Fiber Optics Illustrated Dictionary - Part 25 pps

Fiber Optics Illustrated Dictionary D'Arsonval current A high-frequency, somewhat high-amperage, low-voltage current. D' Arsonval galvanometer See galvanometer, D'Arsonval. D'Arsonval movement A description used in con- texts where apointer associated with adial moves to show a reading when stimulated by direct current. DIAdigital to analog. See DI A conversion. D/ A conversion digital to analog conversion. In the general sense, a process whereby any system or device converts a signal with discrete states (e.g., binary ones and zeroes) into a signal with theoretically infinite states (e.g., a radio frequency signal). In data transmissions, a process or device to convert discrete digital information to a continuous form for transmission over analog circuits, usually through one or more modulation processes. Thus, infonnation from a computer can be converted by a computer modem to 232 analog audio signals that are sent through a phone line, or digital signals from an Internet phone can be converted to analog pulses that can be heard over an analog headset. See AID conversion, modem. D/CLEC, D-CLEC Competitive local exchange carriers (CLECs) that specialize in data delivery services. D/CLEC services mainly arose as acompetitive DSL- based option to expensive TI services for data networking. As such, D/CLEC services have been of particular interest to small businesses. D4 In TI digital transmission lines, D4 is a type of channel bank. Channel banks carry out a variety of interface tasks, including time slot framing and de- tecting and transmitting signaling information. See SuperFrame. DA I. See desk accessory. 2. See destination address. 3. See Directory Agent. 4. See Directory Assistance. 5. discontinued availability. © 2003 by CRC Press LLC DAA See data access arrangement. DAB 1. See digital audio broadcasting. 2. dynamically allocatable bandwidth. DACS See Digital Access and Cross-connect System. daemon A computer process that lurks in the background to handle low priority or intermittent tasks, especially in Unix environments. Daemons carry out many tasks on computer networks, including low- level operating tasks, to automate some aspect of a system administrator's responsibility, and are trans- parent to most users. A daemon may be a continuous background process or intermittent, as needed. Dae- mons are useful as print spoolers, mail message man- agers, and general resource allocators, especially for client/server requests that are invoked irregularly. DAF 1. See Data Administration Forum. 2. See Den- ver Advertising Federation. 3. See Destination Ad- dress Field. Daguerre, Louis Jacques Mande (1789-1851) A French artist and inventor who made significant im- provements in photographic imagery technology in 1839. His early photos, called daguerreotypes, were impressed in silver plated onto copper. They have a very soft, low contrast quality to them, and the clar- ity of the image is affected by the angle at which the plate is held when viewed, due to the reflectivity of the metallic medium. They are fade-resistant, and many of the original daguerreotype images that still survive retain their images. See Talbot, Fox. daisy chain lJ. To connect items individually, one to another in a series, usually through cable or connector hookups. Communication through a daisy chain ofelectronic units may be unidirectional or bidirec- tional. SCSI devices such as hard drives, scanners, cartridge drives, and CD-ROM drives are frequently daisy-chained to one another and to one controller on the logic board. When chaining SCSI devices, care must be taken to assure that each device has aunique ill number (usually from 0 to 7), and that the last member of the series (the one farthest from the SCSI controller) or chain is terminated, either with aphysi- cal connector attached to the outside, or by setting external or internal switches accordingly. Depending on the types ofdevices in the chain, it may be neces- sary for all devices to be turned on for other devices in the chain to function correctly, especially if the chain has been established for transferring electrical power as opposed to data signals. In other situations, devices may be individually turned on or offwithout interrupting the flow of data through the chain. DAL See Dedicated Access Line. Dall testAvariation on the historic Foucault test for assessing the optical quality of lens components that includes a test lens placed in the path of the incident light beam. The test was described by Horace Dall in 1947. It is similar to the Foucault arrangement, but includes a lens placed between a point light source and the paraboloid surface under test to reveal any spherical aberrations opposite in sign to the surface at the radius ofcurvature. Thus, by determining the index of refraction of a lens that is flat on one side and convex on the other and setting the lens at a specific distance between the entry point of the light source and the reflecting mirror or lens under test, the two spherical components can be made to cancel one another out and reveal other characteristics of the reflecting component. This is a type of single reflec- tion null test. When a similar configuration is combined with at least two traversals oflight through the lens, it is ~~~~~~~~~;Er~:~~:t::t~::.a~~~~~~:~l~~:~ .• Dalton, OrvA prominent amateur radio enthusiast who contributed substantially to the design and construction of the first three OSCAR satellites. Callsign K6UEY. See OSCAR. DAMA See Demand Assigned Multiple Access. damped wave Radiantwave oscillations that gradu- ally diminish in amplitude or that are being deliber- ately suppressed so that the amplitude diminishes. damping The process of decreasing the amplitude of wave oscillations. The term is often used in reference to progressively suppressing sound waves (sound damping), though it can generally be used to indicate the suppression ofa variety of types of energy, as electrical oscillations in a circuit. Daniell battery A historic, fairly simple chemical battery, providing approximately 1.1 volts per cell which was suitable for providing constant, depolar- izing current for early telegraphic systems, such as the Morse system. This battery tended to last longer than others in a closed circuit and was widely used during the first three decades of commercial telegra- phy. The cells were usually housed in a box that was placed near a telegraphic station (often under a table or desk). The Daniell battery was comprised of a copper electrode in a copper-sulfate solution on one side of a porous separator, and a zinc electrode in a diluted sulfuric acid orzinc-sulfate solution on the other side housed within glass or earthenware. This two-fluid battery was an important improvement over the voltaic pile. It is named after J .F. Daniell. See cell. Daniell, John Frederic (1790-1845) An English chemist and colleague of Michael Faraday who applied his talents, in the mid-1830s, to the development of batteries that could last longer than those in use at the time. He invented a chemical battery known as the Daniell cell which was used in early telegraph systems. See Daniell battery. DAP See Directory Access Protocol. DAQ 1. data acquisition. 2. delivered audio quality. DAR See digital audio radio. dark conduction The property ofa substance, such as a photosensitive material, to retain electrical con- ductance in darkness. It is usually a residual effect and tends to diminish over time until restimulated by light. dark fiber, dryfiber Plain, unconnected fiber optic cable, not currently carrying a signal. Since fiber is often sold as the hardware portion ofa subscriber service, this phrase was coined to indicate fiber that is sold just as fiber, with the purchaser doing the wiring 233 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary of the components and transmitters. See dim fiber. DARPA See Defense Advanced Research Projects Agency. DARPANET A distributed network of the u.S. De- fense Advanced Research Projects Agency, origi- nating in 1969, from a desire on the part of the u.s. military to exchange information among different sites and to provide redundancy in the event of an at- tack. This project grew to become ARPANET by 1972. In 1983, ARPANET had grown so large that it was split into MILNET, specifically for U.S. military use, and NSFNET (National Science Foundation Net- work), which opened it up to researchers and scien- tists. See ARPANET, Internet, RFC 791. DARS See Digital Audio Radio Service. Darwin See Mac OS X. DAS See Dial Access Switching. DASD See direct access storage device. DAT See digital audio tape. data 1. Constituent basic elements of information that can be formally organized and combined to provide communication, most commonly through written means, though the term is not restricted to written communications. 2. Building blocks that can be ma- nipulated and presented by electronic means, or which are used, interpreted, and organized by human perception and thinking. data access arrangement DAA. A system for inter- facing a communications device to the public switched telephone system. A modem interface is one example ofa DAA that enables computers to transmit and receive data through the telephone system. A DAA may be customer supplied or carrier supplied and may require approval on the part of the common carrier providing the telephone line access. DataAdministration Forum OAF. A division of the British Columbia Advisory Council on Information Management, the OAF encourages corporate data administration and information management activities throughout the Canadian province and is active in the establishment of data administration standards. data awareness Mechanisms for aggregating and switching network data traffic in the appropriate layer. data base See database. data bus See bus. data carrier detect DCD. In telephony, DCD is a signal from the DCE to the DTE, indicating a valid signal between the DTE and DCE devices. It is typically used to set port status for a connection and to gener- ate a signal indicating the loss ofa connection. The DeE is commonly a modem or serially connected printer, and the DTE is the terminal or computer. data circuitA circuit that uses transmission wires and components suitable for the fast transmission of digital information. Data Communications Channel DCC. In SONET networking, a channel related to the OAM&P which includes security and performance information associated with facility and network elements (NEs). Both generic and vendor-specific information can be included. The DCC is incorporated into both the sec- tion and line overhead. 234 data communications equipment DeE.Acategory of devices specified by the Electronic Industries As- sociation (EIA) that typically includes common serial communications peripherals, such as modems and printers. These in turn interface with data terminal equipment (DTE). In Frame Relay networking, DCE has a more specific meaning, as switching equipment that is separate from the various peripheral devices that are connected to a network or workstation. See data terminal equipment. data compression The process of encoding data to store it in a smaller amount of space. Data compression is typically achieved with specialized software tools or with software built into data transmissions hardware. Data compression may be done in advance, if files are to be stored or transmitted later, or it may be done dynamically at the time it is needed, sometimes called realtime or on-the-jly, a capability that is built into some modems. There are many different general-purpose and specialized means of compressing data. Some data compression algorithms are paired with data decompression algorithms, for archiving search and retrieval and audio/video record- ing and playback. Data compression tools do not always make the data smaller. For example, a very tiny icon file, when encoded with an image or general purpose data compression program, may actually be larger than the original by the time the header, decompression, or statistical information about the file is inserted by the compression program. Yet the same tool may quite effectively achieve as much as 60% compression on large images, so the selection of data compression technologies depends on finding the right tool for the job. To overcome this problem, three developers at Western DataCom have developed Ardire- Stratigakis-Hayduk (ASH), a compression scheme that incorporates some of the pattern-matching, and predictive concepts associated with artificial intelligence programming. This scheme attempts to broaden the scope of compression to handle many different types of data in the increasingly media-rich communications that are evolving. See Ardire- Stratigakis-Hayduk, Lempel-Ziv. data compression approaches There are many prac- tical approaches to data compression. One of the sim- plest is to remove redundant data, such as gaps, spaces, or repetitions. This approach is used in encoding voice conversations, which typically have many pauses. It is also useful for compressing graphics that have large areas of similar colors and text documents with repetition and blank spaces. Another means for compressing information is expressing it in a different way. For example, a bitmap image ofa large letter 0 may require 15 kilobytes, whereas the mathematical definition for ellipses that can define the letter 0 may require only 5 kilobytes. A third way to compress data is to try to match the human per- ceptual recognition of the data image ratherlhan the structural data characteristics of the original presen- tation. In other words, there are ways to display graphic images or to play sound files so that they look © 2003 by CRC Press LLC the same or sound the same to the general viewer/ listener, even though the construction and dynamic range of the information may have been altered. Hu- mans have a remarkable ability to conceptually add information or construct a view from a few clues. If you've eVer watched a black and white TV show and "could have sworn" you had seen it in color, you've experienced one aspect of this phenomenon. Data compression can be lossy or lossless, that is, it can retain most of the information in a file or all of the information in a file. A commonly used lossy image format in which most of the information is re- tained is JPEG, often for displaying Web graphics and videoconferencing images. With the ever-growing volume of data unleashed by the capabilities of computer technology, and greater demands for perceptually rich multiple media, the demands for data compression to reduce file space, transmission time, and costs are very high. Some of the most promising recent data compression programs incorporate fractal and wavelet theories into their encoding techniques. See Ardire-Stratigakis- Hayduk, JPEG, Lempel-Ziv, PNG, wavelet theory. data conversion The process of converting computer data stored in one format to another. The three most common reasons for converting data are achieving compatibility (upward, downward, and inter-application), saving space and/or saving time (compression! decompression), and needing to convert between digital and analog forms of information. When software applications are upgraded, they often incorporate new features that are not available in the older versions. Data conversion may be neces- sary to store information in the older or newer file format, and some of the information may be lost in the conversion process. Computer data conversions tend to happen within families of data. Graphics formats are frequently in- terchanged, audio formats are frequently inter- changed, but there isn't much need to convert audio data into visual data, except for experimental applications. That is not to say computer data has to be rigidly defined; it doesn't. For example, the Inter- change File Format (IFF) developed jointly in the mid-1980s by Electronic Arts and Commodore Busi- ness Machines is a broad specification for data definition that can be generically applied to text, sound, and graphics. Similarly, Adobe PostScript fonts, while following specific guidelines, are not just fonts, but rather are shapes that fit in the context ofa larger pic- ture, that ofa page description language which is ca- pable of describing many types of graphical elements besides fonts. Many shareware and commercial data conversion utilities are available, especially for converting among the myriad graphics formats such as PNG, JPEG, Compuserve GIF (which now uses the PNG specification), BMP, ILBM, and TIFF. PNG, JPEG, and GIF are the most commonly used raster graphics formats on the Web, and TIFF is the most widely used graphics format in the publishing and document industry (faxes are also defined within the TIFF specification). ASCII is very widely used in text conversions, and Microsoft's Interchange Format (Rich Text Format or RTF) can be used for text conversions that retain formatting such as bold, indents, fonts, etc. For database information, dBASE formats are often used for converting between one program and another. See data compression, digital to analog conversion. Data CountryCode DCC. In networking, the DCC ~ a a~~~~~~~~;i~~::eSI~~~fiae~~~~i~O:~Z~~1~~~. data country code is in Binary Coded Decimal (BCD) format, contained in two octets, in ISO 3166 format. See Data Network Identification Code. data description In a data dictionary, a unit or group of information which may comprise one or more of the following: a definition of meaning and usage, at- tributes or characteristics, and category or classifi- cation information. data dictionary 1. A reference set of data descriptions that can be machine-processed, and shared by a variety of applications. 2. In database mal;1agement, a lookup reference of data descriptions with a format or relationship such that the database engine can ef- ficiently save, extract, or scan information to/from the dictionary according to the needs of the database. data element A basic unit of information defined generically, or for a specific application. For example, a data element in an employee database might con- sist of a name or job category. A data element may be further defined as including data items components or subcategories. Data Encryption Standard DES. A cryptographic system consisting of an algorithm and a key comprised ofa long series of numbers which are used to- gether to transform data into information which ap- pears unintelligible, and back into data by the per- son for whom the information is intended. DES was developed by the National Bureau of Standards (now the National Institute of Standards and Technology (NIST)) and is intended for public use and for government protection of certain federal unclassified data. See Clipper Chip, Pretty Good Privacy. data entry The act of using a hardware interface to input data to a computing device. Data entry is commonly accomplished through a keyboard and mouse, but voice recognition systems, touch screens, and pen computers are broadening the choice of input devices. Typically, data entry is used to describe repetitive, discrete types of data, like database entries (names, addresses, order numbers, etc.), spreadsheet entries, etc. When the data is more fluid and conceptual and less repetitive, it is still, in its broadest sense, data entry but is more likely to be described in terms of the type of application being used, such as word processing. dataexchange interface DXI. A layer 2, frame-based interface installed between a packet-based router and a SMDS or ATM CSU/DSU. The DXI performs as- sembly and reassembly tasks on behalfofa router that may not have these capabilities. Since most routers now can handle these tasks, use of this particular type of interface is diminishing. 235 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Data General Corporation DG. One of the better known computer companies in the 1970s, Data Gen- eral was founded in 1968 to develop minicomputers and became a Fortune 500 firm a decade later. With almost half a million systems installed worldwide, Data General targeted high-performance computing environments, including scientific, technical, and in- dustrial sites. By the late 1990s, DG was beginning to support server applications for Pentium II processors as well, a strong sign of the convergence of the workstation and personal computer markets. Data General became a division of EMC Corporation after an announced stock swap in 1999. data grade circuitAdistinction made to indicate the more stringent needs ofcomputer data transmission, as compared to voice grade transmissions. Data is transmitted at different frequencies and is more pre- cise and easily interrupted than a phone conversation. Phone conversations use a narrow frequency range and have a great tolerance for pauses, spaces, and extraneous noise, particularly since part of the processing equipment in a voice conversation is the human brain, which understands context and innuendo, as well as just the words. Data, on the other hand, re- quires a cleaner line, less interference and a greater frequency range and has low tolerance for pauses and spaces if they affect the integrity of the information that is being transmitted. Voice grade circuits over phone lines are improved by load coils, a system of looping the wires that are strung along utility poles. Data grade circuits built in the same basic way are hindered by load coils, as they introduce noise at the higher frequencies used. data line card DLC. In a digital telephone network, a link between the transmissions wire connecting the subscriber unit (e.g., modem) on one side and the digital switching matrix (DSM) on the other. Data Link Connection Identifier DLCI. A means of assigning logical connections within a shared physical transmissions path. In Frame Relay networks, the DLCI is a unique 10-bit identification number (address) assigned to a virtual circuit (VC) endpoint that identifies the endpoint within a local access channel. It is a mechanism for keeping track of endpoint devices (e.g., routers) and is used for switching and multiplexing. When a network is configured, addresses are assigned and entry tables are created to map the DLCIs to one another for routing data over the network. If the Frame Relay is linked with a larger network, such as a global public network, the Frame Relay handles the routing offrames through the DLCIs. A router may have multiple DLCIs. If there are multiple ports, identification numbers can be used again. If frame traffic is forwarded across the Frame Relay cloud (shared connectionless resources), the address ID number may change, as the mapping is handled by the Frame Relay switch (note that global IDs are optional Local Management Interface (LM!) extensions and their implementation is not widespread). ISDN frames are similar to Frame Relay frames, and Frame Relay networks may be hooked into ISDN 236 networks. Commercial products exist to interpret ISDN messages and route them through Frame Re- lay networks. In ISDN, the address is a unique 13- bit identifier. See Frame Relay. Data Link Control DLC. A layer in the Open Sys- tems Interconnection (OSI) model, DLC is responsible for a number ofadministrative and error-check- ing functions. In satellite communications, some special adaptations are needed at this level to accommo- date the high bandwidth/delay characteristics of these transmissions. data link layer DLL. In the Open Systems Intercon- nection (OSI) reference model, the layer that ensures transmission of data between adjacent network nodes. Bridges work at the data link layer. See Open Sys- tems Interconnection. Data Link Switching DLS, DLSw. Originally developed by International Business Machines (ffiM), in 1993, DLS was submitted to the IETF as an infor- mational Request for Comments (RFC). DLS defines a reliable means oftransmitting SNA and NetBIOS TCP!IP traffic using IP encapsulation through multi- protocol router networks. See RFC 1795. Data LinkSwitching Workgroup DLSW. The technical group that worked on the development ofa new switching standard for integrating networks over TCP/IP. See Data Link Switching. Data Link Switching Special Interest Group DLSw SIG. Avendor implementation group created in 1993 to address some of the issues raised in regard to RFC 1434 in which International Business Machines (ffiM) provided preliminary information on Data Link Switching. This activity resulted in a new RFC being submitted to the IETF as RFC 1795 which obsoleted RFC 1434. See Data Link Switching. data lump cable A data communications cable that includes integrated circuits (ICs) or other components underneath the cable shielding such that they produce a lump on the profile of the cable. These are popular for interconnecting small handheld devices to computers or computerized components for two reasons. The first is that an added battery source or conversion or translation software or hardware may be needed to make the handheld device operable with other devices. The second is that the shielding from the cable helps protect the components from short circuits from other objects that might be carried in a handbag, briefcase, or pocket. A data lump cable may connect directly to a component or may interface with a cradle into which the component is coupled. data mining The process of seeking out relevant information from a large storehouse ofelectronic data that may be on many different systems in many different formats. Data mining involves using intelligent strategies and algorithms to search for relevant ma- terials based upon various parameters such as previous search history, data patterns, information corre- lations, preferences of individual users, keywords, and other triggers implemented to maximize the relevance of the information retrieved or flagged. Data mining has become a topic of substantial interest © 2003 by CRC Press LLC and development due to the vast amount of information that is flowing onto the Internet. Anyone who has used a search engine and received 300,000 hits, after narrowing a search a couple of times, can see the value in data mining algorithms that can carry out some of the work in advance. Data mining is of particular interest to researchers and marketing profes- sionals and can be an important and legitimate way of searching, sifting, and sorting information for medical research, astronomy, inventing, investing, and information technology (IT) research. Data mining on public networks has become the subject of controversy and political discussions due to the amount of personal information on the Web that is accessible to a wide audience and thus subject to misuse or abuse by individuals with questionable eth- ics or illegal intentions. Many children put personal information on the Web in the process of developing school projects. Much genealogical information is posted on the Web, and there is a great deal of information about people who don't even own or have access to computers, who thus cannot track or trace information that is being distributed about them by third parties. While individual pieces of information may not be harmful in themselves, data mining algorithms have been devised to develop profiles of individuals that indicate where they live, work, and shop, what they buy, how they spend their leisure time, and with whom they associate. These sophisticated profiles have never before in history been possible and there is currently very little legislation to protect individuals from their misuse by thieves, stalkers, or manipulative political or marketing bod- ies operating outside ethical constraints. See Ameri- can Civil Liberties Union, Ask Jeeves, Electronic Frontier Foundation, cybrarian. data multiplexing See multiplexing. DataNetwork Identification Code DNIC. An ITV- T internationally specified system of network host identification that permits individual local networks, tied to public networks, to be located and recognized for internetwork communication, much as a country code and local phone number identifies a phone line. This data network identification scheme, somewhat analogous to a phone number, is used to locate hosts on interconnected public networks by means ofX.75. The DNIC is the first four digits ofa longer 14-digit international code. The first three digits are assigned by the lTV -T to specify a data country code (DCC) and the fourth digit is assigned by the national administration to specify the public data network within that country. Network Terminal Numbers (NTNs) are the responsibility of the administrators of the public network. See X Series Recommendations. Data Numbering PlanArea DNPA. An ITU- T X.25- specified system of endpoint terminal identification implemented in the U.S. using the first three digits ofa 1 O-digit network terminal number (NTN). Data Over Cable Service Interface Specification DOCSIS MCNS/DOCSIS. An interoperable cable service delivery specification developed jointly by the Multimedia Cable Network System partners (Cox, Comcast, TCI, Time Warner) along with CableLabs, MediaOne, and Rogers Cable. Numerous other ven- dors have contributed to the DOCSIS specifications process. The standards facilitate the provision of cable services through intercompatible hardware and business systems. Thus, DOCSIS provides an industry standard for cable Internet access and possible future services. The Motorola SURFboard cable modems installed by AT&T for their cable modem data services are DOCSIS-compliant, for example. DOCSIS 1.0 supports the deployment of high-speed data services through various standards and protocols. An update incorporating Quality of Service (QoS) extensions for improved security and realtime delivery was released as DOCSIS 1.1. The DOCSIS descriptions also encompass existing ratified and de facto standards in the multimedia industry, including some of the ITU-T Series Recommendations, IEEE 802.x, MPEG-2 transport (downstream framing), and DES encryption schemes. DOCSIS is an asymmetric specification, supporting data rates of 27 or 36 Mbps in the downstream direc- tion and 320 Kbps to 10 Mbps in the upstream direc- tion. Modulation is through QAM (downstream) and QPSK and QAM (upstream). Since USB and IEEE 1394 (F irewire) computer interfaces have become popular and are much faster than traditional serial ports, USB and Firewire 1 OBase- T subscriber interfaces are planned. In DOCSIS 1.0, Baseline Privacy is a scheme for encrypting user data using the Cipher Block Chain- ing mode of DES with a 56-bit encryption key (it may be less for international locations). Regular key changes are specified for management purposes. RSA public and private key pairs are installed into cable modems in the manufacturing process. Data is encrypted only for transmission on the cable network and is not intended for high security in other environments. Authentication is not directly supported by DOCSIS 1.0 specifications. DOCSIS 1.1 extends Baseline Privacy with longer RSA keys (1024 bits) and the association ofa digital certificate with each cable modem for authentication. The public and private key pair and digital certificate are installed in the cable modem dwing manufacture. To aid implementors, Kinetic Strategies has prepared a research report, DOCSIS Infrastructure Deployment Forecast: The North American Market for Standards- Based Cable Modem Products and Services 2000- 20004 which is available for a fee. See Multimedia Cable Network System. data overvoice A means of including data on a transmissions line carrying voice signals by using frequency division multiplexing (FDM) to secure and utilize the remaining available bandwidth for the data signals. Thus, for example, a telephony device can be equipped with readouts for data about the call. data processing A broad category of activities en- compassing the manipulation of digital data, as in word processors, spreadsheets, paint programs, etc. In database systems, data processing has a more specific meaning, referring to the creation, access, 237 , • :.) ;.:· ••. : :!.:.'.·~.:t.·; , ~f:f © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary retrieval, manipulation, and analysis of textual and fmancial data. Data processing is commonly used in payroll accounting, statistical analysis and reporting, customer profiling, and many other common tasks related to commerce and business management. data protectionA broad category of actions and systems that are designed to protect data. There are two general categories of data protection: keeping the data available and in its desired form (uncorrupted), and keeping the data safe from unauthorized use. In the first category, data backups, archiving, mirroring, and other means are used to protect data from being lost or corrupted. This can occur at the local applications level, in the hardware, and at the over- all systems level. Many file system directories are duplicated to provide access if corruption occurs in one. Some operating systems allow multiple versions ofa file to be saved automatically, so that there is always a history of recent changes and aprevious ver- sion that can be used if needed. Backup hardware in the form of tapes, cartridges, optical media, and redundant drives are used by many for scheduled or dynamic backups. In the second category, passwords, digital encoding/ encryption, secure channels, proprietary formats, vaults, safe-deposit boxes, data certificates, digital signatures, etc., are all used to protect the data from unauthorized access, use, or abuse. See backup, backup file, encryption, mirroring, Pretty Good Pri- vacy,RAID. data rate Aquantification of the input or transmission of computer data. Data rates are very situation specific. For example, in data entry jobs, the data rate may be the number of fields filled per minute or the number of customer orders entered per hour. In network communications, it may be the number of bits or packets transmitted per second. See baud rate. data service unit DSU. Adevice used in ISDN systems to interconnect computers with digital phone services for end-to-end digital communications. It is similar to a modem in the sense that it fits between the computer and phone line service, but it differs in that it does not perform analog to digital and digital to analog conversions. The DSU is installed in the customer's premises and connects the synchronous communications system through a four-wire line (usually a leased line) to the local central office. The DSU is used in conjunction with a Channel Service Unit (CSU) which is installed at the central office. data set ready DSR. A control signal commonly used in serial network communications and included in the pinout specifications for the ubiquitous RS-232 electrical connector. The DSR indicates whether the communications device is connected and ready to begin handshaking. For example, assume the user has di- aled a BBS or Internet Access Provider (lAP), and the called modem has just picked up the line. The DSR senses the connection and provides a signal that lets the hardware/software know that it can continue to the next step of negotiating a connect speed and beginning the communications. See data terminal ready, RS-232. 238 data striping A means of distributing data across drives in an array. Afault tolerant means of providing data security that is incorporated into redundant array of inexpensive disks (RAID) systems. data terminal equipment DTE. Acommunications data terminal hardware specification. See data communications equipment. data terminal ready DTR. A control signal commonly used in serial communications and included in the pinout specifications for the ubiquitous RS-232 electrical connector. The DTR signals whether the communications device is connected and ready after it has successfully begun handshaking. For example, assume the user has connected with a BBS. The DSR verifies the connection, a connect speed is negotiated, handshaking begins, and the terminal is ready to continue communicating. The DTR signals this state of readiness. DTR serves an output for DTE devices and an input for DCE devices. See data set ready, RS-232. data typing The process ofspecifying or determining the format ofa variable, file, or block of data. data unit DU. A generic term for any modular or connecting unit in a data path, although the abbre- viation is most commonly applied to small, limited- function digital or digitaUanalog units as opposed to more complex systems. Examples include computer modems and passive switchers. data warehousing A term used primarily by large corporations with very large databases until hard drives became bigger and less expensive and the Web made huge databases easily accessible through the In- ternet. Data warehousing is a system ofinformation storage and retrieval for vast databases that are comprised ofsmaller storehouses of databases. Large databases present unique logistical and programming challenges. Many database storage and retrieval systems are limited in the number of records or files they can handle; in other words, many of them are not scal- able. Consequently, various new strategies for data warehousing are being developed. The Internet has made accessibility, through a local Internet Access Provider (lAP), so easy and inexpensive that companies are demanding increased access to databases not only online, but at branch offices in other states and countries. Thus, data warehousing through Web browsers is developing, even though the suitability ofHTML to this task is somewhat limited. With one of the Web-friendly programming environments, such as Sun's Java, the job becomes easier, but the logistical demands of taking geographically divergent databases that may be in a variety of formats to meet local needs, and accessing them all as a conceptual unit over the Net, is an ongoing programming challenge that will probably continue to evolve for some time. Efforts to promote open systems and object-oriented programming strategies may contrib- ute to streamlining the process of data warehousing. See COREA, Open Systems Interconnect. database Any collection of data organized in some form for storage or for storage and retrieval. A database can be as simple as a list of names or as complex as a relational, distributed, multisite archive of © 2003 by CRC Press LLC integrated images, ideas, text, facilities, actions, and processes. A file system hierarchy on a computer storage device is a type of database, as is an employee file that includes pictures, birthdates, addresses, and social security numbers. Database creation and management programs typically have text-based interfaces, graphics interfaces, or both. With text-based interfaces, information is organized into lines and fields and usually listed se- quentially from top to bottom. In more flexible graphical databases, a screen mask or input template can be created almost as though using a paint program to draw the input screen. Lines, boxes, colors, and other visual elements can be used to make the database appealing and its functions and input actions apparent to the user. Fields are then assigned to the graphical elements, and the order of input is defined. More sophisticated databases include scripting or symbolic programming languages to allow automa- tion of the database so error messages, prompts, help windows, and other applications elements can be presented when appropriate. There are many ways to store data in a database: com- pressed or uncompressed, encrypted, encoded, or plain ASCII. The format of the data isn'tusually what creates compatibility problems. More often the orga- nization of the data, which can vary widely, is the hurdle that must be overcome when interchanging data among applications or systems. See data warehousing, expert system. database reports Charts, graphs, lists, and other statistical reports that can be selected or computed from information in adatabase or more than one database. Reports are widely used for financial statements, business plans, demographics, research, etc. database serverA system, computer, or application specifically designed to provide database capabilities, security, and file access to multiple users on a system. There are two aspects to a database usually incorporated into aclient/server model: the application that generates, searches, and retrieves the data and the data itself. Sometimes the data is on the server, and the application is on the individual user's machine. Sometimes it's the other way around, and sometimes all aspects of the database system are handled by the server. It depends on the sophistica- tion of the system and the needs of the users. In high security situations, the server usually handles every- thing. In smaller networks, where security is less of an issue, the applications may be installed on individual machines to run faster, while the data is banked on the server, with password access, number ofuser restrictions, etc., centrally handled by the server software. datagram This term is used in a general sense to mean a unit of information in apacket-switched network without regard to previous or following packets. Depending upon the network architecture, it may also have a defined format within that system. In lay- ered architectures, the datagram may be associated with a specific layer or layers. A datagram may be encapsulated and subsequently decapsulated at the re- ceiving end, for example when tunnelling through different systems. Internet Protocol (IP) datagram transmission over connectionless X.25-based public networks has been defined by avariety of organizations. Source and destination information are typically encoded in association with the datagram. See Point- to-Point Protocol, RFC 877, RFC 998. DataSPAN Frame Relay Service A Nortel Frame Relay digital telephony service aimed at users with virtual private line networks (corporations, educa- tional institutions, etc.) and high-speed interconnec- tions. The DataSPAN service provides bandwidth on demand through space-saving multiple virtual circuits for each access port that connects to the user's equipment. DataSPAN services can transmit over perma- nent connections at DS-O, DS-I (fractional or full) rates. It can support switched access from circuit- switched services at Os-o rates. See Dialable Wide- band Service. . date and time stamp A common function of computer applications and operating systems (OSs) that records when some event occurred. For example, files are usually date- and time-stamped as to the time of their creation or the time they were last updated (or both). Entries to databases are frequently date- and time-stamped, as are computerized physical premises access systems, electronic timecards, and many more. The only problem with date and time systems on computers is that not all computers take the time or date from a reliable source. Some have lithium-battery powered realtime clocks, but many do not. It may be up to the user to set the date and time manually, and a power outage can change the settings. To improve time-stamping, many operating systems now have a utility that can automatically access a computer network and take the time from a reliable source on a network. This is useful for synchroniz- ing with time-sensitive sites (auctions, stock ex- changes) and for general file management. Datapath Loop Extension, Datapath Extension DPX. In telephony networks, a commercial loop extension capability and card manufactured and li- censed by N ortel. Datapath Services enable digital full-duplex synchronous and asynchronous fast transmission rates over standard twisted-pair wiring. The importance ofDatapath Services is that they allow smaller, local exchange carriers (LECs) to provide services previously accessible only to bypass and in- terexchange carriers (IECs). Thus, high-bandwidth telephony applications such as multimedia services (e.g., videoconferencing) can be made more readily available to a broader range ofsubscribers. The DPX may terminate either a channel bank or digital loop carrier, forming a link between the subscriber's connecting device and DS-O channels on DS-I network facilities on a digital public switched telephone network (PSTN), now also known as a public switched digital service (PSDS). The device that connects the user's computer to the wire loop and the DPX unit may be a modem or other similar data unit, acluster control unit, or aterminal 239 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary 25-pin male D connector usedprimarily for serial andSCSI data communications. DBD 1. database drivers. 2. See digital bearing dis- criminator. DBMS See Database Management System. DBS See direct broadcast satellite. DDT 1. database thang. A tongue-in-cheek name for a Berkeley database data structure. 2. defect-based testing. 3. Deutsche Bundespost Telecom. 4. DBT. A versatile, extendible MATLAB Toolbox for radar signal processing suitable for signal intelligence and communications applications, developed by the Swedish Defence Research Establishment. DBT supports modeling of array antennas and simulation of many situations associated with these antennas. DC 1. In telephone communications, Delayed Call. 2. See direct current. 3. disconnect conform. In Open Systems Interconnection (OSI), a transport protocol source, and in 1808 he invented the arc lamp by connecting the terminals of a voltaic cell to a piece of charcoal, resulting in a brilliant light now known as an arc light, or electric arc. Michael Faraday became his laboratory assistant in 1813. dB abbrev. See decibel. DB 1. data bus. 2. See database. DB-9 A common designation for a 9-pin D-shaped computer connector, used on many laptops and desktop computers, especially for serial connections through an RS-232 cable. DB-9 simply describes the physical connecting portion and does not define the electrical relationships of the pins to the wires in the cable to which the connector attaches. RS-232, on the other hand, defines specific pinouts and pathways for various types of signal and information data. DB-IS A common designation for a 15-pin D-shaped computer connector most often used for monitor cables and Ethernet transceivers. DB-15 simply describes the physical connecting portion and does not define the electrical relationships of the pins to the wires in the cable to which the connector attaches. DB-2S A designation for a 25-pin D connecter very widely used for computer data transfer, especially serial cables, and one end of many parallel and SCSI cables. DB-25 describes the physical connecting portion and does not define the electrical relationships of the pins to the wires in the cable connector. Many of the common, inexpensive AlB switchboxes are installed with DB-25 female connectors. 25 14 D-Style Connector - 25-Pin interface unit (e.g., for ISDN connections). Depend- ing on the wire gauge, the distance between the data unit and the DPX may be up to about 18,000 feet. In some situations, the DPX may replace a DLC in a channel bank. Datum Corporation Manufacturers of time and frequency technologies used in computer networks, land-based wireline communications, and wireless satellite and cellular communications. Datum is a world leader in the manufacture of cesium atomic clocks, supplying Europe and most of the American Global Positioning System (GPS). daughterboard, daughtercardjargonA printed circuit board that piggybacks onto a motherboard (which contains the main processing circuitry) in an electronic system. The daughterboard is frequently, though not necessarily, smaller than the motherboard and usually adds some specific type of functionality: more memory, acceleration to the main CPU, a device interface, etc. A fatherboard has further been described as a connection to a motherboard that provides a series of connectors, into which several daughterboards can be connected. DAV See digital audio video. DAVIC See Digital Audio-Video Council. Davies, Donald (1924-2000) A researcher at the Brit- ish National Physical laboratory, Davies developed concepts for distributed digital networks contempo- raneously with Paul Baran in the mid-1960s. It is be- lieved by Baran that each conceived the idea inde- pendently. This important new mode of data transmission became known as packet-switching and was incorporated into the ARPANET which evolved into the Internet. See Baran, Paul. Davisson, Clinton Joseph (1881-1958) An Ameri- can physicist, Bell Laboratories researcher, and win- ner of the Nobel Prize in physics in 1937 along with George Paget Thomson, Davisson studied electron diffraction and theories of electron optics. During World War IT, he researched crystal physics and the theory of electronic devices. Davy, Edward (1806-1885) An English physician who invented the electromagnetic repeater (electric renewer) and created one of the early copper wire- based telegraph systems in the mid-1830s. He dem- onstrated a needle telegraph in 1837 and received a telegraph patent in December 1837, halfayear after Wheatstone and Cooke received their telegraphic patent. Later, after emigrating to Australia, he developed a process for refining copper. Thus, along with Morse, Cooke, and Wheatstone, he was one of the pioneers of telegraphic equipment and perhaps the first to develop a telegraphic relay. See telegraph history. Davy, Humphry (1778-1829) An English scientist and educator who passed a current through potash in 1807, decomposing it and discovering a new element (potassium). Davy subsequently discovered more elements and clarified that some substances considered elements actually were not. He also proposed a theory of electrolysis. In the early 1800s, he observed the properties of carbon when connected to an electrical 240 © 2003 by CRC Press LLC data unit. See Open Systems Interconnect. DCA 1. Defense Communications Agency. A U.S. Government agency involved in military standards development. 2. Document ContentArchitecture. An International Business Machines (IBM) system of specifying a document series from draft to final document. 3. Dynamic Channels Allocation. A wireless concept that is used in DECT PCS services. DCC 1. See data communications channel. 2. See Data Country Code. 3. digital command center. 4. See Digital Command Control Standards. 5. digital communications center. 6. See Digital Communications Conference. 7. See Distributed Computing & Com- munications Laboratory. dcd The name of a freely distributable (under the GNU General Public License) Linux CD player, developed by David E. Smith, that allows CDs to be played on an appropriately configured computer. It provides a means to enter playlists, etc. DCD 1. See Data Carrier Detect. 2. See Document Content Description. DCE 1. See data communications equipment. 2. digital communications media. 3. See Distributed Com- puting Environment. DCM 1. See digital switching matrix. 2. See dynamically controllable magnetic. DCP 1. Department of Consumer Protection. 2. See Digital Communications Protocol. DCS 1. digital communications system. 2. digital cross-connect system. 3. distributed computing system. DCT 1. digital carrier trunk. 2. See discrete cosine transform. DCTI See desktop computer telephony integration. DDB 1. device-dependent bitmap. 2. digital databank. See data warehousing. DDCMP See Digital Data Communications Message Protocol. DDD See Direct Distance Dialing. DDE See dynamic data exchange. DDN See Defense Data Network. DDOS distributed Denial of Service. See Denial of Service. DDS 1. digital data service. 2. digital data storage. 3. distributed data system. DE See Discard Eligibility. de Coulomb, Charles A. See Coulomb, Charles A. de facto standardA format, specification, or design, usually from aself-interested commercial source, that has become widespread. This often confers a large degree ofindustry control to the major stakeholders. Occasionally de facto standards are good, if a public-service body hasn'tprovided a standard, and if the standard brings down the cost of goods to make them more widely available to the general public. Some- times de facto standards are bad, since the specification or products themselves may be of poor quality and may only have become widespread through ag- gressive advertising, consumer trust, or lack of un- derstanding of the technology. By the time the consumer learning curve catches up with the technology, the standard may be too entrenched to change. Very frequently, the cheapest product becomes the de facto standard, not because it is a good product, based on good design principles, but because it's more af- fordable. Sometimes the most convenient product becomes the de facto standard. Ifa product is easier to use, or more portable, it may outsell more flexible or powerful designs. Sometimes the first product to hit the market becomes the de facto standard, and manufacturers are some- E!:B!ll:~~~~;'r;~~~ff:~2~;a:~!~ 1~~·'. dem on the shelves often had a big say in specifying the format for communications and, consequently, a short-term de facto monopoly. de Ferranti, Sebastian (1864-1930) An inventor who collaborated with Elihu Thomson and William Stanley in the development of the transformer. He was also responsible for developing the first high voltage alternating current (AC) distribution system at a time when direct current (DC) distribution systems were prevalent. de Forest, Lee (1873-1961) A highly ambitious American inventor who harnessed the power ofelec- trons by inventing the Audion vacuum tube, which was granted apatent in January 1907 (U.S. #841,387). This very significant technology has been used in electronics in many industries for decades, though eventually transistors superseded vacuum tubes, except for some specialized high frequency applications. Although he was loathe to acknowledge his pre- decessors, de Forest's invention stemmed from the work ofT. Edison and J.A. Fleming. However, he is to be credited with the introduction of the electron tube grid unit, creating a triode, which was a significant advance over the design of the Fleming tube. Lee de Forest's three-electrode vacuum tube made transcontinental communication possible, and the proliferation of vacuum tubes for radio wave detec- tion caused the decline ofcrystal detector radio sets. In the 1920s, de Forest contested E. Armstrong for the invention ofregeneration and won. (Lee was born "De Forest, "but later in life is said to have preferred "de Forest. " His wireless company was spelled "DeForest.") See Audion, DeForest Wireless Tele- graph Company, Edison effect, transistor. de-encapsulation An important aspect of packet- switched networking in which user data field data in an encapsulated packet is extracted upon receipt. dead band In guidance systems, a means of intro- ducing hysteresis by preventing errors from being corrected until they have exceeded a certain specified magnitude, a safety precaution against the guided object reacting prematurely to interference or spuri- ous signals. Dead Link Check DLC. A Perl program to check HTTP links, with reporting capabilities, DLC was released in 1999 by Martial Michel. Since 'dead' links, those that no longer point to avalid address on the World Wide Web, can be aproblem for users of a site, a utility that alerts the Webmaster to problems is a welcome tool. It can be used by itself or as an extension to Public Bookmark Generator. 241 © 2003 by CRC Press LLC . relationships of the pins to the wires in the cable connector. Many of the common, inexpensive AlB switchboxes are installed with DB -2 5 female connectors. 25 14 D-Style Connector - 2 5- Pin interface unit (e.g., for ISDN connections). Depend- ing on the wire gauge, the distance between the data unit and the DPX may be up to about 18,000 feet. In some situations, the DPX may replace a DLC in a channel bank. Datum. CRC Press LLC Fiber Optics Illustrated Dictionary 2 5- pin male D connector usedprimarily for serial andSCSI data communications. DBD 1. database drivers. 2. See digital bearing dis- criminator. DBMS See Database Management System. DBS See direct broadcast satellite. DDT 1. database thang. A tongue-in-cheek name for a Berkeley database data structure. 2. defect-based testing. 3. Deutsche Bundespost Telecom. 4. DBT. A versatile, extendible MATLAB Toolbox for radar signal processing. Fiber Optics Illustrated Dictionary D'Arsonval current A high-frequency, somewhat high-amperage, low-voltage current. D' Arsonval galvanometer