L

L-band
    That part of the electromagnetic spectrum usually applied to satellite and microwave communications.

LAMA
    See local automatic message accounting (LAMA)

LAN
    See local area network (LAN)

LAN interconnect
    A point on a local area network (LAN) where circuits can be routed & administered.

LAN Manager
    A local area network (LAN) management software jointly developed by Microsoft and 3Com that runs under MS-DOS and OS/2. LAN Manager is original equipment manufactured (OEM)ed by a number of vendors, including 3Com, IBM, DEC, Ungermann-Bass, and AT&T.

LAN Manager for Unix Systems
    An implementation of LAN Manager for use with Unix. Known colloquially as LM/X.

LAN network manager
    1) A person who is responsible for managing the local area network (LAN) in an organization. Duties can include adding new users, installing new hardware and software, diagnosing network problems, helping users, performing backups, and setting up a security system. Unlike MIS managers, LAN managers are not always formally trained in LAN and information management. 2) The IBM proprietary network management software for Token Ring networks.

LAN protocols
    A range of protocols supported by a frame relay network, including transmission control protocol/Internet protocol, apple talk, xerox network system (XNS), internetwork packet exchange (IPX), and common operating system used by DOS-based PCs.

LAN segment
    In the context of a frame relay network supporting LAN-to-LAN communications, a LAN linked to another LAN by a bridge. Bridges enable two LANs to function like a single, large LAN by passing data from one LAN segment to another. To communicate with each other, the bridged LAN segments must use the same native protocol. See bridge.
bridge

LAN Server
    IBM's proprietary implementation of Microsoft's local area network (LAN) Manager.

land line
    The traditional telephone services and/or companies.

land mobile service
    Two-way radio service in the 44-50 MHz band.

LAP
    See local access provider (LAP) or Link Access Protocol (LAP)

LAP-B
    See link access procedure-balanced (LAP-B and LAPB)

LAP-D
    See link access procedure-D channel (LAP-D and LAPD)

LAPB
    See link access procedure-balanced.

LAPD
    See link access procedure-D channel (LAP-D and LAPD)

LAPF
    See Link Access Procedure for Frame Relay (LAPF)

LAPM
    See Link Access Procedure for Modems (LAPM)

large scale integration (LSI)
    The depositing of many electronic circuits, including transistors, on a small silicon chip in a single manufacturing process.

laser disk
    A storage device written and read to by laser. See optical disk and video disk.
optical disk video disk

last in/first out (LIFO)
    A queuing technique in which the newest entry in a queue or file is the first to be removed.

last mile
    A circuit, facility, or service used to connect an intercity cable, microwave system, and so forth to a customer of a point of presence (POP).

last number redial
    A private branch exchange (PBX) and electronic key telephone feature in which the last number dialed can be automatically redialed at the touch of a preprogrammed button.

LAT
    See local area transport (LAT)

LATA
    See local access and transport area (LATA)

latency
    The time interval between when a network station seeks access to a transmission channel and when access is granted or received. Same as waiting time. In a bridge or a router, it is the amount of time elapsed between receiving and retransmitting the local area network (LAN) data packet.

LATS
    See Loop Access Test System (LATS)

LAWN
    See local area wireless network (LAWN)

layer
    The seven-layered open system interconnection (OSI) model of network-processing functions. The seven layers comprise a set of rules and standards for successful data communications.

layer 1
The first layer of the open system interconnection (OSI) model, known as the physical layer. Layer 1 represents the actual electrical or mechanical hardware interface that connects a device to a transmission medium.

layer 2
The second layer of the open system interconnection (OSI) model, known as the data link layer. Layer 2 organizes data into frames and sequences for transfer across the physical layer. Layer 2 primarily detects and corrects message errors transmitted across any physical link, and allows the nodes on either end of a connection to signal one another when initiating a communications session. See media access control and logical link control.
media access control (MAC)
logical link control (LLC)

layer 3
The third layer of the open system interconnection (OSI) model, called the network layer. Layer 3 routes data from one element to another until it reaches its final destination.

layer 4
The fourth layer of the open system interconnection (OSI) model, called the transport layer. Layer 4 recovers data that may have been lost due to node failure on the lower network layers, and maps transport and network addresses. Layer 4 also establishes and manages reliable network connections between end nodes, and controls packet flow, size, and sequence restrictions that prevent excessive amounts of data from entering the network

layer 5
The fifth layer of the open system interconnection (OSI) model, called the session layer. Layer 5 is a dialogue between two users or devices during which data is exchanged in a conversational manner.

layer 6
The sixth layer of the open system interconnection (OSI) model, called the presentation layer. Layer 6 defines the way in which information is presented so that the communicating devices interpret it properly. This layer controls the format of screens and files, and defines syntax, control codes, special graphics, and character sets.

layer 7
The seventh layer of the open system interconnection (OSI) model, known as the application layer. Layer 7 applies to the actual meaning rather than the format or syntax of applications, and permits communication between users.

layer management entity (LME)
Defines management information, services, and functions related to specific open system interconnection (OSI) layers.

LCB
See line control block (LCB)

LCD
See liquid crystal display (LCD)

LCDM
See logarithmic companded delta modulation (LCDM)

LCP
Link Control Protocol (PPP usage)

LCR
See least-cost routing (LCR) or automatic route selection (ARS)

LCU
See line control unit (LCU)

LD
See long distance (LD)

LD-1
A fractional digital service hierarchy level 1 with service similar to FT-1, except it is integrated with voice, data, video, and frame relay.

LDM
See limited distance modem (LDM)

LDN
See listed directory number (LDN)

learning bridge
See smart bridge

leased line (LL)
A communications line leased from a public utility common carrier for exclusive use. T1 and T3 are often used as leased lines. Synonyms: private line and full period line

least-cost routing (LCR)
A method of automatically selecting the least costly facility for transmission of a call. Synonyms: most economical route selection, automatic route selection, and flexible route selection

LEC - Local Exchange Carrier.

Local telephone company, providing connections between local points or to long distance carriers for extended connections. Examples are Pacific Bell in California, Illinois Bell in Illinois, GTE in Hawaii, etc.

LEC BAN
See LEC billing account number.
LEC billing account number (LEC BAN)

LEC billing
An arrangement in which the local exchange carrier (LEC) invoices the customer for some or all telecommunications services.

LEC billing account number (LEC BAN)
A three-digit number added to the billing telephone number used as the local exchange carrier (LEC) customer number. A LEC BAN groups all automatic number identifications (ANIs) for a customer.

LEC card
The billing arrangement that enables the caller to bill calls to an authorized calling card issued by a local exchange carrier (LEC).

LEC charges
Charges that are the responsibility of the local exchange carrier (LEC).

LED
See light-emitting diode (LED)

Lempel-Ziv-Welch (LZW)
A data compression algorithm named after its developers.

LEO
See low-earth-orbit (LEO)

LEO satellite
A satellite operating in a low-earth-orbit (LEO). LEO satellites can orbit the earth in less than two hours.

LERG
See Local Exchange Routing Guide (LERG)

Letter of Agency (LOA)
    1) A document that authorizes changing the service provider. See responsible organization.
    2) A letter giving a common carrier the authority to conduct business with a telephone company on behalf of a customer. For example, an LOA would be used to order circuits or 800 numbers for the customer. responsible organization (RO and resporg)

letters shift
    A teleprinter function initiated by the letters-shift character, causing the machine to shift from upper to lowercase.

level
    An expression of the relative signal strength compared to a standard.

levels of abstraction
    1) Relational: Tables with security authorization that may be subsets of database tables or joins of the rows and columns of multiple tables. RDBMS cannot update through views.
    2) Object-oriented: Objects that are a subset or aggregation of the subcomponents of one or more other objects. The underlying business object models encapsulate the data and behaviors that are used by object views. Object views send messages to the encapsulated objects to effect any required updates or to create new object instances.

LF
    See low frequency.
low frequency (LF)

LIBOR
    See London interbank offer rate.
London interbank offer rate

library routine
    A proven routine that is stored in a software program library.

LIDB
    See line information database

life cycle
    The support maintenance of software products

LIFO
    See last in/first out (LIFO)

light pen
    A light-sensitive stylus input device that is connected to a video terminal. The user touches a spot on the screen to edit text, choose from a menu, or draw an image, then presses a button to initiate the computer's response.

light-emitting diode (LED)
    An electronic device that emits light with little generation of heat. LEDs are often used in fiber-optic systems instead of coherent light lasers, particularly when low speeds or short distances are involved.

lightwave transmission
    A laser communication transmitted through the air, rather than through fiber-optic cable.

limited distance modem (LDM)
    A modem that operates on lines of up to six miles.

line
    The transmission path from a non-switching subscriber terminal to a switching center.

line conditioning
    A telephone company service that reduces envelope delay, noise, and amplitude distortion, enabling the subscriber to transmit higher speed data than over traditional telephone lines.

line control block (LCB)
    A storage area containing control information required for managing and scheduling line operations.

line control unit (LCU)
    An amplifier that increases a signal's strength before the signal is placed on the address or data bus, or transmitted. Synonym: datalink control protocol line driver

line driver
    A signal converter that conditions the digital signal transmitted by an RS232 interface to extend reliable communication beyond the 50ft RS232 limit up to several miles. It is a baseband transmission device.

line hit
    Interference that causes a loss or an insertion of unwanted signals into a data path.

line information database
    A local exchange carrier's (LEC) database used to provide calling card validation.

line load control
    An attendant's ability to temporarily restrict least-important stations from initiating outbound calls during excessive traffic load or emergency conditions.

line loading
    A method of minimizing amplitude distortion by equipping a telephone line with loading (inductor) coils.

line lockout
    See lockout

line noise
    This is interference on the telephone lines. It causes garbage to appear on your screen.

line number
    The four-digit XXXX number assigned to a North American telephone within an NXX exchange. See North American numbering plan.
North American numbering plan (NANP)

line preference
    The ability to program a private branch exchange (PBX) to accept calls on a certain line.

line ringing
    A multi-button telephone function that allows incoming call ringing signals to be received on specific lines.

line segment
    The expanse of fiber-optic cable(s) between two network locations.

line signal standards
    In the US: T1 carries data at 1.544Mbit/s and has 24 voice circuits; T1C 3.152 Mbit/s with 48 voice circuits; T2 6.312Mit/s with 96 voice circuits, and T3, 44.736Mbit/s. In Europe, the standards are of the form En. E1 line speed is 2.048Mbit/s with 3C voice circuits; E2 is 8.448Mit/s with 120 voice channels. E3 is 34.368Mbit/s with 480 voice circuits. In the UK, E1 is often referred to as MegaStream, a BT label for its 2Mbit/s leased circuits.

line speed
    The maximum transmission rate of signals over a circuit. Line speed is usually expressed as baud, or bits per second (bps).

line status indication
    A visual indication of line conditions, such as busy, idle, on hold, or ringing.

line switching
    A technique that establishes a connection between incoming and outgoing lines. See message switching.
message switching

line turnaround
    The action required in a communications system to reverse the transmission direction after receiving a data block of transmission so that the receiving device can send data. Line turnaround time in a half-duplex system is usually much longer than in a full-duplex system.

line-side access
    A type of access in which customers making long distance calls must dial extra digits (e.g., up to 17), in addition to the called party's telephone number. (See access.)
access

lines of action
    The presentation of a complex activity as a set of (relatively) independent components.

lines of force
    See flux

link
    A communications path between two devices or nodes in a network. See bridge and gateway.
bridge
gateway

Link Access Procedure for Frame Relay (LAPF)
    The data link layer protocol used by frame relay as defined in ITU-T Recommendation Q.922 and ANSI T1.618.

Link Access Procedure for Modems (LAPM)
    1) A type of error control used by some modems. It is NOT a compression method, though some modem manufacturers have incorrectly advertised it as such. Link Access Procedure for Modems (LAPM) 2) A type of error control used by some modems. It is NOT a compression method, though some modem manufacturers have incorrectly advertised it as such.

link access procedure-balanced (LAP-B and LAPB)
    A Consultative Committee on International Telephone and Telegraph (CCITT)-compatible protocol, similar to link access procedure-D channel (LAP-D.) LAP-B provides error-controlled access to packet-switched networks.

link access procedure-D channel (LAP-D and LAPD)
    A protocol that provides reliable signaling on an ISDN-D channels. LAP-D is a member of the high level data link control (HDLC) protocol.

Link Access Protocol (LAP)
    The Data Link or open system interconnection (OSI) Layer Two protocol specified by the ITU-TS for the X.25 interface standard.

link layer
    The second layer in the open system interconnection (OSI) model relating to data transmission between network nodes.

Link Problem Determination Aid (LPDA)
    A component of IBM's NetView". Requires IBM modems or IBM DSU (a few compatibles are available).

link state algorithm
    A routing algorithm such as OSPF which takes into account lowest delay when choosing a route: link speed and congestion as well as hop count.

link-access protocol-balanced (LAP-B)
    The most common data-link control protocol used to interface X.25 DTEs with X.25. It also specifies a LAP or link access procedure (not balanced). Both LAP and LAP-B are full-duplex, point-to-point bit-synchronous protocols. The unit of data transmission is called a frame. Frames may contain one or more X.25 packets. LAP-B is the data link level of X.25 in a packet switched network. Same as a subset of the asynchronous balanced mode of HDLC.

lip sync
    The maintenance of sound (i.e., speech) exactly in step with movement in a visual image (i.e., faces). Because the processing time for the video portion of the signal in a low-speed codec is about 100 times longer than the audio processing time, codecs usually incorporate adjustable audio delay circuitry to delay-equalize the two signals, to regain lip sync.

liquid crystal display (LCD)
    Liquid crystals are low power consumption devices that produce a non-emissive display system by modulating ambient light rather than emitting light. LCDs have good visibility, even in sunlight.

list processing
    A method of processing data in list form. A list processing language is used primarily for mathematical and arithmetic logic research, rather than for production programming.

listed directory number (LDN)
    Telephone directory numbers used to route private branch exchange (PBX) incoming network calls to an attendant.

live HOBIC
    Live-operator calls that give time and charges information to a hotel. It is used in conjunction with a call accounting system delivered through Hobic lines.

LL
    See leased line (LL)

LLC
    See logical link control (LLC)

LME
    See layer management entity (LME)

LMI
    See local management interface (LMI)

LMI Rev 1
    A synchronous polling scheme used for link management of a frame relay channel where the user polls the network to obtain status information on the permanent virtual circuits (PVC)s configured on the channel. LMI exchanges this information using DLCI 1023.

LMOS
    See Loop Maintenance Operations System (LMOS)

LMS
    See local measured service (LMS)

LMSI
A proprietary interface developed and used by Philips to connect Philips CD-ROM drives to a PC.

LNS
The abbreviation for lines; "#LNS" is the number of lines.

LOA
See Letter of Agency (LOA)

load
    To enter data from external storage, such as a diskette, to computer memory.

load balancing
    The practice of splitting communications on one route into two or more routes to balance traffic on each route. Load balancing makes communications faster and more reliable. In remote internetworking, bridges and routers perform load balancing by splitting local area network (LAN)-to-LAN traffic among two or more wide area network (WAN) links. This permits a combination of several lower speed lines to transmit LAN data simultaneously.

load balancing station/trunk lines
    A private branch exchange (PBX) feature that permits subscribers to change telephone and trunk terminations to allow smother traffic flow on the network.

load sharing
    A multiple-computer system that shares the load during peak hours. During nonpeak periods or normal operation, one computer manages the entire load with the other(s) acting as fallback.

loading
    A system for adding regularly spaced inductance units to a circuit to improve its transmission characteristics.

loading coil
    An inductor used in local loops to improve voice transmission. Loading coils may cause distortion in digital communications.

lobe
    The cable between a Token Ring station and the Trunk Coupling Unit to which it is connected. Lobe length comprises a patch cable from the TCU to the main wiring panel, the length of the main wiring to the user station's location, then a patch cable from a floor/desk socket to the station.

local access
    See local loop

local access and transport area (LATA)
    A geographic service area defined in the AT&T Modification of Final Judgement (MFJ). The Bell operating companies (BOCs) and GTE are restricted to operations within their own areas. Long-distance service within a LATA is provided by the local exchange carrier (LEC). Service between LATAs is provided by an interexchange carrier (IEC). LATAs are represented by a 3-character code. There are approximately 200 LATAs in the U.S.

local access mileage
    A recurring charge rate element based on mileage to the central office (CO). It may be applied to dedicated non-switched private lines.

local access provider (LAP)
    Any organization that is authorized to provide local access. The organization may or may not be the local exchange carrier (LEC).

local area network (LAN)
    A short-distance data communications network. LANs are typically within a building or campus, and to link together computers and peripherals under a standard protocol. The network provides high-bandwidth communications over coaxial cable, twisted-pair, fiber, or microwave media. LANs are usually owned by the user. LANs have two main advantages: users can typically access a centralized database and shared resources, and can send messages to other LAN users. Most notable of LAN topologies are Ethernet, token ring, and fiber-distributed data interface (FDDI).

local area transport (LAT)
    An Ethernet protocol developed by Digital Equipment Corporation.

local area wireless network (LAWN)
    O'Neill Communications' wireless local area network (LAN) for transferring files, exchanging messages, and sharing peripherals among PC users.

local automatic message accounting (LAMA)
    The central office (CO) equipment that records the information required for billing a subscriber's local telephone calls by the local telephone company.

local bridge
    Bridge that links two local LANs: in the same building, for example.

local call 1)
    A call destination that is within the local area of the switching system serving the calling station. 2) A telephone call within the specified local calling area of the telephone exchange. 3) A call that incurs no charges.

local call billing
    A feature in hotel applications that computes the billing for local calls placed by guests based on total message units.

local dynamic routing
    Dynamically controlled routing (DCR) within a local calling area. See dynamically controlled routing.
dynamically controlled routing (DCR)

local echo
    This is when a communications program will send information (either that you type or from a file) to your screen, as well as to the other modem.

local exchange
    The switching center where a subscriber's lines terminate. Synonym: end office

Local Exchange Routing Guide (LERG)
    A document that defines the end offices (EOs) and their relationship to tandem offices. The LERG is produced by Bellcore traffic routing administration (TRA).

local line
    A channel connecting the subscriber's equipment to the line-terminating equipment in the central office exchange. Local lines are usually a two- or four-wire metallic circuit.

local loop
    A channel between a customer's terminal and a central office (CO). The most common form  of loop, a pair of wires, is also called a line.

local management interface (LMI)
    A protocol with four different versions used to control the local interface from a routing device to the wide area network (WAN) switch. A LMI is also used for configuration, flow control, and maintenance of the local connection.

local measured service (LMS)
    A pricing structure for local calls which requires customers to pay according to usage, rather than simply paying a flat monthly fee. LMS generally combines access (fixed rate) and usage (variable rate) charges. The access charge is usually set at a rate substantially less than the prevailing flat rate and may include a usage allowance. Usage charge calculations are typically based on up to four variables: frequency (the number of calls placed); duration (how long calls last); the geographical distance the call covers; and the time of day or week the call was placed.

local network
    A set of telecommunications links within a limited area

local number portability (LNP)
    A customer NPA-NXX number that local exchange carrier agreements permit to be transported with customers when they relocate outside their local exchange, permitting cost savings by virtue of not having to change logos on business literature.

local security
    A security method available for 386 and 486 servers running HPFS386. This method extends LAN Manager security measures to protect the files on a server by restricting access of the users working at the server. With local security, a user must be assigned permissions to access any file or directory in an HPFS386 partition, whether or not the resource is shared as part of a LAN Manager resource.

local service area
    The area within which local or non-toll calls are placed, such as the area service by an exchange.

local serving office (LSO)
    The local office designated by the six-digit code: numbering plan area (NPA) + NXX 6-digit code.

local trunk
    The trunks between local central offices.

LocalTalk
    1) LocalTalk is a 230 kbps baseband network technology that uses the carrier sense multiple access/collision detection (CSMA/CA) media access method.
    2) The cable and connector system used in Apple Computer's AppleTalk networks.

lockout
    To ensure privacy, the attendant and executive override capabilities are not allowed to function on a station line. See privacy and privacy release.
privacy and privacy release

log-in
    See log-on

log-off
    The procedure that ends a terminal session.

log-on
    The procedure that initiates a terminal session.

Log-on script
    A batch program containing LAN Manager, NetWare and other operating system commands used to configure workstations. Logon scripts can be written for one or more users.

Log-on server
    For a domain, a logon server is the primary domain controller and the backup domain controllers. For a user, the server that processes the user's logon request.

log-out
    See-log off.
 
log-off

logarithmic companded delta modulation (LCDM)
    A refined delta modulation (DM) technique that logarithmically scales the input amplitude to increase the dynamic range of coding analog signals into digital signals.

logarithmic companded logic circuit
    An electronic circuit that is used to complete a logical function, and is usually encoded on a chip.

logic errors
    Any change in an originally transmitted signal (for example, a bit change from a 1 to a 0). These errors can be caused by cross-talk, jitter, slips, hardware, software, or human errors.

logic gate
    The basic decision-making circuits in digital equipment, usually having two or more binary inputs, and one binary output.

logical
    A name you can use in place of or as part of a file specification. These names are easier to remember and type than the full file specification.

logical channel
    1) In a packet mode operation, a method of two-way simultaneous transmission across a data link comprising associated send and receive channels.
    2) In data communications, an electrical path connecting a station and a channel.

logical link control (LLC)
    A protocol subset developed by the Institute of Electrical and Electronic Engineers (IEEE) 802.2 committee for data-link-level transmission control. It is the upper sublayer of the data link layer that complements the media access control (MAC) sublayer. It provides a common interface point to the MAC layers, which specify the access method used.

logical port
    Each physical port is configured to a number of logical ports. Logical ports allow bundling of channel bandwidth of a physical port.

logical record
    A record separate from its physical environment. That is, parts of the same record can be located in different locations).

logical ring
    The path a token follows in a fiber-distributed data interface (FDDI) or token ring network made up of all the connected media access control (MAC) sublayers of each node. In FDDI, the physical topology can be a ring, a tree, or a dual ring of trees.

logical unit (LU)
    An IBM SNA network function defined in layers four, five and six (Transmission Control, Data Flow Control and Presentation Services) of the SNA architecture. In the SNA network, corresponding LUs are able to exchange information. Originally, particular types of LU has specific functions: LU1 for printers, LU2 for displays and so on. As SNA has developed, new types of LU have been introduced that support a broader range of communications facilities. LUs are normally associated with particular Physical Units (PUs), or network devices. LUs provide the services required by Applications (APs) in the IBM SNA environment, sitting between the APs and the PUs. A Dependent LU relies on the host for activation, physically and logically, while an Independent LU can initiate a session without host involvement.

London interbank offer rate
    A standard interest rate based on an average of the rates offered by five international banks. Used in agreements as an alternative to the U.S. prime rate, especially in international transactions.

Long Distance (LD)
    A switched public exchange network telephone call that extends beyond the local calling area, making it a toll call.

long haul
    Circuits spanning considerable distances.

long-distance carrier
    A company providing long-distance telephone service between a local exchange carrier (LEC) and local access transport areas (LATAs).

long-haul facilities
    Facilities used for the inter-DSA portion of an interstate circuit (typically beyond 50 miles).

long-haul facility
    A long-distance trunk or intermachine trunk (IMT).

long-term testing
    A type of circuit testing in which a tester's terminal may be logged out of an operations support system without ending the test. The circuit test can be left to run for any amount of time, and the results retrieved at a later date.

longitudinal balance
    A measure of the electrical balance between the two conductors (tip and ring) of a telephone circuit Specifically, the difference between the tip-to-ground and ring-to-ground AC signal voltages, expressed in decibels. If the longitudinal balance of a circuit is insufficient, cross talk, electronic chatter, and miscellaneous noises can occur.

longitudinal redundancy check (LRC)
    A method to make sure data is received correctly. Consists of performing parity checks across the data message.

longitudinal redundancy check bit (LRC)
    A block-checking procedure in which the check characters depend only on bits of corresponding positions in the data characters.

look-up table
    A set of addresses, including source and destination, used by a bridge or router to determine what should be done with a packet. As the packet comes in, its address information is read and compared with the information in the look-up table. Depending on the information, the bridge may forward the packet or discard it. Many bridges and routers can build their look-up tables as they operate.

lookahead routing
    A switching system feature that uses a dedicated communications channel to verify the availability of all links required to switch a call.

loop
    1) A short transmission line connecting the user and the switchboard.
    2) A closed path in which a signal can circulate. A loop may be within a piece of equipment, such as a repeater or carrier terminal, or it may be a complete carrier circuit.

Loop Access Test System (LATS)
    A relay connection matrix that provides access to individual customer four-wire local loops and/or to DS0 (64 kbps) circuits. The matrix is only at DDS hub offices.

loop back
    A diagnostic test in which a signal is transmitted across a medium, is returned to the sending device, and is compared with the original transmission.

loop checking
    A technique for checking the accuracy of transmission data. The received data is returned to the sender for comparison with the original data.

loop circuit
    The circuits linking a subscriber's telephone with the local switching equipment.

loop filter H.261 usage.
    See spatial filtering

Loop Maintenance Operations System (LMOS)
    A Bellcore testing system for voice grade telephone service that records customer trouble reports and tracks subsequent maintenance activitites.

loop network
    A central network topology that includes a continuous circuit connecting all nodes. Messages are routed around the loop to and through a central controller.

loop signaling
    Any of the three signaling methods that use the metallic loop formed by the trunk conductors and the terminating equipment bridges.

loop-back test
    A circuit test that applies a signal and receives the data back for interpretation. A loop-back test is done at a customer device such as a channel service unit (CSU) or at a network device such as DS-0 or DS-1 digital access and crossconnect system (DACS) port.

Loop-start
    A supervisory signal, initiated by an off-hook telephone, received at the private branch exchange (PBX) or local exchange that closes a DC loop and initiates service.

loopback
    See loop back

looping
    A test procedure in which the received signal returns to its source.

LOS
    See loss of signal (LOS)

loss of frame
    A T1 error condition when 175 +/- 75 consecutive zeros are received.

loss of signal (LOS)
    The absence of pulses lasting for one millisecond or more.

loss of signal alarm
    See red alarm

lossless compression
    A process which allows data compression and its expansion to its original form without any loss of information. PKZip, ZOO and ARC are common examples. See also Huffman Coding.

Huffman Coding

lossy compression
    A process which compresses data in such a manner as to make complete recovery of the original data impossible. This process is commonly used for still or motion images where the recovered image only needs to be subjectively acceptable. Examples include JPEG, MPEG, H.261. Note that lossy compression provides significantly greater possible compression than lossless compression. See also Cosine Transform.
cosine transform

lost call
    A call that cannot be completed because of blocking.

loudspeaker paging access
    See paging

low frequency (LF)
    The band of frequencies between 30 and 300 kilohertz.

low level language
    Languages such as Assembler and C that use symbols which are converted directly into machine code.

low speed
    Data communications systems operating at speeds of less than 2,400 bits per second (bps). (See high speed; medium speed.)

high speed

medium speed

low-earth-orbit (LEO)
    Orbit paths between 350 miles and 875 mile above the earth. These values are approximations.

LPDA
    See Link Problem Determination Aid.

Link Problem Determination Aid (LPDA)

LRC
    1) See longitudinal redundancy check or longitudinal redundancy check bit.

longitudinal redundancy check (LRC)

longitudinal redundancy check bit (LRC)

LSI
    See large-scale integration.
large scale integration (LSI)

LSO
    See local serving office.
local serving office (LSO)

LU
    See logical unit.

logical unit (LU)

LU6.2 luminance
    That portion of a composite video signal which represents the monochrome or brightness part of the image.

LZW
    See Lempel-Ziv-Welch (LZW)