Fiber Optics Illustrated Dictionary - Part 28 ppsx

Fiber Optics Illustrated Dictionary authentication, and/or signer authentication services for creating independent, interoperable implementations. The XML Signature is a method of associat- ing a key with referenced data. The XML namespace Uniform Resource Identifier (URI) and prefix for other sub-URIs for the W3C specification is xmlns=/Ihttp://www.w3.org/2000/09/xmldsig#/1 See encryption, JEPI, Pretty Good Privacy, signature. Digital Signature Standard DSS. A draft standard to permit the creation and transmission ofa secure digital signature through a Digital Signature Algo- rithm (DSA) to provide authentication of documents and transactions. Web commerce is eagerly seeking means by which documents can be electronically se- cured in order to use them for trade, banking, stock transactions, contract negotiations, etc. and will prob- ably quickly adopt this or another scheme when sufficient confidence in its efficacy is attained. See Elec- tronic Certification. Digital Steppingstones project A project ~f t~e Tomas Rivera Policy Institute (TRPI) that studies IS- sues of access to technology and telecommunications networks and examines exemplary practical implementations in diverse environments, including librar- ies, schools, and community centers across the U.S. The Digital Steppingstones project focuses on ~der served communities such as low-income and minor- ity communities. An important aspect of this. project is assessing access to the Internet and making rec- ommendations for its broad and practical implementation in publicly accessible facilities. http://www. trpi.org/dss/ Digital Subscriber Line DSL, xDSL. A data transmission service operating over existing copper pub- lic phone lines. Imagine turning o~ your co~put.er, connecting to the Internet, and findIng somethIng In- teresting that you want to explain to a busin~ss colleague or friend. If you have only one phone h~e, ~~ you're using a modem to change the computer s dIgi- tal signals into analog signals that can be sent over the phone line, you would historically have to hang up the modem, wait for a dial tone, and then call your colleague or friend. Digital Subscriber Line is a family of two-way communications services that makes it possible for you to talk to your friend without hang- ing up the computer connection first. You can do both at the same time, which means you can talk through the phone while you navigate the N et toge~h~r, dis- cussing the things that you both can see. This IS how it is done. Phone services historically have been analog systems, and there are millions of miles of copper wires installed around the world to provide these services. With the development of computers, phone switching centers began, in the late 1980s, to convert to digital equipment and software. This enabled vo!ce and d~ta to be carried on one line at the same tzme, and zn- stead of using a modem to change the computer signal to analog, and leaving the voice as an analog signal, it could be done the other way around. In other words, now the voice call is changed to digital and 262 the computersignal remains digital. This opens up a world of possibilitiesforfaster transmission, better compression and security, and simultaneous datal voice communications, without having to replace those millions of miles of copper wires. That sounds very practical, yet relatively few people have switched to DSL services. One of the reasons is distance. While most subscribers are within the 12 000 feet or so in which DSL services can operate at their best speeds, about 20% of the population is not. Crosstalk and other types of interference are problematic as well and are still being resolved. For example, traditional phone lines have loading coils installed at intervals, to extend the signals on voice grade communications. Unfortunately, at higher digital data rates, these coils cause interference. Perhaps more important is the way in which DSL services were deployed. Originally, subscribing to DSL involved having the phone company install a special voice/data splitter on the subscriber premises and, further, installing a special peripheral device in the subscriber's computer. This method was costly and not very practical, and most consumers are resistant to having proprietary peripheral cards installed in their computers. Most prefer the option of choosing a vendor and interface, and also of installing the hard- ware external to the computer, so things can be changed around as needed. For this reason, a number ofcommercial vendors have proposed several variations of DSL services, such as DSL Lite. DSL was first developed by Bell Communications Research Inc. in 1987 to provide a means to deliver interactive TV and video-on-demand over copper wires. The name is somewhat confusing, since it is not the line that is installed, but rather the interfaces at each end of the line. The point of DSL was to cre- ate technology that would make use of existing lines. In fact, a DSL line typically consists of two telepho~e lines. Since the introduction of DSL, further vana- tions have been adapted, as shown in the Digital Sub- scriber Line Services chart. Digital Subscriber Line coding and variations Since DSL is a multichannel service, it is necessary to split the available bandwidth to utilize it efficiently. This bandwidth splitting is typically done with echo cancellation (EC) or frequency division multiplexing (FDM). There are two predominant schemes for subdividing available bandwidth into smaller units to individually evaluate their transmission suitability. This is useful over twisted-pair copper lines, which can vary widely in their characteristics. The two most common are discrete multitone (DMT) and carrierless amplitude and phase modulation (CAP), ~d others are being developed. Some of these modulatIon tec~ niques have descended from the telegraph and radIO broadcast industries, and some, such as wavelet encoding, are relatively new and still being explored. Each of these has various trade-offs in terms of avail- ability, cost, speed, and susceptibility to interference, as shown in the Common Modulation Schemes chart. © 2003 by CRC Press LLC Digital Subscriber Signaling System 1 DSS 1. A telephone signaling system standardized as ITU- T Q.931 that is implemented over Layer 3 of the ISO communication model. It defines protocol for estab- lishing, maintaining, and tearing down calls. DSS 1 is commonly used in local loops providing Integrated Services Digital Network (ISDN) services for transmission over the D channel. Once the DSS 1 signal reaches the local telephone switching office, it is usually transmitted to external nodes using Signaling System #7 (SS 7). See D channel, Integrated Services Digital Network. Digital Subtitle Encoder DSE. A PoliStream second-generation multimedia titling encoder that is capable of transmitting ideographic languages (e.g., Chinese) in varied font styles. It is a Digital Video Broadcasting (DVB) standards-compliant bitmap imaging system designed to transmit multilanguage subtitles. It accepts subtitle data from traditional con- trollers and generates realtime output ofcompressed bitmaps. The data may be sent through regular broadcast channels, satellite broadcast relays, or computer network links (e.g., Ethernet). Digital SupervisoryAudioTone DSAT. A means of signaling using audio tones on cellular data networks. In AMPS cellular signaling, there are three designated supervisory audio tones (SATs). However, in NAMPS, there are instead seven subaudible digital vectors called DSATs. SATs and DSATs are used to verify the correct channel tuning after the channel has been assigned. The central office (CO) notifies as to the new voice channel and vector. digital switching matrix DSM. Amatrix format for digital switching that facilitates signal routing in high bandwidth applications. The device can be concep- tualized as a grid of channels (e.g., 64 ¥ 64 data channels), which may be individually selected to input or output to any other selected channel, or the channels may be grouped into frames for combined processing. In commercial applications, the DSM may ac- cept both digital and analog signals, but the signals will be converted to digital signals before being switched (routed). Pulse code modulation (PCM) is typically used for digitization in telephony applications. A commercial DSM for high-capacity tele- phonic switching is microprocessor controlled and may also include memory, time base, speech, and control components. DSM is used in data switchers for realtime switching between synchronous video sources. It is also used in high-capacity telephone networks for routing calls to various application processing units. In a Datapath architecture digital telephone network, it is the switching unit between the data line card and the digital carrier module or trunk controller. See Datapath Loop Extension. Digital Telephony and Communications Privacy ImprovementsAct Also known as the Digital Tele- phony Bill, this law was passed by the U.S. Congress in October 1994. The terms of the bill, drafted to be supportive of the efforts of law enforcement agencies, would require common telecommunications car- riers to design networks in such a way that law enforcement agencies could access, in realtime, the con- tents of communications on their networks and trans- actional signaling. The Electronic Frontier Founda- tion came out strongly against the bill at the time, but after the events of 11 Sept. 2001, the same issues have been raised again and proposed as changes to the Se- curity and Freedom through Encryption Act developed in the late 1990s. When the Digital Telephony Bill was first introduced, Varieties of Digital Subscriber Line Services Type Abbreviation Speed Notes asymmetric DSL ADSL 6 Mbps + Twisted-pair copper phone wires. The possible maximum rate of transmissions is inversely proportional to distance. Typically uses discrete multitone (DMT) line coding for data; frequency division multiplexing (PDM) or echo cancellation is used to subdivide the bandwidth. high bit-rate DSL HDSL TIlE 1 speeds Symmetric. Longer distances can be supported through the use of repeaters. See high bit-rate Digital Subscriber Line. single line DSL SDSL Still in development. Can be used over a single wire pair. rate adaptive DSL RADSL up to 8.7 Mbps Bandwidth can be tuned to subscriber needs. It works over longer transmission lines. Rate and speed adjust to the line length and quality. very high rate DSL VDSL 13-60 Mbps Used in conjunction with FTTC or FTTB. Different downstream and upstream speeds. (Upstream speed is 1.5 to 2.3 Mbps.) Shorter maximum distance. Still in development. 263 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary wiretapping proponents suggested that electronic wiretapping was just an extension of current wiretapping practices as applied to electronic communications, but technologists and privacy advocates argue that the analogy is not tenable - the capabilities in- herent in tapping into the National Information In- frastructure through electronics opens up far-reach- ing potentialities that are not safeguarded by current or proposed legislation in a way that is comparable to the safeguarding oftraditional wiretaps. As important as it is for law enforcement to find ways to keep up with crimes that are increasingly con- ducted through new electronic technologies, it is also important to examine the implications of expanding law enforcement tapping capabilities. How is electronic wiretapping different from traditional wiretapping? Here is a summary ofjust a few of the many complex issues involved: Visibility. The establishment of an electronic wiretap can be remotely executed and is thus invisible, in contrast to traditional wiretaps where a telecommunications technician has to physically install and deinstall a tap. In a physical connection system, accountability is more readily detennined and traced. Electronic wiretapping, on the other hand, is hard to monitor. A tap can potentially be engaged and disengaged through software that cannot be seen by anyone other than the user. Duration of a Tap. There are laws and warrants de- termining the physical location and effective duration of traditional wiretaps. The physical removal ofa tap or changing ofa switch at a phone company ends the transaction when the court-assigned permit expires. Even with the same court order requirements in place, there are currently few safe- guards consistently integrated into the network systems of Internet Services Providers or the Internet as a whole that can ensure that the process has been ended as required. Due to the nature of software and network communications, an electronic tap could be continued beyond the stated deadline with little chance of discovery. Some proponents feel that accountability needs to be ensured by agents outside of the system requesting the tap. ISPs might be a natural choice, but then the time and economic bur- den of compliance fall on vendors rather than on law enforcement agencies and vendors would, in essence, be entrusted with watching the police, a situation which has historically led to problems such as deal-making and, in some cases, corrupt alliances. Before electronic wiretapping can be fully endorsed and implemented, a new accountability technology needs to be built into global networks. Perhaps commercial vendors looking for new mar- kets can help resolve these issues with innovative products. Eavesdropping on Computer Users Outside the Pur- view of the Tap. A physical wiretap is limited in scope. You must install the tap at or near the premises being tapped. The tap may inadvertently pick up conversations of innocent individuals in the vi- cinity, but this is usually a local rather than a large- 264 scale problem. With an electronic wiretap, the local physical limitation is gone. Theoretically, communications and activities of hundreds of millions of individuals could be monitored with the same mechanism used to monitor the alleged criminal, without any obvious sign that this is happening. This aspect must be addressed before broad-ranging powers are given to officials to tap a medium like the Internet with technology that is exception- ally powerful and generalized in its capabilities. See American Civil Liberties Association, Electronic Freedom Foundation, Security and Freedom through Encryption Act. Digital Television Standard DTS. See ATSC Digi- tal Television Standard. Digital Television Station Project, Inc. DTSP. A collaborative project of the broadcast industry, including almost 300 broadcasters, computer manufacturers, content creators, and service providers. Based in Washington, D.C., the DTSP originated as the Model HDTV Station Project (WHD-TV), established by the Consumer Electronics Manufacturers Association (CEMA) and the Association for Maximum Service Television (MSTV) in 1996, to provide a hands-on educational facility for the implementation of digital television (DTV) technologies. DTSP is the January 2000 follow-up to this project, supported by the original sponsors and also by the National Association of Broadcasters (NAB). DTSP was established with an I8-month mandate to operate WHD- TV, a model digital television station, and provides continued op- portunities to develop and test interactive television, data broadcasting, and overlay services such as closed captioning, lip syncing, etc. See Association for Maximum Service Television. digital trunked radio system DTRS. A radio communications system in which communications channels are shared over trunks, as opposed to individual channels being shared by users as they become available. DTRS systems can be set up as individual or group channels (called talk groups) or combined as groups of groups, making it a useful system for government, emergency/safety, and industrial communications. Priority and encryption can be incorporated into digital trunked systems and they can be config- ured to intelligently manage traffic over the trunks to optimize efficiency of the system. digital to analog conversion The conversion of data stored as discrete units, usually in ones and zeros on computer systems, to modulated analog wave pat- terns. A modem is a common device which performs digital to analog conversion when changing computer signals to modulated analog signals that can be carried electrically through a phone line connection. The process is reversed at the receiving end. See D/ A conversion, modem. Digital Versatile Disc DVD. Also frequently referred to as digital videodisc, when the content is primarily image-based, but the format is not limited to video images, and a large proportion of digital videodiscs also contain audio. DVD is a vendor consortium-developed 180-9660- © 2003 by CRC Press LLC supporting optical disc fonnat specification, similar to compact disc (CD), except that it is designed to store a much larger quantity of data. Standardization has not been a single process. The DVD Forum, a consortium of developers, and Philips and Sony, independent commercial developers, have provided somewhat different versions ofDVD. The Philips and Sony technologies can be licensed on a royalty ba- sis, while the DVD Forum specifications are shared technologies. The DVD fonnat is gradually becoming standardized so that DVD consumer players and DVD-enabled computers can interchangeably use the discs. DVD is quickly growing in support and accep- tance by consumers, especially for entertainment and computer data storage purposes. One of the more recent developments is the release of the DVD+RW fonnat by the DVD+RW Alliance, which is not directly related to the DVD Forum. DVD physical discs are the same diameter as the CDs popularly used for music (120 mm), but very slightly thicker, bringing the recorded surface of the disc a little closer to the laser pickup, pennitting a higher resolution or areal density, i.e., smaller, more precise pits can be used to store the information, depending upon whether the disc is recorded single- or double- layered, and single- or double-sided. DVDs use higher density storage and different modulation and error-correction schemes than CDs. Further flexibil- ity is possible through the use of dual lens apertures in the laser pickup to provide a dual CDIDVD player. While a DVD player can usually play CDs for back- ward compatibility, a CD player does not inherently have the capability to play DVDs. DVD specifies more than the compression and playback format, it also provides functionality to build interactivity into the medium through menus, multiple languages, and other features. This functionality makes it attractive for educational software and games programming and allows movie makers to include extra features. DVD designates MPEG-2 as the digital compression standard for video recorded on DVDs. MPEG-2 is a fast digital motion recording and playback specification. A DVD can be recorded on both sides for up to a total of about 18 Gbytes of data. This capability is very attractive to developers, as it means a two- hour MPEG-2 encoded movie can fit on one side. Sound is encoded in either MPEG audio or Dolby AC-3. DVD can be played on a stand-alone system similar to a combination CD/laserdisc system; it works like a laserdisc player, but is small like a CD player. It can also be played on a computer through a DVD computer peripheral player. DVD can provide many types of audio storage and playback but, because of its high storage capacity, vendors are particularly interested in offering movies on DVD rather than on cassette tapes or the larger- format videodisc formats. The DVD medium is more robust and convenient than tapes or large discs, provides better sound than most tape technoloogies, and can store motion pictures of up to 133 minutes in length, including subtitles. This capacity isn't sufficient to hold every type of movie, so longer films may be offered on two discs or are compressed with a number ofinnovative predictive coding methods. It is probable that DVD, or something like it, will eventually supersede traditional video cassette tape movies. DVD is also a promising technology for electronic books (ebooks). ~~!:~~!~i~; ~~i~I~~:!;l~~~~~ J. Digital Versatile Disc associations Much of the im- petus for DVD development has come from industry alliances to promote and develop the technology. See individual entries for organizations ofparticular interest, which include the DVD Forum and the DVD+RW Alliance. Digital Versatile Disc player, DVD player A consumer electronics component for decompressing and outputting the digital data from a Digit8;1 Versatile Disc (DVD). The player uses a laser light beam to scan and read the infonnation coded into pits and grooves in the DVD medium. DVD players may play a single disc or may be capable of holding multiple discs (similar to CD disc changers and jukebox systems). Most DVD players will also play audio CDs but the DVDs themselves are not directly CD-compatible. The DVD fonnat uses a higher density of storage on the disc and different modulation and error correction schemes than CD. Consumer DVDs are designed in much the same way as laserdisc players and VCRs to play movies (multimedia) and audio recordings on a television set or through a monitor and separate audio components. DVD players are also available for personal computers and may be internal (similar to a CD-ROM player) or external (similar to an external CD-ROM or cartridge player). In the past, DVD players that worked on computers couldn't nec- essarily play the same DVDs as a separate consumer DVD player, but intercompatibility is now more common. DVD players are also being designed as small portable entertainment devices (like small boom boxes or portable television sets). Movies are one of the most popular types ofDVD products being sold and rented. Typically movies played back on DVD players with appropriate monitors support high quality component (Y IC) video at about 500 horizontal lines ofresolution compared with only 210-225 horizontal lines ofresolution in VHS. DVDs are mastered at CCIR6014:2:2 ratios. DVD-Video supports multiple aspect ratios. When movies are played back, the consumer is usually given a choice of aspect ratios, selected from letterbox (4:3), pan and scan (4:3), and anamorphic (wide screen 16:9) fonnats. DVD players sold in Europe often support both PAL and NTSC while those sold in North America may only support NTSC (this is changing as higher definition formats are finally beginning to catch on). Audio frequency response is generally in the range of2 to 44,000 Hz with a signal-to-noise ratio of 110 265 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary decibels or more. The higher-end DVD players tend to have added features, such as progressive scan, various filters, digital video equalizers, surround sound, and slow-motion effects, that are not available on the less expensive models. DVD+RW, one of the newer versions ofDigital Ver- satile Disc technology, is promoted by the DVD+RW Alliance as a storage and digital video recording format that will be compatible with the majority of existing DVD players. It is expected that DVD+RW players/drives will be able to read CD-ROM, CD-RW, DVD-R, DVD-RW, CD-Audio and other similar discs. See Digital Versatile Disc, DVD Forum. digital video DV. Technologies that enable the recording and playback ofdigitally encoded moving image infonnation and sound fall into the category of digital video. Some of the big barriers to inexpen- sive digital video have been the large amount of data that is required to record even small segments of video and the wide bandwidth and processing that is required to quickly display high resolution color images. When still images from film frames using cell animation techniques are individually digitized and stored, each frame may require up to 24 Mbytes, if it is to approximate closely the image quality of35mm film. Since each second of animation requires between 24 and 30 individual still frames, as much as 720 M- bytes may be needed to store a second of video. A full-length movie is usually ca. 7000 seconds or more, requiring more than 5,000,000 Mbytes of storage. That's a lot, and that's not including sound or data that might be added to provide search and retrieval markers interspersed with the images. Digi tal video has been in development longer than most people realize. Much of the research in this area originated in the Research Center in Yokohama, Ja- pan, in 1977. By 1979, Ampex, Bosch, and Sony were introducing technologies based on digital video con- cepts, and there was a move to introduce new standards to encompass emerging digital technologies. In order to make digital video technology possible, a number of innovations and trade-offs have been implemented. Data compression and decompression techniques are used to store images in less space, but Digital Versatile Disc (DVD) Formats Format Description DVD-Audio A CD-like DVD with improvements in audio fidelity and higher capacity than traditional CDs. DVD-R Primarily a read format. It is a professional authoring fonnat in which the DVD data, when ready to master, is recorded once to the DVD and then read as many times as desired. DVD-RAM A type of hard storage with random read-write access. It is a medium that can be written to, with an appropriate DVD drive, more than 100,000 times and functions much like a hard drive, except that the discs are easily swapped and stored (like cartridges). DVD-RAM discs can be used to store up to about 4.7 Gbytes of data. DVD-RW Similar to a DVD-RAM, in that it is a type of hard storage, but rather than random-access, it uses sequential-access, similar to streaming tape storage for up to about 4.7 Gbytes per side. It can be rewritten up to about 1000 times and thus is an appropriate choice for data that doesn't often change, such as backup data, or database information that is read more often than it is written. DVD+RW This rewritable DVD format is one of the more recent versions, promoted by the DVD+RW Alliance. Hewlett-Packard announced the release of its first DVD+RW drive in August 2001. DVD+RW is primarily intended for data storage and recording digital video onto 4.7 -Gbyte discs. The Alliance plans to release software upgrades to support DVD+R as well. This format has been developed without the endorsement of the DVD Forum and is competitive to current formats. DVD-ROM A format for computer data storage drives that is generally available with SCSI and ATAPI interfaces with capacities ranging from about 2.x Gbytes to over 9 Gbytes. Blank rewritable cartridges are available in both single- and double-sided optical cartridges and recordable discs. DVD-ROMs may at some point supersede CD-ROMs. DVD Video Text Sometimes called DVD- Text, an optional means to store and access textual information related to a DVD. It can consist of consumer information for users of DVD players or DVD-ROM drives, text information for content providers or DVD authors, or textual supplements to a video or audio data stream to enhance the value of the main information on the DVD. A guidebook for DVD- Text is available from the DVD Forum. 266 © 2003 by CRC Press LLC the picture quality does not equal film, and fast pro- cessors and frame buffers are required to handle playback in realtime. In spite of its limitations and technical requirements, developers are forging ahead with digital video products partly because digital video can be edited and manipulated in remarkable ways. Special effects that are impossible or difficult to achieve with analog film are possible with digital video. DV also has greatly increased possibilities for interactivity and access through the Internet. Of further importance is the fact that it doesn't have to go through a chemical photo- finishing process before it can be used. See animate, celluloid, D-l, Digital Versatile Disc, interactive video, MPEG, video-on-demand. Digital Video Broadcasting DVB. A family of compatible television/media delivery standards including cable and satellite technologies. Digital Video Broadcasting standards are supported by the Digital Video Broadcasting Group and published by the Eu- ropean Telecommunications Standards (ETSI). In conjunction with the Centre for Electrotechnical Stan- dards (CENELEC) and the European Broadcasting Union (EBU), ETSI has formed ajoint technical com- mittee to handle DVB standards. DVB is gaining ac- ceptance in Europe as a digital video infrastructure and is gradually replacing the traditional analog en- vironment. There are a number of subdivisions of DVB technologies. DVB Technology Subdivisions Subdivision/Description DVB-C cable transmission standard DVB-S satellite transmission standard DVB- T Digital Terrestrial Broadcasting (DTTB) designed to be adapted to the needs of local frequency and geo- graphical environments. It enables the development of single-frequency networks (SFNs). Configurable to support legacy 50- or 60-Hz systems. See Digital Video Broadcasting Group. Digital Video Broadcasting Group DVBG. A Euro- pean trade consortium that provides support and specifications for traditional and emerging broadcast technologies, such as cable and satellites, and schemes for the protection of commercial programming. See Digital Video Broadcasting. http://www.dvb.org/ digital video format A series of digital video standards introduced since the 1980s, including 0-1,0-2, 0-3, etc. See Digital Video Formats chart. Digital Video Interactive DVI. A digital recording and playback chipset technology developed at the David SamoffResearch Center. The technology was acquired by Intel Corporation which subsequently developed it into Indeo 2 and Indeo 3. It is now known as Intel Video Interactive (IVI). See Intel Video In- teractive. digital videodisc See Digital Versatile Disc. digital voice encoding A process of sampling and quantizing voice signals and storing them as digital data. Since voice encoding can require much memory, the information is usually compressed for storage and decompressed when replayed. Fractal and wavelet compression techniques are becoming popular choices. Much of the technology for digital voice encoding has come from the music industry. Research into the sampling and playback of synthesized music can be applied effectively in voice encoding and playback applications. Digital voice encoding is used in speech and voice recognition systems and is used to send conversations over digital voice telephone channels, such as Inter- net telephone applications, ISDN, or digital PCS. To save memory and cut down on transmission time, in- teresting algorithms for removing pauses and spaces are used in conjunction with digital voice. Encryp- tion to ensure privacy is also possible with digital voice communications. Digital voice encoding can provide a data library of sounds, phonemes, words, or other units, for use in digital applications such as automated voice menu systems, speaking computer applications, digital talking books, answering machines, voice mail systems, and more. Digitally encoded voice tends to be more pleasant and natural sounding than mathematically generated voice and, hence, is favored for applications where instructions or responses are given to a human listener. See quantize, silence suppression, speech recognition, voice recognition. Digital8 Adigital video format developed by Sony. It records digital video data and stores it on Hi8/8mm tapes. The recording time is about 45 minutes due to the faster rolling speed of the tape compared to Hi8/ 8mm (which records about two hours). See Hi8mm. Digital-S See digital video format (D-9). digitize lJ. To convert from analog to digital, i.e., to take an analog signal and convert it into data that consists of discrete units, such as ones and zeros, usually by sampling the analog signal at discrete points and assigning a value to the data at that point. Since most broadcast media are transmitted as waves, they are analog systems of communication. However, in order to process the information or interface it with networks, it must be converted to digital format at some point in the transmission process. Thus, digitizing a phone signal makes it possible to add features such as compression, encryption, and voice recognition. Digitizing a video signal allows processing of the image: palette changes, overlays, image com- posites, split screen viewing, videoconferencing and more. A desktop scanner is a type of digitizer, as is a digital camera. See analog, digital, digitizer, pulse code modulation. digitizerA device that converts a signal from analog to digital, usually by sampling the analog signal at discrete points over time and assigning a value to the measurement. Up to a point, the more frequent the 267 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary sampling, the better the encoding represents the original. Digitizers are commonly used in video and audio applications. The sound patches used in electronic music are digitized from analog sound samples. Digitizers are not constrained to only two dimensions. Apen or robot arm can trace contours on a physical object and convert the spatial information into a 3D coordinate system for rendering, ray tracing, or CAD programs. One of the first widely distributed 2D digitizers for microcomputers (1986) was NewTek's DigiView for the Amiga. It is historically significant as the forerunner to the Video Toaster, which initi- ated the desktop video industry. See sampling. Dll See defense information infrastructure. Dijkstra'sAlgorithm InATM networking, an algo- rithm sometimes used in conjunction with link and nodal state topology information to calculate routes. DIM See document image management. dim fiber Fiber optic cable in which the carrier provides the means to carry the signal through the fiber but does not originate the signals at either end of the circuit. See dark fiber. dimmed Lowered in illumination or visibility. Re- duced in intensity. See ghosted. DIMS 1. Document Image Management System. A hypermedia-based document management system jointly developed by HyperMedia and Aramco for the purpose of capturing the geological, geophysical, and reservoir engineering data at Aramco. 2. document and image management system. A system for auto- mating and integrating the management of images of text and graphical documents that are typically scanned and stored as digital data. DIN 1. Deutsches Institute fUr Normung. German institute for standardization, one of the major standards bodies in Europe. DIN is located in Berlin. The DIN specification is a widely used standard for computer connectors. 2. dual inline. DINA 1. Danish Informatics Network in the Agricul- tural Sciences. 2. Distributed Intelligence Network Architecture. 3. Dynamic Intelligent Network Archi- tecture. A research project topic at Carnegie-Mellon University. dinosaur slang A large, obsolete, or aging system that requires excessive resources to keep in operation. Given the speed of technological obsolescence in computer technology, systems quickly become dino- saurs, but firms often hang on to them because the cost ofinstalling and learning new technology may be higher than continuing to use old, slow, but tried- and-true systems. The term dinosaur applies especially to old room-sized supercomputers that are now less powerful than many desktop systems. diode, Fleming tube Historically, an electron tube with only two electrodes, the cathode (electron- emitting) and the anode (electron-attracting) in which Digital Video Formats Type Description D-l A component digital video format approved by SMPTE in 1985 and introduced by Sony as the DVR-l 000 in 1987. It is favored by digital effects videographers. D-l records uncompressed 8-bit video. The cassette tapes are large, weighing up to six pounds. D-2 A composite digital video format introduced by Ampex (1986) and Sony (1988) that enjoyed broad popularity until the late 1990s, superseding composite video tape recorders (VTRs) using 1" tapes. D-2 (CCIR 601) records uncompressed video. The cassette tapes are large, weighing up to six pounds. D-3 A composite digital video format developed by Matsushita and introduced by Pana- sonic as a competitor to Sony's D-2 format. D-3 records uncompressed video. D-5 A component digital video format developed by Matsushita and introduced by Pana- sonic as a competitor to Sony's D-l format. D-5 records uncompressed 10-bit video. D-5 has caught on in High Definition Television (HDTV). Some D-5 systems can play D-3 tapes. Panasonic has introduced a high definition 4: 1 compression ratio version ofD-5. D-6 A high-definition digital video format developed by Toshiba/BTS and introduced by Philips. D-6 records up to 64 minutes per 19 mm cassette through a system of helical scan tracks with a track pitch of 22 micrometers. There is a longitudinal analog cue track, a control track, and a time code track. D-7,DVCPRO (DVCPRO is the more common name, D-7 is the SMPTE designation, but it is placed here for easier comparison with other D- formats.) A component digital video fonnat introduced by Panasonic. D-7 records compressed 8-bit video at 34 mmJ second. D-7 supports 16-bit, 48 kHz digital audio. Streaming video is supported through SDTI and IEEE-1394 (FireWire) standards. D-9, Digital-S A component digital video format introduced by Jve. D-9 records 3.3: 1 ratio compressed 1 O-bit video. Promoted as similar in quality to Digital Betacam. 268 © 2003 by CRC Press LLC the electrons flowed freely and uncontrolled, which wasn't very useful. L. de Forest developed a Fleming tube into a triode, a three-electron tube, which enabled control of the electron flow. In transistor electronics, a diode is a piece of semiconductor material, positive on one side and negative on the other, with a terminal at each end. Where the electrical positive and negative regions come to- gether in the semiconductor, it is called a p-n (positive-negative) junction. Like a two-element electron tube, the electrons normally flow in one direction, serving as a rectifier, enabling conversion of energy from one fonn to another - an essential aspect of communications technology. An old crystal radio rectifier can change radio waves into audio waves that can be heard through earphones. Asemiconductor rectifier can change alternating current (AC) to direct current (DC) for powering electronic equipment. Forcing the flow of electrons to go opposite to the natural direction can be accomplished in some cir- cumstances with sufficient voltage, resulting in re- verse bias. This practice is sometimes useful in semiconductor technology for altering the information stored in a chip. See avalanche breakdown, erasable programmable read-only memory, Zener diode. Fleming Valve - Two-Electron Tube evacuated globe metal sleeve (anode) for attracting electrons filament (cathode) for emitting electrons connecting leads The Fleming valve was an important historical an- tecedent to three-element vacuum tubes but lacked the controlling grid that made itpossible to control elec- tronjlow. diode laser module DLM. See laser diode. diode transistor logic DTL. Acircuit board configu- ration wherein the logic is accomplished with diodes as opposed to resistors, reducing interaction between inputs and permitting many diodes to be used. How- ever, depending on the design and application, there may be a trade-off in speed. DTL calculators during the 1960s can be found that include both transistors and integrated circuits and represent some of the early DTL devices. Sometimes referred to as diode logic (DL), the early implementations tended to be discrete- component bipolar logic circuits (eventually super- seded by integrated circuits). As integrated circuits (ICs) became more commonly used, DTL became an IC implementation of the basic diode logic. See transistor-transistor logic. DIP 1. document image processing. 2. dual inline package. See DIP switch. DIPswitch dual inline package switch. A very small switch that is meant to be toggled to one side or the other (hence "dual"). It can be changed with a pencil or other pointed object. A few DIP switches are large enough to be toggled with the fingers. The early desktop microcomputers, such as the Kenbak -1 and the Altair, were programmed with DIP switches. The smaller DIP switches are often found on SCSI devices, for setting SCSI ill numbers or for adjust- ing other settings. Graphics controller cards sometimes have DIP switches to adjust resolution or scan rate settings. dipole antenna In its basic sense, an antenna with two poles mounted horizontally to produce one long rod that is a subdivision of the length of the wave- length it is designed to receive. Ideally the impedance ( expressed in ohms) of the lead connecting the dipole antenna to the receiving equipment (radio, television, etc.) should match the impedance of the antenna at its strongest point (usually the center in a symmetri- cal antenna). The frequency response ofa simple dipole antenna can be increased by placing the two rods proximate and parallel, and connecting them at both ends to form afolded dipole antenna. DirecPC A commercial service from Hughes Net- work Systems allowing data communications through Very Small Aperture Terminal (VSAT) satellite systems with a personal computer. The satellite signal is received by a two-foot parabolic dish antenna that feeds the transmission to a peripheral card in a computer where the signal is demodulated, demultiplexed, decoded, and then sent to the software interface. The software can also orient the satellite dish. An innovative version of this system for Internet use combines the satellite receiving system and a normal analog connection to an Internet Services Provider (ISP) as an upload/download hybrid system. This service allows the user to connect to an ISP through the normal phone line with a conventional modem, to interact through a Web browser. When files are re- quested, rather than downloading through the ISP phone connection, the files are transferred from the DirecPC network operations to the VSAT satellite, and then to the user's DirecPC dish, thus providing downloads at 400 kbps compared to about 39.6 kbps. direct access storage device DASD. Quick access computer storage devices such as hard drives and memory chips. direct broadcast satellite DBS. Originally this phrase was intended to describe a particular service transmitting in the 12.2 to 12.7 GHz range, a frequency approved by the World Administrative Radio Conference (WARe). However, since the availabil- ity of smaller, more convenient receiving dishes, the term is also used in abroader context, to describe any 269 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary satellite that transmits a signal that can be picked up by individual home and business subscribers, without going through an intermediary station. Since direct broadcasts are transmitted to small consumer dishes, and the size of the dish is related to the length of the radiant waves being received, the higher wavelengths are used, such as Ku-band frequencies. This permits a dish as small as two or four feet in diameter to be effective. Further, as DBS systems are transmitted to the small consumer dishes rather than to a station with a powerful receiving antenna, it was necessary for them to use high power transmissions, usually around 40 to 160 watts, far more than was being used for C-band communications in the 1980s when DBS began to develop. (Today satellites can deliver almost ten times that power.) DBS to the home presents a number of the same moral challenges as widespread access to the Internet. DBS providers are concerned about illegal consumer copy- ing and redistribution of programming, and consumers are concerned about program content. These issues are still being studied and resolved. DBS system guidelines for Europe were originally established by segmenting the 11.7 to 12.5 GHz frequency spectrum into 40 channels, to be shared among the various European member nations. U.S. DBS systems fall under the jurisdiction of the Federal Communications Commission, through guidelines established in the various Telecommuni- cations Acts, and must conform to internationally established technical standards. See C-band, Ku- band, microwave. direct connect modemA type of modem which became popular on desktop systems in the early 1980s, gradually superseding acoustic modems. The acoustic modem was used to connect data and phone lines by setting the handset of a telephone into two suc- tion-cup style holders. Room noise could be a problem and the holders didn't always provide a good connection. In contrast, the direct connect modem was cabled electrically between the computer serial port and the modem, if it was an external modem, or between the computer and the internal modem card to the telephone line through the telephone jack that normally connected to a phone. This system offered faster data rates and was more convenient and effective in reducing noise and ensuring a good connection. See acoustic modem. direct current DC, dc. Amore-or-Iess constant electrical current flowing in one direction. DC current is supplied to many small appliances by batteries. Much of early communications history was based on direct current (DC) as a power source. Telephones had talking batteries and common batteries. The batteries tended to be large, leaky wet cells, that caused inconveniences if moved or subjected to temperature fluctuations. More than fifty years after the inven- tion of the telegraph, AC power for telegraph systems was still considered a novel idea, but the shortage of batteries, and their cost, provoked French and Swiss engineers to experiment with AC generators, as de- scribed in the Annales des Postes, Telegraphes et 270 Telephones in September 1919. Eventually the ad- vantages of AC power were better understood, its use became common, and DC took its place as a current source for small, portable electric conveniences such as calculators, radios, wristwatches, laptops, cameras, etc. See AC to DC converter, alternating current; re- sistance; Tesla, Nicola. Direct Distance Dialing DDD. A commercial name to indicate the capability ofa network to connect a long-distance call without operator intervention. Direct Inward Dialing DID. In the past, calls going through a central office (CO) to aprivate branch extension had to go through an attendant. With increas- ing automation, this routing is now rarely necessary. With DID, the called digits are passed through central office DID lines directly into the private branch exchange (PBX). DID lines do not offer a dial tone and hence cannot be used for direct outgoing calls. Direct Inward System Access DISA. A telephone setup in which outside callers can dial into a telephone system, usually a private branch exchange (PBX), and have the use of the system's services as though they were on the premises using the system from the inside. DirectMarketingAssociation, Inc. DMA. The larg- est trade association for users/vendors in the direct, database, and interactive marketing fields, founded in 1917. Due to the significant impact of the Internet on marketing technology and venues, the Association has also acquired two electronic commerce trade associations, the Association for Interactive Media (AIM) and the Internet Alliance (IA). Telecommuni- cations are an integral aspect of marketing, and thus progress in electronics has shaped and been shaped by the activities of the DMA. The DMA encourages and supports the growth and profitability of its membership and advocates adher- ence to high ethical standards. It provides leadership on behalfof the membership in government and pub- lic affairs. http://www. the-dma.org/ direct memory access DMA. A means to bypass the central processing unit (CPU) in a computer and interact directly with memory. This access is used to reduce processing time and increase speed. direct outward dialing DOD. The capability of a private branch exchange (PBX) to dial calls outside the exchange without first dialing an access code (typically "9"), or going through an operator. Direct Print Protocol DPP. A''thin'' networking protocol for data transfer with a pair of DPP command sets, Direct Print Command, and File Transfer Com- mand, for printing images over an IEEE 1394 serial bus, developed by HCL technologies. As the result ofDPP interoperability events, a number of commercial fIrmS have implemented DPP. There are other protocols that transmit data over IEEE 1394, including IP over 1394, SBP-2, and AV/C. Each of these has its own strengths and limitations in terms of the types of data that are efficiently handled. DPP has mainly been of interest to digital imaging and printing developers. direct sequencing A spread spectrum frequency- changing broadcast technology. Spread spectrum © 2003 by CRC Press LLC broadcasts spread a transmission over a broader range of frequencies than is typically needed to contain the broadcast. A pseudorandom digital sequence directs a phase modulator to distribute the original RF transmission over a bandwidth that is proportional to the clock frequency of that sequence. The receiver must then be synchronized to the same pattern as the broadcast generator in order to remove the phase modulation and recreate the original signal. Although it's not commonly done, it is possible to combine direct sequencing with frequency hopping. This combination would likely be used only in very high security transmissions, as the synchronization and receiving techniques are not trivial. See frequency hopping, spread spectrum. direct set In ATM networking, a set of host interfaces that can establish direct communications at layer two for unicast. direct view storage tube DVST. A cathode-ray display device introduced in the late 1960s to overcome the slow refresh and large storage buffer needs of early vector display monitors. By employing a slow- moving beam and a storage mesh, the DVST design substantially decreased the cost of display devices. See vector display. directional antennaA radio antenna that is designed to concentrate its signal transmission or receiving strength, resulting in a stronger signal, but one which is not of equal magnitude in all directions. Commonly found in AM radio transmissions. directory 1. A list, usually of names and associated information, often sorted or organized and displayed to enhance visual clarity. These may be the names of people, companies, institutions, files, or subdirectories, etc. 2. A table of organizational identifi- ers that provides addresses to individual items within the organizational path. This path is frequently hierarchical in structure. directory, file On a computer file system, an organizational structure, comprising a file storage area, under which there can be further files or directories. A directory listing typically includes other information about the directory and its associated files or subdirectories, such as creation date, permissions, file type, and byte size. The display may be in text or graphical mode, and an icon resembling a file folder is often used to symbolize the directory. Directory Agent DA. 1. A Novell Directory Services (NDS) database service that accepts advertisements for Service Location Protocol (SLP) devices from Service Agents, negotiates registration/deregistration, and answers queries. See Service Location Protocol, Service Agent, User Agent. 2. A database service for negotiating links between buyer and seller agents on a distributed network. Directory Assistance DA. A telephone service in which the caller dials directory assistance to request the number ofa person outside the local calling area, or within the local calling area if the number is not listed. The number may not be listed because the individual just moved in, just changed a phone number, or installed the phone just after the directory was published. Subscriber unlisted numbers may not be given out through Directory Assistance. In most ar- eas, there is now about a $.75 charge associated with a Directory Assistance request, and there is usually a two-number limit on each request. Directory Assistance used to be handled entirely by human operators, but there are now automated systems which will request the city and name of the person whose number is being sought and dispense the number, or hand over the call to the operator to complete the transaction, or to clarify the information provided by the caller. These automated systems combine speech recognition and speech synthesis to carry out their tasks. directory caching A timesaving function of many operating systems that stores the local directory listing in memory so that each time the user accesses the directory information (for listing parts or all of the files), it will be displayed very quickly. For small directories there isn't much difference, but for very long directory listings it can be faster, especially if the information is being output to a shell or MS-DOS text window. Further, the information can be used to access more quickly the files in that directory. Since the directory information is read from memory rather than from disk, transfer time can be faster. Directory Number Call Forwarding DNCF. An interim Service Provider Number Portability (SPNP) which is provided through existing available telephone services, such as call routing and call forwarding. The DN CF is set up so that calls are forwarded to a new number. While it is called Directory Num- ber Call Forwarding, unlisted numbers can also be set up with the service. Directory System Agent DSA. A directory application process in the Open Systems Interconnection (OSI) system. OSI utilizes a system of agents (helper applications) that can query a local database, com- municate with other agents, or hand off requests to other agents when appropriate. The DSA specifically provides associated Directory User Agents with access to the directory information base (Om). directory tree The name given to a hierarchical directory file structure. The "tree" includes the current directory and subdirectories and files associated with the current directory both above it and below it in the hierarchy. Directory tree commands are used for file creation, deletion, renaming, protection, or for search- ing a series of files or all the files located in the tree. Directory UserAgent DUA. A directory application process in the Open Systems Interconnection (OSI) system. OSI utilizes a system of agents (helper applications) that can query a local database, commu- nicate with other agents, or hand off requests to other agents when appropriate. The DSU specifically as- sists and represents the user in accessing a database through a Directory System Agent (DSA). See Di- rectory System Agent. dirty power Electrical power that is spiky, bursty, noisy, or otherwise unreliable. Dirty power can be dangerous to delicate electronic components. Lap- tops plugged in on ferries and trains should be used 271 © 2003 by CRC Press LLC . response is generally in the range of2 to 44,000 Hz with a signal-to-noise ratio of 110 265 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary decibels or more. The higher-end DVD players tend to have added features, such as progressive scan, various filters, digital video equalizers,. ofDigital Ver- satile Disc technology, is promoted by the DVD+RW Alliance as a storage and digital video recording format that will be compatible with the majority of existing DVD players. It is expected that DVD+RW players/drives will be able to read CD-ROM, CD-RW, DVD-R, DVD-RW, CD-Audio and other similar discs. See Digital Versatile Disc, DVD Forum. digital. Video Formats Type Description D-l A component digital video format approved by SMPTE in 1985 and introduced by Sony as the DVR-l 000 in 1987. It is favored by digital effects videographers. D-l records uncompressed 8-bit video. The cassette tapes are large, weighing up to six pounds. D-2 A composite digital video format introduced by Ampex (1986) and Sony (1988) that enjoyed broad popularity until the late 1990s, superseding composite video tape recorders (VTRs) using 1" tapes. D-2 (CCIR 601) records uncompressed video. The cassette tapes are large, weighing up to six pounds. D-3 A composite digital video format developed by Matsushita and introduced by Pana- sonic as a competitor to Sony's D-2 format. D-3 records uncompressed video. D-5 A component digital video format developed by Matsushita and introduced by Pana- sonic as a competitor to Sony's D-l format. D-5 records uncompressed 10-bit video. D-5 has caught on in High Definition Television (HDTV). Some D-5 systems can play D-3 tapes. Panasonic has introduced a high definition 4: 1 compression ratio version ofD-5. D-6 A high-definition digital video format developed by Toshiba/BTS and introduced by Philips. D-6 records up to 64 minutes per 19 mm cassette through a system of helical scan tracks with a track pitch of 22 micrometers. There is a longitudinal analog cue track, a control track, and a time code track. D-7,DVCPRO (DVCPRO is the more common name, D-7 is the SMPTE designation, but it is placed here for easier comparison with other D- formats.) A component digital video fonnat introduced by Panasonic. D-7 records compressed 8-bit video at 34 mmJ second. D-7 supports 16-bit, 48 kHz digital audio. Streaming video is supported through SDTI and IEEE-1394 (FireWire) standards. D-9, Digital-S A component digital video format introduced by Jve. D-9 records 3.3: 1 ratio compressed 1 O-bit video. Promoted as similar in quality to Digital Betacam. 268 ©

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