websters new world telecom dictionary phần 6 pptx

• 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 informa tion- frame 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. Figure L-2 illustrates the LAPD frame and its component fields. Figure L-2 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 begin- ning 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. 1 2 1 or 2 2 1260 Frame Header Frame Trailer Flag Address Control Flag Frame Check Sequence Information Field Length in octets < LAPB (Link Access Procedure, Balanced) 278 74570c12.qxd 9/11/07 12:25 PM Page 278 • 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. Figure L-3 illustrates the LAPF frame and its component fields. Figure L-3 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 begin- ning 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. DLCI (high order) C/R 0/1 EA 0 Flag (01111110) Address field Information field Frame check sequence Flag (01111110) 1 2 0–4096 2 1 octets DLCI (low order) FECN BECN EA 1 DE 279 LAPF (Link Access Procedure for Frame Mode Services) 74570c12.qxd 9/11/07 12:25 PM Page 279 • 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 LAPF (Link Access Procedure for Frame Mode Services) 280 74570c12.qxd 9/11/07 12:25 PM Page 280 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-in-first-out (LIFO) See LIFO. 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. 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, con- centrators, multiplexers, switches, and routers as they variously transmit and retransmit, amplify and ream- plify, 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. 281 launch cable 74570c12.qxd 9/11/07 12:25 PM Page 281 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 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. Layer 1 Referring to the Physical Layer of the OSI Reference Model. See also OSI Reference Model and Physical Layer. Layer 2 Referring to the Data Link Layer of the OSI Reference Model. See also Data Link Layer and OSI Reference Model. Layer 2 Tunneling Protocol (L2TP) See L2TP. 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. 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. 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. law 282 74570c12.qxd 9/11/07 12:25 PM Page 282 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 Net- work 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 negoti- ates 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. 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. 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). LD- CELP 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, rep- resenting 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 normal- izes 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. 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 283 leaky wave 74570c12.qxd 9/11/07 12:25 PM Page 283 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 low- order (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 network- based 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, cal- culators, 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. leaky wave 284 74570c12.qxd 9/11/07 12:25 PM Page 284 legacy Referring to a software or hardware component or element, a system, or a network that is tech- nically 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. 285 lifetime control 74570c12.qxd 9/11/07 12:25 PM Page 285 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 correspon- dence saved is the first retrieved, or at least appears first in the stack. See also FIFO and queue. 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. 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. lightguide An optical fiber. See optical fiber. 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. Lightweight Directory Access Protocol (LDAP) See LDAP. limited distance modem (LDM) See short haul modem. linear predictive coding (LPC) See LPC. 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. LIFO (Last-In-First-Out) 286 74570c12.qxd 9/11/07 12:25 PM Page 286 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 mes- sage 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 con- verting 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 Number- ing 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 287 line powered 74570c12.qxd 9/11/07 12:25 PM Page 287 [...]... (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,... low income consumers.The Low Income Consumers Program is one of four programs established by the Telecommunications Act of 19 96, supported by the Universal Service Fund (USF), and administered by the Universal Service Administrative Company (USAC) See also lifeline service, Telecommunications Act of 19 96, USAC, and USF low frequency (LF) See LF low-order mode A physical path taken by a signal or signal... wavelength Low-Power Radio Service (LPRS) See LPRS 74570c12.qxd 9/11/07 12:25 PM Page 2 96 low-tier 2 96 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 voltage (LV)... (MegaHertz) is a million (1 06) Hertz, Mbps (Megabit per second) is a million (1 06) bits per second, and MBps (MegaByte per second) is a million (1 06) bytes per second In transmission systems, therefore, a million is exactly 1,000,000, since the measurement is based on a base 10, or decimal, number system 3 In computing and storage systems, a MB (MegaByte) is actually 1,048,5 76 (220) bytes, because the... actually 1,048,5 76 (220) bytes, since the measurement is based on a base 2, or binary, number system.The term MB comes from the fact that 1,048,5 76 is nominally, or approximately, 1,000,000 See also byte and M Mbps (Megabit per second) Million (1 06) bits per second A measure of bandwidth in a digital transmission system See also bandwidth, bps, and M MBps MegaByte per second, or million (1 06) bytes per... James P Gordon, and Herbert J Zeiger invented the maser, which earned them the Nobel Prize in Physics in 1 964 Research continued into the 1950s, leading to the optical maser, or laser, for which AT&T Bell Telephone Laboratories was awarded a patent in 1 960 See also laser mashing A process of building new Web-based services from reusable components of other services, mashing is a technique defined in Web... 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,... 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 266 1 See also encryption, ISP, PPP, PPTP, protocol, tunneling, and VPN LU (Logical Unit) In the IBM Systems Network Architecture (SNA), a program that manages communications software for communications... 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... 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 . 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,. by the Telecommunications Act of 19 96, supported by the Universal Service Fund (USF), and administered by the Universal Service Administra- tive Company (USAC). See also lifeline service, Telecommunications. Wavelength Band) The ITU-T standard optical transmission window in the wavelength range of 1, 565 –1 ,62 5 nm. See also wavelength and window. LBE (Large Business Enterprise) A large commercial (i.e.,

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