How Bluetooth Works

Bluetooth is a short-range, frequency-hopping spread spectrum (FHSS) radio service that operates in the same unlicensed 2.4 GHz frequency range as WiFi networks and many cordless telephones, microwave ovens, baby monitors, garage door openers, and other wireless products. Frequency-hopping spread spectrum means that a Bluetooth transmitter splits the radio signal into very small segments and hops among 79 different frequencies 1,600 times per second to reduce interference and fading.

Because a Bluetooth radio is both a transmitter and a receiver, it's sometimes known as a transceiver. In order to further reduce the likelihood of interference, Bluetooth transmitters use very weak signals. The output power of the most common Class 2 Bluetooth radios is just 2.5 milliwatts (0.0025 watts) or less, which gives the signal a range of about ten meters (30 feet) in free space. By comparison, a cellular mobile telephone typically transmits at .6 watts or 3 watts. More powerful Class 1 Bluetooth radios with a maximum range of about 100 meters (320 feet) are also available.

Computers, peripherals, and other Bluetooth devices within range of one another form a piconet (pico is Greek for "really small") that includes one master device and one or more slaves. The master sets the frequency-hopping pattern and clock, and sends a synchronizing signal to all of the slaves, so the master maintains a two-way communications channel with each slave; individual slaves can't communicate with each other, except through the master. The channel is divided into time units called slots that carry data from one device to another between frequency hops.

Each device identifies itself with an address that lets other Bluetooth devices know what it is. For example, all keyboards have addresses within one range, and telephone handsets have addresses in another range. When a host or base station creates a new piconet (or adds a device to an existing piconet), it only communicates with devices whose addresses are within the right range (in Windows, the Bluetooth Properties dialog box specifies the types of slave devices). When a second Bluetooth host communicates with a different address range (say, a mobile phone and headset), it creates another piconet that uses a different frequency-hopping pattern. Most of the time, the two frequency-hopping patterns don't overlap, so the two piconets don't interfere. If they do try to use the same frequency at the same time, both masters back off and try different frequencies. Each piconet is limited to a master and seven slaves, but there's no practical limit to the number of piconets that can run in the same area.

When one Bluetooth device wants to communicate with another device on the same piconet, the master and slave exchange control signals in a format called the link manager protocol (LMP) or access control list (ACL). These signals specify the target device to which the signal is directed and the action that the originating device wants the target to take.

All of this radio communication is invisible to a user. Once you set the Bluetooth configuration options in Windows, the computer should automatically detect nearby Bluetooth devices and automatically establish communication links with them. If there are other Bluetooth devices in the same room, the computer ignores them because their addresses identify them as the wrong type of device.

Tip 

Not every wireless peripheral uses Bluetooth technology. Even if they use 2.4 GHz radios, many keyboards, mice, and other small peripherals communicate with some other kind of wireless link. If you're trying to use a Bluetooth interface on your computer, make sure the peripheral devices all carry a Bluetooth logo.