See Link Control Protocol, Point-to-Point Protocol. link encryption An internetwork security mechanism which, unlike an authentication header CAR), affords some protection from traffic analysis. link MTU A unit describing the maximum packet size which can be conveyed in one piece over a communications link. nection files to specifY a network to be used for the node's AppleTalk connection. When a connection is selected, the LAP Manager routes the communica- tions through the selected link-access protocol and its associated hardware, thus acting as a switching mechanism. See Link Access Protocol, Link Control Protocol. link aggregation token See aggregation token. link attribute In ATM networks, a parameter used to assess a network link state, to determine whether it is a viable choice for carrying a given connection. link connection In ATM networks, a connection which can be used to transmit information without the addition of any overhead. link constraint In ATM networks, a restriction ap- plied to the use ofa specific link. In other words, re- strictions as to whether it may be used as the path for a connection. Link Control Protocol LCP. In order to support a variety of environments, Point-to-Point Protocol (PPP) provides a Link Control Protocol (LCP). When network link access has been initiated, the LCP agrees upon encapsulation format options, handles varying limits on packet sizes, detects common misconfigu- ration errors, and terminates a link. The main phases for establishing, configuring, main- taining, and terminating a link are shown in the Link Control Procedures chart. The Link Control Protocol packet is encapsulated within the information field ofaPoint-to-Point Pro- tocol data link layer frame. In brief, the packet is con- figured as shown in the Link Control Protocol Packet Format diagram. See Compression Control Protocol, link access protocol, Point-to-Point Protocol, SNA Control Protocol, RFC 1171, RFC 1661. Link Control Protocol codes LCP codes. These link- establishment packets are used to establish and con- figure network links. Link Control Packets are as- signed as follows: Fiber Optics Illustrated Dictionary lines of force, lines of magnetic induction The sphere of magnetic influence of a magnet. For ex- ample, in a basic bar magnet, the lines of force can be conceptualized as radiating outward from the poles with no break between the north and south poles. Al- though diagrams of magnetic lines of force show them as discrete lines in one plane, the actual region of influence is three-dimensional. The lines them- selves are a descriptive way ofconveying the prop- erties of the forces that cause them to crowd each other sideways. Areas where these lines converge, as at the poles, indicate areas of stronger magnetic force. The direction of the lines can be defined as the di- rection along the north pole if forces other than the magnetic force were hypothetically neutral. Lines of force can be observed indirectly by sprin- kling iron particles over a bar magnet and tapping the surface until they form patterns around the mag- net. The lines are not fixed in one position; they will change if you sprinkle the filings again, but come about through their interaction, as each piece behaves as a tiny magnet, alternately attracting and repelling near neighbors. Collectively these lines are classed as flux. See flux, magnetic field. link n. I. In its broadest sense, a communications cir- cuit or channel. 2. A specific leg in a circuit, as be- tween two nodes, two networks, or two users. In ATM, it is more specifically defined as a logical link, that is, an entity with a specific topological relation- ship and transport capacity between two specified nodes in different subnetworks. See Ethernet, Frame Relay, asynchronous transfer mode, tunneling. 3. A communications medium over which nodes can com- municate at the link layer. 4. A logical link between software entities, files, or processes, as between an icon and an executable program, or between an alias and a file directory. See alias. link v. I. To form a logical relationship between soft- ware entities, or software and hardware devices. 2. To interconnect hardware devices and/or cables. 3. In software development, while building executable files, to link the compiling source code into appro- priate system resources, headers, or other system soft- ware specified. link access protocol LAP. A generic category of pro- tocols for establishing (setting up) data transmission connections across a wide variety of devices. As the name implies, LAP operates at a lower level called the link level, which is Level 2 in many common net- working architectures. LAP is one ofseveral proto- cols functioning at the link level to establish, control, and take down network links. In frame-based net- works, balanced LAPs are more commonly used now than the earlier basic LAPs. However, there are many different implementations of LAPs for device con- nections including computers, modems, infrared remotes, etc. from different manufacturers. Link Access Protocol Manager LAP Manager. A set of software utilities providing a standard interface between high-level protocols and link-access proto- cols for AppleTalk Link Access Protocol. LAP Man- ager enables a user to select among AppleTalk con- 592 Code Code I Code 2 Code 3 Code 4 Code 5 Code 6 Code 7 Code 8 Code 9 Code 10 Code II Control code assignment Configure-Request Configure-ACK Configure-NAK Configure-Reject Terminate-Request Terminate-ACK Code-Reject Protocol-Reject Echo-Request Echo-Reply Discard-Request © 2003 by CRC Press LLC link state advertisement LSA. A mechanism for describing portions of the routing topology ofa net- work within a network switch fabric. As implemented with Virtual LAN Link State Protocol, there are two basic types of link state advertisements: switch link lists all the functioning switch-re- lated links and the cost associated with the use of each link network link lists all network link adj acent re- lationships Each switch originates an initialization advertisement when it comes online (with no links), followed by one switch link advertisement that describes a complete list of all LSAs each time there is a change in the state ofa neighbor. Switches associated with multi-access links originate a network link advertisement that describes all the functioning fully-adjacent switches attached to the link. See Open Shortest Path First Protocol, Virtual LAN Link State Protocol. See RFC 2328, RFC 2642. link state database LSD. A collection of link state advertisements for an entire network fabric that is maintained within the fabric-associated switches and used to calculate the system of best paths to all other associated switches. link state routingAn approach to routing on distrib- uted networks that was pioneered on the historic ARPANET packet-switching network. McQuillan et al. described ARPANET routing in IEEE Transac- tions on Communications in May 1980. Within a few years, modifications to the basic concept increased the fault tolerance of the routing system and formed a basis for subsequent link state protocols such as Open Shortest Path First Protocol. See link state ad- vertisement. Linux A popular, well-supported, widely imple- mented, freely distributable, open source Unix-like operating system developed in the early 1990s by Linus B. Torvalds in Helsinki, Finland (he has since moved to California). Torvalds has been honored for his contributions to information and telecommunica- tions technology. Linux is mostly POSIX compliant, and features true 32- or 64-bit multitasking, virtual memory, TCPIIP drivers, shared libraries, protected mode execution, and more. Linux supports the X Windows system, conforming to the X/Open standard. It also supports the common Internet protocols, including POP, IRC, NFS, Telnet, WAIS, Kerberos, and many more, as a client or a server. Popular Unix shells work with Linux. Some excellent applications have been ported to Linux, with more being developed all the time. Linux has been ported to many platforms, including PowerPC, Amiga, Macintosh, Intel-based machines, Atari, and others. There are a number of commer- cially available versions ofLinux, including Stac and Red Hat and it is a very popular choice for local area network (LAN) and Internet servers. It is reliable, powerful, and is being adopted by many large-scale research and development firms. The Linux Journal is a good trade magazine for Linux developers and users. Link Control Protocol Packet Format o 1 2 3 o 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 bits +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I Code I Identifier I Length I +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I Data +-+-+-+-+ Link Control Procedures Phase Events Notes Phase 1 Establishment and configuration Configuration packets are exchanged, a Configure-ACK packet sent and received, and the Open state entered. Phase 2 Link quality determination An optional phase in which the link is tested for quality. Phase 3 Network layer protocol configuration Upon completing optional link quality determination, network layer protocols can be configured and maintained or taken down. Phase 4 Link termination The link may be terminated at any time, usually at a user's request, but possibly also because of a physical event. 593 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Linux International An international organization of developers, vendors, and users of the Linux Oper- ating System and its associated programs. http://www.lLorg/ Linux Terminal Server Project LTSP. An open- source project under the GNU Public License to cre- ate administration tools to facilitate the setup ofLinux network diskless workstations. The motivation for the project was to provide a flexible, reliable, low-main- tenance, cost-effective network setup for remote com- puting workstations. liquid crystal display LCD. A low power display technology comprising a number of layers, one of which is made of liquid crystal, sandwiched together. The long crystalline molecules in the liquid crystal layer are used to deflect and polarize light. When exposed to an electric field, the crystals orient them- selves in the same direction, no polarization occurs, and the light is absorbed; the display remains dark. Liquid crystal displays were originally developed by RCA's David SamoffResearch Center and Westing- house in 1963 and 1964. The fIrst calculator to use LCD technology was introduced by Sharp in 1973. Simple LCDs include a set of alphanumeric charac- ters and sometimes some symbols. More sophisti- cated LCDs incorporate raster display technologies, especially when used with computers. LCD panels are popular in phones, calculators, and other low- power, mobile devices such as laptops. External light is needed for the user to see the display unless back- lighting is added. An active matrix LCD includes a transistor at each display point to increase the speed at which the crystals can change state and add color. Tektronix further developed "plasma addressing," which incorporates some of the properties of gas plasma into liquid crystal displays. liquid detector See electrolytic detector. LIS 1. Link Interface Shelf. 2. Logical IP Subnet- work. LISA Laser Interferometer Space Antenna. LISN 1. See line impedance stabilization network. 2. See Low-Incidence Support Network. LISPList Processing. A high level programming lan- guage introduced in 1958 by John McCarthy. It is used in many artificial intelligence applications, and as a macro scripting language in applications like AutoCAD from Autodesk Inc. and EMACS. Golden Common LISP is one of the more common imple- mentations of LISP. LISP code is syntactically dif- ferent from languages such as C, and has been retro- actively called an acronym for Lots of InSignificant Silly Parentheses by subsequent generations of pro- gramming students due to its nested statements. LISTSERV A significant discussion list software product introduced in 1986 that became prevalent on BITNET, Internet, and local area networks. As of spring 2001, L-Soft International, Inc. 's LISTSERV program was being used to manage over 170,000 lo- cal and public lists. LISTSERV is available in com- mercial and free LISTSERV Lite versions. See dis- cussion list, Majordomo. list server A computer file distribution system used 594 for managing email, newsgroups, discussion lists, and other types of files received from one or many sources, and distributed to one or many subscribers to the service. Using servers to manage the traffic in a centralized manner can cut down on administrative overhead and provide a means to implement security and selective filtering as needed. listed address LA. A location identifier, such as an email address on the Internet. The concept of the listed address has become important to Internet Ser- vices Providers (ISPs) in their management of"spam" (unsolicited bulk email messages). Checking to see if the source of spam is a listed address is one of the ways in which mail can be filtered or selectively pro- cessed. LIT See line insulation test. Litespan Alcatel's next generation network digital loop carrier multiservice (narrow/wide/broadband) access platform, the most widely deployed integrated digital loop carrier with DSL in North America, with a greater than 420/0 market share. The Litespan family of products provides advanced SONET-based Next Generation Digital Loop Carrier (NGDLC) capabilities, providing ATM xDSL and TOM narrow/wideband subscriber services. Litespan 1540 is a flexible multiservice access platform en- abling the delivery of multiple access services to a region from a single node. See GEMINAX. Iithium-tantalate A synthetic crystal first grown in the Bell Laboratories, lithium-tantalate was the first really practical alternative to natural or synthetic quartz for the development ofa number of commu- nications-related components such as filters. This substance has practical applications for high band- width transmissions. See quartz crystal, quartz crys- tal filter. Iittle-endian Stored or transmitted data in which the least significant bit or byte precedes the most signifi- cant bit or byte. Many file incompatibilities between computer systems, in which the file formats are oth- erwise almost identical, are due to platfonn conven- tions which store the data in big-endian or little- endian form. . Littman-Metcalfconfiguration A setup in which a resonating cavity external to a laser (often in the form ofa diffraction grating) is used to reflect a selected portion of the incident light at low angles called graz- ing angles. This configuration may be used to pro- duce a mode-locked, stable beam tuned to a desired frequency. See grazing incidence, grating. Littrow configuration In an optical grating filter, a configuration in which light diffracted from the grat- ing travels back along the same path as the incoming (incident) light. This occurs, for example, when a light beam ofa specific wavelength hits a reflective planar surface at 90°. Imagine light traveling down the shaft of an imaginary invisible flagpole that is anchored in a flat reflective base. When the light hits the base, it is reflected back again along tIte direc- tion of the shaft in the same path but in the opposite direction. Awareness ofLittrow effects is important to technolo- © 2003 by CRC Press LLC If the light reflected from a surface travels back along the same path through which it came (A), it is calleda Littrow Configuration The absolute angle is not the keyfactor, but ratherthe angle of the incident light beam relative to the surfaceplanefrom which it is reflected, as illustrated when the surface is rotated to the same degree as the incident light beam (B). gies where back reflection can interfere with signal transmission. For example, ifa fiber light source hits the endface of a fiber filament conducting core (or any bonding materials that may have been used in the joint) at an angle that causes the light to be reflected back to the source, the signal will not propagate through the light pipe. Changing the shape of the endface, the method of fusing, or the angle of propa- gation may help mitigate back reflection. See grat- ing. See acceptance angle, back reflection. Contrast with grazing incidence. Littrow Configuration LivermoreAutomatic Research Calculator/Com- puter LArc. A supercomputer developed for the Lawrence Livermore National Laboratory (LLNL) in the mid-1950s by the Sperry-Rand Corporation. The LArc had multiple noninterleaved core memory boxes and data, instruction, and DMA access slots; the random access memory (RAM) was mechanical in nature. LLC See Logical Link Control. LLC encapsulation LLC encapsulation is a means to "envelope" a set of packets so that several proto- cols can be carried over the same virtual circuit (VC). Type 1 and Type 2 are defined for connectionless and connection services. See Logical Link Control. LLC/SNAP LLC/SubNetwork Attachment Point. An encapsulation protocol used in Logical Link Con- trol. In an ATM environment, this is the default packet fonnat for Internet Protocol (IP) datagrams. See TU- LIP, TUNIC, RFC 1483. LMI Local Management Interface. See Frame Re- lay extensions. LMOS Loop Maintenance Operations System. LMS 1. See Local Measured Service. 2. Local Mes- sage Switch. 3. Location and Monitoring Service. LMSS Land Mobile Satellite Service. LMST See lightweight multiband satellite commu- nications terminal. LMU Line Monitor Unit. LNA See low noise amplifier. LNP See Local Number Portability. LNWResearch CorporationA California company that sold microcomputers assembled and in kit form in the early 1980s. The LNW-80 was a TRS-80-com- patible computer arguably better than both the Apple II and the TRS-80 in that it supported a faster CPU, more memory, and higher resolution color graphics. Unfortunately, the LNW computers didn't catch the attention of the public and never had the impact of the better-known brands. load coil See loading coil. load distribution unit LDU. A device for distribut- ing electrical loads which may include fault detec- tion circuits, alarms and control systems connected with the load. A load distribution unit can be used for disconnecting system loads to prevent damage to backup systems (e.g., batteries) during prolonged AC power outages or from low voltage exposure. Load Number The Canadian counterpart to the U.S. Ringer Equivalence Number (REN) system. As there may be more than one phone device attached to a single line (modem, fax, answering machine, etc.), a system was established to determine and indicate ~:e~~~~~:~~~ ~~ti~ ~~~:c;~~:~f~:fp~~~: •• In this way, the Load Numbers can be summed to show the total load, and make sure the line was not overloaded. In the REN system, most lines can handle a load of up to 5.0. In Canada, the concept is the same, but the scale uses larger units; in other words, a standard phone might have a load of about 10 to 20 points, with the total load for a single line being about 100 points. Many electronic devices will show the Load Num- ber somewhere on the main circuit board. load-balancing system LBS. A system for dynami- cally balancing the execution ofjobs to be processed, usually accomplished through software or a combi- nation of software and firmware (as in routers). LBSs are particularly useful in environments where tasks are farmed out to various systems, as on a distributed computer network. A load-balancing system is also used for dynamically balancing the transmission of traffic through a com- munications pathway, as in telephone circuits. In general, load-balancing systems are used where prior knowledge of what resources may be available at any given time cannot be easily anticipated. LBSs are intended to facilitate and automate task or traffic delegation as transparently and seamlessly as pos- sible. Load balancing is becoming increasingly important as global voice and data networks serve greater num- bers of users. Load balancing is distinct from load sharing in that balancing involves the efficient, well- distributed use of resources as opposed to the gen- eral availability ofa resource to multiple users or pro- cesses. Internet Service Providers (ISPs) use load balancers with Internet servers to efficiently manage response times and site access availability. Load balancers can be quite sophisticated, not only distributing user reflective surface A reflective surface light beam 595 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary requests across multiple servers, but also rerouting requests if changes occur in the available servers. De- pending upon their sophistication, LBSs may auto- matically adjust to server changes without powering down the system and may support security features associated with the service. In CORBA-based architectures, load balancing en- ables distribution ofclient processes among servers on a number of levels to balance network, operating system, and middleware-related processing loads. Othman, O'Ryan, and Schmidt have suggested a number of load balancing architectures to support CORBA systems. McArdle et a1. have described two types of load-balancing strategies for CORBA-based Service Control Points (SCPs), including a novel ant- based algorithm and a distribution method based on mathematical minimization of expected communica- tion flows. SGI has aNetwork Load Balancing software prod- uct that collects Ethernet devices under a single IP address and balances the input and output loads across the Ethernets. Thus, the IP address of the load-bal- ancing device is advertised (usually through its do- main name) and individual IP addresses of the asso- ciated devices are not. The variety of approaches to load balancing is reflected in products from Foundry Networks' Serverlron line, which include Server Load Balancing (SLB), Global Server Load Balanc- ing (GSLB), and Firewall Load Balancing (FLB). The Network Load Balancing component in Windows 2000 scales by distributing IP traffic across multiple cluster hosts. Effective load balancing can lead to higher quality of service for busy multiuser systems or services, such as popular Web sites. Quick response times can make the difference between keeping or losing a potential customer. Fast network transfer speeds are not suffi- cient in themselves to solve all the aspects of quick access and adding more servers may not be economi- cally feasible. Efficient delegation of tasks and traf- fic direction through good LBSs is one way to maxi- mize the effectiveness of an existing system. Load balancing is as much art as science. The sys- tem must anticipate and adapt to a changing environ- ment and the analysis of the effectiveness of load- balancing algorithms is, in itself, a challenge. State aggregation and decomposition are two means of as- sessing dynamic load balancing. In 1997, H. Lin pro- posed a combination of these methods and introduced the concept of a correlation window for analyzing dynamic LBS policies. The Parallel Programming Laboratory at the University of Illinois conducts re- search in load balancing, particularly in object mi- gration and seed load balancing, concepts ofinterest in parallel computing systems. There are a variety of commercial LBSs for telephone trunks and Internet use along with several Internet- related open-source software such as Queue, Supersparrow (for wide area networks), and ANTS load-balancing systems are distributed under the terms of the GNU Public License. See ant, Supersparrow. 596 loading coil A small electromagnetic induction de- vice which helps prevent attenuation of the signal on a wireline. Loading coils were developed in the early 1900s to improve long-distance transmissions in tele- phone lines and are still commonly used. By calcu- lating the optimum size and spacing the loading coils carefully along a wireline, it was possible to extend a circuit by several times. With the advent of data communications, loading coils have become a mixed blessing. While they improve transmissions in voice grade lines, they tend to add noise and distortion in the higher frequencies used in data transmissions. LOB See line of bearing. Lobe Attachment Module LAM. In a Token-Ring network, an expansion device to extend the number of users that can be accommodated on a single seg- ment ofa Token-Ring network. Multiple LAMs (up to a specified number) can be connected to a Con- trolled Access Unit (CAU). It may also be used with aMultistation Access Unit (MSAU). See Controlled Access Unit, Multistation Access Unit, Token-Ring network. LOe 1. line of communication. 2. See Loss of Cell. Local Access and TransportArea LATA. The terms of the 1984 AT&T divestiture resulted in the creation ofnumerous geographically local telecommunica- tions service areas, of which there are now more than 200 in the U.S. LATAs are determined, to some ex- tent, by population densities. Originally, local Ex- change Carriers were not permitted to connect calls across LATA boundaries, as that was the privilege of Inter Exchange Carriers (IXCs). Since that time, the rules have been modified to some extent. See Inter Exchange Carrier, Modified Final Judgment. Local Area Augmentation System LAAS. A VHF- based Global Positioning System (GPS) augmenta- tion system which functions along with the Wide Area Augmentation System (WAAS) to provide high-ac- curacy satellite-based navigation services for avia- tion, LAAS provides navigation and landing data where WAAS cannot be used. LAAS meets Category II/III aviation requirements in specific locations. Fur- ther details are available through the Federal Avia- tionAdministration (FAA). See Wide Area Augmen- tation System. http://gps.faa.gov/ local area network LAN. A computer network within a specified geographical space, such as a build- ing or region, or within an institutional entity such as a classroom or department. The network links com- puters and a variety of shared resources, typically files, application programs, and peripheral devices such as printers, fax machines, modems, and scan- ners. Connections between the computers are through wires, fiber optic cables, or wireless signals. LAN s are typically connected directly by telephone wire or coaxial cables, and thus are somewhat con- strained in physical size and number of users, due to lower transmission speeds, network topology, signal reduction (see attenuation), and fixed-bandwidth limitations, than wide area networks (WANs). There are a number of common ways in which LANs are connected, with various commercial, shareware, © 2003 by CRC Press LLC and freeware products to handle the software tasks associated with networking. One of the most popu- lar server products is Apache, which is robust, freely distributable, and very widely installed. Commercial products include IBM Token-Ring, Microsoft NT, and Novell Netware. As technology advances and becomes less expensive, direct cabling will probably decrease and wireless solutions become more common, thus reducing the distinction between LANs and WANs. See Token- Ring, virtual LAN. local area data transport LADT. Data transmission services offered over existing local phone lines. In general, it is a low-speed, low-cost option practical for many casual users of data services. In some cases, the system has been enhanced to provide simulta- neous voice and data (e.g., ISDN services). The grow- ing availability of U.S. ISDN (USDN) services is increasing the likelihood that local ISDN-based LADTs may eventually provide transport among LADTs. The abbreviation also refers more specifi- cally to AT&T's commercial LADT offerings, which include protocol conversion. See ISDN. LocalArea SignalingServices LASS. Commercial adjunct processor-based services used to create ini- tial orders. A Bulk Calling Line Identification (BCLID) service provides private branch exchanges (PBXs) with information on calls from outside the PBX group. Local Area Transport LAT. Aproprietary commu- nications protocol for terminal-to-host transmissions, developed as VAX systems by Digital Equipment Corporation (DEC). local area wireless network LAWN. A local home, business, community organization, campus, or other phone or data network which uses wireless technol- ogy to provide the links. The communication is of- ten carried out over radio frequencies (RF) in the fre- quency modulated (FM) or infrared range. Tech- niques common to LAWN include spread spectrum, originally developed for government operations, and narrowband. See Industrial Scientific Medical, spread spectrum. LocalAutomatic Message Accounting LAMA. An automatic message accounting (AMA) system used in local telephone switching centers in conjunction with number identification information to collect bill- ing data. It also automates the routing oflong-distance calls through more than one local office. local battery Equipment which draws power from a local source, rather than drawing current from the line to which it is attached. Most phones draw current from the phone line sufficient to operate the phone, but if they have extra features (LCD display, speak- erphone, etc.), they may require additional power which comes from a battery or local wall socket. Laptop computers use local battery peripherals, such as PCMCIA modems, rather than those which require separate power, e.g., desktop modems, to maximize convenience and portability. Local Bus Computer processors require a way to communicate with the many devices that make a computer useful: storage, printers, modems, input de- vices, scanners, etc. The local data bus is an inter- face that links the motherboard and various control- ler cards and other interface connectors that comonly communicate with external devices. Many different standards are defined for bus transmissions, and the bus speed does not always match the CPU speed, cre- ating a processor bottleneck for some types of pro- cesses and activities. A Local Bus is one of the newer, faster buses begin- ning to supersede other common buses, including EISA, VESA, and PCI. local callA telephone service phrase referring to calls placed through the local exchange, billed on the subscriber's predetermined regular service plan. These are generally geographically close. There are three common types of billing systems for local calls: 1. unlimited calls for a flat rate monthly fee, 2. flat rate up to a certain number of calls, then a per-call charge beyond that, or 3. a per-charge call which may or may not be scaled according to the total number of calls for the month. The first option is widespread in Canada and the U.S., while many places in Europe use the third option. For contrast, see long-distance. local differential GPS LDGPS. An implementation of the Global Positioning Service (GPS) designed to improve local accuracy of the data. A single GPS re- ceiver is placed at a known location where it can re- ceive GPS signals. It becomes a reference station which forms a scalar correction for GPS satellites within current view; broadcasting the correction in- formation is provided to local users. Since there is degradation over distance, a series of"cells" would be needed to apply this system over a large geographi- cal area. See differential GPS, wide area differential GPS. Local Exchange Carrier LEC. A designation for a local telephone company, now more commonly dis- tinguished as an Incumbent Local Exchange Carrier (LEC) or a Competitive Local Exchange Carrier (CLEC). This is defined in the Telecommunications Act of 1996, and published by the Federal Communications Commission (FCC), as meaning: " any person that is engaged in the provision of telephone exchange service or exchange access. Such term does not include a person insofar as such person is engaged in the provision ofa commer- cial mobile service under section 332(c), except to the extent that the Commission finds that such service should be included in the definition of such term." See Federal Communications Commission, Telecom- munications Act of 1996, United States Telephone Association. Local Exchange Carrier duties The Federal Com- munications Commission (FCC) stipulates a number of duties in the Telecommunications Act of 1996 as shown in the Telecommunications Act chart. local injection and detection LID. In fiber optics splicing and assembly, LID is a fusion splicing align- ment process. Two fibers to be joined are lined up 597 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary end-to-end. Light is then injected into the conduct- ing core of the fibers that are to be joined. One or both of the fibers are adjusted spatially in fine increments until there is optimum transmission of light through the dual-fiber assembly - somewhat like focusing a camera back and forth to determine the point at which the image is sharpest, except that light conduction rather than image focus is determined. The fibers can then be fused into a single lightguide which hopefully retains the good alignment of the unfused fibers. The splice is then inspected. See fusion splice. local loop In telephone installations, a physical link through a wire pair connection between the sub- scriber, which may be an individual, a business, or a private branch system, and the switching office. The local loop once included the connection right to the subscriber's phone, but now the demarcation point is usually a patch panel or exterior connections box (although to-the-phone is still available for a fee). Local Management Interface LM!. In Frame Re- lay networks, an extended specification for informa- tion exchange between devices. See Frame Relay Extensions for more detail. Local Measured Service LMS. Atelephone billing system in which subscribers pay according to the number of calls made or received (or both), rather than according to a flat monthly rate. Measured ser- vice is sometimes provided at a flat rate up to a speci- fied number of calls, and then a per-call fee above that number (many banks set up checking charges this way as well). Generally, LMS is interesting to those who make very limited use of the phone or who have a line primarily for incoming calls. In some countries, all service is measured and even calls that are unan- swered or that result in a busy signal may be billed in some areas. local multipoint distribution service LMDS. A pro- posed terrestrial wireless communication service de- signed to send video over small cells. This would be competitive with urban cable TV services. The Fed- eral Communications Commission (FCC) released a Notice ofProposed Rule Making on the LMDS pro- posal in December 1992. This led to various FCC proposals to segment the transmissions band, with the effect that the primary spectrum would be limited to the 27.5 to 29.5 GHz range. Long-term advocates of satellite communications were concerned that this approach would limit future evolution and growth of satellite communications de- ployment. Local Number Portability LNP. Atelecommunica- tions service to enable local telephone numbers to be retained even ifasubscriber's carrier is changed. LNP was implemented in the mid-1990s and made it easier for phone services to compete. The management of LNP services is, in part, dependent upon the mainte- nance ofregional databases containing the numbers and the vendors servicing those numbers. See Local Number Portability Administrator, Location Routing Number. Local Number Portability Administrator A man- aging authority tasked with administering the Local Number Portability database for tracking and updat- ing local numbers and the telecommunications car- riers servicing those numbers. NeuStar, Inc. (formerly Lockheed-Martin Information Management Service) is currently the administrator for the Local Number Portability Administration Center (NPAC). See Lo- cal Number Portability. local service ordering guidelines LSOG. Require- ments published by individual communications carriers to instruct customers on how to pre-order or Telecommunications Act - Stipulated Local Exchange Carrier Duties "Each local exchange carrier has the following duties: (I) RESALE The duty not to prohibit, and not to impose unreasonable or discriminatory conditions or limitations on, the resale of its telecommunications services. (2) NUMBER PORTABILITY The duty to provide, to the extent technically feasible, number portability in accordance with requirements pre- scribed by the Commission. (3) DIALING PARITY The duty to provide dialing parity to competing provid- ers of telephone exchange service and telephone toll ser- vice, and the duty to permit all such providers to have non- discriminatory access to telephone numbers, operator ser- vices, directory assistance, and directory listing, with no unreasonable dialing delays. (4) ACCESS TO RIGHTS-OF-WAY The duty to afford access to the poles, ducts, conduits, and rights-of-way of such carrier to competing providers of telecommunications services on rates, terms, and condi- tions that are consistent with section 224. (5) RECIPROCAL COMPENSATION TIle duty to establish reciprocal compensation arrange- ments for the transport and termination of telecommuni- cations." 598 © 2003 by CRC Press LLC order local connection products and services. LSOGs typically identify Order and Billing Forum (OBF) industry guidelines and carrier-specific requirements. After presenting the guidelines, the carrier will usu- ally offer a variety of forms to the customer to sign up for services. Gone are the days when you could simply ask a phone company to install a phone. Now you must read guidelines and fill out forms to identify various choices of equipment leasing or buying, various car- riers for different services, optional "value" or "fea- ture" packages, optimum proportion of data services, phone services, number of extensions, extra lines, etc. Many carriers now publish the forms online. This is a great convenience except that many of them offer the forms only in Microsoft Word (a proprietary com- mercial product) instead of using a format such as PDF, for which the vendor (Adobe) offers freely downloadable readers. Hopefully more vendors will use file fonnats with free readers for distributing Web forms to potential customers. local service request LSR. The process by which a telecommunications customer requests local service. F or example, a Competitive Local Exchange Carrier (CLEC) would carry out LSR procedures to order loops from Bell Telephone. Many carriers now pro- vide the request forms on the Internet for download. They can then be mailed or, in some cases, emailed or otherwise submitted electronically. When a local service request is combined with other requests in order to set up an enhanced loop service, the proce- dure and forms are collectively known as an access service request (ASR). An administrative LSR may be required to identify a CLEC. Local Services Provider LSP. A telecommunications provider that specializes in local services (e.g., local telephone services) rather than regional or national services (e.g., long-distance calling). local signal transferpoint LSTP. In mobile personal communications services, one or more local message routing translation and screening points. A local sig- nal transfer point is associated with a LATA geo- graphical area connecting to multiple service switch- ing points (SSPs). One or more LSTPs may be con- nected to a regional signal transfer point. Mobile switching centers (MSCs) are connected to the sig- naling network (e.g., SS7) through a local signal transfer point. LOCAL TV Act of 2000 An act which created a $1.25 billion federal loan guaranteed fund to assist residents in rural areas to receive urban and local TV broadcast signals through satellite transmissions. Thus, both profit and nonprofit organizations could access resources for offering local broadcast signals, with the government guaranteeing a portion of the loan. An amendment cleared the way for electric co- operatives and telephone systems to participate in the loan guarantee program. LOCAL TV is an acronym for Launching Our Com- munities' Access to Local Television. The Act was drafted in part because the 1999 Satellite Home Viewer Improvement Act (SHVIA) did not include legislation to help rural Americans receive local broadcasting. See Rural Local Broadcast Signals Act, Satellite Home Viewer Improvement Act. LocalTalk Aproprietary local area network Apple- Talk-compatible protocol developed by Apple Com- puter and used on Macintosh computers and periph- erals. LocalTalk is not a fast protocol, but all Macs come networkable right out of the box, with a simple serial cable, and something can be said for ease of use and convenience, particularly in school and work environments. See AppleTalk. Location Area Identity LAI. A subscriber identity allocated and assigned on a location basis as part of the Temporary Mobile Subscriber Identity (TMSn used in a Global System for Mobile Communications (GSM). Location Routing Number LRN. A 10-digit num- ber to uniquely identify a switch in a telephone cir- cuit. In the 1990s, LRNs facilitated the implementa- tion of competitive services such as Local Number Portability (LNP). Various regulatory agencies sup- ported this AT&T/Lucent system and it became an ~~~~~riaEi:~~~~~~~~~~~fo:~~~t~~ ~. LODE See Large Optics Demonstration Experiment. lodestone, loadstone A natural magnetic material called magnetite, an oxide of iron. All magnetite can be readily magnetized. This material was used to cre- ate early compasses and was called magnes lapis, magnetic stone, or magic stone. It is called lodestone when it comes out of the ground already exhibiting magnetic properties. Lodestone is probably the same stone mentioned by Plato in Timaeus as " the Heraclean stone," since the Heraclean stone was paired in the same sentence with the attractive prop- erties of amber. See amber, magnet. Lodge, Oliver Joseph (1851-1940) An English physicist who demonstrated that radio waves could carry a signal over distance, in 1894. Lodge is also known for his experiments with tuning in radio waves, ideas developed by succeeding scientists. Lodge-Muirhead detector A simple, early type of self-restoring radio-frequency detector built by hob- byists and commercial manufacturers in the early 1900s. It employs a small steel revolving wheel, with the outer edge sharpened to a very fine edge, sup- ported between slots on frame posts on either side of a rod passing through the center of the wheel. A small motor supplies the power to quickly turn the wheel. A hard rubber cylinder with a slot cut in the top sits directly under the wheel. Mercury is poured into the slot and makes contact with a binding post threaded in from the outside of the rubber. A thumbscrew is installed in the rubber to raise or lower the mercury level. When the wheel revolves, it makes contact with the mercury and the signal is translated through the mercury, by means of brushes, to the binding post which connects to the rest of the circuit, including a receiver. The battery power to the motor is controlled by means ofa potentiometer. See detector. LOF 1. See Loss of Frame. 2. loss of fix. The loss 599 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary ofa desired target or calibration setting. 3. lowest op- erating frequency. LOFAR See Low Frequency Array. log-periodic antenna A periodic antenna is one in which the input impedance varies as the frequency varies. Log-periodic antennas have a variety of ar- rangements of active interconnected dipoles to pro- vide broadband, high-gain capabilities. They are use- ful for very high frequency (VHF) signal ranges. logarithm, log Amathematical means ofexpressing an exponent - the power to which a number is raised. For example, in the expression x = 10" the symbol n stands for the log of x. As an example, loge I 0) = 1. A number ofphenomena have logarithmic qualities, such as sound volume, pH scales, and earthquake magnitude scales. These phenomena are not sensed on a linear scale, but rather on a logarithmic scale, in which the magnitude of the previous point on the scale is not linearly related (e.g., twice as much each time), but exponential. However, expressing the mag- nitude of these phenomena in increasingly huge num- bers can be cumbersome, so an antilog may be ap- plied to "compensate" for the logarithmic increase to create a scale with numbers that are smaller and easier to understand. Thus, a Richter scale scale for express- ing earthquake magnitudes uses small manageable numbers, but it must be remembered that it represents a logarithmic phenomenon and thus the difference between 5 and 6 on the Richter scale is much greater than the difference between 4 and 5. Logs are expressed in terms ofa reference base and it is best to specify which one is used. Thus, log) would read as "log, base 2, ofthree." A log may 6e classified as natural (commonly used in chemistry and physics) or common. Natural logs have a base of e (2.71828) and common logs have a base of 10. Log tables are published to aid in lookup of logs for ex- ponents other than powers of 10, since these are time- consuming to calculate. Many calculators now have log keys to simplify the process. See antilog. logic bomb A software program designed to penetrate a system, present a message (sometimes through graphics or sound), or damage memory or stored data, when some particular logical operation happens. A time bomb is a type of logic bomb which can, for example, wipe all the data offa hard drive when the bomb detects that it is April I. Logic bombs are not always malicious, but they are seldom appreciated. They generally fall into the category of practical jokes, which are usually funnier to the perpetrator than to the intended object of the joke. See virus. logical block In storage devices, such as hard drives and magneto-optical devices, the smallest address- able unit. Each block is associated with a unique num- ber, usually starting with 0, and incrementing for each succeeding block. This allows the system to locate data, read and write to the device, partition the drive, etc. in an organized manner. Logical BlockAddress LBA. A means for saving and retrieving information by accessing block addresses on a storage medium, rather than by using cylinder- head-sector addressing schemes. The blocks on the 600 storage medium are addressed sequentially, usually starting with zero. SCSI peripherals use this address- ing method. Some IDE drives are now beginning to use this method, but it may be necessary to request LBA mode explicitly. logical driveA drive configured separately from the physical configuration of the drives. For example, a computer may have three drives, each with several partitions, but logically, the system may organize them into four logical drives with various partitions from different drives in such a way that the user "sees" four drives rather than three, of sizes set by software. Conversely, a system may have three drives, each with a couple of partitions, which are aggregated into one drive. Thus, the storage space appears to the user as one large drive. Not all operating systems can organize drives in this way. Some of the lower-end personal computers have limitations in the configu- ration oflogical drives. logical link A link between nodes or devices based upon an abstract rather than a physical topology. Thus, virtual LAN s, logically direct connections, and other types ofpaths can be set up in association with the physical connections. Logical Link Control LLC. The upper sublayer of the layer 2 Open Systems Interconnection (OSn pro- tocol. LLC provides data link level transmissions control. It is the default multiplexing layer for Inter- net Protocol (lP) over AAL5.It was developed by the IEEE 802.2 committee to provide a common access control standard for networking which is independent ofpacket transmission methods. It includes address- ing and error checking capacities. Logical Storage Unit LSD. Abuffer unit which con- nects to PBX systems to store call information. Also known as a buffer box or poll-safe. logical topology In a network, the connections and relationships between computers and various devices may not map in a one-to-one relationship with the physical topology. Thus, logical topologies, organized and managed in software, may be administered and diagramed separately or as an overlay to the physi- cal connections. In large networks, logical topologies often require tracking and display programs to con- figure and troubleshoot the logical connections. From a user's point of view, many aspects ofa logical to- pology can make computing more efficient and en- joyable. As a simple example, there are workstation networks which allow hard drives to be mapped in such away that they appear as one giant drive to the user, even though the data may physically be spread over a num- ber of partitions and drives. Thus, the user doesn't have to worry about whether there is enough space, which drive to use, or on which drive or partition that old file was stored. The operating system takes care of all the housekeeping involved in managing the sys- tem. On a larger scale, the same can be said of whole systems. The user may be using an application pro- gram located on a machine in another room, another building, or another state, but it can be logically mapped in so that it appears to be running on the 10- © 2003 by CRC Press LLC cal machine. Commercial software programs with graphical user interfaces exist to help manage logi- cal topologies and, in some cases, various manage- ment utilities come with the operating system. logical unit number LUN. The LUN is an identifi- cation system used with SCSI devices which allows the computer and controller to distinguish and com- municate with up to seven devices including the con- troller for each SCSI chain. Each device must have a unique ill so that the controller can administer more than one device when they are chained together. logiciel Software, in French. login script, logon scriptA login script is a file that includes commands or variable settings pertaining to the login and initial setup. The two most common functions ofa login script are: 1. to set up the system to the specifications ofaparticular terminal, and 2. to set up the preferences for a particular user or set of users. In the first case, the login may include infor- mation about the characteristics of the terminal, such as hardware-specific keyboard mapping, graphics card settings, etc., as well as environmental variables, patches, and other initialization parameters. In the second case, the settings may include the user screen size and color preferences, preferred fonts, frequently used applications, permissions, and other character- istics of the user environment. See batch file, JCL, Perl. LOLA The Language of Temporal Ordering Speci- fication (LOTOS) Laboratory. See Language ofTem- poral Ordering Specification. LON local operating network. See LONWORKS. Long Distance Internet Provider LDIP. Coopera- tive services offered by companies that operate un- der restrictions set by the Modified Final Judgment (MFJ) who wish to provide Internet services. long haul communication A call which extends be- yond the local exchange area. longkeyA key held down for longer than a prescribed period which signals an event separate from a short press of the same key. For example, a long key on a computer keyboard might cause the character asso- ciated with the key to repeat, or to initiate a key-re- lated process. long tone In telephony, a long key is one which sig- nals for longer than other keys, in order to commu- nicate through various automated phone menu sys- tems. That is why many phone system menus instruct you to press the pound key (#) after typing in a num- ber, so that a long key signal will be transmitted. long-distance call, toll call A telephone service re- ferring to any call outside the local selVice area. Long- distance calls are frequently completed and charged through a carrier other than the local service, and in- clude extra digits in order to identify the desired des- tination. Long-distance calls are usually billed in a cooperative arrangement through the local carrier, although some services may be billed separately. Sometimes called a trunk call, though this phrase is less common. The first recognized long-distance call is said to have been a one-way message from Alexander Graham Bell's father in Brantford, Ontario, Canada, to Bell in Paris, Ontario, in 1876. The first two-way long- distance call was between Bell and Watson in Cambridgeport and Boston, respectively. The first transcontinental phone line went into ser- vice in 1915, connecting San Francisco and New York City, with Edison and Watson conducting the first conversation over this line. About a decade later, long-distance radiotelephone service was established across the Atlantic Ocean. Some of the important inventions which made long- distance communications possible were Pupin's load- ing coils, de Forest's triode, Armstrong's regenera- tor circuit, and microwave antennas introduced in the early 1950s. Contrast with local call. long-distance carrier IXC. Local long-distance pro- viders which are competitive with incumbent local exchange carriers. longitudinal redundancy check LRC. A data trans- mission error checking technique incorporating a block check on a group of data. An accumulated Block Check Character (BCC) is compared to the sending BCC; if they match, the block is considered to have been transmitted without errors. See cyclic redundancy check. LONMarkCertificationA program to ensure inter- operability among functional LONWORKS devices, developed by the LONMark Interoperability Asso- ciation. See LONWORKS. LONMark Interoperability Association A trade organization of companies supporting and promot- ing the LONWorks control automation system, estab- lished in 1994. The association aids in the integra- tion of multi vendor systems for building, home, trans- portation, and industrial environments. Permission to display the LONMark logo is awarded by the asso- ciation to firms that successfully complete the LONMark Certification conformance tests. http://www.1onmark.org/ LONTalk A communications protocol associated with LONWORKS automation products that enables intelligent control to be associated with motors, fans, switches, sensors, valves, and other industrial, transportation, and residentiallbusiness automation applications. The protocol is designed to be able to support more than 500 transactions per second and priority levels may be assigned. It follows the Open Systems Inter- connection (OSI) reference model and is currently the only product of its kind to implement all seven lay- ers of the model. LONTalk is favored in part because of its reliability. Unlike some automation systems with one-way com- munications only or two-way communications with- out acknowledgments, LONTalk supports end-to-end acknowledgments and automatic retries. Third-party developers of LONTalk have further developed "heartbeat" techniques to enable individual nodes to "check in" with a main or management controller to let the controller know all systems are functional. Alarm conditions can be triggered if nodes do not respond as expected. 601 © 2003 by CRC Press LLC . 1 bits +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ -+ I Code I Identifier I Length I +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ - +-+ -+ I Data +-+ - +-+ -+ Link. code assignment Configure-Request Configure-ACK Configure-NAK Configure-Reject Terminate-Request Terminate-ACK Code-Reject Protocol-Reject Echo-Request Echo-Reply Discard-Request © 2003 by CRC. injection and detection LID. In fiber optics splicing and assembly, LID is a fusion splicing align- ment process. Two fibers to be joined are lined up 597 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary end-to-end.