Fiber Optics Illustrated Dictionary - Part 26 pot

Fiber Optics Illustrated Dictionary dead spot Aphrase often used in broadcast communications (radio, TV, cellular) to describe a region in which there are no signals, due to terrain or other obstructions. Cellular customers in particular are sus- ceptible to dead spots, because they may be constantly moving between buildings, boulders, mountains, etc. deadlock In competition for resources such as printers on a network, a route on the Internet, or space for a car on a road, deadlock occurs when there is more demand than there is supply and the contention is un- resolved. Contention is commonplace on networks and there are many ways to handle the situation, but sometimes a system lacks a mechanism for resolv- ing unexpected contention which may result in the system "freezing," looping (making repeated, unful- filled requests), or losing the directive for resources (e.g., a document disappearing from a print queue). The term deadlock is usually reserved for contentious situations where all the parties involved in the competition for resources experience some type of pro- tracted slowdown or lockup which is difficult or im- possible to resolve except by extraordinary measures (e.g., a reboot). See contention. debug v. To rid a system systematically ofproblems or bugs, especially in computer software. In software, bugs include syntax errors, looping errors, logical errors, and user interface design ergonomics problems. This process has become easier with the availability of debugging software and higher level programming languages, but it is still an arduous, exacting, painstaking activity. In the programming com- munity, not all good programmers are good debuggers, and code is sometimes passed on to a pro- grammer who has aparticular mindset and talent for this exacting, detail-oriented work. Software databases for tracking and reporting bugs are becoming popular and automated programs to test software, in order to find problems and report bugs, may be in- cluded in the debugging arsenal. decay Reduction through processes such as absorption, attenuation, erosion, or corrosion. Light or sound signals will decay over time and space due to interaction with the environment as energy is converted into other forms (e.g., heat). Physical structures, such as cables, will decay from abrasion and chemical interaction from heat, humidity, acidic or base solutions, or ultraviolet radiation from the Sun. The term tends to be associated with conditions or phenomena that decline gradually, often in a somewhat smooth manner, as opposed to those that decline very quickly or in ways that would be graphed as spikes rather than a smooth curve. The measure of the speed of decay is somewhat related to the phenomenon itself. Light is a fast-moving phenomenon and thus decay patterns in light are much faster than those in a corroding cable. In a fiber optic cable, dopants may be incorporated into the filament to influence the reflection of the light and wavelengths that are allowed to pass through the lightguide. The tradeoff is that the signal will decay more rapidly over any given distance as compared to an undoped cable with the same characteristics. 242 Since decay is related to distance, and transmission over distance is important in telecommunications, many methods to minimize decay and amplify signals have been devised, including electrical and optical amplifiers. See absorption, amplifier, attenuation. DECCa Defense Commercial Communications Of- fice (U.S.). decibel dB. One tenth ofa bel. Adimensionless ratio of two powers, which, in electricity related to telecommunications circuits, may be referenced to mil- liwatts. More familiarly in acoustics, a decibel may be calculated and expressed in pascals as a ratio of the sound pressure to a reference pressure. Since that's a little difficult to understand without knowing the formulas and individual frames of ref- erences, it may be easier to understand decibels in terms of examples in acoustics. Sound volume is typically expressed in decibels, on a logarithmic scale (the bigger it gets, the proportionally louder it sounds) with lower numbers representing lower volumes. At the high end of the scale, around I 75±25 dB, perma- nent hearing damage occurs. Even at sustained levels of 100 to 150 dB (and sometimes lower), loss may occur. Sudden contrasts from low to high volume sounds can be especially harmful to human hearing. Environmental noises tend to range from about 5 to 100 decibels, with low volume sounds emanating from appliances and traffic, and high volume sounds coming from horns, explosions, collisions, etc. See bel, neper. decimal system Based on a base 10 numbering system, the most common one in human culture, probably due to the fact that we have 10 digits each on our hands and feet. Computers, on the other hand, are commonly based on binary systems, base 2, due to the fact that electrically powered computers are easy to design around systems that use two states: on or off, high or low, etc. decimetric wave An ultra-high frequency (UHF) electromagnetic wave in the approx. 300- to 3000- MHz (3- to 30-GHz) range. Waves in this range are detected during solar flareups and those at the lower end of the scale are used in certain wireless voice and data communications. Wireless decimetric systems are used in military applications, aerospace, and communication systems ofdeveloping nations. In fiber optic cables, amplitude modulation oflaser light in the decimetric range may be used for testing embedded optical fibers (e.g., for strain tests) with resulting reflection from the fiber analyzed by a UHF- capable network analyzer. DECNet Digital Equipment Corporation's propri- etary Ethernet-based local area network (LAN). See Digital Equipment Corporation. decollimate To cause to diverge from a parallel direction. For example, if a collimated laser light hits the endface ofa fiber optic cable at the wrong angle or encounters dopants within the material, the light may be deflected from its original parallel path and be reflected in many directions. Lenses can be used to spread or concentrate a collimated !,Jeam. For © 2003 by CRC Press LLC example, a diffusing lens would spread a collimated beam, while aFresnel lens could concentrate a beam by focusing it over an area that is smaller than the source beam. See collimate. decompression The process of decoding, expanding, and otherwise reverse-engineering the information in a compressed file. Decompression is usually done to restore a file to its original state, or to restore the information to a form that is comprehensible to a viewer/listener/reader, but which may approximate, rather than duplicate the original state. Decompres- sion may happen in advance of using the information or may be carried out as the data is being read, as in MPEG animation playback systems. See compression, data compression. DECT See Digital European Cordless Telecommu- nications. Dedicated Access Line, Dedicated Line DAL, DL. Aprivate network connection between a business or individual and a phone carrier or network service provider. Calls through a dedicated line are automatically routed to the phone carrier or other service provider. Dedicated lines can be installed on various types of circuits, ranging from copper phone lines to fiber optic. dedicated array processor DAP. A processor in a redundant array ofinexpensive disks (RAID) system that specifies various array-specific tasks related to management of the multiple disks and the information and organization of the information on the disks. This information is particularly important if a problem has occurred with the data, and the disk array needs to be adjusted or rebuilt. Dedicated Short-Range Communications DSRC. An international standards effort for short-range communications that is ambitious in the sense that it must contend with aheterogenous installed base, compet- ing commercial interests, and varying levels of federal support. The standards effort has produced a number of drafts for two independent but related standards, including DSRC using microwave frequencies and a DSRC data link standard. The long-term goal is to support an interoperable North American transportation system, and there is still discussion as to whether interoperability should be mandated or promoted. In the future, it is hoped that DSRC can provide a consistent framework for communications, including travel information systems, navigation aids, tracking and routing, and a wide variety ofservice-to-vehicle services that are possible through wireless data information services (maps, trip planners, emergency locators, etc.). DSRC has applications for a wide range oftranspor- tation users, including casual travelers, professional travelers, trucking companies, emergency services, and transit authorities. The Transit Standards Consor- tium, for example, has investigated the use ofa se- lected set of radio frequencies for DSRC applications for transit vehicles. See intelligent vehicle highway systems. Deep Space Network DSN. Around-the-clock communications network that facilitates communication between Earth-based stations and space stations and interplanetary probes. Originally established in 1958, it is managed and operated by the Jet Propulsion Laboratory (JPL) with treaty agreements for foreign complexes. The three complexes that comprise the network are located in Goldstone, California; Robledo, Spain; and Canberra, Australia. See Gold- stone Deep Space Communication Complex, NASA. ~;:~~~o~tiliC;:i:e~:~r:/~~:~:~:t1~:e~f ·1. remote electron microscopy by Berkeley Lab re- searchers Parvin, O'Keefe et a1. in August 1995. This historic achievement is important because electron microscopy is very sensitive to changes that can occur in a split second and thus is not tolerant of transmission delays while the viewer makes adjustments, waits for images to display, etc. The solution to this problem was to have a system onsite with the microscope that can monitor the environment and make adjustments as needed to take some of the burden off the remote viewer and reduce the time it takes to com- municate commands and data offsite. However, this first successful experiment was a single-user solution. What if numerous users wanted to view and control the same microscope over the Web, for example in the same manner as many pan-and-tilt Web cams? Parvin, O'Keefe, and Taylor subsequently developed a system that came to be called Deep View, wherein multiple microscopes with different characteristics could be accessed by multiple remote participants. It is a scalable, distributed network concept for a common interface for accessing multiple microscopes that may change from time-to-time. The system also facilitates the exchange of information and data analysis. Combined with image processing that enables images to be improved, augmented, and combined, Deep View is apowerful system for coordinated microscope-based research. Deep Space Network Parabolic Antenna The 70-meterparabolic antennafor space communications located at the Canberra Deep Space Net- worksite in Australia, Jan. 1990. [NASAIJPL image.] default The initial setting, factory setting, reset setting, 243 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Satellite Sensor Sensitivity Regions The Defense Meteorological Satellite Program (DMSP) has been serving weather data collection needs for the U.S. military operations for several de- cades through 2.3-ton satellites operating in polar orbits at about 458 nautical milesfrom the Earth. This diagram illustrates the two regions of sensitivity of the two onboard sensors of the Operational Linescan System on board the satellite. in monitoring and predicting weather patterns, especially potentially dangerous weather systems such as thunderstonns and typhoons. It also provides information on electromagnetic fields that could influence long-range communications systems and ballistic- missile early warning radar systems based upon electromagnetic sensing systems. Data are relayed from tracking stations in New Hamp- shire, Greenland, and Hawaii to the military meteorological center in Nebraska. The data can also be relayed directly from the satellites to appropriately- equipped transportable military tactical units. See De- fense Support Program, Milstar. Electro-optical i 1 11 - 2 (microns) I 4 I 6 I 8 Infrared 1'1 I 12 10 Defense Satellite Communications System DSCS. A global military communications satellite network operating in super high frequency bands. DSCS is administrated by the DISA. Defense SupportProgram DSP. A satellite data and defense support program operated by the U.S. Air Force Space Command as part of North America's early warning system. DSP satellites are intended to detect missile and space launches and nuclear deto- nations. See Defense Meteorological Satellite Pro- gram, Milstar. Deflate Protocol DP. A means of compressing PPP encapsulated packets that was proposed in the mid- 1990s. DP is based on the deflate compression format which was, in tum, based on Lempel-Ziv LZ77 compression. Deflate has a good compression ratio, is openly specified as zlib source code by Gailly and Adler, and is widely implemented in such popular compression programs as PKZIP and gzip. See Point- to-Point Protocol, RFC 1979. deflect vt. To divert something from its original course, preferred direction, or current position. The term is most often used to describe the redirection of something that is moving, such as radiant energy or an object. Deflection is intrinsic to many semiconductor assembly processes and the functioning of communication systems. Reflective shielding is used to deflect electromagnetic parameters, or configuration; the state or parameters that are in effect at the start ofa program, available in a dialog box, or stored in a file. Sometimes the user can modify the default settings. Many customizable software applications or hardware devices have a button that the user can press to restore default settings in case erroneous or confusing settings have been subsequently established. In some systems, in- terrupting power to the system (e.g., removing a PRAM battery) can restore system defaults. default carrier In telephone communications, the long-distance carrier assigned to handle calls for customers who have not specified an alternate carrier. Defence Science and Technology Organisation DSTO. A division of the Australian Department of Defence, the DSTO has been investigating future technologies for use in defence applications, along with aiding in procurement, developing new capabilities, and enhancing existing capabilities since 1910. The DSTO has developed a trial high-frequency (HF) radio data network, as well as virtual reality simulations of naval and aeronautic systems. http://www.dsto.defence.gov.au Defense Advanced Research Projects Agency DARPA. Fonnerly ARPA, an agency of the U.S. De- partment of Defense (DoD) that handles research and development in basic and applied research, and which takes on imaginative, high-risk/high-payoff projects which may result in dramatic technological advances on behalfof the DoD. DARPA was established in 1958 to assure U.S. global technological advancement in coordination with, but independent of, the U.S. military research and development establishment. DARPA has, as part of its mandate, a design that is a deliberate counterpoint to traditional research ideas and approaches. It man- ages a budget of about $2 billion. DARPA's technical staff is rotated every few years to encourage new perspectives and is drawn from industry, academia, and government laboratories. Defense Data Network DDN. The packet switching network of the U.S. Department of Defense, established in 1982, which later became MILNET. DDN was separated from ARPANET (which evolved into the Internet). It includes classified and unclassified sections (DISNETx and MILNET). defense information infrastructure DII. The facili- ties, networks, software and information that comprise both peacetime and wartime resources of the u.S. Department of Defense. Defense Meteorological Satellite Program DMSP. A program initiated by the u.S. Department of De- fense (DoD) in the mid-1960s, DMSP satellites sur- vey environmental features such as pollution, clouds, water, snow, and fIre, in the visual and infrared spec- trums. This data is downloaded to ground stations and processed for U.S. military operations. The primary sensor on the DMSP is the Operational Linescan System (OLS), which provides continuous visual and infrared spectra images of cloud cover over a sensing breadth of about 1,600 nautical miles. The data from the system aids military weather forecasters 244 © 2003 by CRC Press LLC radiation away from sensitive components. Many components are designed to deflect some wavelengths and allow others to pass, providing filtering capabilities. Light travels in astraight line unless other forces act upon it to change its direction. Thus, the cladding in a fiber optic cable, which has a slightly lower refrac- tive index than the conducting core, deflects/reflects the light back into the conducting core, enabling it to continue along the core and to follow paths that may curve. See reflect, refraction. deflection modulation On a scanning cathode-ray tube (CRT), a system of deflecting the scan vertically from a base horizontal path. This makes the signal appear like a series of peaks above the baseline. deformer A tool for defonning the shape of an object. It may be a physical tool that uses pressure or heat or some other mechanism for defonning an object, or it may be an algorithmic or virtual tool for deforming a digital or conceptual object. degauss To demagnetize; to bulk erase; to provide electrical current-carrying coils to neutralize magne- tism. Cathode-ray tube (CRT) monitors are equipped with magnets to influence the direction of the electron beams. Some monitors will have a degaussing button to reset the magnetic environment in the monitor. Named after Karl F. Gauss. Delaney lamp detector A novel solution to creating a radio wave detector is the Delaney lamp detector, devised by u.S. Naval electrician Delaney. It was an electrolytic detector that consisted of an incandescent lamp bulb with the top broken off, the filament removed, and a 20% solution of nitric acid poured into the globe. This detector was found to respond well to signals originating nearby, without burning out from the oscillations ofa strong signal coming from a nearby wireless station, a problem that was common to other types ofelectrolytic detectors. See detector. delay Lag, hysteresis, gap, retardation ofa signal or phenomenon, time lapse. Delay is a relative tenn in that it implies a lag as compared to other processes or compared to what was expected. Delay is inherent in many types of telecommunications technologies, and systems design and operation must take into consideration a variety of types of delay. Transatlantic phone callers used to experience ap- preciable delays between the time one side of the conversation was heard and understood and the other side responded. This type of delay is now rare, or within acceptable levels. In the 1980s, the delay between sending and receiving email correspondence was typically about a week, now the delay is often only seconds or minutes. Delay can be a serious problem in systems that use more than one line to transmit associated phenomena, as in many videoconferencing systems that transmit graphics and sound over separate communications lines. In packet switched networks in which packets are split, sent through different routes, and reassembled at the receiving end, portions of the transmission might delayed, affect the reconstitution of the message at the destination point. In systems where synchronization of pulses is important, delay must be taken into consideration or may be deliberately introduced into faster aspects of the system to align the pulse timing of two or more components. Delay is sometimes deliberately introduced in data transmission systems to prevent overly quick reac- tions to situations that might be nonmeaningful (as in guidance systems or briefpower brownouts). De- lay is inherent in store-and- forward systems that may collect email messages, or other types of network traffic, and dispatch them in batches or when CPU time is at a low ebb. See cell rate, hysteresis, jitter. delay distortion In transmission technologies, distortion of the expected shape or characteristics of the signal as some aspects of it are delayed over other component aspects. Delay distortion tends to be cu- mulative over time and distance. In mathematical tenns, it can be described as the different Fourier components ofa signal traveling at different speeds relative to one another. Delay distortion is more prevalent at certain wavelengths and with waves comprised of multiple frequencies. It may also be more significant at higher data rate speeds where processing of the signal at the destination must be handled more quickly and where there may not be as much time to respond to and cor- rect distortion. In fiber optic cables, delay distortion arises from light dispersing within the confines of the lightguide cable such that different wavelengths arive at the destination at slightly different times. See chromatic disper- sion. delay encoding A means of encoding within the bit period ofa signal, a method that is more prevalent in radio signaling. A signal level transition can occur in the middle of the bit period or may be delayed to the end of the bit period. Binary data is represented by two signal levels representing zero (0) or one (1). If a zero is initially transmitted, the signal level does not change. However, ifa second zero follows a previous zero, the signal changes at the end of the bit period. Ifit changes in the middle of the bit period, it represents a one. See time-delay modulation. delimiter Any symbol, code, character, or other data used to signal a gap, break, boundary, or stopping/ starting point. Delimiters are very important in both human and computer communications. People use spaces, commas, paragraphs, and other punctuation and symbology to serve as delimiters in written text. It greatly enhances the ease with which the infonna- tion can be understood. Computer programs use symbols, punctuation, and spaces as common delimiters for arrays, lists, paths, and other types ofinforma- tion. Radio broadcast systems use spaces as delimiters between songs, or tones as delimiters before and after emergency announcements. Dellinger fade-out In radio communications, sun spot activity may be associated with highly absorb- ing areas in the ionosphere which can impair short wave transmissions. 245 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary delta channel See D channel. delta matched antenna See Y antenna. delta modulation, difference modulation A common method for converting/encoding analog signals into digital signals (AID conversion) for storage on digital media or transmission over data networks. This scheme was developed during the 1940s for te- lephonyapplications. In delta modulation, a scanned value is compared to the previously scanned value, to see if it is greater or less than the previous value, sending a one (1) if it is greater, and zero (0) ifit is less. (Actually the one and zero values can be reversed, just as long as the sender and receiver agree on which is assigned to the greater or lesser result.) Described another way, the sinusoi- dal analog wave is sampled, each sample is compared to the previous sample, and the modulated signal reflects the relationship by encoding it as a one or zero. Thus, the frequency of the sampling will influence the encoded signal in terms of how much information can be conveyed and how true it is to the original signal. This scheme is relatively simple and re- sults in very fast encoding. There are many different versions of delta modulation (e.g., linear and adaptive), and many are not com- patible. Pulse code modulation (PCM) is the other common method for converting from analog to digital. Because more information is encoded, it's not as fast as delta modulation, but it can be more readily converted between different versions of PCM. Un- like delta modulation, PCM doesn't require an inter- mediary analog stage to convert between different versions, and thus it is more popular than delta modulation. See AID conversion, delay encoding, sigma- delta modulation, pulse code modulation, sampling. delta testing Final stage of testing a finished product on endusers who would normally be customers or users of the product. This testing is more commonly known as User Acceptance Testing. See alpha testing, beta testing, gamma testing, User Acceptance Testing. Demand Assigned Multiple Access DAMA. In wireless communications, on-demand channel sharing. It is accomplished by assigning a call to a channel that is currently idle, or to an unused time slot, sometimes called Bandwidth On Demand. Thus, a single tran- sponder can support hundreds or thousands of subscribers, though actual capacity depends upon the DAMA implementation. DAMA is common on mesh communications satellite networks. Prototype OAMA systems were developed in the mid-1990s by the U.S. Defense Information Systems Agency (DISA). DAMA is a newer technology intended to increase channel capacity over methods common in the 1990s. It is typically used in single- hop satellite connections for a variety of point-to- point or multipoint voice, facsimile, or data services. DAMA assigns channels automatically, rather than manually or by establishing fixed channels. When a specific OAMA transmission is terminated, the channel is returned to a pool of available resources. DAMA is used in commercial satellite systems and 246 on the U.S. Navy's UFO communications satellites (designed to replace lower capacity FLTSAT systems). Mini-DAMA is a type of U.S. Navy DAMA system that includes encryption and data transfer capabilities previously handled by devices outside the DAMA system. demand circuits Data network segments whose costs are related to usage. Demand PriorityProtocol DPP. A client-server protocol mechanism that handles message exchange and sensing by multiple devices in IEEE 802.12 (1 OOV G- AnyLAN) networks. The process is governed by the Media Access Control (MAC) states. The client requests access to the communications medium and the server evaluates the request and returns permission to the client, if the request is granted. Once permission is granted, the requester goes into a transmit frame mode and sends the data to the network. demarcation point The point at which a telecommunications carrier's equipment or responsibility ends, and the subscriber's equipment or responsibility be- gins. This point may be at ajunction box on the side ofa building or a patch panel within a building. Resi- dentiallines tend to be demarcated outside, while multiline phone systems in businesses tend to be demarcated at a patch panel or terminal block inside the premises. The block itself is sometimes called a demarcation strip. demodulation The process of taking a signal that has been manipulated to carry information, and extract- ing that information from a carrier wave or math- ematically recreating that information from sidebands or other parts of the original transmission. A crystal detector uses a crystal as a type of one-way valve to detect and demodulate radio waves. A modem is a device that demodulates an analog phone signal to convert it to digital information that can be understood by a computer. A broadcast receiving station takes modulated airwaves and demodulates them for local transmission through cable or aiIwaves. See modulation. demon dialer, war dialerA function ofa program, usually for telecommunications or telemarketing, that automatically calls a phone number repeatedly or calls down through a list of phone numbers, cycling back to the top until one answers. DEMS 1. digital electronic management system. 2. digital electronic message service. demultiplexing 1. In a multiplexed signal, a process for recovering signals combined within it, usually to restore distinct channels contained within the transmission. See carrier wave, band, channel. 2. In ATM networking, a function performed by a layer entity that identifies and separates Service Data Units (SDUs) into the individual connections of which they are comprised. DENet Denmark's Ethernet network which intercon- nects academic institutions. Denial of Service DoS. A network administration term for situations in which users are unable to access the system. DoS may be intentionally implemented by an administrator or may be the result of © 2003 by CRC Press LLC undesired network configuration problems, vandal- ism, or security breaches such as hacking or flooding. With the Internet being increasingly used for sensitive government and financial transactions (especially time-sensitive stock and auction transactions), DoS situations arising from deliberate tampering have become a significant security concern. Safeguards against unauthorized DoS are difficult to implement without also affecting legitimate users; sometimes security measures to prevent one type of abuse (e.g., excessive junk email) can leave a system more vul- nerable to other types of abuse (e.g., DoS attacks), making it a challenge to configure a system for opti- mum use while keeping it secure. See Denial of Ser- vice attack. Denial of Service attack DoS attack. A network service security breach in which access to legitimate users is blocked or significantly impeded by unauthorized users. Sometimes DoS attacks are launched to crash a system in order to take it offline or to re- veal other security weaknesses through which an in- vader might further infiltrate the system. Denial of Service attacks can be directed toward different aspects ofa computer network. Filling up the available disk space, planting a program that greed- ily consumes system resources, or overwhelming the connection algorithms are various strategies that can cause DoS problems. SYN-flooding is the transmission ofa barrage ofre- quests for a network connection to deliberately com- promise normal use. When the server unsuccessfully tries to connect to an overwhelming number of un- reachable return addresses (those that don't return a connection acknowledgment), a DoS condition re- sults. Denial of Service attacks have existed as long as there have been computer networks, but they achieved prominence in the media and in industry and government discussion groups in February 2000 when an ordered series of attacks was launched against some of the most prominent commerce sites on the World Wide Web. See Land attack, Trojan horse, virus, RFC 2505, RFC 2827. dense wavelength division multiplexing DWDM. A transmission system capable of producing and si- multaneously transmitting many wavelengths over a single fiber optic cable. On DWDM multiple channels can be carried on the same transmissions medium, in essence a virtual "fiber bundle" within a single conducting core. This substantially increases broadband capacity and is ofparticular interest to firms installing long-distance cables over difficult terrain such as mountains or ocean bottoms (subma- rine cables). DWDM is speed and protocol independent, meaning it can carry a wide variety of transmission protocols, including SONET, Internet Protocol (IP), Ethernet, and SDH. Different types of communications can be carried at different speeds over the same physical network. DWDM was commercially introduced in the mid- 1990s and Sprint was one of the first high-profile companies to use it, implementing the technology for long-distance telephone selVices. Due to cost and implementation, DWDM was originally favored for long-haul networks, but was be- ginning to be used in metropolitan area installations in 2000. DWDM can increase capacity over time division multiplexed (TDM) systems by a factor of four to eight times. Metro 1500 systems from Cisco, for example, can accommodate up to 32 wavelengths (channels) per strand of optical fiber. Gigabit Ether- net transmission speeds are readily implemented over DWDM optical networks. By 2001, it was possible to transmit 1.60 Tbps over a distance of 2000 kilometers. By March 2002, Lu- cent Technologies had demonstrated a 64-channel differential phase shift keying (DPSK) system that could transmit over a long haul of 4000 Ian (2500 miles) with each channel transmitting at 40 Gbps through 100 km spans. Testing and maintenance technologies for DWDM were also evolving in the late 1990s, with equipment capable of measuring wavelength, frequency, channel spacing, signal-to-noise ratio, and power being offered by companies such as Wavetek Wandel Goltermann in 2000. See lambda switching. densitometerA photoelectric instrument for measuring the opacity or relative degree of light absorption ofa material. Darker materials absorb more light and thus have a higher optical density. There are different types of densitometers, some that measure transmission and some that measure reflection. They are commonly used in the printing industry to monitor consistency and quality. See hygroscope. density A general descriptive or comparative term describing the proximity of individual elements, hence the total number that can be fit within a specified area. Density is used to describe optical and magnetic storage capacities, screen and printer resolu- tions, compression efficiencies. As a general rule of thumb, especially for storage media, the denser the information capacity, the higher the cost. Denver Telephone Dispatch Company One of the earliest phone companies in the United States, established by Frederick O. Vaille in early 1879. DepartmentofHealth DOH. State, federal, and cor- porate health agencies have been devoting substan- tial resources to evaluating various distance educa- tion and physician support technologies and databases that can aid in health and safety. With telemedicine, for example, a surgeon in one region can oversee and direct an operation in a remote location. With shared databases, diagnosis and treatment research and information can be collectively used and managed. The applications of telecommunications to medical management are far-reaching, and we've currently only begun to implement the technologies. depletion layer Abarrier region in a semiconductor in which the mobile carrier charge density is not suf- ficient to neutralize the charge density of donors and acceptors. The donors contain impurities to facilitate electron activity, and the acceptors include trivalent impurities, forming "holes" to accept the electrons. 247 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary depolarization Demagnetization or the counteraction of polarization. In many transmission systems, the radiant wave signal may be horizontally or vertically polarized and may gradually lose that polarization as it passes through various media (moisture, particles, terrain obstructions, etc.). The wave may also be deliberately depolarized at the reception point. One of the important evolutionary steps in the history of the battery is the depolarizing characteristics of the Daniell battery. Polarization and depolarization are significant in many technologies and are of particular interest to quantum physicists, antenna manufacturers, astrono- mers, and photographers. When light waves vibrate in more than one plane (e.g., sunlight), they are considered to be unpolarized. When the vibrations occur in only one plane, the light is considered to be polarized. This can happen through scatter, reflection, or refraction or can occur or be deliberately effected in a transmitter. Reflection offa highly reflective surface can yield unpolarized light. The angle ofreflec- tion offa less reflective surface will affect the amount of polarization to a different extent. deregistration The process of dissociating two enti- ties, which may be names, processes, objects, etc. Deregistration is a process that happens often in computer applications, particularly those associated with networks. The process of creating virtual links between applications, icons, and processes allows for keeping track or creating shortcuts. Often these vir- tuallinks are removed or reorganized, particularly in a dynamic network environment. In public or pay systems, deregistration may be associated with billing or security, in that a subscriber, cellular handset, computer, or other entity is regis- tered in order to assure service to the subscriber, or to provide privacy and authentication. In these systems, registration is often with a unique identifier, which must be deregistered before services can be discontinued, or before the identifier can be assigned to another. deregulation A process of removing authority from a governing body in general, or of removing authority of the government body over specific jurisdictions. As examples, control over banking, telecommunications, or other large public or commercial systems have at various times been removed or reduced in order to stimulate competition and innovation. Some- times deregulation has the intended positive effect. Sometimes deregulation causes a change in the system that attracts unscrupulous members of society, who seek new loopholes that can be used to take ad- vantage of the system to the detriment of others. DES 1. See Data Encryption Standard. 2. destination end station. descramble Scrambling is mixing up a signal or digital data file so that it cannot be used without fIrst being processed. Descrambling is recreating the original in lossless or lossy format so that it is human-read- able or viewable. Scrambling provides security and protection from unauthorized viewing or copying and is a common way to commercially protect cable or 248 satellite TV programs from being viewed by those who haven't paid for the service. In most regions, unauthorized descrambling devices are illegal. See encyrption. Design Layout Record DLR. A database record of telephony design layout. A DLR may be searched by a circuit type with additional parameters supplied such as the U.S. state in which the circuit is located. A DLR search yields administrative and design information specific to the queried circuit. Design Structure Matrix, Dependency Structure Matrix DSM. A system and project analysis tool for modeling information flow, as in the product development process. Parameters are charted in a matrix representation containing subsystems or activities and corresponding information exchange and dependency patterns. Research on DSM is ongoing at MIT. Design System Language DSL. A predecessor to the PostScript page definition language, DSL originated in the mid-1970s at Evans & Sutherland Computer Corporation. Evans & Sutherland (E&S) became well known for their pioneering work in flight simulation and other 3D graphics software, and the Design Sys- tem was one of the outcomes of a research project using an interpretive language to build complex graphics databases. The Design System Language was later put to use in CAD applications. See Post- Script. Designated Transit List DTL. In ATM networking, DTL is network routing policy data consisting ofa list of nodes and any associated link identifiers, which specifies the path across a PNNI peer group. desktop A broad term in the computer industry describing any common workspace found in a home or office (or garage) where the various user tools and appliances are contained. Desktop computers are those that can fit comfortably on a desk (as opposed to dishwasher-sized minicomputers or room-sized supercomputers). See desktop metaphor. Desktop Management Interface DMI. A specifica- tion developed by the Desktop Management Task Force (DMTF) to allow easier, more consistent access to management database information and applications through Management Interfaces (MIs). The DMTF is a consortium of hardware and software ven- dors. desktop metaphorA phrase to describe the conceptual rationale for the design and display of information on a computer output device, usually a monitor. The idea is to take objects and actions that are famil- iar to workers and home users and represent them in an easily recognizable form on the computer. desktop publishing DTP. Using desktop computers to design, layout, and print files. It is now straight- forward to create a file on a home computer and send it directly, via FTP, to a commercial printer. Many of the pioneer tools for the evolution of desktop publishing were developed at Xerox PARC, subsequently implemented and popularized on the Macintosh computer in the mid-1980s, and enhanced through the development of laser printers. DTP has had a resounding impact on the publishing industry © 2003 by CRC Press LLC and has put publishing into the hands of tiny companies and individual publishers. Quark XPress, Adobe PageMaker, Adobe Frame- Maker, and TeX are DTP software applications widely supported by users, service bureaus, and publishers. Ventura Publisher was popular in the 1980s. Quark is favored for ad layout, posters, and other large-format or complex color projects and, to a lesser extent for books, mainly those that have many illustrations and those that don't require a lot of exten- sive indexing and cross-referencing. Design professionals like Quark. TeX is preferred for sophisticated technical symbols as are found in mathematics and physics treatises and textbooks and, although its interface is less intuitive, it has capabilities in symbol representation and for- matting beyond most general purpose desktop publishing programs. Corel has been promoting its products as desktop publishing tools, and for illustrations it can work well; however, many service bureaus and professionals prefer some of the other products, at least at the present time. For creating and inserting PostScript EPS (encapsulated PostScript) illustrations into other desktop publishing programs, many people swear by Corel Draw! There are also a number of other publishing programs available and word pro- cessors that are sometimes stretched to perform basic desktop layout tasks. desktop video Just as the Macintosh computer facilitated the development ofa new mass market field called desktop publishing, the Amiga computer facilitated the development ofa new mass market field called desktop video. Within two years of its debut in 1985, many video genlock and broadcast video products were introduced for the Amiga, the most significant being the N ewTek Video Toaster, which is still used today by many local cable companies and some of the large broadcast networks. A desktop video-equipped computer enables the development, editing, and merging of computer images, video taped footage, and live broadcasts at a fraction of the cost of using older video equipment. desktop computer telephony integration DCTI. The integration of computer processing and database search and retrieval functions, especially, with telephone communications equipment and procedures. For example, a database that is used to qualify and select potential or existing customers can be used to automatically dial on the behalfof sales agents. If the call is successfully connected, the next available agent can be notified and the information in the database can be displayed on the sales agent's terminal. This type of integration and automation is becoming increasingly prevalent. Videoconferencing is another example ofDCTI. desktop videoconferencing Video phone capabilities provided on a small desktop dedicated system or microcomputer equipped with a microphone, video camera, and fast network or phone access. Businesses are considering videoconferencing as an option to expensive travels, especially for meetings involving participants who are widely distributed geographi- cally. Videoconferencing systems are affordable now, but the speed of the line greatly affects the refresh rate of the image. If the transmission rate is slow, the image will be blurry and slow to update, and it will look more like a series of still shots than natural movement. End-to-end fiber optics networks are well suited for the transmission of audio/video confer- ences. ~~:au:s::~~nS~~el~~l~~f~:~~O~~~~~s~~:~~nge: .[. in a specified direction, usually a region on the Earth. There were early electronically despun antenna experiments on u.S. military satellites in the late 1960s which met only limited success. Later, mechanically despun antennas were also tried. destination address DA. A flag, field, or other indi- cator commonly used to designate the receiving point for a data transmission, call, or physical correspondence. In Token-Ring, Ethernet, and Fiber Distrib- uted Data Interface (FDDI) networks, the DA is a data field sent in the direction of the recipient that de- scribes a unique Media Access Control (MAC) address. See address, Destination Address Field, domain name, Media Access Control. Destination Address Field DAF. In frame-based networking, the DAF specifies the intended destination for the frame, which may be one or more stations (singlecast or multicast). In Ethernetworks, the DAF is usually implemented as a six-byte address, and, if the DAF contains all ones, it is considered to be an all-stations broadcast. In the IEEE 802.3 Ether- net standard, the Destination MAC Address follows the Start Frame Delimiter and precedes the Source MAC Address. See destination address. destination end station DES. In ATM networking, the end or termination point ofa transmission. It is used as a reference point for available bit rate (ABR) services. Since ATM has a number of traffic flow control mechanisms, which often depend on cells ar- riving at the destination at a certain time or in a certain manner, ABR and constant bit rate (CBR) dis- tinctions are important with relation to the defined end station. See cell rate. Destriau effect When exposed to an alternating electric field, certain phosphorescent inorganic materials suspended in a dielectric medium will luminesce, emit electroluminescent light rather than incandescent light. Certain semiconductors can be designed and excited to emit carrier injection luminescence. detect Sense, become aware of, register a reading as aresult of some influence, respond to. detector 1. In radio electronics, a device to appre- hend or detect radio waves. In early schemes, many materials were tried, including barretters, natural rec- tifying crystals, synthetic crystals, and electrolytic cups. The challenge with most detectors was solving the problem of amplifying the signal, or shifting the range of frequencies into the audible range. Current electronics have sophisticated and effective ways of achieving it, but in the late 1800s and early 1900s, ways of capturing and amplifying the waves were just being developed. 2. A substance or circuit that reacts 249 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary when exposed to electromagnetic waves such as radio waves or light. The detector may also convert or rectify the electromagnetic oscillations so they can be incorporated into a circuit with practical applications. A crystal detector works as a type of one-way valve to rectify radio waves, without outside power sources. Later, vacuum tubes were designed that could accomplish this task in ways that allowed great selectivity over the frequencies detected (tuner circuits) and that could amplify the signal so the broadcast could be played through a speaker rather than through small headphones. Fiber optics are now incorporated into many detectors. They are useful as detecting and imaging probes and as lightguides to draw a light signal from one part of the system to another. See crystal detector, Delaney lamp detector, electrolytic detector, electron tube, Lodge-Muirhead detector, magnetic detector, Massie Oscillaphone, optical detector, photodetector, Shoe- maker detector, silicon detector. Deutsch, Peter Along with Alan Emtage, one of the developers of the Archie system created originally at McGill University and a cofounder of "the Archie group," a group of volunteers dedicated to support- ing and enhancing the Archie project. Archie is very widely used on the Internet. Deutsche Telekom The German telephone services authority. DT includes a Technology Center, which engages in research, development, and demonstra- tions of innovative telecommunications systems. http://www.telekom.de/ device configuration management In networks, the configuration, tracking, and management of various devices, ports, and interface cards. These devices are increasingly software configurable, and the software used to manage them will often show actual images of the physical switches through a graphical user interface. Device Control Protocol DCP. An assigned well- known port 93 on TCP/UDP systems. device driver Computer software that controls, or provides an interface to, one ofa variety of devices such as hard drives, printers, video cards, CD-ROM players. A device driver is a software interface through which the operating system interacts with various peripherals attached to the system. See display driver. DFA See doped fiber amplifier. DGEP See dynamic gain equalization processor. DGPS Differential Global Positioning System. See Differential GPS, Global Positioning System. DGPT Department General of Posts and Telecom- munications, Viet Nam. DGT Direccion General de Telecommunicaciones. Spanish telecommunications authority. DBCP See Dynamic Host Configuration Protocol. Dhrystone A relative performance test intended to evaluate system program execution other than float- ing point and input/output operations. As with many benchmarks, the information is useful only within a narrow viewpoint, as in controlled experiments. The Dhrystone was developed in the Ada programming 250 language by R.P. Weicker of Siemens and is described in overview with other benchmarks in IEEE Com- puter, Dec. 1990. See benchmark, Rhealstone, Whetstone. diagnostic programs Application programs used to run hardware test suites, to evaluate processor functioning, or both. Most computers will run a systems check of the hardware before booting up the higher level operating functions. Network servers now often have diagnostic tools with graphical user interfaces to show virtual and physical connections, traffic flow, routers, switches, and various configuration settings. Diagnostic software is available for many computers to self-check and indicate possible problems in processing rates or connections. Many diagnostic routines are intended to run on a regular sched- ule, to locate potential problems before they become serious. Engine diagnostics used to mean getting grandpa to stick his head under the hood of the car to listen to the engine. Now it means taking the car into a shop and getting it hooked up to a computerized engine evaluation system that displays various system parameters on a computer monitor with charts and graphs. diagnostic techniques Various means of ascertain- ing and measuring the properties ofa system or circuit in order to understand its characteristics, monitor its behavior, or detect any problems or anomalies. Diagnostic techniques form a part of troubleshoot- ing a system and are often part of or combined with installation and testing. Some systems incorporate automated diagnostics that will check systems on startup, at random intervals, or at scheduled intervals. See ammeter, tap, voltmeter, Wheatstone bridge. dial A circular, movable mechanism for entering a number or other code. While dials are used in many electromechanical devices, they are most commonly associated with rotary telephone sets. In a telephone system, turning the dial causes a pulse ofa prede- termined length to be sent along the line to indicate the desired destination address. This process allows the system to set up an end-to-end connection for a conversation or data communication. The dial was also historically known as afinger wheel. The dial superseded the hook on old telephones and effectively obsoleted human operators for local calls within a few years. Dials were commonly used until the 1970s, when touchtone phones began to become prevalent in North America (many other countries still use dial phones). See Strowger switch. Dial Access Switching DAS. A Cisco commercial network switching system that provides dial services (and dial backup services) for Frame Relay perma- nent virtual circuits (PVCs) in Cisco wide area networks (WANs). The PVCs are activated by ISDN calls to the switching network. DAS is comprised of the DAS Server Shelfhardware and Dial-Up Frame Relay software. dial tone An audible signal that indicates that a phone line is active and ready to be dialed. The dial may be provided by the local phone carrier or by a private branch system. A different type of dial tone, called a © 2003 by CRC Press LLC stutter dial tone, has recently been introduced by phone companies offering voice messaging services to their subscribers, to indicate that messages are available to be retrieved. dial-around serviceA competitive telephone service in which the user dials an access code prefix in order to hook into a third-party service by bypassing the nonnallocallong-distance carrier. The 1010 (ten ten) long-distance service marketed through television advertising is an example. Dialable Wideband Service DWS. A sophisticated digital telephony circuit-switched data service from Nortel that enables public telephone networks to of- fer a wide variety of digital services to their subscribers. DWS provides on-demand, trackable variable bandwidth connections which can be implemented over public telephone networks within the international number plan. See DataSPAN Frame Relay Ser- vice. Dialed NumberIdentification Service DNIS. A service to display the number the caller dialed on an in- coming phone call. This service is useful for regional numbers that are rerouted to a central administration or sales system. For example, an economic region may have several area codes, but there may be one sales office handling all the calls. The DNIS allows the person answering the call to see whether the caller dialed the local number, the toll-free number, or a long-distance number, giving some feedback as to which lines are being used. DNIS does not provide the number of the calling party; a service like Caller ill is needed to provide this additional information. dialing parity Defined in the Telecommunications Act of 1996 and published by the Federal Commu- nications Commission (FCC), as " a person that is not an affiliate ofa local exchange carrier is able to provide telecommunications services in such a manner that customers have the ability to route automatically, without the use of any access code, their telecommunications to the telecommunications services provider of the customer's designation from among 2 or more telecommunications services providers (including such local exchange carrier)." This measure was designed to level the playing field and encourage fair practices. See Federal Commu- nications Commission, Telecommunications Act of 1996. dialup networking DUN. In the days before Tl and cable modem access to the Internet, most access to computer networks and bulletin board systems (BBSs) was through a modem and a phone line. Be- cause modems and computer time were both expensive until the late 1980s, dialup networking was usually done on an as-needed basis, with the phone connection live only as long as was necessary to carry out the necessary data transfers (e.g., file uploads and downloads, reading mail, leaving messages). The earliest dialup networks were available only to corpo- rations and government officials but, by the late 1970s, 300 baud modems and desktop computer- based bulletin boards made dialup networking popular with hobbyists. Passwords were rare, courtesy was common, and various software programs were developed to facilitate the process and keep phone connections to a minimum. As modem technology improved, 1200 baud and 2400 baud modems were sold in the 1980s and, by the 1990s when Internet access was growing by leaps and bounds, 33 Kbps and 56 Kbps modems were generally available at reasonable prices. At this point many people added extra phone lines so they could stay online longer without tying up their voice lines, and service providers added extra modems to support the high demand. With the availability of cable modem, T 1, and other 24-hour access technologies in the late 1990s, dialup networking is becoming less prevalent, although it will probably still be available for a number of years. At some point, 24-hour fiber optic connections or satellite connections to every home and business might be standard, just as it is in some motels and educational institutions now. DUN connecations are facilitated through scripts and files that store information for initiating a dialup connection. DUN files include phone numbers, modem settings, and other relevant parameters. diaphragmA thin, flexible sheet or membrane, usually with a curved surface. In electronics, a diaphragm usually is designed to respond to vibrations and has an interface to a device that can interpret those vibrations into electrical signals. Or conversely, it may take electrical pulses and interpret them into vibrations. Diaphragms are common in microphones and speakers. dichroic Literally, two-colored, though the term is often applied to phenomena with multiple-color in- teractions. A dichroic substance can cause an incident electromagnetic wave to pass some frequencies and reflect others - a very useful and often very beauti- ful characteristic. The iridescence associated with a hummingbird's feathers, when viewed from the right angle, gives some idea of the type of color effects that can be deliberately created with dichroic materials. Light propagated from a dichroic interaction has its associated electric field vector vibrating in a single direction. See absorption, anisotropic, dichroic filter, polarization. dichroic filter/reflector There are a variety of reflectors that exploit dichroic phenomena to reflect some wavelengths and allow others to pass through. For example, areflector may reflect only light in the vis- ible range and filter out other frequencies by allow- ing them to continue on through the reflector. Dich- roic filters differ from gel filters in that they reflect rather than absorb undesired wavelengths. This gives dichroic filters low susceptibility to heat degradation. Dichroic reflectors can be fabricated through vapor deposition processes in which fine layers of different materials are deposited within a thermal cham- ber. The layered composite may subsequently be stamped or cleaved into smaller components. Depending upon their construction and the frequencies with which they will be interacting, they can 251 © 2003 by CRC Press LLC . Fiber Optics Illustrated Dictionary dead spot Aphrase often used in broadcast communications (radio, TV, cellular) to describe a region in which there are no signals, due to terrain or other obstructions. Cellular customers in particular are sus- ceptible to dead spots, because they may be constantly moving between buildings, boulders, mountains, etc. deadlock In competition for resources such as printers on a network, a route on the Internet, or space for a. powered computers are easy to design around systems that use two states: on or off, high or low, etc. decimetric wave An ultra-high frequency (UHF) electromagnetic wave in the approx. 30 0- to 300 0- MHz ( 3- to 30-GHz) range. Waves in this range are detected during solar. characteristics could be accessed by multiple remote participants. It is a scalable, distributed network concept for a common interface for accessing multiple microscopes that may change from time-to-time. The system also facilitates the exchange of information and data analysis. Combined

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