Fiber Optics Illustrated Dictionary ITU-T V Series Recommendations Number. Descdption : d::>: ::':. : >' : ::: <' :: >, ':':: ' ,:: ~ '\' -' ,,' ,, ': '" :,:' ",::,':'. \:: ::.' '.' ", -, " :,' , ': "':::" V.21 A1984 dial\1p lllodem standard.supportin.gdata rates up to 300 bps. Mostmodems in the late 1970s and early 1980s were acoustical couplers, with direct conneetmodems just beginning toicateh on in the mid-1980s. V.22 A1988 dialup modem standard supporting data rates up to 1200 bps with fallback to 600 bps. It is interesting to note that in the 1980s, many technical people insisted it was impossible to support speeds faster than 1200 bps over standard phone lines, and that this was probably the fastest speed at which modems would ever transmit data. V.22 bis A1988 update to the V.22 standard supporting data rates up to 2400 bps through frequency divisi()n techniques, with link negotiation fallback to 1200 bps, and fallback to ~22. V.23 Dialup modems supporting data rates up to 1200 bps with fallback to 600bps and a 75-bps back channel·orreverse channel. V.26~1~84stattdard for.t.Uodem stlppomng (lata rates up to 2400 bps on· four-wit'eleased telephone lines. V.26 bis Ai1984 update to V.26 supporting 2400.and 1200 bps over public phone dialup lines. V.26 ter A .1988uPdclte to ~26 and ~26 bis thatsupports both echo cancellation (public phone lines), and point-to-point two-wire leased phone lines. V.27 A1988 standard for modem data rates up to 4800 bps with manual equalizer for use with leased telephone lines. V.27 bis A1984 update to Y.27 that supports data rates of 4800 and 2400 bps with automatic equalizer for use with. leased telephone lines. V.27 ter Al9~4update to V.27 and V.27 bisthatsupports data rates of 4800 and 2400 bps over public phone dialup lines. V.29 .A 1988 standard for modem data rates of96()O bps for four-wire leased tel~ph9ne lines. V.32Astan~dfor public dialup and two-wire leasedline modemswitb ratesup to 9600 bps with fallback 1048,800 bps. This standard was approved in ·1984. When modems incorporating Vl32Weretirstiptroducedin the late 1980s, itwD.~not unusual for a singl~modem. to cost between $600 and $2,000. Modems of this speed can now be found for $10, as they have been superseded by much faster data rates 0(33,600 bps. Many of the ~32 modems were dual-standard modems that supported V.32 in addition to various proprietary protocols from individual manufacturers. See V.32 bis. V.32 bis A1991 standardfor public dialup and two-wire leased line modems that supports rates up to 14,400 bps with fallback to 12,000,9600, 7200, and 4800 bps. Like V.32, a number of these modems are dual-standard modems with proprietary protocols included, and they are generally all backward compatible withV.32. Many of them support MNP 2 to MNP-4 error control and MNP-S data compression, in addition to V:42N42 bis error control and data cOlllp~ession. V.32 ter ~Hupda~toV.32 bis designed by AT&T thatsupports data rates up to 19,200,andfallback to16~800andV~32 bis. This fonnatisfre.elydistributableandhasbecomea de: facto stanQard. The c'?rrespondingITU-T approved fOrmat for 19,200 is V.34. V.33 A 1988 standard for modem data rates of 14,400 bpsforfour-wire leasedtelephone lines. V.34 A.1996 standard for dialup modems andfaxlmodemswitb rates up to 33,600 bps in full duplex, and up to 28,800 bps in halfduplex for facsimile transmissions. (These are sometimes also called VFast, or VFe, but those are actually slightly differentinterim protocols developed by vendors who were anxious, in the mid-1990s, to get products to market while the work on the \1.34 standard was being completed. Some modems became V.34NFast hybrids.) V.34 modems are designed to adapt to the line by probing the connection and adjusting according to quality and capacity. They interact with the telephone circuit through handshaking. Optional control data. can be .sent through an. auxiliary signaling channel. V.34 modems are not as subject to noise as earlier modems, due to the implementation of • multidimensionaltrellis coding. V.34 bis ,A19~6 standardth~tsupports the higher data rates of 56,000 bps and 31,200 t>ps. 972 © 2003 by CRC Press LLC V.ltO v: 120 V.130 V.230 Digital Modems, Digital/Analog Hybrids, Wideband, & Parallel Transmission Modems ~19 A19~ standardfor modems for paralleldata transmission overtelephone~ignaling frequencies. V.36 • A. 1 988 •• standardfor widebandsynchronoustransmissionmodems.using 60 to 108 kilohertz group band circuits. V.37 A1988 standard for wideband synchronous transmission modems for signaling rates higher than 72,000 bps, using 60 to 108 kilohertz group band circuits. V.38 A 1996 standard for wideband data circuit-terminating equipment for rates of 48,000, 56,000, and 64,000 bps for use on digital point-to-point leased circuits. V:70 A 1986 standard for digital simultaneous voice and data (DSVD) modems in which a data transmission and a digitally encoded voice transmission can be sent atthe.sflllletime over a single dialup phone line. DSVD are typically downward compatible with standard dialup 1Tl~detnS based on more recent high-speed technologies (e.g.,V:34).'[histypeof te chnology lends itselfwell to teleconferencing applications,·and anumber or vendors have incorporated \1:70 to this end. V.7S A 1996 standard for digital simultaneous voice/data (DSVD) transmission .termil1alcolltrol procedures. See \Z70. . V.76 A 1996 standard for multiplexing using V.42LAPM-based procedures. v: 76 multiplexing has been incorporated into V. 70 systems, although it is not limited to V. 70. \1:80 A standard for the application interface for communications through data terminal equipment (DTE} of a synchronous H.324 bit stream,.such as video, for asynchronous transmission over public switched telephone networks. V.80 works in conjunction with·a number of other related technologies and standards. H.324 is an ITU- T approved standard that provides the foundation for combining video, voice, and data•communications on a single analog phone linell~ing a28,800 bps data .ratecon~~tion.Tbisopensthe doortoavari~~~rpracti~~l, reasonably .pricedstand-alone.and cotnputer.baseavideoconferencing:J'roal1ct§<ti1at display up tolS video frames per second. H.763 andG.723 are video and voice compression schemes used in H.324. V:90 A 1998 standard for digital to/from digital ordigital to/from analog conneetions, supporting download data rates up to 56,000 bps with uploaddata rates up to 33,600. During development it was known as V:pcm and some vendors call it PCM. This standard reflects the gradual conversion of data communications to digital. format. This standard luls its own Web site at http://www.v90.coml Half-Duplex Modem ITU-T-RecommendedTransmission Standards V:17 A two-wire scheme for facsimile machines l1l1 d fax/modems used in conjunction with extended Group 3· facsimile standards for image transfer at rates of 12,000 bps and 14,400 ~~.,. / •. .·,i. " ·.··.i . ····i •. ii,,· ·.i· \.fi V.27 .A modulation scheme for dialup facsimile. machines andfaxlmodemsusedinconjllDctiot1 With~t1p3. facsimile standards fo~imag~~~fe~at~~s o~24<JO~~~~i~~.~P!!' V.29 A modulation scheme for dialup facsimile machines and faxlmodems used in conjunction with Group 3 facsimile standards for image transfer at rates of.7200 bps anci9600bps. ISDN- and Digital Communications-Related V.I00 A 1984 standard for interconnection between public data networks (pDN s) and public switched telephone networks (PSTN). A 1996 standard for ISDN support of data tenninalequipment (OTE) with. V-series interfaces. Listed also as I series 463. A 1996 standard for ISDN support of data terminal. equipment (DTE) with V-series interfaces with provisionfor statistical multiplexing. Listed alsoaslseries46~. A ·1995 standard for an ISDN tenninaradaptor framework. .AI 988 specification for a general data comtnlUlicationinterface layer 1. 973 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary The chart on the following pages includes a number ofV Series Recommendations ofparticular interest. Note that while alargernumber often denotes a more recent standard, there are exceptions, and the catego- ries are intermingled resulting in the number schemes series being interleaved. V5 A telephony standard adopted by the European Telecommunications Standard Institute (ETSI) for use in digital local exchanges in Europe, Australia, South America, and the Far East. Exchanges around the world are gradually being upgraded to V 5. V-band A frequency band commonly used in radar applications. See the band allocations listing for a chart and details of frequencies. V.Fast A vendor-developed format which was brought out while the ITU -T standards bodies were developing the V.34 protocol. V.Fast is similar to V.34, but not the same, and consequently some V.Fast modems are not compatible with V.34, while others are hybrid and support both. See V Series Recom- mendations listing under V.34. VAC See vehicle access control. vaccine A whimsical name for a virus protection soft- ware program that signals the user if anomalies or known viruses are present, and allows the virus to be disabled or deleted. As quickly as virus detectors and vaccines are written and disseminated, virus creators come up with new ways to create mischief or out- right destruction on people's computer systems. With the Internet foreshadowing a not-too-distant day when every computer is linked to the net, perhaps 24 hours per day, more opportunities for vandalism ex- ist, and education about virus detection and protec- tion needs to be made known to the computing public. vacuum An enclosed space in which the gases are at pressures below atmospheric pressure. Since many vacuums are actually near-vacuums, categories of vacuums from low to ultrahigh, depending upon the pressure, have been described. The discovery and creation ofvacuums have aided scientists in making important discoveries and provide environments in which various phenomena take place, or fail to take place, due to the absence of gases. For example, a filament in a bulb can bum much longer in a vacuum. ITU- T.V Series RecOIDDlendations, COOL Error Contro.landData Compression Protocols V.41 A 1972 code-independent error control. standard. V.42 A1996enorcontrolstandardthatgreatlytenhancestllefunctiollingofmodemsioverstandard phonelipes.Phonelines~dto benoisy,slow,andsome",hat unreliable for high speed data c011U11unications. In the past,. iftherewel'~err()~s.ontheline, .·mQdemswouldreact by ~xJJi.,i~~ line noise, losing the connecti0!l' or aborting the current operation. With new error c0l1tro1capabilitiesbuiltin,. some of these problems are overcome througtr filtering and selective.·retransmission. \r.42includes two error control protocolsfor dialup modems. These are linkaceess proc:edures for modems (LAP-M) and Microcom Networking Protocol (MNP-4), such that connections with a variety of modems are supported. V.42 uses a filtering process somewhat like the error correction schemes incorporated into a number of file transfer protocols, such as)'Modem and ZModem. V.42 bis A 1990standard for datacircuittenninatingequipment (DeE). MiscelianeousModemrrransmissionlNetwork~Related Standards and Brotocols ••. ~8 . ·A J994 guide to procedures for starting Sessions of data tnmsmissionsand:settingup eoWJ~tiQns •• ~etel'$.Qver g~I1er~l~",il~~~.l!elepp()Ile .t1etw~rks. V:S bls A 1996 guide to procedures for theidentificationand selection of common modes of op~ration between data circuit-terminatingeqpipment(DCE) and betweenliatatenninal equipment (DTE) over general switched telephone networks, and on leased point-to-point tel~phone.type circuits. V.18 Interoperability guidelines for communications services for the hearing impaired. V.24 A1996set of defmitions for interchange circuits between data terminal equipment (DTE) anddatacircuit tenninatingequipn1~nt (DCE)that specifies the characteristics of interfaces, including pinout circuitry. This· is similarto theRS-232 specifications. V.15 bls A;199~Qommandset designed for synchronous communications through serialports and V.54 AtnodelD diil.gnostics standard implementeditl high speed mogems 974 © 2003 by CRC Press LLC vacuum column In some magnetic tape drivers, there may be a vacuum mechanism to control the tape loop. This provides a lower air pressure 'suction' next to the tape and the drive mechanism. vacuum gauge An instrument for measuring the de- gree of vacuum in an enclosed space. There are a number of types of vacuum gauges, including ma- nometers, ionization gauges, and thennal conductiv- ity gauges. Evacuated Electron Tubes Vacuum tubes fulfilled thousands of roles in elec- tronicsfor overfiftyyears, before the invention of the transistor and modern semiconductor technologies. Even now they are appropriate for certain high-fre- quency applications. The most significant step in the evolution of vacuum tubes was the triode, which in- cluded a controlling grid to harness electron energy. These excellenthistoric examples arefrom the Ameri- can Radio Museum collection. vacuum tube A ubiquitous, essential, and versatile electron tube that was common until about the mid- 1960s, after which it was superseded by various early electronic transistors, and later more sophisticated semiconductor technologies. The vacuum tube in its basic fonn consists of an elec- tron-emitting filament (cathode) and a metal plate (anode) to which the electrons are attracted. Various types of control grids might be interposed between the cathode and the anode (or in other locations), with the whole thing sealed in a glass vacuum tube to con- trol the internal environment and to prolong the life of the filament. The first vacuum tubes adapted for radio broadcasting equipment were developed in the early 1900s. While most small vacuum tubes have passed out of use, cathode ray tubes (eRTs) are still widely used in television sets and desktop computer monitors. There are still situations in which vacuum tubes can be a better solution than the solid-state components now commonly used in electronics. For high power- level, high-frequency applications, vacuum tubes are sometimes more efficient and still merit consider- ation. See Audion, cathode ray tube, electron tube. vacuum tube amplifier An important development stemming from Lee de Forest's invention of the Audion, developed by Harry DeForest. The vacuum tube amplifier was incorporated into telephone re- peating units, which extended communications dis- tances. VAD See voice activity detection. Vail, Alfred (1807-1859) An American scientist and inventor, and associate of Samuel Morse, from whom Morse adapted a number of ideas related to the build- ing of telegraph systems. Vail was very mechanically apt and continued over the years to make technical improvements to the technology. In 1837 Vail made an agreement with Morse to tum over the rights for his inventions in return for a share of the commer- cial rights. In 1848, due to the mounting workload, and the lack of sufficient appreciation and compen- sation, Vail decided to tenninate his relationship with Morse. Morse code may be one of Vail 's biggest con- tributions. When Morse was designing a system for coding messages, he initially created a complex nu- meral-letter relationship that required a time-consum- ing dictionary lookup to decode the words. Vail ap- parently came up with the simpler system, after al- tering the mechanics ofa telegraph instrument so it moved vertically rather than horizontally, to allow the instrument to leave spaces (thus yielding dots and dashes) when recording a message. See Morse code, telegraph history. Vail, Theodore N. (1845-1920) Theodore Vail was from the same Vail family that had a close associa- tion with Samuel Morse at the time of his invention of the telegraph, and Vail learned telegraph code from the elder Morse. Vail became a telegraph operator, and later devoted most of his life to the management and promotion of universal telephone services. Vail became company general manager of the first Bell company in 1878 in Boston. He was the first president of the Telephone Pioneers of America and a cofounder of Junior Achievement Inc. (1919). He was instrumental in continually expanding the company's offices and services, and directed the building of the frrst transcontinental telephone line, completed in 1914 and officially opened in 1915. ~~::e ~~~:~~~::~ ~f:~~~:~~~l~~:lf;:d i. service, Vail resigned in 1887 due to his disgust at the narrowly commercial vision of the Bell financiers. In 1907, he was induced to return to AT&T as presi- dent, at a time when Bell was rapidly buying inde- pendents. Bell also purchased Western Union stock, and installed Vail as president of what had once been Bell's chief rival. With the death of J.P. Morgan in 1913, Vail voluntarily took steps to reduce AT&T's monopolistic buyouts and control of the long distance networks, in order that reorganization could be done from the inside rather than being imposed by regula- tory authorities. Vail retired as president in 1919. See Kingsbury agreement. valance band In semiconductors, a region represent- ing the energy states of bound electrons, those that are not free to move readily within the material, as opposed to a conduction band representing the en- ergy states of electrons that may move more readily to provide energy change/conduction. 975 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary In a diode semiconductor component, the valance and conduction bands are separated by a gap or "electron holes" to provide a p-njunction for directional move- ment of electrons within a material. The application of current can cause a shift in energy levels such that energy is released as the material returns to a state of rest. This energy, in the form of photons, may be di- rected outside the component to create semiconduc- tor lasers, for example. See light-emitting diode, p-n junction, semiconductor laser. value In imagery, the relation ofa color to white or black, or levels of gray. Sometimes called lightness. In color, the intensity of the color. value-added Services offered, usually for a fee, in addition to standard product or subscriber services. Options. VAN value added network. VanAllen radiation beltA region of space surround- ing Earth in which there is high-intensity particle ra- diation which is sufficiently destructive that it is avoided for communications satellite orbits. This re- gion is roughly between the low Earth orbit (LEO), starting at about 1000 kilometers, and medium Earth orbit (MEO). It is named after James A. VanAllen. vanadate (symb. - VO 4) A rare earth crystalline sub- stance grown by various means, sometimes two or three bars at a time. Vanadate may be doped with a variety of materials such as neodymium (Nd), which is a component of dydimium (neodymium is also used in electromagnet read/write components in magnetic storage disks). Vandate is grown in labs, often using the Czochralski technique. Once grown into glasslike rods in a vari- ety of colors, it can be annealed, cut into shapes that include beam-splitting cubes or birefringent wedges. It may then be polished, and incorporated into fiber optic components such as diode lasers. Nd:YVO (neodymium-doped yttrium-vanadate) has a short absorption depth compared to Nd:YAG (neodymium-doped yttrium-aluminum-garnet). Highly birefringent, transparent YVO 4 has been de- veloped for use in passive fiber optic components such as interleavers, isolators, compensators, and cir- culators. See Czochralski technique, doping, YAG Van de GraaffgeneratorA device for creating elec- trostatic effects by charging insulated electrodes to high energy potentials. Van de Graaff generators are popular exhibits in science museums. They typically are configured as melon-sized metallic spheres atop a central pole, and visitors can place their hands on the globe and watch their hair stand on end. It is named after Robert J. Van de Graaff, an American physicist of the early 1900s. vapor deposition A process used in the fiber fabri- cation industry for creating a preform cylinder (typi- cally silica glass) from which fibers are pulled. There Vapor Deposition Preforms - Modified Chemical Vapor Deposition (MCVD) In modifiedchemicalvapordeposition (MCVD), oxygen is bubbled through chemical solutions chosen to influence the physical and opticalproperties of the fabricated material. The resulting vapors are drawn into a tube (A) that is attached to a lathe (B). A torch (C) is drawn along the tube as it rotates. Inside the tube the vapors undergo chemical reactions and fuse to form glass on the inside of the tube. Rotating the preform tube helps consolidate the preform blank with an even consistency. The preform blank is fabricated with the correct core-to-cladding ratio and subsequently put into a high-temperatun! furnace as part of a fiber drawing assembly in which the molten medium is dropped from the top of the assembly so that gravity can draw down the glass to form a long, slenderfiberfilament. 976 © 2003 by CRC Press LLC are a number ofdifferent processes for vapor depo- sition based upon chemical vapor reactions that oc- cur in the presence of heat, including modified chemi- cal vapor deposition (MCVD), outside vapor depo- sition (OVD), plasma chemical vapor deposition (PCVD), and vapor axial deposition. The types and combinations of vapors vary, but argon or argon and helium are commonly used as a carrier gas in the deposition process and helium is used to sweep out impurities in the preform consolidation of the vapors. See diagram. See boule, helium recovery, preform. van Heel, Abraham C.S. (1899-1966) A physicist from the Netherlands, van Heel had the opportunity to spend 1925 at the Institute of Optics in Paris, work- ing with Fabry, after whom the Fabry-Perot interfer- ometer is named. He also studied at Leiden Univer- sity and took aposition at Delft University in applied physics where he led the optics group from 1947 to 1966. In 1948, van Heel became cofounder of the Interna- tional Commission for Optics (ICO). In the 1950s he did significant research into the loss of light in plain optical filaments. His observations led to the land- mark development of cladding, an outer layer that could contain light within a lightguide through inter- nal reflection. In 1954 van Heel published his find- ings in Nature magazine and the fiber optics indus- try developed dramatically from that point on. See Hopkins, Harold. van Musschenbroek, Peiter (1692-1761) A Dutch educator and experimenter who created the Leyden jarcondenser (named after the region) in 1745, inde- pendently of other inventors, including E.G. von Kleist. It was a simple but practical capacitor, and effective enough to harm a person not careful in its handling. Van Musschenbroek also described a way to carry out experiments with the jar so that a person would not be mortally shocked. Many subsequent ex- perimenters, including Benjamin Franklin, devised variations on the basic Leyden jar. vaporware Aderogatory term for software that has been announced prior to completion, or prior to dis- tribution. The negative aspects of the term stem from two sources, truth in advertising and the fact that many announced software products never hit the shelves. Because the software industry is new and confusing to a great portion of the public, unsupported product announcements have not been as strongly condemned and regulated as in other industries, a situ- ation which may change as the buying public be- comes more familiar with the new technologies and tired ofunsubstantiated promises. variable bandwidth Bandwidth that can be tailored, usually on an on-demand basis, to the capacity needs of the current transmissions. The ability to adjust ca- pacity allows the system to more efficiently allocate resources and provides a mechanism for setting up accounting systems that bill on an as-used basis. variable bit rate VBR. A data transmission com- monly represented by irregular groups of bits or cell payloads, followed by unused bits or payloads. VBR traffic is generated by most media other than voice. In an ATM environment, a VBR service can be real- time or non-realtime, and is guaranteed sufficient bandwidth and quality ofservice (QoS). See cell rate for a chart. variometer An instrument for measuring magnetic declinations, particularly of the Earth. See declina- tion. Varian, Russen H. (1899-1959) An American inven- tor, Varian worked in the physics laboratories at Stan- ford and, in 1948, cofounded Varian Associates with his brother Sigurd and Edward L. Ginzton. He served as the President and a Board member until his death. He was awarded numerous patents related to thermi- onic tubes, magnetic resonance, and various radar technologies. Varian was a member of several promi- nent engineering organizations and a Director of the West Coast Electronic Manufacturer's Association. He was a Sierra Club Committee member and ac- quired land to further conservation efforts connected with the Sierra Club. He and his wife Dorothy set the groundwork for Castle Rock to become a state park in 1968. Varian had an early introduction to television tech- nology through Philo Farnsworth and became well- known for the co-development, with Sigurd Varian and William Hansen, of the Klystron tube technol- ogy that is used to this day in broadcast TV. This im- portant invention in the late 1930s enabled the gen- eration ofultra-high frequency radio waves (micro- waves) and supported the evolution of radar and com- munications technologies. See Klystron. Varian, Sigurd F. (1901-1961) An American engi- neer and commercial pilot, Sigurd shared in many collaborative inventions and business ventures with his brother Russell. He helped cofound Varian Asso- ciates and invented a number of types ofpractical de- vices, including pumps, filters, heaters, and a preci- sion high-speed drill press. His interest in navigation was a good match for his co-invention of the Klystron, an electron tube to generate ultra-high fre- quency radio waves (microwaves) that could be used to improve navigation through radio direction- find- ing technologies. See Klystron. Varian Associates, Inc. A pioneering microwave technologies development firm, founded by the Varian brothers (Russell and Sigurd) and Edward Ginzton in 1948 in San Carlos, California. Dorothy Varian, Russell's wife, was also active in the found- ing, development, and operations of the company, and served as the treasurer until 1951. Varian Associates built upon the Klystron tube in- vented by the Varian brothers and William Hansen and developed further by Ginzton, Marvin Chodorow, and others. Varian Associates and the Sperry Gyro- scope Company's lab contributed to important devel- opments in radar sensing, navigation, and microwave communications technologies over the following de- cades. Varian was also instrumental in creating some of the components important in chemical spectros- copy and medical imaging. Varian, Inc. was spun off from Varian Associates in 1999 to focus on life sci- ences and health care products. 977 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Varian Associates was the first company to move into the Stanford Industrial Park, in 1953, the heart of the emerging Silicon Valley. There were a number of di- visions within the firm, including the Microwave Power Tube Division and the Electron Devices Group, which separated from Varian Associates to become Communications and Power Industries (CPI) of Palo Alto, California. The Microwave Power Tube Division of CPI continues the tradition of develop- ing and improving klystron technology. See Ginzton, Edward; Klystron; Varian, Russell and Sigurd. Varley loop test A type of diagnostic procedure which uses resistance through a bridge to locate a fault in a length of circuit. Variable resistance is con- nected in series with the resistance of the broken or defective line. It is similar to the Murray loop test, but with a third wire, and more commonly used. See Murray loop test; Wheatstone bridge. Varley, Cromwell Fleetwood A British researcher and technician who investigated ionization, and was involved in early Atlantic telegraph cable installation. He was hired by Western Union to evaluate the tele- graph system in the U.S. Varley standardized many of the lines, systems, and diagnostic techniques. The Varley loop test is named after him. See Wheatstone, Charles. VAX Virtual Access Extension. A series of minicom- puters from Digital Equipment Corporation (DEC) that was developed in the mid-l 970s as the succes- sor to the DEC PDP-x series. The VAX computer sys- tem was released in 1977 with 32-bit computer ca- pabilities compared to the 18- or l6-bit systems char- acteristic of the PDP- line. The VAX 11/780 was popular in educational institutions around the world, often replacing or being added to the still-popular PDP-II machines. Both dumb and smart remote ter- minals (ADS, Gigi, Tektronix, Ramtek, and others) could be readily interfaced with the VAX to support one or two hundred users at a time. The VAX 11/750 followed, along with several other models, including the MicroVAX. The early VAX machines were generally running VMS as the operating systems, but Unix also became apopular VAX option over time. See PDP VBE VESA BIOS extensions. A high-resolution VESA BIOS standard that can be implemented in hardware or software to provide control of video graphics displays on a computer, typically an Intel- based International Business Machines (IBM) or third-party licensed computer. vBNS very high speed Backbone Network Service. A research network established in 1995 by the Na- tional Science Foundation. VBR See variable bit rate. VBX Visual BASIC Extension. VC virtual connection, virtual circuit. A generic term for a logical communications medium established on request. A connection typically includes a concatena- tion of channels forming an end-to-end path. A cir- cuit refers to transmission in both directions. Three types of VCs include permanent (PVC), smart/soft permanent (SPVC), and switched (SVC). In an ATM 978 environment, data to be transmitted by a VC is seg- mented into 53 octet quantities called cells. This con- sists of5 octets of header, and 48 octets of data. VCC virtual channel connection. A generic term to describe a logical connection. In an ATM environ- ment, a virtual channel refers to the unidirectional transport of ATM cells associated by a common unique identifier value. Virtual circuits (VCs) can be combined to form a virtual channel. VCI Virtual Channel Identifier. A value in the header of each ATM connection cell which identifies that connection. See VC, VCC, virtual channel. VCL Visual Component Library. Alibrary used for applications development for Borland Delphi prod- ucts. VCSEL See vertical-cavity surface-emitting laser. vector Any quantity having both magnitude and di- rection. vector display A cathode ray tube (CRT) vector dis- play is one in which the sweep ofa beam follows a vector (line or stroke) and illuminates (and refreshes) the specific part of the display that is needed to ren- der the desired colors and shapes. In other words, the beam doesn't follow the sawtooth scan characteris- tic ofraster displays. This is in contrast to a televi- sion screen, in which a beam constantly sweeps the screen to form a frame in which the beam travels across the entire display area on a constant, repeti- tive basis. In a vector monitor, a vector generator takes the co- ordinates supplied by the processor and converts them into analog voltages that are used to control the di- rection of the beam as it excites the phosphors coated on the inside of the CRT. Vector graphics appear very 'crisp' and clean, but refreshing large areas or the entire screen is generally slow and impractical for many applications. Vector monitors were prevalent during the 1960s and '70s, but have largely been replaced by mass market raster monitors. Tempest was an early video game that employed vector graphics, whereas Space Invaders used raster graphics. vector font A set of textual characters or symbols defined by vector algorithms (usually lines, spline curves, and arcs) rather than by relative positioning of dots within a grid (raster or bitmap font). Avector font looks smooth and appealing at almost any size, except very tiny sizes, and displays at the highest resolution available to the output device on which it is being displayed or printed. In general, the higher the resolution, the smoother the lines and more at- tractive the overall look of the font. Contrast with raster font. vector quantization See quantization, vector. vehicle access control VAC. Control ofvarious de- vices related to transport vehicles. Examples include garage doors, access gates, and other types ofbarri- ers that may be encountered and opened with a de- vice in the vehicle, such as aradio-controlled remote control. Newer VACs often include security features and frequency programming for customizing the fre- quency emitted by the devices (this is handy in storage © 2003 by CRC Press LLC facilities where many doors are close together). The frequencies available for these types of devices vary, depending upon the country. Vendor's ISDN Association See ISDN associations. verifying punch A punch card perforator that also verifies the punches to each card to see that the per- forations are correct, automatically replacing those that are defective. Veronica Named tongue-in-cheek for a comic-book character reference to arelated tool that queries FTP sites named Archie. What Veronica does for Gopher information is similar to what Archie does for FTP sites. Veronica is an Internet keyword query tool that searches Gopher sites, sometimes called Gopher- space, and typically displays the results of the search in the format of a Gopher menu. Users are linked transparently to the source, and may not see the lo- cation of the source unless they explicitly ask. Veronica was introduced in 1992 by S. Foster and F. Barrie of the University ofNevada (Reno). Computer programmers love to come up with acro- nyms, and Veronica is no exception. It's astretch, but it has been said that Veronica stands for Very Easy Rodent-Oriented Netwide Index to Computerized Archives. See Archie, Anarchie, Gopher, Jughead, WAIS. Versatile Interface Processor VIP. A Cisco Systems commercial router interface card that provides mul- tilayer switching. Version Fast Class VFC. A vendor-developed in- terim format for modem-based serial communica- tions, which was commercially implemented while the lTU- T was working out the V.Fast standard. See V.Fast, V Series Recommendations. versorium A device for detecting electrical proper- ties ofvarious materials, invented by William Gil- bert in the late 1500s. The versorium resembles a compass needle in that it is a horizontal movable needle balanced on a small support stem, but differs in that the needle itself is not made of magnetic ma- terial, like lodestone, but rather of wood or a nonmag- netic metal. Gilbert used this sensitive device for evaluating attractive properties of different materi- als when altered by rubbing. Descendants of this in- strument are now called electroscopes. See Gilbert, William. vertical-cavity surface-emitting laser VeSEL. (pron. vixel) In general, traditional lasers emit light from the edges of the structure, and the laser cavity runs along the horizontal length of the device. veSEls differ in several important ways. The light is emitted from the surface rather than from the edges, veSEL Implementation of an Optical Neural Network veSEL array N /' 19a 'mumJnated II area t~ tQb enable ~J 1111 output An example of an opticalneuralnetworkimplementedwith a VeSEL array that isspacedapartfrom a maskthat is, in turn, spaced apartfrom an array of optical detectors with the same configuration of rows/columns as the laser array. This implementation was proposed by R. Webb of British Telecom, in a June 1998 patent application for a telecommunications switch (U.S. patent # 6,307,854). 979 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary positioning and quantity of quantum wells can be used to control the optical properties associated with VCSEL devices. In current confIgurations, VCSEls can be "tuned" to between about 400 - 1600 om through the selection, combination, and thickness of the layers and doping of the quantum wells. One big advantage ofsurface-emitting laser compo- nents is that the assembly can be tested before it is stamped out of the fabrication materials, in contrast to edge-emitting lasers that must be stamped out be- fore the emissions can be tested. This represents a sig- nificant time/materials/cost savings. They are also more efficient in terms of the amount oflight that is emitted in relation to the power drawn from the cir- cuit. In addition to this, the light beam is narrower and rounder than that from regular edge-emitting la- sers. This is significant in terms of the suitability of VCSELs for fIber optic networks. If you get more light for the same amount ofpower and the beam fIts into a smaller space, it becomes eminently suitable for use with tiny optical fIber waveguides. Because they are surface-emitting, whole new classes of circuits can be constructed with VCSELs. They can be mounted on primed circuit boards or other suit- able substrates, or can be mounted directly on other semiconductor chips (e.g., for optical detection sys- tems), depending upon their function. They can also be organized into arrays. Commercial and experimental VCSEls have been developed for a number of uses. There are now com- mercial VCSEL-based laser diodes in the 780 and 850 nm ranges. VCSEL multimode fIber optic transceiv- ers are available for Fast Ethernet, ATM, FDDI, Fi- bre Channel, and Gigabit Ethernet networks, provid- ing transmissions up to about 2000 m. VCSEl array optical 'neural' networks have been proposed as well. Edge-emitting lasers still have some important advan- tages, however. Currently, their maximum optical power is higher than that ofVCSELs, and the wave- lengths emitted are more suitable for optical fIber transmissions. VCSEls currently are suitable for very short distances, but some development needs to oc- cur before they can be used in place of edge-emit- ting lasers for longer transmission lines. See laser, quantum cascade laser. Vertical Service Code VSc. In telephony, sub- scriber-dialed codes that access value-added features and services from various telecommunications car- riers. Examples include Call Forwarding, Call Trace, Automatic Callback, and others. These services are invoked by typing in a number or 'star code' (a num- ber prefixed by an asterisk). The Industry Number- ing Committee has developed guidelines for the as- signment ofVSCs. Care must be taken, when request- ing VSCs for new services, that the number does not conflict with codes established in other regions within the North American Numbering Plan (NANP). Examples of some of the more commonly available VSC assignments are listed in the VSC Assignment Examples chart. The North American Numbering Plan Administration (NANPA) publishes a more complete list. See chart. http://www.nanpa.coml Surface-Emitting Semiconductor Laser Layer (1) is the substrate supporting the lower re- flective layer (2). The reflective layer bounces light back up into the central quantum well region through an undoped spacer layer (3). Between the lower and upperreflective layers is the quantum wellactivelayer (4) composed of an undopedquallfum well layer and a barrier layer. Above this is anotherspacer layer and the optical confinement layer (6). The thickness from the bottom of the lower spacer layer (3) to the top of the upper spacer layer (5) is an integralmultiple of the oscillation wavelength divided by the refractive index. This physical/mathematical relationship fosters a standing wave between the re- fleCTive SIll/aces such that the antinode of maximum light illfensityoccursat the quallfum well active layer. '~==8 the laser cavity is vertical rather than horizontal, and the beam is narrower. Otherwise, the general operat- ing principles are similar to regular lasers. VCSELs were fIrst reported in the mid-1960s, by Melngailis, who coauthored many articles related to optics, especially infrared technologies, between 1965 and 1969 and has coinvented semiconductor-based opti- cal wave guide devices since that time. The commercial implementations of VCSELs are fairly recent, however. It was not until the late 1970s, in Japan, that wavelengths suitable for telecommu- nications applications began to be generated with VCSEL technology. Room temperature VCSELs be- gan to be developed by the mid-1980s and practical laser devices based upon VCSELs were beginning to emerge in the early 1990s. A cross-section of a VCSEL-based sUI/ace-emitting semiconductor laser showing the layered structure. This configuration was designed by N. Ueki of Fuji Xerox Company and submittedAug. 1998for a u.s. patellf (#6,320,893). The devicefeatures a stabletrans- verse mode, reduced threshold current, and higher output than previous semiconductor laser embodi- ments. In VCSELs, light is emitted from the surface through the sandwiching ofreflective mirrors around the la- ser cavity, which is usually only a few wavelengths long. Metallic mirrors were not found to be very ef- fective but distributed Bragg reflectors are effective in reflecting a high percentage of the reflected light to be guided back toward the laser cavity. VCSEls may be either optically or electrically pumped. The 980 © 2003 by CRC Press LLC vertigo A perception by an individual that the envi- ronment is moving around the individual, or the in- dividual is moving in relation to the environment when there is no physical motion or a misperceived physical motion. For example, watching abig-screen movie w1th realistic action may cause aviewer to feel as though he is moving, when in fact he is sitting still in a theater seat. The brain's misinterpretation of motion as it relates to body position causes a dizzy- like feeling and may cause or exacerbate motion sick- ness. It can even make a person feel like he or she is falling. Illness, nervous tension, or migraines can make vertigo more acute. (Vertigo is often confused with the dizziness that occurs when blood supply to the brain is reduced.) Sensitive individuals may ex- perience vertigo after long periods ofwatching text scroll by on a computer. Since a fairly significant number ofpeople experi- ence vertigo at one time or another, developers of multimedia products involving moving platforms, headsets, and other motion-controlling or body-worn devices need to take this into consideration when de- signing and testing their systems. Games developers have discovered that a certain number of people be- come 'seasick' from playing fast action video games. See burnout, carpal tunnel syndrome. Very High Density Cable Interconnect VHDCI. A type of compact computer peripheral connection cable that enables multiple 68-pin SCSI connectors to be connected to a computer backplate. See P con- nector. very high frequency VHF. Electromagnetic waves in the approximate range of 50 to 300 MHz, part of which is allocated for amateur use (50 to 54 MHz and 144 to 146 MHz) with some regional variations. Very Large Array VLA. A system of 27 movable astronomical telescopes in a V-shaped configuration 50 miles west of Socorro, New Mexico. It comprises agiant virtual telescope in that the signals can be in- terferometrically combined to provide a bigger pic- ture of the cosmos covering the viewable extents of the separate telescopes. Thus, the VLA has ~ effective Vertical Service Code (VSC) Assignment Examples Number Name Notes *51 Who Called? Provides the subscriber with the directory number and date/time of unanswered calls. This is similar in function to Caller ill devices that log incoming calls and date-/timestamp them. *57 Customer Originated Trace Provides the recipient ofa harassing or threatening call with the ability to request a trace of the last call received. *60 Selective Call Rejection Enables the subscriber to reject incoming calls from a selection ofcalling parties. *61, *81 Distinctive Ringing/Call Wait. Activate/Deact. Enables the subscriber to have incoming calls from a select list ofcalling parties identified using a ringing sound that is distinctive and recognizable. *66, *86 Automatic Callback, Activate/Deactivate Enables a subscriber to automatically have a call placed to the last number dialed when that number becomes available (no longer busy). *70 Cancel Call Waiting Enables the subscriber to cancel call waiting. There are several different types of Call Waiting services, including regular, deluxe, and selective Call Waiting. *72,*73 Call Forwarding, Activate/Deactivate Enables the subscriber to control the redirection of calls to another number/ station. This is handy if you are expecting an important call and are planning to be at another location. *77,*87 Anonymous Call Reject., Activate/Deactivate Enables the subscriber to set up the rejection ofcalls from callers who are using anonymous dialing (those that don't show up on Caller ID services). 981 © 2003 by CRC Press LLC . spectros- copy and medical imaging. Varian, Inc. was spun off from Varian Associates in 1999 to focus on life sci- ences and health care products. 977 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Varian Associates was the first company to move into the Stanford Industrial Park, in 1953, the heart. that it is avoided for communications satellite orbits. This re- gion is roughly between the low Earth orbit (LEO), starting at about 1000 kilometers, and medium Earth orbit (MEO). It is named after James A. VanAllen. vanadate (symb. - VO 4) A rare earth crystalline sub- stance grown by various means, sometimes two or three bars at a time. Vanadate may be doped with a variety of materials such as neodymium (Nd), which is a component of dydimium (neodymium is also used in electromagnet read/write components in magnetic storage disks). Vandate is grown in labs, often using the Czochralski technique. Once grown into glasslike rods in a vari- ety of colors, it can be annealed, cut into shapes that include beam-splitting cubes or birefringent wedges. It may then be polished, and incorporated into fiber optic components such as diode lasers. Nd:YVO (neodymium-doped yttrium-vanadate) has a short absorption depth compared to Nd:YAG (neodymium-doped yttrium-aluminum-garnet). Highly birefringent, transparent YVO 4 has been de- veloped for use in passive fiber optic components such as interleavers, isolators, compensators, and cir- culators. See Czochralski technique, doping, YAG Van de GraaffgeneratorA device for creating elec- trostatic effects by charging insulated electrodes to high energy potentials. Van de Graaff generators are popular exhibits in science museums. They typically are configured as melon-sized metallic spheres atop a central pole, and visitors can place their hands on the globe and watch their. C.S. (189 9-1 966) A physicist from the Netherlands, van Heel had the opportunity to spend 1925 at the Institute of Optics in Paris, work- ing with Fabry, after whom the Fabry-Perot interfer- ometer is named. He also studied at Leiden Univer- sity and took aposition at Delft