Fiber Optics Illustrated Dictionary An email server at the local domain handles the processing of mail once it is received from the ISP, and it can route it according to the needs of the lo- cal system. The domain name is expressed alphanumeric ally with dots between each of the levels or portions of the domain, with the assigned domain as the last two parts of the name. Since 1995, the number of registered domain names has risen from 100,000 to over 10 million. Within the U.S., domain names are subdivided into various categories. Outside the U.S., the extension is usually a designator for the country. The North American Domain Name Extensions chart shows a sample of familiar domain name extensions. The ap- pendix includes a more complete list of over 200 In- ternet domain name extensions from around the world. See Internet Corporation for Assigned Names and Numbers. domain name registrationA necessary administra- tive step for individuals and organizations who wish to have a unique domain name on the Internet. Un- like state registrations of company names, or com- panies in different industries that share a common name, there can be only one ofa particular name on the global Internet. This fact has made domain names hot commodites, with companies buying and selling domain names the way logos are sometimes bought and sold. To register a domain name, you must have a site ready to come on line, and you will usually go through your ISP to establish the name for the site. In 1995 registration changed to a fee system; in 1998, it was about $100 for initial registration and $50 per year to retain the name. Now it's much less. See Do- main Name System, InterNIC, lANA. domain name server A computer with server capa- bilities that communicates with the Domain Name Service (DNS) on the Internet after having registered its own unique ID. The serving computer provides an Internet Protocol (IP) address to the DNS for a do- main name that is not fully qualified (does not end in a dot). For individual users, this is usually handled by a serving computer at a local Internet Services Pro- vider (ISP). For campus/commercial users, it may be handled by a local campus backbone server. The lo- cal Start of Authority is delegated to provide Domain Name Server services for the assigned domain. This information is essential to providing connections be- tween individual systems or local area networks (LANs) and the Internet at large. See Domain Name System. Domain Name System, Domain Name Service DNS. A domain name distributed database established in the early 1980s at the University of Wisconsin. DNS provides mapping between host names and In- ternet Protocol (IP) addresses. DNS evolved out ofa need for a distributed system to handle a very large number of domain names. In older systems, host files were regularly distributed, until they became too large to be managed on most systems. To become a node on the Internet, it is necessary to fonnally register a unique domain name. The extensions 282 familiar to Intemetusers as .com, .edu, .net, .biz, and . info are part of the domain naming scheme called zones. Each domain name is stored in a central re- pository on the Net, and addresses are resolved through this database. Demand for domain names on the Internet has risen from a trickle to a flood. Businesses are realizing that the unique name requirement is different from tradi- tional business naming schemes. For example, it is possible for two businesses in the same state to have the same business name if they are in different lines of business. On the Internet, however, there can be only one of each name in the world, assigned on a first-come, first-serve basis. This exclusivity has cre- ated an unprecedented demand for names, leaving second comers with little choice but to come up with a less desirable or memorable name or to change their signs, stationery, and other marketing materials, an expensive proposition. To relieve some of the demand for domain names, new domain name extensions were introduced in 2001, theoretically increasing the availability of names to other comers. However, the demand prob- lem wasn't fully resolved for two reasons. First, a new extension doesn't necessarily aid certain types of business (e.g., a book store isn't helped much by the fact that there are new . aero extensions available for aeronautics companies). Second, big corporations with large marketing budgets typically register mul- tiple versions of their names, thus leaving smaller companies out of the running even when additional extensions are added. See domain, domain name server, InterNIC, RFC 830. Domain Name System security extensions DNSSE. Specifications developed to improve the weak secu- rity aspects of the Domain Name System. As the In- ternet Domain Name System became increasingly important for the transmission of secure documents, such as sensitive government communications and financial information and services, it was felt that extensions for the use of cryptographic digital sig- natures should be developed. See RFC 2065. domain organization In its general sense, the orga- nizational structure ofa digital network domain in- cludes the operations, devices, and other elements under the general control ofa processor, system, or network. The overall controlling and administrating entity may be called a host and may function as a server or have jurisdiction over a number of servers. The host maintains some type of access and manage- ment mechanism to a database or other record of other computers and devices on the system. This entity may be a single computer or software program, or a logi- cal amalgamation of several computers or software programs. Similarly, security mechanisms are gen- erally orchestrated by the host or other controlling member of the domain. The organization of the ele- ments associated with the host and contained within the domain can vary substantially with the type of network and various devices that are included. domestic arcA portion of an orbiting satellite's path or range that provides transmissions between the © 2003 by CRC Press LLC satellite and the country it is serving. There are a num- ber of domestic satellites in use that specifically are launched to cover aparticular country or territory, as in Alaska and India. Others serve domestic needs during a particular portion of their orbit. See satellite. dominant carrierAdesignation for a long-distance telecommunications provider that dominates a par- ticular region or market. In most cases, a dominant carrier is more stringently regulated in order to bal- ance a monopolistic advantage with opportunities for competition. dominant mode 1. The most effective or most preva- lent mode of transmission or conduction within a material. For example, conduction (rather than con- vection or radiation) may be the dominant mode for heat transmission in a particular material. In fiber pulling systems in a furnace with a gaseous environ- ment, thermal radiation has been found to be the dominant mode for heat transfer. 2. In a single-mode waveguide or one that can carry more than one wave- length at a time (e.g., multimode fiber) the mode that has the best propagation characteristics for that waveguide, which will depend upon the frequencies used and physical characteristics of the guide. Thus, the mode with the least loss or distortion of signal over the link is the dominant mode or the wavelength that is not cut off below the V parameter. See numeri- cal aperture, V number. DOMSAT Domestic Communications Satellite. A geosynchronous broadcast relay satellite. NOAA uses it, for example, in conjunction with the Geosta- tionary Operational Environmental Satellite (GOES) system, for relaying meteorological data. dongleA small hardware device or security key used for software or system security. Dongles were widely introduced on microcomputers to protect commercial products in the 1980s and were raucously opposed by hackers, technical users, and enough general us- ers that their use has been almost abandoned in North America. donorAn n-type dopant, such as phosphorus, which is used in solar photovoltaic devices. The dopant puts an additional electron into an energy level near the conduction band to increase electrical conductivity. Doping is a common way ofmanipulating the prop- erties of electromagnetic materials. See doping. door A term often used on computer bulletin board systems (BBSs) to indicate a category ofuser access activity that is external to the BBS software itself. Thus, a separate software program, such as a game or quiz, external to the BBS software is launched when the user selects a specific door. dopant An industrial chemical added in minute amounts to pure semiconductor materials, usually to improve the conducting properties of the materials. See donor, doping. doped fiber amplifier DFA. A fiber optic cable that has been impregnated or doped with substances, usu- ally rare earths, which alter its transmission proper- ties. See doping. dopingA means of adding small amounts ofmateri- als with particular properties to another in order to enhance it for the purpose for which it is being used. For example, in semiconductor manufacture, mate- rials are doped to enhance or inhibit particular ten- dencies to give up electrons or form "holes." Optical fibers are often doped with rare earth elements to al- ter their transmission characteristics. See germanium. Doppler shift Aperceived or measured shift in fre- quency when the source of radiant energy moves rela- tive to the position of the observer or receiver. DoS See Denial of Service. DOS See disk operating system. double density DD. A term for the physical configu- ration of magnetic particles on a floppy diskette, and hence the maximum amount of digital data it can store. Three and one-half inch, double density dis- kettes hold approximately 720-880 kilobytes of data, depending on the platform. The diskettes themselves are interchangeable between systems, provided they are formatted for the operating system on which they are being used. Double density diskettes have given way to high density diskettes (1.4 Mbytes), SuperDisks (100+ Mbytes), higher capacity cartridge formats, and USB and FireWire devices. double-pass autocollimation testA type of null test in the sense that it tests a mirrored optical assembly, such as a parabolic mirror, against itself. This is a very accurate test with the light path reflecting off the mir- rored surface twice and is useful for assessing tele- scoping components. A Ronchi grating, with evenly spaced lines and facets, can be used as the reference image. Any spherical aberrations in a flat mirror under test will usually manifest as concave or con- vex out-of-focus images. if the mirror is parabolic, the bands will be curved and should be compared against an ideal reference. See Ronchi test. double-superheterodyne DSH. A technology used in audio tuners as a conversion method for maximiz- ing the selectivity of tuners and reducing distortion as much as possible in order to take full advantage of programming from broadcast sources that pack a lot of programming into limited frequencies and from shortwave sources. See superheterodyne. DOV data over voice. Technology that allows data transmissions to be carried over traditional phone connections, usually copper twisted pair. See ISDN. down converterA technique and device used in com- munications in which the incoming frequencies are shifted. There are two common reasons for doing this. By shifting incoming frequencies so they are differ- ent from the outgoing frequencies, it is possible to reduce interference between the two sets of signals. Some frequencies are shifted up when they are trans- mitted, in order to put them into a particular broad- cast band or slot. A down converter is then needed at the receiving end to downshift the frequencies back to levels that can be used by the playing or viewing equipment that provides the information to the user. downlink The common name for the satellite to Earth portion ofa transmission, and the uplink is the Earth to satellite portion. The downlink is often frequency-shifted from the uplink in order to reduce interference between the two sets of signals. Uplink 283 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary and downlink services may be carried by different providers and may be subject to different usage re- strictions or billing arrangements download To receive computer data from a source on another system, usually through a network or modem connection. Downloading is typically done with a communications program file transfer proto- col (ZModem is commonly used), FTP client, or a Web browser. Common types of downloads include files from BBSs or Internet file archives and World Wide Web images and text. When using browsers on the Web, the information that you view is typically downloaded or cached on your machine (because loading the source code of the Web page on your local computer allows the browser to redisplay the previously viewed pages more quickly.) This is usually convenient, but it has disad- vantages as well and constitutes a security hole on your system. It may also fill up your hard drive. Make sure your cache is flushed (erased) when you are fin- ished with the Web files. You can always save ones of interest in appropriate directories for later view- ing while offline. While downloading files from FTP sites or Bulletin Board Systems, make sure you don't accidentally overwrite an existing file of the same name. Not all download software will inform you ofa duplicate file- name. It is a good policy, at any time, to download into a separate directory or even a separate partition until you have run a virus checker on the downloaded files. All foreign files should be suspected of possible viruses until you have determined that they are prob- lem- and virus-free. See FTP, upload. downstream A designation for any of the systems, nodes, legs, or hops in a transmissions pathway that are subsequent to the current one. Thus, a printer is generally downstream from a computer, a radio lis- tener is downstream from the radio broadcaster, and various workstations may be downstream from a server. See download. Contrast with upstream. downtime The block of time during which a system is nonfunctional. Downtime on a computer may be caused by software or hard drive crashes, broken net- work connections, etc. Downtime on phone systems may be caused by power outages, overloaded lines, or breaks in the lines. downwardly compatible Software or hardware de- signed to work in some way with older software or hardware, sometimes called legacy applications or equipment. Often the downward compatibility is only partial. For example, a software program may be able to export a file in the older format, but it may not in- clude all the characteristics of the file when loaded into the earlier application. Similarly, a new computer may work with an older monitor, but that doesn't mean the monitor can support all the graphics modes that might be built into the graphics controller on the new computer. Downward compatibility is a way of safeguarding a financial investment and of maintain- ing a minimum level of continued data access and use of existing software. Contrast with upwardly com- patible. 284 DP 1. See data processing. 2. See Deflate Protocol. 3. See demarcation point. 4. Dial Pulse. A standard Hayes modem command with the letter "p" used to designate a pulse dial setting for subsequent dialouts, thus ATP (ATtention, dial Pulse). See ATD. DPA 1. Dearborn Protocol Adapter. A commercial Microsoft Windows PC board for interfacing host computers with automotive communications net- works, from the Dearborn Group, Inc. 2. Defence/ Defense Procurement Agency. 3. Demand Protocol Architecture. An architecture for dynamically load- ing protocol stacks as they are required. 4. digital port adapter. 5. See Differential Power Analysis. 6. See Digital Performance Archive. 7. Disabled Persons Assembly. Telecommunications technologies are an important tool for enabling disabled persons to lead fuller, more active lives as well as being a means for people with similar problems to intercommunicate and support one another through difficult challenges. http://www.dpa.org.nZ/8. distributed processing ar- chitecture. 9. See Document Printing Application. DPBX digital private branch exchange (PBX). Most private branches in North America are becoming digi- tal, so the D is now commonly assumed when using PBX. See private branch exchange. DPCM See differential pulse code modulation. DPE distributed processing environment. See distrib- uted computing. DPLB Digital Private Line Billing. DPNSS See Digital Private Network Signaling System. DPO Direct Public Offering. See Initial Public Of- fering for description of this specific state-regulated subcategory for securities offerings. DPP 1. See Demand Priority Protocol. 2. See Direct Print Protocol. 3. Director of Public Prosecutions (Australia). 4. Distributed Pipe Protocol. A client- server protocol that was implemented over TCP us- ing remote method invocation (RMI) by Wang and Ouyang in 2000. 5. Distributed Processing Periph- eral. 6. See distributed programming platfonn. DPX See DataPath Loop Extension. DQDB See Distributed Queue Dual Bus. Dr. Dobb's Journal of Software Tools This journal has been a perennial favorite with programmers, pro- viding technical information on a wide variety of plat- forms and programming languages since 1976. It originated as a newsletter in 1975 documenting Tiny BASIC. The name is somewhat a collapse of the originators' first names, Dennis and Bob. Draft RFC A formal stage in the Request for Com- ments standards and information distribution process in which the proposal is submitted for evaluation and comment. On the Internet, this process is widely used to encourage open standards and professional and public participation. See Request for Comments. drag line A wire, rope, or other line for threading wire and cable through narrow channels (pipes, conduits, walls, etc.). The drag line may be preinserted during building construction and left for later use. See birdie. dragon A program running low-level secondary sys- tems tasks, especially on Unix systems, which are © 2003 by CRC Press LLC generally transparent to the user. Monitors and sta- tistical programs are often run in the background as dragons, and the results oftheir activities may be viewable by the system administrator or those with sufficient security clearance. See daemon. DRAM 1. digital recorder, announce mode. 2. See dynamic RAM. Draper,Henry(1837-1882) An American physician and inventor and the son of J. William Draper, Henry Draper was a pioneer in spectral analysis, carrying on research in astronomy and photography that was begun by his father. Henry made major contributions to spectroscopy and astronomical photography and created the first photographs ofstellar spectral lines, in 1872. See Draper Catalogue, Henry. DraperCatalogue, Henry The Henry Draper Cata- logue is a spectral classification of stars established in 1890 by Edward Pickering, Anna Palmer Draper, Willamina Fleming, Annie Jump Cannon et a1. in honor of Henry Draper. By 1915, more than a quar- ter million stars had been cataloged, and, in 1918, the first volume of the Catalogue was published. The Catalogue is still an important astronomical reference and has now been published online. Draper, John (ca. 1943- ) See Cap'n Crunch. Draper, John William (1811-1882) An English- born American scientist, educator, and historian who developed some of the early photographic processes. Draper researched incandescent substances and his son, Henry Draper, carried on this line of research to become a pioneer in spectrum analysis. Some of Draper's photographic research in the late 1830s apparently predated Daguerre's. He investi- gated photography of the very small and of the very distant (e.g., the Moon). His book Human Physiol- ogy (1856) contained the first-published micro-pho- tographs. He is credited with creating the first Ameri- can portrait, in 1840. Draper's ten-minute exposure was a big improvement over previous techniques that required many hours ofexposure. Draper was an associate of Samuel Morse and they cooperated on some projects. In 1862, he published The History of the Intellectual Development of Eu- rope, in 1874, The History of the Conflict Between Religion and Science and, in 1878, Scientific Memoirs. drift 1. Variation from adesired signal or current over time from factors other than line, load, environment, or warmup period. See calibration. 2. In radio tech- nology, signal drift is not uncommon. For example, if you set an analog radio to a favorite station and gradually lose the setting as the tuning changes, this is drift. DRiP See Duplicate Ring Protocol. drop pointAprotrusion on an insulator or other ob- ject designed to channel moisture away from sensi- tive components or electrical currents. Drop points (sometimes called drip points) are typically on the lowest protruding edge where water is channeled and may have a variety of shapes and spacings. See in- sulator. drive type The make, model, and configuration of a hard drive. A drive can conform to one of the common desktop standards, SCSI or IDE, for example, and, once it is formatted, it is important for the system to have a record of the drive type in order to keep track of sectors, blocks, partitions, and files, the various data configurations that can be set up dynamically on the drive during use. The drive type is really a com- bination of the cabling, data bus characteristics, physical properties of the drive, and magnetic data configuration which is superimposed on the magnetic recording surface. Some types of drives can be used in combination with one another, and others cannot. For example, SCSI and IDE drives are not mixed on one data bus. A dif- ferent controlling mechanism is used for each type. SCSI devices can chain up to six devices (the con- troller counts as the seventh device), while IDE drives can chain up to two, with one designated as a master and the other as a slave. When a formatting software program is run with a new drive, or one that is being formatted for another computer system, it may query the drive for infor- mation about its characteristics and display that in- formation on the screen. For example, it may show the brand, model, and size of the drive, and whether there are any existing partitions. Many drives now come preformatted, but it may still be necessary to set up partitions, if desired. driver 1. In software, a program that includes code that can translate commands into instructions recog- nizable by a specific device, such as a facsimile mo- dem, printer, scanner, hard drive, etc. Desktop publishing programs typically include a directory full of drivers for various printers which translates the print instructions from the software into the closest approximation possible by the printer through the print driver software. 2. In software event-process- ing, code that receives commands and distributes them appropriately for execution. droid colloq. abbrev. android. 1. An anthropomor- phic robot, generally more human than machine which may be a combination ofbiological and me- chanicaVelectronic parts. 2. A company drone, some- one who unquestioningly follows instructions and mechanically goes about the business of work (or liv- ing) without much enthusiasm orintrospection, pos- sibly due to apathy or unquestioned acceptance of authority. 3. Aperson hired as a human robot to do a mindless, repetitious, production-line job that offers few opportunities for variety or creative interaction. These are the kinds of positions that should be handled by machines, in order for people to have more leisure and creative time. drop Ashort cable connection, often between a util- ity pole and a building, or between one panel and another, or a panel and other distribution entity, com- monly used for supplying telephone, cable TV, or computer network services. drop frame In television video broadcast recording and playback, North American television was de- signed to play at 30 frames per second. Then when color signals were introduced, the differences be- tween black and white and color technology resulted 285 '.,t, ' ••. ;:.: '."'.'.,. I '.(;I' ::w © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary in acompensatory adjustment of the frame rate down to 29.97 frames per second, a situation that altered the time code and reduced its usefulness for station timing. Drop frame mode, also known as compensated mode, is a technique in which the system skips ahead avery small amount at specified intervals, skipping over the fIrst two bits in each minute. However, this is mi- nutely too much of an adjustment, so each ten min- utes, only a single bit is skipped. It's similar to the way in which we adjust our calendar to celestial events by introducing leap years (except that a day is added rather than skipped in a leap year) where needed, to better synchronize them. Drop frame modes are important to broadcasters be- cause programming is interspersed with commercial announcements, shorts, special programming, and other timing-related items, that need to be sequenced in a smooth, seamless way. See SMPTE time code. drop loop In telephone wiring between the switch- ing office and local subscribers, the circuit is called a loealloop and the specific section of the circuit from the utility pole or other nearby junction point to the subscriber's home or office is the drop loop. dropout, drop-out An undesirable low-level, irregu- lar loss of information when transferring from one system, medium, or format to another. Dropout happens, for example, in video editing, when copy- ing or editing tapes, especially with less robust for- mats and inexpensive equipment. Dropout can some- times be seen as white dots appearing somewhatran- domly on the screen. They are especially noticeable if the screen has large areas of dark or solid colors. dropout, transmissionAshort interruption in atrans- mission, usually caused by aproblem in the transmit- ting or receiving equipment. Differentindustries have different objective measures for the length of inter- ruption that constitutes a dropout. dropped call A call terminated without the express desire of the parties engaged in the call. In radio phone communications, dropped calls are not uncommon, as the signal can easily be interrupted by terrain, weather, or a stronger signal from another source or distance. dry cable, raw cable, dark cable Conductive cable or wire with no added electronics and no signal pass- ing through. The cable you find on spools in the hard- ware store is dry cable. Raw fiber optic cable is called dark cable. dry cell A common, compact type of battery de- scended from the wet cell, but differing in that it em- ploys nonliquid electrolytes in the form of paste or gel. Dry cells were invented by Gassner in 1888 and manufactured in the early 1900s. Since they do not Digital Transmission Speed Categories Signal Level/Description Os-o A 64,000 bps standard for transmitting digital data through pulse code modulation (PCM). A sampled signal is quantized and transmitted with bits that represent quantization levels being transmitted separately. A standard used in telephone systems. DS-I A frame format standard for transmitting data at 1.544 Mbps, developed in 1962. Used on Tl systems. It incorporates time division multiplexing (TOM) to combine 24 OS-O signals, and adds a single framing bit. Signals are transmitted with bipolar (B8ZS) pulses or alternate mark inversion (AMI). In 1969, the standard was extended to SuperFrame to increase the signal-to-noise ratio, and later it was further modified to create Extended I SuperFrame which is more robust. Europe uses a 32-channeI2.048 Mbps system whi~h is somewhat similar but incorporates different synchronization and signaling formats. See Extended SuperFrame, SuperFrame. OS-IC This signal system was designated 1C because it fits somewhere between OS-1 and OS-2 in terms of its 3.152 Mbps signaling rates. Used on TIC systems. It was introduced by AT&T in 1975. OS-1 signaling bits are bit-interleaved into the information bits. OS-2 A frame format developed for longer transmission lines and to accommodate AT&T's Picturephone technology (which was developed many years before the technology to use and support it became sufficiently widespread). Used on T2 systems. It combines four OS- 1 signals or 96 OS-O signals, employs two framing stages, and transmits at 6.312 Mbps. Europe uses a different lTV-defined system that operates at 8.448 Mbps (2.048 Mbps primary rate). OS-3 A frame format developed for signaling over broad bandwidth signaling systems. U sed on T3 systems. OS-3 uses Bipolar with Six Zero Substitution (B3ZS). The OS-3 signal combines 7 OS-2 or 672 OS-O signals, is framed in two stages, and transmits at 44.736 Mbps. Through multiplexing, the asynchronous signals are transmitted over synchronous links. Europe uses a different lTV-defined system that operates at 34.368 Mbps (2.048 Mbps primary rate). 286 © 2003 by CRC Press LLC use liquid acids, they are easier to handle, and more portable than wet cells, and can be used in any orien- tation. They are commonly used in flashlights, small appliances, and many handheld devices. Many dry cells include toxic chemicals and heavy metals and should be recycled through local centers, not thrown in the trash. See Gassner, wet cell. DS 1. Dansk Standardiseringsrad. The Danish Stan- dards Institute, located in Hellerup. 2. digital system. 3. See Distributed Single Layer Test Method. DS- A series of signal speeds for transmitting digital data through a variety of modulation and multiplex- ing schemes, designated DS-l through DS-4, with higher numbers representing faster possible transmis- sion speeds. This system is primarily used in North America and Japan. Asimilar system, which differs in data rates, encoding, and numbers of channels, the E-system, is used in Europe. The DS-system first was initially used by phone carriers for connecting main switching centers. Gradually, as the technology be- came less expensive, it began to be used in the back- bones of larger private branch exchanges, and now it is used in telephone feeder plants, and local area net- work backbones as well. See Digital Transmission Speed Categories chart. See E-carrier. DS Facility Acategorization system for describing digital transmission capacity. See DS-, Digital Trans- mission Speed Categories chart. DSA 1. data service adapter. 2. Digital Signature Al- gorithm. See Digital Signature Standard, Electronic Certification. 3. See Direct Selling Association. 4. See Directory System Agent. DSAT 1. digital satellite. 2. digital satellite TV. 3. See Digital Supervisory Audio Tone. DSC 1. See digital selective calling. 2. See Digital Subtitle Encoder. 3. Disability Statistics Center. 4. document supply center. 5. distributed statistical computing. DSCS See Defense Satellite Communications System. DSE 1. See Deep Sky Exploration. 2. See Distrib- uted Single-Layer Embedded. 3. distributed software/ systems engineering. See distributed programming platfonn. 4. See Dynamic Systems Estimation library. DSH See double-superheterodyne. DSL See Digital Subscriber Line. DSLAM DSL access multiplexer. See add/drop mul- tiplexer, Digital Subscriber Line. DSM 1. See Design Structure Matrix/Dependency Structure Matrix. 2. See digital switching matrix. DSP See Defense Support Program. DSR See data set ready. DSRC See Dedicated Short-Range Communications. DSRR See Digital Short-Range Radio. DSS 1. digital satellite system 2. See Digital Signa- ture Standard. 3. direct satellite system. See direct broadcast satellite. 4. direct selling support. 5. direct station selector. A consumer broadcast system com- ponent that enhances channel selection for desired programs; the term is especially applied to satellite programming services selection. Dstl An organization formed in July 2001 when the U.K. Defence Evaluation and Research Agency (DERA) was split into a private company and the Dstl agency, which supports a number of sites supporting research in science and technology. Dstl traces some of its lineage back to organizations originating in the 15th century. DSTO See Defence Science and Technology Organisation. DSU See Digital Service Unit ~S2~:'~~~~:~i~~rvj~~::t:~~;lf~~~~~i:i; :r.illr,', transmission line and a router. Some units combine a digital modem, router, and terminal server, a com- bination that is popular with Internet Services Pro- viders. See digital service unit, channel service unit. DSX panel See digital signal cross-connect panel. DT See Deutsche Telekom. DTE See Data Terminal Equipment, End Device. DTL 1. database template library. 2. See Designated Transit List. 3. See diode transistor logic. 4. distance . teaching and learning. DTMF See dual tone multifrequency. DTMX digital trunk manual (telephony) exchange DTP See desktop publishing. DTR 1. See Data Terminal Ready. 2. detailed trans- action report. 3. discrete tone relations. DTRS 1. digital tape recording system 2. digital te- lemetry recording system 3. Digital Trunked Radio System. DTS Digital Television Standard. See ATSC Digital Television Standard. DTSR 1. Dial Tone Speed Recording. 2. digital tape system recording 3. digital temporary storage record- ing. DTT 1. digital tape transfer. 2. digital telecommuni- cations/telephone trunk. 3. digital tie trunk. A digital telephony trunk line providing a direction connection between private branch exchanges (PBXs), in other words, tying them together. 4. digital trunk testing. DTTB Digital Terrestrial Broadcasting. See Digital Video Broadcasting. DTU 1. digital test unit. 2. digital trunk unit. Du Fay, Charles Fran~ois de Cisternay (1698-1739) A French soldier and scientist who discovered that electricity had two basic attracting and repelling prop- erties, which could be demonstrated, for example, by rubbing amber with wool and rubbing glass with silk. He called these resinous electricity and vitreous elec- tricity, making the distinction in a context that had eluded previous researchers. Du Fay developed some of the ideas first investigated and described by S. Gray in England and made some important observations about the composition of the materials of the conducting medium. Perhaps most important ofOll Fay's observations is that " an electrified body attracts all those that are not themselves electrified, and repels them as soon as they become electrified by the electrified body." DUA See Directory User Agent. dual attachment concentrator DAC. A connecting device used in double ring Fiber Distributed Data Interface networks which employ a token-passing scheme over a redundant ring network. Dual ports and 287 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary attachment points are used in connection with the concentrator to reroute data ifa problem arises. See Fiber Distributed Data Interface for more detailed information. Dual Attachment Station DAS. A configuration of a Fiber Distributed Data Interface (FDDI) token-pass- ing, dual attachment network. The dual attachments provide fault tolerance. They consist ofa primary ring and a secondary ring, the first of which is usually used for data transmissions, and the second as a backup in case of problems. A Class A, dual attachment station (DAS) connects to both rings and a concentrator which, in turn, ensures the ring transmission is not interrupted. Failure in a ring causes a series ofadap- tations such that the ring wraps back on itself and tem- porarily eliminates the failed station. See A port, B port. dual cable A two-cable configuration, usually in a local area network (LAN), often implemented to pro- vide redundancy and fault tolerance, as in ring-based Fiber Distributed Data Interface (FDDI) systems. See Fiber Distributed Data Interface. dual homing 1. A means of providing backup and fault tolerance on a network system, particularly char- acteristic of Fiber Distributed Data Interface (FDDI) networks. FDDI networks utilize stations, which can be eliminated through rerouting ifa problem is found, without interrupting transmissions. Dual homing uti- lizes two concentrators, a primary and a secondary, with the secondary used as backup that is automati- cally activated ifa problem occurs. See Dual Attach- ment Station, optical bypass. 2. In a Frame Relay net- work, a means of providing fault tolerance by using dual port connections in different locations. dual mode There are many dual mode devices in tele- communications. Many modems are dual mode in order to support both vendor proprietary protocols and industry standard protocols. Dual mode monitors will sometimes support both NTSC and RGB signals. Some phones have dual pulse and tone dialing capa- bilities. Many cellular phones are now designed to support traditional analog signals and emerging tech- nologies that use digital signals. Dual mode devices tend to come about when there are competing stan- dards, or when technology is transitioning from one stage to the next. Some are autosensing, switching to the correct setting unaided, and some have to be explicitly set with a switch or software. dual output fiber optic sensor A device with two sensors in one assembly. Thus, dual independent read- ings may be taken for redunancy or comparison. The sensors may be analog, digital, or hybrid. dual tone multifrequency DTMF. Touchtone signal- ing on a phone system. The tones are actually a com- bination of two frequencies that are variously com- bined to provide unique codes for each key on the telephone, when pressed. This signal is sent through the line to indicate the desired number to be dialed or the desired selection from an automated phone menu system. The tones were chosen for frequencies that carry well on voice-grade lines. Phone phreakers used to exploit these tones for dialing 288 unauthorized long-distance numbers with small de- vices called blue boxes. With the increased use of out- of-band signaling systems, such as Signaling System 7, which send the signals separately from the conver- sational information, this practice is becoming less prevalent and will eventually be impossible. See touchtone phone. Dublin CoreA standard metadata scheme for describ- ing document-like informational objects in order to facilitate data discovery on computer networks. Thus, core objects such as Date, Creator, and Description can be quickly and easily located and utilized for in- dexing and archiving purposes. The Dublin core is intended to supplement rather than supplant existing Web metadata searching and indexing methods and can be applied to physical as well as electronic ob- jects. The Dublin Core Metadata Element Set (DCMES) became the first IETF metadata standard arising from this scheme. See Government Informa- tion Locator Service, International Development Markup Language. Dublin Core Metadata Initiative DCMI. An open forum first held in Dublin, Ohio, in 1995 as a result of discussions in October 1994 at the 2nd Interna- tional World Wide Web Conference. DCMI promotes the understanding and development of interoperable online metadata standards and specialized vocabular- ies intrinsic and related to the Dublin Core. Consen- sus is evaluated and administered by the Dublin Core Directorate. http://www.dublincore.org/ duct Protective pipe or tube through which lines or fluids are run. See conduit. ductileA property of being malleable, a material that can be shaped, drawn out, flattened, or othelWise bent or manipulated without significant stress or breakage. Duddell, William du Bois (1869-1942) An English experimenter and engineer who discovered that elec- tric arcs created in a circuit with coils and condensers could generate very high-frequency audible tones in the low radio wave frequencies. Duddell fashioned a keyboard connection to control the oscillations, thus varying the pitch and creating the "Singing Arc," in 1899, arguably inventing the fIrst electronic instru- ment and creating possibilities for wireless commu- nications. The Duddell Medal and Prize was estab- lished in 1923 by the Council of the Physical Soci- ety. See Poulsen arc. Dulbecco's observation The observation that credit for a scientific discovery seems to go to the one who gets the most publicity as opposed to the one who cre- ated the original invention or made the original ob- servation. As stated by Nobel laureate Renato Dulbecco, "Credit generally goes to the most famous discoverer, not to the first." A closely related senti- ment was expressed earlier by William Osler as "In science the credit goes to the man who convinces the world, not to the man to whom the idea first occurs." To emphasize the importance of credit to a scientific career, Broad and Wade have stated, "It is difficult for a nonscientist to appreciate the overriding impor- tance to the researcher ofpriority of discovery The desire to win credit, to gain the respect ofone's peers, © 2003 by CRC Press LLC is a powerful motive for almost all scientists." Priority for scientific discovery carries not only rec- ognition and personal satisfaction, but has a highly significant bearing on future scientific opportunities and funding for future research. dumb switch A switching device that channels sig- nals through a desired pathway as needed without automation through digital intelligence. Dumb switches are commonly manually set passive devices. A dumb AlB switch can be used for two computers to share one printer or modem. Dumb switches are very common, particularly as AlB or AlBIC switches, because they are inexpensive and easy to set up and use. They make no logical or electrical evaluations or decisions about the incoming or outgoing data; they simply route it mechanically. In its simplest form, a dumb switch does not alter or boost the electrical connection, though some may be equipped to amplify or condition a signal, without changing its infonnational content. Thus, switches may be electrically passive or active. In video, where a great deal of switching occurs, banks of both pas- sive and active switches channel the desired video or audio feeds into the broadcast or recordin~ channel. In contrast, on automated networks, "smart' switches may evaluate the incoming data and perform some rudimentary routing. Some switches are so smart, in fact, that the distinction between switches and rout- ers at the high end is blurred. On layered networks, switches typically operate at the second layer. Other than the inconvenience of threading cables be- hind desks and through walls, installing a dumb de- vice switch is pretty straightforward. Ensure that the interconnected devices are compatible, and use gen- der benders and converters that are the right sizes and numbers of pins to hook everything together. Most dumb switches for computer applications have 25- pin female D connectors. Make sure all the systems are powered offbefore making any connections, and test new connections individually rather than all at once, so that a problem can be isolated and corrected right away. See AlB switchbox. dumb terminal A minimally configured computer tenninal, with no direct processing capabilities, that isn't very useful unless it is networked to a central processing system. Universities and libraries often use dumb terminals to provide cost-effective user ac- cess to the main system. The advantage is that the dumb terminals are inexpensive, easy to maintain, and not highly subject to abuse or vandalism. In their simplest form, they consist only of a touchscreen monitor or teletype interface; one step up is a key- board or mouse and a monitor. Some not-quite-as- dumb terminals will include both keyboard and mouse, monitor, and sometimes a floppy drive or CD- ROM drive, but they still rely on a remote system for actual processing of data and commands. Hybrid ter- minals may include a processor for simpler tasks but still rely on the remote system for most of their computing power. dumpster diving Searching for mechanical parts, discarded electronics, trade secrets, software and hardware manuals, access codes, login names, and passwords in large outside trash cans called dumpsters. Dumpster diving is practiced by corpo- rate espionage agents and computer hackers. The practice has been around for a long time, but the term gained media prominence in the mid-1980s when a group of teenagers in San Diego was apprehended for computer piracy and revealed some of the ways by which it had come into possession of source code, computer passwords, phone numbers of BBSs and timeshare systems, and other confidential materials. Shredders are employed by many companies to pro- tect sensitive documents from prying eyes and some will incinerate the documents rather than disposing of them in a dumpster. In the military, even greater precautions against theft are taken, with codebooks and other sensitive documents sometimes being printed in inks that will disappear if photocopied or exposed to moisture. DUN See dialup networking. Dunwoody, Henry Harrison Chase Dunwoody pat- ented the use of carborundum in early radio wave crystal detectors in 1906 (U.S. patent #837,616). Car- borundum (silicon carbide) was robust in the sense that it could be clamped down, and thus it was used in portable wireless telegraphy (radio) units. Tube radios, which could be readily amplified, eventually superseded crystal detectors. duopoly A market situation in which two major sell- ers greatly influence the market, though they may not necessarily control it. In industries requiring licenses, a situation where exclusive operating licenses are is- sued to two businesses rather than one. duplex Double, bidirectional. duplex connection, duplex transmission Data trans- missions in which a message can be sent in both di- rections along the same transmissions line or path. In many duplex systems, the messages can be alter- . nately sent in one direction or the other, but not in both directions simultaneously, whereas in full du- plex, the messages can be sent in both directions at the same time. Serial communications software of- ten has half- and full-duplex settings. duplex telegraphy A historic innovation in tele- graphic communications in which two messages were sent in opposite directions, at the same time, over the same line by varying the strength of the current. This innovation was first put into practical use in the 1850s and was thereafter of great interest, since it could sig- nificantly improve the efficiency of telegraphic com- munications, in effect doubling the capacity of the line in the 1800s. Since two-way or duplex telegra- phy had many commercial advantages, there was much interest in this idea and in putting it into com- mercial use. J. W. Gintl, an Austrian telegraphic director and physi- cist, was one of the first to propose apractical means of duplex communications using two batteries. Two years later, Siemens & Halske were to patent a du- plex telegraph, using only a single battery. In North America, Joseph B. Stearns refined the concept and Thomas Edison extended it, patenting a quadraplex 289 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary telegraph (U.S. #480,567) that could transmit two signals in each direction by varying not only the strength of the current, but the direction as well (fol- lowed by patents for a number ofrefinements and variations ). Duplex or multiplex communications are now gen- erally the norm for many types of communications, although wireless communications over the same fre- quency are still sometimes carried out in one-way or simplex mode, as are some simple homebrew com- munications systems. See Farmer, Moses; Frischen, Carl; Gintl, Julius; Stearns, Joseph; telegraph history. duplexer A switching device that provides alternat- ing transmitting and receiving through the same trans- missions system (data line or antenna). Duplicate Ring Protocol DRiP. A Cisco protocol for Virtual LAN (VLAN) switches and routers that iden- tifies active Token-Ring VLANs. DRiP data is uti- lized for detecting duplicate configurations and for all-routes explorer filtering. DRiP has a Cisco HOLC protocol type value ofOxO 102. See Token-Ring network. DVB See Digital Video Broadcasting. DVBG See Digital Video Broadcasting Group. DVCPRO See D-7. DVD See Digital Versatile Disc. DVD Forum An international association of software and hardware developers and manufacturers of Dig i- tal Versatile Discs (DVD) technology. It was origi- nally founded as the DVD Consortium in 1995. The original founding companies included Hitachi, Matsushita Electric Industrial, Mitsubishi Electric, Pioneer Electronic, Royal Philips Electronics, Sony, Thomson Multimedia, Time Warner, Toshiba, and Victor. There are now more than 200 member com- panies, with headquarters in Tokyo, Japan. The Forum promotes acceptance of DVD-related products in the entertainment, IT, and consumer electronics industries. It defines DVD format speci- fications, publishes educational materials, creates DVD format books, and administers the DVD Veri- fication Laboratories. Membership is open to orga- nizations engaged in DVD research and development or manufacturing. See Digital Versatile Disc, Digital Versatile Disc player. http://www.dvdforum.org/ DVD+RW Alliance A voluntary alliance of indus- try-leading personal computer and optical storage developers and manufacturers, including Hewlett- Packard, MCCNerbatim, Philips Electronics, Ricoh Company, Sony, Thomson multimedia, and Yamaha. The Alliance disseminates information relating to DVD+RW technologies through various events and publications. It has been working independently of the DVD Forum. In spite of its quick acceptance and widespread popu- larity, the DVD format will likely be superseded by other optical formats, most probably blue laser tech- nologies. Blue laser diodes have the potential to write optical media with more than five times as much data as current DVD formats. Thus, about 40 hours of broadcast recording or 2 hours of high quality cin- ematic entertainment could be recorded on a single side. See DVD Forum. http://www.dvdrw.com/ 290 Dvorak keyboard A type of keyboard layout de- signed by August Dvorak and William Dealey after they studied the natural movement of fingers and of the hand over typewriter keys and researched ways in which to conform the key positions to the com- fortable hand use, rather than conforming the hand to unnatural keyboard lettering layouts. DVST See direct view storage tube. DWDM See dense wavelength division multiplexing. DWS See Dialable Wideband Service. DXC See digital cross-connect. DXI See data exchange interface. dynamic bandwidth allocation The process of as- signing bandwidth on demand or according to algo- rithms to maximize the efficiency of the system, rather than transmitting on particular frequencies or at particular times. dynamic beam focusing In cathode-ray tubes (CRTs), the sweep of the beam from an electron gun across the inside surface of the screen that displays the image. This beam forms a series of motions which, ifkept equal, are curved. To keep the distance equal across the sweep of the beams, earlier televi- sion screens and computer monitors were curved to match the length of the beams. Early flat screen moni- tors were rare and expensive. With more sophisticated hardware and software algorithms, manufacturers have devised ways of compensating the travel dis- tance of the beam to adjust to the characteristics of a flat surface. One of these techniques is dynamic beam focusing, adjusting the beam focus as needed, de- pending on which part of the screen is illuminated, and its distance from the gun. dynamic data exchange DDE. Any process in which data is transferred between systems or between applications without intermediary steps, such as sav- ing the information and transmitting with adifferent application. See drag and drop, Object Linking and Embedding. Dynamic Host Configuration Protocol DHCP. An expanded client/server configuration protocol de- scended from, and downwardly compatible with, Bootstrap Protocol (BOOTP). DHCP provides manual, automatic, and dynamic allocation of IP ad- dresses and a complete set of TCP/IP configuration values. It utilizes ports 67 and 68 and retains BOOTP's bootrequest and bootreply packet formats. See RFC 1533, RFC 1541. dynamic IP addressing When logging onto the In- ternet, or any system using the Internet Protocol (IP), it is necessary for a unique number to be assigned to the session to handle the flow of data to and from the user. Dynamic IP addressing is a scheme for automat- ing the process of assigning an address when a user connects to an Internet Access Provider (lAP) or other network access point. As part of the user login, a unique number is assigned for that session. The num- ber is typically freed when the user logs off, so the lAP can reassign it to the next user, ifneeded. Free- ing the address is an important part of the process on large distributed networks like the Internet, where there may be millions of users online, with some of © 2003 by CRC Press LLC the large lAPs handling tens of thousands of simul- taneous users. dynamic gain equalization processor DGEP. An electronic processor for providing dynamic gain-flat- tening and tilt control in optical amplifiers. In com- mercial systems, DGEP provides gain equalization through patented all-fiber acousto-optical tunable fil- ter (AOTF) technology. In single-mode optical fibers, acoustic waves can be applied directly to produce what Novera Optics calls notch filters for optical sig- nal transmission. Notches may be shaped by control- ling the frequency and amplitude of the acoustic wave. See acoustic wave. Dynamic Link Library DLL. In software program- ming, a Microsoft product format for consolidating a number of frequently used routines, or routines that may not be available by default in an application, such as Visual BASIC. The DLL is an organizational pro- gramming tool that allows a 'library' of routines to be written once, bundled together, and thereafter linked into a program and called by the application program as needed. dynamic RAM DRAM. Random access memory that requires a supply ofcurrent through the chip at all times in order to retain and refresh the stored in- formation. When you tum a computer off, the data currently in RAM is lost. RAM is one of the most prevalent types of dynamic fast storage used in com- puters. Most systems these days require about 16 or 32 Mbytes of RAM for basic functioning. This amount is in stark contrast to desktop computers in the 1970s, which could run telecommunications pro- grams, word processors, and spreadsheets in less that 8 kilobytes (not megabytes) of RAM, and systems in the mid-1980s, which could run music and graph- ics simultaneously in a fully multitasking environ- ment in only 4 megabytes of RAM. See static RAM. Dynamic Random Access Memory See dynamic RAM. dynamic range A range ofintensities, between the minimum and maximum extremes. It's a phrase that is often applied to concepts of light or sound. In im- agery, the dynamic range ofa scanner, for example, is the range of light levels, from the brightest high- light to the darkest shadow, that can be picked up and transmitted. In music, the dynamic range of a re- corded symphony performance is the range from the softest note to the loudest, expressed in terms of deci- bels. Dynamic range is sometimes described more objectively in terms of the maximum and minimum levels ofa parameter as measured by an instrument designed for that use. See gamut. dynamic resource allocation In various types of communications, the administration, allocation, and dynamic reallocation of resources, such as frequen- cies, channels, processes, programs, and access to shared peripheral devices. Dynamic resource alloca- tion usually entails intelligent algorithms for deter- mining authorizations, priorities, and needs, and of- ten includes sophisticated queuing, routing and multi- tasking capabilities. dynamic routing In general, the creation and adjust- ment of communications paths on an as-needed or as- optimized basis, so paths will change to fit the needs ofasituation as specified. In data networks, dynamic routing allows the system as a whole to stay online even if individual systems or routes change or are unavailable. This is accomplished through routers, which can communicate with other routers, usually those topologically nearby, and which may increase, or modify routing tables as needed. Dynamic routing works well on large, changeable, packet-switched systems like the Internet. Routers can relay data around distressed or suddenly unavail- able systems or trunks. On small systems, the over- head of dynamic routing may not be worth the loss of speed that the processing takes. Static routing may be used quite effectively on small systems with known, stable characteristics. See router, Routing Information Protocol. dynamic sector repair A fault correction and pre- vention system built into hard drive systems, particu- larly multiple disk arrays such as RAID, that seeks faulty sectors on a disk, repairs the data ifpossible, and records bad sectors to prevent the system from trying to write to those sections in the future. See re- dundant array ofinexpensive disks, SMART. dynamic storage In computing, the allocation of tem- porary or permanent storage space in an intelligent manner, so unused space can be optimally used, and unneeded data is removed to allow the reuse of stor- age for other applications. It may also involve occa- sional reorganization ofinformation if extra process- ing cycles are available. See garbage collection. Dynamic Systems Estimation library DSE. An ob- ject-oriented noncommercial software library for studying multivariate time series analysis techniques and forecasting models. DSE runs in Splus and R The library is suitable for applications such as studying the statistical implications of equivalence among dif- ferent model representations, studying the forecast- ing properties of models, or studying small sample properties of estimators. dynamically controllable magnetic DCM. Mag- netic materials that can change permeability in real- time when stimulated by a magnetic field. DCM ma- terials that have this property have been found in the VHF to microwave frequency ranges. Some commu- nications antennas need to be transparent at some fre- quencies and reflective at others, and DCM materi- als are being tested for their effectiveness for this use. dynamo The historical name for what is now termed a generator. A dynamo is a machine that converts me- chanical energy into electrical energy (direct current). A friction-based bicycle light is an example of a simple dynamo. When the cyclist pedals, the wheel spins and rubs against the contact point for the gen- erator. The generator takes this mechanical energy and converts it into light, so the cyclist can see and be seen at night. See alternator; generator; Siemens, Werner. 2. An energetic, dynamic individual. dynamometer, electrodynamometer A sensitive current, voltage, and power detecting instrument similar to aD' Arsonval meter except that it uses a 291 © 2003 by CRC Press LLC . processor DGEP. An electronic processor for providing dynamic gain-flat- tening and tilt control in optical amplifiers. In com- mercial systems, DGEP provides gain equalization through patented all -fiber acousto-optical tunable fil- ter (AOTF) technology. In single-mode optical fibers, acoustic waves can be applied directly to produce what Novera Optics calls notch filters for optical sig- nal transmission. Notches may be shaped by control- ling the frequency and amplitude of the acoustic wave. See acoustic wave. Dynamic. concentrator DAC. A connecting device used in double ring Fiber Distributed Data Interface networks which employ a token-passing scheme over a redundant ring network. Dual ports and 287 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary attachment points are used in connection with the concentrator to reroute data ifa problem arises. See Fiber. downlink is often frequency-shifted from the uplink in order to reduce interference between the two sets of signals. Uplink 283 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary and downlink services may be carried by different providers and may be subject to different usage re- strictions or billing arrangements download To receive computer data from a source on another system, usually