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Bluetooth devices will normally operate at 2.4 GHZ

in the license free, globally available ISM radio

band. The advantage to this band includes worldwide

availability and compatibility. A disadvantage to

this 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 Bluetooth

communication more robust and more secure.

Frequency hopping

Frequency hopping is basically jumping from frequency

to frequency within the ISM radio band. After a

bluetooth device sends or receives a packet, it

and the device (or devices) it's communicating with

hop to another frequency before the next packet is

sent. This scheme offers three advantages:

1. Allows Bluetooth devices to use the

entirety of the available ISM band, while never

transmitting from a fixed frequency for more than a

short period of time. This helps insure that

Bluetooth conforms to the ISM restrictions on the

transmission 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 next

frequency.

3. Provides a base level of security as

it's very hard for an eavesdropping device to predict

which frequency the Bluetooth devices will use

next.

The connected devices however, must agree upon the

frequency they will use next. The specification

in Bluetooth ensures this in two ways. First, it

defines a master and slave type relationship between

bluetooth devices. Next, it specifies an algorithm

that uses device specific information when

calculating the frequency hop sequences.

A Bluetooth device that operates in master mode can

communicate with up to seven devices that are set in

slave mode. To each of the slaves, the master

Bluetooth device will send its own unique address

and the value of its own internal clock. The

information sent is then used to calculate the

frequency hop sequences.

Because the master device and each of the slave

devices use the same algorithm with the same initial

input, the connected devices will always arrive

together at the next frequency that they have agreed

upon.

As a replacement for cable technology, it's no

wonder that Bluetooth devices are usually battery

powered, such as wireless mice and battery powered

cell phones. To conserve the power, most devices

operate in low power. This helps to give Bluetooth

devices a range of around 5 - 10 meters.

This range is far enough for wireless communication

but close enough to avoid drawing too much power

from the power source of the device.