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Bluetooth is an omnidirectional wireless technology that provides
limited-range voice and data transmission over the unlicensed
2.4-GHz frequency band, allowing connections with a
wide variety of fixed and portable devices that normally would
have to be cabled together. Up to eight devices—one master
and seven slaves—can communicate with one another in a socalled
piconet at distances of up to 30 feet.
Applications
Among the many things users can do with Bluetooth is swap
data and synchronize files merely by having the devices
come within range of one another. Images captured with a
digital camera, for example, can be dropped off at a personal
computer (PC) for editing or a color printer for output on
photo-quality paper—all without having to connect cables,
load files, open applications, or click buttons.
The technology is a combination of circuit switching and
packet switching, making it suitable for voice as well as
data. Instead of fumbling with a cell phone while driving, for
example, the user can wear a lightweight headset to answer
a call and engage in a conversation even if the phone is
tucked away in a briefcase or purse.
While useful in minimizing the need for cables, wireless
local area networks (LANs) are not intended for interconnecting
the range of mobile devices people carry around
everyday between home and office. For this, Bluetooth is
needed. And in the office, a Bluetooth portable device can be
TABLE B-1 Performance Characteristics of Bluetooth Products
Feature/Function Performance
Connection type Spread spectrum (frequency hopping)
Spectrum 2.4-GHz ISM (industrial, scientific, and
medical) band
Transmission power 1 milliwatt (mW)
Aggregate data rate 1 Mbps using frequency hopping
Range Up to 30 feet (9 meters)
Supported stations Up to eight devices per piconet
Voice channels Up to three synchronous channels
Data security For authentication, a 128-bit key; for
encryption, the key size is configurable
between 8 and 128 bits
Addressing Each device has a 48-bit Media Access
Control (MAC) address that is used to
establish a connection with another
device
in motion while connected to the LAN access point as long as
the user stays within the 30-foot range.
Bluetooth can be combined with other technologies to
offer wholly new capabilities, such as automatically lowering
the ring volume of cell phones or shutting them off as
users enter quiet zones such as churches, restaurants, theaters,
and classrooms. On leaving the quiet zone, the cell
phones are returned to their original settings.
Topology
The devices within a piconet play one of two roles: that of
master or slave. The master is the device in a piconet whose
clock and hopping sequence are used to synchronize all other
devices (i.e., slaves) in the piconet. The unit that carries out
the paging procedure and establishes a connection is by
default the master of the connection. The slaves are the
units within a piconet that are synchronized to the master
via its clock and hopping sequence.
The Bluetooth topology is best described as a multiplepiconet
structure. Since Bluetooth supports both point-topoint
and point-to-multipoint connections, several piconets
can be established and linked together in a topology called a
“scatternet” whenever the need arises.
Piconets are uncoordinated, with frequency hopping
occurring independently. 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. Although synchronization of different piconets is
not permitted in the unlicensed ISM band, Bluetooth units
may participate in different piconets through Time Division
Multiplexing (TDM). This enables a unit to sequentially participate
in different piconets by being active in only one
piconet at a time.
With its service discovery protocol, Bluetooth enables a
much broader vision of networking, including the creation of
personal area networks, where all the devices in a person’s
life can communicate and work together. Technical safeguards
ensure that a cluster of Bluetooth devices in public
places, such as an airport lounge or train terminal, would
not suddenly start talking to one another.
Technology
Two types of links have been defined for Bluetooth in support
of voice and data applications: an asynchronous connectionless
(ACL) link and a synchronous connection-oriented
(SCO) link. ACL links support data traffic on a best-effort
basis. The information carried can be user data or control
data. SCO links support real-time voice and multimedia
traffic using reserved bandwidth. Both data and voice are
carried in the form of packets, and Bluetooth devices can
support active ACL and SCO links at the same time.
ACL links support symmetric or asymmetric packetswitched
point-to-multipoint connections
used for data. For symmetric connections, the maximum data
rate is 433.9 kbps in both directions, send and receive. For
asymmetric connections, the maximum data rate is 723.2
kbps in one direction and 57.6 kbps in the reverse direction.
If errors are detected at the receiving device, a notification is
sent in the header of the return packet so that only lost or corrupt
packets need to be retransmitted.
SCO links provide symmetric circuit-switched point-topoint
connections, which are typically used for voice. Three
synchronous channels of 64 kbps each are available for voice.
The channels are derived through the use of either Pulse
Code Modulation (PCM) or Continuous Variable Slope Delta
(CVSD) Modulation. PCM is the standard for encoding
speech in analog form into the digital format of ones and
zeros. CVSD is another standard for analog-to-digital encoding
but offers more immunity to interference and therefore
is better suited than PCM for voice communication over a
wireless link. Bluetooth supports both PCM and CVSD; the
appropriate voice-coding scheme is selected after negotiations
between the link managers of each Bluetooth device
before the call takes place.
Voice and data are sent as packets. Communication is
handled with Time Division Duplexing (TDD), which divides
the channel into time slots, each 625 microseconds (ìs) in
length. The time slots are numbered according to the clock of
the piconet master. In the time slots, master and slave can
transmit packets. In the TDD scheme, master and slave
alternatively transmit. The master starts its
transmission in even-numbered time slots only, and the
slave starts its transmission in odd-numbered time slots
only. The start of the packet is aligned with the slot start.
Packets transmitted by the master or the slave may extend
over as many as five time slots.
With TDD, bandwidth can be allocated on an as-needed
basis, changing the makeup of the traffic flow as demand
warrants. For example, if the user wants to download a large
data file, as much bandwidth as is needed will be allocated
to the transfer. Then, at the next moment, if a file is being
uploaded, that same amount of bandwidth can be allocated
to that transfer.
No matter what the application—voice or data—making
connections between Bluetooth devices is as easy as powering
them up. In fact, one advantage of Bluetooth is that it
does not need to be set up—it is always on, running in the
background, and looking for other devices that it can communicate
with.
When Bluetooth devices come within range of one
another, they engage in a service discovery procedure, which
entails the exchange of messages to become aware of each
other’s service and feature capabilities. Having located
available services within the vicinity, the user may select
from any of them. After that, a connection between two or
more Bluetooth devices can be established.
The radio link itself is very robust, using frequencyhopping
spread-spectrum technology to overcome interference
and fading. Spread spectrum is a digital coding technique
in which the signal is taken apart or “spread” so that
packets are sent
over time slots of 625 microseconds (ìs) in length between the master and
slave units within a piconet.
It sounds more like noise as it is sent through the air. With
the addition of frequency hopping—having the signals skip
from one frequency to another—wireless transmissions are
made even more secure. Bluetooth specifies a rate of 1600
hops per second among 79 frequencies. Since only the sender
and receiver know the hopping sequence for coding and
decoding the signal, eavesdropping is virtually impossible.
For enhanced security, Bluetooth also supports device
authentication and encryption.
Other frequency-hopping transmitters in the vicinity will
be using different hopping patterns and much slower hop
rates than Bluetooth devices. Although the chance of
Bluetooth devices interfering with non-Bluetooth devices
that share the same 2.4-GHz band is minimal, should non-
Bluetooth transmitters and Bluetooth transmitters coincidentally
attempt to use the same frequency at the same
moment, the data packets transmitted by one or both devices
will become garbled in the collision, and a retransmission of
the affected data packets will be required. Anew data packet
will be sent again on the next hopping cycle of each transmitter.
Voice packets, because of their sensitivity to delay,
are never retransmitted.
Points of Convergence
In some ways, Bluetooth competes with infrared, and in
other ways, the two technologies are complementary. With
both infrared and Bluetooth, data exchange is considered to
be a fundamental function. Data exchange can be as simple
as transferring business card information from a mobile
phone to a palmtop or as sophisticated as synchronizing personal
information between a palmtop and desktop PC. In
fact, both technologies can support many of the same applications,
raising the question: Why would users need both
technologies?
The answer lies in the fact that each technology has its
advantages and disadvantages. The very scenarios that leave
infrared falling short are the ones where Bluetooth excels,
and vice versa. Take the electronic exchange of business card
information between two devices. This application usually
will take place in a conference room or exhibit floor where a
number of other devices may be attempting to do the same
thing. This is the situation where infrared excels. The shortrange
and narrow angle of infrared—30 degrees or less—
allow each user to aim his or her device at the intended
recipient with point-and-shoot ease. Close proximity to
another person is natural in a business card exchange situation,
as is pointing one device at another. The limited range
and angle of infrared allow other users to perform a similar
activity with ample security and no interference.
In the same situation, a Bluetooth device would not perform
as well as an infrared device. With its omnidirectional
capability, the Bluetooth device must first discover the
intended recipient. The user cannot simply point at the
intended recipient—a Bluetooth device must perform a discovery
operation that probably will reveal several other
Bluetooth devices within range, so close proximity offers no
advantage here. The user will be forced to select from a list
of discovered devices and apply a security mechanism to prevent
unauthorized access. All this makes the use of
Bluetooth for business card exchange an awkward and needlessly
time-consuming process.
However, in other data-exchange situations, Bluetooth
might be the preferred choice. Bluetooth’s ability to penetrate
solid objects and its ability to communicate with other
devices in a piconet allow for data-exchange opportunities
that are very difficult or impossible with infrared. For example,
Bluetooth allows a user to synchronize a mobile phone
with a notebook computer without taking the phone out of a
jacket pocket or purse. This would allow the user to type a
new address at the computer and move it to the mobile
phone’s directory without unpacking the phone and setting
up a cable connection between the two devices. The omnidirectional
capability of Bluetooth allows synchronization to
occur instantly, assuming that the phone and computer are
within 30 feet of each other.
Using Bluetooth for synchronization does not require that
the phone remain in a fixed location. If a phone is carried
about in a briefcase, the synchronization can occur while the
user moves around. This is not possible with infrared because
the signal is not able to penetrate solid objects, and the
devices must be within a few feet of each other. Furthermore,
the use of infrared requires that both devices remain stationary
while the synchronization occurs.
When it comes to data transfers, infrared does offer a big
speed advantage over Bluetooth. While Bluetooth moves data
between devices at an aggregate rate of 1 Mbps, infrared
offers 4 Mbps of data throughput. Ahigher -speed version of
infrared is now available that can transmit data between
devices at up to 16 Mbps—a four times improvement over the
previous version. The higher speed is achieved with the Very
Fast Infrared (VFIR) Protocol, which is designed to address
the new demands of transferring large image files between
digital cameras, scanners, and PCs. Even when Bluetooth is
enhanced for higher data rates in the future, infrared is likely
to maintain its speed advantage for many years to come.
Bluetooth complements infrared’s point-and-shoot ease of
use with omnidirectional signaling, longer-distance communications,
and capacity to penetrate walls. For some users,
having both Bluetooth and infrared will provide the optimal
short-range wireless solution. For others, the choice of
adding Bluetooth or infrared will be based on the applications
and intended usage.
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