Fiber Optics Illustrated Dictionary - Part 70 pot

Fiber Optics Illustrated Dictionary Mellon, and the University of Michigan. The original NeXT was grayscale, with color added in later versions. The first NeXT was cube-shaped, with later hardware resembling more conventional desktop flat systems, known as NeXT stations. The first NeXT was based on the Motorola 68030 processor with a built-in 68882 math coprocessor, and it came standard with 8 MBytes of RAM. At the time, most computers had 1 to 4 MBytes of RAM. The original price was $6500 and marketing efforts were aimed at higher education institutions, although business owners expressed early interest due to the networking capabilities of the system. The frrst edition of CRC's Telecommunications Il- lustrated Dictionary was written on a NeXT computer, and even though the basic technology is over 10 years old, the computer hardware and operating system have stood the test of time in essentially their original form. The simple, stunningly aesthetic graphical user interface still beats most systems hands-down; the powerful object-oriented Unix- based operating system and shell connect seamlessly with the Internet, and the multitasking operating system allows dozens of processes to run happily at the same time. In over 3 years of 24-hour a day operations running multiple desktop publishing, Web browsing, and illustration programs at the same time, the author's machine didn't crash once. That's an enviable track record. After using a dozen different types of computers daily for over 20 years, the author has seen few systems that equal it (Sun systems provide similar performance). The NeXT is an excellent networking computer, con- necting easily to the Internet, other NeXT systems, and other types of computers through TCP lIP. It is also an excellent Internet portal, with a full comple- ment of Unix tools, including Telnet, FTP, and others easily downloadable from the Net. OmniWeb, by Lighthouse Design, Ltd., is a powerful Internet graphical browser for NeXTStep that preceded many well-known graphical browsers. In 1997, Apple Computing, Inc. bought out NeXT, Inc. and continued developing the operating system software under the development name of Rhapsody, now better known as Mac as X. See Jobs, Steven. Next Generation Digital Loop Carrier NGDLC. Developed in the 1980s as an evolution of Digital Loop Carrier systems, NGDLC is based on very large scale integration (VLSI) technology. ISDN was developed and promoted at about the same time that NGDLCs were implemented, so many were developed to accommodate ISDN. Whereas Digital Loop Carriers were designed to provide services over tra- ditional copper phone lines, NGDLC was designed to work in conjunction with fiber optic cables or fiber/copper hybrid systems. See Digital Loop Carrier. Next Generation Internet NGI. A U.S. federal mul- tiagency research and development initiative established in 1997. NGI works with industry and academia to develop, test, and demonstrate advanced networking technologies and applications. The fol- lowing federal networks are used as testbeds for the 682 NGI initiative: • NSF's very high performance Backbone Net- work Service (vBNS) • NASA's Research and Education Network (NREN) • DoD's Defense Research and Education Net- work (DREN) • DoE's Energy Sciences network (ESnet) (proposed beginning in FY 1999) NGI is coordinated by the NGI Implementation Team, coordinated by the Large Scale Networking Work- ing Group of the Subcommittee on Computing, In- formation, and Communications (CIC) research and development of the U.S. White House National Sci- ence and Technology Council's Committee on Tech- nology. http://www.ngi.gov/ Next Hop Resolution Protocol NHRP. An internet- working architecture, which runs in addition to routing protocols and provides the information that enables the elimination of multiple Internet Protocol (IP) hops when traversing a Next Hop Resolution Protocol network. Aims at resolving some of the la- tency and throughput limitations of Classical IP. See Next Hop Server, ROLC, NBMA. Next Hop Server NHS. In an NHRP networking environment, the Next Hop Server locates an egress point near a given destination and resolves its ATM address, enabling the establishment ofa direct ATM connection. See Next Hop Resolution Protocol. NGDLC See Next Generation Digital Loop Carrier. NGI See Next Generation Internet. NGSO nongeostationary orbit. NGSO FSS nongeostationary orbit fixed satellite service. NHRP See Next Hop Resolution Protocol. NICI See National Information and Communications Infrastructure in Appendix G. nickel metal hydride NiMH. A rechargeable battery commonly used in portable devices. Hydride is a hy- drogen compound. nickel-cadmium cell NiCd, NiCad. A very common, sealed, rechargeable power cell that works well in low temperatures. The positive electrode is nickel and oxide, and the negative electrode is cadmium, with the plates immersed separately in a potassium-hy- droxide electrolyte solution. NiCad batteries have been used in many small, portable telecommunications devices but have the disadvantage ofa "memory effect," that is, they will not fully recharge unless first fully discharged, thus reducing the useful time ofthe battery. nickname An easy-to-use, easy-to-remember substi- tute or secondary name. In most cases, it's a short name, generally one easy to remember because it is familiar or matches the personality or properties of the person or obj ect for which it is designated. A nickname can also be a name given as a term of intimacy and affection between two people who are closely acquainted. Nicknames may also be names that are easy to type, to save time, as on public discussion areas of the Internet. See handle, NICname. © 2003 by CRC Press LLC NICname, Nicname, Nickname On Internet Relay Chat (IRC), a name that can be set with /nick <putnamehere>. Only one person can have a specific nickname at anyone time on IRC. See Internet Re- lay Chat, WhoIs. Nicol, William (ca. 1768-1851) AScottish educator and physicist who developed the Nicol prism from Iceland spar. There are few records about his early life, but he began publishing his research in 1828 and spent the latter part of his life studying crystals and fossils. He apparently developed his own lenses and invented new methods for grinding samples for microscopic inspection. Nicol's sister, who was about five years younger, married Edward Sang, a promi- nent mathematician and engineer. Nicol prism A refractive component consisting of two blocks ofIceland spar (calcspar) cemented together along the diagonal plane with Canada balsam, a material derived from bark ooze that was a common bonding agent in optics for about 200 years. The Nicol prism was one of two common birefringent prisms, along with the Ahrens prism. The unusually distinct birefringent properties of Ice- land spar were described in 1670 by Rasmus Bartholin, but the mathematics of its properties were not worked out until almost 150 years later, by Tho- mas Young. William Nicol put two equally shaped calcspar blocks together to produce the Nicol prism, described in 1828. The device was used in polarimeters as early as the 1840s. Historic polarimeters consisted of a sample tube mounted horizontally between an analyzing Nicol prism and a polarizing Nicol prism. Monochromatic light could be shone through the tube with the analyzing prism rotated to produce two light sources for comparison. Nicol prisms were incorporated as substage polarizers into European scientific microscopes, beginning around the 1850s or 1860s. Arotating Nicol prism polarization analyzer could be mounted between the microscope nosepiece and the objective lens or over the eyepiece, depending upon the instru- ment. By selectively rotating the prism and the sample, the polarizing characteristics of the sample could be discerned. Nicole prisms continued to be common in microscopes as recently as the 1950s. In the 1880s, William Thompson (Lord Kelvin) used a Nicol prism in his lectures and demonstrations on the polarization of light. Edwin Land credits his invention ofpolarizers at the age of 19 to a demonstra- tion he saw as aschoolchild ofa Nicol prism. His invention led to a patent and the establishment of the Polaroid Corporation. Polarization is now typically accomplished with films and coatings. Polarizing beam splitters somewhat resemble Nicol prisms, having two blocks combined with a refractive coating layer, but the blocks are equi- lateral triangles and the resulting component is cubi- cal. See Bartholin, Rasmus; Iceland spar; polarization, Wollaston prism. NIDS See network intrusion detection system. NIF See network interface function. ~~~::~~;g~~;~~ir~f:~~~~~:::~ • ordinate, and regulate private sector participation in the provision of telecommunications services. http://www.ncc.gov.ng/ NIMBUS A satellite program initiated in the early 1960s by the National Aeronautics and Space Admin- istration (NASA), and now operatedjointly with the Oceanic Atmospheric Administration (NOAA). Nicol Birefringent Polarizing Prism Icelandsparis a doubly-refractive (birefringent) mineralwith interestingprismaticproperties. TWo calcspar blocks can be bonded along the diagonal with a material of slightly lower refraction index (b) toform a Nicol prism which makes itpossible to more widely separate the two courses of light through the material and thus isolate the one of interest. In constructing the prism, the natural rhombus angle of -72 0 can be ground down to 68 0 (a) for more effective results. In this example, incident light (I) enters the material and is split into two beams, one slightly stronger than the other. The ordinary beam (0) is reflected off the seam due to its angle and the fact that it has a slightly lower refractive index than the bonding material (much as light reflects off the cladding in fiber optic filaments). The "extra-ordinary" beam (E) hits the seam almost straight on andpasses essentially unimpeded across the bonded seam. Thus, the planepolarized light (E) exits in the same general direction as the incident light. 683 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary NIMBUS is used for research and development by atmospheric and Earth scientists. NiMH See nickel metal hydride. Nimrod Routing Architecture A scalable network routing architecture intended to support dynamic, heterogenous intemetworks of arbitrary size, Nimrod was originally suggested by Noel Chiappa. It was thereafter refined by the IETF Nimrod Working Group and formally described in the mid-1990s. Nimrod achieves scalability by representing and ma- nipulating routing information at multiple levels of abstraction to accommodate expanding, diversifying networks. Nimrod is characterized by maps that rep- resent internetwork connectivity and services, user route generation and selection, and user packet for- warding along established paths. It is applicable to routing within single and multiple routing domains in TCP/IP and OSI environments. See RFC 1992. NIOD Network Inward/Outward Dialing. Nipkow Disc - Historic Image Scanner This Nipkow disc shows the spiral series of holes through which the light is beamedas the disc rotates. This historic example isfrom theAmericanRadio Mu- seum collection. [Classic Concepts photo.} Nipkow, PaulGottlieb A German experimenter who developed a rotating dial with a spiral arrangement of holes that he patented in 1884. It was an early elec- tromechanical television system. This was later incorporated into television transmitting and receiving units. See Nipkow disc. Nipkow disc A rotating disc with a sequential heli- cal pattern of holes used by many early television 684 experimenters to attempt the projection of television images. The perforated disc was rotated in front of the image to be transmitted in order to quantize the signal, in a primitive sense, by segmenting the image into lines. The photosensitive material selenium was placed behind the disc to register the dark and light areas of the image. Unfortunately, systems for amplifying the signal for transmission didn't exist at the time, and didn't become practical until other tech- nological developments occurred. The disc is named after its inventor, Paul Nipkow. Modem versions of the Nipkow disc are used in a variety of applications and typically are fabricated from plastic or glass, with thousands of pinholes or microlenses embossed into the substrate. They resemble translucent CDs and are used in optical scanners and confocal microscopy. In a confocal microscope capable of resolving very tiny images, the perforated disc rotates between a beam-splitter and a lens, providing expanded depth imaging through a form of optical sectioning. A rotation of the disc enables an XY section of the specimen to be acquired in realtime. Height can be evaluated through grab- bing a frame and processing the image and Z data combined with it to provide a 3D topographical "map" of the microscopic specimen. See Baird, John Logie; television history. Nippon Advanced Mobile Telephone System NAMTS. An early, first-generation, analog FM-based mobile phone system with digital processing, first introduced in Japan. Communication Services Lim- ited (CSL) made the system publicly available in 1984. Nippon Telephone and Telegraph Corporation NTT. The major Japanese telephone company and largest phone company in the world. In 1997, deregu- lation allowed NTT to begin operating internation- ally, and its first international subsidiary, NTT World- wide Telecommunications Corporation, began servic- ing overseas corporate customers. See Arcstar; Japan Telecom Co. Ltd.; KDDI Corp. NIS 1. Network Imaging Server. 2. See Network In- formation Service. NISDN 1 See National ISDN-l in Appendix G. NIST See National Institute of Standards and Tech- nology in Appendix G. NITF National Image Transfer Format. NIUF See North American ISDN Users Forum. NJE See Network Job Entry. NL port, end loop port In a Fibre Channel network, a port on an endstation that enables it to be connected to the Fibre Channel loop. The NL port is assigned the lowest addresses and thus has the highest prior- ity in terms of obtaining control ofa loop. See F port, FL port, N port. NLA See Network Layer Address. NLANR National Laboratory for Applied Network Research. NLC See nematic liquid crystal. NLPID Network Layer Protocol ID. NMAA National Multimedia Association of Ameri- can. http://www.nmaa.org/ © 2003 by CRC Press LLC Emily Noether - Mathematician EmilyNoetherdevelopedmathematicalgroup theories that are still widely used by physicists. NMACS Network Monitor and Control System. NMD See nonintrusive measurement device. NMP 1. See Network Management Processor. 2. See Network Management Protocol. NMR See nuclear magnetic resonance. NMS Network Management System. NNI 1. network node interface. 2. Nederlands Normalisatie-Instituut. A Netherlands standards organization established in 1959, located in Delft. NNTP See Network News Transfer Protocol. NOAA See National Oceanic and Atmospheric Ad- ministration. noble gas A rare or inert gas. Examples include argon, krypton, neon, and helium. These gases are useful in illuminated signs and laser technologies. See argon, krypton, neon. nodal clock In a network, a reference clock source for major timing functions associated with a node. A valid clock reference may be extracted from a variety of sources, including a Primary Linkage and Co- ordination Program (PLCP). The best nodal clock in a facility can be selected to provide a unified timing source for Building-Integrated Timing Supply (BITS) such that a group of network switches would appear as a node in the hierarchical network scheme. node Junction, confluence, meeting point, terminal, intersection. A connection point in a network, which may consist ofa router, switcher, dialup modem, computer, or other interconnecting device supporting the same protocol, or converting to the needed protocol. Together the nodes, equipment, and pathways con- stitute the network topology. See backbone, leaf node. node port See N port. Noether, Emily "Emmy" (1882-1935) A brilliant German-born mathematician, Emily Noether developed mathematical group theories which underlie many subsequent representations of modem physics. Einstein praised her contributions and offered to write her obituary. Noetherian Rings are named after her and grandmaster chess player Emanuel Lasker proved some Noetherian algebraic concepts (Lasker-No ether decomposition theorem). noise Meaningless or otherwise unwanted sounds or signals interfering with the desired information or transmission in electromagnetic or acoustic communication systems. Noise can arise from bad shield- ing, wires too close together, overlapping transmissions, weather disturbances, irregular/reflective terrain, incorrect operation, deliberate human interference, random varying velocity, or faulty or incom- patible hardware. Noise in fiber optic networks is different from noise in wired networks. Electrical disturbances, voltage surges, and ground loops can have a significant impact on wired networks but may have little or no impact on fiber optic cables, especially end-to-end optical networks. :~~:~~:~n!~1~!~=~:~i£~~:~:~~~! 'a thermal noise or noise from back reflection and are vulnerable to noise anywhere there is ajunction with electrical components. Thermal noise may be especially problematic in systems using p-i-n photodetec- tors. Fiber optic networks may experience beat noise at the receiving end from amplification, depending upon the frequencies or from relative intensity noise from fluctuations in the emissions from the source illumination. Another source of noise in optical networks is modal noise, which arises when the emitted light (e.g., laser light) travels through a multimode fiber in slightly different reflective paths, resulting in slightly varying distances for the total path or phase delay. The light reaching the output end may exhibit fluctuat- ing interference patterns and may obscure whether the light pulse is on or off. Microbends and pits may increase modal noise. Modal partition noise results from a fluctuation in intensity from the laser source affecting the longitudinal modes of a multi mode transmission. Dispersion of the different modes can result in varying speeds along the travel path, caus- ing fluctations at the destination. Intersymbol interference noise may result from un- even fluctuations of light pulses that overlap in the process of traveling toward the destination. Quantum noise resulting from the particle nature of light can contribute to shot noise. There is Poisson variance in the number of photons received in any given bit pe- riod such that the photonic energy fluctuates. This may increase as the optical power increases. See crosstalk, garbage, interference, loss (includes dia- grams), intersymbol interference. 685 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary noise, modal See dispersion, noise. noise canceling Techniques and technology to reduce or eliminate noise. This may be background noise or noise on the transmissions pathway. Noise cancellation can be through digital algorithms which analyze the information and screen out calculated noise (a feature now found on digital cellular phones), or may be through conditioning circuits in transmitters or receivers. Noise cancellation is sometimes achieved by adding noise, creating a "white" noise that may be less objectionable than bursty, intermittent noise. noise filter An electrical circuit designed to detect or evaluate and exclude extraneous signals passing through a circuit. In digital circuits, fairly sophisti- cated analysis may take place. In analog circuits, simple exclusions ofparticular patterns or frequencies may be used. non-facility-associated signaling NFAS. In ISDN networks, a type ofsignaling in which the D channel is at a separate primary rate interface (PRI) from an associated set ofB channels. Multiple PRI lines can be supported through a single D channel using NF AS. In contrast, in facility-associated signaling, the D channel is at the same PRI as the associated B channels. nonintrusive measurement device NMD. A device used to measure various parameters in analog voice transmissions over communications networks. Mea- sured parameters include noise level, speech level, echo path loss, and echo path delay. nonionizing That which does not cause ionization or change the ion environment around it. A number of transmissions media, including visible light rays and radio waves, do not cause ionization, but can be propagated by ionized particles with which they come in contact. nonlinear distortion In an optical waveguide, the distortion that occurs over distance when more than one wavelength, with different transmission characteristics, or more than one pulse, which may reflect at different angles in the waveguide, travels at different speeds. Thus, a signal sent together doesn't always arrive at its destination in synch with its other components and the effect is cumulative over distance. The gradual loss ofsynchronization can have delete- rious effects not only at the destination point, but en route, as well, where crosstalk may result from nonlinear waveform interaction. Nonlinear distortion is influenced by the character of the original pulse, the breadth of the waveguide, the composition of the wavelengths (which may be prone to chromatic dispersion), the number of bends or obstacles (doping) in the lightguide, and the means of amplification. See chromatic dispersion, Raman scattering. See noise. nonreturn to zero NRZ. A simple binary encoding scheme in which ones and zeros are represented by high and low voltages, and there is no return to a zero level between successive encoded bits, hence the name. Since transitions mayor may not occur at each successive bit cell, the NRZ signal has spectral energy and, consequently, a direct current (DC) component that is a nonzero energy at DC. It is thus one 686 type ofbaseband signal. See Manchester encoding. nonvolatile memory Circuits or components that retain their data, even if the electrical current is shut off. In computer circuitry, volatile memory is installed in greater quantities than nonvolatile memory. Non- volatile memory is typically used for configuration settings (e.g., video parameters). See EPROM, read- only memory. Contrast with dynamic random access memory. nonwireline carrier Also called an ABlock carrier, for alternate carrier; that is, a competitive phone services carrier that is not the established local phone company (usually a Bell carrier, hence B Block carrier). NORC Network Operators Research Committee. normal An imaginary line in a direction describing the perpendicular to another line or plane. In other words, for any given line or plane, there is arelation- ship that is perpendicular to that line or plane for any given point ofreference. The term "normal" is unfamiliar to many people and the word "perpendicular" is often substituted. The use of the word "normal" appears to date back to an Old English term for "rectangular" or "right." Since a right angle is 90° perpendicular to the reference angle by definition, this may account for the evolution of the term as it is now used. For simplicity, picture a very slender flagpole anchored squarely in a flat concrete slab; assuming that any point in the center of the flagpole at a specified distance from the slab is equidistant from the slab in any direction (imagine invisible equal-length guy wires all the way around the pole), it is considered normal to the slab, even if the slab is removed from the ground and tilted in different directions - as long as the flagpole remains firmly anchored in the same position relative to the surface of the slab, it is considered normal to the plane of the slab. If you took away the flagpole and substituted an imaginary line, this line expresses the slab's normal geometry, even if the line were extended out the other side of the slab (poking into the ground like a pylon representing the negative direction of the normal vector). While normal is usually visualized as a line, it should be remembered that the line could be imagined as being anywhere perpendicular to the reference surface, not just in the middle. For example, if you moved the flagpole from the center of the concrete slab to the edge, while still keeping it straight (i.e., maintaining the even lengths of the imaginary guywires), the flagpole is still considered normal to the plane of the slab. Thus,jor some shapes, normal can be seen as aplanar concept expressed as a line in the context of a specific given point ofintersection with the reference plane. To understand the planar aspect, imagine walls in a house and how they are set at 90° angles relative to the plane of the floor. You can pick any point of intersection with the floor along the bottom of the baseboard ofa wall and there will be an imaginary line normal to the floor concurrent with the plane of the wall. The relationship holds true for other selected points along the wall. The wall © 2003 by CRC Press LLC could be pivoted around one of those points, using it as an anchor, and any other point where the wall touches the floor in its new orientation will still be normal to the floor. However, the pivoting relationship between the wall and the floor only holds con- sistently for flat surfaces. It is fairly straightforward to grasp the concept of normal for straight lines or flat planes, but what if the reference line or surface is curved? Imagine a toothpick sticking partway into a perfectly round orange. If the line of the toothpick is perpendicular to the tan- gent plane of the orange relative to where the toothpick intersects the curved surface of the orange, the toothpick is normal to the surface of the orange, whether it's the outside convex surface of the orange or the inside concave surface. In aperfect sphere, the relationship holds if the toothpick passes through the center and pokes out the other side. However, if the line of the extra long toothpick is offcenter, or the orange isn't perfectly round, normal cannot be assumed to be the same on both sides of the orange through a single line. Normal must be calculated relative to one point of intersection on asingle reference surface if the calculations are to be generalizable to any surface. Ifyou're having trouble visualizing the angle at which the toothpick must pierce the round orange to be normal, imagine equal-length guywires around the toothpick as you did with the flagpole. Now picture the stakes holding the guywires as spinning around the toothpick to mark a circle like a compass and slicing offa chunk of orange through the marks. If you place the round piece of orange on your table- top, the toothpick will now be sticking straight up, perpendicular (normal) to the surface of the table. For a natural, asymmetric orange (or eggplant), normal will be related to the contours on the fruit's surface where the toothpick pierces the skin. Determining normal for a point on a bumpy surface like the Earth's terrain or an optical diffraction grating is a little more complicated, since it will change every few inches or Jlm, but here is a way to visual- ize the relationship. Imagine flying a small aircraft (or aflight simulator) a few feet off the ground in the desert and maintaining that distance from the ground over a series of sand dunes. As you pull back, the nose of the aircraft pulls up. As you push forward, the nose of the aircraft pushes down. The relationship of the steering stick may not be perfectly perpendicular to the surface of the sand beneath it (due to lag), but it's close and it gives you away to picture approximately where normal is for a given surface on a complex plane like desert terrain. The concept ofnormal, when applied to bumpy surfaces, has a fractal nature in the sense that bumpy surfaces often appear more complex as they are more closely examined. Benoit Mandelbrot used the example of a coastline to describe this relationship in fractal geometry and he developed equations based upon self-similarity leading to the observation that the closer you look at a coastline, the longer it gets, due to the fact that slight indentations and protrusions can be more readily seen. Determining normal for a bumpy surface is somewhat like that. The closer a surface is scrutinized at the point where you want to determine the normal relationship, the more the quan- tity or shapes of the bumps and indentations might influence the angle of an imaginary steering stick or toothpick. Thus, normal is dependent, in part, upon the scale and type of measuring apparatus (and geometry) used to determine the relationship of the point of intersection to the reference plane. Returning to the guywire anal- ogy, imagine atoothpick placed normal to the surface of an irregular eggplant rather than aperfectly round orange; the angle of the toothpick would likely be different if the guywires were very close to the toothpick as opposed to some inches away from the toothpick. The concept of normal, which is a way of conceiv- ing perpendicular relationships in 3D space in any orientation, is essential in many aspects of geometry, theoretical physics, robotics, industrial fabrication (especially telescopes and microscopes), mapping, geology, computer-aided design and drafting, and much more. It is frequently used to describe grating surfaces, layered semiconductorstructures, and other industrial fabrications. The angle ofincidence of a reflected wave is generally described using surface normal as a reference. Thus, normal is useful for describing light paths, reflectivity, and diffraction. Sur- prisingly, many technical references gloss over or don't mention the term at all. See incidence, normal wave. normal distribution A theoretical construct based upon observations of the distribution of certain traits in sufficiently large populations. From these observations, it is possible to create a statistical represen- tation of frequency distribution to form abell-shaped curve with certain consistent mathematical properties across the curve. It is also called a Gaussian distribution after the observations of Karl Gauss. Thus, relationships such as the mode and the mean fall in the center of the curve and are equal and frequency tails off to either side through a number of symmetric standard deviations until it becomes zero. Once it had been observed that many traits seem to follow ageneral statistical distribution pattern (many ofwhich came out of studies or assumptions about human intelligence), the normal distribution or normal curve was then assumed to apply to many other frequency distributions, with this assumption then being built into tools designed to measure traits in a population (which has some self-fulfilling and circu- lar aspects that may interfere with the development ofobjective measuring instruments). Thus, it is used as a basis for describing probabilities (though some might say possibilities). Many students ask profes- sors ifa class is being graded on a "bell curve." In fact, with small class sizes, there aren't sufficient numbers in the population set to justify the assumption of a normal curve but many instructors apply normal curve concepts to assigning grades anyway (or get nervous if exam results don't naturally follow characteristics of a normal curve). 687 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary In its most general sense, in a sufficiently large, natural population, a normal distribution describes a sample set in which there are many members with similar traits, then a gradually lessening number with slightly different traits diverging in either direction from the mean. For example, there may be a large number of men who are 5'10" tall in the U.S., with diminish- ing numbers who are smaller or taller until a point is reached where there are no more people outside the sample set. When plotted in Cartesian coordinates, this trait maybe geometrically illustrated as a statistical curve resembling a symmetric bell shape, with the mean (average) and mode (most frequent) cen- tered around those who are 5'10',. In communications, the concept may be useful for some aspects of assessing, estimating, or predicting network demographics, peak time use, etc. normal wave Energy in wave form (e.g., radio waves) that travels normal (perpendicular) to a reference plane or line. Since normal is atheoretical relationship (an imaginary line), the concept ofa normal wave is not dependent upon whether the wave impacts the reference surface which may, in fact, be imaginary. In practical applications, a normal wave may fail to impact, reflect from, or travel through a reference surface (e.g., a receiving antenna). If the reference surface is reflective, and the wave reaches the surface, the wave would be reflected back in the direction from which it came, a situation called a Littrow condition. See normal. Norman, RobertAresearcher who published a clear statement of the laws governing magnetic attraction and repulsion in 1581. See magnet. Normes Europeenne de Telecommunications NET. An organization providing compliance testing for commercial telecommunications products for sale throughout the European Union to determine whether they conform to mandatory standards. Nortel Northern Telecom Limited. A leading global digital network provider providing commercial data, voice, and video services. Nortel is a dominant public switching equipment supplier in Canada de- scended from Northern Electric. It is also known for manufacturing and distributing radar sets based on magnetron tube technology, particularly during the second world war. Nortel technology is leased by other companies. See Qwest. NorthAmerican area codes See the Appendix for a chart of area codes for Canada, the U.S., and U.S. ter- ritories. North American Basic Teletext Specification NABTS. An Electronic Industry Association (EIA) and lTU standard that describes a means to modu- late data onto a vertical blanking interval (VBI), the transition time when the electron beam in a video dis- play travels from bottom right to top left with the electron beam turned off so as not to interfere with the image that is currently displayed. This is usually associated with an NTSC signal, as is standard in North American television broadcasting. NABTS can be adapted to transmit Internet Protocol (IP) data so that broadcast companies can send vari- 688 ous data services along with atelevision signal. RFC 2078 describes a one-way 36-byte packet structure that can be encoded into a single horizontal scanline of a television signal. Synchronization packets are located at the beginning, followed by address, index, and parameter information, followed by 26 bytes of user data, finally ending with forward error correc- tion (FEC) data. The full NABTS specification is described in EIA- 516. The entire NABTS specification is not always implemented. See NTSC, RFC 2728. NorthAmerican CellularNetwork NACN. A commercial provider of international cellular roaming services through their network backbone, serving over 7500 cities worldwide. Supported protocols are System Signaling 7 (SS7), X.24, GSM and IS41. North American Digital Cellular NADC. A commercial digital mobile phone service launched in 1991, NADC was introduced as asecond-generation system. IS-54 supports data rates of 48 kbps at a bandwidth of 30 kHz using digital phase shift keying (DPSK) modulation. IS-136 is based upon time division multiple access (TDMA). See DAMPS, time division multiple access. NorthAmericanDirectoryPlan NADP. An X.500- based client/server Directory System for providing global electronic directory and address book capabil- ity, distributed by ISOCOR. North American ISDN Users Forum NIUF. See ISDN associations. North American Network Operators Group NANOG. An association oflntemet Service Provid- ers which meets several times ayear to discuss technical issues regarding the administration and operation ofInternet-connected services. North American Numbering Council NANC. A Federal Advisory Committee established and char- tered with the U.S. Congress in 1995 by the Federal Communications Commission (FCC) to assist in adopting a model for administering the North Ameri- can Numbering Plan (NANP). This identification scheme is used for many telecommunications networks around the world. NANC advises the FCC and other NANP member governments on general number issues and on issues of number portability (e.g., for mobile telephones). See North American Num- bering Plan. http://www.fcc.gov/ccb/Nanc/ NorthAmerican NumberingPlan NANP. A system of assigned codes and conventions introduced in 1947 for routing North American (World Number Zone 1) calls through the various telephone trunks of the public telephone network. In 1995, significant changes were made to the NANP, mainly due to increased demand for area codes, including changing the middle digits from 1 and 0 to 2 through 9. See Area Codes chart in the Appendix. NorthAmerican NumberingPlanAdministration NANPA. A working group that develops and advises the North American Numbering Council (NANC) on processes for selecting a neutral NANP Administra- tor. It oversees a number of task forces and coordinates with them on issues related to cost recovery for © 2003 by CRC Press LLC the NANP administration. North American Telephone Association NATA. Now known as the MultiMedia Telecommunications Association, this is an open public policy, market development, and educational forum for telecommunications products and services developers and resell- ers. http://www.mmta.org/ north geographic pole The point at which the imaginary lines of latitude converge at the north pole relative to the shape of the Earth and its alignment in its orbit around the Sun. The general direction in which north-seeking compass needles point is near "geographic north" in northern Canada. See north magnetic pole. north magnetic pole A point in northern Canada, near the north geographic pole, to which the north- seeking tip ofa compass points. What is called magnetic north could be seen as Earth's south pole, if the "north" end of a magnet were used to determine "north" on a compass or alternately as the north pole ifa north-seeking (south) pole ofa compass is used to determine the direction. If that sounds confusing, consider that the designation of north or south on a magnet is not an absolute measure but one assigned relative to the polarity of the Earth, for which we have already designated north and south geographic poles. Thus, "magnetic north" is the direction toward which a north-seeking (south) pole ofa compass points. The north magnetic pole is not the same as the Earth's north geographic pole, despite its proximity, because the planet is a dynamic ecosystem whose magnetic properties change over time, whereas geographic north is a cartographically fixed point. See north geographic pole. NOTIFY, DNS NOTIFY A mechanism for the prompt notification of network zone changes that was proposed as a Standards Track Comment in 1996. NOTIFY is a DNS opcode that enables a master server to advise slave servers ofa change in data so that they may initiate a query to discover the new data. Traditionally many networks were configured to poll the server in order to discover any changes within the zone. This was atrade-off in terms of load on the system vs. the currentness of information. A NOTIFY transaction, on the other hand, establishes a means to initiate and expedite the update process when SOA RR changes occur (and, theoretically, other RR changes), thus reducing delay without imposing ex- cess load on the system. NOTIFY uses a subset of the fields in the DNS Message Format. See RFC 1035, RFC 1996. NOTIFY Set In distributed networks using DNS NOTIFY, the NOTIFY Set encompasses servers to be notified if changes to a zone have occurred that should be queried to enact updates. The set defaults to those listed in the NS RRset but, in some cases, additions or overrides may be possible to accommodate special circumstances or stealth servers that are not listed in the NS RRset. See NOTIFY. NOV News Overview. Novell One of the significant companies providing networking software (Novell Netware) to the business market. Novell is a public company established by a buyout ofNDSI by Ray Noorda in 1983. In 1998, Novell began promoting Novell Directory Service (NDS) as a means to tie different networking plat- forms together. In the 2000s, Novell acquired Cam- bridge Technology Partners and SilverStream Software. NOWT Netherlands Observatory for Science and Technology (Nederlands Observatorium van Wetenschap en Technologie). Noyce, Robert N. (1927-1990) An American elec- tronics engineer and significant pioneer of semiconductor technology, Noyce received the first Ameri- can semiconductor patent (#2,981,877) and more than a dozen other patents. Early in his career, Noyce did research at the Philco Corporation. In 1956, he joined the Shockley Semiconductor Laboratory where he worked with transistors and soon met Gordon Moore, his longtime associate and business partner. Together they founded Fairchild Semiconductor and later, in 1968, Intel Corporation. Noyce was president of Intel until 1975 and then served as chairman of the board until 1979 . I Noyce narrowly missed winning the Nobel Prize in physics. Although his patent was the first to be awarded for a pioneer integrated circuit (IC) invention, a patent application and verifiable invention by Jack Kilby at Texas Instruments was determined to predate Noyce's by just a few months. Kilby was awarded the Nobel Prize in 2000, a decade after Noyce's death. Nevertheless, Noyce made a mean- ingful contribution with his version of the new concept, as his design was commercially practical and his company, Intel, grew to be one of the foremost chip design and manufacturing firms in the world. The Robert N. Noyce award is presented annually to outstanding contributors by the IEEE society. See integrated circuit; Intel Corporation; Kilby, Jack; Moore, Gordon. NPA 1. National Pricing Agreement. AT&T agreement. 2. See Numbering Plan Area. A three-digit area code. NP As include special, reserved, and unassigned numbers. NPR See National Public Radio. NPSTC See National Public Safety Telecommuni- cations Council. NRC 1. See National Research Council. 2. See Net- work Reliability and Interoperability Council. 3. non- recurring charge. NREN See National Research and Education Network. NRIC See Network Reliability and Interoperability Council. NRSC National Radio Systems Committee. NRZ See nonreturn to zero. NSAI National Standards Authority of Ireland. A standards body for Ireland established in 1961, located in Dublin. NSAP network service access point. NSF See National Science Foundation. NSFNET National Science Foundation Network. A network established by the Office of Advanced Sci- entific Computing through the National Science Foundation, which is used for the civilian computing 689 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary operations of the U.S. Department of Defense. See National Science Foundation. NSIE See Network Security Infonnation Exchange. NSlnet NASA Science Internet, a network of the National Aeronautics and Space Administration. NSP 1. See National Internet Services Provider. 2. Native Signal Processing. NSSN National Standards Systems Network. http://www.nssn.org/ NSTAC National Security Telecommunications Ad- visory Committee. NT Northern Telecom, Inc. NTSC National Television System Committee. An organization fonned by the Federal Communications Commission (FCC) which set black and white standards for the emerging television broadcast industry in 1941. By 1953, after the proposal and consider- ation of several television systems, the FCC adopted a 525-line color standard developed by Radio Cor- poration of America (RCA), which was downwardly compatible with previous black and white technologies. This is, in part, why luminance and chrominance infonnation are carried separately. This system was accepted by the FCC and is widely used in North America and parts of South America. In NTSC broadcasts, color, intensity, and synchronization infonnation are combined into a signal and broadcast as 525 scan lines, in two fields of 262.5 lines each (Europe typically uses 625 lines). Only 480 lines are visible; the rest occur during the vertical re- trace periods at the end of each field. NTSC is considered to run at 30 frames per second, although, in color television broadcasts, the actual playing rate is approximately 29.97 frames per second. See High Definition TV, PAL, SECAM. NuBus NuBus is a simple Apple Computer 32-bit backplane card slot standard (ANSI/IEEE P 1196) for the connection of peripherals to Apple Macintosh computers. The clock is derived from a 10-MHz reference. NuBus backplane space is limited to 74.55 x 11.90 mm (even though some models have larger slots). NuBus slots can support up to 13.9W of power per card, although more can be used if other slots are not filled. nuclear magnetic resonance NMR. A technology used to reveal the inside of structures or biological organisms through a series of magnetic scanners or a magnetic field enveloping the body. It is used in addition to, and as an alternative to, X-rays in medi- cal research and diagnostic imaging. null Empty, having no value. A dummy value, character, symbol, or marker. Null values are sometimes used as delimiters to indicate the beginning and/or end ofa value or data stream. Null characters sometimes are used as padding, to even out the size of blocks or to provide extra time for synchronization. Null is a very useful concept, regularly used in pro- gramming and network transmissions protocols. null attachmentconcentrator NAC. In a Fiber Dis- tributed Data Interface (FDDI) network, there are a number of types of node configurations, including single, dual, and null attachment concentrators. A null 690 attachment concentrator does not contain any A, B, or S ports but is configured with multiple M ports. The NAC may be used in a simple tree configuration. It does not support a secondary path for redun- dancy and thus cannot be inserted into a dual ring network. See Fiber Distributed Data Interface. null modem A serial transmissions medium which functions in many ways as a modem, as it uses the same software, protocols, and serial transmissions media, except that there is no modem. In other words, instead of the signal going from a computer to a modem through a phone line to another modem and to the destination computer, the signal goes from the fIrst computer through a serial cable with no modem connected to the second computer, and back again. The transmit (Tx) and receive (Rx) lines are swapped (usually lines 2 and 3). This provides fast local file transfer capabilities between machines. null modem cable There are many ways to config- ure a modem cable, as long as the two computers talking to each other are talking the same language (transmissions protocol), but the most common configuration for a null modem cable is to take a standard RS-232c cable and cross (swap) the transmit and receive lines on one end, that is, lines 2 and 3. Or, rather than taking apart a cable, it is usually easier to get a null modem connector that swaps the lines. It looks very similar to an extender or other small coupler. See null modem. number portability NP. A service which enables subscribers to retain a geographic or nongeographic telephone number when they change their location, their services provider, or their type of service. This is defined with regard to switching services in the Telecommunications Act of 1996, and published by the Federal Communications Commission (FCC), as: " the ability of users of telecommunications services to retain, at the same location, existing telecommunications numbers without impainnent of quality, reliability, or convenience when switching from one telecommunications carrier to another." See Federal Communications Commission, Telecom- munications Act of 1996. NumberingAdvisoryCommittee NAC. This name is used by a number of telecommunications advisory bodies worldwide including Australia, Hong Kong, Zaire, and others. In general, these committees provide forums for the exchange of views on numbering issues, including assignment, reassignment, stor- age, access, and the dissemination of public and gov- ernmental infonnation for developing Number Plans. Recently, these committees have given increased at- tention to the allocation and availability of numbers for mobile telecommunications services. Numbering Plan Area NPA. A three-digit geographic telephony area code. NP As include special, reserved, and unassigned numbers. Within each NPA, there are 800 possible NXX Codes (also known as central office codes). NPAs are divided into two general categories: © 2003 by CRC Press LLC Category Description Geographic NPA The Numbering Plan Area code associated with a specific region. Service Access Code Nongeographic NPAs associated with specialized services that may be offered over multiple area codes, such as toll free numbers, 900 numbers, etc. numeric keypad Any compact block of functionally related touchtone telephone, typewriter, calculator, or computer input keys. The most common type ofnu- meric keypad is a group of about 10 to 18 numerical or function keys arranged in a block. These are usually physically organized to facilitate touch-typing or, in some cases, physically organized to slow down typing! On early touchtone phone systems, there was no point in entering numbers quickly as the switching on the network could not be accomplished as quickly as the numbers could be typed, so the digits were re- versed to slow down digit entry. The numeric keypad on a computer keyboard typically consists of 18 keys, with the numerals zero through nine, and symbols, usually consisting of pe- riod, plus, minus, asterisk (star), and enter keys. The remaining three keys differ widely on various computer platfonns, but usually include symbols such as the tilde, slash, or pipe (vertical bar). See keymap, keypad. numerical aperture NA. In a fiber lightguide, a quantitative description of the lightguiding capabilities of the "light pipe." If fibers were made ofjust one pure light-propagating material and were always straight and perfectly aligned with the incoming light source (also assuming there is no back reflection from the endface), the numerical aperture would probably be expressed in terms of the diameter of the fiber filament. However, fiber optic cables may be made from a number of materials (e.g., glass or plastic) with different refractive indexes and are not perfectly straight nor necessarily perfectly aligned with the source light. Cladding is laminated around the conducting core to reflect the light back into the filament and has a different refractive index from the conducting core. Mathematically expressing these different factors in relation to one another provides a numerical aperture rating that gives an idea of the light-guiding capac- ity ofa fiber cable assembly. See acceptance angle. Numeris, Numeris The name given to their line of data telecommunications services by France Telecom. Numeris is based upon Reseau Numerique al'integration de Service (RNIS) standards to provide its customers " service that opens doors the world over for intelligent telephony, fast Internet access, and the efficient transfer of information." Data rates on analog lines are 33 Kbps (56 Kbps in some circumstances). Access to Numeris can be established through D ISDN (16 Kbps) or B ISDN (dual 64 Kbps) channels. Numeris Duet offers dual lines for simul- taneous phoning and data communications (e.g., fac- simile or Internet). Numeris Commerce offers D channel services for ecommerce applications such as bank card transactions. nutating field In radar tracking, an oscillating feed from an antenna that produces an oscillating deflec- tion of the radar beam. Nutt,Emma N. Credited as the first female telephone operator. See operator, telephone, for additional information and history. nuvistor A type of electron tube in a ceramic enve- lope with cylindrical, closely spaced electrodes. nVabbrev. nanovolt. NV See Network Video. nW abbrev. nanowatt. N x64 A digital network channelized data transmissions system. As an example, a Tl line can be split into multiple Nx64k circuits for transmission across different ATM-based virtual circuits (Ves). When implemented through High-bit-rate Digital Sub- scriber Line (HOSL), it enables a network access provider to provide data services via two pairs of local ~:~~~::~E~fr~~~ri~!~~iA~:~~£;~~;~~ :. Nx64 interface A hardware network interface that enables Nx64-compliant connections at speeds between 64 and up to about 2048 Kbps, in multiples of 64 Kbps. This type of interface, in conjunction with Nx64 data formats, is commercially promoted to provide high bandwidth access to packet-switched backbone networks such as Frame Relay, X.21, and X.25. Thus, it is of interest, for example, to businesses with Frame Relay access to the Internet. A number ofITU- T V Series Recommendations relate to Nx64 standards (e.g., V.35). See Nx64. NXX NXX is also known as Central Office Code, or CO Code. It is an industry abbreviation designating the three digits ofa phone number preceding the last four (EXTN). A ten-digit number is expressed with symbolic characters as: NPA-NXX-EXTN. NXX is a reference to the exchange that services that specific area. N refers to any integer between 2 and 9 and X refers to any integer between 0 and 9. Each NXX Code contains 10,000 station numbers. See North American Numbering Plan, RNX. NYNEX Corporation One of the Regional Bell Operating Companies (RBOCs) formed as a result of the mid-1980s AT&T divestiture, Nynex comprised several related companies, including the New York Telephone company (NY), the New England Tele- phone company (NE), and others (X) such as NYNEX Information Resources, NYNEX Mobile Communications, NYNEX Business Information Systems, and more. In the early 1990s, NYNEX was developing interac- tive video network technology in cooperation with other vendors. In 1994, NYNEX representatives 691 © 2003 by CRC Press LLC . Fiber Optics Illustrated Dictionary Mellon, and the University of Michigan. The original NeXT was grayscale, with color added in later versions. The first NeXT was cube-shaped, with later hardware resembling more conventional desktop flat systems, known as NeXT stations. The first NeXT was based on the Motorola 68030 processor with a built-in 68882 math coprocessor, and it came standard with 8 MBytes of RAM. At the time, most computers had 1 to 4 MBytes of RAM. The original price was $6500 and marketing efforts were aimed at higher education institutions, although business owners expressed early interest due to the networking capabilities of the system. The frrst edition of CRC's Telecommunications Il- lustrated Dictionary was written on a NeXT computer, and even though the basic technology is over 10 years old, the computer hardware and operating system have stood the test of time in essentially their original form. The simple, stunningly aesthetic graphical. hydride NiMH. A rechargeable battery commonly used in portable devices. Hydride is a hy- drogen compound. nickel-cadmium cell NiCd, NiCad. A very common, sealed, rechargeable power cell that works well in low temperatures. The positive electrode is nickel and oxide, and the negative electrode is cadmium, with the plates immersed separately in a potassium-hy- droxide electrolyte solution. NiCad batteries have been used in many small, portable telecommunications devices but have the disadvantage ofa "memory effect," that is, they will not fully recharge unless first fully discharged, thus reducing the useful time ofthe battery. nickname An easy-to-use, easy-to-remember substi- tute or secondary name. In most cases, it's a short name, generally one easy to remember because it is familiar or matches the personality or properties of the person or obj ect for which it is designated. A nickname can also be a name given as a term of intimacy and affection between two people who are closely acquainted. Nicknames may also be names that are easy to type, to save time, as on public discussion areas of the Internet. See handle, NICname. ©. brilliant German-born mathematician, Emily Noether developed mathematical group theories which underlie many subsequent representations of modem physics. Einstein praised her contributions and offered to write her obituary. Noetherian Rings are named after her and grandmaster chess player Emanuel Lasker proved some Noetherian algebraic concepts (Lasker-No ether decomposition theorem). noise Meaningless or otherwise unwanted sounds or signals interfering with the desired information or transmission in electromagnetic or acoustic communication systems. Noise can arise from bad shield- ing, wires too close together, overlapping transmissions, weather disturbances, irregular/reflective terrain, incorrect operation, deliberate human interference, random varying velocity, or faulty or incom- patible hardware. Noise in fiber optic networks is different from noise in wired networks. Electrical disturbances, voltage surges, and ground loops can have a significant impact on wired networks but may have little or no impact on fiber optic cables, especially end-to-end optical networks. :~~:~~:~n!~1~!~=~:~i£~~:~:~~~! 'a thermal noise or noise from back reflection and are vulnerable to noise anywhere there is ajunction with electrical components. Thermal noise may be especially problematic in systems using p-i-n photodetec- tors. Fiber optic networks may experience beat noise at the receiving end from amplification, depending upon the frequencies or from relative intensity

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