Fiber Optics Illustrated Dictionary the device or fiber. Depending upon the application, particles outside the acceptance cone and sometimes large-angle particles within the acceptance cone are excluded from processing or further transmission. See acceptance angle. See Acceptance Cone diagram. acceptance pattern 1. In antennas, a diagramatic plot of off-axis power as it relates to on-axis power as a function of the position or angle of the antenna for a given plane (e.g., horizontal plane). 2. In fiber op- tics, a diagramatic plot of the total transmitted power as it relates to the launch angle of the transmission. acceptance periodA period, usually ofa few weeks, during which a product or service is evaluated by the receiver as to its conformance to the agreed-upon specifications. It is more commonly a stipulation of custom installations than of off-the-shelf products. Acceptance differs from a warranty in that it applies mainly to initial configuration during the ramp-up or installation period, whereas a warranty may cover other factors and last several months or years after purchase and installation are complete. acceptance testA test, which usually follows instal- lation, that demonstrates that the product or services purchased conform to the agreed-upon specifications. An acceptance test may be contractually required by the purchaser before making final payments on the purchase. accepted signal See call accepted signal. acceptor An entity capable of receiving another en- tity, such as a compound, substance, or atomic par- ticle. The providing entity may be called a donor. Donor-acceptor relationships are fundamental pro- cesses and may be used to polarize a molecule, for example, or be associated with certain photochemi- cal processes. Donor-acceptor emissions are found in semiconductor technologies. access 1. n. The point through which a circuit or com- munications device is entered, or the point at which the communications process is entered and initiated. 2. lJ. To gain entry into a circuit or communications device. Phones are generally accessed by dialing a number, although an access code may be required on a secure system. Dialing "9" first to obtain an out- side line is a common access procedure. Account codes are sometimes used to assign billing to specific departments or individuals. Access codes may be used by installation or maintenance technicians to initiate services or procedures not available to the subscriber. Secured computer systems are accessed by logging in with or without a password. See access code. access arm The positioning mechanism that supports a read/write head for reading from or writing to mag- netic or optical storage media. On a computer hard drive, the access arm moves across the disk and po- sitions the head directly to within thousandths of an inch of the area of magnetic particles to be read (or written). See seek time. access attemptAn attempt to gain entry to a system, facility, or process. In computer networks, success- ful and unsuccessful access attempts may be logged to provide information useful for improving network 12 efficiency or security. See authentication, firewall. access carrier An interconnect agreement through which a telecommunications carrier can gain access to the services and network facilities of another car- rier. access charge 1. The charge made for access to a computer system or network. An access charge may be assessed on a periodic basis or per time or vol- ume of use. Internet Service Providers typically charge flat monthly rates, although some will assess extra charges for storage, peak-time connects, access to chat areas, or special online services. 2. The Modi- fied Final Judgment (MFJ) which broke up the Bell system included the rationale and stipulation that us- ers should be able to choose a long-distance carrier, thus changing the way in which long-distance access charges were structured. Compensatory restructuring resulted in two categories of access charges: Cus- tomer Access Line Charges (CALCs), and Carrier Access Charges (CACs). The first applies to local phone loops and varies according to the subscriber (residential or business) and the characteristics of the service. The latter applies to service providers con- necting to the local exchange circuits and varies ac- cording to factors such as distance. Adjustments and modifications in order to implement the many changes have subsequently occurred. See Telecom- munications Act of 1996. access code One or more characters that must be en- tered in order to obtain use authorization to a system such as a phone or network. Access codes are gener- ally used for security, monitoring, and billing pur- poses. They can also be used by technicians to set up a system for use with specified features and, more recently, to program a telephone system. Some typi- cal telephone access code implementations include (1) dialing codes to access an outside line or to dial a long-distance number (dialing "9" is common), (2) dialing an access code to bill the call to a particular line or department, (3) dialing a code to obtain au- thorized use on a privileged system. access control I. The policies, procedures, and sys- tem configurations controlling security or utilization of resources. Access control operates on many lev- els, including building access, system access, appli- cations access, network access, device access, and computer operations access. See access code. 2. A physical or virtual control point, gateway, or other filter or security system that selectively allows data to pass through according to general or specific pa- rameters, which may include priority level, data char- acteristics, sender, receiver, etc. access control field An informational field in the header ofa synchronous multimegabit data service (SMDS) cell which provides access to a shared bus, which in turn provides access to the SMDS network. access control key A physical device, which may be electronic, mechanical, or both, for gaining or con- trolling access to a system, physical structure, or pro- cess. Common access control keys include traditional serrated house, auto, or office 'keys, more recent magnetic keys that resemble metal house keys, and © 2003 by CRC Press LLC Acceptance Cone and Acceptance Angle in Fiber Optic Lightguides A The acceptance cone is a conicalregion within which radiant energy can entera spherical or cylindricalconduct- ing medium, whether it be an acoustical microphone or a fiber optic lightguide. Any radiant energy spreading beyond this region fails to be conducted within the waveguide. If the conducting core is narrow and the radiant energy source spreads at a wide angle, most of the light will be outside the cone of acceptance region (A). Narrow- ing the beam and/or increasing the size of the conducting core (B) can balance a smaller radiant beam with a broader acceptance cone for less loss at the endface of the opticalfiber. Interposing a lens between the source of illumination and the fiber endface (C) is a common way of concentrating the beam to fall more closely within the acceptance cone so less of the light is lost. LIGHT ~ The acceptance angle is a closely related concept, but more commonly refers to the reflective path of the light as it enters andcontinues along the wave guide, as opposed to the beamsinitialpath as ittravelsfrom the light source toward the iightguide. If a straight lightguide is aligned with a coherent beam, all or most of the light will travel through the waveguide, with a minimum of reflection and loss. However, this is rarely the case. There may be slight misalignments between the light source and the lightguide and the lightguide itself is often so slender that it bends (which is useful for installation and routing the light). Thus, much of the beam will reflect back and forth within the cladding, which hasa slightly higherrefractive index than the conducting core, butsome of the light will exceedthe angle at which the refractive index of the claddingis able to reflect the light beam backinto the core to keep it within the waveguide. The light that is not lost through the cladding is said to be within the reflective angle of acceptance. 13 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary card keys that resemble credit cards but contain ac- cess infonnation in the ink or on magnetic strips or miniature circuit boards. Many hotels and workplaces now use electronic rather than metal serrated keys. access control list ACL. A list, table, or database that provides a reference for various levels of security within a system. It can be as simple as alist of names in the hand ofa doorway security guard, or as sophis- ticated as a tiered database of levels of security for different people and processes on a computer net- work. On bulletin board systems (BBSs), many access con- trol lists include a series of software flags for each user which can be toggled individually by the sysop to control user access to services such as chat, email, doors, downloads, etc. The "twit bit" is a flag that basically labels a user as a"loser," in BBS parlance. In other words, the twit is someone who has been flagged for limited access because slbe exhibits im- mature behavior and can't be trusted with access to any of the powerful features. access control method A system for controlling ac- cess to systems, processes, or devices on a network. A variety of general guidelines and specifically de- fined systems for particular types of networks and protocols have been developed. Access control can be set up "by user," "by workstation," "by applica- tion," "by file," "by network," or a combination of these, which may be hierarchical. Examples of par- ticular types of access control on specific types of networks include carrier sense multiple access (CSMA) on Ethernet systems and token passing schemes on ffiM Token-Ring networks. See Media Access Control. access coupler A connecting device used between physical network segments, such as fiber optic cable legs, to allow signals to be passed to the next leg. Access couplers are sometimes used in conjunction with relays and amplifiers, depending upon the type of signal and the distance being traversed. See SC- connector, ST- connector. access delay Ina packet-switched network, a perfor- mance measure for polling systems, calculated from the time of arrival of a data packet to the time it is retransmitted. access group A group of accounts or individuals who have specific, defmed levels and types of privileges within a system, which may be different from indi- vidual privileges and from other groups. For example, on a private branch exchange (PBX), a group of man- agers may be designated as having access to long- distance or outside lines, whereas a new employee may be assigned to a group with limited privileges until an evaluation period has passed. On a computer network, an access group may have certain read and write file privileges which differ from individual privileges and the privileges of other groups. Thus, they may be permitted to run only certain applica- tions, look at certain directories, etc., according to settings established by the system administrator. access line 1. The physical link between the sub- scriber box and the local telephone switching center. 14 From the subscriber box to the telephones is consid- ered inside wiring and may be installed by the sub- scriber or, for a fee, by the phone company. See 10- cal loop. 2. In BBSs, the line through which the caller accesses the BBS modem. There may be multiple lines, sometimes with different baud rate capabilities. Historically, BBSs have been accessed through phone lines but, increasingly, BBSs are interfacing with the Internet to provide online access through telnet. 3. In frame relay systems, a communications circuit that connects a frame relay device to a frame relay switch. access link A link. A connection point in a network that enables access to resources on the other side of the link. In Signaling System 7 (SS7) it specifically refers to a dedicated signaling link that is not associ- ated with any particular physical link. access mechanism A device for moving and posi- tioning an access arm, usually on random access read and/or write media. access method Logical guidelines established by International Business Machines (ffiM) in the 1960s for input and output access to computing resources, particularly those that are share~ as on local area net- works (LAN s). By consolidation of instructional se- quences in common procedures, functions, and sub- routines, the overall structure can be simplified. access module AM. In general, an included or op- tional interface enabling transmission lines or periph- erals to connect to a main unit such as a computer, microscope, milling machine, or telephony service unit. Depending upon the unit, it may include spe- cific connectors, voltage or data selection switches, and sometimes signal conversion, compression or decompression electronics. In microscopy, access modules can allow video re- corders, still cameras, or projection devices to be at- tached. In computing, printers, modems, and other peripherals are commonly connected through periph- eral cards, a type of access module. In wireless and wireline services, a mechanism to provide electrical or optical connections with various transmission cable ends (RJ-x jacks, BNC connectors, ST connec- tors, 25-pin D connectors, etc.) so they can be readily connected to service supply units. Access Network Termination ANT. Typically a de- vice that provides coupling to the user end of the com- munications line. Depending upon the type of trans- missions service, such a device may provide signal splitting, framing and de framing, cell header process- ing, encryption/decryption services, and other termi- nation/translation functions needed at the subscriber end of the communications circuit. Thus, a cable modem is an example of an ANT device as it pro- vides access to a fiber optic line and termination/ translation services for interfacing a personal com- puter. See access module. access software provider Defined in the Telecom- munications Act of 1996, and published by the F ed- eral Communications Commission (FCC), under SUBTITLE A-Telecommunications Services as: " a provider of software (including client or server software), or enabling tools that do anyone © 2003 by CRC Press LLC or more of the following: (A) filter, screen, allow, or disallow content; (B) pick, choose, analyze, or digest content; or (C) transmit, receive, display, forward, cache, search, subset, organize, reorganize, or translate content." access tandem AT. Acentral office telephony switch- ing system that provides distribution services for outgoing or incoming traffic between End Offices. access tandem switches Specific types of switches that are used to connect End Offices to Interexchange Carrier (IXC) switches or to interconnect central of- fice (CO) switches. access time 1. The interval between a signal or in- struction to access information or a device, and the time it takes to successfully retrieve that information, or interact with the device. Depending upon the sys- tem, the access time mayor may not include the time it takes to display the requested information to the user. For example, in adatabase query on acomputer system, the access time may be two seconds to search and retrieve a long list of names and addresses, but it may take twenty additional seconds to fully display all the listings, and the access time may not include the display time or may include the display time for the initial information, but not the time during which the software may be building additional viewable in- formation below a scrolling window. Access time is described in terms of units appropriate to the aver- age time and device involved. For example, access time for a process may be described in CPU clock cycles, for data access on storage media by seek time in milliseconds or fetching in nanoseconds. 2. In magnetic storage devices, the interval during which the access mechanism, once information is requested, moves across the medium to the desired location and successfully reads the data. access unit AU. 1. In Token-Ring networks, a wir- ing concentrator that connects the end stations. The AU provides an interface between the Token-Ring router interface and the end stations. Also known as Media Access Unit (MAD). 2. In many X.400-based commercial software applications, the AU works in conjunction with mail servers to provide synchroni- zation between post offices and other services, such as directories, address books, etc. 3. In building ac- cess or financial system access, access units are typi- cally locks, card readers, or biometric scanning de- vices. Since these units are increasingly being moni- tored by remote terminals and even video cameras, fiber optics networks may be interfaced with access units to provide fast response and monitoring of ac- cess events. See access unit interface. access unit interface The portion of an access unit that provides abridge between the entity seeking ac- cess and the structure, process, or system to which the entity seeks access. The configuration and size of the interface depends upon the type ofaccess sys- tem in place. A deadbolt is an example of a simple, traditional, mechanical access unit interface. Credit or ATM card-reading machines are common magnetic access unit interfaces. Iris, retina, or hand scanners are more recent electro-optical access unit interfaces. See access unit. ~:~:~~~~:V~I~;~~~~i~~~~:a~~~~l ~~~:~~~~~ • tal Equipment Corporation and Philips Semiconduc- tors, for connecting peripheral devices such as mice, keyboards, card readers, scanners, etc. to computers through a four-wire serial bus. See Universal Serial Bus. Accessible Information on DevelopmentActivities AIDA. Formerly known as IDA!, AIDA is a Devel- opment Gateway (DG) pilot project that encompasses tens of thousands of individual projects related to the International Development Markup Language (IDML). The DG provides support software and documentation to facilitate the conversion of data into IDML format. See International Development Markup Language. accounting server A software application, some- times operating from a dedicated, secured computer that monitors network usage, stores the information, and may assess charges for usage based on CPU time, real time, time of day, department, or some other measure appropriate to the type of use. ACCS Automatic Calling Card Service. accuracy 1. Degree ofconformity to a stated or ob- served value considered to be optimal or correct. See calibration. 2. Precision. 3. Degree of freedom from error. ACIDC, AC-DC 1. alternating current/direct current. See AC to DC converter. 2. An electrical appliance that can operate on alternating current (AC) or direct current (DC). 3. Euphemistically, a designation for something or someone operating in two modes or "swings both ways," especially in alt.xxx USENET discussion lists. See AC to DC converter. ACIDC ringing A telephone ringing signal control mechanism that uses both AC and DC power sources. Ringing is usually provided through low voltage AC current transmitted through the line from a ringing generator to the subscriber's premises. The ringing generator may be powered by a -48V or -24V DC battery. Ringing cessation, amplification, special tones, or other phone options may be powered through DC sources associated with the line or with the telephone set. See ringing generator, ringing sig- nal. ACD See Automatic Call Distribution. ACF Advanced Communication Function. ACG See automatic call gapping. achromatic 1. Uncolored; unmodulated; neutral; black and white; grayscale. 2. In the visual spectrum, lightwaves that are not dispersed or singled out ac- cording to a particular wavelength. achromatic lens An optical lens designed to com- pensate for the chromatic aberrations that result from different wavelengths in the optical spectrum having different focal points, which can lead to blurring of an image. Multiple elements are used to construct achromatic lenses for precision applications such as astronomy and microscopy. The elements may be of different types of glass. Chemical coatings can 15 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary further be used to improve lens quality by suppress- ing glare. ACIA See asynchronous communications interface adapter. ACK See acknowledge. acknowledge, acknowledgment ACK. A message or signal from the receiver to the sender confirming re- ceipt of data or accurate receipt of data. In handshak- ing, ACK sometimes also signifies that the receiver is ready for further data. ACK and NACK are com- monly used on bidirectional communications sys- tems, in which data can transmit in only one direc- tion at a time. See negative acknowledge. ACL I. See access control list. 2. Applications Con- nectivity Link. 3. Association for Computational Lin- guistics. ACLU See American Civil Liberties Union. aclastic Having the property of not refracting light. Contrast with refraction. ACM I. See Association for Computing Machinery. 2. Automatic Call Manager. An administrative and operations system that handles inbound and outbound calls integrated with a database. Telemarketing, tele- research, and collection agencies make use of these types of systems. 3. Address Complete Message. A call setup message in ATM networking that is re- turned to indicate that the address signals required for routing the call have been received by the called party. The ACM is sometimes sent in conjunction with other routing messages. ACO I. Additional Call Offering. 2. alarm cutoff. A switch that suppresses an audible alarm, while not af- fecting a corresponding visual alarm. ACOnet Austrian Academic Computer Network. An ATM-basedAustrian research network funded by the Austrian Ministry of Science, Transport, and Art. ACOnet interconnects about a dozen universities and provides international links to other countries through EBS-Vienna. http://www.aconet.atl Acorn Tube The acorn tube, developedfor ultra-highfrequency (UHF) applications in the late 1930s, is distinctive for its lackofa base andthe leads protrudingthrough a centralglass girdle. acorn tube A very small vacuum tube named for its 16 squat, rounded shape. It has electrodes leading di- rectly through the glass on several sides. It was de- veloped for use at extremely high frequencies (e.g., UHF) in the late 1930s. While most vacuum tubes have been superseded by transistors and other mod- em electronics, there are still high frequency appli- cations where vacuum tubes are practical. ACOST Advisory Council on Science and Technol- ogy. A U.K. organization which superseded ACARD in 1987, ACOST is a government advisory and co- ordinating body on policy and research. acoustic Relating to sound phenomena, the science of sound, as well as biological structures and non- biological apparatus for generating, conveying, controlling, or apprehending sound. In music, an acoustic instrument doesn't typically have an elec- tronic pickup because it depends upon its physical structure and the sUITounding medium to create and convey the desired sounds at the desired intensity. See acoustic wave. acoustic converter Any device that is designed to interface with an audio sending and/or receiving cir- cuit to provide amplification or conversion between analog and digital audio signals. The coupler is usu- ally designed to exclude extraneous noise that could interfere with a signal and may be a self-contained unit or peripheral. Acoustic couplers that resemble large suction cups were incorporated into early external modems to pro- vide a way to interface telephone handsets with com- puters. The coupler was designed so that the outbound modem signal played into the mouthpiece micro- phone and the inbound signal played from the ear- piece speaker into the modem. See acoustic modem. acoustic delay line I. In general, a transmission line designed to delay acoustic signal propagation. Why would you want to slow down a signal? Delay may reduce loss in systems where acoustic signals can be generated or travel faster than the transmissions sys- tem can handle them and "excess" data could be lost due to congestion or overrun. It may also be useful when sound is transmitted in conjunction with slower/higher bandwidth signals, such as video data in videoconferencing systems - a compensation sys- tem enables associated signals to be presented to- gether even ifthey don't transmit together. 2. In his- toric computing machine design and programming, mercury acoustic delay lines were used for memory access timing purposes to create wait time before execution of subsequent instructions. An electrical pulse would be converted to a sound pulse as it trav- eled through a mercury tube and converted back to an electrical pulse at the end of the tube. If neces- sary, the signal would be sent more than once through the tube and sometimes mUltip,le tubes were used to represent separate "registers.' In historic calculators from the late 1960s, electromechanical delay line systems were used for register storage. They func- tioned by sending a series of pulses through a coiled wire calculated to provide a specific "tuned" rate sent to a transducer that converted the signal back into electrical energy. This was a more compact and effi- © 2003 by CRC Press LLC cient evolution of the bulky mercury acoustic delay lines in early computers. Delay lines have been su- perseded by integrated circuits in most applications. acoustic echo canceller AEC. In many telephony devices, the microphone is only inches away from the speaker. Thus, sounds from the speaker can reach the microphone and interfere with the clarity of com- munications. Unpleasant screeching, echoes, or howling may occur. An acoustic echo canceller re- duces or eliminates this unwanted interference. Echo cancellation is accomplished in a number of ways with one or more electronic filters placed in various positions in the signal path, depending upon the sys- tem. In some advanced systems, ambient background sounds are also filtered and 'cancelled' to improve the clarity of the audio signal. acoustic feedback See feedback. acoustic model In software applications, a means to apprehend and interpret sound input, such as speech, by breaking it down into smaller units and then us- ing those together to build an aural representation or interpretation of input combined from these units into larger words and speech patterns. Early attempts at speech recognition were hit-and- miss, and very person-specific, but new programs can transcribe speech into text with a useful degree of accuracy up to about 70 words per minute. Comput- erized speech used to be characterized by very flat, mechanized sounds but, with faster processors and better methods and sound samples, natural sounding voices can be generated. Many automated phone voice applications now use speech generation for messages, queries, and instructions. See phonemes, sampling, speech recognition. acoustic modem A modulating/demodulating com- puter peripheral that converts digital signals created by a computer into audible tones that can be coupled with the transmitting end ofa telephone handset or other audio transmissions device so they can be sent through an analog phone line. The device then con- verts the audible tones generated by the other end of the transmission back into digital signals for the re- ceiving computer to interpret. The modem is usually attached to the computer by means of an RS-232 (EIA -232) interface, although some acoustic modems designed for the early Apple computers were con- nected through the joystick port. Acoustic 300-baud modems were prevalent on per- sonal computer systems in the late 1970s. These were gradually superseded by direct connect modems in the 1980s. In the 1990s, all-digital systems began to supersede analog phone lines. Acoustic modems have many limitations. They tend to be bulky, as they need sufficient shielding around the transmitting and receiving electronics to prevent the tones from crossing over and interfering with one another. They are subject to interference from exter- nal noises. They work well only with old-style phone handsets - the newer, flatter ones don't provide suffi- cient shielding or contact with the couplers to trans- mit clean tones, and they do not generally employ any sophisticated data compression capabilities, resulting in slow transmission speeds. See acoustic coup ler, direct connect modem. acoustic streaming Gradients of acoustic pressure in nematic liquid crystals giving rise to acousto-op- tic effects. This phenomenon has practical applica- tions in light modulation and liquid crystal-based light valves. See acousto-optic, acousto-optic effect. acoustic telegraph Messages conveyed by sound, such as bells ringing specific tones or sequences (still used in many European churches), drumbeats, horns, or shouts that are passed from one person to the next (still used on large sailing vessels, railroad lines, or other areas where other means of distance commu- nication are not available). In the early days of electrical telegraphs, a number of inventors were seeking ways to use tones to con- vey more information over a single line than was possible with the simple on/off system that was gain- ing widespread use. Experiments in trying to send tones led to the invention of the telephone, and so the technology leap- frogged over the acoustic telegraph. Basic telegraphic systems were used for a long time concurrently with the evolution of telephones. acoustic transducer A mechanism that converts acoustic energy into other forms of energy or other forms of energy into acoustic energy. See acoustic wave, sonar. acoustic velocity (symb. - a) The speed of sound (technically, velocity is the rate of motion in a direc- tion). A measure of acoustic velocity is valuable in many communications fields and also facilitates the determination of other measures such as distance or the 'thickness' or permeability ofa medium or par- ticulate environment. acoustic wave Longitudinal wave movement through an elastic medium (solid, liquid, gaseous, etc.). An acoustic wave or sound wave travels through a com- pressible medium through pressure and density changes along the direction of motion. The displace- ment of molecules within the acoustic medium pro- duces high pressure (condensation) and low pressure (rarefaction) regions. The speed with which the wave travels through a medium depends upon the inertial and compressibility characteristics of the medium and may be calculated if these measures are known. Sounds emanating from an acoustic wave may be audible or inaudible. Inaudible sounds may be sub- sonic (below human hearing) or supersonic (above human hearing). Differences in media result in vari- ous pitch and tonal qualities, some of which may be discerned by human hearing. Inaudible characteris- tics may sometimes be heard by other species or by acoustic measuring instruments. Since a vacuum is an absence of any medium and thus is not subject to physical perturbations, sound waves do not propa- gate in a vacuum. A surface acoustic wave (SAW) is a wave that propa- gates along the surface of an elastic medium. Ripples along the surface ofa pond provide an example of visible SAW motion but SAWs may be so small as to be undiscernible by the human eye (and may be particularly difficult to detect on rough surfaces). 17 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Holographic interferometry techniques are being de- veloped for the detection of SAWs. Recent advancements in SAW science have resulted in a number of new electronic and chemical technolo- gies. In microelectronics, Japanese researchers have been experimenting with SAWs as tiny linear motors using Rayleigh waves, SAW bandpass filters are used in digital radio modulation, and Sandia Labs has been turning SAWs into tiny chemical sensors for making handheld chemical detectors. SAW research has also resulted in some interesting discoveries related to X waves. In fiber optic networks, an acoustic wave can be sent over a strand of fiber to produce a notch filter for con- trolling gain and tilt. See dynamic gain equalization processor, X wave. acoustical Doppler effect When a moving object such as a train generates sound, there may be some interesting effects created by the compression and de- compression of the sound waves relative to a station- ary listener some distance from the train. The engineer on the train hears the sound of the whistle at a constant pitch. The stationary listener hears the pitch increase as the train approaches and decrease as the train passes. This effect is due to the compression of the sound waves relative to the stationary listener as the train approaches, which increases the frequency (pitch) of the sound wave. Conversely, as the train passes, the distance between sound waves becomes greater, re- sulting in a decrease in frequency (pitch). When taking acoustic measurements of moving ob- jects, this effect must be taken into consideration. Doppler effects, sometimes called Doppler shift, are important in remote sensing and guidance systems. Acoustical Society of America ASA. A scientific society founded in 1928 after an initial meeting at the Bell Telephone Laboratories in New York. It began publication of its professional journal in 1929. ASA merged with three other societies in 1931 to form the American Institute of Physics. ASA has been in- volved in research, development, promotion, and standardization efforts in the field of acoustics. http://asa.aip.org/ acoustics 1. The art and science of sound production, transmission, and reception. 2. The sound-carrying capacity, in terms of quality, fidelity, and loudness, of an environment such as a concert hall or record- ing studio. See anechoic. acoustics, engineering The art and science of sound control in electronic structures, including amplifica- tion, propagation, dampening, and the harnessing of sound to carry information, as in data and broadcast transmissions. acousto-optic A-a. Incorporating acoustical and optical properties or providing conversion from acoustical to optical energy or vice versa. Acousto- optic devices are routinely used in acoustical, laser switching, and imaging applications. Acousto-optic solid state components can be used to control laser beams for imaging, surveillance, and materials processing systems. Radio frequency control 18 signals are effective in accurately adjusting the angle oflaser beams through acousto-optical deflection. In frequency-shifting applications, an acoustic wave can be used to diffract an incident optical wave to down- or upshift the laser frequency. Piezoelectric transducers can be used with acousto- optic crystals to generate acoustic waves with a pe- riodic variation in the refractive index. Aportion of the incident beam (acoustic or optical) will be effec- tive, providing a modulation mechanism. See acousto-optic modulation. acousto-optic deflector In laser technology, a vari- able frequency, acousto-optic mechanism for control- ling the angle of a coherent beam. Control signals deflect the beam within acertainrange of angles and in certain specific directions, within the resolution parameters of the deflector. Acousto-optic X-Y scan- ners can be configured from two or more deflectors arranged to address both X and Ycoordinate planes. Acousto-optic deflectors are used in spatial measur- ing, graphics, mirror tracking systems, and molecule manipulation applications. See Bessel beam. acousto-optic effect The influence of an acoustic wave upon an optical phenomenon such that the op- tical energy is refracted. This effect can be quite pre- cisely controlled and thus has practical applications for deflecting and modulating light beams. See acousto-optic deflector, acousto-optic modulation. acousto-optic modulation A technique which can be used quite effectively for color control, dimming, and blanking in laser light beams. The beam is shone through an acousto-optic crystal. The modulation is applied with electrical impulses to the crystal to in- fluence the intensity of the beam. Three beams can be used, red, green, and blue, as in a cathode-ray tube, to provide color modulation. This is known as poly- chromatic acousto-optic modulation. Other applica- tions ofacousto-optics can be used to tune filters. See acousto-optic, modulation. acousto-optic switch In fiber optics, a switch that provides faster, more precise rerouting of signals trav- eling through an optical fiber network than is avail- able through traditional mechanical switches. Experi- mental systems include acousto-optic switches that diffract light from individual incoming fibers to se- lected outgoing fibers. acousto-optic antenna control An antenna control system based upon acousto-optic technologies. An innovative system with both transmit and receive capabilities was designed by Nabell A. Riza in the late 1980s to form the first all-optical phased array antenna controller. The system was extended to work with wideband signal processing applications while Riza worked with General Electric. Riza demon- strated how antenna arrays could be controled with nematic liquid crystal-based phase and time dealy control mechanisms which could, in turn, be applied to radar control systems. Riza was one of the first to suggest that optical micro-electromechanical systems (MEMS) could be used in the design of fiber optic switches for phased array systems. See multiplexed optical scanner technology. © 2003 by CRC Press LLC ACP See activity concentration point. acquisition 1. The gathering, receipt, and possession of data. 2. The process of orienting toward and ac- quiring data, that is, seeking a source; setting up the necessary protocols; aiming an aerial; scanning net- work inputs or broadcast frequencies; and receiving the transmission. 3. The process of gathering equip- ment, software, or businesses for a certain purpose. acquisition and trackingA data detection or receiv.;. ing system such as radar that seeks out a signal, locks in on it, and orients toward the source of the signal while receiving. acquisition time The time required to seek out and lock on to the source of the desired signal. Commonly used in microwave transmissions such as radar and satellite communications. ACR 1. abandon call and retry. 2. allowed cell rate. In ATM, an available bit rate (ABR) service param- eter that describes the current allowable sending rate in cells per second. See cell rate. 3. attenuation to crosstalk ratio. ACRFNET See Academic Computing Research Fa- cility Network. Acrobat An Adobe Systems commercial page lay- out software application used for creating documents containing text and graphics. The Acrobat Distiller takes PostScript code and interprets it into objects that can be displayed on a variety ofplatfonns support- ing the freely available Acrobat Reader program. The Acrobat Portable Document File (PDF) is popularly used to distribute documents on the Web. Since it re- tains font/image fonnating, the PDF files can be dis- played or printed in professional layout formats. PDF is also becoming popular for fonns handling, since the files can be designed to be selectively edited by a user. Thus, they are useful for electronic, paperless processing of government fonns such as business li- cense applications, for example. In addition to software downloadable from the Net, a number of word processing and desktop publish- ing programs will export Acrobat-distilled PDF files and the resulting files may be several times smaller than the original, depending upon the contents. Dis- tilled PDFs typically use a .pdjfile name extension. See Portable Document Fonnat. acronym A word fonned by taking the first letter or letters from each successive word in a phrase or com- pound tenn. An acronym is a special fonn of abbre- viation that has wordlike cadence and sound proper- ties and may, because of its ease of pronunciation and mnemonic qualities, eventually become part of the general vocabulary. Examples include scuba (self- contained underwater breathing apparatus), radar (ra- dio detecting and ranging), and BASIC (Beginner's All Purpose Instruction Code). ACS 1. See Advanced Communication System. 2. See Australian Computer Society. 3. automatic call sequencer. A simple form of automated phone call handler which hands off calls to available agents. ACSE See Association Control Service Element. ACSL See Advanced Continuous Simulation Language. ACSnet The Australian Computer Society national computer network, which maintains links to national networks in other countries. There are close ties be- :~~~!;~~~~~!~=:r~l~t~~~~~ - resenting Australia in the International Federation for Infonnation Processing (IFIP). ACSR aluminum conductor steel-reinforced. Al- though aluminum is light and a good conductor, alu- minum cables with steel cores tend to be bulkier and heavier than copper. ACT 1. See Applied Computer Telephony. 2. See Authorization Code Tab Ie. ACTA See America's Carriers Telecommunications Association. ACTAS See Alliance of Computer-Based Telephony Application Suppliers. actinism Aproperty ofradiant energy in the X -ray, ultraviolet, and visible parts of the spectrum to pro- mote chemical changes. actinometry The measurement ofthe radiation in- tensity emanating from a body. Actinometry is use- ful in assessing radiant energy from photochemical reactions and astronomical bodies. ACTIUS See Association of Computer Telephone Integration Users and Suppliers. activation fee, setup fee In many communications services, there is an activation or setup fee associated with starting a new account. This fee covers the ser- vice provider's administrative costs of installing the account and providing the new user with operating instructions, passwords, etc. Sometimes providers will waive activation fees in order to attract new sub- scribers. active communications satelliteA communications satellite that employs transponders (a type of repeater) or other means of amplifying and forwarding (relay- ing) a signal, usually with the frequencies shifted so the uplink and downlink transmissions do not inter- fere with one another. Unlike the larger passive sat- ellites launched in the 1960s, newer active satellites can amplify a signal without the extra bulk needed in earlier systems. Virtually all current satellites are active. active jamming The deliberate interposition of sig- nals intended to disrupt communications such as ra- dio or radar transmissions. active line A communications channel that is cur- rently being used. While no human communication may be taking place, if it is a data line, there may nev- ertheless be meaningful activity on the line, such as computer processes interacting with one another. active lines Ina television image, those lines that are visible to the viewer at anyone time. Since a frame consists of many sweeps of the beam, only some of the possible lines may be seen by the viewer at any one time, but because the transition is so fast, the image is perceived as continuous. Those lines that are not active are blanked. See blanking, frame, scan line. active matrix display Usually a liquid crystal dis- play (LCD), active matrix is a means of brightening 19 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary an electronic display by adding transistors to indi- vidual elements to maintain the image between suc- cessive scans or refreshes of the screen. Thus, the screen appears to refresh more quickly and gives a crisper, more contrasting appearance that aids in leg- ibility. Color laptops frequently incorporate this tech- nology and active matrix screens are gradually replac- ing passive matrix screens. ActiveX Descended from Microsoft's Object Link- ing and Embedding (OLE) but intended to run over the Internet and to compete with Sun's Java, ActiveX facilitates adding animation, sound, and interactive element into Web documents. ActiveX components are similar to browser plugins, or Java applets. See ActiveX Controls, Java. ActiveX Controls Microsoft ActiveX controls are in- teractive objects created individually by developers. They can be embedded in various Web-related appli- cations. ActiveX controls can be programmed in a variety oflanguages, including Visual BASIC, Java, or C++. They can then be readily shared with other programmers. A number of commercial vendors of authoring and page layout display systems have in- corporated ActiveX Controls into their software. Advanced Communications Technology Satellite ACTS. the Advanced Communications Technology Satellite. was a project of the late 1980s and 1990s in which the Space Shuttle was used to deploy a commu- nications satellite with large reflective sll/faces. Top left: ACTS relationship to the shuttle and the Earth. illustrated in 1987. Top right: ACTS in Earth orbit over the ocean afterrelease by the Space Shul/le Dis- covery in the early 1990s. Bottom: Advanced Com- munications Technology Satellite (A CTS) parabolic antennas. [NASAlGRC image; NASAIJSC image de- tail; NASAIGRC image. August 1996.] activity concentration point ACP. A place in a 20 network at which there is a high traffic load, in other words, a focal point for higher activity than is ordi- narily found in other locations in the system, as in computer network servers. activity reports Automated usage logs generated by computing devices that are usually accessible through a file or printout. Activity reports can provide infor- mation about times and types of use, errors, and sometimes transmitter/recipient information. Activ- ity reports are commonly available on facsimile ma- chines, high-end printers, and some electronic pho- tocopiers. There are disadvantages to the glut of in- formation and statistics that can quickly and easily be generated by electronic systems; there aren't suf- ficient hours in the day to evaluate all of it, nor is there sufficient storage space to keep hardcopy versions, and there is the danger ofturning people into human robots, doing the drudge work of feeding informa- tion into computers or trying to evaluate the output all the time rather than using them to increase free time and quality of life. ACTRIS See Association for Cooperation in Tele- communications Research in Switzerland. ACTS 1. See Advanced Communications Technolo- gies and Services. 2. Advanced Communications Technology Satellite. 3. See Association of Com pet i- tive Telecommunications Suppliers. 4. Automatic Coin Telephone Service. An automated system for handling payphone traffic, it directs the user on how much money to insert, handles calling card calls, pro- vides diagnostic and tuning information to techni- cians, etc. In areas without ACTS service, calls are handled by TSPS operators. Actual Measured Loss AML. A telephony system evaluation statistic used to express traffic manage- ment efficiency. It is sometimes compared to Ex- pected Measured Loss (EML) to provide a measure ofdeviation from desired or mandated service effi- ciency levels. actuate To initiate or activate into mechanical mo- tion. Rotational motion, signal transmission, or me- chanical button or switch positions are examples of devices that may be actuated. actuator A mechanical or electromechanical posi- tioning or regulating mechanism. Actuators have many uses, such as: (1) aiming antennas to remotely or automatically scan the arc of a satellite, (2) con- trolling active optics deformable telescopic mirrors, (3) controlling signal or chemical flow through regu- lating valves, or (4) providing access control in fiber alignment devices. Actuators have become increasingly small and so- phisticated and multiple actuators, ranging from a few to 500,000, may be used to control telescopic mir- rors, for example. Piezoelectric actuators are compact components typi- cally created from multiple layers of ceramics. They are useful for beam deflection, sensors, valves, fiber- to-waveguide alignment devices, and optical and electrical switches. In NASAIHR TextronlU.S. Armed Forces flight tests, fiber optics were found to sufficiently reliable and © 2003 by CRC Press LLC maintainable for use with smart actuator technolo- gies. See AS-Interface. actuator bus A transmission link for delivering ac- tuator-related digital signals. See INTERBUS, PROFffiUS. ACUTk See Association of College and University Telecommunications Administrators. ADA 1. A high level, structured, data-typed program- ming language, somewhat like an extended Pascal, developed by and mandated within the Department of Defense, but not popular outside of this circle. It has been criticized by some programmers as being cumbersome and difficult to use. The language is named after Ada Lovelace, the technically astute daughter of Lord Byron. There have since been varia- tions on ADA, includingADA++, which in tum has been superseded by ADA 95. See Lovelace, Ada. 2. Average Delay to Abandon. The average length of call duration for a caller held in a queue who hangs up before being connected with the callee. ADACC Automatic Directory Assistance Call Completion. adapter A device to connect one type of component, system or connector to other components, systems, or connectors to provide physical and electronic com- patibility on each end of the connection. An adapter is used when the two connections do not naturally couple with one another. Related to adapters are con- nectors, which are most often small passive devices, simply passing information or current through, while adapters tend to be combined with active, signal-pro- cessing or enhancing components, or with gender changers, extenders, or splitters. See adaptor, adopter. adapter card See peripheral card. adaptive antenna array A series of antennas grouped and arranged so the combination of anten- nas provides enhanced reception or transmission over individual antennas. An antenna array can be config- ured to monitor signals or signal conditions or to use input from other sources, such as computers, and to adapt to them as appropriate. For example, in a di- rectional antenna array, if the signal shifts due to movement on the part of the sending antenna, the ar- ray may be able to move or swivel to optimize the signal level (as in elliptical satellite orbit communi- cations). For wireless communications, adaptive ar- rays with horizontal extensions are being installed on existing cell towers to direct beams towards mobile terminals in order to increase capacity. Sector antennas may sometimes be housed in the same physical structure as adaptive array antennas to reduce the footprint of the assemblage, but care must be taken to arrange them so they do not cause distor- tion. See Butler antenna; antenna, smart. adaptive channel allocation ACA. In systems where transmissions channels are dynamic for purposes of congestion control, resource allocation, or security, there may be mechanisms to change channel alloca- tions between or during transmissions based upon scheduled, estimated, or computed loads. Commer- cial multiplexed wireless communications systems use ACA to maximize resources and respond to sub- scriber demands. Many secured military communi- cations systems use ACA for privacy and to prevent jamming. adaptive communicationA communications system that incorporates intelligence and feedback mecha- nisms to optimize signal or data transfer. In telephony, a cordless phone may automatically switch channels to find abetter signal if the current one deteriorates. In the telephone switching system, a phone call may be routed through another trunk if congestion is de- tected. In computer network systems, the system may reroute packets if one of the hops in ajourney changes or becomes unavailable. adaptive differential pulse code modulation ADPCM. An ITU-T standard for voice digitization and compression in which sample rate speeds are re- lated to the variation in the samples, thus using fewer bits than pulse code modulation (PCM), which is commonly used in digital voice coding, if the sample speeds are slow. An analog voice can be carried on an up-to-32 Kbps channel. ADPCM can be used over digital networks such as frame relay systemS. adaptive equalizationA means of compensating for distortion or loss of detail in digital data or in the digi- tal signals transmitted through a network link. Adap- tive equalization can compensate for a number of types of distortion including delay, echo, and co- channel interference. Trained adaptive equalization relies upon coopera- tive transmissions between the sender and the re- ceiver to enable the receiver to make adjustments based upon the training interval. Blind adaptive equalization is based upon only the data received, without the benefit oftraining data. Experiments have shown that adaptive equalization can improve high-speed data transmissions over mul- timode fiber at certain frequencies. In the mid-1990s, Drewes et al. assessed adaptive lin- ear equalization and decision feedback equalization for wireless ATM systems. Wireless data transmis- sions are subject to impulse prolongation from mul- tipath propagation leading to intersymbol interfer- ence; one means of dealing with the problem is to use time division multiple access (TDMA) with adap- tive equalization. Drewes et al. found that different algorithms were suitable for uplink and downlink transmissions, suggesting that least squares algo- rithms were best for radio broadcast channels that are different for each time slot. In graphics and visualization applications, adaptive equalization is utilized to increase clarity of visual details without changing the color information by reducing overall dynamic range. adaptive predictive coding APC. A means of con- verting analog signals to digital data through a pre- dictive algorithm built upon linear functions of pre- vious values for the sampled, quantized signals. The basics of speech encoding were developed in the 1930s, and predictive coding was applied to televi- sion broadcasts a couple of decades later. Improve- ments in the form of adaptive algorithms were de- veloped by M. Shroeder et al. in the 1960s for speech 21 © 2003 by CRC Press LLC . modulation. This is known as poly- chromatic acousto-optic modulation. Other applica- tions ofacousto -optics can be used to tune filters. See acousto-optic, modulation. acousto-optic switch In fiber optics, a switch. A modulating/demodulating com- puter peripheral that converts digital signals created by a computer into audible tones that can be coupled with the transmitting end ofa telephone handset or other audio transmissions device so they can be sent through an analog phone line. The device then con- verts the audible tones generated by the other end of the transmission back into digital signals for the re- ceiving computer to interpret. The modem is usually attached to the computer by means of an RS- 232 (EIA -2 32 ) interface, although some acoustic modems designed for the early Apple computers were con- nected through the joystick port. Acoustic 30 0-baud modems were prevalent on per- sonal computer systems in the late 1970s. These were gradually superseded by direct connect modems in the 1980s. In the 1990s, all-digital systems began to supersede analog phone lines. Acoustic modems have many limitations. They tend to be bulky, as they need sufficient shielding around the transmitting and receiving electronics to prevent the tones from crossing over and interfering with one another. They are subject to interference from exter- nal noises. They work well only with old-style phone handsets - the newer, flatter ones don't provide suffi- cient shielding or contact with the couplers to trans- mit clean tones, and they do not generally employ any sophisticated. Fiber Optics Illustrated Dictionary the device or fiber. Depending upon the application, particles outside the acceptance cone and sometimes large-angle particles within the acceptance cone are excluded from processing or further transmission. See acceptance angle. See Acceptance Cone diagram. acceptance pattern 1. In antennas, a diagramatic plot of off-axis power as it relates to on-axis power as a function of the position or angle of the antenna for a given plane (e.g., horizontal plane). 2. In fiber op- tics, a diagramatic plot of the total transmitted power as it relates to the launch angle of the transmission. acceptance