L2TP (Layer 2 Tunneling Protocol)
A Layer 2 protocol that can work in a non-IP enterprise environment. L2TP is used primarily by service providers to encapsulate and carry VPN traffic through their backbones.

LAN (local area network)
(1) A system for linking terminals, programs, storage, and graphic devices at multiple workstations over relatively small geographic areas. (2) A network that is limited to a small area, for example the premises of an office building or plant.

LANE (LAN Emulation)
An ATM Forum standard for emulating a LAN across an ATM network.

LAPB (Link Access Protocol Balanced)
A modified form of HDLC that the ITU-T chose as the link-level protocol for X.25 networks. LAPB provides for the reliable transfer of a packet from a host to an X.25 packet switch, which then forwards the packet on to its destination.

laser (light amplification by simulated emission of radiation)
A device that converts electrical energy into radiant energy in the visible or infrared parts of the spectrum, emitting light with a small spectral bandwidth. Lasers are widely used in fiber-optic communications, particularly as sources for long-haul links.

LATA (Local Access and Transport Area)
Geographic regions within the United States that define areas within which the Bell operating companies (BOCs) can offer exchange and exchange access services (local calling, private lines, and so on).

latency
The delay associated with the time it takes a packet to travel from entry point to exit point.

layer
In the OSI reference model, a collection of related network-processing functions that comprise one level of a hierarchy of functions.

L-band
The portion of the electromagnetic spectrum commonly used in satellite and microwave applications. L-band operates in the 390MHz to 1550MHz range, and it supports various mobile and fixed applications.

LCD (liquid crystal display)
A graphic display on a terminal screen using an electroluminescent technology to form symbols or shapes.

LDAP (Lightweight Directory Access Protocol)
The standard directory server technology for the Internet. LDAP allows retrieval of information from multivendor directories.

LDP (Label Distribution Protocol)
An MPLS signaling protocol.

leased line
A communication channel contracted for exclusive use from a common carrier, frequently referred to as a private line.

LEC (local exchange carrier)
The incumbent local telephone company. There was originally no competition among LECs, but as soon as competition in the local loop picked up, LECs were segmented into ILECs, CLECs, and DCLECs.

LED (light-emitting diode)
A semiconductor junction diode that emits radiant energy and is used as a light source for fiber-optics communications, particularly for short-haul links. Also used in alphanumeric displays in electronic telephones and calculators.

LEO (low earth-orbit) satellite
A satellite that orbits at about 400 to 1,000 miles (640 to 1,600 kilometers) above the earth.

lightwave communications
A term sometimes used in place of optical communications to avoid confusion with visual information and image transmission, such as facsimile or television.

limited-distance modem
A device that translates digital signals into analog signals (and vise versa) for transfers over limited distances; some operate at higher speeds than modems that are designed for use over analog telephone facilities.

line
(1) The communications path between two or more points, including a satellite or microwave channel, also referred to as the transmission line. (2) In data communication, a circuit connecting two or more devices. (3) The transmission path from nonswitching subscriber terminal to a switching system.

line hit
Electrical interference that causes the introduction of undesirable signals on a circuit.

line of sight
(1) A characteristic of some open-air transmission technologies (such as microwave, infrared, and open-air laser-type transmissions) in which the path between a transmitter and a receiver must be clear and unobstructed . (2) A clear, open-air, direct transmission path that is free of obstructions such as buildings but may in some cases be impeded by adverse weather or environmental conditions.

line speed
The maximum data rate that can be reliably transmitted over a line.

link
(1) A physical circuit between two points. (2) A conceptual (or virtual) circuit between two users of a packet switched (or other) network that allows them to communicate, even when different paths are used.

link redundancy level
The ratio of actual number of paths to the minimum number of paths required to connect all nodes of a network.

link state
A state in which each router is aware of the topology of the entire network. Each router sends out information about the links that the router has to all other routers on the network. The final routing table is based upon the shortest path to each destination. Most new routing protocols are based on this algorithm.

link-state protocol
A generic class of routing protocols in which information about the status of the entire network is propagated to every node and used in routing decisions. OSPF, IS-IS, and NLSP are link-state routing protocols.

LLC (logical link control)
A protocol developed by the IEEE 802, common to all of its local network standards, for data link-level transmission control. The upper sublayer of the IEEE Layer 2 (OSI) protocol that complements the MAC protocol (IEEE 802.2). LLC 1 is a minimal function LLC that supports connectionless link layer service. LLC 2 supports connection-oriented data link service.

LMDS (Local Multipoint Distribution Service)
A technique for supplying broadband access via a point-to-point microwave digital system. Referred to as Multipoint Video Distribution service in Europe, it operates over a very large frequency allocation, a 1.3GHz band that’s generally located somewhere in the range of 28GHz to 45GHz, depending on the country. It is a popular technique for deploying wireless local loop.

LMI (local management interface)
A Frame Relay specification for the method of exchanging status information between the user (e.g., bridge or router) and the network.

loading
Adding loading coils to a transmission line to minimize amplitude distortion.

loading coil
An induction device used in local loops, generally those exceeding 3.4 miles (5.5 kilometers) in length, that compensates for the wire capacitance and serves to boost voice grade frequencies. They are often removed for new generation, high-speed, local loop data services, because they can distort data signals at higher frequencies than those used for voice.

local
Pertains to a device that is connected directly to the computer without using a WAN communication line.

local exchange
The switching center in which subscribers’ lines terminate. The exchange has access to the other exchanges and to national trunk networks. Also called central office, end office, serving office, and Class 5 office.

local exchange trunk
A trunk between the CPE and the local exchange. Also referred to as central office trunk.

local loop
A line connecting a customers’ telephone equipment with the local telephone company exchange. Often referred to as a subscriber line, an access line, or the last mile.

local number portability
A service that enables you to keep your own telephone number when you move to a new location.

local service area
The area within which the telephone operating company uses local rates for calling charge.

LocalTalk
Apple Computer’s proprietary 230.4Kbps baseband CSMA/CA network protocol.

location-based online services
Services provided over a wireless infrastructure that are based on the location of the user. The location of the user can be determined by global positioning systems (GPS) or by cellular networks. Radio signals emitted from cellular phones can be tracked from cellular towers and triangulated, yielding locations nearly as accurate as those from a GPS receiver.

logical address
An address that is used to identify the communications program by “name” to the protocol stack with which you are working. No matter where your program is put in the network, your logical address will remain the same, even though your physical address may change.

long-haul
Long-distance, describing (primarily) telephone circuits that cross out of the local exchange.

loop
(1) A local circuit between an exchange and subscriber CPE, either residential (single line telephone) or business (PBX). Also called subscriber loop, local line, and local loop. (2) In programming, a sequence of computer instructions that repeats itself until a predetermined count or other test is satisfied.

loop back
A diagnostic procedure used for transmission devices; a test message is sent to a device being tested, which then sends the message back to the originator for comparison with the original transmission. Loop-back testing may be performed within a locally attached device or conducted remotely over a communications circuit.

loop circuit
The circuit connecting the subscriber’s equipment with the local exchange switch. Also called metallic circuit and local loop.

loop signaling systems
Any of the three methods of transmitting signaling information over the metallic loop formed by the trunk conductors and the terminating equipment bridges. Transmission of the loop signals can be accomplished by (a) opening and closing the DC path around the loop, (b) reversing the voltage polarity, or (c) varying the value of the equipment resistance.

loop start
The most commonly used method of signaling an off-hook condition between an analog phone set and a switch, whereby picking up the receiver closes a wire loop, allowing DC current to flow, which is detected by a PBX or local exchange and interpreted as a request for service.

loss
A decrease in energy of signal power in transmission along the circuit as a result of the resistance of impedance of the circuit or equipment.

lossless compression
In data compression, the process by which the information is recovered without any alteration after the decompression stage. This technique is used for computer-based data or programs. It may also be required in certain multimedia applications where the accuracy of the information is essential, such as in medical imaging. Lossless compression is also called bit-preserving or reversible compression. Examples of lossless compression include run-length encoding or Huffman encoding.

lossy compression
In data compression, the case in which the decompressed information is different from the original uncompressed information. This mode is suitable for most continuous media, such as sound and motion video, as well as for many images. That the decompressed information is different from the original in lossy compression does not imply that the perceptual response of an observer is different. Also called irreversible compression.

low frequency
Generally indicates frequencies between 30KHz and 300KHz.

low-level language
A programming language in which instructions have a 1-to-1 relationship with machine code.

LPC (linear predictive coding)
A vector-quantization-based compression scheme for speech. It can compress speech down to 2.4Kbps.

LSR (label-switching router)
An MPLS-enabled router and/or MPLS-enabled ATM switch. As each packet enters the network, an ingress LSR assigns it a label, based on its destination, VPN membership, ToS bits, and other considerations. At each hop, an LSR uses the label to index a forwarding table.