What's in a Bluetooth System?

Bluetooth is a short-range wireless technology that allows both voice and data to be transmitted between electronic devices. Originally conceived as a means to implement hands-free mobile phone use without a cable between the handset and the headset, companies have developed some very exciting, futuristic applications for Bluetooth. With this technology, people really could send and receive voice or data to or from any device or person in a network using a small lapel-mounted device a la Star Trek.

The Bluetooth Special Interest Group (SIG), which was founded in 1998 by Ericsson, IBM, Nokia, and Toshiba, authored the current standard. Today, over 2000 members of the Bluetooth SIG as well as vendors of both integrated circuits and development tools have helped designers to create Bluetooth end products.

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There are four basic parts to any Bluetooth system: a radio frequency (RF) component, a baseband or link control unit, link management software, and the supporting application software.

Bluetooth Radio—The Bluetooth radio is a short-distance, low-power radio operating in the unlicensed spectrum of 2.4-gigahertz (GHz). The radio uses a nominal antenna power of 0-dBm (1-mW) and has a range of 10 meters (33-feet). Optionally, a range of 100 meters (about 328-feet) may be achieved by using an antenna power of 20-dBm (100-mW). Data is transmitted at a maximum rate of up to 1-Mbits-per-second (Mbps). However, communication protocol overhead limits the practical data rate to a little over 721-Kbits-per-second (Kbps).

Figure 1:  Block diagram of a typical Bluetooth radio.

The Bluetooth radio employs spectrum spreading, whereby the transmission hops among 79 different frequencies between 2.402- and 2.480-GHz at nominal rate of 1600-hops/s. Spectrum spreading minimizes interference from other devices in the 2.4-GHz band, such as microwave ovens and other wireless networks. If a transmission encounters interference, it waits 1/1600^th of a second (625-µsec) for the next frequency hop and retransmits on a new frequency. Frequency hopping also provides data security because two packets of data are never sent consecutively over the same frequency, and the changing frequencies are pseudo-random.

Bluetooth Baseband and Link Controller—The second part of a Bluetooth system is the Link Controller, the supervisory function which handles all the Bluetooth baseband functions, that include encoding voice and data packets, error correction, slot delimitation, frequency hopping, radio interface, data encryption, and link authentication. In addition, the Link Controller also executes Link Management software.

The Bluetooth standard supports two link types: the synchronous connection oriented (SCO) link, used primarily for voice communications, and the asynchronous connection-less (ACL) link, used for packet data. Each link type supports 16 different packet types. Any two devices in a Bluetooth system may use either link type and may change link types during a transmission. Over these links, Bluetooth supports an asynchronous data channel, three synchronous voice channels at 64-Kbps, or simultaneous asynchronous data and synchronous voice channels. The asynchronous channel can support an asymmetric link of 721-Kbps in either direction and 57.6-Kbps in the return direction or a 432.6-Kbps symmetric link.

Link Manager—The third component of a Bluetooth system, the Bluetooth Link Manager software governs the communication between various Bluetooth devices, such as a phone and a PC. The Link Manager's control includes setting up the communication link and performing authentication, configuration, and other protocols.

Application Software—The fourth component of a Bluetooth system is the application software embedded in the end product (e.g., PDA, mobile phone, or keyboard). All Bluetooth devices are required to have Bluetooth-compatible sections in the application software so that any Bluetooth device will work with any other one.