Bluetooth technology
- a standard for wireless networking of small peripherals
Bluetooth has now established itself in the market place enabling a variety
of devices to be connected together using wireless technology. Bluetooth
technology has come into its own connecting remote headsets to mobile phones,
but it is also used in a huge number of other applications as well.
Beginnings ...
Bluetooth technology originated in 1994 when Ericsson came up with a concept to
use a wireless connection to connect items such as an earphone and a cordless
headset and the mobile phone. The idea behind Bluetooth (it was not yet called
Bluetooth) was developed further as the possibilities of interconnections with a
variety of other peripherals such as computers printers, phones and more were
realised. Using this technology, the possibility of quick and easy connections
between electronic devices should be possible.
It was decided that in order to enable the technology to move forward and be
accepted as an industry standard that it needed to be opened up as an industry
standard. Accordingly, in Feb 1998, five companies (Ericsson, Nokia, IBM,
Toshiba and Intel) formed a Special Interest Group (SIG). Three months later in
May 1998, Bluetooth was publicly announced with the first specification
following on with the first release of the standard in July 1999. Later more
members were added to the group with four new companies, Motorola, Microsoft,
Lucent and 3Com, joining the group. Since then more companies have joined and
the specification has grown and is now used in a large variety of products.
The name
The name of the Bluetooth standard originates from the Danish king Harald
Bl�tand who was king of Denmark between 940 and 981 AD. His name translates as
"Blue Tooth" and this was used as his nickname. A brave warrior, his main
achievement was that of uniting Denmark under the banner of Christianity, and
then uniting it with Norway that he had conquered. The Bluetooth standard was
named after him because Bluetooth endeavours to unite personal computing and
telecommunications devices.
Basics
Bluetooth is a wireless data system and can carry data at speeds up to 721 Kbps
in its basic form and in addition to this it offers up to three voice channels.
Bluetooth technology enables a user to replace cables between devices such as
printers, fax machines, desktop computers and peripherals, and a host of other
digital devices. Furthermore, it can provide a connection between an ad hoc
wireless network and existing wired data networks.
The technology is intended to be placed in a low cost module that can be
easily incorporated into electronics devices of all sorts. Bluetooth uses the
licence free Industrial, Scientific and Medical (ISM) frequency band for its
radio signals and enables communications to be established between devices up to
a maximum distance of 100 metres.
RF system
Running in the 2.4 GHz ISM band, Bluetooth employs frequency hopping techniques
with the carrier modulated using Gaussian Frequency Shift Keying (GFSK).
With many other users on the ISM band from microwave ovens to Wi-Fi, the
hopping carrier enables interference to be avoided by Bluetooth devices. A
Bluetooth transmission only remains on a given frequency for a short time, and
if any interference is present the data will be re-sent later when the signal
has changed to a different channel which is likely to be clear of other
interfering signals. The standard uses a hopping rate of 1600 hops per second.
These are spread over 79 fixed frequencies and they are chosen in a
pseudo-random sequence. The fixed frequencies occur at 2400 + n MHz where the
value of n varies from 1 to 79. This gives frequencies of 2402, 2404 �.. 2480
MHz. In some countries the ISM band allocation does not allow the full range of
frequencies to be used. In France, Japan and Spain, the hop sequence has to be
restricted to only 23 frequencies because of the ISM band allocation is smaller.
During the development of the Bluetooth standard it was decided to adopt the
use of frequency hopping system rather than a direct sequence spread spectrum
approach because it is able to operate over a greater dynamic range. If direct
sequence spread spectrum techniques were used then other transmitters nearer to
the receiver would block the required transmission if it is further away and
weaker.
Modulation
The way in which the data is modulated onto the Bluetooth carrier was also
carefully chosen. A form of frequency shift keying known as Gaussian Frequency
Shift Keying is employed. Here the frequency of the carrier is shifted to carry
the modulation. A binary one is represented by a positive frequency deviation
and a binary zero is represented by a negative frequency deviation. It is then
filtered using a filter with a Gaussian response curve to ensure the sidebands
do not extend too far either side of the main carrier. By doing this it achieves
a bandwidth of 1 MHz with stringent filter requirements to prevent interference
on other channels. For correct operation the level of BT is set to 0.5 and the
modulation index must be between 0.28 and 0.35.
Transmitter power levels
The transmitter powers for Bluetooth are quite low, although there are three
different classes of output dependent upon the anticipated use and the range
required. Power Class 1 is designed for long range communications up to about
100m devices, and this has a maximum output power of 20 dBm, Next is Power Class
2 which is used for what are termed for ordinary range devices with a range up
to about 10m, with a maximum output power of 4 dBm. Finally there is Power Class
3 for short range devices. This support communication only to about 10cm and it
has a maximum output power of 0 dBm.
There are also some frequency accuracy requirements for Bluetooth
transmissions. The transmitted initial centre frequency must be within �75 kHz
from the receiver centre frequency. The initial frequency accuracy is defined as
being the frequency accuracy before any information is transmitted and as such
any frequency drift requirement is not included.
In order to enable effective communications to take place in an environment
where a number of devices may receive the signal, each device has its own
identifier. This is provided by having a 48 bit hard wired address identity
giving a total of 2.815 x 10^14 unique identifiers.
Data transfer
There are two ways in which data is transferred. The first is by using what is
termed an Asynchronous Connectionless Communications Link (ACL). This is used
for file and data transfers. A second method is termed a Synchronous
Connection-orientated Communications Link (SCL). This is used for applications
such as digital audio.
The ACL is enables data to be transferred via Bluetooth at speeds up to the
maximum rate of 732.2 kbits/sec. This occurs when it is operating in an
asymmetric mode. This is commonly used because for most applications there is
far more data transferred in one direction than the other. When a symmetrical
mode is needed with data transferred at the same rate in both directions, the
data transfer rate falls to 433.9 kbits/sec. The synchronous links support two
bi-directional connections at a rate of 64 kbits/sec. The data rates are
adequate for audio and most file transfers. However the available data rate is
insufficient for applications such as high rate DVDs that require 9.8 Mbit/sec
or for many other video applications including games spectacles.
Data is organised into packets to be sent across a Bluetooth link. The
Bluetooth specification lists seventeen different formats that can be used
dependent upon the requirements. They have options for elements such as forward
error correction data and the like. However the standard packet consists of a 72
bit access code filed, a 54 bit header field, and then the data to be
transmitted which may be between 0 and 2745 bits. This data includes the 16 bit
CRC if it is needed.
As it is likely that interference will cause errors, error handling is
incorporated within the system. For asynchronous links packet sequence numbers
are transmitted. If an error is detected in a packet then the receiver can
request it to be re-sent. Error coding using a 16 bit CRC is also available. For
the synchronous links packets cannot be re-sent as there is unlikely to be
sufficient bandwidth available to re-send data and "catch up". However it is
possible to include some forward error control.
Communication nets
In order that Bluetooth devices may communicate with each other they form small
clusters or nets. These are termed "piconets" and comprise up to eight devices.
Within a piconet, one of the devices assumes the role of "Master" while the
others become slaves.
If more than eight devices are available to join the net, eight are allowed
in, and the others need to remain in an inactive standby state. They may be
requested to join the net at a later state if required.
To enable a net to eb set up, the master transmits an enquiry message every
1.28 seconds to discover whether there are any other devices within range. If a
reply is received then an invitation to join the net is transmitted to the
specific device that has responded. After this the master allocates each device
a member address and then controls all the transmissions.
All Bluetooth devices have a clock that runs at twice the hopping speed and
this provides synchronisation to the whole net. The master transmits in the even
numbered time slots whilst the slaves transmit in the odd numbered slots once
they have been given permission to transmit.
As security is becoming an important issue, especially where links to
computers are concerned, secure communications are possible over Bluetooth with
the devices encrypting the data transmitted. A key up to 128 bits is used and it
is claimed that the level of security provided is sufficient for financial
transactions. However in some countries the length of the key is limited to
enable the security agencies to gain access if required.
Summary
Bluetooth is well established, but despite this further enhancements are being
introduced. Faster data transfer rates, and greater flexibility. In addition to
this efforts have been made to ensure that interoperation has been improved so
that devices from different manufacturers can talk together more easily.
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