Bluetooth Technology
How Bluetooth Technology Works
"Connective convenience"
Bluetooth is a high-speed,
low-power microwave wireless link technology, designed to connect phones,
laptops, PDAs and other portable equipment together with little or no work
by the user. Unlike infra-red, Bluetooth does not require line-of-sight
positioning of connected units. The technology uses modifications of
existing wireless LAN techniques but is most notable for its small size and
low cost. The current prototype circuits are contained on a circuit board
0.9cm square, with a much smaller single chip version in development. The
cost of the device is expected to fall very fast, from $20 initially to $5
in a year or two. It is envisioned that Bluetooth will be included within
equipment rather than being an optional extra. When one Bluetooth product
comes within range of another, (this can be set to between 10cm and 100m)
they automatically exchange address and capability details. They can then
establish a 1 megabit/s link (up to 2 Mbps in the second generation of the
technology) with security and error correction, to use as required. The
protocols will handle both voice and data, with a very flexible network
topography.
This technology achieves
its goal by embedding tiny, inexpensive, short-range transceivers into the
electronic devices that are available today. The radio operates on the
globally-available unlicensed radio band, 2.45 GHz (meaning there will be no
hindrance for international travelers using Bluetooth-enabled equipment.),
and supports data speeds of up to 721 Kbps, as well as three voice channels.
The bluetooth modules can be either built into electronic devices or used as
an adaptor. For instance in a PC they can be built in as a PC card or
externally attached via the USB port.

Each device has a unique 48-bit address
from the IEEE 802 standard. Connections can be point-to-point or multipoint.
The maximum range is 10 meters but can be extended to 100 meters by
increasing the power. Bluetooth devices are protected from radio
interference by changing their frequencies arbitrarily upto a maximum of
1600 times a second, a technique known as frequency hopping. They also use
three different but complimentary error correction schemes. Built-in
encryption and verification is provided.
Moreover, Bluetooth devices won't drain
precious battery life. The Bluetooth specification targets power consumption
of the device from a "hold" mode consuming 30 micro amps to the
active transmitting range of 8-30 milliamps (or less than 1/10th of a watt).
The radio chip consumers only 0.3mA in standby mode, which is less than 3 %
of the power used by a standard mobile phone. The chips also have excellent
power-saving features, as they will automatically shift to a low-power mode
as soon as traffic volume lessens or stops.
Bluetooth devices are
classified according to three different power classes, as shown in the
following table.
Power Class |
Maximum Output |
Power |
1 |
100 mW |
(20 dBm) |
2 |
2.5 mW |
(4 dBm) |
3 |
1 mW |
(0 dBm) |
But beyond untethering devices by replacing
the cables, Bluetooth radio technology provides a universal bridge to
existing data networks, a peripheral interface, and a mechanism to form
small private ad hoc groupings of connected devices away from fixed network
infrastructures. Designed to operate in a noisy radio frequency environment,
the Bluetooth radio uses a fast acknowledgment and frequency hopping scheme
to make the link robust. Bluetooth radio modules avoid interference from
other signals by hopping to a new frequency after transmitting or receiving
a packet. Compared with other systems operating in the same frequency band,
the Bluetooth radio typically hops faster and uses shorter packets. This
makes the Bluetooth radio more robust than other systems. Short packages and
fast hopping also limit the impact of domestic and professional microwave
ovens. Use of Forward Error Correction (FEC) limits the impact of random
noise on long-distance links. The encoding is optimized for an uncoordinated
environment.
Bluetooth guarantees security at the bit
level. Authentication is controlled by the user by using a 128 bit key.
Radio signals can be coded with 8 bits or anything upto 128 bits. The
Bluetooth radio transmissions will conform to the safety standards required
by the countries where the technology will be used with respect to the
affects of radio transmissions on the human body. Emissions from Bluetooth
enabled devices will be no greater than emissions from industry-standard
cordless phones. The Bluetooth module will not interfere or cause harm to
public or private telecommunications network.
The Bluetooth baseband protocol is a
combination of circuit and packet switching. Slots can be reserved for
synchronous packets. Each packet is transmitted in a different hop
frequency. A packet nominally covers a single slot, but can be extended to
cover up to five slots. Bluetooth can support an asynchronous data channel,
up to three simultaneous synchronous voice channels, or a channel, which
simultaneously supports asynchronous data and synchronous voice. It is thus
possible to transfer the date asynchronously whilst at the same time talking
synchronously at the same time. Each voice channel supports 64 kb/s
synchronous (voice) link. The asynchronous channel can support an asymmetric
link of maximally 721 kb/s in either direction while permitting 57.6 kb/s in
the return direction, or a 432.6 kb/s symmetric link.
Modes of operation
An interesting aspect of
the technology is the instant formation of networks once the bluetooth
devices come in range to each other. A piconet is a collection of devices
connected via Bluetooth technology in an ad hoc fashion. A Piconet can be a
simple connection between two devices or more than two devices. Multiple
independent and non-synchronized piconets can form a scatternet. Any of the
devices in a piconet can also be a member of another by means of time
multiplexing. i.e a device can be a part of more than one piconet by
suitably sharing the time. The Bluetooth system supports both point-to-point
and point-to-multi-point connections. When a device is connected to another
device it is a point to point connection. If it is connected to more that
one (upto 7 ) it is a point to multipoint connection. Several piconets can
be established and linked together ad hoc, where each piconet is identified
by a different frequency hopping sequence. All users participating on the
same piconet are synchronized to this hopping sequence. If a device is
connected to more than one piconet it communicates in each piconet using a
different hopping sequence. A piconet starts with two connected devices,
such as a portable PC and cellular phone, and may grow to eight connected
devices. All Bluetooth devices are peer units and have identical
implementations. However, when establishing a piconet, one unit will act as
a master and the other(s) as slave(s) for the duration of the piconet
connection. In a piconet there is a master unit whose clock and hopping
sequence are used to synchronize all other devices in the piconet. All the
other devices in a piconet that are not the master are slave units. A 3-bit
MAC address is used to distinguish between units participating in the
piconet. Devices synchronized to a piconet can enter power-saving modes
called Sniff and hold mode, in which device activity is lowered. Also there
can be parked units which are synchronized but do not have a MAC addresses.
These parked units have a 8 bit address, therefore there can be a maximum of
256 parked devices.

Voice channels use either a 64 kbps log PCM
or the Continuous Variable Slope Delta Modulation (CVSD) voice coding
scheme, and never retransmit voice packets. The voice quality on the line
interface should be better than or equal to the 64 kbps log PCM. The CVSD
method was chosen for its robustness in handling dropped and damaged voice
samples. Rising interference levels are experienced as increased background
noise: even at bit error rates up 4%, the CVSD coded voice is quite audible.
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