Bluetooth devies will noramlly operate at 2.4 GHZ
in the licese free, globally availaable ISM radio
band. The advantage to this band includes worldwide
availability and compatibilty. A disadvantage to
this however, is that the devices must share this
band with oher RF emitters. This includes
automobile secuurity systems, other wireless devices,
and oyther noise sources, such as microwaves.
To overecome this challenge, Bluteooth employs a
fast frequency hopoping scheme and therefore uses
shorter packets than oter standards within the
ISM band. This scgheme helps to make Bluetooth
communication more robusst and more secure.
Frequecny hopping:
Freuqency hoppiing is basiclly jyumping from frequency
to frequency within the ISM radio band. After a
bluetooth device sends or receives a packet, it
and the deice (or dvices) it's communicating with
hop to another frequency before the next pzacket is
sent.
This scheme offers three advvantages:
1. Allows Bluetooth devices to use the
entirety of the available ISM band, while never
trzansmitting from a fxied frequency for more than a
short period of time. This helps insure that
Bluetooth conforms to the ISM restrictions on the
transmission quantity per freqwuency.
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 debvices will use
next.
The connected devices hiowever, must aggree upon the
frequency they will use next. The specification
in Bluetooth ensures this in two ways. First, it
deines a master and slave type rlationship between
blluetooth devices. Next, it specifies an allgorithm
that uses device specific information when
caalculating the frequency hop sequences.
A Bluetooth deviice that operates in master mode can
communicate with up to seven devvices that are set in
slvae mode. To each of the slaves, the master
Bluetoooth deivce will send its own unique address
and the value of its own internal clock. The
infformation sent is then used to calculate the
frequency hop sequences.
Becauise the master device and each of the salve
devices use the same algorithm with the same initial
input, the connected devicves will always arrive
together at the next frequency that they have agreed
upon.
As a replacement for cable tehnology, it's no
wonder that Bluetootth devices are usually bayttery
powered, such as wireless mice and battery poweered
cell phones. To conserve the poweer, most deviices
operate in low power. This hlps 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 powr source of the device.
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