In free-space optics FSOs, where diode lasers trans-mit signals through the air to connect buildings with one another or with a nearby fiber backbone, a lens can spread the light beam so
Trang 1Fiber Optics Illustrated Dictionary
batteries (TIS 96-2528 - 1985) refers to
Leclanche-type cells
LEeSLAN Emulation Configuration Server ALAN
software server that maintains configuration
informa-tion that enables network administrators to control
which physical LANs are combined to form VLANs
SeeLEC
LEDSee light-emitting diode
left-hand circular polarizationLHCP A
polariza-tion orientapolariza-tion associated with antennas, for
ex-ample, satellite antennas Left-hand refers to a
coun-terclockwise direction The left or right sense of the
polarization is dependant upon various factors,
in-cluding the transmitter type and transmission
fre-quency Some systems can be switched from
right-to left-hand and may benefit from this flexibility Left
or right orientations are also relevant to other types
ofpolarization besides circular polarization, as in
left-or right-hand slantpolarization.
left-hand rule,Ampere's ruleAhandy memory aid,
once widely used to determine an axis of rotation or
direction of magnetic flow in a current It originally
came from Ampere's description of a person
swim-ming in the same direction as the current (in a wire)
When the swimmer looks left, it's the same direction
that the north-seeking end of a compass will point if
it is in the vicinity ofthe current-carrying wire Since
then, it was decided it was easier to use the left hand
and actually look at the thumb and fingers, rather than
imagining a swimmer Extend the thumb and fingers
of the left hand so that the fingers are held together
and point straight in one direction, with the thumb at
a right angle to the fingers, in an "L" shape Now curl
the fingers around a conductive wire, so that the
thumb points in the direction of the current The
rection ofthe curled fingers is said to indicate the
di-rection ofthe magnetic field associated with the
cur-rent
Some ofthe confusion associated with left- and
right-hand rules stems from the fact that pioneer physicists
did not originally know in which direction current
was flowing in a circuit between negative and
posi-tive terminals In fact, it was not always important to
know, as long as the terms ofreference were kept
con-sistent in one direction or the other
Sometimes a distinction is made between current in
a motor and current in a generator By this
reason-ing, using the left hand, the fingers will show the
di-rection ofthe current for a conductor in the armature
of a motor Using the same hand relationship for the
right hand will show the direction for a conductor in
the annature of a generator Since the universe
ap-pears to be right-handed in its general orientation,
some physicists will assert thatitmakes sense to use
the right-hand rule See right-hand rule
leg1 Aportion of a trip, broadcast, or transmission
2 The transmission segment in a network between
two physically distinct entities (such as a
worksta-tion, switch, router, node, etc.), between addressable
entities, or a combination of the two
legacyThat which is inherited, or which remains from
a predecessor
legacy equipment/software Existing equipment, software, or operating procedures that are becoming dated but are still actively used are called legacy sys-tems For economic reasons, most legacy systems are maintained and enhanced, rather than scrapped in
favor of new systems Even when it's not
economi-cally practical, legacies are sometimes retained be-cause managers are reluctant to let go ofthe emotional investment tied up in existing systems and proce-dures A more practical reason for retaining legacy systems is that staff training costs time and money, and staffmembers may be reluctant to switch to a new system Most computer operating systems are legacy systems, incorporating downward compatibility in or-der to work with olor-der equipment
Legacy to the FutureL2F Aframework for provid-ing an integrated modelprovid-ing system oflegacy and new code components to support simulations in the Los Alamos National Laboratory (LANL) L2F is imple-mented using JAVA and CORBA to facilitate com-munication among objects written in different lan-guages The system includes a setup server and an associated database, application servers, clients and optional security services L2F has been cleared for open source distribution
Lemelson, Jerome "Jerry" H.(1923-1997) Apro-lific American inventor and engineer, Lemelson re-ceived more than 500 patents by the time ofhis death, with more patents pending
Lemelson began inventing gadgets in childhood and turned to technology in the mid-1950s.Inthe 1960s,
he designed magnetic recording and beam switching and production line automation systems.Inthe 1970s,
he began designing miniaturized devices such as por-table audio/video systems and earned patents for op-tical circuits and display devices
Sometimes the patents took many years to process For example, a patent application for a videotape re-cording system for rere-cording frame animation on tape media was submitted by Lemelson in 1962 and not granted until 1980
Lemelson is credited with developing technology that led to the ubiquitous barcode reader and he sold many data processing patents to ffiM
In 1994, Lemelson and his wife founded the Lemelson Foundation to promote and support Ameri-can invention and innovation
Lempel-ZivLZ A pair of coding compression for-mats described in IEEE articles "Compression of In-dividual Sequences via Variable-Rate Coding," in
1977 (LZ77), and "A Universal Algorithm for Se-quential Data Compression," in 1978 (LZ78) They are named after their developers Abraham Lempel and Jakob Ziv LZ77 was presented by Ziv and Lempel as a dictionary-based scheme for lossless data (text) compression LZ78 sends pairs of pointer and character data These two important schemes are so universal and powerful that a significant number of data compressors have been based on Lempel-Ziv concepts
LZ77 became the basis for LArc, LZARI, LHarc, and others Lempel-Ziv schemes have been widely
Trang 2gies, but the early versions were not developed for
the demands of multimedia networks with widely
ranging data characteristics (which scarcely existed
at the time) Another limitation of the original
Lempel-Ziv techniques was in performing efficient
searches for previous matching strings
Enhance-ments and variations are addressing some of these
limitations and adaptations to special circumstances
In 1994, Jung and Burleson proposed a parallel
al-gorithm, architecture, and implementation for
Lempel-Ziv compression exhibiting a scalable,
regu-lar structure suitable for VLSI array implementation
This has practical applications for data compression
in portable digital data communications and wireless
local area networks (LANs) where effective
compres-sion/decompression schemes can significantly
im-prove throughput and reduce connection time costs
See Ardire-Stratigakis-Hayduk, differential
Ziv-Lempel, LArc, LHarc
Lempel-Ziv-Stac LZS A data compression system
developed by Stac Electronics, Inc., largely based
upon the LZSS algorithm See
Lempel-Ziv-Storer-Szymanski
Lempel-Ziv-Storer-Szymanski LZSS Asliding
dic-tionary compression scheme descended from LZ77
by Lempel and Ziv LZSS was developed in the early
1980s by James Storer and Thomas Szymanski
Storer filed for a patent for data
compression/decom-pression in 1987 (U.S #4,876,541) LZSS differs
from its predecessor mainly by using bit flags to
iden-tify the subsequent data as a literal or an offset, which
can result in more compact data compression and
faster decompression Anumber ofdevelopers
devel-oped variations and optimized versions ofLZSS, one
of which evolved into LArc See LArc
Lempel-Ziv-Welch LZW A dynamic dictionary,
lossless data file compression scheme based on the
Lempel-Ziv method, first described by Terry A
Welch in 1984 following the publication of the
Lempel-Ziv format in the late 1970s The developer
community generated a substantial amount of debate
overuse ofLZW, since it was distributed as an open
standard for many years before the community at
large was informed that LZW was being patented
Many programmers incorporated LZW into their
soft-ware, thinking it was in the public domain LZW has
been incorporated into ARC and PKZIP PKZIP is a
particularly widespread archiving utility
A patent for the technology is held by IBM
(U.S #4,814,746), with a similar one held and
en-forced by Unisys Corporation (U.S #4,558,302).
Some claim these two patents cover virtually the same
technology, or that the Unisys patent is a subset of
the ffiM technology A similar patent has also been
held since 1989 by British Telecom
The enforcement of the patent rights caused ripples
ofunease in the programming community, as several
modem file protocols use LZW Many graphics
terchange formats incorporate LZW compression,
in-cluding TIFF, which is widely used in desktop
pub-lishing programs, and the Compuserve 8-bit GIF
for-In telephone technology, LZW is used in Northern Telecom's Distributed Processing Peripheral (DPP) for the transmission ofcompressed data At one point, Unisys issued a statement exempting freeware au-thors from paying license fees on the use ofLZW in their programs in order to quiet the concerns of soft-ware developers who were distributing softsoft-ware
with-out commercial gain Later, Unisys asserted that all
software developers would be subject to a minimum royalty ($.10) in order to protect the patent, with ex-ceptions only for charitable institutions See Ardire-Stratigakis-Hayduk, Lempel-Ziv, LZC
lens Acomponent with optical properties such that it lets light pass through while altering its path Amag-nifying glass is a simple convex lens that spreads the light coming through from the object observed such that the object appears larger when it hits the human eye or an imaging apparatus There are many differ-ent types oflenses Aprism is a type oflens that sepa-rates white light into its wavelength components so that the colors may be seen A Fresnel lens diffracts light to concentrate or diffuse it
Lenses may be combined in linear or planar arrrays
to control light passage over a larger area than would
be possible with one lens It is usually not practical
to make a single optical lens larger because the cur-vature causes image aberrations nearer the edges
In free-space optics (FSOs), where diode lasers trans-mit signals through the air to connect buildings with one another or with a nearby fiber backbone, a lens can spread the light beam so that birds, tennis balls,
or other obstacles that may briefly interrupt the trans-mission are likely to only pass through part of the beam rather than obstructing it completely See face-plate; fiber optic; Fresnel lens; lens, lenticular; trans-fer lens
lenticular Historically, lenticular meant having a lens shape, especially a lentil-shaped, double-convex lens
as in a traditional magnifying glass The term is now more often applied to lens configurations with arrays ofclosely aligned lens shapes designed to refract light
at particular angles in succession as the viewer or the lens is rotated through a plane It also no longer nec-essarily implies a double convex lens and some lens arrays with straight surfaces have been called lenticulars, though they are probably better referenced
as gratings See lenticular lens lenticular image Aspecialized optical image imprint-ing technique in which a series of two or more re-lated 2D images is interlaced and embedded under a ridged surface such that the net viewing effect is simi-lar to a stereographic image or animation exhibiting depth or movement
Early commercial lenticular images (sometimes called lenticulars) bonded an image that was printed
on card stock behind the refracting lens Now, with computerized image processing techniques, it is pos-sible to take successive frames ofan image, interlace the image at tolerances to match the lens density and imprint them on the back of the lens material itself such that the strips of the frame associated with the
·., •.•:••.,., :"• • ,,:'• •.•.1
./<;
~}~
Trang 3Fiber Optics Illustrated Dictionary
refracted light from a viewing surface are
succes-sively visible
The source of the effect is the embedding of a series
of frames behind the ridged surfaces of the base
ma-terial to exploit the refractive properties of the
indi-vidual lens facets, somewhat like a cross between a
holographic image and a traditional cell animation
Thus, as the viewing angle changes, the frame
asso-ciated with the ridges that can be seen from a
par-ticular angle becomes visible Some have described
it as an evolutionary descendent of stereograms It
has some advantages over stereograms in that many
frames can be displayed in succession and no
spe-cial viewing apparatus is required
Lenticular images can be designed to move in
hori-zontal or vertical directions The number of possible
frames in the series varies with the size of the
len-ticular surface, the differences between frames, and
the orientation
Lenticular images are created by taking a series of
image frames, deciding on the orientation of the
frames when viewed, digital image processing the
frames to determine which parts must be imprinted
on which surfaces ofthe lenticular lens, and
imprint-ing them, usually through a lithographic process,
serigraphic, or photographic process
For many purposes, lenticulars have advantages over
holograms Lenticular lens bases may be more robust
than holographic bases They are typically easier to
see in certain light conditions The colors are
photo-graphic, as opposed to the subtle prismatic colors
typical of holograms They have advantages in size,
as it is fairly straightforward to impress lenticular
images as large as a small billboard
Not surprisingly, lenticular images are popular as
novelty items and point of purchase marketing
dis-plays However, there are many scientific applications
in which a lenticular image can help reveal the
struc-tures of an object or convey 3D information in an
educational environment For example, a lenticular
image can help illustrate the 3D geometry
ofbiologi-cal specimens or celestial bodies
lenticular lens A flattish lens array designed such
that, when viewed at particular angles, it reflects light
in certain predetermined directions through succes-sive, closely-aligned surfaces within the viewing scope of the user or imaging device Each individual lenslet in the array is called a lenticule
Commercial lenticular lenses are typically fabricated from coated plastic sheets (PVC, APET, styrene, etc.) that are flat on one side, ridged on the other They are availableinvarious shapes, sizes, and resolutions
A plastic rectangular sheet may have a resolution of about 10 to 75 lenticular ridges per inch (RPI) and a thickness of about 0.0625 in Optimum viewing angles depend upon density and application but are generally around35° For offset printing processes,
higher densities are available As the density of the lenses and overall size increase so, generally, does the difficulty of laminating the material
Many lenticular lens sheets are sold to the publish-ing industry for novelty products and signage How-ever, lenticular lenses composed of arrays of cylin-drical lenslets are also sold as diffusion lenses for scientific purposes See Fresnel lens, lenticular image
LEOSee low Earth orbit
LESLAN Emulation Server A local area network (LAN) software server which provides Media Access Control (MAC) address-to-ATM address resolution services for LAN Emulation (LANE) See LECS, LEe
letterboxing A video display technique that preserves
the aspect ratio of a wide screen cinematic produc-tion even when it is displayed on a different system such as a TV screen Wide screen movies are often modified to fit a TV screen ratio, but then informa-tion on either side ofthe image is lost.Inletterboxing,
it may appear as if some of the image is gone, be-cause there are larger black areas at the top and bot-tom ofthe image, but in fact, it is the letterboxed ver-sion that shows theentire image and retains the
fi-delityof the original picture The difference can be quite dramatic For example, in a number of scenes
in the beautiful filmBaraka,the nonletterboxed
ver-Lens for Concentrating Light Toward a Cable
c
Depending upon the light source and the nature of the light emitted (e.g degree ofspread) a lens may be used to concentrate the light (b) emitted by a light-emitting diode (LED - a)Jor example The lens focuses the light (d) to a point within the cladding diameter (e) of the lightguiding fiber optic cable F,vm there the light reflects within the core (f) and propagates along the waveguide (g).
Trang 4three See anamorphic, aspect ratio.
levell cacheAsmall, fast static RAM buffer On an
Intel Pentium central processing unit (CPU) chip, a
16-kilobyte cache memory is incorporated into the
chip
level2cacheAnexternal, fast, static RAM buffer
On an Intel Pentium central processing unit (CPU)
chip, a cache memory is incorporated into the
pro-cessor in addition to the level 1 cache included with
the CPU On some of the Pentium chips, the level 2
cache is layered into the CPU (for faster access), and
in some, it is a separate section, with a bus allowing
it to communicate with the CPU Level 2 caches may
vary in size from 256 kilobytes to 1MByte
Leviton Manufacturing Company, Inc.One ofthe
world's largest companies specializing in the design,
development, and production oftelecommunications
wiring devices The Leviton Voice&Data Division
provides system solutions for network infrastructures
with fiber and copper technologies.Itproduces
con-nectors, cabling assemblies, panels, fiber optic test
equipment, and other related products The Voice&
Data Division is located in Bothell, Washington
Leyden Jar - Historic Capacitor
conducting rod
insulating
stopper
insulating
container
wire or chain
from rod to
bottom metal
surface
thin metal
coating inside
and out
The Leyden jar was an important historic storage
tankfor electrical experimentation, acting as an
elec-trical condenser/capacitor [American Radio Museum
collection.]
Leyden jar, Leiden jarA device that concentrates
and stores electrical energy, thus serving as an
elec-trical condenser; an early capacitor The Leyden jar
menter, in 1745.Itconsisted ofa nail in a bottle con-nected to a terminal of an electrical device, with the jar held in von Kleist's hand; he received an unpleas-ant shock from his experiment Ayear later, Cunaeus and Peiter van Musschenbroek created a condenser consisting of ajar mostly coated inside and out with metal foil, with the inner coating in contact with a conducting rod that passed through the stopper (in-sulator)
The foil would typically cover about two-thirds or a little more ofthe swface ofthe jar, and the rod would
be inserted through a stopper ofcork or rubber Some-times a chain was attached to the bottom end of the metal rod The jar was named after the town ofLey-den (LeiofLey-den) in The Netherlands It was subsequently discovered that a Leyden jar charge could be sent through wires over distance Benjamin Franklin con-ducted numerous experiments with Leyden jars in his attic laboratory, and they remained prevalent for an-other 150 years
LFACSSee Loop Facility Assignment and Control System
LFAPSee Light-weight Flow Admission Protocol
LGCLine Group Controller
LGRSLocal Government Radio Service
LHA A lossless compression scheme developed by Yoshizaki In terms of compression performance for text files, LHA was an improvement over the origi-nal LZ77 by Lempel and Ziv, through static Huffman coding, but not as efficient as the popular GZIP.Itis also known as LZH due to shared lzh/.lha file ex-tensions LHarc also uses lzh file extensions, so there may be some confusion, but they are from the same family ofcompression utilities and somewhat down-wardly compatible LHA has been widely used in Japan and is a popular utility on the Amiga computer
See LHArc, LZHUF
LHarcAfast, efficient dictionary-style compression!
archiving utility descended from LZSS LZSS was developed into LArc by Okumura and Moo in the late 1980s Okumura then created LZAR!, which incor-porated adaptive arithmetic compression, from LZSS
Yoshizaki subsequently modified LZAR! with adap-tive Huffman coding instead of adapadap-tive arithmetic coding to create LZHUF, which was then rewritten
in assembler with an updated interface to create LHarc, which was fast and popular
LHarc is a full archiving utility, allowing multiple files to be stored, listed, added to, or removed from a single archive LHarc was especially popular on the Amiga, edging out earlier favorites due to its speed and versatility; there are versions for mM-licensed PCs, Macs, Ataris, and Unix machines as well See Lempel-Ziv, RAR
LHCPSee left-hand circular polarization
LICSee light-guide interconnect cable
Licklider,J.C.R "Lick"(1915-1990) Acomputing pioneer who was instrumental in supporting a num-ber ofimportant early developments, including time-sharing and theARPANET.He is best remembered for his inspiration and enthusiasm, and his ability to
; 1 : :
'., :";,••,,.• ··.s:~.:.,~.•, [,.•i.,.,t,"" "
'::;1,
Trang 5Fiber Optics Illustrated Dictionary
get the funding and other resources necessary for
vari-ous computer pioneers to bui ld the stuff of dreams
LIDBSee Line Identification/Information Database
LIFOlast in, first out A descriptive term for the
or-der in which data are processed in a queue For
ex-ample, picture a stack of dinner plates in a plate well
in a buffet; when the stack is refilled by the
restau-rant staff, the last plate on the stack (the one on top)
is the first one removed by the next customer Data
can be handled in the same way See FIFO, FILa,
GIGO, LILa
hav-ing wavelengths in the approximate range of 390 to
750 nm, that is, the transition to ultraviolet at one end
of the spectrum and infrared at the other end The
phrase white light is used to describe light with a
mix-ture of frequencies The speed of unimpeded light is
3.00 x 10.8meters per second, symbolized as c in
mathematical calculations
It was discovered that light could be broken up into
its component wavelengths with prisms, and this
aided researchers in understanding the nature oflight
and the colors associated with particular frequency
ranges The visible spectrum is specific to human
perception; other mammals and insects have broader,
shifted, or more specific perception of color ranges
A flower that to humans appears yellow may have
other colors in the ultraviolet or infrared spectrum that
are perceived by pollinating insects Most dogs and
cats are insensitive to colors as humans perceive them
(Siamese cats reportedly can perceive color)
Light is the primary stimulus for sighted individuals
sense to forms, other beings, and their orientation and
movement in three-dimensional space The
interac-tion ofthe light waves hitting various objects,
bounc-ing back through eyes and processed by a brain,
con-stitutes the complex phenomenon called sight Some
creatures can see beyond the range ofhuman-visible
light Dolphins use sonar (sound waves) to detect
objects which may not be visible to humans, and thus
can "see" inside some objects in a way not possible
for humans without mechanical aids See fiber optic,
infrared, lamp, laser, light-emitting diode, spectrum,
ultraviolet
optical fiber The material provides a conduit or
chan-nel by which the light can be directed The idea is
similar to the concept of wave guide
specified point to another by electromagnetic
radia-tion in the optical freuquencies.Ingeneral, light
trav-els in a straight line, but multiple reflected straight
line segments can be combined to form a curved path
See lightpath
fi-ber optic cables Laser light will travel along a
fila-ment the size of a hair, and filafila-ments can be bundled
to provide more light This is very handy for
illumi-nating hard-to-reach places like small pipes or
in-side the human body for medical research or
pro-cedures
transmis-sion medium in a fiber optic network Coded electri-cal pulses from computer systems, existing phone systems, and other sources are used to stimulate a light source such as a light-emitting diode (LED) or
an injection-laser diode (lLD) to generate the light pulses that are then funneled through a lens, corrected with prisms, if necessary, and transmitted through a cladded (shielded) fiber waveguide The light propa-gates through the waveguide through a process called total internal reflection (TIF) The cladding layer around the fiber reflects beams that are within a cer-tain range ofangles back into the fiber core, preserv-ing the signal over distance At the receivpreserv-ing end, the light pulse is usually translated back into an electri-cal signal for digital decoding and other processing Along the transmission path, the light pulse may be amplified or regenerated through a variety of elec-trical or optical means See light-emitting diode
miles/sec (299,800 kilometers/sec) When traveling through air, water, a fiber optic waveguide, or other dielectric (conductive) material, the matter will im-pede the speed oflight and slow it down.Infiber op-tic cables, for example, the propagating light travels about 124,100 miles/sec
In general, the higher the refractive index of a mate-rial through which the light travels, the more it im-pedes the propagation oflight This characteristic can
be exploited in cable construction The cladding around an optical fiber core is made from a material with a slightly lower refractive index than the core such that the light beams are reflected back into the core (within certain effective angles) Sometimes, as
in multimode cables, the refractive index is graded
to selectively control light speed based upon its dis-tance from the core, in order to compensate for cer-tain types of frequency dispersion See dispersion, graded index, stepped index
semicon-ductor p-n junction structure used in many electronic displays, particularly small ones The LED lights up when a current is provided LEDs are common in digital clocks, calculators, microwave readouts, elec-tronic instrument displays, and much more The LED typically resembles a small illuminated knob with a semiconductor within the knob (which is actually a lens), and leads coming out from the semiconductor/ knob arrangement into the device circuitry
LEDs are now also used to provide the light rays for certain types of fiber optic transmissions, especially
in multimode cables (more precise and more expen-sive laser lights are used for single mode fiber)
light-guide cable interconnect (LCI) A light-guide is
a conduit for directing and containing a light-based transmission and an interconnect cable is one which specifically interconnects various equipment and de-vices LICS are used in fiber optics transmission net-works and typically have connection ends 'that can
be more readily connected and disconnected for the purposes of installing and reconfiguration network physical topologies See loose cable
Trang 6protocol from Cabletron which allows an external
Flow Admission Service (FAS) to manage flow
ad-mission at the network switch, allowing flexible FAS
to be used by a vendor or user without unduly
bur-dening the switch See RFC 2124
LIGHTCONNECTA leading global supplier of
diffractive MEMS-based dynamic optical
network-ing components such as gain equalizers
lightlineAmultiple fiber filament assembly in which
one or both cable ends is set in a row in a supporting
frame to produce a line of light Commonly one end
ofthe assembly is round (to couple with the next link
or the light source) and the other spreads the filaments
side-by-side into a long line (essentially the shape of
a window washing tool, round at one end [handle]
and long and narrow at the other [wiper blade])
Ap-plications include machine vision illumination,
sens-ing, signage, artworks, and commercial lighting For
imaging or display applications, the individual fibers
in a lightline can be calibrated to different intensities
lightpathApoint-to-point optical link that may pass
transparently through intermediary nodes Thus, it
comprises a logical one-hop link, even ifthere is more
than one physical hop
LightSARlight synthetic aperture radar See
syn-thetic aperture radar
lightwave communicationsOptical
communica-tions systems using high frequencies This term helps
distinguish optical communications from very short
wave microwave communications Fiber optic cables
are used as the physical medium for transmission
This is distinct fromlightwave transmissions, which
involve transmission through air or space rather than
through a cable as the physical medium When
ho-modyne or heterodyne detection schemes are used,
they are called coherent optical communications
systems
lightwave transmission Optical communications
space, without using a cable as a physical medium
This is a wide bandwidth, line-of-sight, short distance technology which is relatively inexpensive.Itis suit-able for building-to-building installations where it is impractical to string wires This system is subject to loss and is somewhat dependent on weather, and thus specialized in its practical applications
Lightweight Directory Access ProtocolLDAP.A
front-end client/server standard intended to provide
a lightweight complement to Directory Access Pro-tocol (DAP), LDAP is based somewhat on ITU-T
X.SOOand can accessX.SOOdirectories.Itis a distrib-uted, hierarchical protocol for accessing network en-tities and repositories and is more scalable for some implementations than, for example, Network Infor-mation Services (NIS) LDAP was developed in the early 1990s at the University of Michigan and sub-mitted as a joint Standards Track RFC with the ISODE Consortium and Performance Systems
In LDAP, the protocol elements bypass some of the t:~~~~;~:~:F~~~~~~~~~~l!::J:k:~~ ••
ments are simply encoded as strings A lightweight best error rate (BER) encoding is used for protocol elements Extensions to the fonnat such as authenti-cation and server discovery are being discussed and developed See RFC 1777
LightWeight Encoding RulesLWER LWER com-prise one of three major encoding schemes used in open architecture development, developed in the early 1990s LWER provide a means for creating encodings optimized for encoding and decoding CPU cycles and apply better to wide bandwidth commu-nications between similar architectures than to slower mixed-system networks See Basic Encoding Rules, Packed Encoding Rules
Light-Emitting Diode Light Source
c
Lasers and light-emitting diodes (LEDs) are two ofthe most important sources oflightforfiber optic cables In this
example, an LED (a) provides a source ofspreading light similar to aflashlight beam (b) which isfocused (d) by a
lens (c) towards the lightguide where it propagates by total internal reflection (TIR) by bouncing offthe cladding (e),
which has a lower refractive index than the conducting core (j) Depending upon the application, multiple LEDs may
be used.
Trang 7Fiber Optics Illustrated Dictionary
like new A subjective term describing a product that
has been opened and stored or used, but is physically
unmarred and functionally in good working order
The phrase is not intended to imply that the product
will last as long as a brand new product in normal
use, hence the phraselike newinstead ofnew. See
certified, refurbished, used
LIM Link Interface Module.
LINeLaboratory Instruments Computer One ofthe
earliest small computers, developed at the
Massachu-setts Institute ofTechnology (MIT) in 1963, the LINC
was the inspiration for Digital Equipment
Corporation's (DEC's) PDP-8
line code violation LCV In EI/T! networks, an
in-dicator of bipolar violations (BPVs) or excessive zero
(EXZ) errors Aline err second (LES) is a second time
duration in which one or more LCVs are detected A
severely err second (SES) is one in which serious
defects are detected such as 2048 LCVs or more for
nonCRC signals
line err second LES See line code violation.
line of bearing LOB In radio direction-finding
sys-tems, the direction and general position of a target or
positional goal A radio direction-finder is a device
for tracking down the source ofa radio frequency
sig-nal Flashing lights or beeps are sometimes used on
portable systems to indicate the direction or
proxim-ity of a target (e.g., wildlife with a radio collar) In
mounted systems, a monitor may indicate a line of
bearing relative to a selected reference point Most
direction-finding devices will have a certain margin
oferror, expressed in degrees, that can be used to
cal-culate the region of error of a bearing over distance
Lines of bearing are sometimes graphically plotted
by hand on a chart or map A line of bearing is
some-times a calculated position based on a series
ofread-ings within a general range In direction-finding,
in-tersecting lines of bearing may indicate the position
of a transmitter at or near the intersecting point
Consumer systems for determining lines of bearing
are more sophisticated compared to the old handheld
beeping antennas prevalent for many years It is now
possible to use a Global Positioning System device
with an automatically adjusting antenna to carry out
direction-finding and transmitter tracking functions
line of sightAnunimpeded direct line of connection
through "free space" as it relates to a particular
trans-mission technology Line of sight is a relative term
The line of sight for a flashlight beam is usually
lim-ited to 10 or 15 feet, as the light falls off quickly and
stops if it encounters physical objects in its path The
line of sight for a coherent laser beam in the infrared
spectrum (e.g., a VCR remote control) is a little longer
than a flashlight beam and can pass through some
types of objects The line of sight for a handheld
ra-dio is a couple of miles because the rara-dio waves can
pass through walls and many other solid objects, but
the signal is gradually scattered by very rough
ter-rain, many buildings, and reflective surfaces Line of
sight is an important concept in radio
communica-tions and free-space optics as these are important
options for linking people to fiber optic backbones
in regions where the fiber optic cables cannot be in-stalled all the way to the premises See free-space optics, Photophone
line speed The maximum or actual speed of data
transmission through a wire or cable There are dra-matic variances in line speeds depending upon the type of cable, the distance or character of the path, and the interface or modulation devices used to con-vert or send/receive the data
Line speed for digital data transmissions is commonly expressed in terms of the number of bits or charac-ters transmitted per unit of time (usually seconds) For comparison, a typical 64K ISDN connection to
a page on the World Wide Web might download an average-sized image at 7500 cps Many factors can reduce the line speed from its theoretical maximum, but ISDN and cable modem throughputs are signifi-cantly slower than 10M Ethemet, for example, which could download the same data at speeds of up to about 480,000 cps
line trunk controller LTC A telecommunications
trunk controller that provides a meanstouse equip-ment contained within a central office The LTC is capable ofgiving interfaces to outside ports; it is one
of a number of peripheral modules that can provide trunk interfaces to a digital multiplex system (DMS)
Itmay interface with multiple lines which, inturn, may be linked to a network by multiple speech links
line utilization monitor LUM A diagnostic and
ad-ministrative utility providing statistical logs or dis-plays of transmission line use
line switching unit LSU A generic phrase for a
va-riety of devices that range from simple passive switchers to complex active voice or data transmis-sion line switchers A line switching unit can be used
on a single computer or local area network (LAN) to switch between multiple peripheral devices such as printers and scanners or, in a more sophisticated ar-chitecture, between a disabled system and a standby system In telephony, line switching units are typi-cally floor-size cabinets with multiple lines and elec-tronic switching circuits to manage those lines and their connections LSUs may be manual or automatic Automatic units are more likely to be used in remote locations or for backup or emergency systems trig-gered by alarms or fault conditions
linear modulation A modulation scheme developed
at Bristol University in 1991 The technology began
to be used in the U.S in the mid-1990s The advan-tage oflinear modulation is that it enables voice and data transmissions to be carried over narrow chan-nels (e.g., 5 kHz), thus making it possible to increase capacity up to five times that of traditionally wider frequency-modulated (FM) channels A pilot tone is inserted into the audio baseband, essentially splitting the band, enabling linear modulation to be achieved without distorting the signal
The development of new modulation schemes that can support higher capacities and bit-rate densities has become particularly important in recent years The demand for mobile services is growing, while the pool of available frequencies remains essentially
Trang 8eral Communications Commission (FCC) has issued
a requirement that spectral efficiency for mobile
de-vices be improved by the year 2005 Linear
modula-tion is seen as one of the key technologies that may
aid developers in meeting this requirement See tone
in band
linear predictive codingLPC Asystem for digitally
encoding speech at low bit rates while retaining good
clarity and recognition without extensive computing
overhead In LPC, speech signals are analyzed as to
various common aspects, including resonance,
inten-sity, pitch, etc Fonnants (resonances or boosted
fquencies) are filtered and stored in addition to the
re-maining signal, which is tenned the residue Portions
of the speech are sampled at the rate of about 40
frames per second, resulting in manageable file sizes
for storage and intelligible speech when the encoded
data is reconstructed In the encoding/decoding
pro-cess, estimates are used and errors between a
pre-dicted signal (based on previous samples) and an
ac-tual signal are minimized for the series of sequential
samples LPC methods result in lower infonnation
bit rates than adaptive predictive coding (APC)
meth-ods Since speech and music share many common
aspects, LPC has been useful for computer music
encoding for storing and extracting time-varying
fonnant information
LPC concepts have also been applied to the
compres-sion of imagery in that predictive techniques have
been used to provide linear approximations of
vec-tor infonnation which, inturn,can be used for
loss-less or lossy compression algorithms for geospatial
imagery See sampling
linear programmingAlgorithmic symbols,
proce-dures, and strategies in which a problem or statement
can be expressed in a standard form within certain
variable, sign, and coefficient constraints to find a
solution to a problem Both the program constraints
and the problem itselfare linear While a limited fonn,
many types of problems can be solved or expressed
with this type of programming environment
Variables within a linear programming problem can
be seen as corresponding to decision factors in the
problem to be solved Linear programming is useful
for solving or generating optional optimization
solu-tions for production lines, transmission paths, or
in-vestment or management scenarios
linear search & track processorLSTP Asystem for
processing radar search and track signal data The
Na-val Research Laboratory Collaboration has been
port-ing LSTP functionality to parallel computport-ing platfonns
linear tape-openLTD A tape storage technology
developed jointly by Hewlett-Packard, IBM, and
Seagate LTG is an open fonnat that incorporates
lin-ear multichannel, bidirectional tape fonnats with data
processing enhancements such as error checking, data
compression, etc.Inspite ofthe many storage options
available for data management that rely on optical or
hard disk fonnats, there are still advantages to the use
of tape, including high capacity and low cost
Hun-dreds of GBytes can be stored on a single tape
car-The LTO technology has been optimized into two
open tape versions specified as fast access or high capacity This decision was based on the observation
that some storage needs are read-intensive (requir-ing speed ofaccess) and some are write-intensive (re-quiring high capacity) The formats are respectively called Accelis and Ultrium Licenses for third-party developers were made available as ofApril 1998 Commercial implementations vary, but transfer rates
of 30 Mbps are available IBM offers low voltage differential (LVD) and high voltage differential (HVD) versions for SCSI and Fibre Channel (FC) Quantum's Super DLTtape technology is somewhat competitive with LTD See Super DLTtape line, communications Inits most general sense, any path or transmissions link between two or more com-municators, including any subscriber line, switches, routers, cables, etc which might comprise the main
transmissions pathway The term line usually implies
a physical connection, or series of physical connec-tions including wire or fiber optic cable Hybrid sys-tems including wired and wireless connections are sometimes also called lines Complete wireless con-nections are usually referred tointerms of ainvays
rather than lines
line, electricA circuit connection physical conduc-tor consisting of shielded or unshielded wire/cable line conditioningImprovements and enhancements
to a communications line to reduce interference and improve the quality of the signal Some phone com-panies offer higher quality line service as an option, which may be important to those doing a lot of data communications over a phone line
line finder Anevolutionary improvement in step-by-step telephone switching systems that eliminated the need for a separate switch selection for each sub-scriber line When the caller picked up the phone a relay would be used to find an available line-finder switch, hunting for the caller's terminal would be ini-tiated, and the caller would be given a dial tone when the line was connected with the switching system Line IdentificationlInformation DatabaseLID.A national system of telecommunications information databases first deployed in the early 1990s It is de-signed so that subscriber and carrier information can
be readily accessed and cross-referenced This infor-mation is used for inforinfor-mation-querying, validation, and alternate billing administration See Local Num-ber Portability
line impedance stabilization networkLISN A di-agnostic instrument for measuring emissions, a LISN stabilizes the line impedance so that tests can be re-peated against a reference at more than one point This enables the device tested to be isolated from an ex-ternal power source, for example There are a vari-ety oftypes ofLISNs with low-pass or high-pass fil-ters, depending upon their purpose
LISNs have connectors for attaching the tested de-vice and may also have a connection for additional diagnostic equipment or displays, such as a spectrum analyzer or electromagnetic interference (EM!) meter
Trang 9Fiber Optics Illustrated Dictionary
They are designed to test within specific frequency
ranges (e.g., 10 kHz to 30MHz).
LISNs are commonly used to see ifdevices are
gen-erating signals (radio frequency interference) that
may affect their operation or nearby devices They
also can help determine whether the emissions are
above certain maximum levels established as
indus-trystandards (e.g., ANSI) or in conformance with
levels established by regulating bodies such as the
Federal Communications Commission (FCC)
Elec-tromagnetic disturbance characteristics and standards
are encompassed within CISPR 11 to CISPR 28
CISPR 16, in particular, specifies radio interference
measuring apparatus and measurement methods
per-tinent to LISNs Multiline ISNS are also known as
V-Networks See International Special Committee on
Radio Interference
line insulation test LIT In telephony, a diagnostic
test performed from the central office to determine
line resistance and line voltages
line light A linear source of illumination (long and
narrow) as might be created with a fluorescent bulb
or neon light In most cases, the distribution of light
is fairly even along the breadth of the light, though
this is not necessarily so Aline light can also be
cre-ated with an array ofpoint lights, as from an array of
light-emitting diodes or lasers and may be fed to a
remote location through a fiber optic cable See
linelight
line noise Electrical noise in a communications line
which interferes with voice communications, or
which causes spurious characters to show up in a data
transmission.Inolder modems used over phone lines,
line noise sometimes resulted in strange characters
being displayed in the telecommunications software
terminal window Severe line noise can interrupt a
data file transfer, or even cause the connection to be
disconnected With newer phone line services and
newer, error-correcting modems, this problem is
di-minishing
line of sight Many communication technologies
re-quire an unobstructed straight line of travel for the
signals to reach the intended destination or be seen
by the appropriate people, so this is an important
con-cept that affects the design ofmany communications
technologies The term is generally used in the
con-text ofair- or spaceborne communications, though it
is also applicable to sonar and may even be stretched
to include nonvisible electromagnetic phenomena
traveling through evenly particulate matter (e.g.,
x-ray probing in soft dirt)
Semaphore systems, for example, require that
signal-ing arms or flags be visible to the receiver Beamed
light from a ship requires that no impediments block
the way or bounce the beam in a direction away from
the recipient (fog can rapidly diffuse a light beam)
Very short wave radio signals, which may be readily
reflected or absorbed by objects and even small
par-ticles, require a clear line ofsight to project over
dis-tances Line of sight thus refers to a straight, clear,
direct, and generally unobstructed travel path The
line of sight for a particular technology is usually
expressed as an expected range or as an expected maximum
Since some types of phenomena or wavelengths are more readily absorbed or reflected than others, line
of sight is a context-sensitive designation The line
of sight for a flashlight beam or infrared remote in-side a dark house may only be 15 ft depending upon where the visible or infrared light beam hits a wall, another person, or a piece of furniture The line of sight for a 2.4 GHz home entertainment system trans-ceiver might be about 150 ft, traveling through walls and people, but not having sufficient power to reach the entire neighborhood to interfere with all the other home entertainment transceivers (thoughitmay cause strange images on the television next door) The line
of sight for a radio modem or small handheld radio may be up to a couple of miles depending upon the hills and trees in the terrain and how densely the buildings are spaced Thus, line ofsight is dependent upon the type of phenomenon, its power, and its wavelength
In order to extend line of sight, many communica-tions technologies rely on repeater stacommunica-tions for propa-gating a signal past obstructions and over longer dis-tances Radio transmissions often use the Earth's ionosphere as a type of"repeater station" in the sense that the waves are directed at the ionosphere at a par-ticular angle and bounced back toward the Earth to span longer distances than would be possible using
a horizontal line of sight GPS systems rely on mul-tiple orbiting satellites that are spaced in relation to each other such that at least three are usually "vis-ible" within the radio range ofa receiver at any given time The different coordinates obtained by the sat-ellites within the line of sight are used to triangulate
a location
Mobile communications systems that rely on radio
or light waves are subject to line ofsight limitations For this reason, many mobile data and voice services hook into landlines for a significant portion of the transit distance The landlines have traditionally been copper wire phone lines, but increasingly broadband fiber optic land links are used
line powered Any device that receives its power from the main system or transmissions medium to which
it is attached For example, most basic phones with-out extra features do not require a power supply or battery because they are powered by the current in the phone line Some laptop peripheral devices, to minimize size and weight, derive their power from the laptop itself See talk battery
line printer A printing device that prints one line of characters, or a full line at a time This phrase has been applied rather generically to most impact printers that print a line of characters by typing each character successively, but, technically, these are actually char-acter printers True line printers compose and "stamp"
an entire line oftext (they're quite fast and often very noisy), and compose the next line in the printer memory buffer while the current line is being printed Line printers tend to be used in institutional and industrial environments where speed is more
Trang 10impor-line status indicator On a telephone, modem, or
other appliance that can connect to more than one line
at a time or perform a variety of functions on one or
more lines, there may be a character display or
vari-ous lights to inform the user of the status of the line
On multiline telephones, a light usually shows which
lines are in use so that the user can avoid barging in
on a call inadvertently On a modem, line status
indi-cators may flash to show whether data are sent or
re-ceived, and may indicate whether or not a carrier is
present On network ports, status indicators may
in-dicate loss offrame (LOF) or loss of signal (LOS)
line switchingSee circuit switching
Line Terminating EquipmentLTE In SONET
net-works, an element that originates and/or terminates
a line signal.Itcan originate, access, or modify the
line overhead, or terminate it, ifneeded See SONET,
Synchronous Transport Signal
linear transponderAdevice commonly used in
com-munications satellites and radio relay stations, which
takes a small segment of frequencies, amplifies the
signal strength across the range of frequencies, and
retransmits them at a slightly different frequency
range (by shifting or multiplying), so that the whole
segment is adjusted up or down This is often done
to prevent the transmitted signals from interfering
with the received signals See store and forward
re-peaters
lineman, line workerIn the early days of the
tele-graph, when cable was being strung across continents,
linemen were assigned to dig holes, cut down trees
for poles, set the poles, climb them, and attach the
wires, gradually working their way through
wilder-ness, native encampments, and mountain ranges, until
the coasts and settlements were interconnected Once
the lines were installed, they would test them, often
with portable telegraph keys, and maintain the lines
through inclement weather over hostile terrain
The work was hazardous, no insurance or benefits
were available, and linemen injured by electrocution,
falls, and other hazards were dependent on the
good-will of their employers for assistance
Maintaining the increasing number ofwires and poles
involved the dedication of many 24-hour crews and,
until the mid-1960s in North America, much of the
work was done by climbing the poles with belts and
cleated boots (lineman's climbers), securing in at the
point that needed repair, and doing the work
manu-ally with simple tools Since that time, power tools,
sophisticated testing equipment, and cranes with
buckets (cherry pickers) for the line workers have
in-creased in use to the point that it is uncommon to see
a worker scale a pole in urban centers
The line workers now are also responsible for
dig-ging, diagnosing, and installing underground
trans-missions lines,inaddition to managing lines on
util-ity poles
lineman's climbers, line climbersAvariety ofpole
climbing equipment used over the last century to
al-low installation and repair workers to scale utility
poles in various types of weather These range from
on the boots, used in conjunction with a heavy hip belt (sometimes called a "scare strap") that helped the line worker stay secured and oriented to the pole at a comfortable angle In urban areas, line climbers have mostly been superseded by mechanized cranes, some-times called "cherry pickers," although climbers are still needed in some circumstances, especially in ar-eas of rough terrain See lineman
Magnetic Lines of Force
Iron filings sprinkled around a magnet on a light-colored surface or piece ofglass will reveal particu-lar types ofpatterns like those shown in this diagram, depending upon the shape ofthe magnet These pat-terns will change each time the filings are sprinkled, since they areformed not because there are lines ema-nating from the magnet, but because the magnetic forces associated with the magnet cause the particles
to interact with the magnet and one another in spe-cific ways.
Lines offorce are related to the shape and orienta-tion ofthe objects with which they are associated, and the currentflowing through those objects, in the case ofelectromagnets Above are some examples oflines offorce associated with different structures.
~> ,
,::t