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.
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