Fiber Optics Illustrated Dictionary batteries (TIS 96-2528 - 1985) refers to Leclanche- type cells. LEeS LAN 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. LED See light-emitting diode. left-hand circular polarization LHCP. A polariza- tion orientation 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 of polarization besides circular polarization, as in left- or right-hand slantpolarization. left-hand rule,Ampere's ruleA handy 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 ofa 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 ofa compass will point if it is in the vicinity of the 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 di- rection of the curled fingers is said to indicate the di- rection of the magnetic field associated with the cur- rent. Some of the 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 of reference 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 of the current for a conductor in the armature ofa 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 that it makes sense to use the right-hand rule. See right-hand rule. leg 1. A portion ofa 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. legacy That which is inherited, or which remains from a predecessor. 582 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 of the 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 staff members 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 older equipment. Legacy tothe Future L2F. A framework for provid- ing an integrated modeling system of legacy 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) A pro- lific American inventor and engineer, Lemelson re- ceived more than 500 patents by the time of his death, with more patents pending. Lemelson began inventing gadgets in childhood and turned to technology in the mid-1950s. In the 1960s, he designed magnetic recording and beam switching and production line automation systems. In the 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. F or example, a patent application for a videotape re- cording system for recording 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-Ziv LZ. A pair of coding compression for- mats described in IEEE articles "Compression ofIn- 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 © 2003 by CRC Press LLC adapted and used in network and modem technolo- gies, 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 (LAN s) 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. A sliding 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. A number of developers devel- oped variations and optimized versions ofLZSS, one of which evolved into LArc. See LArc. Lempel-Ziv-Welch LZW. A dynamic dictionary, loss less 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. Asimilar patent has also been held since 1989 by British Telecom. The enforcement of the patent rights caused ripples of unease in the programming community, as several modem file protocols use LZW. Many graphics in- terchange formats incorporate LZW compression, in- cluding TIFF, which is widely used in desktop pub- lishing programs, and the Compuserve 8-bit GIF for- mat popular on the Web. In telephone technology, LZW is used in Northern Telecom's Distributed Processing Peripheral (DPP) for the transmission of compressed 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 software 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 A component with optical properties such that it lets light pass through while altering its path. A mag- 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 of lenses. A prism is a type of lens 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 of closely 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 A specialized 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 of an 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 583 ·., •.• : ••. ,., :" •.•. ,,:' • •.•. 1 .• ,:., ./<; ~}~ © 2003 by CRC Press LLC Fiber 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 ofpossible 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 of the 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 ofpurchase marketing dis- plays. However, there are many scientific applications in which alenticular 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 available in various 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 around 35°. 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. LEO See low Earth orbit. LES LAN 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 of the image is lost. In letterboxing, it may appear as if some of the image is gone, be- cause there are larger black areas at the top and bot- tom of the image, but in fact, it is the letterboxed ver- sion that shows the entire image and retains the fi- deli ty of the original picture. The difference can be quite dramatic. For example, in a number of scenes in the beautiful film Baraka, 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 of spread). a lens may be used to concentrate the light (b) emitted by a light-emitting diode (LED - a)Jor example. The lensfocuses the light (d) to a point within the cladding diameter (e) of the lightguidingfiber optic cable. F,vm there the light reflects within the core (f) andpropagates along the waveguide (g). 584 © 2003 by CRC Press LLC sion shows only two people rather than the original three. See anamorphic, aspect ratio. levell cache A small, fast static RAM buffer. On an Intel Pentium central processing unit (CPU) chip, a 16-kilobyte cache memory is incorporated into the chip. level 2 cache An external, 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 1 MByte. Leviton ManufacturingCompany,Inc. One of the world's largest companies specializing in the design, development, and production of telecommunications wiring devices. The Leviton Voice & Data Division provides system solutions for network infrastructures with fiber and copper technologies. It produces 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 jar A device that concentrates and stores electrical energy, thus serving as an elec- trical condenser; an early capacitor. The Leyden jar was devised by E.G. von Kleist, a German experi- menter, in 1745. It consisted 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. A year later, Cunaeus and Peiter van Musschenbroek created a condenser consisting ofajar 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 of the swface of the jar, and the rod would be inserted through a stopper of cork or rubber. Some- times a chain was attached to the bottom end of the metal rod. The jar was named after the town of Ley- den (Leiden) 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. LFACS See Loop Facility Assignment and Control System. LFAP See Light-weight Flow Admission Protocol. LGC Line Group Controller. LGRS Local 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. It is 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 of compression 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. LHarcA fast, 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 adaptive 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. LHCP See left-hand circular polarization. LIC See light-guide interconnect cable. Licklider,J.C.R. "Lick" (1915-1990) A computing pioneer who was instrumental in supporting a num- ber of important early developments, including time- sharing and the ARPANET. He is best remembered for his inspiration and enthusiasm, and his ability to 585 ;1.:: '., :";, •• ,, .• ~.:.,~ .• , [, .• i.,.,t, "" " .··.s: '::;1, © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary get the funding and other resources necessary for vari- ous computer pioneers to bui ld the stuffof dreams. LIDB See Line Identification/Information Database. LIFO last in, first out. Adescriptive term for the or- der in which data are processed in a queue. For ex- ample, picture a stack ofdinner 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. light Radiant energy visible to the human eye 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 ofunimpeded light is 3.00 x 10. 8 meters 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 ofcolor 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 of the 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. light guide Light-conducting material such as an optical fiber. The material provides aconduit or chan- nel by which the light can be directed. The idea is similar to the concept of wave guide. light path In general, the path traveled from one specified point to another by electromagnetic radia- tion in the optical freuquencies. In general, light trav- els in a straight line, but multiple reflected straight line segments can be combined to form a curved path. See lightpath. light piping Bringing light into an area through fi- ber optic cables. Laser light will travel along a fila- ment the size ofa hair, and filaments 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. light pulse The basic information-carrying transmis- 586 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 receiving 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 ofelec- trical or optical means. See light-emitting diode. light speed The speed oflight in a vacuum is 186,292 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. In fiber op- tic cables, for example, the propagating light travels about 124,100 miles/sec. In general, the higher the refractive index ofa mate- rial through which the light travels, the more it im- pedes the propagation of light. 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. light-emitting diode LED. An inexpensive 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 interconnect cable LIe. Also called 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. © 2003 by CRC Press LLC Light-weight Flow Admission Protocol LFAP. A protocol 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. LIGHTCONNECT A leading global supplier of diffractive MEMS-based dynamic optical network- ing components such as gain equalizers. lightlineA multiple 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 of the 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. lightpathA point-to-point optical link that may pass transparently through intermediary nodes. Thus, it comprises a logical one-hop link, even if there is more than one physical hop. LightSAR light synthetic aperture radar. See syn- thetic aperture radar. lightwave communications Optical 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 from lightwave 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 systems based on transmitting a beam through air or space, without using a cable as a physical medium. This is a wide bandwidth, line-of-sight, short distance technology which is relatively inexpensive. It is 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 Protocol LDAP. 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.SOO and can access X.SOO directories. It is 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 Inter- national. I 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 Rules L WER. L WER 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 of the most importantsources oflightfor fiber optic cables. In this example, an LED (a) provides a source of spreading light similar to aflashlight beam (b) which isfocused (d) by a lens (c) towards the lightguide where itpropagates by total internal reflection (TIR) by bouncing off the cladding (e), which has a lower refractive indexthan the conducting core (j). Dependingupon the application, multipleLEDs may be used. 587 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary like new Asubjective term describing aproduct 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 phrase like new instead of new. See certified, refurbished, used. LIM Link Interface Module. LINe Laboratory 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 ofbipolar violations (BPVs) or excessive zero (EXZ) errors. A line 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 ofatarget 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 of error, 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. Aline of bearing is some- times a calculated position based on a series ofread- ings within a general range. In direction-finding, in- tersecting lines ofbearing may indicate the position of a transmitter at or near the intersecting point. Consumer systems for determining lines ofbearing 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 sight An unimpeded direct line ofconnection through "free space" as it relates to aparticular trans- mission technology. Line ofsight is a relative term. The line ofsight for a flashlight beam is usually lim- ited to 10 or 15 feet, as the light falls offquickly and stops if it encounters physical objects in its path. The line ofsight 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 ofobjects. The line ofsight for a handheld ra- dio is a couple ofmiles because the radio 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 588 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 ofcable, 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 ofbits 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 means to use equip- ment contained within a central office. The LTC is capable of giving interfaces to outside ports; it is one ofa number ofperipheral modules that can provide trunk interfaces to a digital multiplex system (DMS). It may interface with multiple lines which, in turn, may be linked to a network by multiple speech links. line utilization monitor LUM. Adiagnostic and ad- ministrative utility providing statistical logs or dis- plays of transmission line use. line switching unit LSU. Ageneric phrase for a va- riety of devices that range from simple passive switchers to complex active voice or data transmis- sion line switchers. Aline 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 Amodulation 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., 5kHz), thus making it possible to increase capacity up to five times that oftraditionally wider frequency-modulated (FM) channels. Apilot 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 ofavailable frequencies remains essentially © 2003 by CRC Press LLC the same. To complicate the picture further, the Fed- eral 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. linearpredictive coding LPC. A system 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 fre- quencies) 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, in turn, can be used for loss- less or lossy compression algorithms for geospatial imagery. See sampling. linear programming Algorithmic 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 itself are 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. linearsearch & track processor LSTP. A system for processing radar search and track signal data. The N a- val Research Laboratory Collaboration has been port- ing LSTP functionality to parallel computing platfonns. linear tape-open LTD. 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. In spite of the 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- tridge in compressed fonnat. 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 of access) 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 of April 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 In its 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 to in terms of ainvays rather than lines. line, electric A circuit connection physical conduc- tor consisting of shielded or unshielded wire/cable. line conditioning Improvements 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 finderAn evolutionary 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 Database LID. 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 information-querying, validation, and alternate billing administration. See Local Num- ber Portability. line impedance stabilization network LISN. 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 of types ofLISNs with low-pass or high-pass fil- ters, depending upon their purpose. LISN s 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. 589 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary They are designed to test within specific frequency ranges (e.g., 10 kHz to 30 MHz). LISNs are commonly used to see if devices 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- try standards (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. A line light can also be cre- ated with an array of point 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. In older 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 of many communications technologies. The term is generally used in the con- text of air- 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 of sight 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 590 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 (though it may 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 of sight 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 stations 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 of sight limitations. F or 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 printerA 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 of text (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 impor- © 2003 by CRC Press LLC tant than cost or low noise levels. 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 switching See circuit switching. Line Terminating Equipment LTE. In SONET net- works, an element that originates and/or terminates a line signal. It can originate, access, or modify the line overhead, or terminate it, if needed. See SONET, Synchronous Transport Signal. lineartransponderA device 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 worker In 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 of wires 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, in addition to managing lines on util- ity poles. lineman's climbers, line climbersA variety of pole 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 climbing irons or spurs strapped on the legs, to cleats 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 of glass will reveal particu- lar types of patterns like those shown in this diagram, depending upon the shape of the magnet. These pat- terns will change each time thefilings are sprinkled, since they are formed 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 of force are related to the shape and orienta- tion of the objects with which they are associated, and the currentflowing through those objects, in the case of electromagnets. Above are some examples of lines of force associated with different structures. 591 '''' • :' .• : ,; • , .• ; •.• ; . ~> , ,::t © 2003 by CRC Press LLC . Fiber Optics Illustrated Dictionary batteries (TIS 9 6-2 528 - 1985) refers to Leclanche- type cells. LEeS LAN 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. LED See light-emitting diode. left-hand. 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 (LAN s) 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. A sliding dic- tionary compression scheme descended from LZ77 by Lempel and Ziv. LZSS was developed in the early 1980s by James Storer. 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. A sliding dic- tionary compression scheme descended from LZ77 by Lempel and Ziv. LZSS was developed in the early 1980s by James Storer