Everything about wireless networks


What is bluetooth

Bluetooth is an omnidirectional wirelessdownload a large data file, as much
technology that provides limited-rangebandwidth as is needed will be allocated
voice and data transmission over theto the transfer. Then, at the next
unlicensed 2.4-GHz frequency band,moment, if a file is being uploaded,
allowing connections with a wide varietythat same amount of bandwidth can be
of fixed and portable devices thatallocated to that transfer.
normally would have to be cabledNo matter what the application—voice
together. Up to eight devices—oneor data—making connections between
master and seven slaves—canBluetooth devices is as easy as powering
communicate with one another in athem up. In fact, one advantage of
socalled piconet at distances of up toBluetooth is that it does not need to be
30 feet.set up—it is always on, running in the
Applicationsbackground, and looking for other
Among the many things users can do withdevices that it can communicate with.
Bluetooth is swap data and synchronizeWhen Bluetooth devices come within range
files merely by having the devices comeof one another, they engage in a service
within range of one another. Imagesdiscovery procedure, which entails the
captured with a digital camera, forexchange of messages to become aware of
example, can be dropped off at aeach other’s service and feature
personal computer (PC) for editing or acapabilities. Having located available
color printer for output onservices within the vicinity, the user
photo-quality paper—all without havingmay select from any of them. After that,
to connect cables, load files, opena connection between two or more
applications, or click buttons. TheBluetooth devices can be established.
technology is a combination of circuitThe radio link itself is very robust,
switching and packet switching, makingusing frequencyhopping spread-spectrum
it suitable for voice as well as data.technology to overcome interference and
Instead of fumbling with a cell phonefading. Spread spectrum is a digital
while driving, for example, the user cancoding technique in which the signal is
wear a lightweight headset to answer ataken apart or “spread” so that
call and engage in a conversation evenpackets are sent over time slots of 625
if the phone is tucked away in amicroseconds (ìs) in length between the
briefcase or purse. While useful inmaster and slave units within a piconet.
minimizing the need for cables, wirelessIt sounds more like noise as it is sent
local area networks (LANs) are notthrough the air. With the addition of
intended for interconnecting the rangefrequency hopping—having the signals
of mobile devices people carry aroundskip from one frequency to
everyday between home and office. Foranother—wireless transmissions are
this, Bluetooth is needed. And in themade even more secure. Bluetooth
office, a Bluetooth portable device canspecifies a rate of 1600 hops per second
be TABLE B-1 Performance Characteristicsamong 79 frequencies. Since only the
of Bluetooth Products Feature/Functionsender and receiver know the hopping
Performance Connection type Spreadsequence for coding and decoding the
spectrum (frequency hopping) Spectrumsignal, eavesdropping is virtually
2.4-GHz ISM (industrial, scientific, andimpossible. For enhanced security,
medical) band Transmission power 1Bluetooth also supports device
milliwatt (mW) Aggregate data rate 1authentication and encryption.
Mbps using frequency hopping Range Up toOther frequency-hopping transmitters in
30 feet (9 meters) Supported stations Upthe vicinity will be using different
to eight devices per piconet Voicehopping patterns and much slower hop
channels Up to three synchronousrates than Bluetooth devices. Although
channels Data security Forthe chance of Bluetooth devices
authentication, a 128-bit key; forinterfering with non-Bluetooth devices
encryption, the key size is configurablethat share the same 2.4-GHz band is
between 8 and 128 bits Addressing Eachminimal, should non- Bluetooth
device has a 48-bit Media Access Controltransmitters and Bluetooth transmitters
(MAC) address that is used to establishcoincidentally attempt to use the same
a connection with another device infrequency at the same moment, the data
motion while connected to the LAN accesspackets transmitted by one or both
point as long as the user stays withindevices will become garbled in the
the 30-foot range. Bluetooth can becollision, and a retransmission of the
combined with other technologies toaffected data packets will be required.
offer wholly new capabilities, such asAnew data packet will be sent again on
automatically lowering the ring volumethe next hopping cycle of each
of cell phones or shutting them off astransmitter. Voice packets, because of
users enter quiet zones such astheir sensitivity to delay, are never
churches, restaurants, theaters, andretransmitted.
classrooms. On leaving the quiet zone,Points of Convergence
the cell phones are returned to theirIn some ways, Bluetooth competes with
original settings.infrared, and in other ways, the two
Topologytechnologies are complementary. With
The devices within a piconet play one ofboth infrared and Bluetooth, data
two roles: that of master or slave. Theexchange is considered to be a
master is the device in a piconet whosefundamental function. Data exchange can
clock and hopping sequence are used tobe as simple as transferring business
synchronize all other devices (i.e.,card information from a mobile phone to
slaves) in the piconet. The unit thata palmtop or as sophisticated as
carries out the paging procedure andsynchronizing personal information
establishes a connection is by defaultbetween a palmtop and desktop PC. In
the master of the connection. The slavesfact, both technologies can support many
are the units within a piconet that areof the same applications, raising the
synchronized to the master via its clockquestion: Why would users need both
and hopping sequence.technologies?
The Bluetooth topology is best describedThe answer lies in the fact that each
as a multiplepiconet structure. Sincetechnology has its advantages and
Bluetooth supports both point-topointdisadvantages. The very scenarios that
and point-to-multipoint connections,leave infrared falling short are the
several piconets can be established andones where Bluetooth excels, and vice
linked together in a topology called aversa. Take the electronic exchange of
“scatternet” whenever the needbusiness card information between two
arises.devices. This application usually will
Piconets are uncoordinated, withtake place in a conference room or
frequency hopping occurringexhibit floor where a number of other
independently. Several piconets can bedevices may be attempting to do the same
established and linked together ad hoc,thing. This is the situation where
where each piconet is identified by ainfrared excels. The shortrange and
different frequency-hopping sequence.narrow angle of infrared—30 degrees or
All users participating on the sameless— allow each user to aim his or
piconet are synchronized to this hoppingher device at the intended recipient
sequence. Although synchronization ofwith point-and-shoot ease. Close
different piconets is not permitted inproximity to another person is natural
the unlicensed ISM band, Bluetooth unitsin a business card exchange situation,
may participate in different piconetsas is pointing one device at another.
through Time Division MultiplexingThe limited range and angle of infrared
(TDM). This enables a unit toallow other users to perform a similar
sequentially participate in differentactivity with ample security and no
piconets by being active in only oneinterference. In the same situation, a
piconet at a time.Bluetooth device would not perform as
With its service discovery protocol,well as an infrared device. With its
Bluetooth enables a much broader visionomnidirectional capability, the
of networking, including the creation ofBluetooth device must first discover the
personal area networks, where all theintended recipient. The user cannot
devices in a person’s life cansimply point at the intended
communicate and work together. Technicalrecipient—a Bluetooth device must
safeguards ensure that a cluster ofperform a discovery operation that
Bluetooth devices in public places, suchprobably will reveal several other
as an airport lounge or train terminal,Bluetooth devices within range, so close
would not suddenly start talking to oneproximity offers no advantage here. The
another.user will be forced to select from a
Technologylist of discovered devices and apply a
Two types of links have been defined forsecurity mechanism to prevent
Bluetooth in support of voice and dataunauthorized access. All this makes the
applications: an asynchronoususe of Bluetooth for business card
connectionless (ACL) link and aexchange an awkward and needlessly
synchronous connection-oriented (SCO)time-consuming process.
link. ACL links support data traffic onHowever, in other data-exchange
a best-effort basis. The informationsituations, Bluetooth might be the
carried can be user data or controlpreferred choice. Bluetooth’s ability
data. SCO links support real-time voiceto penetrate solid objects and its
and multimedia traffic using reservedability to communicate with other
bandwidth. Both data and voice aredevices in a piconet allow for
carried in the form of packets, anddata-exchange opportunities that are
Bluetooth devices can support active ACLvery difficult or impossible with
and SCO links at the same time. ACLinfrared. For example, Bluetooth allows
links support symmetric or asymmetrica user to synchronize a mobile phone
packetswitched point-to-multipointwith a notebook computer without taking
connections used for data. For symmetricthe phone out of a jacket pocket or
connections, the maximum data rate ispurse. This would allow the user to type
433.9 kbps in both directions, send anda new address at the computer and move
receive. For asymmetric connections, theit to the mobile phone’s directory
maximum data rate is 723.2 kbps in onewithout unpacking the phone and setting
direction and 57.6 kbps in the reverseup a cable connection between the two
direction. If errors are detected at thedevices. The omnidirectional capability
receiving device, a notification is sentof Bluetooth allows synchronization to
in the header of the return packet sooccur instantly, assuming that the phone
that only lost or corrupt packets needand computer are within 30 feet of each
to be retransmitted.other.
SCO links provide symmetricUsing Bluetooth for synchronization does
circuit-switched point-topointnot require that the phone remain in a
connections, which are typically usedfixed location. If a phone is carried
for voice. Three synchronous channels ofabout in a briefcase, the
64 kbps each are available for voice.synchronization can occur while the user
The channels are derived through the usemoves around. This is not possible with
of either Pulse Code Modulation (PCM) orinfrared because the signal is not able
Continuous Variable Slope Delta (CVSD)to penetrate solid objects, and the
Modulation. PCM is the standard fordevices must be within a few feet of
encoding speech in analog form into theeach other. Furthermore, the use of
digital format of ones and zeros. CVSDinfrared requires that both devices
is another standard forremain stationary while the
analog-to-digital encoding but offerssynchronization occurs.
more immunity to interference andWhen it comes to data transfers,
therefore is better suited than PCM forinfrared does offer a big speed
voice communication over a wirelessadvantage over Bluetooth. While
link. Bluetooth supports both PCM andBluetooth moves data between devices at
CVSD; the appropriate voice-codingan aggregate rate of 1 Mbps, infrared
scheme is selected after negotiationsoffers 4 Mbps of data throughput.
between the link managers of eachAhigher -speed version of infrared is
Bluetooth device before the call takesnow available that can transmit data
place.between devices at up to 16 Mbps—a
Voice and data are sent as packets.four times improvement over the previous
Communication is handled with Timeversion. The higher speed is achieved
Division Duplexing (TDD), which divideswith the Very Fast Infrared (VFIR)
the channel into time slots, each 625Protocol, which is designed to address
microseconds (ìs) in length. The timethe new demands of transferring large
slots are numbered according to theimage files between digital cameras,
clock of the piconet master. In the timescanners, and PCs. Even when Bluetooth
slots, master and slave can transmitis enhanced for higher data rates in the
packets. In the TDD scheme, master andfuture, infrared is likely to maintain
slave alternatively transmit. The masterits speed advantage for many years to
starts its transmission in even-numberedcome. Bluetooth complements infrared’s
time slots only, and the slave startspoint-and-shoot ease of use with
its transmission in odd-numbered timeomnidirectional signaling,
slots only. The start of the packet islonger-distance communications, and
aligned with the slot start. Packetscapacity to penetrate walls. For some
transmitted by the master or the slaveusers, having both Bluetooth and
may extend over as many as five timeinfrared will provide the optimal
slots.short-range wireless solution. For
With TDD, bandwidth can be allocated onothers, the choice of adding Bluetooth
an as-needed basis, changing the makeupor infrared will be based on the
of the traffic flow as demand warrants.applications and intended usage.
For example, if the user wants to



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