L2F (Layer 2 Forwarding)
A tunneling protocol developed by Cisco Systems. L2F encapsulates Point-to-Point Protocol (PPP) packets within Internet Protocol (IP) packets. This provides the significant benefit of being able to use private IP addresses across a virtual private network (VPN) by hiding them (via encapsulation) from the public network. This concept is important because many enterprises make use of private addressing schemes that conflict with public Internet addressing. See tunneling, PPP and VPN.

L2TP (Layer 2 Tunneling Protocol)
A protocol that combines the features of Microsoft's Point-to-Point Tunneling Protocol (PPTP) and Cisco Systems' Layer 2 Forwarding (L2F) to support multiprotocol or private-address Internet Protocol (IP) traffic across a public IP network. L2TP is a mature Internet Engineering Task Force standard that has been widely implemented. (See L2F and PPTP.)

LAN (local-area network)
A geographically limited communication network that connects users within a defined area. A LAN is generally contained within a building or small group of buildings and is managed and owned by a single enterprise. The shorter distances within a building or campus enable faster communications at a lower cost than wide-area networks (WANs). Although an increasing number of LANs use Internet standards and protocols, they are normally protected from the public Internet by firewalls. LANs are generally used to perform the following functions:

LANE (Local-Area Network Emulation)
An asynchronous transfer mode (ATM) standard proposed by the ATM Forum. It makes ATM networks resemble Ethernet or token ring local-area networks (LANs). LANE operates at the media access control layer of the Open Systems Interconnection (OSI) stack, and does not recognize the network layer at all. See ATM, LAN and OSI.

LAPD (Link Access Procedure — D Channel)
An Integrated Services Digital Network (ISDN) data link layer procedure, used for signaling and control on the D channel of an ISDN network.

laptop computer
A portable personal computer (PC) that can be easily used in multiple environments. The portability of the laptop enables individuals to take all of their software, files and applications with them, whether they are in the office, at home or on an airplane. At minimum, laptops include a keyboard, a flat display screen, a central processing unit, a battery and AC adapter, and a hard disk drive. They are frequently also equipped with compact disc read-only memory (CD-ROM) or floppy disk drives, modems and a variety of communication ports. Laptops usually have smaller keyboards and display screens than those used on larger desktop computers; however, they usually have ports to connect external devices such as monitors or keyboards. See PC and CD-ROM.

laser
A device that emits a highly coherent beam of light. The term stands for "light amplification by stimulated emission of radiation." A typical laser has an active medium, which emits light, and a cavity structure, which selects certain wavelengths and directions for the emitted light. Lasers convert electrical energy into radiant energy in the visible or infrared parts of the spectrum, emitting light with a small spectral bandwidth. For this reason, they are widely used in fiber-optic communications, particularly as sources for long-haul links.

laserdisc
A storage medium that uses laser technology to record and retrieve data.

LATA (local access and transport area)
A local telephone network area controlled and operated by a U.S. local-exchange carrier (LEC). Circuits and calls within a LATA (intra-LATA) are generally the sole responsibility of the LEC, while interexchange carriers (IXCs), such as AT&T or MCI, handle circuits and calls that cross LATA boundaries (inter-LATA). See LEC and ICX.

latency
The time interval during which a network station seeks access to a transmission channel and access is granted or received.

layer
In the Open Systems Interconnection (OSI) reference model, a layer is a collection of related network-processing functions that constitute one level of a hierarchy of functions. See OSI.

Layer 2
The term used to describe the data link layer of the Open Systems Interconnection (OSI) reference model. See OSI.

Layer 2 switching
The transparent connection of at least two local-area network (LAN) segments to form a single, logical LAN. In this type of switching (also known as "LAN switching"), local traffic stays within its respective segment, and only traffic destined for another segment is transmitted by the switch. Common switching protocols include 802.1p and 802.1q (see 802.p and 802.1q).

Layer 3
The term used to describe the network layer of the Open Systems Interconnection (OSI) reference model, where protocols such as Internet Protocol (IP) operate. See OSI and IP.

Layer 3 switching
A form of switching that can offer routing at lower costs and higher speeds than traditional routers. Switching normally operates at the data link layer (Layer 2) of the Open Systems Interconnection (OSI) stack, where packets are forwarded according to their Media Access Control address. Layer 3 switches act as routers, forwarding packets in hardware at the network layer, typically using Internet Protocol (IP). See Layer 2 switching, OSI and IP.

Layer 4
The transport layer of the Open Systems Interconnection (OSI) reference model, where such protocols as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) operate. See OSI, TCP and UDP.

Layer 4 switching
An industry term that is misleading and incorrect. While it is physically impossible to switch in Layer 4, Layer 4 "switching" is, by definition, the control of traffic through the use of policies or filters using Layer 4 (transport layer) protocol port IDs to differentiate and act on application-specific traffic.

LCD (liquid crystal display)
A low-powered, flat-panel display technology. LCD displays create images using liquid crystal molecules controlled by an electrical field.

LCMS (learning content management system)
An integrated set of technologies used to manage all aspects of learning content, including authoring or acquisition, content history, auditing, replacement and deletion. An LCMS requires that content be broken into "learning objects," which represent learning events such as classes. These objects can be organized into courses, courses into curricula, curricula into student learning agendas, and so on. Traditional content management software can perform these tasks, but requires customization for the e-learning focus; hence, a new LCMS software category has emerged. See e-learning.

LCR (Lifetime Clinical Record)
A computer-based patient record system from Siemens Medical Solutions Health Services. LCR provides a longitudinal view of patients' lifetime clinical histories.

LDAP (Lightweight Directory Access Protocol)
A server-to-server interface for directory information exchange among directories, devised as a low-cost, simpler implementation of the X.500 Directory Access Protocol. It facilitates the implementation of replication and chaining among dissimilar directories. Proposed by the University of Michigan, it was adopted by Netscape in 1996 for directory lookup, and has become the preferred access path for looking up directory information not only in X.500 directories, but also in many other directory structures on the Internet.

LDMS (LANDesk Management Suite)
An Intel desktop management tool for software distribution, inventory and remote control.

leased line
A dedicated circuit, typically supplied by the telephone company or transmission authority, that permanently connects two or more user locations and is for the sole use of the subscriber. Also called a private line, tie line or dedicated facility.

LEC (local-exchange carrier)
The carrier or local telephone company that handles local access and transport area (LATA) traffic (for example, a Bell operating company or independent local telephone company). See LATA.

LED (light-emitting diode)
A semiconductor that produces light when activated.

legacy
Term used to describe enterprise applications or systems installed in the distant past, but still being used. Typically, they are characterized by outdated technologies, but are still critical to day-to-day operations. Replacing legacy applications and systems with systems based on new and different technologies is one of the IS professional's most significant challenges. As enterprises upgrade or change their technologies, they must ensure compatibility with old systems and data formats that are still in use.

Lempel-Ziv
A compression algorithm developed in the 1970s and selected as the basis for the International Telecommunication Union's V.42 bis compression standard.

LEO (low earth orbit)
A satellite constellation category. LEO satellites orbit at a height of between 800 to 1,200 miles above the earth's surface.

LEP (light-emitting polymer)
A display technology based on the ability of certain plastics to glow when charged with an electric current. Still in the early stages of development, the technology has the long-term potential to enable the creation of flexible displays that could be rolled up and placed in a jacket pocket.

LF (low frequency)
A term generally used to describe frequencies between 3 and 300 kilohertz.

LIDM (line impact dot matrix)
A printing technology that uses a ribbon and an array of impact elements.

life cycle
The complete span of an enterprise's use of an IT asset, from procurement through management and disposal.

LIFO (last in, first out)
A type of message queuing that puts the last message received first in line.

LIMS (laboratory information management system)
Applications used to manage the collection of samples, collection and formatting of test results, and the reporting of results by sample or product category. LIMS applications may be environmental-, medical- or research-focused.

line
A communications path between two or more points — for example, a satellite or microwave channel, or transmission path from a nonswitching subscriber terminal to a switching system (see leased line).

line balancing
The optimization of the assignment of operations to workstations in an assembly line to minimize idle time and the number of workstations required.

line driver
A communications transmitter/receiver used to extend the transmission distance between terminals and computers that are directly connected. It acts as an interface between logic circuits and a two-wire transmission line.

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

link state protocol
Any routing protocol derived from the Dijkstra algorithm such as Shortest Path First (SPF), Open Shortest Path First (OSPF) and Intermediate System to Intermediate System (IS-IS). Routers that use link state protocols update each other and learn network topology by periodically broadcasting link state data across the network. The data from each router includes only the cost and identification of those networks directly connected to that router. All routers on a network compile tables of routers and connections and can then calculate optimal paths from themselves to each link. See SPF, OSPF and IS-IS.

Linpack
An early benchmark for scientific applications, since replaced by System Performance Evaluation Cooperative (SPEC) benchmarks. See SPEC.

Linux
A version of Unix developed by Linus Torvalds, who sought to champion openness and counter the closed nature of vendor-proprietary operating systems. The Linux credo says that the software must be made available as open-source code, enabling anyone to read the coding rules and submit improvements. The source code is accessible free on the Web or is otherwise virtually given away, and vendors derive their revenue from utilities and support.

LISP (List Processing)
An object-oriented programming language.

little-endian
A method of storing or transmitting data where the most significant bit or byte is presented last. (The name is an allusion to Jonathan Swift's "Gulliver's Travels," which contained characters so named because they believed that boiled eggs should be eaten from the "little end" first.) See big-endian.

LMDS (local multipoint distribution service)
A microwave-based wireless technology that operates at around 28 gigahertz. In the United States and other countries, it is used for fixed high-speed data, Internet access, and advanced telephone and entertainment services in metropolitan areas.

LMS (learning management system)
A full infrastructure on which e-learning can be built and delivered. An LMS has six main components:

See courseware, e-learning and LCMS.

LNP (local number portability)
The ability to change to a different local phone service provider while retaining the same phone number.

LNTT (line nonimpact thermal transfer)
A category of printer technology.

load balancing
The ability of processors to schedule themselves to ensure that all are kept busy while instruction streams are available.

LOB (line of business)
A corporate subdivision focused on a single product or family of products.

LOC (line of code)
A unit used in measuring or estimating the scale of programming or code conversion efforts.

local exchange
The exchange in which subscribers' lines terminate. It has access to other exchanges and to national trunk networks. Also called local central office or end office.

local loop
A local circuit between a local exchange and a subscriber telephone station. Also called a subscriber loop or local line. See local exchange.

location service
A service that enhances applications for mobile telephones and devices by exploiting information about where the user is located. This information is detected through technologies deployed in the carrier's infrastructure and in the handset. Location methods used by these technologies include cell of origin, time of arrival (TOA), angle of arrival (AOA), Enhanced Observed Time Difference (EOTD) and Global Positioning System (GPS). See cell of origin, TOA, AOA, EOTD and GPS.

log-on
The procedure by which a user begins a system or network session (sometimes presented as "logon"). It often involves the entry of a user name and password for authentication purposes. Synonyms include log-in and sign-on.

logistics network
A supply network that, like a computer network, can be configured by different hubs, routers and transmission lines. (The logistics network corollaries are warehouses, replenishment rules and trucking companies.) Tools exist to model and optimize the performance of the network. These systems rationalize and quantify the role of each warehouse in satisfying corporate objectives.

log-off
The procedure by which a user ends a system or network session (sometimes presented as "logoff"). Synonyms include log-out and sign-off.

LOINC (Logical Observation Identifier Names and Codes)
A system of names and codes for identifying laboratory and clinical observations.

long-haul
Long-distance — telephone circuits that cross out of the local exchange.

look and feel
The appearance and behavior of a graphical user interface to the end user.

loosely coupled
An application integration term. A loosely coupled messaging communication model is intermediated by a queue. The two parties in communication are not connected directly; instead, they are both connected to the intermediary. Such an intermediary may be a persistent queue, as in most message-oriented middleware (MOM); a network, as in Hypertext Transport Protocol (HTTP); or an in-memory queue, as in Bea Systems' Tuxedo and most MOM. The data that is exchanged is a message that is usually associated with a unique identifier. See application integration, MOM and HTTP.

loss
The decrease in the energy of signal power in transmission along a circuit due to the resistance or impedance of the circuit or equipment.

lossless compression
A data compression technique where the recovered data after decompression is precisely the same as before compression.

lossy compression
A data compression technique where the recovered data after decompression is different from the data that was compressed, though it will not be perceived as such by an observer.

LPAR (logical partition)
The division of a single computer (such as a mainframe or midrange server) into two or more independent environments, each running its own operating-system instance. An LPAR enables formerly separate systems to be consolidated on a single machine.

LSI (large-scale integration)
Between 3,000 and 100,000 transistors on a chip.

LTL (less than truckload)
A logistics term denoting a segment of the shipping market, wherein volume of each shipment is less than the volume of a single truckload.

LTO (Linear Tape Open)
A tape storage format, originally developed by Hewlett-Packard, IBM and Seagate Technology.

LTO-CM (Linear Tape Open Cartridge Memory)
A Linear Tape Open (LTO) tape format feature, which consists of an embedded, noncontacting, passive radio frequency interface that accesses 4 kilobytes of nonvolatile memory. Similar to the memory-in-cassette (MIC) feature used in Advanced Intelligent Tape (AIT) drives, LTO-CM can be used to capture metadata about the contents of the tape cartridge without consuming drive resources. See LTO, MIC and AIT.

LU (Logical Unit)
IBM's term for a logical node that is the user's "port" into a Systems Network Architecture (SNA) network. LU 1 is a high-performance print and 3270 data stream. LU 6 enables host-to-host data exchange, and provides a peer-to-peer data stream. See SNA.

LU 6.2 (Logical Unit 6.2)
An IBM Systems Network Architecture (SNA) protocol. LU 6.2 is a device-independent process-to-process protocol; it also supports asynchronous (store-and-forward) networking. It provides the facilities for peer-to-peer communication between programs. See SNA.

LUN (logical unit number)
A number used in a small Computer Systems Interface (SCSI) bus to identify a peripheral device (such as an attached storage system). See SCSI.