An open specification for short-range wireless transmission of voice and data that is currently under development. Bluetooth provides a simple, low-cost method of linking Personal Digital Assistants (PDAs), cellular phones, laptops, and other information appliances.
Bluetooth can be used for bridging data networks, connecting peripherals to devices, and forming ad hoc connections between groups of information appliances. Bluetooth is the initiative of a consortium called the Bluetooth Special Interest Group (SIG), whose original members include industry leaders Ericsson, IBM, Intel, Nokia, and Toshiba. More than 850 vendors support it.
Bluetooth supports transmission of voice and data over 2.4-GHz radio frequencies, which is the unlicensed Industrial-Scientific-Medical (ISM) band, using a frequency-hopping scheme with a maximum of 1600 hops per second, resulting in a new frequency being used to transmit each packet.
This scheme allows for smooth operation—in spite of fading due to reflecting obstacles or excessive distance, and in spite of noise due to electromagnetic interference (EMI), such as that generated by microwave ovens. In addition, Bluetooth uses short packets and fast acknowledgments to increase reliability and employs forward error correction to reduce the effects of random noise.
The range of transmission for Bluetooth is typically between 0.1 and 10 meters but can be as much as 100 meters using higher transmission power. The system's automatic power adaptation adjusts transmission power to the minimum needed for reliable transmission in any given situation, which reduces the chance of eavesdropping. Bluetooth also includes encryption and authentication mechanisms. The entire Bluetooth technology is implemented in a single 9-millimeter-by-9-millimeter chip.
Bluetooth data transmission normally takes place over an asynchronous channel that provides 721 Kbps in the forward direction and 57.6 Kbps in the return direction, but synchronous data transmission at 432.6 Kbps in both directions is also supported. Time-division duplexing (TDD) is employed to alternate transmission between the two directions and thus provide full-duplex communication.
Each TDD slot normally carries one packet, but packets can be spread across up to five slots. Signaling is baseband and uses a binary FM scheme. Channels can be routed by using a combination of circuit switching and packet switching.
Bluetooth voice transmission can use up to three concurrent synchronous 64-Kbps voice-only channels or one channel that simultaneously supports both asynchronous data and synchronous voice transmission. The voice channels use the continuous variable-slope delta modulation coding scheme.
Bluetooth supports concurrent connections among up to eight devices, forming what is called a piconet. Each device is temporarily assigned a unique 3-bit MAC address for the duration of the connection.
A master/slave relationship exists between one device and all other devices for the duration of the connection for the purpose of establishing clocking and the hopping sequence. In all other respects, the devices operate as peers during a connection. Unconnected devices are in standby mode and listen for connection attempts every 1.28 seconds on each of 32 preassigned hopping frequencies.
Link setup and authentication is performed using the Link Manager Protocol (LMP), which uses the link controller services built into the chip. Connections between devices can be either point-to-point or point-to-multipoint, and piconets can be joined, with each piconet having a different hopping sequence.