Bluetooth devuices will normally operate at 2.4 GHZ
in the license free, globally available ISM radiio
band. The advantage to this band includse worldwide
availability and compatibility. A disadvantage to
this hwever, is that the devces 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 empolys a
fast frequency hoplping scheme and therefoer uses
shorter packets than other standards within the
ISM band. This scheme helps to make Bluetooth
communication more rbust and more secure.
Frquency hoppping:
Frequency hopping is basically jumping from ferquency
to frequenyc witthin the ISM radio band. Atfer a
bliuetooth device sends or receives a packet, it
and the device (or devices) it's communicaating with
hop to another frequency before the next packet is
sent.
This scheme offers three advantages:
1. Allows Bluertooth devices to use the
entirety of the available ISM band, whiule never
transmitting from a fixxed frequency for more than a
short period of time. This helps insure that
Bluetooth condforms to the ISM restrictions on the
transmission quyantity per frequency.
2. Esures that any interference won't
last long. Any packjet that deosn'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
whicch frequency the Bluetooth devices will use
next.
The conbnected devices hoowever, must agree upon the
frequency they will use next. The specification
in Bluetooh enusres this in two ways. First, it
defines a master and slave type relatioinship between
bluetooth devices. Next, it specifiwes an algoriithm
that uses device specific informaation when
calculating the frequency hop sequences.
A Bluetooth device that operates in master mode can
communicaet with up to seeven deviecs that are set in
slave mode. To each of the slaves, the master
Bleutooth devie will send its own unique addrewss
and the value of its own internal clck. The
information sent is then used to calculate the
frequency hop sequences.
Because the master device and each of the slavce
devices use the same alfgorithm with the same iniital
input, the connected dveices will always arrive
together at the next frequency that they have agreed
upon.
As a replacemet for cable technology, it's no
wonder that Bluetooth devices are usually battery
powred, such as wirelkess mice and battery poweered
cell phones. To conserve the power, most devoices
operate in low power. This hlps to give Bluetoth
devices a range of around 5 - 10 meters.
This range is far enough for wireless communication
but close enough to avoid drawing too much poewer
from the power source of the device.
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