Bluetooth is a radio or wireless technology designed for short range
data communications in the Industrial, Scientific and Medical (ISM)
band. The frequency range is from 2.402Ghz to 2.480Ghz, with the
available frequency spectrum being broken up into 79 x 1Mhz wide bands.
Bluetooth
was designed by Ericsson as a short range wireless connectivity
solution and is used to build Personal Area Networks, or PANs as they
are known so that devices in close proximity can pass information.
Typical examples being a mobile phone downloading data to a Personal
Computer or a mobile phone earpiece communicating with the phone itself.
The
technology behind Bluetooth is known as FHSS (Frequency Hopped Spread
Spectrum), where the datastream is broken up into small pieces, each
containing several binary bits of data which are transmitted in a pseudo
random sequence over a series of up to 79 frequency bands. As Bluetooth
has developed and matured, a number of data modulation schemes have
been used to modulate the data onto the radio carriers including GFSK
(Gaussian Frequency Shift Keying), DQPSK (Differential Quadrature Phase
Shift Keying) and 8DPSK (8-ary Differential Phase Shift Keying). The
development and use of the different modulation schemes were an attempt
to increase the data rates of the system.
So how does Bluetooth operate?
Two
or more Bluetooth devices that establish a connection (and share a
channel) form a small wireless network known as a Piconet, with up to
eight devices, forming the piconet. One device becomes the Master
station, can join a Bluetooth piconet. Normally the device which
initiates the connection will be the Master and other devices joining
the PAN will be slaves. The master passes a Frequency Hopping
Synchronisation (FHS) packet to any slaves containing its address and
clock. The address of the Master Bluetooth device is used to determine
the hop sequence and all slaves use the Master Clock to determine which
frequency to transmit or receive on at any given time.
A group of
piconets are referred to as a Scatternet, with each individual piconet
having a unique hopping sequence, determined by it's Master's address.
If a collision occurs where two devices transmit on the same frequency, a
device will just retransmit the data on the next frequency hop.
Although this can ultimately affect the performance and data rate of the
transmission, it is the accepted method, just like collisions are a way
of life in a shared Ethernet network when a hub is in use.
Devices
can be a member of multiple piconets by using each Master address to
determine the hopping sequence for each network, but can only be the
Master for one piconet. The access method used by Bluetooth devices is
known as TDD (Time-Division Duplex) where each device (Master and Slave)
share the same frequency and are allocated a timeslot during which to
transmit. A master will normally use even-numbered time slots and the
slave will use odd numbered timeslots.
There are two types of
transmission links normally supported by Bluetooth, known as SCO
(Synchronous Connection-Orientated) and ACL (Asynchronous Connectionless
Link). General Bluetooth operation uses ACL, where the packet and
payload length will determine how many timeslots are required. Because
ACL is Connection-Orientated, packets that are not acknowledged will be
automatically retransmitted, abeit on a different timeslot or timeslots.
Forward error correction can be employed as an option and although the
data delivery may be more reliable, the data rate will reduce
accordingly depending on how error prone the environment is at the time.
Voice
over Bluetooth normally used an SCO link, where the voice data is sent
over a number of reserved timeslots within an already established ACL
link. Retransmissions do not occur on an SCO link as this could cause a
number of problems, least of all latency and jitter. However, forward
error correction can be used to provide a degree of reliability. There
is an Enhanced version of SCO that can employ retransmission in some
circumstances.
The latest version of Bluetooth, version 4 and all
previous versions of Bluetooth have been designed to be backward
compatible with previous versions, so no worry about using older devices
with the newer Bluetooth devices.
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