Fiber Optics Illustrated Dictionary - Part 29 ppt

Fiber Optics Illustrated Dictionary with power conditioners and good quality surge sup- pressors. DISA 1. Data Interchange Standards Association. 2. Defense Infonnation Systems Agency. 3. Direct In- ward System Access. A subscriber option for external access to a private branch exchange (PBX), usually through a security code. disable 1. v. To prevent from functioning. There are many ways to disable a process or mechanism, but the two most common are interrupting the power or the transmission. 2. n. A signal, tone, or command sent through a circuit to disable a device at the end of the transmission. Telephone companies have the capability ofelectronically enabling and disabling a number of phone services. Disabilities Rights Office DRO. An office of the Federal Communications Commission (FCC) that helps to ensure that people with communications-re- lated disabilities are given the same opportunities as others to access and use communications services. http://www.fcc.gov/cib/ drol disaster recovery The procedures and resources for bringing a system back online after a disaster. Disas- ter recovery applies to everything from repairing or replacing facilities and equipment damaged by floods, hurricanes, or bombing to recovering data from a hard drive damaged by a lightning strike or a toddler poking paper clips into the circuit board. Data recovery is a fact of life. Whether or not you archive your data, your drive will fail at some point, or you will inadvertently overwrite an important file. Pre- vention is usually better but, if recovery of data be- comes necessary, there are many shareware and com- mercial tools to assist with the task. disc A round, flat data storage and retrieval medium. Usually these are write once/read many (WORM) devices, since optical media are not as easy to write, and rewrite, as floppy disks. Although in English disc and disk are used somewhat interchangeably, in com- puter parlance, disc tends to be used more often to designate optical storage media such as audio CDs, laserdiscs, etc., whereas disk is used more often as a contraction of diskette to designate an encased floppy diskette (flexible magnetic medium). See disk. discard To throwaway, be rid of, toss aside. This is usually said of something that is of no interest or fur- ther use (or anticipated future use) or that cannot be dealt with or used at aparticular instance in time. The concept ofdiscard is important in networks since there are many instances when too much data or bad data may create bottlenecks or other types of problems. discard-eligible OE. In data-segmented networking systems, there are often packets or frames that are dis- carded for various reasons, including redundancy, congestion, timing out, low priority status, etc. Some- times the decision to discard the packet is made on the part of the network's lower level functions, and sometimes the discard status is explicitly represented by a signal bit. In Frame Relay, there is a bit in the HOLC frame address field that can be set to mark the frame as expendable in case of congestion. See leaky bucket. 272 disclaimerArefutation of responsibility or liability. Software license agreements often have disclaimers of liability for any damage to computers on which the software is run. It's pretty unusual for software to damage hardware (yes, it is possible), but it doesn't happen under normal operating conditions. Neverthe- less, attorneys prefer to have all bases covered. disco tech slang A rather good pun for disconnect technician. disconnect 1. To separate two discrete units or de- vices from one another, usually with the implication that the separation broke some type ofelectrical, in- ductive, or communications link between the two. 2. To terminate acommunication, as in a phone call. 3. To break a circuit. Discoverer XIVAhistoric satellite launched in Aug. 1960, it was the first satellite to be ejected from an orbi ting space vehicle and recovered in midair. The satellite was launched into a north-south polar orbit in an orbit of 116 miles altitude at the perigee and 502 miles at the apogee. On the 17th pass, the Agena ejected Discoverer XIV from its nose and the re-en- try vehicle fired retrorockets to slow the return trip. Once in Earth's atmosphere, it parachuted toward Earth and was recovered by a C-119 on the third pass over the parachute. discrete Individual, separate, distinct. discrete cosine transform DCT. A mathematical means ofmanipulating information by 'overlaying' cosines in order to analyze or use it from another point of view. OCT techniques are used in a number of digi- tal compression schemes, including a lossy compres- sion technique that provides a practical balance be- tween a high degree of compression and relatively good perceptual clarity in the decompressed image (although very high compression ratios may create a blocky effect). An adaptive version of this technique is used in JPEG image compression. JPEG is one of the primary image representation formats used to dis- play graphics on the World Wide Web. One of the dis- advantages ofDCT is a visual artifact called Gibb's effect that manifests as ghostly ripples running along distinct edges. See discrete wavelet multi tone, F ou- rier transform, fractal transfonn, JPEG, MPEG, lossy compression, wavelet. discrete line illumination, line illumination DLI. Alight source with a narrow spectral width. Discrete line illuminators are useful in many technologies, including communications devices and telescopic and industrial quality assurance calibration systems. Some have connections for fiber optic cables for wavelength calibration of fiber-equipped spec- trometers. DLI sources include single-mode laser lights and cooled, very low pressure spectral calibration lamps. Spectral lamps are available commercially to provide avariety ofdefined wavelengths in materials such as neon, argon, xenon, and mercury. discrete multitone OMT. Amulticarrier technology used for transmitting multiple media over existing copper wires. It lends itself to avariety of data deliv- ery services, including Digital Subscriber Line (OSL) © 2003 by CRC Press LLC and Hybrid Fiber-Coax (HFC). It has been selected by the American National Standards Institute (ANSI) as a standard line code for T 1.413. DMT uses discrete Fourier transforms for creating harmonics along the main lobes (and demodulates at the receiving end). Thus, DMT uses harmonics, or tones, as a means to divide the bandwidth into sub- channels. The number of bits per tone is dependent upon the frequency of the tone since signal-to-noise ratios vary at different pitches. The utilization of these tones is somewhat like FM broadcasting; some tones are used for data and some are used as guard bands. A signaling tone is used in each data stream for tim- ing. By dividing the available bandwidth into smaller units, portions of the available bandwidth can be in- dividually tested to evaluate speed, availability, and suitability for transmission. This allows the optimi- zation of transmissions over existing twisted-pair in- stallations, which can vary widely in their character- istics from region to region. This scheme is currently used in ADSL installations and is being evaluated as a potential standard for VDSL. See carrierless am- plitude and phase modulation, Digital Subscriber Ser- vices, discrete wavelet multi tone. discrete wavelet multitone DWMT. Amulticarrier technology developed by Aware, Inc. for transmitting multiple media over existing copper wires, based on the same general principles as discrete multitone. DWMT is being promoted for use with Digital Sub- scriber Line (DSL) and Hybrid Fiber-Coax (HFC). DWMT divides the available bandwidth into smaller units to utilize suitable portions. It differs from DMT, which uses Fourier transforms, by using wavelet transforms for encoding subchannel bits. DWMT produces lower energy harmonics than DMT, mak- ing it easier to demodulate the encoded signal at the receiving end, where a fOlWard fast wavelet transform (FWT) is used. It is less susceptible than other schemes to channel distortion and requires less over- head. See discrete multitone, wavelet. discussion listA mechanism for people to intercom- municate and debate topics of interest over computer networks. In the early days, when most communica- tions were facilitated by email transmissions among academic institutions and individual subscribers to a list, they were called mailing lists. However, now it is best to call them discussion lists to distinguish them from mailing lists that are used for bulk commercial or junk email messages containing product informa- tion or solicitations (similar to bulk mail delivered by the postal service). Discussion lists are ubiquitous and account for a significant proportion of commu- nications over local networks. Because participation in discussion lists is so high, they are typically hier- archical and many are now moderated, to minimize abuse, commercial messages, and personal slurs. The content of most discussion lists is text messages sent through email. In recent years, however, Web- based lists are growing in prevalence, and some lists now incorporate HTML tags and graphics. As band- width and transmission speeds increase, it is likely that video images and sound will become established in discussion lists. See mail distributor, USENET. dish colloq. Common terminology for a parabolic satellite or terrestrial receiving or transmitting dish. dish aperture The diameter of a parabolic commu- nications receiving dish. In general, the larger the aperture, the broader the scope of signal apprehen- sion (there are exceptions depending upon the nature of the signal and the shape and materials of the dish). The amount of aperture is also partly dependent upon the size and position of the feed apparatus, as it will block some of the signals. dish focal point The distance, in a parabolic antenna, from the reflective surface to the focusing point of the signal. It is important to know this point in order to position the feed mechanisms efficiently. The fo- cal point is dependent on the breadth and CUlVature of the dish, with flatter dishes generally having a fo- cal point that is farther away from the reflective sur- face than more concave dishes of the same diameter. disk, diskette A round, flat, flexible, encased me- dium, typically used for data storage and retrieval. The flexible medium inside the case is coated with magnetic particles that can be rearranged, thus pro- viding read/write capabilities. Common Modulation and Signal Subdivision Schemes Modulation or Subdivision Scheme Abbreviation/Notes carrierless amplitude/phase modulation CAP Carrier signal is suppressed and reassembled at the receiving end. Single channel makes it more susceptible to interference. quadrature amplitude modulation QAM Variations in signal amplitude are used to represent data. discrete multi tone DMT Frequencies are divided into discrete subchannels in order to optimize the throughput of each channel; faster, less susceptible to interference. discrete wavelet multitone DWMT Provides some interesting means of implementing better performance and low-loss compression with less susceptibility to interference. 273 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary In the late 1970s and early 1980s, desktop comput- ers were equipped with 8" disk drives, and the ac- companying diskettes were flexible and highly sub- ject to damage. By the mid-I980s, 3.5" disk drives were becoming popular, even though the diskettes were expensive at $6.00 each, because they had a hard shelled protective covering. Prices dropped until dis- kettes of double the capacity were only $.30 each. By the mid-1990s, superdisks were starting to be devel- oped, but it was not until 1998 that they were widely announced. The superdisks are downwardly compat- ible with regular floppy disks but can hold over 100 Mbytes each. Diskettes are being superseded by USB and FireWire storge devices Disk also is short for disk drive, a high capacity hard storage medium. See disc, diskette, hard drive. disk controllerA hardware peripheral circuit provid- ing an interface between acomputer's main circuitry and a floppy or hard disk drive. The most common formats for hard disk controllers are SCSI, IDE, USB, and FireWire. SCSI is predominantly used on Sun, Amiga, Macintosh, DEC, HP, NeXT, SGI, and server- level ffiM-licensed desktop computers. IDE is pre- dominantly used on ffiM-licensed consumer-model desktops and some recent Macintosh computers. SCSI allows up to six devices to be chained to each controller port (the controller is considered the sev- enth device). IDE allows up to two devices, desig- nated master and slave, to be chained to each con- troller port. SCSI disk controllers can be used to in- terface with hard drives, scanners, cartridge drives, CD-ROM drives, and redundant array of inexpensive disk (RAID) systems. IDE can interface with hard drives, cartridge drives, some CD-ROM drives, and some scanners. To enhance the limited capabilities of IDE, an Enhanced IDE specification has been de- veloped. See FireWire, Universal Serial Bus. disk mirroring See mirror, RAID. disk operating system DOS. The low level operat- ing functions ofa computer that are used to read or write a floppy disk or hard disk and are engaged when the system boots up. The phrase has since taken on a more general meaning to encompass all the low level operating functions ofa computer. In the late 1970s, there were several disk operating systems made avail- able for the various microcomputers. For example, the Radio Shack TRS-80 Model I could be run with CP/M, TRS-DOS, LDOS and others, and you could select the one you wanted to run by reading it from tape (technically, a tape operating system), or from a floppy disk (and later from a hard drive). Gradually, however, as microcomputers came into the main- stream, people began to associate the operating soft- ware with the computer on which it came, and many do not realize that each computer hardware platform is capable of running a variety of operating systems. For modem computers, there are many choices, in- cluding Rhapsody, BeOS, Windows, MacOS, Open- Step, Linux with The X Windows System, etc. The user doesn't have to be tied to the operating system that comes bundled with the computer. In the future, if more developers adapt the various emerging inter- 274 national standards for open systems, particularly ob- ject-oriented technologies, applications may finally become operating system independent, and then us- ers will be able to choose different computers the way they choose different cars, with the common object format being the "gas" that is standardized to power them all. disk server A system dedicated to file storage, re- trieval, and handling on a network with shared disk resources. It may also implement user access pass- words, file locking, license restrictions, and other administrative tasks associated with dynamic file sharing on a multiuser network. On large computer systems, the disk server may manage many dozens or hundreds of disks and may require a room of its own with special fast cabling, such as Fibre Channel cabling. See RAID. diskless computer A computer with RAM access only and no disk drives or which has no user-acces- sible hard disks. Typically, diskless PCs are simpli- fied systems for inexperienced users or secured sys- tems for preventing vandalism, theft, or the introduc- tion of viruses. Diskless PCs are generally attached by a network cable to a secure console housing a stor- age medium such as a hard drive. Some, however, may allow the user to insert removable flash storage. Diskless systems are often found in kiosks, copying centers that offer access to computers by the hour, public shopping centers, libraries, amusement parks, and science and technology museum exhibits. DISN See Defense Information Systems Network. dispatch 1. ~ To send out a verbal or written com- munication or package. 2. n. A missive, telegram, or other written or electronic communication, usually intended for someone out of normal hearing range, or intended to provide a record of the communica- tion. 3. n. A communication intended for a group of recipients, such as field workers, taxi drivers, law enforcement officials, etc. Often sent verbally as a radio communication. dispersion The spreading and gradual loss of signal strength that occur in electromagnetic or optical data transmissions over distance. Different media have different dispersion characteristics. Dispersion is usu- ally, though not always, undesirable, and steps are taken to minimize it in most transmission technologies. There are a number of types of dispersion related to the wavelength composition, characteristics, and angles of reflectance of propagating light beams. In fiber optics, multimode cables are more subject to dispersion than single-mode cables, and hence have a shorter effective distance (this can be mitigated somewhat with graded index fiber). In airwave broadcasts, undesirable dispersion, called scattering, can occur as a result of moisture, particles, and terrain. See diffraction, graded index, reflection, stepped index. dispersion, chromatic In fiber waveguides, the dis- crepancy in the rate of propagation of light beams of varying wavelength. Since light is made up ofa wide variety of wavelengths which have different properties in terms of speed, visibility, and ability to travel © 2003 by CRC Press LLC through "solid" matter, light pulses of differing wave- length regions will interact in specific ways with the conducting core and the reflecting cladding resulting in varying times of arrival and varying amounts of power (Le., some wavelengths may be absorbed more than others), especially over longer cable runs. Graded index fiber that has been doped to customize its light-carrying property may be used to mitigate some of the effects of chromatic dispersion. See dis- persion, modal. dispersion, modal In fiber waveguides, the pulse spreading that occurs as the different beams of light, reflecting from the cladding back to the core, vary in how far they travel in a given period of time. In mul- timode fibers and those with larger core diameters, there is a greater prevalence of dispersion of the time of arrival ofa light pulse due to the larger area through which the beams may reflect and disperse from one another. Graded index fiber, which has been doped to influence its light-carrying properties, may be used to mitigate the effects of model dispersion by match- ing the indexes ofportions of the fiber to the beams that will be traveling through. See dispersion, chro- matic. dispersion, wave The separation of an electromag- netic wave into its component frequencies. For ex- ample, light rays passing through a prism will be dis- persed into individual color frequencies. See diffrac- tion, dispersion. display n. Apresentation ofvisual information, usu- ally on a display medium such as a screen, wall, or computer display device such as a cathode-ray tube (CRT), plasma display, liquid crystal diode (LCD), or light-emitting diode (LED) surface. See cathode- ray tube, light-emitting diode, liquid crystal diode, multisync. display driver The software that translates instruc- tions from a computer processor to the correct oper- ating instructions for an attached display device. A display driver may be written to control one specific type of display, a family of devices, or a wide vari- ety of devices. Most computers have a set of generic display drivers compatible with the operating system. However, as there are many different types of prod- ucts, with new ones all the time, many vendors will provide display driver software with their product. Sometimes the display driver will be incorporated into the peripheral card that interfaces with the dis- ~~!s ~~=~lO~:~~~:~~~; ~:~~~~~~=~~}~~=;II mation to the driver about their capabilities and con- figuration parameters, so the driver can configure it- self to optimally take advantage of the installed device. display interface card Aperipheral card that is in- terfaced between the computer processor and the physical display device in order for electrical signals and control data from the device driver to be trans- mitted between the computer and the display device. Sometimes the peripheral card itself comes installed with display driver software. Some display interface cards have hardware settings to indicate the parameters of the installed display de- vice. Thus, there may be a dial, jumpers, or dip switches that have to be set to configure the screen resolution, number of colors, refresh rate, and other parameters. Display Support Protocol nsp. An assigned well- known port 33 on TCPIUDP systems. Display Systems Protocol nsp. An assigned weI/- known port 246 on TCPIUDP systems. distance learning, distant learning Receiving edu- cation through telecommunications media, including mail correspondence, email, Web sites, audio and video tapes, videoconferencing, etc. A number of edu- cational institutions are providing course content, ref- erences, etc., on the Internet, especially on the Web, and with digital signatures they may also eventually provide testing, assignments, and critiques through electronic means. See audiographics, videoconfer- encing, whiteboarding. Modal Dispersion Concepts _ !iiiliidi riM • ;&1 2 a b • • Light beams traveling along afiberoptic lightguide do not all reflect atthe same angles at the same time. Imagine two pulses sent through afiber core at slightly different angles (1 a and1 b) with 1aparallel to the reflective cladding and 1b at an angle such that it reflects backandforth in relation to the cladding and the conducting core but in the same general line of travel as the straight beam. If you were to stretch out (calculate) the paths of two beams starting at the same time (2), b would be longer, and the discrepancy between the time of arrival of a and b would increase as the diameter of the core and/or the length of the transmission link increased. Light travels veryfast, so the effects are subtle, but may become significant over long haul multimode networks. 275 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary distinctive dial tones This is a method by which dial tones are distinguished from one another by proper- ties such as pitch, so that the caller can tell what type of call is being connected. This is particularly useful on private branch systems (PPBX) where internal and external calls require different sequences of numbers. For example, it is common to dial "9" to get an out- side line on private systems, and the change in tone helps the caller know the call has been given outside access and will now accept an outside number. distinctive ringing A subscriber option or feature of some phones that uses a different tone or ringing se- quence to identify incoming calls as coming from inside or outside a branch system, or to signal another extension. Different phone carriers offer this option under a variety of names, such as Feature Ring and Ident-a-Call. While distinctive ringing is common in businesses with private exchanges, there are now phones and peripherals for residential and small business users to distinguish calls on separate lines, or to identify another callee on a single line, with codes that make a different ringing sound or sequence. It's handy if you get many calls for teenagers. distort To defonn, contort, warp or pervert out ofa normal sound, shape, or condition. Distortion is sometimes intentional, as in distorting the sound from an electric guitar or synthesizer to create some spe- cial effect. In most cases, however, distortion of a transmission is an undesirable fault condition. distortion An undesirable change in the basic char- acteristics of a wave or data transmission sufficient to interfere with the information or its perception. Extraneous noise is not technically considered to be distortion. In visual images, distortion usually in- volves undesirable aberrations in the basic character- istics of the image, such as color, shapes, or lines that misform or obscure the original features, or of the entire image, in which case the outer contours may become squeezed or twisted. In sound distortion, the pitch, speed, or timbre of the sound may be altered, making it difficult to discern the content or source of the sound. distribute To apportion, spread out, scatter, dole, deal out, or dispense; to give out, broadcast, or deliver to members in a group. distributed backbone A backbone is a central artery or trunk in a network. A distributed backbone is one in which network segments are interconnected through hubs joined with backbone cables. Thus, there may be multiple segments or rings joined to one another through a backbone segment. See backbone, collapsed backbone. distributed computing A system in which the com- puting processes are divided, parcelled out, or other- wise handled simultaneously, or in which computing services are apportioned, broadcast, or delivered to users. A local area network (LAN) or wide area net- work (WAN) are examples of distributed computing environments. Users of individual workstations can work independently of one another on tasks that are frequently carried out, yet they can share common 276 files, applications, and devices distributed around the system. This system permits more open and efficient use of resources. A render farm is another example of distributed com- puting in which a centralized system parcels out in- dividual tasks related to constructing 3D models and images. These are assigned to various machines, and the completed processes or objects are reintegrated to create a completed rendering, thus potentially speeding up processing time. The Internet represents an example of distributed computing wherein rout- ing and transmitting of messages occur through a co- operative network of many interconnected systems, and many common resources (Web sites, archives, applications, search utilities, online chats) are shared among users. Distributed Computing & Communications Labo- ratory DCC. An experimental research lab at Colum- bia University that develops fundamental, novel net- working technologies and exports them to academic and industrial organizations. Projects include mobile agents; programmable, active network systems; adap- tive, self-managed network systems; and manage- ment technologies for Quality of Service (QoS). The DCC is under the umbrella of the Columbia Network- ing Research Center (CNRC). http://www.cs.columbia.edu/dcc/ Distributed Computing Environment DCE. A middleware set of components from The Open Group that includes procedure, directory, file, and security services which may be bundled into avendor's oper- ating system or integrated into a system by a third- party developer to fulfil the above-listed functions. Distributed Computing Environment DCE. A commercially developed set of services from Digital Equipment Corporation (DEC) that supports the de- velopment, maintenance, and use of distributed com- puter applications. It is based on the Open Software Foundation's DCE. (The Open Software Foundation is now The Open Group.) Distributed-Feedback Laser Diode DFB lasers have a layered, corrugated structure above theactive lasingregion. The spacing of the cor- rugations is related to the refractive index of the ma- terials, allowing the corrugations to serve as a grat- ing filter that selectively reflects the desired wave- length into the cavity from which it is emitted. The component is distributed in the sense that the light reflection andfilteringprocessoccurs across the length of the component. © 2003 by CRC Press LLC distributed-feedback laser DFB. A compact semi- conductor laser component used in lightwave trans- mission systems. DFB lasers are based upon Fabry- Perot (FP) lasers, but have an extra grating filter that provides a sharply concentrated wavelength output without the lower powered sidewaves that are char- acteristic ofFPs. This makes DFBs suitable for tasks such as optical amplification and dense wavelength division multiplexing (DWDM) applications where finely concentrated signals allow closer proximity of neighboring signals without interference. L- and C-band transmission applications are common. DFBs are sold in various configurations in terms of power requirements, connector options, and the optical wavelength emitted. See Distributed-Feedback laser Fundamentals diagram. See Fabry-Perot laser. distributed file system A file system that is distrib- uted across more than one partition, more than one ~;~~:;.~~::~~~::n~~~~::~~~~:~~ ~f:~~~ '. Distributed-Feedback Laser Fundamentals pump P outp~t A distributedfeedback (DFB) semiconductor laseris constnlctedout of a number of carefully organizedactive and passive layers, depending upon the intended size andfunction of the component. DFB lasers can be housed inside traditional computer chip packaging or may befabricated of thin film layers for insertion into other assemblies. Silicon-based compounds are commonly used under the grating layer and as base substrates. Whatever shape or materials are selected, the basic concepts are the same - the organization of the layers and the grating period (distancefrom one corrugation to the next) serves as a reflectivefiltering component, causing somewavelengths to be reflected back andvery specific tuned wavelengths to pass through to be output at the other end of the assembly. This is a highly simplified cross-section of the gratingfilter within a semiconductor component, illustrating the layered composition and corrugated structure of the grating. The grating period would be much finer than shown here andthe number andthickness of the layers vary with the materials and fabrication techniques selected. Energy appliedto one end of the componentbounces alongwithin the conductive layers andis selectivelyback-reflected and filtered as it passes across the length of the component (hence the name distributed-feedback laser) leaving only a very na1TOW wavelength range by the time it exits the end as laser light. Altering the grating period allows the wavelength to be tuned to a specificfrequency and the signal is very stable, qualities that are extremely useful for communications applications and thefabrication of scientific instruments. 277 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary uted file system across partitions or across disks, in such a way that a directory or set of files appears to the user as though it were all in one place, thus mak- ing it transparent, appearing as a single logical unit, even though the physical underpinnings are different devices. See redundant array of inexpensive disks (RAID). DistributedManagement TaskForce,Inc. DMTF. A trade organization promoting the development, unification, and adoption of management standards and initiatives for desktop, enterprise, and Internet environments. The organization seeks to enable in- tegrated, cost-effective technologies to be used effi- ciently rather than in a crisis mode. http://www.dmtf.org/ distributed network administration DNA. 1. A wiring and services distribution scheme that divides up the administration and actual physical connections into smaller units, which may be one per department, one per floor, or some other arrangement. This is con- venient for systems where there are significant physi- cal limits on the length of individual transmissions cables (as in some fiber optic installations), where operations or maintenance personnel are assigned to different sections of the building, or where different types of wiring are used for different departments or areas of the premises. See distribution, distribution frame. 2. A wireless services system in which admin- istration is subdivided into particular regions or sec- tions of the service (a local area, city, or state). This is a common administrative system for nationwide wireless systems that cover a large area of territory, but through which many users roam from region to region. distributed programming platform DPP. A pro- gramming platfonn connected to a network such that the programming process is carried out transparently over a number of machines or CPU s to whatever ex- tent is desired. Traditionally, programming has been done on discrete platfonns or on connected platfonns whereupon the programming process itself is not sig- nificantly different because of the connectivity. A DPP, on the other hand, is one in which the connec- tivity to other processors, in a distributed fashion, is exploited to take advantage of the increased power and redundancy supplied by the distribution. Thus, attributes such as parallel processing, background processing, fault-compensation, and other factors can aid in the programming. DPP also implies there is a means to evaluate the performance in terms of la- tency, reliability, and other functional trade-offs and priorities. DistributedQueue Dual Bus DQDB. A connection- less packet-switched network protocol with a 53-oc- tet cell and header/information structure somewhat like ATM, used for telecommunications services in Metropolitan Area Networks (MANs). DQDB is de- scribed in IEEE 802.6. It is descended from Queued Packet and Synchronous Exchange (QPSX). DQDB supports isochronous and nonisochronous commu- nications. DQDB employs the slot as the basic unit of data 278 transfer. A slot is further subdivided into a one-octet Access Control Field (ACF) and a 52-octet segment. Distributed Single-Layer Embedded (Test Method) DSE. In ATM networking, an abstract method for test- ing a protocol layer, or sublayer, which is part of a multiprotocol Implementation Under Test (IUT). distribution 1. A cabling tenn referring to the deliv- ery of services through cables and/or wires. A distri- bution system is the combined media, connections, and topology that provide services through wires or cables, usually consisting of electricity, voice, or data network services. Many different types of distribu- tion arrangements exist and are described through- out this reference. See distribution frame, horizontal distribution. 2. Apportioning, assigning, sending out, delivering of products, services, or computing pro- cesses. distribution frameA centralized circuit management structure for creating, troubleshooting, and accessing a variety of incoming and/or outgoing lines, some- times in the thousands if it is a commercial switch- ing system. Adistribution frame may be for supply- ing electrical power or may organize lines and con- nections for data communications. Frames are often built into closets, floors, or ceilings, depending upon available space and whether frequent access is re- quired. The frame may include blocks, conduits, and other physical structures to facilitate cabling. distribution panel A grid-like frame, usually of metal, with rows and columns of punched out holes through which cables can be threaded and mounted. These are often designed to fit standard 19-in. rack mounts. distribution roller See stringing roller. DistrictTelephone Company Historically, one of the first entrepreneurial efforts to incorporate A. Graham Bell's telephone invention into a commercial prod- uct and service. George W. Coy, Herrick P. Frost, and Walter Lewis opened the District Telephone Com- pany in New Haven, CT in 1878. Initially there were only eight conducting lines strung up wherever it was possible, and fewer than two dozen subscribers. The simple switchboard, designed and built by. George Coy, was mounted on a table. District Telephone be- came Connecticut Telephone and then Southern New England Telephone, in 1882. dithering A term that the imaging field has adapted from physical appliances in scientific labs where dith- ering devices are used to shake up items to minimize clogging, settling, or the effects of friction. In com- puter imaging, dithering is the blending and combin- ing of pixels to simulate the effect ofa shade of gray or a color that may not be available on the system. divest To dispossess of authority, property, or juris- diction; to take away, to deprive, to disinherit; to dis- mantle. divestiture The act of breaking up, dispossessing, or otherwise dismantling an institution. In the U. S. le- gal system, divestiture is a process wherebylhe U.S. Justice Department oversees the alteration of a company's organizational and asset structure in or- der to deconcentrate power or enforce fair competitive © 2003 by CRC Press LLC restrictions. It is not a common procedure, but it oc- curs when acompany's activities are monopolistic in nature or when a company has engaged in unethical or illegal practices that provide the company with an advantage. The process ofdivestiture thus is intended to re-establish safeguards to competitive access by new or smaller contenders. In the telecommunications industries, the three most high profile Justice Depart- ment actions against major vendors include the AT&T divestiture in the mid-1980s and the investigations into the competitive policies and practices ofMicro- soft Incorporated and Intel Corporation. At the time ofwriting, no conclusions have been drawn, and no divestiture proceedings have taken place, but dives- titure is one of the possible steps the Justice Depart- ment may follow if any of the allegations of unfair practices are found to be true. For infonnation on the AT&T divestiture, see Consent Decree of 1982, Judge Green, Kingsbury Commitment, Modified Final Judgment. DL 1. See distant learning, distance learning. 2. dis- tribution list. A list of parties receiving correspon- dence, items, or processes. Email and discussion fo- rums are often handled through distribution lists. DLC 1. Danish Lithosphere Centre. 2. See data line card. 3. See Data Link Control. 4. See Dead Link Check 5. See Digital Loop Carrier. 6. Disability Law Center. http://dlc-ma.orgl DLCI See Data Link Connection Identifier. DLL 1. See data link layer. 2. See Dynamic Link Li- brary. DLM diode laser module. See distributed-feedback laser, laser diode. DLP Digital Light Processing. See Digital Micromirror Device. DLR 1. See Design Layout Record. 2. Deutschen Zentrum fUr Luft- und Raumfahrt. Gennan Air and Space Center involved in planetary exploration and other space sciences. DLR supports educational pro- grams, space forums, asteroid research, satellite re- search, robotics, and more. The DLR Asteroid Re- search Team is known as DART. DLS 1. See Data Link Switching. 2. Distance Learn- ing Section of the American Library Association & Association of College and Research Libraries. 3. Distributed Learning Subcommittee of the Univer- sity of British Columbia within the Centre for Edu- cational Technology. 4. Division ofLaboratory Sys- tems of the U.S. Centers for Disease Control and Pre- vention. DLSw See Data Link Switching. DLSW See Data Link Switching Workgroup. DM See delta modulation. DMA 1. Dayton Microcomputer Association. http://www.dma.org/ 2. See Direct Marketing Asso- ciation, Inc. 3. See direct memory access. 4. See Document Management Alliance. DMCA See Digital Millennium Copyright Act. DMD 1. differential mode delay. 2. See Digital Micromirror Device. DMI 1. See Desktop Management Interface. 2. Digi- tal Multiplexed Interface. An AT&T interface that interconnects and multiplexes transmissions between Tl trunks and private branch exchanges. DML Development Markup Language. See Inter- national Development Markup Language. DMS 1. data management system. 2. decision man- agement systems. 3. Digital Multiplex System. A se- ries of programmable communications switches. Northern Telecom (NTI) provides one of the switches in common use in telecommunications, called the DMS-250 (there are other models as well). 4. dimin- ishing manufacturing sources. 5. Division of Math- ematical Sciences of the National Science Founda- tion. The DMS supports small-group projects aimed at exploring and developing the properties and applications of mathematical structures. http://www.nsf.gov/mps/divisions/dms/ 6. Dutch Meteor Society. DMSP See Defense Meteorological Satellite Program. DMT See discrete multi tone. DMTF 1. Desktop Management Task Force. 2. See Distributed Management Task Force, Inc. I DMUX digital multiplexer. See multiplexer. DN See Directory Number. DNA 1. See Digital Network Architecture. 2. See dis- tributed network administration. DNC 1. direct numerical control. Data connectivity between a computer processor and a computer-com- patible numerical control (NC) device. 2. distributed networking computing. See distributed computing. 3. dynamic network control DNCF See Directory Number Call Forwarding. DNIC See Data Network Identification Code. DNIS See Dialed Number Identification Service. DNRDynamic Network Reconfiguration. Afeature on commercial International Business Machines (mM) network that allows network address designa- tions to be reorganized without closing down the system. DNS See Domain Name System. DNSSE See Domain Name System Security Exten- sions. DOCSIS See Data Over Cable Service Interface Specifications. document colloq. In computerese, atext file in a for- mat specific to the word processor that created the file. The .doc file name extension is often used to identify this type of file. Document files in this sense of the word are not the same as standardized ASCII text files. Doc files provide fonnatting, font, and color information specific to the applications program that created the file. One of the best formats for exchang- ing files between applications that are not intercom- patible is the Interchange format, also known as Rich Text Format (RTF). It holds more information than ASCII text files (font names and sizes, and text at- tributes such as bold and underline are retained), and it is widely supported on many platforms and by ev- ery major word processor and desktop publishing pro- gram. In fact, it would be great if more people would use RTF files on the Web for document downloads, instead ofusing word processor-specific document files that can't be loaded by many users. 279 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary document camera A camera specially adapted for photographing documents, usually for historical pres- ervation, archiving, or replication. This type of cam- era is mounted and optimized for a suitable focal length and artificial lighting conditions. It may be combined with a bank of lights to illuminate the docu- ments evenly. It is typically attached to a stand so that it can be readily raised or lowered like an enlarger. More recently, document cameras have been equipped with electronics that enble the signal from the camera to be fed into a computer system. The camera may be a digital still camera, digital cam- corder, or analog camcorder. In this way, documents can be digitized, stored directly on networks, or used in videoconferencing sessions. Document Content Description DCD. A structural schema facility proposed in 1998 to the World Wide Web Consortium for specifying rules for the struc- ture and content ofXML documents. DCD utilizes a subset of the XML-Data Submission. DCD defines document constraints in XML syntax, using an RDF vocabulary, much the same as traditional CML DTDs. HTML coders will recognize the basic angle-bracket nested tag structure ofa DCD. However, rather than the common paragraph, anchor, and image tags found in HTML, the contents of the DCD tags include ele- ment definition types and parameters. The DCD se- mantics are intended to be a superset ofXML DTDs while maintaining conformance with RDF. document database A digital repository of document information, usually consisting of document images which show documents as individual pages as they might be viewed in print form, or individually stor- ing the elements of the documents (pictures, text, mathematical formulas, etc.), in such a way that the information can be manipulated, stored, and retrieved with word processors, drawing programs, etc. The design and programming of document databases is a great challenge, as it is not always known how the information might be used in the future. It is also becoming imperative to design systems that can handle vast amounts of information yet still search and retrieve the information with reasonable speed. document image management DIM. Electronic storage, access, processing, and retrieval of documents stored in image format. Image format is a very com- mon way to archive information that would othelWise require massive space. It means simply that text, graphics, and everything else is stored together as a picture, with no explicit differentiation of textual con- tent. Microfiche is one of the systems designed to store image directory lists and image "snapshots" of newspapers,journals, documents, certificates, etc., in order to provide reasonably quick lookup and to save storage space. document image processing DIP. The process of taking printed documents and converting them to a visual digital form to be manipulated and/or viewed electronically. In the past, converting text information to image for- mat somewhat limited what could be done with the images. It wasn't possible to apply search/retrieval, 280 and editing to the text itself, because it was not in a form amenable to database software. That is no longer true. While work still needs to be done on document recognition systems, there are now programs which can take information stored in image format and process the characters into text; some can even rec- ognize which parts of the page are images, and which parts are text, in order to intelligently handle the con- tent of the page. See document image management; zone, optical recognition. Document Management Alliance DMA. A compre- hensive standard for interoperability among elec- tronic document management systems, sponsored by the Association for Information and Image Manage- ment. It is related to the ODMA, a standard for inter- operability between desktop applications and elec- tronic document management systems. Document Object Module An interface model for allowing programs and scripts to dynamically access and process documents independent of language or platform. Document Printing Application DPA. A standard for printing within an open, distributed network en- vironment (ISO/IEC 10175) originally published in 1996. The Internet Printing Protocol (IPP) is a "lighter" version with some of the functionality of the comprehensive DPAstandard. documentation Text and/or images that, taken to- gether, describe how a system works and how to use it. Good documentation goes beyond simply describ- ing system features and functions; it describes them in such a way as to provide a context as to when and why they should be used. Documentation is often sub- divided into tutorial and reference sections. This is generally a good idea, as tutorials are useful when you are first learning a system, and references are useful once you have grasped how to use a system. The worst computer documentation is a manual that simply repeats and describes the contents of the menus. The better ones tell you what the program does, and how, why, and when you should do it. DoD See Department of Defense. DOD See direct outward dialing. DOH I. See Department of Health. 2. See Document Object Module. Dolbear, Amos Emerson (1837-1910)A Tufts Col- lege professor for almost three decades, beginning in 1874, Dolbear was an associate of Thomas Edison. Dolbear was awarded a patent for an induction-based wireless telegraph in 1886 (filed in 1882). He also developed an electrostatic version of the telephone, in the early 1880s, apparently improving upon the concepts of Philip Reis, but it lost out to the Bell phone in both utility and recognition. In the 1890s, Dolbear described discoveries related to the effects of temperature on the properties of met- als and their relationship to conductivity and stated " that at absolute zero their electrical conductiv- ity becomes infinite, or, as it is more generally stated, the electrical resistance of metals becomes zero so it seems altogether probable that the qualities and states of matter so familiar to us as © 2003 by CRC Press LLC solids. liquids and gases depend absolutely upon temperature and that at absolute zero there would be neither solid, nor liquid, nor gas, and that elec- trical and magnetic qualities would be at a maxi- " mum. Dolbear wrote many magazine articles and books, in- cluding The Art of Projecting: A Manual of Experi- mentation in Physics, Chemistry, and Natural His- tory, with the Porte Lumiere and Magic Lantern, in 1883, and Matter, Ether, and Motion, in 1894. The Amos Emerson Dolbear Scholarship was established in 1947 to aid students of exceptional promise in elec- trical engineering and physics. The New England Wireless and Steam Museum, Inc. includes wireless artifacts from Benjamin Dolbear, Amos Dolbear's son. Amos Dolbear - Physicist, Writer, Educator Dolbear was an associate of Thomas Edison and a talented inventor in his own right. He described im- portant aspects of conductiVity and developed a num- ber of telephone devices. Dolby-NR Dolby Noise Reduction. A system devel- oped by Ray Dolby to improve sound quality by re- ducing noise. Quality levels are designated with let- ters (Dolby SR is professional level quality). Dolby- NR is widely applied in the sound portion of the mo- tion picture industry. See THX. domain Sphere of influence or activity, province, dominion; the set ofprocesses, items, or actions that constitute a sphere ofinfluence or activity. There are two aspects to the concept of domain in telecommu- nications. The first is recognition and access to and from the domain. How does the domain provide a presence to other systems, and how do they recog- nize and acknowledge its presence? The second is what constitutes a domain, what is included within the organizational grouping that is called a domain? See domain identification and domain organization. domain identification In most data network archi- tectures, a domain is perceived from outside a sys- tem as a unique address or identifier describing the logical and sometimes physical access points to a system, since systems external to the domain frequently know very little about the size and composition of elements within the domain. Since computers work with binary digits, the domain on a computer network that allows it to be identified and signaled by other domains is usually an address that can be expressed as a numeral or alphanumeric series. This address will often have a name or sym- bol associated with it, to make it easier for humans to recognize and remember it. Information about the nature of the domain, such as its basic function or geography, are sometimes expressed in the name. For example, on the Internet the alphanumeric expression ofa domain name typically indicates the type of do- main (commercial, not-for-profit, educational, etc.) and the location (.ca [Canada], .au [Australia], etc.). This naming scheme is not completely consistent, but it is helpful in most circumstances. It also is not a guarantee of location, as a system may remotely dial into a domain in another country. See domain name, for further details on Internet naming, and domain name server, Domain Name System, firewall, host, Internet, server. domain name A unique identifier, usually expressed in alphanumeric characters, to a computer domain. Local networks that are not connected to outside en- tities can setup this domain name in any way they choose or according to the domain naming param- eters of the network software that is running on the system. On the Internet, however, which is a global distributed network, it is necessary to maintain an extensive database of domains in order that each one is uniquely identified. Thus, a domain name on the Internet must be registered with one of the assigned registration entities, the oldest of which is InterNIC. See domain name, Internet. domain name, InternetAglobally unique identifier for a domain that is continually or occasionally online on the Internet through an Internet Access Provider (lAP), which may provide additional services as an Internet Services Provider (ISP). This identifier is used by the system to locate a domain in order to send and receive files, email, messages, routing informa- tion, and other network traffic. A certain flexibility is inherent in this scheme in that levels below the lAP can be rearranged according to the needs of the local network. The domain name is actually a name resolved to an Internet Protocol (IP) address, which is composed of numbers, but the name is what is familiar to most users, and it is automatically converted by the sys- tem. Once an Internet domain name has been as- signed, such as ourdomain.org, subdomains can be locally assigned, such as accounting.ourdomain.org administration.ourdomain.org sales.ourdomain.org Similarly, usernames associated with email addresses can be expressed in avariety of ways: max@sales.ourdomain.org bighoncho@ourdomain.org 281 © 2003 by CRC Press LLC . Scheme Abbreviation/Notes carrierless amplitude/phase modulation CAP Carrier signal is suppressed and reassembled at the receiving end. Single channel makes it more susceptible to interference. quadrature amplitude modulation QAM Variations in signal amplitude are used to represent data. discrete multi tone DMT Frequencies are divided into discrete subchannels in order to optimize the throughput of each channel; faster, less susceptible to interference. discrete wavelet multitone DWMT Provides some interesting means of implementing better performance and low-loss compression with less susceptibility to interference. 273 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary In the late 1970s and early 1980s, desktop comput- ers were equipped with 8" disk drives, and the ac- companying diskettes were flexible and highly sub- ject to damage. By the mid-I980s, 3.5" disk drives were becoming popular, even though the diskettes were expensive at $6.00 each, because they had a hard shelled protective covering. Prices dropped until dis- kettes of double the capacity were only $.30 each. By the mid-1990s, superdisks were starting to be devel- oped, but it was not until 1998 that they were widely announced. The superdisks are downwardly compat- ible with regular floppy disks but can hold over 100 Mbytes each. Diskettes are being superseded by USB and FireWire storge devices Disk also is short for disk drive, a high capacity hard storage medium. See disc, diskette, hard drive. disk. calibration systems. Some have connections for fiber optic cables for wavelength calibration of fiber- equipped spec- trometers. DLI sources include single-mode laser lights and cooled, very low pressure. Fiber Optics Illustrated Dictionary with power conditioners and good quality surge sup- pressors. DISA 1. Data Interchange Standards Association. 2. Defense Infonnation Systems Agency. 3. Direct In- ward System Access. A subscriber option for external access to a