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How Bluetooth Works

Bluetooth devices will normally operate at 2.4 GHZ in the license free, globally available ISM radioband. The advantage to this band includes worldwideavailability and compatibility. A disadvantage tothis however, is that the devices must share this band with other RF emitters. This includes automobile security systems, other wireless devices,and other noise sources, such as microwaves.

To overcome this challenge, Bluetooth employs a fast frequency hopping scheme and therefore uses shorter packets than other standards within the ISM band. This scheme helps to make Bluetoothcommunication more robust and more secure.

Frequency hoppingFrequency hopping is basically jumping from frequencyto frequency within the ISM radio band. After a bluetooth device sends or receives a packet, it and the device (or devices) it's communicating withhop to another frequency before the next packet issent. This scheme offers three advantages: 1. Allows Bluetooth devices to use theentirety of the available ISM band, while never transmitting from a fixed frequency for more than ashort period of time. This helps insure that Bluetooth conforms to the ISM restrictions on thetransmission quantity per frequency. 2. Ensures that any interference won't last long. Any packet that doesn't arrive safely to its destination can be resent to the nextfrequency. 3. Provides a base level of security asit's very hard for an eavesdropping device to predictwhich frequency the Bluetooth devices will usenext.

The connected devices however, must agree upon thefrequency they will use next. The specification in Bluetooth ensures this in two ways. First, itdefines a master and slave type relationship betweenbluetooth devices. Next, it specifies an algorithmthat uses device specific information when calculating the frequency hop sequences.

A Bluetooth device that operates in master mode cancommunicate with up to seven devices that are set inslave mode. To each of the slaves, the masterBluetooth device will send its own unique addressand the value of its own internal clock. Theinformation sent is then used to calculate thefrequency hop sequences.

Because the master device and each of the slavedevices use the same algorithm with the same initialinput, the connected devices will always arrive together at the next frequency that they have agreedupon.

As a replacement for cable technology, it's no wonder that Bluetooth devices are usually batterypowered, such as wireless mice and battery poweredcell phones. To conserve the power, most devicesoperate in low power. This helps to give Bluetoothdevices a range of around 5 - 10 meters.

This range is far enough for wireless communicationbut close enough to avoid drawing too much powerfrom the power source of the device.

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