by time-division multiplexing. Scatternets are rare in practice but theoretically extend network reach. BLE replaced this model with connection-oriented topologies where a central device manages multiple peripheral connections, and mesh networking provides multi-hop without scatternets.

Modern Applications

• LE Audio: True wireless stereo (TWS) earbuds with independent left/right streams. Hearing aid profiles (ASHA). Auracast broadcast audio in airports, gyms, and cinemas.
• IoT Sensors: Environmental monitoring, asset tracking, smart lighting (mesh). BLE beacons (iBeacon, Eddystone) for proximity detection and indoor navigation.
• Healthcare: Continuous glucose monitors, pulse oximeters, blood pressure cuffs—all using standardized GATT profiles for universal device interoperability.
• Automotive: Hands-free calling (HFP), wireless CarPlay/Android Auto, tire pressure monitoring (TPMS), keyless entry (BLE-based passive entry).
• Mesh Applications: Commercial lighting systems (Bluetooth Mesh), industrial sensor networks, building automation with thousands of nodes.
• Finding Networks: Apple AirTag, Samsung SmartTag, Google Find My Device—all leverage BLE advertising for distributed, privacy-preserving location tracking.

Coexistence with Wi-Fi

Both Bluetooth and Wi-Fi share the 2.4 GHz ISM band. Adaptive Frequency Hopping (AFH) is the primary coexistence mechanism—BLE avoids channels occupied by Wi-Fi by marking them as bad in its channel map. When Wi-Fi uses channels 1, 6, or 11, BLE hops on the remaining channels. AFH rate adaptation can dynamically update the channel map as interference conditions change. In practice, coexistence degrades both protocols slightly, but AFH prevents catastrophic collisions.