If you have ever had a tangle of computer wires clutter your desk, you can appreciate Bluetooth, which sends data wirelessly over short distances. Invented in 1994, the Bluetooth communications standard uses the same radio wavelength and frequencies as cordless telephones and baby monitors. Bluetooth uses software and a range of radio frequencies to dynamically encode data.
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Bluetooth uses radio frequencies between 2.4 and 2.483Ghz. The Federal Communications Commission set this range aside for low-power general use, called the industrial, scientific and medical band. The Bluetooth standard breaks the ISM radio range into 79 separate channels and sends data over combinations of them.
You can calculate the wavelength for any radio signal by dividing the speed of light in meters per second by the signal's frequency. For Bluetooth, divide 300,000,000 by 2,400,000,000 to obtain .125 meters or 12.5 cm. The relatively short wavelength allows for a correspondingly short antenna, saving space in Bluetooth devices.
As Bluetooth is designed to transmit signals only up to 10 meters or 33 feet, its radio broadcast power level is very low – 2.5 milliwatts, or thousandths of a watt. It is intended to connect devices such as headsets to cellphones and fax machines to computers, so long distance power is not necessary. Devices such as cellphones need a range of several miles, so they broadcast with a few watts of power, a thousand times as much as Bluetooth does.
Adaptive Frequency Hopping
A computer chip selects the 79 channels of the ISM band automatically to prevent eavesdropping and interference with other equipment. Though your cordless phone uses the same frequency band, Bluetooth avoids channels already in use. Through a technology called adaptive frequency hopping, Bluetooth changes channels 1,600 time per second to make the best use of local radio conditions.