There is a particular angle at which unpolarized light, when encountering a dielectric surface, reflects as maximally polarized, calledBrewster's angle, ex-pressed as8p •The angle ofrefl
Trang 1Fiber Optics Illustrated Dictionary
Brewster's Angle Applied to Light
different degrees, depending upon the angle, with Brewster's angle being the relationship where polar-ization is at its maximum If the incident light is al-ready polarized in a direction parallel to the dielec-tric surface, it will not be reflected
There is a particular angle at which unpolarized light, when encountering a dielectric surface, reflects
as maximally polarized, calledBrewster's angle, ex-pressed as8p •The angle ofreflection, relative to the dielectric surface normal, is complementary to the angle ofthe refracted light, relative to normal Con-versely, if light that is already polarized (in a direc-tion parallel to the surface plane) is incident to the surface, the polarizing effect ofthe dielectric cancels out the polarized incident light and no polarized light
is reflected.
Brewstersangle is consistentfor incident light and dielectric materials with the same characteristics, but varies with the wavelength and index ofrefraction of the dielectric with which the incident light interacts.
polarized reflected light
Ci E
o
c
unpolarized incident light
Brewster's lawWith respect to electromagnetic en-ergy encountering a dielectric material, the relation-ship between the angle ofpropagation ofthe reflected energy relative to the perpendicular of the surface (nonnal), and the refractive index ofthe dielectric ma-terial, can be described in an equation derived by
Brewster, known as Brewster slaw
sO,6pr·
4 •••·••::·•••·.~}·PQ~f.ip. •••~·••il~fl8p
In thisequation,ep(ih~:~lli'gieai~hich the polariza-tion effect for the reflected energy is at its maximum
for a given material and wavelength) is Brewsters
angle.The index of refraction ofthe dielectric mate-rial is expressed as n.Thus, the refractive index is equal to the tangent of the angle of the reflected en-ergy (relative to surface normal) This information is useful for designing components that are intended to maximally polarize unpolarized radiant energy or to not reflect already-polarized energy (polarized par-allel to the surface plane) The accompanying dia-grams help clarify these relationships
Examples ofapplications that exploit Brewster's law
which excessive voltage is detected and the circuit
tripped or broken in order to prevent overload,
elec-trical fires or damage to appliances or the main panel
2.Inradio communications, anyone who drops in on
a channel to communicate when others are already
engagedinconversation 3.Inpublic online forums,
a person who breaks into an ongoing conversation or
thread with irrelevant or unkind and unwanted
com-ments
breaking strength 1.Anindustrial measure of the
force needed to break a specific structure or material
(or a combination of the two) Used in structural and
safety design and selection and installation
ofappro-priate wires and cables
breakoutAn exit point for electrical conductors
(wires, cables, etc.) along the length of the circuit,
between the endpoints of the circuit Breakouts can
be used for additional installations or testing and
di-agnosis of the circuit Breakouts are usually covered
or cappedinsome way to prevent interference with
the circuit and shock or fire hazards
breakout boxA diagnostic instrument used to tap
into an existing circuit to evaluate its functioning, to
see whether individual lines within a group are
cor-rectly connected, and, in some cases, to detect which
signals are being transmitted Breakout boxes often
have indicators such as tones or light-emitting diodes
(LEOs) to help assess the line and may include
jump-ers to temporarily switch connections One useful
application of a breakout box is to test a serial
cir-cuit, since computer equipment manufacturers don't
consistently follow the RS-232 or RS-423
specifica-tions.Inthe case of a temporary need for a null
mo-dem cable (e.g., for transferring information from one
computer to another through telecommunications
software), a breakout box can be used to cross the
transmit and receive lines See breakout.
Brewster's angleIn the context of an
electromag-netic wave encountering a dielectric material, there
is a particular angle, called Brewster's angle, at which
the polarization effect of the interaction is at a
maxi-mum, whether this is the polarization and reflection
of unpolarized radiant energy (e.g., white light from
the Sun) or the "cancellation" of energy through
ab-sorption, ifitis already polarized parallel to the
di-electric surface before contact
Different materials refract light to different extents
Much of David Brewster's research centered on
studying the different optical properties of crystals,
including their reflective and refractive properties
The specific angle at which Brewsterian effects
oc-cur was found to be related to the refractive index of
the dielectric material encountered by the
propagat-ing incident wave and the polarization and other
char-acteristics of the incident radiant wave Brewster's
angle will be between 0° - 90° since no polarization
occurs if the radiant energy hits the dielectric at an
angle perpendicular to the material (0° - straight on)
or travels parallel to the dielectric's contact surface
(900
- thus not being reflected)
Ifthe incident light is unpolarized when it encounters
the dielectric, the reflected light will be polarized to
Trang 2sunglasses, radio signal conditioners, fiber optic
cou-plers, photographic films, and a wide variety
ofcom-ponents for scientific instruments such as
micro-scopes
Brewster's law has broad and consistent applications
in optics that are very useful but may sometimes be
a hindrance Novel ways to overcome some of its
ef-fects have been recently developed In the 1990s, it
was found that highly birefringent, layered, polymer
film could be assembled in alternating layers to
con-trollight so that the net effect was to avoid
polariza-tion of the reflected light that is characterized by
Brewster's angle This led to the development
oflay-ered film components capable ofreflecting light
uni-formly, which has many potential applications in
communications, astronomy, and medical imaging
See acceptance angle, blaze angle, Brewster's angle,
dichroic, incidence angle, Malus's law, Snell's law
Brewster, David (1781-1868) Scottish-born British
author, qleric, educator, and scientist who built
tele-scopes as a child and entered university as a young
adolescent Brewster fulfilled his early promise and
demonstrated talent in many fields of endeavor He
was admitted to the Royal Societies ofEdinburgh and
London and received many medals and honors
dur-ing his lifetime
He served as editor of the Edinburgh Encyclopedia
for two decades and as president of the University of
Edinburgh for almost a decade near the end ofhis life
was in the field of optics, with a focus on the struc-tural components of the eye, as well as the interac-tion of light with the optical properties of crystals which, inturn,led to the formulation of Brewsters
lawto describe properties related to polarization Around 1811-1813, he noted that there was a
particu-lar angle, Brewstersangle,at which the interaction
of light with a mineral substance has a maximal ef-fect related to polarization Over the next two de-cades, he extended his studies into the spectral char-acteristic of glasses and gases and studied fluores-cence in chlorophyll
Brewster balanced his experimental observations with his talent for building things He invented a com-pact kaleidoscope that was granted a patent in 1817, and also established the foundation for a new optical lens system that could transmit bright light suitable for lighthouses, in1835 See Brewster's angle BRI See Basic Rate Interface
bridge 1 A link that provides a connection across a physical or conceptual gap This link mayor may not
be intended to affect the quality or format of the ob-jects or information crossing the gap 2 In networks,
a device commonly used to handle communications between separate local area networks (LANs) which mayor may not use the same protocols Thus, To-ken-Ring networks and Ethernet networks may be connected via a bridge In Frame Relays, a bridge encapsulates LAN frames and feeds them to a Frame Angles of Incidence, Reflection, and Refraction
polarized reflected light
"ii E
o c
~~
refracted ' "
B
incident light
Incident
light
A
This diagram shows the basic relationships between incident electromagnetic energy interacting with dielectric materials with different properties Imagine that the propagating wave is white lightfrom the Sun traveling through air (which is itselfa dielectric) to encounter a solid dielectric material Materials with different refractive indexes (A and B) are shownfor comparison The collision ofthe incident light with the surface ofthe dielectric (a material that does not readily conduct energy) causes the direction oftravel and polarity ofthe light to change Depending upon the properties of the dielectric solids, some of the light will be reflected, some will be absorbed, and some will continue on down through the dielectric but at a different angle, called theangle of refraction.
In both (A) and (B), the angle ofthe incident light (the light hitting the suiface) and the reflected light are the same relative to the suiface normal (perpendicular to the suiface) and there is a direct relationship between the index of refraction and the direction oftravel ofthe scattered light As the refractive index and the angle ofthe incident light increases, so does the angle ofthe reflected light, up to the point at which it is parallel to the surface (in which case
it doesn~reflect).
Depending upon the properties ofthe reflective material, not all the light will be reflected Some may be absorbed and some may be refracted through the material The angle of refraction (light that passes through the material rather than reflecting offofit) is related to the angle ofthe incident light and its interaction with the refractive material.
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Basic Categories of Network Bridges
bridge number BN Aring network designator used
in source routing Together a segment number (SN)
and bridge number (BN) comprise a route
designa-tor When a destination is located on the ring itself,
the bridge number is 0 (zero)
Bridge Protocol Data Unit BPDU.InATM
network-ing, a message type used by bridges to exchange
man-agement and control information BPDU is a Media
Access Control (MAC) management control
proto-col used to implement Spanning Tree Protoproto-col (STP)
Itprovides a mechanism for routing data traffic
Relay switch for subsequent transmission It also
re-ceives frames from the network, strips the Frame
Relay frame, and passes the LAN frame on to the end
device
Bridges in computer networks are associated with the
Media Access Control (MAC) layer (or in OSI, the
MAC sublayer) Bridges fall somewhere between
repeaters and routers, although with increasing
tech-nological sophistication the distinction between
bridges and routers is less clear Bridges can be
clas-sified into general categories
See brouter, eXtranet, Media Access Control address,
repeater, router, spanning tree algorithm
bridge, acousticInacoustic instruments, the bridge
elevates and spaces the strings and transfers
vibra-tions to the body (soundbox) of the instrument In
electronic instruments, the bridge elevates and spaces
the strings and transfers vibrations to the body of the
instrument where they are, in tum, converted into
electrical signals, usually by an energy-conversion
device called a pickup
Type of Bridge
transparent
source routing
source routing
transparent
Description
A general purpose bridge widely used on Ethernet networks that supports multiple bridges in a spanning tree configuration A transparent bridge builds its own MAC address table based on source information from incoming traffic Reference IEEE 802.1
Specific to Token-Ring networks Routing is determined at the source, rather than at the bridge and, hence, incoming frames contain routing information and an indicator as to whether it should be used Reference IEEE 802.5
A less common hybrid configuration used in
a small percentage of implementations
through a single conduit when more than one physical conduit exists (e.g., a backup loop for use if the root loop fails) The BPDU consists of flags, Hello time, and root, port, bridge, protocol, delay, and aging in-formation
bridger switching A technique for improving low return transmissions, as in cable networks, by sequen-tially turning on and off each leg of the distribution circuit It is sometimes used in conjunction with high pass filters While improvements in reliability can be attained in this way, it is at the cost of greater com-plexity and, hence, greater expense
bridging clip A small piece of conducting apparatus used to connect nearby terminals, contacts, or other circuit elements that are close together, either for the purpose of changing a circuit (usually temporarily)
or for testing it
Bright, Charles Tilson (1832-1888)AnEnglish in-ventor and chief engineer for the Magnetic Telegraph Company Bright was the first to undertake under-ground cable installation with gutta-percha as an in-sulating material When Edison was installing the historic Washington-Baltimore line in the 1830s, problems with insulation and ground-breaking caused the line construction to be changed from underground
to overhead, so Bright's success with an underground line was important Later, Bright installed the first cables to be laid in deep water, first with a shallower line across the English Channel in 1851, and two years later a deepwater line between England and Ire-land Further lines around the world followed brightness The level of luminosity or amount of il-lumination emanating from a surface or display me-dium Luminance is used to describe the lightness or brightness component of a television broadcast sig-nal Brightness across the visible spectrum is not equally perceived for different colors See contrast Brillouin, Leon (1889-1969) AFrench-born Ameri-can quantum physicist who studied band structures
in crystalline solids Brillouin is best known for de-scribing Brillouin scattering, a quantum effect in acoustic modes in refractive materials
In the I920s, Brillouin made important discoveries
in quantum dynamics, developing a means of ap-proximating solutions to the Schrodinger equation
He accepted a position at the Sorbonne in the late 1920s In the 1930s, he was associated with the Born Institute in Goettingen Inthe 1940s, he took posi-tions at American universities and authored Wave Propagation in Periodic Structures.In1948, he be-came director of Electronic Education at IBM Brillouin then joined Columbia University at a time when exciting evolutionary advancements in lasers were being made by scientists such asC.Townes See Brillouin scattering
The Laboratoire Leon Brillouin is named in his honor Brillouin scattering A frequency change/phase shift effect that occurs in scattered light from compres-sions/decompressions in an elastic, refractive mate-rial Thus, photons are interacting with vibrational structures at the quantum level within the light-re-fracting medium as opposed to simply encountering
Trang 4Brillouin scattering occurs when the light-refracting
structure exhibits transverse and pseudo-transverse
elastic compressibility (calledacoustic modes due to
the compression/decompression character of sound
waves traveling through a medium) Brillouin effects
are complex and subtle interactions that can be
chal-lenging to measure and that depend upon many
fac-tors, including density, refractive index, elasticity,
temperature, humidity, vibrational intensity, the
di-rection of the incident light, etc Neverthless,
scien-tists are endeavoring to characterize and harness
Brillouin effects for use in fiber optic and other
tech-nologies
This effect was first studied by Leon Brillouin, ca
1920 The mathematics of the scattering effect was
not well understood until the study and development
of lasers and fiber optics in the early 1970s gave it a
wider context in which to be researched This, in turn,
furthered development of nonlinear fiber optics and
the understanding of other optical phenomena and
modulation technologies
Brillouin scattering may impose limitations For
ex-ample, in optical fibers, increasing the power of the
light in a cable causes it to reach a threshold at which
point Brillouin effects impose physical limitations on
light propagation Acoustic noise arising from
Brownian molecular motion can stimulate
spontane-ous Brillouin scattering causing back reflection
Brillouin scattering is a useful tool for researching
molecular excitation and Brillouin effects may be
deliberately stimulated for use by testing and
correc-tion components The effect can also be exploited in
the development of fiber-based sensors S Burgett
et al have described the use ofBrillouin optical time
domain reflectometry to carry out strain measurments
in precision-wound optical fibers and L Thevenaz et
al have described strain and temperature sensing
us-ing embedded optical fibers See acoustical Doppler
effect, Bragg's law, Raman scattering, Rayleigh
scat-tering, Stoke's shift
British Academy of Film and Television Arts
BAFTA Established by Alexander Korda in 1946 as
a club for the British film elite, the club evolved into
the British Film Academy, founded in April 1947 It
later merged with the Guild of Television Producers
and Directors to form the Society of Film and
Tele-vision Arts BAFTAsupports the Film Awards as well
as Television and Children's Awards programs
http://www.bafta.org/
British Approvals Board for Telecommunications
BABT AU.K.-based telecommunications regulatory
organization, the BABT provides third-party
accred-ited regulatory and certification services BABT
Cer-tification marks are commonly recognized in Europe
In AprU 2001, the BABT announced its affiliation
with TUV SiiddeutschlandAG The merger resulted,
in part, in the creation of BABT Product Service
USA, which focuses on telecommunications
confor-mity assessment
BABT Product Service also has centers in several
Asian countries http://www.babt.co.uk!
broadcast provider since the late 1920s, when it be-gan its fITst experimental television transmissions, the BBC began widespread public broadcasting from London in 1936
British Computer SocietySee computer societies, national
British Film InstituteBFI An independent Royal Charter body, founded in 1933, that promotes under-standing of the moving image arts, including televi-sion and film It fulfils the cultural and educational roles of the Film Council http://www.bfi.org.uk! British Indian Submarine Telegraph CompanyAn underwater telegraph cable-laying company founded
by John Pender, a British merchant, in 1869, not long after the laying of the first oceanic telegraph cable traversing the Atlantic Ocean Through his involve-ment with the transatlantic cable project, Pender was inspired to propose a cable connection between Brit-ain and India, where the British had established large centers oftrade and colonization.In 1872, he also es-tablished the Eastern Telegraph Company Within a decade, the company supported a majority of tele-graph traffic to India and had expanded to include Australia, China, and Japan through overland and underwater cables Eventually Pender achieved his ambitious dream when London, England and Bombay, India were linkedin1879
British National CorpusBNC Avery large linguis-tic collection of written and spoken British English compiled from 1991 to 1994 under the management
of an academic/industrial consortium led by Oxford University Press In terms of technology, the collec-tion is of interest in the fields of speech recognicollec-tion and synthesis (telephony, computer text dictation and generation, etc.) and artificial intelligence (especially natural language synthesis and processing) BNC Online is hosted by theBritish Library Initiativesfor Access Programme The BNC is licensed material,
but its associated SARA Client is freely available, as
is limited demo access to the database
British Standards InstitutionBSi, BSI AU.K stan-dards body which provides input to various interna-tional standards associations, including ISO and
ITU-T It originated as a Committee within the Council ofthe Institute ofCivil Engineers in 1901, appointed
to consider standardization in the steel industry The Committee of engineers and naval architects was joined by the Institute ofElectrical Engineers in 1902
In 1918, it became the British Engineering Standards Association, granted a Royal Charter in 1929 The current name was adopted in 1931 BSI currently sup-ports about 19,000 live standards It owns the
Kitemark quality mark http://www.bsi-global.com/
British TelecomBT, British Telecommunications pIc Originally affiliated with the British Ministry of Defence and British Post Office, the telephone net-work and telephony research arms separated from the Post Office to become British Telecom BT publishes the quarterlyBT Technology Journal (technical
pa-pers) andsphere (BT technology) with copies
avail-able on the Web
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British Telecom Research LaboratoriesBTRL The
research lab ofBritain's largest telecommunications
provider, based in Marlesham Heath, Suffolk,
En-gland British Telecom is involved in many
interna-tional collaborative projects including research with
the Massachusetts Institute of Technology in
artifi-cial intelligence natural language processing
British thermal unitBtu The unit ofthennal energy
required to raise the temperature of 1 pound of
liq-uid water by1degree Fahrenheit at sea level 1Btu
=251.996 calories=1054.8 Joules Optical sensors
have been developed to measure Btu's While
gener-ally used to express the energy-producing or
trans-ferring capability ofheating/cooling systems, it is also
used to express the heat generated by large-scale
com-puting systems to facilitate the design ofbuilding
cir-culation systems associated with their installation
brittleAquality ofa physical substance lacking
elas-ticity, one that is vulnerable to breakage Substances
may be brittle in one set of circumstances and not
another, e.g., electrical components or connectors
may be vulnerable below or above certain operating
temperatures
broadbandA band of frequencies wide enough to
be split into narrower bands, each capable of
indi-vidual use for a variety oftransmissions or for
trans-missions by a variety of users Broadband
transmis-sion requires suitable hardware and cabling, capable
ofquickly transmitting and receiving a large amount
of infonnation Fiber optics are often used The
en-tire breadth of the band is not necessarily used for
transmissions, depending upon supply and demand
Also, there may be gaps between bands to prevent
interference Cable TV is a ubiquitous example of
broadband transmissions where the band is split into
all the different channels to which the recipients have
subscribed.Asin many broadcast media, broadcast
technologies tend to be one-way, or mostly one-way,
but with the increased demand for interactivity, more
two-way communications over broadband are being
developed See baseband, telecomputer, wideband
Broadband Connection-Oriented Bearer See
BCOB
broadband digital cross-connect systemB-DCS A
digital cross-connect system that accepts a variety of
optical signals and is used to tenninate SONET and
DS-3 signals B-DCS accesses STS-l signals and
switches at this level and is appropriately used as a
SONET hub for routing and other functions B-DCS
is commonly implemented with node tennination via
add/drop multiplexers (ADMs) and B-DC switches
Some newer optical systems that provide SONET-like
capability can transmit without the B-DC switches
See wideband digital cross-connect system
Broadband High Layer InformationBHLI.An
ATM infonnation element that uniquely identifies an
application (or session layer protocol of an
applica-tion) BHLI is implemented in various ways,
depend-ing upon whether the codepoint is user-specific,
ven-dor-specific, or ISO
broadband integrated gatewayBIG Acomponent
ofHFC (Hybrid Fiber Coax) networks that converts
an ATM transmission into a signal that can be trans-mitted over the HFC Working in conjunction with a connection management controller (CMC), the BIG strips infonnation from ATM cells and orders and addresses them for further transmission See connec-tion management controller, HFC
Broadband ISDNSee B-ISDN
Broadband Lightwave Sources and SystemSee BLISS
broadband line terminationB-LT Optical or elec-tricalline tennination (LT) on a broadband network that provides a physical and link between an access network and a local digital exchange The tennina-tion unit will convert signals as needed and, in some cases, provide multiplexing functions for multiple attached devices
Broadband Passive Optical NetworkBPON, Broadband paN.Anoptical communications net-work capable of providing high bandwidth services Commercial implementations ofconsumer program-ming via BPON provide a range of up to about 20 kilometers on a single fiber serving a couple ofdozen
or more customers
Broadband Telecommunications Architecture BTA.Anarchitecture introduced by General Instru-ment for multimedia networking
broadband terminal adapterBTA Adata commu-nications device that interfaces a broadband ISDN (B-ISON) connection to other terminal equipment that
is not directly compatible with B-ISDN
broadband transport managerBTM In telephony,
a transport mechanism for long-distance portions of
a connection In the late 1990s, Tellabs, Inc., planned field trials of a BTM for ATM networks
BROADBANDLOOPBBL.AnACTS project for defming and testing a concept for a cost-effective broadband access network allowing fiber to be inte-grated into local loops as telecommunications band-width demand increases The target user base for the project is residential subscribers and small- and me-dium-sized businesses Field trials were set up for Denmark, Poland, and Portugalwithsome trials con-sisting of overlays to existing telephony and CATV networks See BLISS, UPGRADE, WOTAN broadcastv To transmit sound, images, or data over distance, in the context ofmore-or-Iess simultaneous receipt by a larger audience Transmission can occur through a variety of media, over airwaves, satellite links, wire or fiber, or a combination of these Re-ception can often be enhanced with antenna, cable,
or satellite hookups Radio, television, and Internet chat channels are common broadcast channels Commercial and high power broadcasting is regu-lated.Inthe United States, the Federal Communica-tions Commission (FCC) is the primary regulatory body and has jurisdiction over the allocation ofbroad-cast frequencies In Canada, the Canadian Radio Tele-vision and Telecommunications Commission (CRTC) handles many of the same functions
In most ofNorth America, very low power broadcast-ing is permitted without a license; otherwiseit wouldn't be possible for people to use cordless
Trang 6out being licensed Generally these low power
broad-casts are limited to a signal strength of 250
micro-volts per meter, as measured three meters from the
transmitter for FM transmissions, and 0.1 watts on a
maximum three meter antenna for AM transmissions
This effectively limits the broadcast distance to 100
feet or so for FM and a couple ofblocks for AM
Tra-ditional broadcasts are typically in the range of 535
to 1605 kHz
Commercial entertainment broadcasts are often
fi-nanced by revenues from sponsors which are aired
in the fonn ofcommercials Since this revenue model
has been successful for quite some time in the
televi-sion and radio industries, it is not surprising that many
broadcasters are turning to the same ideas in
design-ing infonnation to be viewed over the Web In
con-trast to commercial stations, however, the Web is far
less regulated and has many more participants, and
it will be interesting to see howWebcastingevolves
over the next several years See television, radio,
multicast, narrowcast, unicast
Broadcast and Unknown ServerBUS InATM
net-works, this server handles data sent by an LE client
to the broadcast Media Access Control (MAC)
ad-dress, all multicast traffic, and initial unicast frames
which are sent by a LAN Emulation (LANE) client.!t
encompasses the functions that support establishment
of a virtual circuit (VC) connection See Media
Ac-cess Control address
broadcast data triggerAdditional services are
some-times delivered with audio/video broadcast
program-ming Closed captioning or subtitles are examples of
traditional broadcast services and others are
becom-ing available as digital broadcastbecom-ing over computer
networks increases Depending on the medium,
cer-tain standardized broadcast triggers have been
de-fined and categorized, including transport type Aand
B triggers Broadcast triggers are realtime data events
associated with enhanced TV broadcasting delivered
in a textual syntax based on the EIA-746A standard
presented through the ISO-8859-1 character set (U.S
ASCII or Latin-I) By adding triggers to the data
stream, the viewing box or software program receives
a signal to interpret and present the additional
infor-mation or services In recent years, attributes have
been added to the triggers to accommodate Internet
broadcasts; these generally require two-way
commu-nications The presence oftwo-way communications
makes it possible to incorporate triggers as part ofan
on-demand interactive TV system
The ATVEF has defined two modes ofdata transport
that incorporate triggers TransportAdelivers
trig-gers by the forward path and pulls data by the return
path Transport B delivers triggers and data by the
forward path, but the return path is optional
See ATVEF Enhanced Content Specification
broadcast list1 On computer networks, a list
ofus-ers to whom broadcast messages are sent, usually by
a system operator (sysop) or other privileged
admin-istrator See broadcast message 2 On fax machines,
a list ofrecipients to whom the same fax will be sent
or underlying electromagnetic phenomenon that pro-vides a conduit for electrical or other signals trans-missions Electromagnetic phenomena are the most common broadcast media, though a disturbance in a medium (metal, water, wood, etc.) that produces sound waves can also constitute a broadcast medium (albeit slower and less effective than electromagnetic media) In some networks, a physical layer capable
of supporting broadcast messages
broadcast message 1.A message sent to a selected group of users (or all users) on a computer or radio network A common computer network broadcast message infonns users that the system will shut down
in 5 or 10 minutes This message allows users to save work, close files, and finish up before being logged out On networks, broadcast functions are usually available only to those with system privileges, as it
is a capability that is easily abused Schools are be-ginning to use broadcast phone or email messages to infonn parents of registration, reporting, or meeting events with respect to their school-aged children See allcall, anycall, broadcast list 2 A message broad-cast over a public broadbroad-cast medium, such as a news flash or Emergency Alert System (EAS) message
3 A message broadcast over a paging or public ad-dress (PA) system
broadcast over networkIn ATM networking, data transmissions to all addresses or functions on the sys-tem
Broadcast Pioneers LibraryAneducation and re-search resource located in the Hornbake Library at the University of Maryland, College Park, founded
in 1972 The collection includes correspondence, books, film, video, periodicals, historic photographs, scripts, and transcripts More infonnation is available online through the Pioneers' Web site http://www.lib.umd.edu/UMCPILAB/
broadcast standardsEstablished in the late 1930s
in the U.S., professional standards still exist as im-portant guidelines for ethical business practices, safety standards, and standardized broadcast formats
in the broadcast industry See Canadian Broadcast Standards Council, Federal Communications Com-mission
broadcast stormA broadcasting clamor that is ex-cessively busy, frequent, or powerful that it overrides other communications In radio broadcasts, a broad-cast storms occur in times of emergency, when nu-merous operators simultaneouslytryto call for help
or send messages to friends and relatives.In end-to-end systems, such as analog wireline telephone sys-tems, broadcast storms as such don't occur (except, perhaps, in a different sense, on a party line), because excessive calling will result in a fast busy being sent
to the caller, indicating that no trunks are available, rather than in many people talking at once In data networks, however, a broadcast stonn can occur as a fault condition in which some process goes wild and starts broadcasting to all workstations and disrupting user interactions and work Astonn may occasionally
be caused on unsecured networks by a virus
Trang 7Fiber Optics Illustrated Dictionary
distributed by a vandal See allcall, broadcast
mes-sage
Broadcast Wave FormatSee EBU Broadcast Wave
Format
broadcasting satellite serviceBSS One of two
di-visions into which Ku-band satellite broadcast
ser-vices have been split BSS operates in the 12.2- to
12.7-GHz range The other is fixed satellite service
(FSS) See ANIK, Ku-band
broadside array antennaAphased array (with
har-nesses) of antennas with the maximum radiation
di-rected perpendicularly to the plane that holds the
driven elements This antenna arrangement can be
configured as a billboard antenna by adding a
reflect-ing sheet behind the array See billboard antenna
BroadwayThe internal development code name for
The X Window System 11 Release 6.3 (XIIR6.3)
from The Open Group See X Window System 11
Release 6.3
Brocade Fabric Aware programBFA Atesting and
configuration initiative for fostering end-to-end
inter-operability for storage area networks (SANs) in
multi-vendor, heterogenous environments Participating
firms agree to specify, test, and support pretested SAN
configurations with a mix of servers, switches, and
storage subsystems Brocade Fabric Aware
qualifi-cations may be presented after the completion
ofrig-orous testing for interoperability within specified
multivendor configurations Brocade
Communica-tions Systems, Inc., has set up an interoperability
laboratory testing environment to support the Fabric
Aware program
bronzeAnalloy consisting primarily of copper with
tin and occasionally other elements added
Brooks' lawAdding manpower to a late software
project makes it later From Frederick P Brooks,
au-thor ofThe Mythical Man-Month, a much-quoted
provocative book about the engineering development
culture
brouter, b-routerbridge router Combination
de-vices that function as links between different
net-works The combination ofa bridge and a router
pro-vides the physical and logical connections between
networks, which mayor may not have different
pro-tocols, and routing tables to facilitate the efficient
transmission ofinformation to the desired destination
A brouter typically performs its functions based on
information in the data link layer (bridging) and the
network layer (routing) See bridge, router
Brown&Sharpe Wire GaugeSee American Wire
Gauge
Brownian movement, Brownian motionBotanist
R Brown observed in early 1827 that pollen grains
suspended in water were in a continual state of
agi-tated motion This motion has been widely observed
for small particles suspended in fluids.Itis said that
the molecules ofthe suspension medium continually
buffet the particles, resulting in the characteristic
movement Einstein later provided a mathematical
explanation of Brownian motion Theoretical
mod-els for queueing and aggregated connectionless
net-work traffic, based on fractional Brownian motion,
have been proposed
brownout 1.When power is partially, but not com-pletely lost Some companies use an industry-specific definition for a brownout, usually based on a relative
or specific drop in voltage Complete loss of power
is called a blackout 2 In cellular systems, a security precaution used by some companies to prevent fraudulent use When brownout is in effect, there may
be roaming areas in which a subscriber's system will not function
browser 1.Anobject-oriented software development tool for inspecting a class hierarchy 2 A software utility for displaying and traversing files and direc-tories 3 Asoftware client for accessing the resources
of the World Wide Web See browser, Web browser, WebA historic milestone in software ap-plications, designed to make it easy to access World Wide Web client/server resources stored on a variety
of computers on the Internet Prior to the develop-ment of Web browsers, there were many publicly available and valuable data repositories on the Inter-net, but access was through inscrutable line com-mands or uninspiring textual menus The repositories themselves sometimes included images, but the im-ages were seldom directly viewable over a remote link They had to be copied and then loaded into a compatible application supporting the various for-mats in order to be useful on the local machine Web browsers simplified this process by providing a 'front end' that transparently automated, integrated, and standardized access to the more sought-after types of information stored for distribution on the Internet The Web browser client/server model was originally developed for NeXTStep by Time Berners-Lee, in
1989 As the concept spread through various systems and became associated with the Internet, another im-portant change was taking place, the commercializa-tion of the Internet from a research system to a sys-tem that could be used for commerce; this develop-ment got the attention of the general public, even those with no previous interest in computers
A variety of Web browsers were quickly developed
to meet the growing demand for Web access from different types ofcomputers, resulting in phenomenal growth and interest over the next decade and beyond Within a few years NCSA Mosaic, OmniWeb, Lynx (text browser), AWeb (Amiga Web), Netscape Navi-gator, and Internet Explorer had become popular browsers for 'surfing' the Web
The Web gained in commercial prominence in the early 1990s, and browsers developed into practical tools for accessing, traversing, and displaying files
on the Internet A browser interprets standardized HTML tags that are used to describe the Web page, and displays the results on the user's system Sun Microsystems Java applets are used by many devel-opers to further enhance the capabilities ofa browser with programming algorithms that aren't directly sup-ported by HTML tag interpretation
Browsers typically download HTML pages onto the local drive, so they can be more quickly redisplayed when the user moves back through previously viewed
Trang 8through File Transfer Protocol (FTP) and other
ex-ternal utilities Some browsers incorporate email
functionality as well or will launch the email utility
of choice when an email anchor is selected on a Web
page Most browsers support plugins,
browser-com-pliant applications for processing multimedia file
for-mats from a variety of vendors Plugins are popular
for enabling the user to play music, movies,
stream-ing video news broadcasts, and other television-like
fonns of infonnation and entertainment
A Web browser's simple, accessible means
ofmak-ing available infonnation on the Internet has resulted
in an explosion ofinterest and participation,
increas-ing from a handful ofusers in 1989, to more than 30
million in 1998 The number of data respositories
available on the Internet has grown in conjunction
with the increase in users
One of the issues that becomes more important, as
Internet commerce grows and data repositories
in-clude more and more personal information, is
secu-rity Security enhancements have been gradually
added to Web browsers since the mid-1990s, but these
are not impervious to skilled hackers Web browsers
were originally designed to be open and easy to use,
and no one fully anticipated how quickly and
aggres-sively banks, stores, and other institutions of
com-merce would set up their entire service line and
cus-tomer access databases to be Web-accessible As Web
browsers become more powerful, they become more
and more like operating systems that can be
poten-tially accessed and controlled from remote sources
in sophisticated ways Already, by 1999, there were
companies using their Web browsers to 'look' at
fonnation on the desktop of the individuals who
in-stalled the browsers on their computers The
compa-nies didn't necessarily inform the user of this
intru-sion, or did so in small print or vague ways not
un-derstood by those not technically acquainted with
computers
If computers are to be secure in the future, it is
im-portant for the community at large to understand the
potential for abuse and hold companies to high
ethi-cal standards in the matter ofcomputer and individual
privacy associated with Web browsers Unfortunately,
many people don't understand the technical or
po-litical issues and others don't consider the long-tenn
consequences While they may not be willing to give
out name and address information on a
moment-to-moment basis, they can often be persuaded to do so
when offered the possibility of winning million
dol-lar sweepstakes Once their personal information gets
into an unethical Web respository, it can be
redistrib-uted to millions of other computers within seconds;
there's currently no way to undo this type of
infor-mation theft, which may include social security
num-ber, credit card numbers, and more Think twice
be-fore volunteering information to unfamiliar Web sites
through your Web browser
Another security-related browser issue is the use of
cookies, identifiers within a browser that enable a
Web site to recognize automatically a returning user
tity a second time Users should take time to under-stand what cookies are, how they work, and whether they should be explicitly turned off in the browser to help safeguard privacy Always read the privacy policy statements associated with each site before providing personal information and boycott sites that require more information than you feel they need to carry out a transaction
Web browsers have opened up a world of communi-cation, educommuni-cation, and opportunity for millions of people and will likely be an essential aspect of the Internet for a long time to come Many Web brows-ers are freely available for download and Netscape Navigator is open source software that can be down-loaded and modified by developers See FTP, Java, HTML, HTTP, Internet, NCSA Mosaic, PDA mac-robrowser, PDA micmac-robrowser, SGML, World Wide Web
browsingSearching or scanning through data for in-formation or to get a general feel for the format or contents of a body of information The information may take a variety of forms: text, files, directories, images, sounds, etc See browser
brushA conducting structure that provides an elec-trical connection between a motor and its power source
brute force 1 A problem-solving method that in-volves trying every possible combination and permu-tation This method is only practical for small prob-lems of limited scope and is usually unwieldy for larger or more complex problems Sometimes it is used in conjunction with other problem-solving meth-ods such as heuristics 2 A programming approach that involves reliance on a system's basic capabili-ties and processing power, rather than on efficient algorithms and elegance of design and concept A brute force application generally does not run quickly
on legacy systems
brute force attackAnattack on a security system using every possible combination, password, login name, or other entry data rather than using a targeted strategy Brute force attack data are often generated automatically with computer software This type of attack is usually easily detected and is often not very effective
BS1 See backscatter.2.band signaling 3 See base station.4.See beam splitter
BSAMSee basic sequential access method BSCCBellSouth Cellular Corporation Acorporation serving about 10% of the U.s wireless market, formed in 1991
BSDSee Berkeley Software Distribution
BSE 1 back-scattered electrons 2 Basic Service Element 3 Basic Switching Element In packet switched networking, a basic unit which may be com-bined with other BSEs to emulate a larger switching topology
BSFbit scan forward An assembly language bit manipulation in which a bit string is searched for a set or cleared bit, from low-order to high-order See BSR
Trang 9Fiber Optics Illustrated Dictionary
BSFTSee Byte Stream File Transfer
BSISee British Standards Institution
BSLSee British Sign Language
BSMS1 billing and subscriber maintenance service
2 Broadcast Short Message Service
BSMTPSee Batch Simple Mail Transfer Protocol
BSP1 Bell System Practice Bell internal policies
and procedures for creating instructional manuals for
the servicing, support, and operation ofphone
equip-ment 2 See byte-stream protocol
BSR1 bit scan rate 2 bit scan reverse.An
assem-bly language bit manipulation in which a bit string is
searched for a set or cleared bit, from high-order to
low-order See BSF
BSS1 Base Station System 2 See broadcasting
sat-ellite service 2 Business Support System
BSVC1 Broadcast Switched Virtual Connections
2 The name for an object-oriented, generic
micro-processor simulation framework for building a
vir-tual computer that evolved from a Motorola 68000
simulator supporting the 6850 UART
BT 1 See British Telecom 2 Burst Tolerance In
asynchronous transmissions mode (ATM)
connec-tions supporting variable bit rate (VBR) services, BT
is the limit parameter of the GCRA See cell rate
BT cut crystalAtype ofcrystal with vibratory
quali-ties that makes it suitable for crystal radios
BT PhonebaseAservice ofBritish Telecom since the
early 1990s that enables subscribers to make
direc-tory enquiries through a computer modem The
da-tabase is updated continuously and is more up-to-date
than a yearly paper-based telephone directory The
call is billed at long-distance phone rates but is
gen-erally still less expensive than Directory Enquiry
(U.K Directory Assistance service) The typical
con-nect speed is 2400 baud, though higher rates became
available in some areas as of 1999 See TeleDirectory
BTA1 See Basic Trading Area 2 See Broadband
Telecommunications Architecture 3 See broadband
terminal adaptor
BTag Beginning Tag In ATM, a one-octet field of
the CPCS_PDU used in conjunction with the Etag
octet to fonn an association between the beginning
of a message and end of a message
BTBT band-to-band tunneling Direct transfer of
electrons from filled valence band (VB) states to
empty states or recombination ofelectrons with holes
in the valence band
BTE1 Boltzmann Transport Equation 2 broadband
terminal equipment
BTl1 British Telecom International 2 Broadband
Trial Integration.AnACTS project to demonstrate the
role of Quality of Service (QoS) on Internet
Proto-col (IP) over ATM in order to develop optimization
data for networks and to improve user perception of
network services The project involves development
of an integrated IPv6 and switched ATM
multicast-ing network with QoS support of user-controlled
bandwidth and delay The project is designed in three
phases, the establishment of the technical platform,
implementation ofsignaling and management
ofrout-ers and switches, and the development of integrated
protocols International connections will be through PVC-based ATM networks using UNI 4.0 SVCs for bandwidth management See BID, BLISS, BONA-PARTE, BOURBON, UPGRADE, WOTAN BTLBell Telephone Laboratories See Bell Labora-tories
BTMSee broadband transport manager
BTNSee Billing Telephone Number
BTRL1 BTheory Research Labs 2 Breward Teach-ing&Research Laboratories 3 See British Telecom Research Laboratories
BTS1 Base Transceiver Station In mobile communi-cations, an end transmission point 2 bit test and set BtuSee British thennal unit
BTUbasic transmission unit
BTU InternationalAmajor supplier ofthermal pro-cessing systems to the electronics industry, primarily semiconductor packaging and printed circuit boards bubble memoryA type of nonvolatile memory; it doesn't have to be constantly electrically refreshed
to retain the data Bubble memory, as used in com-puters, consists of a thin layer of material that has magnetic properties Amagnetic field is used to ma-nipulate a circular area such that the diameter be-comes smaller, forming a bubble
bucket truckSee cherry picker
buffer1 Acircuit or device designed to separate trical circuits one from another 2 Aphysical or elec-tronic storage device designed to compensate for a difference in the rate of use or flow of objects or in-formation Generally a buffer is intended to increase speed ofaccess and efficiency In a computer, a buffer
is often used as a storage area for frequently accessed information, so the software doesn't have to con-stantly access slower storage devices such as a hard drive, if sufficient fast access chip memory (e.g., RAM) is available Cut and paste functions make use ofa buffer Data in a buffer tends to be temporary and volatile See cache, frame buffer, RAM disk buffer, cableA layer of material to protect inner or outer components from abrasion, moisture, pressure, flexing, or tampering In fiber optic cable assemblies, the buffer layer encircles the coating, cladding, and inner light-conducting core to provide protection from the elements See swelling tape
buffer boxSee Logical Storage Unit
buffer condenserA condenser installed in an elec-tronic circuit to provide protection to other compo-nents by reducing excessive voltages, especially surges
buffer memory, buffer storageElectronic memory, usually RAM, used for information storage and re-trieval, particularly for applications programs which make use ofchunks ofinformation that are frequently recalled See buffer, cache
bugA small, concealed listening device used in sur-veillance and espionage Placing a covert bug in a room or on a phone line is almost always illegal The term is also used in conjunction with small, hobbyist transceiver projects for electronics education, wire-less intercoms, child monitors, and other legitimate uses See bug, software/hardware; wire tapping
Trang 10error that adversely affects operations or user
inter-action Grace Hopper is credited with relating the first
story about a computer bug that was found by a
tech-nician, and for preserving the bug itselfin a log book
This story has long been a part of hacker lore as the
origin ofthe term "bug" in computer technology The
bug in the story apparently was moved to the
Smith-sonian Institution in the early 1990s (after an earlier
unsuccessful attempt to have it accepted) but was not
immediately exhibited However, there are earlier
an-ecdotes about bugs in industrial settings that indicate
the term may go back decades, if not longer
Removing bugs from software (debugging) is an art
form, and not all programmers who are good at
writ-ing code are good at findwrit-ing and correctwrit-ing bugs
Unfortunately for developers, removing one bug
of-ten introduces one (or more) elsewhere
Unfortu-nately for consumers, some commercial software
vendors release products knowing they are full of
bugs, and there are no specific regulations
prohibit-ing it Because computer technology is technical, the
user may not know whether a problem is from bugs
or from incorrect use of the software
Another unfortunate aspect to bugs is that companies
often combine software enhancements with bug fixes
and sell the new product as an upgrade with no
guar-antee that it is more robust than the previous version
(sometimes it is less so) This situation is like
buy-ing a $15,000 car with a faulty engine, and havbuy-ing
the manufacturer refuse to fix it and, instead, advise
you to pay $5,000 to upgrade to next year's model
When you do, you find that the engine's been fixed,
but the axles are defective, and the car has racing
stripes that you didn't want in the first place This
situ-ation in the software industry won't change until
con-sumers stop buying substandard software and
en-hanced upgrades, and support instead the more
re-sponsible vendors who provide patches for bugs
sepa-rately from releases of enhanced versions
bug, telegraphA telegraph lever which, depending
upon its position, can be used to send dots or dashes
to partly automate transmission
build 1 An increase in diameter of a line or object
attributable to insulating materials 2 In software
de-velopment, the process of combining, compiling, or
linking code so as to build an application
bulbThe sealed glass enclosure for an incandescent
or fluorescent lamp Bulbs provide protection for the
gaseous environments and the delicate filaments that
they enclose See Edison, Thomas Alva; lamp
bulk encryptionSimultaneous encryption ofa group
or set ofcommunications, such as multiple data
mes-sages or multiple channels on a broadcast medium
bulk eraser, bulk degausserAn electromagnetic
de-vice designed to save time by clearing the data from
a large number of floppy disks at one time By
rear-ranging the particles on the physical disk, the
elec-tronic infonnation is destroyed.Itis handy for
recy-cling the diskettes or for providing a measure of
se-curity with data that needs to be destroyed.Itis wise
to keep magnetic storage media away from computer
ently erase or damage the data on them Alarge-scale pirate software vendor, who was apprehended in Van-couver, B.C., is rumored to have had a bulk eraser in
a storage cabinet wired to a button under the service counter to destroy evidence in the case of a police raid See diskette
bulk storageMedia on which large amounts ofelec-tronic data can be stored The amount of storage that constitutes large keeps increasing In the mid-1970s,
a tape holding 100 kilobytes was considered bulk storage! In the mid-1980s, a writable optical disk holding 600 MBytes was bulk storage Now hard disks and tapes holding 4 GBytes or more are being bundled with consumer machines
bulletin board systemBBS The forerunner to the Internet, BBS systems are typically individual com-puters set up for public or private modem access, by
a number of users, on which there are shared files, mail, and chat services The administrator is usually called the SysOp (System Operator) In the late 1970s and early 1980s, it was extremely rare for a BBS to
be password-protected; there was open access to all Unfortunately, persistent abuse has made this type of BBS almost extinct.Inthe mid-80s, there were still many BBSs running on TRS-80s, Color Computers (CoCo), Commodore 64s, Apple lIe's, and Amigas, with only 5 or 10 MBytes of hard drive storage for the entire system BBSs have since become more so-phisticated, offering credit card payment options, and increasingly are being linked to the Internet through telnet See FidoNet
bump contactsSmall conductive lumps on electronic circuits that protrude to enhance electrical contact, such as those that allow chips to touch terminal pads bunch strandingAtechnique used to combine wires
so they fit tightly together, with individual strands retaining the same directional relationship to one an-other to form a stranded wire Stranded wire is use-ful in situations where flexibility is desired or when the electrical properties of the wires are influenced
by proximity to others
bunchingAn alternating convection-current effect in
an electron stream caused by velocity modulation Bunching is quantified as a parameter based on the relationship of the depth of velocity modulation to the absence ofmodulation Used in electron tubes to generate ultrahigh and microwave frequencies See klystron
Bundesamt fur Zulassungen in der Tele-kommunikation BZT A German telecommunica-tions approval authority established in the early 1980s
bundled 1 Combined products or services, some-times from a variety of manufacturers, offered at a combined price Phone and cable companies often have bundled or packaged deals, such as regular tele-phone service and Caller ID-related services offered
at a flat rate, or movie and educational channels bined Software products are often bundled with com-puter systems Operating systems are almost always bundled with computers, often along with various