What is Bluetooth?
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.
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.
The Bluetooth technologies have allowed us to provide fast data communications between
devices that are in close proximity (within a few metres) without the need for a cable running
RS-232 protocol for example and so have provided us with mobility free from the constraints
imposed with the use of copper wiring.
David Christie is MD at NSTUK Ltd, a Technical Training and Consultancy company based
in the Northeast of England. David delivers technical training in the area of Data Communications
and Telecoms and also provides consultancy and Training Needs Analysis. The company runs an
ecommerce website specialising in the sale of Networking hardware and consumer electronics.
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