Lambda Calculus, considered to be the mathematical basis for programming language, is a calculus developed by Alonzo Church and Stephen Kleene in the 1930s to express all computable functions. In an effort to formalize the concept of computability (also known as constructibility and effective calculability), Church and Kleene developed a powerful language with a simple syntax and few grammar restrictions. The language deals with the application of a function to its arguments (a function is a set of rules) and expresses any entity as either a variable, the application of one function to another, or as a "lambda abstraction" (a function in which the Greek letter lambda is defined as the abstraction operator). Lambda calculus, and the closely related theories of combinators and type systems, are important foundations in the study of mathematics, logic, and computer programming language.
Lambda Switching (sometimes called photonic switching, or wavelength switching) is the technology used in optical networking to switch individual wavelengths of light onto separate paths for specific routing of information. In conjunction with technologies such as dense wavelength division multiplexing (DWDM) - which enables 80 or more separate light wavelengths to be transmitted on a single optical fiber - lambda switching enables a light path to behave like a virtual circuit.
Although the ability to redirect specific wavelengths intelligently is, in itself, a technological breakthrough, lambda switching works in much the same way as traditional routing and switching. Lambda routers - which are also called wavelength routers, or optical cross-connects (OXC) - are positioned at network junction points. The lambda router takes in a single wavelength of light from a specific fiber optic strand and recombines it into another strand that is set on a different path. Lambda routers are being manufactured by a number of companies, including Ciena, Lucent, and Nortel.
Multiprotocol Lambda Switching is a variation of Multiprotocol Label Switching (MPLS , confusingly, the abbreviation for both variants) in which specific wavelengths serve in place of labels as unique identifiers. The specified wavelengths, like the labels, make it possible for routers and switches to perform necessary functions automatically, without having to extract instructions regarding those functions from IP addresses or other packet information.
Lambda switching gets its name from lambda, the 11th letter of the Greek alphabet, which has been adopted as the symbol for wavelength. In networking, the word is used to refer to an individual optical wavelength.
LAMP is an open source Web development platform based on Linux, Apache, MySQL, and PHP, a programming language for which Perl or Python is sometimes substituted. The term was coined in Europe, where these programs are commonly used together and have become something of a standard development environment. The name derives from the first letters of each of the programs. Each program is an open source standard in its own right: Linux is the operating system; Apache is the most commonly-used Web server; mySQL is a relational database management system (RDBMS) with add-on tools for Web-based administration; and PHP is a popular object-oriented scripting language that encompasses the best features of many other programming languages to make it efficient for Web development. Developers that use these tools with a version of a Windows operating system instead of Linux are said to be using WAMP.
LAN - Local Area Network:
See Local Area Network.
Last Mile Technology:
Last-mile technology is any telecommunications technology, such as wireless radio, that carries signals from the broad telecommunication along the relatively short distance (hence, the "last mile") to and from the home or business. Or to put it another way: the infrastructure at the neighborhood level. In many communities, last-mile technology represents a major remaining challenge to high-bandwidth applications such as on-demand television, fast Internet access, and Web pages full of multimedia effects.
Today, in addition to "plain old telephone (dial-up) service", last-mile technologies to deliver voice, data, and TV can include:
* ISDN, a somewhat faster technology than regular phone service
* Digital Subscriber Line (DSL) over existing telephone twisted pair lines
* Cable and the cable modem for data, using the same installed coaxial cable that already is used for television
* wireless, including services such as DirecTV
* Less frequently because of the installation expense, optical fiber and its transmission technologies.
1) In a network, latency, a synonym for delay, is an expression of how much time it takes for a packet of data to get from one designated point to another. In some usages (for example, AT&T), latency is measured by sending a packet that is returned to the sender and the round-trip time is considered the latency.
The latency assumption seems to be that data should be transmitted instantly between one point and another (that is, with no delay at all). The contributors to network latency include:
* Propagation: This is simply the time it takes for a packet to travel between one place and another at the speed of light.
* Transmission: The medium itself (whether optical fiber, wireless , or some other) introduces some delay. The size of the packet introduces delay in a round trip since a larger packet will take longer to receive and return than a short one.
* Router and other processing: Each gateway node takes time to examine and possibly change the header in a packet (for example, changing the hop count in the time-to-live field).
* Other computer and storage delays: Within networks at each end of the journey, a packet may be subject to storage and hard disk access delays at intermediate devices such as switches and bridges. (In backbone statistics, however, this kind of latency is probably not considered.)
2) In a computer system, latency is often used to mean any delay or waiting that increases real or perceived response time beyond the response time desired. Specific contributors to computer latency include mismatches in data speed between the microprocessor and input/output devices and inadequate data buffers.
Within a computer, latency can be removed or "hidden" by such techniques as prefetching (anticipating the need for data input requests) and multithreading, or using parallelism across multiple execution threads.
3) In 3D simulation, in describing a helmet that provides stereoscopic vision and head tracking, latency is the time between the computer detecting head motion to the time it displays the appropriate image.
Layer 2 refers to the Data Link layer of the commonly-referenced multilayered communication model, Open Systems Interconnection (OSI). The Data Link layer is concerned with moving data across the physical links in the network. In a network, the switch is a device that redirects data messages at the layer 2 level, using the destination Media Access Control (MAC) address to determine where to direct the message.
The Data-Link layer contains two sublayers that are described in the IEEE-802 LAN standards:
* Media Access Control (MAC) sublayer
* Logical Link Control (LLC) sublayer
The Data Link layer ensures that an initial connection has been set up, divides output data into data frames, and handles the acknowledgements from a receiver that the data arrived successfully. It also ensures that incoming data has been received successfully by analyzing bit patterns at special places in the frames.
Layer Two Tunneling Protocol - L2TP:
Layer Two Tunneling Protocol (L2TP) is an extension of the Point-to-Point Tunneling Protocol (PPTP) used by an Internet service provider (ISP) to enable the operation of a virtual private network (VPN) over the Internet. L2TP merges the best features of two other tunneling protocols: PPTP from Microsoft and L2F from Cisco Systems. The two main components that make up L2TP are the L2TP Access Concentrator (LAC), which is the device that physically terminates a call and the L2TP Network Server (LNS), which is the device that terminates and possibly authenticates the PPP stream.
PPP defines a means of encapsulation to transmit multiprotocol packets over layer two (L2) point-to-point links. Generally, a user connects to a network access server (NAS) through ISDN, ADSL, dialup POTS or other service and runs PPP over that connection. In this configuration, the L2 and PPP session endpoints are both on the same NAS.
L2TP uses packet-switched network connections to make it possible for the endpoints to be located on different machines. The user has an L2 connection to an access concentrator, which then tunnels individual PPP frames to the NAS, so that the packets can be processed separately from the location of the circuit termination. This means that the connection can terminate at a local circuit concentrator, eliminating possible long-distance charges, among other benefits. From the user's point of view, there is no difference in the operation.
Layer 3 refers to the Network layer of the commonly-referenced multilayered communication model, Open Systems Interconnection (OSI). The Network layer is concerned with knowing the address of the neighboring nodes in the network, selecting routes and quality of service, and recognizing and forwarding to the Transport layer incoming messages for local host domains.
A router is a layer 3 device, although some newer switches also perform layer 3 functions. The Internet Protocol (IP) address is a layer 3 address.
LCP - Link Control Protocol:
See Link Control Protocol.
LDAP - Lightweight Directory Access Protocol:
See Lightweight Directory Access Protocol.
LDIF - Lightweight Directory Interchange Format:
See Lightweight Directory Interchange Format.
Learning Management System - LMS:
A learning management system (LMS) is a software application or Web-based technology used to plan, implement, and assess a specific learning process. Typically, a learning management system provides an instructor with a way to create and deliver content, monitor student participation, and assess student performance. A learning management system may also provide students with the ability to use interactive features such as threaded discussions, video conferencing, and discussion forums. The Advanced Distance Learning group, sponsored by the United States Department of Defense, has created a set of specifications called Shareable Content Object Reference Model (SCORM) to encourage the standardization of learning management systems.
LexiBot is a specialized search tool developed by BrightPlanet, as a means of searching the deep Web (the hidden part of the Web that may contain 500 times the content accessible to conventional search engines). LexiBot uses what BrightPlanet calls a comprehensive and intelligent search technology that enables users to conduct searches using simple text, natural language, or Boolean queries on hundreds of databases simultaneously, filter and analyze data, and publish the results as Web pages. LexiBot was designed to perform complex searches to identify and retrieve content from all areas of the Web, and to process the information.
Deep Web content has been accessible only to those who knew the correct URL for the Web site. Then, even once a user had connected to a database, its data could only be accessed by single direct queries. LexiBot acts as an automated direct query engine to make dozens of queries simultaneously to multiple databases. Searches are supported on close to 600 databases. Once the links are found, LexiBot downloads the links, analyzes them, removes the irrelevant ones, then downloads the text portions of the documents to the user's hard drive. The application can be used with desktop databases as well as the Internet, is customizable to user preferences, and can be set up for either simple or advanced usage.
Lightweight Directory Access Protocol - LDAP:
Lightweight Directory Access Protocol (LDAP) is a software protocol for enabling anyone to locate organizations, individuals, and other resources such as files and devices in a network, whether on the public Internet or on a corporate intranet. LDAP is a "lightweight" (smaller amount of code) version of Directory Access Protocol (DAP), which is part of X.500, a standard for directory services in a network. LDAP is lighter because in its initial version it did not include security features. LDAP originated at the University of Michigan and has been endorsed by at least 40 companies. Netscape includes it in its latest Communicator suite of products. Microsoft includes it as part of what it calls Active Directory in a number of products including Outlook Express. Novell's NetWare Directory Services interoperates with LDAP. Cisco also supports it in its networking products.
In a network, a directory tells you where in the network something is located. On TCP/IP networks (including the Internet), the domain name system (DNS ) is the directory system used to relate the domain name to a specific network address (a unique location on the network). However, you may not know the domain name. LDAP allows you to search for an individual without knowing where they're located (although additional information will help with the search).
An LDAP directory is organized in a simple "tree" hierarchy consisting of the following levels:
* The root directory (the starting place or the source of the tree), which branches out to
* Countries, each of which branches out to
* Organizations, which branch out to
* Organizational units (divisions, departments, and so forth), which branches out to (includes an entry for)
* Individuals (which includes people, files, and shared resources such as printers)
An LDAP directory can be distributed among many servers. Each server can have a replicated version of the total directory that is synchronized periodically. An LDAP server is called a Directory System Agent (DSA). An LDAP server that receives a request from a user takes responsibility for the request, passing it to other DSAs as necessary, but ensuring a single coordinated response for the user.
Lightweight Directory Interchange Format - LDIF:
Lightweight Directory Interchange Format (LDIF) is an ASCII file format used to exchange data and enable the synchronization of that data between Lightweight Directory Access Protocol (LDAP) servers called Directory System Agents (DSAs). LDAP is a software protocol for enabling anyone to locate organizations, individuals, and other resources such as files and devices in a network. An LDAP directory can be distributed among many servers. LDIF is used to synchronize each LDAP directory.
The first step in synchronizing LDAP directories is extracting the full contents of or a portion of the original LDAP directory and formatting the contents into an LDIF file. The LDIF file is then mailed to a directory synchronization robot called DIRBOT. After several different steps, a final LDIF file is compared to the original LDIF file. The update instructions on what records to add, delete, or modify in the original directory are decided. These updates are then used to synchronize all LDAP directories.
Linear Tape Open - LTO:
Linear Tape-Open (LTO) is an open standard for a backup tape system, providing formats for both fast data access and high storage capacity, developed jointly by Hewlett-Packard, IBM, and Seagate. IBM released the first LTO products in August, 2000.
One of the tape industry's biggest problems has been a lack of standards, with each vendor providing its own technology. Standardization means that different manufacturers' tapes and tape drives will interoperate (just as audio tape cassettes work in all tape players). Like existing tape systems, LTO uses a linear multi-channel bi-directional format. LTO adds to existing technologies timing-based servo (a device that automates a process of error correction for a mechanism), hardware data compression, enhanced track layouts, and efficient error correction code.
LTO was developed in two different formats - one for fast data access and another for greater storage capacity. The Accelis format uses 8mm-wide tape on a two-reel cartridge that loads at the mid-point of the tape to provide fast data access, specifically for read-intensive applications, such as online searches and retrieval functions. The Ultrium format uses a single reel of half-inch wide tape to maximize storage capacity, specifically for write-intensive applications, such as archival and backup functions. Early products using the Accelis format offer a 25 gigabyte capacity for uncompressed data, while Ultrium based-products offer a 100 gigabyte capacity. Both formats provide transfer rates of 10 - 20 Mbps . While these figures are not unheard of in other technologies, LTO specifications include plans for expected increases that will double current rates with each of the next three generations of products.
Link Control Protocol - LCP:
In the Point-to-Point Protocol (PPP), the Link Control Protocol (LCP) establishes, configures, and tests data-link Internet connections. Before establishing communications over a point-to-point link, each end of the PPP link must send out LCP packets. The LCP packet either accepts or rejects the identity of its linked peer, agrees upon packet size limits, and looks for common misconfiguration errors. Basically, the LCP packet checks the telephone line connection to see whether the connection is good enough to sustain data transmission at the intended rate. Once the LCP packet accepts the link, traffic can be transported on the network; if the LCP packet determines the link is not functioning properly, it terminates the link.
LCP packets are divided into three classes:
1. Link configuration packets used to establish and configure a link.
2. Link termination packets used to terminate a link.
3. Link maintenance packets used to manage and debug a link.
Link encryption (sometimes called link level or link layer encryption) is the data security process of encrypting information at the data link level as it is transmitted between two points within a network. Data, which is plaintext in the host server, is encrypted when it leaves the host, decrypted at the next link (which may be a host or a relay point), and then reencrypted before it continues to the next link. Each link may use a different key or even a different algorithm for data encryption. The process is repeated until the data has reached the recipient.
Link encryption takes place in the lowest protocol layers (layers 1 and 2 in the OSI model). Because the process protects the message in transit, link encryption is very useful in situations where the security of the transmission line is not assured. However, because the message is decrypted at each host in the transmission path, vulnerability can arise at a link when the message must be transmitted between hosts that are not known to be secure.
Link encryption has been used successfully within organizations, including the military, where the security of each link can be assured. It isn't feasible over the Internet, because intermediate links are neither accessible nor secure.
Linkrot is the tendency of hypertext links from one Web site to another site to become useless as other sites cease to exist or remove or reorganize their Web pages. A recent survey showed that almost one of every four Web pages in the survey sample contained a bad link. Observers note that the amount of linkrot that occurs can be correlated closely (and not surprisingly) with the passage of time. Links to the home page of large companies like IBM and Microsoft appear to be the least likely to "rot." However, links to pages within companies often generate "Not found" messages as a result of site page restructures or "old" material being removed. Links to pages created by students often no longer work after the student graduates.
A good practice followed by many sites is to leave the old pages in place with a message identifying the new page address or Uniform Resource Locator and specifying that an automatic refresh or redirection to the new page take place within a few seconds. A number of Web site development and testing products provide the ability to test all the links in a Web site and report on the failing links.
In Hyper-G and possibly other hypertext systems, a link type is the specification of the nature of the information object being linked to. A single link can have any number of defined link types. For example, for any word from which one might link to another information object, that object could be an example of that word, or a graphic illustration of it, or a definition of it, or it might be associated with one person's comments on that subject.
For example, in a page on transmission technologies, the term "ISDN" might be highlighted as a link. When the link is created, the author might choose to let the reader link to an illustration of how ISDN works, or just a brief definition of it, or a whole book about it. Or a reviewer (rather than the author) might create the link for review purposes and the link would be to a particular reviewer comment. (Later, the author or other reviewers would find the link and the linked-to comments and perhaps create additional comments.)
If link types are made generic and all links exist as separate objects apart from the files in which they are used, then different link types and links can be easily added, removed, or changed throughout a set of files. The examples described here are hypothetical and not necessarily exactly how Hyper-G or any other hypertext system works.
Linux (often pronounced LIH-nuhks with a short "i") is a UNIX-like operating system that was designed to provide personal computer users a free or very low-cost operating system comparable to traditional and usually more expensive UNIX systems. Linux has a reputation as a very efficient and fast-performing system. Linux's kernel (the central part of the operating system) was developed by Linus Torvalds at the University of Helsinki in Finland. To complete the operating system, Torvalds and other team members made use of system components developed by members of the Free Software Foundation for the GNU project.
Linux is a remarkably complete operating system, including a graphical user interface, an X Window System, TCP/IP, the Emacs editor, and other components usually found in a comprehensive UNIX system. Although copyrights are held by various creators of Linux's components, Linux is distributed using the Free Software Foundation's copyleft stipulations that mean any modified version that is redistributed must in turn be freely available.
Unlike Windows and other proprietary systems, Linux is publicly open and extendible by contributors. Because it conforms to the Portable Operating System Interface standard user and programming interfaces, developers can write programs that can be ported to other operating systems. Linux comes in versions for all the major microprocessor platforms including the Intel, PowerPC, Sparc, and Alpha platforms. It's also available on IBM's S/390. Linux is distributed commercially by a number of companies. A magazine, Linux Journal, is published as well as a number of books and pocket references.
Linux is sometimes suggested as a possible publicly-developed alternative to the desktop predominance of Microsoft Windows. Although Linux is popular among users already familiar with UNIX, it remains far behind Windows in numbers of users.
LISP, an acronym for List Processing, is a programming language that was designed for easy manipulation of data strings. Developed in 1959 by John McCarthy, it is a commonly used language for artificial intelligence (AI) programming. It is one of the oldest programming languages still in relatively wide use.
In LISP, all computation is expressed as a function of at least one object. Objects can be other functions, data items (such as constants or variables), or data structures. LISP's ability to compute with symbolic expressions rather than numbers makes it convenient for AI applications.
LMDS - Local Multipoint Distribution System:
See Local Multipoint Distribution System.
LMS - Learning Management System:
See Learning Management System.
LNP - Local Number Portability:
See Local Number Portability.
Local Area Network - LAN:
A Local Area Network (LAN) is a group of computers and associated devices that share a common communications line and typically share the resources of a single processor or server within a small geographic area (for example, within an office building). Usually, the server has applications and data storage that are shared in common by multiple computer users. A local area network may serve as few as two or three users (for example, in a home network) or many as thousands of users (for example, in an FDDI network).
The main local area network technologies are:
* token ring
* FDDI (Fiber Distributed Data Interface)
Typically, a suite of application programs can be kept on the LAN server. Users who need an application frequently can download it once and then run it from their local hard disk. Users can order printing and other services as needed through applications run on the LAN server. A user can share files with others at the LAN server; read and write access is maintained by a LAN administrator. A LAN server may also be used as a Web server if safeguards are taken to secure internal applications and data from outside access.
Local Multipoint Distribution System - LMDS:
Local Multipoint Distribution System (LMDS) is a system for broadband microwave wireless transmission direct from a local antenna to homes and businesses within a line-of-sight radius, a solution to the so-called last-mile technology problem of economically bringing high-bandwidth services to users. LMDS is an alternative to installing optical fiber all the way to the user or to adapting cable TV for broadband Internet service. Depending on the implementation, LMDS offers a bandwidth of up to 1.5 Gbps downstream to users and 200 Mbps upstream from the user. A more typical data rate is 38 Mbps downstream. Some services offer both downstream and upstream service (symmetrical service); others offer downstream only (asymmetrical service) with upstream being obtained using wire connections.
In addition to the investment by service providers for transmitters, users need to install transceivers costing about $125-225. However, the cost of installing LMDS is considered far lower than installing fiber optic cable or upgrading cable TV systems. The first markets for LMDS are seen as:
* High-speed data transmission for businesses
* Interactive television and streaming multimedia from Web sites
* Voice service (usually as a supplement to other services)
Because LMDS requires a more expensive and possibly larger transceiver than can conveniently be packaged in a handheld device, LMDS is not viewed as a replacement for or alternative to mobile wireless technologies such as cellular and GSM . On the other hand, LMDS offers much higher data rates because of its use of a higher range of frequencies with their wider bandwidths. In general, LMDS is for fixed locations and offers higher data rates; cellular digital such as GSM is for mobile users at lower data rates (although these will increase with technologies leading up to UMTS).
LMDS uses the range of electromagnetic radiation spectrum in the vicinity of 28 GHz, with the allocated range differing slightly between the U.S., Canada, and other countries. In Europe, ETSI sponsors an equivalent technology. In Canada, it is called Local Multipoint Communication Service (LMCS). Like cellular telephone technologies, LMDS is point-to-multipoint. It is viewed as a future component in the convergence of data and telephony services. Ericsson's LMDS system uses either Ethernet, ATM, or T-carrier system/E-carrier system network interfaces at the user end. ATM allows the user to select and pay for varying qualities of service.
Local Number Portability - LNP:
Local Number Portability (LNP) is the ability of a telephone customer in the U.S. to retain their local phone number if they switch to another local telephone service provider. The Telecommunications Act of 1996 required that the local exchange carriers (LECs) in the 100 largest metropolitan markets provide this capability by the end of 1998. The idea is that by removing the personal inconvenience of having to get a new phone number when changing service providers, competition among providers will be increased. LNP is one of the prices that local carriers must pay in order to be allowed to compete as well in the long-distance market.
LNP is made possible by the Location Routing Number (LRN). In the future, phone number portability may be extended so that customers can retain their phone number when they move to another locality. LNPs and LRNs are supervised by the Number Portability Administration Center, operated by Lockheed Martin under the appointment of the Federal Communications Commission (FCC). When a customer moves their local service to a competitive local exchange carrier (CLEC), a new LRN is assigned to the telephone number being ported. Each local exchange and long distance carrier needs to know what that new LRN is so when someone in an another area dials the number being ported, the carrier knows what LRN to route to. This is accomplished through Local Service Management System (LSMS) databases distributed among the exchange carriers. The NPAC updates all of these databases with the newly assigned LRN. Thus, when the call is made from another area, that carrier refers to its LSMS database to obtain the current LRN for the number dialed.
Location Routing Number - LRN:
In the U.S., a Location Routing Number (LRN) is a 10-digit number in a database called a Service Control Point (SCP) that identifies a switching port for a local telephone exchange. LRN is a technique for providing Local Number Portability (LNP). Using LRN, when a phone number is dialed, the local telephone exchange queries a routing database, usually the SCP, for the LRN associated with the subscriber. The LRN removes the need for the public telephone number to identify the local exchange carrier. If a subscriber changes to another telephone service provider, the current telephone number can be retained. Only the LRN needs to be changed.
In addition to supporting service provider phone number portability, an LRN also supports the possibility of two other types of number portability: service portability (for example, ordinary service to ISDN) and geographic portability. LRN is an alternative to the current NPA-NXX format described in the North American Telephone Numbering System (NATNS). LNPs and LRNs are supervised by the Number Portability Administration Center, operated by Lockheed Martin under the appointment of the Federal Communications Commission (FCC).
In a computer program, a logic bomb, also called slag code, is programming code, inserted surreptitiously or intentionally, that is designed to execute (or "explode") under circumstances such as the lapse of a certain amount of time or the failure of a a program user to respond to a program command. It is in effect a delayed-action computer virus or Trojan horse . A logic bomb, when "exploded," may be designed to display or print a spurious message, delete or corrupt data, or have other undesirable effects.
Some logic bombs can be detected and eliminated before they execute through a periodic scan of all computer files, including compressed files, with an up-to-date anti-virus program . For best results, the auto-protect and e-mail screening functions of the anti-virus program should be activated by the computer user whenever the machine is online. In a network, each computer should be individually protected, in addition to whatever protection is provided by the network adminstrator. Unfortunately, even this precaution does not guarantee 100-percent system immunity.
Logical Link Control Layer:
In the Open Systems Interconnection (OSI) model of communication, the Logical Link Control layer is one of two sublayers of the Data-Link layer and is concerned with managing traffic (flow and error control) over the physical medium. The Logical Link Control layer identifies a line protocol, such as SDLC, NetBIOS, or NetWare, and may also assign sequence numbers to frames and track acknowledgements.
The other Data-Link sublayer is the Media Access Control layer.
A Logical Partition (LPAR) is the division of a computer's processors, memory, and storage into multiple sets of resources so that each set of resources can be operated independently with its own operating system instance and application s. The number of logical partitions that can be created depends on the system's processor model and resources available. Typically, partitions are used for different purposes such as database operation or client/server operation or to separate test and production environments. Each partition can communicate with the other partitions as if the other partition is in a separate machine. Logical partitioning was first studied by IBM in 1976 and later introduced by Amdahl and then IBM. Hitachi and Sun Microsystems also use forms of logical partitioning. Today, both IBM's S/390 (now z/900 Series) and AS/400 products support logical partitioning.