Introduction to Bluetooth

Bluetooth Origins

Before Bluetooth, connecting a mouse, keyboard, or printer to a computer often required separate cables and interfaces such as serial, parallel, or PS/2 ports. The different uses and limitations of these interfaces restricted mobility and complicated interoperability. This created demand for a universal, low-cost, and low-power wireless standard for short-range communication.

The technology developed at Ericsson, a Swedish telecommunications company, in the early 1990s as a wireless alternative to cumbersome RS-232 data cables. The name comes from the 10th-century Danish king Harald "Bluetooth" Gormsson. Harald Bluetooth is associated with the unification of Denmark and Norway. The creators of the technology saw a parallel: their standard was intended to bring different communication protocols and devices into one coherent ecosystem. In 1996, Intel, Ericsson, and Nokia planned to standardize the short-range technology, and in 1998 the Bluetooth Special Interest Group (SIG) was formed with companies including Ericsson, IBM, Intel, Nokia, and Toshiba. Cooperation among SIG members led to broad deployment of short-range wireless connectivity.

WPAN as a Personal Area Network

Bluetooth operates within the domain of a Wireless Personal Area Network (WPAN). To understand what a WPAN is, it helps to compare it with other types of networks you might be familiar with:

• WAN (Wide Area Network): This is a massive network that covers a huge geographical area, like a country or even the entire globe. The Internet is the ultimate example of a WAN.
• LAN (Local Area Network): This network covers a limited area like a single home, office building, or a school campus. Wi-Fi is the most common technology used for wireless LANs (WLANs).
• WPAN (Wireless Personal Area Network): It connects devices used by one person, such as a smartphone and wireless earbuds, a laptop and wireless mouse, or a smartwatch and phone. In typical use the range is intentionally short, often up to 10 meters, which suits devices you carry with you or use at your desk.

Bluetooth is one of the technologies used for WPANs because it was designed for short connections: it offers low-power modes, can be integrated into small devices, and includes mechanisms for sharing spectrum with other wireless technologies. The range of a particular product depends among other things on power class, antenna, PHY, and radio conditions.

Sharing the 2.4 GHz ISM Band with FHSS

For Bluetooth to work, its radio signals need a dedicated frequency band to travel on. The frequency chosen for Bluetooth is the widely available ISM band, specifically the 2.4 GHz band (ranging from approximately 2.400 to 2.4835 GHz). In many jurisdictions, use of this band does not require an individual frequency licence, but devices must still meet local emission requirements.

However, this advantage is also its biggest challenge. The 2.4 GHz band is heavily occupied. Your Wi-Fi network, your neighbor's Wi-Fi network, microwave ovens, some cordless phones, and even some baby monitors all operate in this same frequency space. This creates a high potential for interference, where signals from different devices can collide and corrupt each other.

To operate reliably in this noisy environment, Bluetooth Classic BR/EDR uses a technique called Frequency-Hopping Spread Spectrum (FHSS). Instead of transmitting on a single, fixed frequency, devices in a BR/EDR connection hop from one channel to another in a seemingly random pattern. Bluetooth LE uses a separate channel layout and channel-selection mechanism. The BR/EDR hopping process works as follows:

• Channel Division in Bluetooth Classic BR/EDR: In this mode, the 2.4 GHz band is divided into 79 separate channels, each 1 MHz wide.
• Rapid BR/EDR Hopping: A pair of connected Bluetooth Classic devices (for example, your phone and headset) rapidly hops between these 79 channels up to 1600 times per second.
• Shared Pattern: The hopping sequence is not fully random but is a pseudorandom pattern determined by the Central, historically called the master, in a BR/EDR connection. The Central and Peripheral, historically called the slave, know this pattern and hop in synchronization.
• Interference Avoidance: If a particular channel is momentarily blocked by interference from a Wi-Fi network or a microwave oven, a single BR/EDR transmission may be disrupted for only a small fraction of a second (about $1/1600$ of a second). The devices then move to the next channel in the sequence, while retransmissions and AFH can help maintain communication. This limits the effect of interference but does not guarantee that it will be removed completely. You can think of it as two people having a conversation in a noisy room by quickly moving between 79 different spots in a pre-agreed order.

An Organized BR/EDR Network: Central, Peripheral, and the Piconet

In the Bluetooth Classic BR/EDR model, devices form an organized miniature network called a Piconet. In Bluetooth Classic BR/EDR, a piconet is formed when devices share a physical channel controlled by the Central. It contains one Central, historically called the master, and up to seven active Peripherals, historically called slaves. All devices share a frequency-hopping channel.

The Core Specification also uses piconet for Bluetooth LE. An LE piconet physical channel is shared by exactly two connected devices, a Central and a Peripheral. One LE device can belong to more than one piconet, so the seven-device limit, LT_ADDR, and parked states described below belong to the BR/EDR model.

For example, when you connect a Bluetooth Classic headset to your smartphone, the smartphone typically acts as the Central and the headset as the Peripheral. In this BR/EDR piconet, the phone defines the communication rules.

The Limits of a Piconet

A BR/EDR piconet has a specific size limit: it can contain only one Central and up to seven active Peripheral devices. This limit is a direct result of the BR/EDR addressing scheme. Each active Peripheral is assigned a 3-bit Active Member Address (LT_ADDR), and a 3-bit address allows for $2^3 = 8$ possible values. Since the value 000 is reserved for broadcast messages, this leaves 7 unique addresses for active Peripheral devices.

A BR/EDR piconet can support more than 7 known devices by placing some of them in low-power "parked" states. These devices remain synchronized to the piconet but do not have an active member address and do not participate in traffic until the Central reactivates them.