L

L band - 1. In IEEE terminology, the portion of the microwave radio spectrum in the range of 1–2 GHz. See also IEEE and microwave. 2. In ITU-R terminology, the portion of the microwave radio spectrum in the range of 1.610–1.6255 GHz.Applications include global positioning satellite (GPS), low-Earth orbiting (LEO) satellites, Search for Extraterrestrial Intelligence (SETI), and telemetry. See also electromagnetic spectrum, GPS, ITU-R, LEO, SETI, and telemetry.

L-Band (Long Wavelength Band) - The ITU-T standard optical transmission window in the wavelength range of 1,565–1,625 nm. See also wavelength and window.

L - Symbol for Inductance. See inductance.

L2TP (Layer 2 Tunneling Protocol) - A tunneling protocol used for secure node-to-node communications by Internet service providers (ISPs) and other virtual private network (VPN) service providers in support of multiple, simultaneous tunnels in the network core. End users gain access to the service provider on an unencrypted basis, with the service provider assuming the responsibility for encryption at the edge of the packet network. L2TP is an extension to the Point-to-Point Protocol (PPP) that evolved from a combination of Microsoft’s PPTP and Cisco’s Layer 2 Forwarding (L2F) protocol. L2TP is described in IETF RFC 2661. See also encryption, ISP, PPP, PPTP, protocol, tunneling, and VPN.

Label Distribution Protocol (LDP) - See LDP.

Label Edge Router (LER) - See LER.

Label Switched Path (LSP) - See LSP.

Label Switching Router (LSR) - See LSR.

label - 1. A set of data attached to and providing identification or other information relative to a larger data unit, such as a packet or message. See also packet and message. 2. In Multiprotocol Label Switching (MPLS), the set of data attached to a packet and used by a Label Switched Router (LSR) to select a link across which to forward that packet.The initial packet is inserted by a Label Edge Router (LER). Each LER along the path swaps the label associated with the incoming packet for a new label associated with the outgoing packet to be used by the adjacent downstream router in making the next link selection. See also downstream, LER, link, LSR, MPLS, packet, and router.

LACNIC (Latin-American and Caribbean Network Information Center) - The Regional Internet Registry (RIR) responsible for assigning Internet Protocol (IP) addresses variously to National Internet Registries (NIRs) or directly to Local Internet Registries (LIRs) in Latin America and the Caribbean. See also IP, IP address, LIR, NIR, and RIR.

Lamarr, Hedy (1913–2000) - A famous actress and dancer of pre-war (WWII) fame, and co-inventor of spread spectrum (SS) radio. Born Hedwig Eva Maria Keisler in Vienna, Austria, her first husband (of six) was Friedrich Mandl, an arms manufacturer who socialized with Adolph Hitler and Benito Mussolini. She reportedly hated her overly possessive husband and his Nazi friends and escaped to London before WWII. Using knowledge she gained while married to Mandl, Lamarr developed spread spectrum radio, which she initially used to remotely synchronize multiple player pianos in radio-controlled piano concerts that reportedly were quite popular in those much simpler times. The United States Patent and Trademark Office issued patent #2,292,387 (1942) to Ms. Lamarr and George Antheil, a film-score composer to whom she had turned for help in perfecting the idea, for a Secret Communication System that was, in effect, a spread spectrum radio. In the Pacific Theater during World War II, the Allies used that patented technology extensively to prevent the Japanese from jamming radio-controlled torpedoes.This primitive system used a mechanical switching system much like a piano roll to shift frequencies faster than the Axis military could follow them. Subsequently, spread spectrum combined with digital technology for spy-proof and noise-resistant battlefield communications. During the 1962 Cuban nuclear missile crisis, Sylvania installed SS on U.S. warships sent to blockade Cuba, where the technology provided improved security as well as prevented signal jamming. Ms. Lamarr never requested nor received any royalties from the use of her invention. Ms. Lamarr was an exotic beauty and talented actress. She delighted and shocked prewar audiences by dancing nude in the movie Ecstacy (1933). Her greatest screen role was as Delilah, opposite Victor Mature as Sampson in Cecil B. DeMille’s Sampson and Delilah (1949). Hedy Lamarr has a star on the Hollywood Walk of Fame at 6247 Hollywood Boulevard. In 1997, the Electronic Frontier Foundation (EFF) gave her an award for inventing SS radio, which is the basis for code division multiple access (CDMA), which is used in cellular and other radio networks. See Antheil,George. See also CDMA,cellular radio, EFF, radio, and SS.

lambda (λ) - The Greek letter used by physicists to denote wavelength. See wavelength.

LAN (Local Area Network) - A LAN is a packet network designed to interconnect host computers, peripherals, storage devices, and other computing resources within a local area, i.e., limited distance. LANs conform to the client/server architecture, a distributed computing architecture that runs applications on client microcomputers against one or more centralized servers, which are high-performance multiport computers with substantial processing power and large amounts of memory.A LAN might serve an office, a floor of a building, and entire building, or a campus area, but generally does not cross a public right-ofway such as a street.The distance limitation generally is in the range of a few kilometers, at most, although that is sensitive to the transmission media employed, which include coaxial cable, twisted pair, optical fiber, infrared (IR) light, and radio frequency (RF) systems. Raw bandwidth ranges up to 10 Gbps, although actual throughput generally is much less. LANs generally are private networks, although public wireless hotspots offering wireless Internet access currently are popular. Most LAN standards are set by the 802 Working Group of the Institute of Electrical and Electronic Engineers (IEEE), with examples being 802.3 (Ethernet) and 802.11a/b/g (Wi-Fi).A personal area network (PAN) such as Bluetooth, is much more limited in geographic scope than a LAN. LANs and LAN segments can be interconnected over a metropolitan area network (MAN) or wide area network (WAN). LANs operate at Layer 1, the Physical Layer, and Layer 2, the Data Link Layer, of the OSI Reference Model. See also 802.3, 802.5, 802.11, architecture, bandwidth, Bluetooth, client/server, coaxial cable, Data Link Layer, Ethernet, hotspot, IEEE, IR, MAN,optical fiber, OSI Reference Model, PAN, Physical Layer, RF, throughput, Token Ring, twisted pair, WAN, and Wi-Fi.

LAN emulation (LANE) - See LANE.

LAN emulation client (LEC) - See LEC.

LAN emulation configuration server (LECS) - See LECS.

LAN emulation server (LES) - See LES.

LAN - switch An intelligent, active hub that establishes, maintains, and changes logical connections over physical circuits. Switches flexibly connect transmitters and receivers across networks of interconnected ports and links, thereby allowing network resources to be shared by large numbers of end users. LAN switches are packet switches that can support multiple simultaneous transmissions, reading the destination address of each frame and forwarding it directly to the port associated with the target device. LAN switches are classified as workgroup switches or backbone switches.A workgroup switch serves the needs of a group of users who work together and, therefore, share common resources and intercommunicate frequently. Through the use of workgroup switches, a great deal of traffic can be confined to relatively small LAN segments and overall LAN congestion can be reduced considerably. High capacity backbone switches interconnect workgroup switches and provide connections to routers, as required. LAN switch architectures are several. A shared bus switch has a single high-speed bus that is shared by all incoming and outgoing ports on a time-division multiplexing (TDM) basis. A matrix switch contains multiple interconnected high-speed internal buses in a multi-bus switching matrix that can provide full bandwidth to multiple, simultaneous transmissions. If there are congestion issues in a matrix switch, it may have the ability to subdivide its capacity, with the buses becoming shared buses.A cut-through switch quickly reads the address of the data frame and quickly flows it through the switching matrix, bit by bit. A store-andforward switch temporarily buffers and examines each incoming frame for errors through a CRC check before forwarding it through the switching matrix to the output port. Fragment-free switching, is similar to cut-through except for the fact that the switch stores the first 64 octets of the frame before forwarding it.As most errors occur at the beginning of a frame, this approach eliminates the possibility that runt frames (i.e., truncated frames) will be transmitted.

landline - 1. A traditional telephone connected to the PSTN by a traditional wire (or fiber) local loop that terminates in a fixed location, rather than a cellular mobile telephone connected to a cellular network via radio technology. A cordless telephone is considered part of a landline as the local loop terminates in a fixed base station on the subscriber premises, even though the connection to the base station is wireless. A wireless local loop (WLL) is considered a landline, as it is terrestrial and connects two fixed points. See also local loop and WLL. 2. A telecommunications system that uses traditional terrestrial cabled, or conducted, transmission media such as copper or fiber optics, and wireless systems such as microwave, rather than mobile wireless radio technologies such as cellular or, especially, non-terrestrial satellite.

LANE (LAN Emulation) - A specification (January 1995) from the ATM Forum (since merged into the MFA Forum) for an ATM service in support of native Ethernet (802.3) and Token Ring (802.5) local area network (LAN) communications over an ATM network. Software in the end systems (e.g., ATM-based hosts or routers, known as proxies), of the ATM network emulates a native LAN environment. LANE acts as Layer 2 bridge in support of connectionless LAN traffic, with the connection-oriented ATM service being transparent to the user application. In LANE, a LAN emulation client (LEC) connects to the ATM network over a LANE user-to-network interface (LUNI). The network-based LAN emulation server (LES) registers the LAN medium access control (MAC) addresses and translates them into ATM addresses using the address resolution protocol (ARP). Each LEC is assigned to an emulated LAN (ELAN) by an optional network-based LAN emulation configuration server (LECS). Each LEC also is associated with a broadcast and unknown server (BUS) that handles broadcast and multicast traffic, as well as initial unicast frames before address resolution. LANE traffic generally is Class C variable bit rate (VBR) traffic in message mode, and is supported over ATM Adaptation Layer Type 5 (AAL5). See also 802.3, 802.5, AAL5, ARP, ATM, ATM Forum, broadcast, BUS, Class C ATM traffic, connectionless, connection-oriented, ELAN, emulation, Ethernet, host, Layer 2, LEC, LECS, LES, LUNI, MAC, message mode service, MFA Forum, multicast, proxy, router, Token Ring, unicast, and VBR.

LANE user-to-network interface (LUNI) - See LUNI.

LAPB (Link Access Procedure, Balanced) Pronounced lap bee. A bit-oriented protocol that is part of the X.25 protocol suite, LAPB runs at Layer 2, the Data Link Layer, of the OSI Reference Model.The LAPB frame comprises a header and trailer that encapsulate the packet formed by the X.25 packet layer protocol (PLP) at the Network Layer. The LAPB frame provides a mechanism for transporting packets across a link, ensuring that frames of data are ordered correctly and are free from error. LAPB is a balanced protocol that operates in asynchronous balanced mode (ABM), referring to the fact that the devices have a balanced, rather than a master/slave, relationship. LAPB is a derivative of the High-level Data Link Control (HDLC) protocol, which, in turn, is based on the IBM Synchronous Data Link Control (SDLC) frame. The fields in the LAPB frame are as follows:

• Flag: A specific one-octet field that delimits (i.e., establishes the limits or boundaries of) the beginning and end of the frame.This octet is always 01111110 in binary notation (7E in hexadecimal notation), which is prevented from appearing in the payload by bit stuffing.

• Address: A one-octet field that indicates the sending station in command frames and the receiving station in response frames. User addressing is the responsibility of the logical channel identifier (LCI) field contained in the packet layer protocol (PLP), which is one layer above LAPB in the X.25 protocol stack.

• Control: A one-octet field that identifies the frame type.An information frame carries upper-layer information and some control data.A supervisory frame carries control information such as informationframe acknowledgement, request for retransmission, and flow control.An unnumbered frame carries control data such as disconnection request, acknowledgement, and frame rejection.

• Data: A variable-size field that contains an encapsulated packet.

• Frame Check Sequence (FCS): A two-octet cyclic redundancy check (CRC) field that provides error detection.

See also ABM, asynchronous, bit-oriented protocol, CRC, Data Link Layer, flag, HDLC, header, master/slave, Network Layer, OSI Reference Model, PLP, SDLC, trailer, and X.25.

LAPD (Link Access Procedure, D channel) - Pronounced lap dee.A bit-oriented protocol that is part of the ISDN protocol suite, LAPD runs at Layer 2, the Data Link Layer, of the OSI Reference Model. LAPD defines the ISDN data (D) channels, which are designated for out-of-band signaling and control purposes in all ISDN implementations. LAPD evolved from the LAPB protocol used in X.25 networks. As the Signaling System 7 (SS7) signaling and control network specified for ISDN employs the X.25 packet format, consistency is maintained in the packet format, end-to-end. LAPD is a balanced protocol that operates in asynchronous balanced mode (ABM), referring to the fact that the devices have a balanced (i.e., peer-to-peer) relationship, rather than a master/slave relationship. The fields in the LAPD frame are as follows:

• Flag: A specific one-octet field that delimits (i.e., establishes the limits or boundaries of) the beginning and end of the frame.This octet is always 01111110 in binary notation (7E in hexadecimal notation), which is prevented from appearing in the payload by bit stuffing. Flags also fill idle time on the line.

• Address: A two-octet field known as the Data Link Connection Identifier (DLCI).The first octet is the Service Access Point Identifier (SAPI), which identifies the destination service access point, each of which can support multiple terminal devices.The second octet is the Terminal Endpoint Identifier (TEI), which is the address of the destination terminal device.

• Control: A one- or two-octet field that identifies the LAPD frame type.An information (I) frame carries upper-layer information and some control data.A supervisory (S) frame carries control information such as information-frame acknowledgement, request for retransmission, and flow control.An unnumbered (U) frame carries control data such as disconnection request, acknowledgement, and frame rejection.

• Information: A variable-size field with a maximum of 260 octets comprising upper-layer information.The size of the field is system-dependent. Only information frames include an information field.

• Frame Check Sequence (FCS): A two-octet cyclic redundancy check (CRC) field that provides error detection.

See also ABM, asynchronous, bit-oriented protocol, CRC, Data Link Layer, flag, frame, D channel, ISDN, LAPB, master/slave, OSI Reference Model, out-of-band signaling and control, protocol suite, SS7, and X.25.

LAPF (Link Access Procedure for Frame Mode Services) - Pronounced lap ef.A bit-oriented protocol that is part of the frame relay protocol suite, LAPF is a subset High-level Data Link Control (HDLC) and runs at Layer 2, the Data Link Layer, of the OSI Reference Model. The LAPF frame comprises a header and trailer that frame the user payload. The fields in the LAPF frame are as follows:

• Flag: A specific one-octet field that delimits (i.e., establishes the limits or boundaries of) the beginning and end of the frame.This octet is always 01111110 in binary notation (7E in hexadecimal notation), which is prevented from appearing in the payload by bit stuffing.

• Address: A two-octet (default) or four-octet (option) field that includes the necessary control information in the Data Link Connection Identifier.The address field also contains a Command/Response field, Address Field Extension, Forward and Backward Explicit Congestion Notification fields, and Discard Eligibility data.

• Data Link Connection Identifier (DLCI, pronounced delsey): 10 bits (or 20 in the 4-octet header) that identify the data link, the virtual circuit (VC), and its service parameters, which include frame size, Committed Information Rate (CIR), Committed Burst Size (B _c ), Burst Excess Size (B _e ), and Committed Rate Measurement Interval (T_c ).

• Command/Response (C/R): 1 bit reserved for use of the frame relay access device (FRAD) in order to facilitate the transport of polled protocols (e.g., SNA), which require a command/response for signaling and control purposes. • Address Field Extension (EA): 2 bits that signal the extension of the addressing structure beyond the 2-octet default.The use of EA must be negotiated with the carrier when the service is established.

• Forward Explicit Congestion Notification (FECN): Pronounced feckon. A 1-bit field available to the network to advise downstream devices that the frame has experienced congestion. Should the receiving FRAD determine that subsequent frames were discarded or corrupted in transmission, it is advised that recovery may be required in the form of requests for retransmission.

• Backward Explicit Congestion Notification (BECN) Pronounced beckon.A 1-bit field used by the network to advise devices of congestion in the direction opposite of the primary traffic flow. If the target FRAD responds to the originating FRAD in the backward direction, the BECN bit is set in a backward frame. If there is no data flowing in the backward direction, the frame relay network creates a frame in that direction, setting the BECN bit. If the originating FRAD is capable of reducing the frame rate of its transmissions, it is advised to do so on the indicated DLCI, as the network may discard frames once the notification is posted.

• Discard Eligibility (DE): A 1-bit field indicating the eligibility of the frame for discard under conditions of network congestion.Theoretically, the user equipment sets the DE in consideration of the acceptability of the application to loss. Should the user equipment not set the DE, the network switches may do so on a random basis or when traffic exceeds subscribed levels (CIR). SDLC and real-time voice and video traffic demand high priority and do not tolerate loss, and so must not be marked DE.

• Information Field: Contains user information, either in the form of payload data or internetwork control information.Although this field can be as much as 4,096 octets in length, it generally is restricted to a maximum size of 1,600 octets.

• Frame Check Sequence (FCS): A two-octet cyclic redundancy check (CRC) field that provides error detection.

See also carrier, CRC, Data Link Layer, DLCI, downstream, FRAD, frame, frame relay, FCS, HDLC, header, octet, OSI Reference Model, payload, protocol suite, real-time, SDLC, signaling and control, SNA, trailer, and VC.

LAPM (Link Access Procedure for Modems) - Pronounced lap em. An error control mechanism defined in the ITU-T V.42 Recommendation, LAPM employs a cyclic redundancy check (CRC) for error correction and automatic repeat request (ARQ) for error correction. In the event the LAPM fails, a V.42 modem falls back to the similar, but slower, Microcom Networking Protocol version 4 (MNP4). See also ARQ, CRC, error control, ITU-T, and V.42.

large business enterprise (LBE) - See LBE.

laser (light amplification by stimulated emission of radiation) - A device that produces an intense, coherent, collimated, focused, and nearly monochromatic beam of radiated optical energy by stimulating electronic, ionic, or molecular transitions to lower energy levels. A laser comprises an active medium, or gain medium, and a resonant cavity. An external power source, or pump, in the form of electricity or another laser, energizes the gain medium, which absorbs the energy. Some of the particles in the gain medium are excited into quantum high-energy states.When a critical level of energy is achieved, a light signal passing through the medium produces more optical energy than is absorbed, and the signal is greatly amplified.The resulting radiated optical signal is highly coherent, i.e., consistent in phase and polarization, and virtually monochromatic.Through a resonating cavity and either mirrors or a diffraction grating, the signal is narrowly channeled and collimated, i.e., the rays are lined up so that they are virtually parallel. Fiber optic transmission systems (FOTS) in long haul applications employ semiconductor diode lasers, generally Fabry-Perot lasers or distributed feedback (DFB) lasers. Short haul transmission systems such as those associated with local area networks (LANs) more commonly employ light-emitting diodes (LEDs) or vertical cavity surface-emitting lasers (VCSELs) as light sources. (Note:The laser was patented by AT&T Bell Telephones in 1960 as the optical maser.) See also coherence, collimation, DFB laser, Fabry-Perot laser, LED, maser, pump laser, radiation, and VCSEL.

laser diode - A type of light source that resembles a light-emitting diode (LED) in structure, although much more difficult and expensive to manufacture, much less durable, and much more capable. Laser diodes are associated with more expensive and complex supporting electronics, but generally have much faster cycle times and, therefore, offer much more bandwidth. Diode lasers offer significant mechanical and optical coupling efficiency.Therefore, they can mechanically couple to a singlemode fiber (SMF) and can tightly focus a high-speed optical signal for presentation to its core, which has a diameter of only 5–10 microns. Diode lasers also are capable of generating tightly defined optical signals in very small spectral ranges, or windows. Diode lasers also generate signals at wavelengths longer than 850 nm. In these higher transmission windows at 1310 nm or 1550 nm, the signals attenuate much less and, therefore, can travel much farther without being repeated or amplified. In long-haul, high speed, carrier-class fiber optic transmission systems (FOTS), these narrowly defined windows allow the multiplexing of a number of wavelengths through a process known as wavelength division multiplexing (WDM). Fabry-Perot lasers and distributed feedback (DFB) lasers are two specific types of diode laser commonly used in such networks. See also attenuation, bandwidth, cycle time, DFB laser, Fabry-Perot laser, FOTS, laser, LED, SMF,VCSEL, wavelength, window, and WDM.

last mile - Generally referring to the telco local loop, which is the link between the central office (CO) at the edge of the telco network and the user premises. In a broader contemporary context, the term applies to the physical connection between the edge of any service provider’s network and the end user’s premises. In practice, the last mile is often much longer than a mile. In the United States, UTP local loops are generally 12,000 feet or less, but often are as long as 18,000 feet. Passive optical network (PON) standards allow for local loops as long as 12 miles (20 km).Whether it is the first mile or the last mile, which is a matter of perspective, it is seldom exactly a mile. Sometimes referred to as the first mile. See also central office, local loop, and PON.

last-in-first-out (LIFO) - See LIFO.

LATA (Local Access and Transport Area) - A geographical area defined by the United States Federal Communications Commission (FCC) as a result of the Modified Final Judgement (MFJ) that broke up the AT&T Bell System on January 1, 1984. The Regional Bell Operating Companies (RBOCs) and their component Bell Operating Companies (BOCs) were prevented from offering interLATA toll services, i.e., long distance calling services that crossed LATA boundaries. Initially, the BOCs had the exclusive rights to offer intraLATA toll service, also known as local long distance, within the confines of the 196 defined LATAs. LATAs now serve primarily as reference points for call rating and routing. See also BOC, FCC, MFJ, and RBOC.

latency - Delay.The total time required for a signal to travel from one point to another, generally from a transmitter through a network to a receiver. Propagation delay, a fundamental factor in latency, is dependent on the nature of the electromagnetic signal, as not all signals travel through a medium at the same speed. Propagation delay also is influenced by the distance between the two points, the density of the medium, the presence of passive devices such as loading coils that might increase the impedance of the medium. Latency also is affected by any processing time associated with devices such as repeaters, transponders, concentrators, multiplexers, switches, and routers as they variously transmit and retransmit, amplify and reamplify, time and retime, shape and reshape, code and decode, compress and decompress, encrypt and decrypt, and otherwise process signals and manipulate data. Latency also is affected by any time that data packets spend in queues due to issues of network congestion and any time required to retransmit packets errored or lost in transit. See also jitter.

Latin-American and Caribbean Network Information Center (LACNIC) - See LACNIC.

launch cable - A jumper cable of known good quality that is attached to a light source on one end and a fiber under test on the other end, calibrated for output power, and used for testing optical loss in a fiber optic cable.The launch fiber provides an opportunity for the signal to achieve modal equilibrium, i.e., even power distribution across all modes, and eliminates any possibility that fiber anomalies near the light source can affect the test results.A mandrel wrap sometimes is used with a multi-mode fiber (MMF) launch cable to assist in the achievement of modal equilibrium. Synonymous with injection fiber. See also jumper, mandrel wrapping, MMF, modal equilibrium, optical fiber, power, and source.

law - A statement of scientific fact, phenomena, or relationships that occur with unvarying uniformity under given conditions. See also theory.

lay length - The physical length of something laid flat.The lay length of a twisted pair cable, for example, refers to the length of the individual conductors if the pairs in the cable were to be separated and untwisted, and the individual conductors were to be laid flat. In a Category 5 (Cat 5) data grade cable, for example, each of the four pairs has a slightly different twist ratio, i.e., each pair is twisted slightly more or less than any other pair in the cable.Twisted, the pairs are the same length; untwisted and laid flat, they are not. See also Cat 5 and twisted pair.

Layer 1 - Referring to the Physical Layer of the OSI Reference Model. See also OSI Reference Model and Physical Layer.

Layer 2 Tunneling Protocol (L2TP) - See L2TP.

Layer 2 - Referring to the Data Link Layer of the OSI Reference Model. See also Data Link Layer and OSI Reference Model.

Layer 3 - Referring to the Network Layer of the OSI Reference Model. See also Network Layer and OSI Reference Model.

Layer 4 - Referring to the Transport Layer of the OSI Reference Model. See also OSI Reference Model and Transport Layer.

Layer 5 - Referring to the Session Layer of the OSI Reference Model. See also OSI Reference Model and Session Layer.

Layer 6 - Referring to the Presentation Layer of the OSI Reference Model. See also OSI Reference Model and Presentation Layer.

Layer 7 - Referring to the Application Layer of the OSI Reference Model. See also Application Layer and OSI Reference Model.

layer - Referring to the protocol or protocols operating at a particular level within a network architecture. Such an architecture commonly is detailed in a protocol stack, such as the OSI Reference Model, or protocol suite, such as the TCP/IP protocol suite.The OSI Reference Model is a full seven-layer stack, of which the top layer, the Application Layer, addresses applications and end user processes.The bottom layer, the Physical Layer, deals with physical and mechanical aspects of the interface between a device and a transmission medium. Although each layer addresses different functions and responsibilities, the layers work together, as a whole, to enable an application or end user process. See also application,Application Layer, OSI Reference Model, Physical Layer, protocol, and TCP/IP.

LBE (Large Business Enterprise) - A large commercial (i.e., for-profit) organization.There is no formal definition of the size of an LBE, although it would be larger than a medium enterprise, which generally is defined as having less than 250 employees. See also SME.

LCP (Link Control Protocol) - Referring to a protocol responsible for negotiating Link Layer (Layer 2) details.The Point-to-Point Protocol (PPP), for example, is based on a Link Control Protocol and a Network Control Protocol (NCP).The LCP is responsible for setting up a link between two computers over a circuit-switched telephone connection, and for resolving any issues of authentication.The NCP negotiates any parameters specific to the Network Layer. See also authentication, Data Link Layer, IP, link, NCP, Network Layer, PPP, and protocol.

LCR (Least Cost Routing) - Synonymous with automatic route selection (ARS). See ARS.

LD-CELP (Low Delay Code Excited Linear Prediction) - A voice compression algorithm defined in ITU-T G.728, and used in voice over frame relay (VoFR) and voice over Internet Protocol (VoIP). LDCELP is geared to a rate of 16 kbps, although bit rates as low as 12.8 kbps can be achieved.The low level of delay suggested by the designation is due to the fact LD-CELP accumulates only 5 PCM samples, representing 625µs of a voice stream, in a buffer. A key element of CELP and its variants is the construction and maintenance of a codebook, which comprises binary descriptions of sets of voice samples. CELP employs silence suppression to remove periods of silence and redundancy from the data set, and normalizes the volume level. LD-CELP compares the resulting data set to a set of candidate shapes in the codebook, selects the shape that most closely match the actual data. LD-CELP then transmits the index number of the selected code description and the average loudness level of the set of samples. Every 625µs, the code is sent across the network in a block of 10 bits, yielding a data rate of 16 kbps, which is a compression ratio of 4:1, compared with toll quality PCM voice over the public switched telephone network (PSTN) at 64 kbps. At the receiving end of the transmission, the transmitted code is compared to the codebook, the PCM signal is reconstructed, and, eventually, the analog signal is reconstructed.As compared to CELP, the more frequent transmission of the shorter data blocks yields lower levels of delay through faster processing by the digital signal processors (DSPs), and the compression technique yields more efficient use of bandwidth. LD-CELP yields quality that generally is considered to be on a par with Adaptive Differential Pulse Code Modulation (ADPCM). See also ADPCM, algorithm, analog, binary, buffer, CELP, circuit switching, compression, CS-ACELP, DSP, PCM, PSTN, silence suppression, and toll quality.

LDAP (Lightweight Directory Access Protocol) - A subset of X.500 that can run over TCP/IP networks, LDAP was developed to simplify the demands of the X.500 DAP. Described in the IETF RFCs 1777, LDAP can run as a standalone directory system or can be used as a means of accessing an X.500 directory or other directory. LDAPv3 (RFC 3377) supports non-ASCII and non-English characters for international directories, and can sort through multiple directories on the basis of a single request. See also X.500.

LDM (Limited Distance Modem) - See short haul modem.

LDP (Label Distribution Protocol) - In Multiprotocol Label Switching (MPLS), a protocol used by a Label Edge Router (LER) to distribute labels, or tags, to each Label Switching Router (LSR) in the network core, identifying the treatment that should be afforded all packets in the flow on that particular Label Switched Path (LSP). See also core, flow, label, LER, LSP, LSR, packet, and protocol.

lead balloon - There is no such thing. There was, however, a great rock band in the 1960s and 1970s known as Led Zeppelin, but it’s not the same thing. See also paperless office.

leaky mode - A physical path traveled by a leaky wave in an optical waveguide, such as an optical fiber. A leaky mode has an electromagnetic field that decays monotonically (i.e., steadily) for a finite distance in the transverse direction (i.e., at a 90 degree angle to the plane or axis of the fiber), but becomes oscillatory beyond that finite distance. See also leaky wave.

leaky wave - In an optical fiber, an electromagnetic wave that travels outside the core and in the cladding or even beyond, usually as a result of an incident wave injected into the core at a severe angle less than the critical angle. Leaky waves are detached from the main body of the signal, are not guided, and, therefore, are valueless. Leaky waves travel high-order modes; waves traveling low-order modes generally remain confined within the core. See also cladding, core, critical angle, high-order mode, and low-order mode.

least cost routing (LCR) - Synonymous with automatic route selection (ARS). See ARS.

least significant bit (LSB) - In a binary number or bit sequence comprising one or more bytes, the loworder (i.e., least) bit in the sequence.The LSB conveys the least amount of information, and usually is the right-most bit.The approach is known as big-endian. In some data representation schemes, the LSB is the left-most bit.This approach is known as little-endian. See also big-endian, bit, bit robbing, byte, endianess, and little-endian.

LEC - 1. Local Exchange Carrier. A company providing local telephone service. The incumbent LEC (ILEC) is the local telephony company (telco) in place prior to competition, which introduced one or more competitive LECs (CLECs). See also carrier. 2. LAN Emulation Client. In LANE (LAN Emulation), an end system or endpoint.The LEC connects to the ATM network over a LANE user-to-network interface (LUNI). See also ATM, LANE, and LUNI.

LECS (LAN Emulation Configuration Server) - In LANE (LAN Emulation), an optional networkbased server that assigns a LAN emulation client (LEC) to an emulated LAN (ELAN). See also LANE and LEC.

LED (Light-Emitting Diode) - A semiconductor light source in the form of a transistor with a positive and a negative layer of particular semiconducting materials and a p-n (positive-negative) junction between them.The negative layer is doped with impurities to create extra electrons, which are negatively charged.The positive layer is doped to create extra holes into which electrons can migrate when a charge is applied, which has the effect of adding extra positive particles.When current is applied and the electrons move across the junction, from the n semiconductor layer to the p semiconductor layer, and settle into the holes, they release energy in the form of photons, i.e., light. (Note: A diode conducts current in only one direction, like a one-way gate.) The composition of the semiconductor material determines the color of light, how much of it is absorbed, and how much of it is released. LEDs manufactured with aluminum gallium arsenide (AIGaAs) are used in infrared (IR) applications such as fiber optics.Various other compounds, most including gallium (Ga), to create other colors in the visible and ultraviolet (UV) spectrum.Visible light-emitting diodes (VLEDs) operate in the visible light spectrum, and are found in clocks, watches, calculators, gauges, meters, and a wide variety of other devices.The infrared LEDs used in fiber optic transmission are, of course, much more sophisticated. LEDs pulse on and off relatively slowly, as specified by the cycle time, i.e., rise and fall times of signal intensity. Therefore, LEDs are relatively bandwidth-limited. LEDs also generate signals of relatively broad spectral width in the 850 nm region (850 Band), which wavelength attenuates substantially over relatively short distances. Slower LEDs emit light from an area etched into the surface of a semiconductor chip, while the faster LEDs emit light from the edge of the chip. The physical design of LEDs is such that they couple efficiently only to the relatively broad inner core of multimode fiber (MMF), and they do not tightly focus a collimated beam of light as does a laser diode. LEDs are used in local area networks (LANs), where they support transmission rates of up to 1 Gbps over relatively short distances. LEDs also are used in certain other short-haul transmission systems, including some passive optical networks (PONs). LEDs are relatively inexpensive and durable. See also 850 Band, coupling efficiency, cycle time, diode, IR, LAN, laser diode, MMF, PON, spectral width, transistor, UV, VCSEL, visible light, VLED, wavelength, and window.

leg - A segment or portion of an end-to-end path associated with a call. For example, an international voice telephone call via the public switched telephone network (PSTN) has an originating leg supported by a local exchange carrier (LEC) in one nation, a terminating leg supported by a LEC in another nation, and an international leg supported by an interexchange carrier (IXC) that interconnects the two national LECs. See also IXC, LEC, and PSTN.

legacy - Referring to a software or hardware component or element, a system, or a network that is technically outdated, although often compatible with current technology. In consideration of the evolutionary nature of technology, systems and standards development generally places considerable emphasis on backward compatibility, i.e., compatibility with legacy technology.

Lemple-Ziv (LZ) - See LZ.

Lemple-Ziv-Welch (LZW) - See LZW.

LEO (Low Earth Orbit) - A satellite or satellite constellation (i.e., system) operating at an altitude of 644–2,415 kilometers. Although the term is not precisely defined, Little LEOs involve a relatively small number of satellites, and operate at frequencies below 1 GHz in support of low bit-rate data traffic, such as telemetry, vehicle messaging, and personal messaging. Big LEOs are bigger networks that operate at higher frequencies in support of voice and higher-speed data communications. Unlike a geosynchronous Earth orbiting (GEO) satellite, LEO and MEO (Middle Earth Orbit) satellites do not remain in a fixed position relative to the Earth’s surface, so are referred to as mobile satellite systems (MSSs), as opposed to the fixed satellite systems (FSSs) in geostatic orbit. See also FSS, GEO, MEO, MSS, and satellite.

LER (Label Edge Router) - In Multiprotocol Label Switching (MPLS), a router at the edge of the carrier’s network.The ingress LER examines the packet header and attaches a label, or tag, that identifies the Label Switched Path (LSP) that the packet is to travel through the MPLS network.The label is distributed to Label Switching Routers (LSRs) in the network core to ensure that the packet travels that path, link by link.The egress LER strips the tag away as the packet exits the network. See also header, label, link, LSP, LSR, MPLS, packet, path, router, and switch.

LES (LAN Emulation Server) - In LANE, a network-based server to which a LAN emulation client (LEC) connects to the ATM network over a LANE user-to-network interface (LUNI).The LES registers the LAN medium access control (MAC) addresses and translates them into ATM addresses using the address resolution protocol (ARP). Each LEC is assigned to an emulated LAN (ELAN) by an optional network-based LAN emulation configuration server (LECS). See also ARP, ATM, ELAN, LANE, LEC, LECS, LUNI, MAC, and server.

leverage - A euphemism for reuse. A considerable number of definitions in this book are leveraged from other books I have written for Wiley. I spent so much time writing these beautifully worded definitions over the last 10 years that I figured there was no point in trying to reword them and twist them out of shape in the process. Some things just don’t make sense. See also euphemism.

LF (Low Frequency) - LF radio is in the frequency range of 30–300 kHz and has a wavelength of 10 km – 1 km. LF radio has applications in navigation, maritime communications, information and weather systems, and time systems. See also electromagnetic spectrum, frequency, Hz, and wavelength.

liberalize - Referring to reforming the telecommunications environment by reducing or eliminating the monopoly of the national carrier and creating a competitive environment. Liberalization generally is associated with some level of privatization, which involves transferring all or some portion of the telephone utility from government to private ownership. See also privatize and utility.

lifeline service - Referring to discounted basic telephone service provided to low-income subscribers that meet certain criteria. In the United States, the federal Universal Service Fund (USF) subsidizes the Link-Up Program (installation charges) and Lifeline Program (monthly service charges) for subscribers who have an income that is at or below 135 percent of the federal poverty guidelines or who participate in any of a number of federal assistance programs. See also USF.

lifetime control - Referring to Network Layer protocol mechanisms that limit the life of a packet.The Internet Protocol (IP) and Connectionless Network Protocol (CLNP) packets both contain a time-to-live (TTL) field for this purpose. See also CLNP, IP, Network Layer, packet, protocol, and TTL.

LIFO (Last-In-First-Out) - A buffering method in which the entity that first exits the buffer is the one that last entered.Thereby, the entity served (e.g., processed or switched) is the one that waited the shortest period of time. LIFO is commonly used in file systems and e-mail systems, as the last file or correspondence saved is the first retrieved, or at least appears first in the stack. See also FIFO and queue.

light carrier - A company that leases, rather than owns, the network facilities it uses to provide telecommunications transport services. A heavy carrier is a facilities-based carrier, i.e., a company that owns the switching and transmission systems that compose the network it uses to provide transport services. See also carrier.

light detector - A device used in an optical transmission system to detect an optical signal generated by a light source and propagating through a medium.A light detector essentially is an optical receiver that is paired with an optical transmitter, both of which are connected to electrically based devices or systems. So, the source converts electrons to photons and the detector converts photons to electrons. Detectors take several forms. A positive-intrinsic-negative (PIN) diode detector is paired with a light-emitting diode (LED) or vertical cavity surface-emitting laser (VCSEL) light source over a multimode fiber (MMF) in a fiber optic transmission system (FOTS) or a free space optics (FSO) system. An avalanche photodiode (APD) is paired with a laser diode in a single-mode fiber (SMF) system. See also APD, FOTS, FSO, laser diode, LED, light source, PIN, and VCSEL.

light source - The source of the optical signal in an optical transmission system, which can take the form of a fiber optic transmission system (FOTS) or free space optics (FSO) system. Light-emitting diodes (LEDs) are used in multimode fiber (MMF), which usually is glass optical fiber (GOF), but also can be plastic optical fiber (POF).Vertical cavity surface-emitting lasers (VCSELs) also can be used in MMF systems. Laser diodes are used in single-mode fiber (SMF) systems.Virtually all light sources emit signals in the infrared (IR) spectrum.The exception is LEDs used in POF systems, which operate in the red region of the visible light spectrum.A light source essentially is an optical transmitter that is paired with an optical receiver, both of which are connected to electrically based devices or systems. So, the source converts electrons to photons and the detector converts photons to electrons. See also laser diode, LED, light detector, FOTS, FSO, GOF, MMF, POF, SMF, and VCSEL.

light-emitting diode (LED) - See LED.

light - Electromagnetic energy with a waveform having a frequency above the upper limit of the radio range of 300 GHz and equal or less than the lower limit of the X-ray range of 30 PHz.At the low end of the range is infrared (IR) light, which operates at 30–300 Hz, and at the upper end of the range is extremely high frequency (EHF) radio. See also electromagnetic spectrum, frequency, and Hz.

lightguide - An optical fiber. See optical fiber.

Lightweight Directory Access Protocol (LDAP) - See LDAP.

limited distance modem (LDM) - See short haul modem.

line coding - The manner in which data bits, or blocks of data bits, are represented on a line. Examples include 4B3T, 4B/5B, alternate mark inversion (AMI), and Manchester. See also 4B3T, 4B/5B, AMI, Manchester coding, and quadbit.

line discipline - In the context of a protocol, the sequence of network operations between devices that actually transmits and receives the data, controls errors in transmission, deals with the sequencing of message sets, and provides for confirmation or validation of data received. See also handshaking and protocol.

line doubler - Also known as line extender. A device that more or less doubles the maximum physical reach of a digital subscriber line (DSL) or ISDN service. A line doubler essentially is a repeater. See also HDSL, repeater, and xDSL.

line driver - A type of interface converter used to extend the distance of a digital connection by converting the digital signal to a low-voltage, low-impedance signal that can transmit more effectively and over longer distances on dedicated, specially conditioned twisted-pair circuits.The RS-232 specification (more correctly known as EIA-232), for example, generally limits the distance between devices to 50 feet at transmission rates of 56 kbps.At lower speeds, line drivers can reshape the digital pulses to extend that distance considerably.At speeds of up to 9.6 kbps, for example, line drivers can extend that limitation to 500–5,000 feet over Category 3 (Cat 3) unshielded twisted pair (UTP). Line drivers are unidirectional and operate over simplex circuits. Line drivers can generically be classified as modems, as they change the format of the signal. See also Cat 3, EIA-232, impedance, modem, signal, simplex, and voltage.

line extender - Also known as line doubler.A device that extends, and more or less doubles, the maximum physical reach of a digital subscriber line (DSL) service. A line extender essentially is a DSL repeater. See also xDSL and repeater.

line finder - A component of an electromechanical step-by-step (SxS) or panel switch, or electromagnetic crossbar (Xbar) circuit switch, that identifies lines that go off-hook to request service. A line finder allows many lines (e.g., 100 or so) to share a bank (10 or so) of selectors, depending on the activity levels of the individual subscribers. See also line, off-hook, panel switch, selector, SxS, and Xbar.

line frequency - Also known as lines per inch (lpi). See lpi.

line interface - The total of hardware and firmware that serves to interconnect a line and a switch, router, or other device, and to facilitate their interoperation. Such interfaces are specific to the physical layer protocols, which address such factors as transmission medium, physical dimensions of the medium, signal frequency and wavelength, signal format, and signaling speed. See also firmware, hardware, line, physical layer, and protocol.

line number - The trailing digits of a telephone number. As defined by the North American Numbering Plan (NANP), the line number can consist of any four digits (XXXX), and corresponds with a port on a switch that connects to a circuit or channel over a local loop that serves the physical premises of a subscriber. In the case of a cellular telephone number, there is no local loop, as such, and the number is associated with a handset, rather than a physical premises. See also NANP.

line of sight (LOS) - See LOS.

Line Overhead (LOH) - See LOH.

line powered - In reference to equipment that is electrically powered by the telecommunications circuit to which it connects, thereby eliminating the need for local power.The analog single line local loop circuits in the public switched telephone network (PSTN) are powered from batteries in the central office (CO) at -48 volts DC (Direct Current), which is sufficient to power conversation over a simple analog telephone set. Ringing current is 110 volts AC (Alternating Current) at 20 Hz in North America.As the typical telephone company CO has an uninterruptible power supply (UPS) comprising multiple power utility circuits, substantial battery packs, and backup diesel generators, the typical analog residential telephone still works even if the lights go out.The telephone companies are said to be the largest power utilities in the world, although that is not the focus of their business. See also UPS.

line setup - A very basic protocol issue addressing the manner in which a circuit is set up between devices.There are three alternatives: simplex, half-duplex, and full duplex. See also simplex, half duplex, and full duplex.

line sharing - 1. A technique by which two voice grade services and two different telephone numbers can coexist on the same local loop through frequency division multiplexing (FDM).The two numbers can be associated with two different voice numbers or one number can be a fax number and another a voice number, with distinctive ringing patterns distinguishing fax calls from voice calls.The fax machine can be programmed to recognize the fax ringing pattern and to answer those calls automatically. See also distinctive ringing, FDM, local loop, and voice grade. 2. A technique defined by the Federal Communications Commission (FCC) by which an incumbent local exchange carrier (ILEC) provides voice grade telephone service over a local loop, and shares that loop with a competitive local exchange carrier (CLEC) that provides digital subscriber line (DSL) service. Line sharing is accomplished through frequency division multiplexing (FDM). See also CLEC, FCC, FDM, ILEC, local loop, and voice grade.

line side - In telephone company (telco) terminology, line describes the user side or local loop side of the central office (CO).The customer-facing side of the public switched telephone network (PSTN). In other words, the line side is the side of the network to which users connect in order to access the network core, or backbone.The line side is synonymous with the local loop access circuits, with the demarcation point being the local loop interface at the CO.The trunk side involves the high-capacity trunks that serve to interconnect the various telco switching centers in the core of the carrier network. See also CO, line, local loop, PSTN, and trunk side.

line status lamp - A lamp that indicates the status of a line, usually steadily lit to indicate a line in use and flashing to indicate a line on hold. Line status lamps are used in key telephone systems (KTSs), where they may appear on an individual telephone set or a busy lamp field. See also busy lamp field and KTS.

line-switched ring - A SONET/SDH topology that employs either two or four fibers.The single-ring configuration comprises two fibers, one of which is active and the other of which is held in reserve.Traffic moves in one direction across the active fiber. In the event of a network failure, such as a node failure or a cable cut, the backup ring is activated to enable transmission in the reverse direction, as well.A fourfiber configuration duplicates this approach in a second ring. Line-switched rings up to 1,200 kilometers in route distance offer standard restoral intervals of 50 milliseconds or less. See also optical fiber, path-switched ring, SDH, SONET, and topology.

line - 1. A station line refers to the circuit between a private branch exchange (PBX) switch and a station user’s terminal equipment, which usually is in the form of telephone, although it could be a computer workstation, a printer, a facsimile machine, or some other device. 2. In rate and tariff terminology, line refers to a local loop connection from the telephone company central office (CO) switch to the user premises in support of customer premises equipment (CPE) other than a switch. Such CPE can be in the form of a single-line residence or business set, a multiline set, or the common control unit of a key telephone system (KTS). Such a line is single-channel in nature, i.e., supports a single conversation and is voice grade, i.e., provides enough bandwidth to support a voice conversation, and has a single associated telephone number.A line may be thought of as a tributary of a trunk. See also line side, trunk, and trunk side.

linear predictive coding (LPC) - See LPC.

lines per inch (lpi) - See lpi.

link access procedure for frame mode services (LAPF) - See LAPF.

link access procedure for modems (LAPM) - See LAPM.

link access procedure, balanced (LAPB) - See LAPB.

link access procedure, D channel (LAPD) - See LAPD.

Link Control Protocol (LCP) - See LCP.

link rot - The decomposition of hyperlinks as the linked sites are renamed, moved, or withdrawn from the Web.As the Web is a dynamic application, hyperlinks must be continuously updated to keep them fresh and viable. See also hyperlink.

link-local address - In Internet Protocol version 6 (IPv6), a type of unicast address intended for local use, only.A link-local address is similar to an IPv4 private IP address, as it is not meant to be routed and not intended for use in a public domain such as the Internet, but confined to a single link, or LAN segment. See also domain, Internet, IPv4, IPv6, LAN, private IP address, site–local address, unicast, and unicast address.

link-state protocol - A routing protocol that calls for each router to build a database of the names of its neighboring routers and the cost to connect to each. Once all routers in a network have done so, each has a map of the entire network and can calculate the costs of each available route, from end to end. Open Shortest Path First (OSPF) is an example of a link-state protocol. See also database, OSPF, protocol, and router.

link - 1. A two-point segment of an end-to-end physical circuit.A circuit may consist of a single link, as would be the case between a host computer and a directly attached peripheral, such as a printer. A circuit commonly comprises multiple links. For example, a telephone set may connect across a link to a central office switch at the edge of the carrier network, that central office switch may connect to another central office switch across a link, and to yet another central office switch across a link, and finally to another telephone set across a link. In this scenario, two terminal devices connect via an end-to-end circuit that comprises four links interconnected by three central offices. Link sometimes is used interchangeably with line or circuit. 2. A conceptual two-point segment of an end-to-end circuit that connects two end users and enables them to communicate, even when two separate physical paths are used. In a satellite radio link, for example, there is an uplink from the Earth station (i.e., antenna) to the satellite and a downlink from the satellite to the Earth station. In a cellular network, the uplink is the upstream radio link from the mobile station to the base station and the downlink is the downstream link from the base station to the mobile station. See also antenna, circuit, downlink, downstream, uplink, physical, and upstream. 3. In hypertext, the hyperlink, or logical connection between discrete data elements. See also hyperlink , hypertext, and link rot. 4. A logical connection, association, or relationship between two or more things.

Linux (Linus Unix) - A UNIX-like computer operating system (OS) developed by Linus Torvalds, and numerous collaborators worldwide, that was designed to run on PCs powered by Intel processors. Linux is free, open source software that anyone can modify, although at one’s own risk. Many companies package the Linux kernel with a number of utilities and other programs into commercial versions that include documentation and support. See also kernel, open source, OS, program, UNIX, and utility.

LIR (Local Internet Registry) - The local organization responsible for assigning Internet Protocol (IP) addresses to Internet Service Providers (ISPs). The LIR receives IP address assignments from a National Internet Registry (NIR) or Regional Internet Registry (RIR) that receives address assignments from the Internet Assigned Numbers Authority (IANA). See also IANA, IP, IP address, ISP, NIR, and RIR.

LIS (Logical IP Subnetwork) - In Classical IP (CIP) over ATM, the term for a virtual LAN (VLAN). See also CIP and VLAN.

little-endian - Referring to the orientation of a computer system, application, or network design with respect to the placement of most significant bit or digit in a coding scheme. Little-endian places the most significant bit, digit, or byte in the last, or rightmost, position. Big-endian places the most significant bit, digit, or byte in the first, or leftmost, position. Bi-endian systems can work either way. Motorola processors employ the big-endian approach, whereas Intel processors take the little-endian approach.Telephone numbers, for example, are big-endian, beginning with a country code, followed by an area code, a central office prefix, and a line number. Bit robbing in T1 systems involves the least significant bit (LSB), which is the eighth bit in a byte, which is a little-endian approach.The terms derive from Jonathan Swift’s Gulliver’s Travels, in which the Big-Endians were a faction of people on the islands of Lilliput and Blefuscu who defied the emperor’s decree that soft-boiled eggs should be broken at the small end before being consumed. See also bi-endian, big-endian, bit, byte, endianess, and LSB.

liveware - Slang for people, distinguishing them from software, firmware, and hardware. Synonymous with grayware. See also firmware, grayware, hardware, and software.

LLC (Logical Link Control) - According to the IEEE model, the upper sublayer of the Data Link Layer, described in the OSI Reference Model.The LLC sublayer is concerned with issues of multiplexing, flow control, and detection and retransmission of dropped frames.The IEEE 802.2 Working Group sets standards and develops recommendations for LLC. See also 802.2, Data Link Layer, flow control, IEEE, multiplexer, and OSI Reference Model.

LLU (Local Loop Unbundling) - See unbundled service.

LMDS (Local Multipoint Distribution Service) - A wireless local loop (WLL) technology developed by Bernard B. Broussard for wireless cable television (TV), referring to premium wireless subscription TV rather than traditional free broadcast TV or cable TV. Broussard, with Shant and Vahak Hovnanian, formed a firm that provided 49 TV channels in New York City, and later added high speed Internet access.The technical rights to LMDS technology later were spun off into a separate company, and the U.S. Federal Communications Commission (FCC) auctioned the first LMDS radio licenses in early 1998.The A Block has a width of 1.15 GHz in the frequency bands of 27.5–28.35 GHz, 29.1–29.25 GHz, and 31.0–31.15 GHz.The B Block has a width of 150 MHz in the spectrum between 31.15 GHz and 31.3 GHz. Outside of North America, LMDS operates in the 20 GHz and 45 GHz bands. LMDS requires line of sight (LOS), has a maximum cell diameter of 10–15 miles, and compensates for rain fade through the use of adaptive power controls. LMDS can carve a 360° cell into four quadrants of alternating antenna polarity, and supports both point-to-point and point-to-multipoint service configurations. LMDS is flexible enough to support local loops ranging from 1.544 Mbps (T1) to 622 Mbps, and in either symmetric or asymmetric configurations. Multiplexing access methods include frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA).TDMA modulation options include phase modulation (BPSK, DQPSK, QPSK, and 8PSK) and amplitude modulation (QAM, 16-QAM, and 64-QAM). LMDS is considered obsolete in the United States, having been replaced by the IEEE 802.16 standards, commonly known as WiMAX. See also 8-PSK, 802.16, asymmetric, antenna, BPSK, CDMA, DQPSK, FCC, FDMA, frequency band, LOS, phase modulation, point-to-multipoint circuit, point-to-point circuit, polarity, QPSK, rain fade, spectrum, symmetric, TDMA, WiMAX, and WLL.

LMI (Local Management Interface) - In frame relay, a protocol that provides operational support for the user network interface (UNI).The LMI is a polling protocol between the frame relay access device (FRAD) and the network, which periodically verifies the existence and availability of the permanent virtual circuit (PVC), as well as the integrity of the UNI link. See also FRAD, frame relay, link, polling, protocol, PVC, and UNI.

LNP (Local Number Portability) - Referring to the ability to port, i.e., move, a local telephone number from one service or service provider, i.e., local exchange carrier (LEC), to another in a competitive environment. In the United States, the Telecommunications Act of 1996 mandated LNP, and established the Local Number Portability Administration (LNPA) to oversee the development and deployment of the necessary mechanisms. LNP is accomplished through the use of local routing numbers (LRNs) that point the originating exchange to the correct terminating carrier and exchange. LNP eventually is expected to support geographic portability. In 2003, the Federal Communications Commission (FCC) required wireless number portability (WNP), extending portability to cellular telephone numbers. LNP and LRNs are supervised by the Number Portability Administration Center (NPAC). See also LEC, LRN, Telecommunications Act of 1996, and WNP.

LNPA (Local Number Portability Administration) - The organization formed to oversee the development and deployment of a mechanism for Local Number Portability (LNP). See also LNP.

load balancing - The process of distributing a load across multiple resources. For example, traffic can be distributed across multiple circuits to multiple switches or routers and, therefore, across multiple physical paths in order to mitigate issues of network congestion and enhance overall network performance. Or, competition for computational resources can be mitigated if a computing load is distributed among multiple processors. Local area network (LAN) segmentation can reduce congestion and, therefore, improve overall network performance through the application of routing bridges, switches, and routers that variously restrict traffic to physical segments and distribute traffic across multiple physical paths. See also network management and traffic.

load coil - See loading coil.

load - 1.The amount of traffic placed on a circuit or system. 2. In reference to a load coil, or loading coil, which acts as a lumped inductor to compensate for capacitance on a long twisted pair copper circuit. See also loading coil. 3. The amount of force or weight on a cable. Dynamic load refers to the short-term load placed on a cable during installation. Static load refers to the long-term load placed on a cable, such as a riser cable, which hangs vertically. See also bend diameter and tensile strength.

loading coil - A toroidal (i.e., ring shaped or donut-shaped) device comprising a powdered iron core, or sometimes a soft iron wire core, around which copper wire is wound. A loading coil is spliced into an unshielded twisted pair (UTP) copper local loop, where it functions as a lumped inductor, which is to say that at a specific point in the circuit the process of inductance takes place, to compensate for the distributed capacitance between the two parallel wires. In effect, the loading coil tunes the copper circuit, optimizing it for mid-voiceband performance.The loading coil also functions as a low-pass filter, increasing loss above the voiceband cutoff frequency of 4 kHz, while reducing mid-voiceband attenuation by as much as 80 percent. Loading coils are passive, i.e., not electrically powered, devices commonly placed on local loops that exceed approximately 18,000 feet (5.5 km) in length.The first loading coil is placed approximately 3,000 feet (.9 km) from the central office (CO) and at intervals of 6,000 feet (1.8 km) or so, thereafter.The presence of loading coils renders local loops unusable for ADSL, ISDN,T-carrier and other loops operating at high data rates, as they filter out the high frequencies associated with those higher data rates. Where such services are to be deployed, the local loops must be properly conditioned, which entails removing the loading coils, bridged taps, and other impediments.The presence of a loading coil also has the effects of increasing the impedance of the circuit, which significantly reduces the velocity of propagation (Vp), i.e., speed of signal propagation, to 10,000–12,000 miles per second.This speed penalty is not of particular significance in short voice grade local loops. If the loops are long, however, loading coils can create unacceptable problems with echo, or signal reflection. See also bridged taps, capacitance, conditioning, echo, impedance, inductance, propagation, and Vp.

Local Access and Transport Area (LATA) - See LATA.

local area network (LAN) - See LAN.

local battery - A battery that provides electrical power to stations, or terminal devices, as distinguished from the common battery that provides loop current, i.e., electrical energy to the line. See also battery, common battery, electricity, and loop current.

local call - An imprecise term describing a telephone call that is local, rather than long distance, in nature. Such a call incurs no distance-sensitive long distance, or toll, charges, although there may be a charge per call or per minute, perhaps for calling activity that exceeds a threshold.A local call typically is confined to a metropolitan area or a central office exchange (COE) area.

local exchange carrier (LEC) - See LEC.

local exchange Central office exchange (CO or COE) - See CO.

Local Internet Registry (LIR) - See LIR.

local long distance - A rate and tariff term that describes an intraLATA long distance call. See also intraLATA, LATA, long distance, and tariff.

local loop unbundling (LLU) - See unbundled service.

local loop - An access circuit from the network edge to the customer premises, a local loop is a short haul circuit for access to a local exchange.The most common example of a local loop is an electrically based, two-wire, copper access circuit between a telephone company central office (CO) switching center and a residential or small business premises. Such a circuit is provisioned over a single unshielded twisted pair (UTP), within which two wires are required to complete the electrical circuit, with the current in one wire opposite to the current in the other, and with both wires carrying the information signal.The two conductors comprise an electrical loop, with one wire carrying the go signal and the other carrying the electrical return signal. In the broader contemporary sense, any access circuit between the customer premises and the edge of the telco network, or that of any other service provider, is termed a local loop, whether it is electrically based or employs optical or radio energy. See also last mile.

local management interface (LMI) - See LMI.

Local Multipoint Distribution Service (LMDS) - See LMDS.

local number portability (LNP) - See LNP.

Local Number Portability Administration (LNPA) - See LNPA.

local routing number (LRN) - See LRN.

location-based services - Services offered by cellular radio providers that are sensitive the physical location of the terminal device. Such services include descriptions of and directions to restaurants and other retail establishments in proximity. See cellular radio.

log (logarithm) - In mathematics, the exponent expressing the power to which a fixed number (base) must be raised to equal a given number (antilogarithm). Logarithmic functions are the inverses of exponential functions. In an equation expressed as: b ⁿ = x where b and x are known, a logarithm can be used to discover n, and is expressed as n = log _b (x) For example, log ₁₀ (1000) = 3 because 10 ³ = 1,000.

logarithm (log) - See log.

logical IP subnetwork (LIS) - See LIS.

logical reach - The maximum length of a fiber optic link, without regard to the loss budget, or the allowable amount of signal attenuation that the system can withstand. See also attenuation, fiber optic, link, loss budget, physical reach, and signal.

logical topology - In reference to the manner in which devices logically interconnect in a network.The logical topology of a network may differ considerably from the physical topology. LAN and WAN topologies variously include bus, mesh, partial mesh, ring, star, and tree. See also LAN, logical, logical topology, mesh topology, partial mesh, physical, ring, star, tree topology, and WAN.

Logical Unit (LU) - See LU.

logical - 1. Based on facts, rational thought, and clear reasoning. 2. Referring to something that does not exist, but has the appearance or effect of physical presence. In the context of telecommunications, a logical circuit, for example, does not have a physical presence in the sense that it is not tangible. Rather, it is defined as some amount of bandwidth provided over a physical, i.e., tangible, circuit that may support many logical circuits. Similarly, a channel may be in the form of regular time slots provided over a digital circuit.The time slots are brief moments in time during which electromagnetic signals convey bits of information associated with a given data transfer. Such a channel is purely logical, with no material presence and no physical presence except to the extent that the electromagnetic energy is the stuff of the science of physics. See also channel, circuit, digital, physical, physics, signal, and time slot.

login - Synonymous with logon. Referring to the process by which one identifies oneself to a computer, and typically comprising the entry of a user identification (user ID) name and password. See also password.

logodaedaly - Pronounced log-a-DEE-da-lee. From the Greek logos, meaning words, and French legerdemain, meaning sleight of hand, hence, any artful deception or trick.The arbitrary or capricious coinage of words. For example, comatext is defined as text that pleases the person who wrote it, but puts others into a coma.Telecrastination is defined as letting the phone ring more than twice before you pick it up, even if it is only six inches away. I don’t include such funsense in this dictionary.

logon - Synonymous with login. See login.

logophobia - From the Greek logos, meaning words, and phobos, meaning fear. Fear of words. Logophobia symptoms include breathlessness, excessive sweating, nausea, dry mouth, feeling sick, shaking, heart palpitations, inability to think clearly, a fear of dying, becoming mad or losing control, a sensation of detachment from reality, or a full blown anxiety attack. If you are experiencing any of these symptoms, put this book down immediately and seek medical attention. Note: Neither the author nor the publisher assumes any liability whatsoever for any ill effects, real or imagined, for the logophobic experience as a result of reading the words in this book. (Honestly, I don’t make these things up.Well, OK, I made up the part about the liability, but I don’t make all of these things up, at least not entirely.) See also Hellenologophobia.

LOH (Line OverHead) - In a SONET or SDH frame, overhead of 18 octets that controls the reliable transport of payload data in the Synchronous Payload Envelope (SPE) between any two network elements (NEs). LOH and Section Overhead (SOH) between adjacent NEs compose Transport Overhead (TOH). See also frame, link, octet, overhead, SDH, SOH, SONET, and TOH.

long distance - An imprecise rate-and-tariff term that describes a call or circuit that connects two relatively distant parties or systems.A long distance call terminates outside the central office exchange (COE) area, i.e., the serving area of a central office (CO), and outside the municipality. In the United States, an intraLATA long distance call is termed local long distance.

long haul circuit - Within the core, or backbone, of a Wide Area Network (WAN), transport circuits, or long haul circuits, carry data over long distances. Long haul traditionally is defined as distances equal to or greater than 50 miles (80 kilometers). See also access circuit, short haul circuit, and WAN.

long wavelength (LW) - See LW.

long wavelength band (L-Band) - See L-Band.

longitudinal redundancy checking (LRC) - See LRC.

loop current - Referring to the electrical current provided by a common battery and powering the electrical loop in telecommunications applications.The term typically is used in the context of a local loop, and the current typically is 48 volts (V) direct current (DC). See also battery, common battery, DC, local loop, power, and V.

loop start - A signaling technique used in single line, multiline telephones, and key telephone systems (KTSs) to start, or seize the line between the terminal and the central office switch.With loop start signaling, the telco central office switch provides battery.When the handset of the telephone is lifted, the electrical loop is completed and dc current flows across the circuit.The central office switch detects that fact, bridges the line, assigns a register, and returns dial tone for an outgoing call, or connects an incoming call, as appropriate. Loop start also is used in some PBXs, although it is unusual due to issues of glare, or collision between incoming and outgoing calls. See also glare, ground start, loop, and register.

loop - 1. An electrical loop, i.e. closed electrical circuit.The two conductors of an electrical loop compose one wire carrying the go signal and the other carrying the electrical return signal.The circuit is closed and the loop is completed when the conductors are connected. 2. A local loop. In the broader contemporary sense, any access circuit between the customer premises and the edge of the telco network, or that of any other service provider, is termed a local loop, whether it is electrically-based or employs optical or radio energy. See also local loop. 3. A physical configuration used in residential or small business inside wire installations. Such a configuration connects multiple voice telephone jacks to one or two pairs of wires in a continuous, shared electrical loop. See also home run.

loopback address - In Internet Protocol (IP), a type of unicast address used when a host needs to send a packet back to itself for test purposes. See also host, IPv6, packet, unicast, and unicast address.

loopback test - A test of a local loop initiated by a local exchange carrier (LEC) at the network edge and causing a contact closure at the network interface unit (NIU) on the customer premises.The loopback test thereby allows the testing of the local loop in the forward direction, through the NIU, and looping in the backward direction, all in isolation from the customer premises equipment (CPE) and wiring. See also CPE, LEC, local loop, and NIU.

loose-tube cable - A fiber optic cable configuration involving a semi-rigid hollow plastic tube that houses and protects a number of optical fibers.The fibers can be either individually coated or organized into ribbons coated with protective acrylate and the tube is flooded with a water blocking gel.The tube or tubes are then helically stranded around a dielectric (e.g., fiberglass) or steel central strength member that supports the handing weight of the cable and prevents buckling.There also commonly is an aramid yarn strength member in the cable core.An outer polyethylene jacket surrounds the entire cable. If armoring is required for protection from rodents and other critters, a metallic tape or mesh is formed around the jacket and another jacket is formed to surround the armoring. Loose-tube cable is used in outside plant (OSP) applications, where extremes of temperature, rough handling, and mechanical disturbances make tight buffered cable unsuitable. See also aramid, cable, dielectric, fiberglass,optical fiber, OSP, polyethelene, strength member, tight buffered cable, and water-blocking gel.

LOS (Line-of-Sight) - 1. Optical LOS. Line of vision. A direct imaginary line between two points, as though it were from the center of the eye to the center of the object viewed.A direct non-guided path in the form of a straight line between a transmitter and receiver, uninterrupted by physical matter other than that suspended in the atmosphere. Opaque objects such as mountains, buildings, and trees interrupt optical LOS, as does the horizon over long distances due to the natural curvature of the Earth. Optical LOS is critical in free space optics (FSO) transmission systems and high-frequency radio system. Optical LOS is always preferable in radio systems, even those operating at low frequency. 2. Radio LOS. A direct nonguided path between a transmitting antenna and a receiving antenna.The criticality of LOS is sensitive to the radio frequency (RF) employed.Very low frequency (VLF), and low frequency (LF) signals tend to be travel between the Earth and the ionosphere. LF and medium frequency (MF) signals propagate as ground waves, which tend to follow the curvature of the Earth. Signals at the high end of the MF range and in the high frequency (HF) range benefit from ionospheric refraction, a phenomenon in which the density gradient in the atmosphere acts like a lens and tends to bend radio beams back towards the Earth. At very high frequencies (VHF) and above (i.e., ? 30 MHz) true optical LOS is considered essential, absent special modulation techniques combined with space division multiplexing techniques such as multiple input/multiple output (MIMO). See also electromagnetic spectrum, ground wave, HF, ionosphere, LF, MF, MIMO, near-LOS, NLOS, radio, refraction, RF, space division multiplexing, VHF, and VLF.

loss budget - Referring to the calculated amount of signal attenuation that a transmission system will tolerate between two points.The calculation considers the power level of the transmitter and the gain, i.e., sensitivity level, of the receiver. In a fiber optic transmission system (FOTS), the loss budget establishes the total amount of tolerable loss attributable to the propagation of the signal through splices, connectors, and the optical fiber cable itself. See also attenuation, connector, FOTS,gain, loss,optical fiber, power, propagation, signal, and splice.

loss - 1. The energy or power expended without having accomplished any useful work. In electrical systems, such loss is described in watts (W). See also watt. 2.The attenuation of signal level in a communications medium. Such loss generally is expressed in decibels (dB). See also attenuation, dB, and signal. 3. The disappearance of packets or other message units in network transit. Packet loss can be caused by errors introduced into address fields, which affects the ability of the various network switches, routers, and other devices to properly forward the message units. Loss also can be caused if device buffers overfill and message units are erased. See also address, buffer, switch, and router.

lossless compression - Referring to compression techniques that enable faithful reproduction of the signal, with no data loss, although compression rates tend to be relatively low. See also artifact, compression, lossy compression, and signal.

lossy compression - Referring to compression techniques that tend to produce artifacts, which are unintended and unwanted distortions or aberrations that result in a degraded signal, but supports very high compression rates. In video systems and communications, the artifacts often show up as jagged blockings or tiling effect known as aliasing, banding of colors, white spots, and even dropped frames. Although the picture is degraded as a result, the compression ratios can be as high as 200:1.The MPEG standards, for example, specify lossy compression in the form of discrete cosine transform (DCT). See also artifact, compression, DCT, lossless compression, MPEG, and signal.

low delay code excited linear prediction (LD-CELP) - See LD-CELP.

low earth orbit (LEO) - See LEO.

low frequency (LF) - See LF.

Low Income Consumers Program - In the United States, a program that provides lifeline subsidies to reduce the installation and monthly costs of basic telephone service for low income consumers.The Low Income Consumers Program is one of four programs established by the Telecommunications Act of 1996, supported by the Universal Service Fund (USF), and administered by the Universal Service Administrative Company (USAC). See also lifeline service, Telecommunications Act of 1996, USAC, and USF.

low voltage (LV) - See LV.

low-order mode - A physical path taken by a signal or signal component that is either parallel to or relatively modestly transverse to the waveguide. Some signal components travel directly through the center of the waveguide, at least theoretically, and, therefore, travel the shortest possible distance between the point at which they enter the waveguide and the point at which they exit the waveguide. Other modes take more transverse paths, striking and reflecting off of the interface between the core and cladding as they propagate through an optical fiber, for example. Low-order modes take parallel or modestly transverse paths, while high-order modes take considerably more transverse paths. See mode for more detail. See also cladding, core, critical angle, high-order mode, and waveguide.

low-pass filter - A device that passes all signals below a designated frequency (electrical) or wavelength (optical) band, but absorbs, attenuates, blocks, rejects, or removes all other signals. See also absorption, attenuation, band, band-pass filter, electrical, frequency, high-pass filter, optical, signal, and wavelength.

Low-Power Radio Service (LPRS) - See LPRS.

low-tier - In wireless telecommunications, referring to systems intended for pedestrian, in-building, oncampus, and wireless local loop (WLL) application. Examples include wireless LANs (WLANs) and wireless office telecommunications systems (WOTS). See also high-tier, WLAN, WLL, and WOTS.

low-water-peak fiber (LWPF) - See LWPF.

LPC (Linear Predictive Coding) - A method of digitally encoding analog signals that predicts the value of a signal at a specific point in time to be a linear function of the past values of the quantized signal. Code excited linear prediction (CELP) and its derivatives are examples of LPC methods. See also analog, CELP, digital, encode.

lpi (lines per inch) - Also known as line frequency.A measure of the vertical resolution of material printed in grayscale or halftone. See also dpi.

LPRS (Low-Power Radio Service) - In the United States, a private one-way, short distance communication service designed to provide auditory assistance to persons with hearing disabilities, persons who require language translation services, and persons in educational, health care, law enforcement, and various other settings.Two-way communications are prohibited. LPRS is an unlicensed service operating in the 216.75–217.0 MHz band.The Federal Communications Commission (FCC) regulates LPRS, which is in the family of personal radio services. See also FCC and personal radio services.

LRC (Longitudinal Redundancy Check) A parity checking error control method that improves on the simple vertical redundancy check (VRC) by viewing data as a block, or data set. This approach is characterized in terms of the manner in which human beings add numbers, not only in columns, but also in rows across columns, as though the devices were viewing data set in a matrix format. This additional technique of checking the total bit values of the characters on a longitudinal (i.e., horizontal) basis employs the same parity (i.e., odd or even) as the vertical check technique in the example in Table L-1. LRC and VRC are easily and inexpensively implemented in devices employing asynchronous transmission. LRC adds a significant measure of reliability when used in conjunction with VRC, although compensating errors still can occur in non-adjacent characters.The LRC is sent as one or two extra characters, known as block check characters (BCCs), at the end of each data block. See also error control, parity check, and VRC.

BitN/Value	C	O	N	T	R	O	L	Odd Parity
1	1	1	0	0	0	1	0	0
2	1	1	1	0	1	1	0	0
3	0	1	1	1	0	1	1	0
4	0	1	1	0	0	1	1	1
5	0	0	0	1	1	0	0	1
6	0	0	0	0	0	0	0	1
7	1	1	1	1	1	1	1	0
8 (Odd Parity)	0	0	1	0	0	0	0

Longitudinal Redundancy Check (LRC)

LRN (Location Routing Number) - In the United States, a 10-digit number that identifies a switch port for a central office (CO), an LRN is used to provide local number portability (LNP).The LRN is maintained in a Services Management System (SMS) database in a Service Control Point (SCP), which is part of the Advanced Intelligent Network (AIN).When a caller dials a telephone number, the originating CO queries a routing SMS database in an SCP for the associated LRN.The network uses that number to direct the call to the proper physical exchange, which may be one of several operated by competing local exchange carriers (LECs). LRNs are supervised by the Number Portability Administration Center (NPAC). See also AIN, CO, database, Directory Number, LEC, LNP, SCP, and SMS.

LSB (Least Significant Bit) - See least significant bit.

LSP (Label Switched Path) - In Multiprotocol Label Switching (MPLS), the physical path selected by a Label Edge Router (LER) for a flow of packets between common endpoints and based on the Forwarding Equivalence Class (FEC) associated with the packet flow. Using a Label Distribution Protocol (LDP), the LER distributes to Label Switched Routers (LSRs) in the core the individual link selections that comprise the end-to-end path selection.The LER inserts labels into the headers of the individual packets in the flow. Each LSR in the core uses the label of the incoming packet to select the outgoing port and link, and swaps that label for a new label, which the downstream LSR uses to select the next port and link, and so on. See downstream, FEC, flow, header, label, LDP, LER, link, LSR, MPLS, packet, path, and port.

LSR (Label Switching Router) - In Multiprotocol Label Switching (MPLS), a router in the network core that forwards packets along paths consistent with the labels inserted into packet headers by Label Edge Routers (LERs) as they entered the network.The LERs employ a Label Distribution Protocol (LDP) to distribute labels to the LSRs in the network core.As the LSRs receive the packets, they examine the short labels, compare them against a label database, switch the existing label for a new one, and quickly forward the packet across an appropriate link to the next LER. The process is repeated by each LSR until the packet reaches the egress LER, which strips the tag away as the packet exits the network. See also core, header, link, LSP, LDP, LER, MPLS, packet, path, router, and switch.

LU (Logical Unit) - In the IBM Systems Network Architecture (SNA), a program that manages communications software for communications with end users. A logical unit session originally was defined as an end-to-end communication between an end-user terminal and the originating application residing in the host. Later versions, such as LU 6.2, support peer-to-peer communications between intelligent devices, without requiring the host to assume responsibility for communications support activities. LU 6.2 is also known as Advanced Program-to-Program Communications (APPC).

Luddite - A pejorative term for someone who opposes technological or industrial innovation, especially if it automates manual functions or processes.The term originates in an unconfirmed folk legend about a feebleminded English (Nottinghamshire or Leicestershire) textile worker named Ted Ludd who smashed (c. 1779) two stocking knitting frames with a hammer to protest beatings by his master. The Luddites (1811–1816) in England were groups of textile workers who protested the use of automated wide-frame textile looms that could be operated by cheap, unskilled labor.The movement was so strong that it led to labor riots and battles with the British army.At least 17 Luddites were convicted of industrial sabotage and executed for that crime. Many more were convicted and shipped off to penal colonies in Australia. Note: I wrote this definition (and this entire book) on a computer that I seriously considered smashing with a hammer on multiple occasions.

luminance - The measure of intensity, or brightness, as measured in candelas per square meter. Luminance can vary within an image.An analog video transmission varies the luminance by varying the power level, or amplitude, of the signal, with high power representing black and low power representing white. See also candela and luminous flux.

luminiferous ether - See ether.

luminous flux - Radiant flux evaluated with respect to its luminous (brightness) efficiency. See also flux, luminance, and radiant flux.

lumped amplification - A type of amplification that occurs in a single, discrete location.Also known as discrete amplification. Erbium-doped fiber amplifiers (EDFAs), and most amplifiers, perform the process of amplification on a lumped, or discrete, basis. See also amplifier, distributed amplification, EDFA, and Raman amplifier.

LUNI (LANE User-to-Network Interface) - Pronounced loonee. In the LANE (LAN Emulation) environment, the interface by which an end system, known as a LAN Emulation Client (LEC), connects to the ATM network. See also ATM, LANE, LEC, and UNI.

LV (Low Voltage) - 1.A relatively low amount of electromotive force (emf).The LV grid runs at 110 volts at 50–60 Hz in North America, and 220 volts at 50 Hz in Europe and most of the rest of the world.Access broadband over power line (Access BPL) technology can make use of those LV lines as a portion of a local loop for broadband data communications. See also Access BPL, emf, HV, Hz, MV, volt, and voltage. 2. According to the International Electrotechnical Commission (IEC), alternating current (AC) voltage of 50–1,000V, or direct current (DC) voltage of 120–1,500V. Extra low voltage (ELV) is AC voltage less than 50V or DC voltage below 120V. Unlicensed personnel can safely install ELV wiring. See also AC, DC, and IEC.

LW (Long Wavelength) - Referring to fiber optic systems operating in the 1300, 1310, and 1550 nm ranges, with the IEEE 802.3ae specification for 10GBase-SR, LW being one example. See also 10GBase-SR, LW, and SW.

LWPF (Low-Water-Peak Fiber) - Single-mode fiber (SMF) manufactured with low levels of hydroxyl (OH) ions in order reduce the attenuation peak in the 1400 nm window, which is in the E-band (1360–1460 nm).The traditional SMF manufacturing process introduces hydroxyl (OH) ions into the fiber core.Wavelengths in the region around 1400 nm attenuate about 2 dB/km as a result of their interaction with those ions.As traditional single-wavelength fiber optic transmission systems (FOTS) employing SMF operate in the 1310 nm or 1550 nm window, water peak attenuation does not affect them. However, 4 of the 18 channels in coarse wavelength division multiplexing (CWDM) systems fall within the E-band and, therefore, are rendered unusable by water peak attenuation. Zero-water-peak fiber (ZWPF) contains near zero hydroxyl contamination and, therefore, suffers near zero water peak attenuation. See also attenuation, CWDM, dB, dB/km, E-band, FOTS, hydroxyl, SMF, water peak, wavelength, and ZWPF.

LZ (Lemple-Ziv) - A compression algorithm used in some modems, LZ achieves compression ratios of better than 5:1 for some forms of text and numerical data. LZ compression allows the data terminal equipment (DTE) to operate at speeds up to 128 kbps while the analog link between the modems remains at 38.4 kbps or less over a 4 kHz channel. LZ is named after its inventors, Abraham Lempel and Jacob Ziv. See also analog, channel, compression, DTE, link, modem, and LZW.

LZW (Lemple-Ziv-Welch) - A lossless data compression algorithm used in the Graphics Interchange Format (GIF) and optionally used in Tagged Image File Format (TIFF) files. LZW is named after its inventors, Abraham Lempel, Jacob Ziv, and Terry Welch. See also analog, channel, compression, DTE, GIF, link, lossless compression, modem, TIFF, LZ, and LZW.