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Computer networks that usually cover a limited range, say, within the boundary of a building. A computer network is two or more computers that communicate with each other through some medium. The primary usage of local-area networks (LANs) is the sharing of hardware, software, or information, such as data files, multimedia files, or electronic mail. Resource sharing provided by local-area networks improves efficiency and reduces overhead. See also Digital computer; Electronic mail; Multimedia technology.

Four basic types of media are used in local-area networks: coaxial cable, twisted-pair wires, fiber-optic cable, and wireless. Each medium has its advantages and disadvantages relative to cost, speed, and expandability. Coaxial cables provide high speed and low error rates. Twisted-pair wires are cheaper than coaxial cables, can sustain the speeds common to most personal computers, and are easy to install. Fiber-optic cable is the medium of choice for high-speed local-area networks. Wireless local-area networks have the advantage of expandability. See also Coaxial cable; Communications cable; Fiber-optic circuit; Optical communications; Optical fibers.

The topology of a local-area networks is the physical layout of the network. For wired local-area networks, there are four basic topologies: bus, ring, star, and mesh. The most widely used local-area network topology is the bus, where the medium consists of a single wire or cable to which nodes are attached. A message transmitted over a bus propagates in both directions along the bus, passing each tap until it is finally absorbed at the ends.

There are a number of ways in which nodes can communicate over a network. The simplest is to establish a dedicated link between the transmitting and receiving stations. This technique is known as circuit switching. A better way of communicating is to use a technique known as packet switching, in which a dedicated path is not reserved between the source and the destination. Data are wrapped up in a packet and launched into the network. In this way, a node only has exclusive access to the medium while it is sending a packet. During its inactive period, other nodes can transmit. A typical packet is divided into preamble, address, control, data, and error-check fields. See also Packet switching.

An access protocol is a set of rules observed by all the nodes in a local-area network so that one node can get the attention of another and its data packet can be transferred. Two common protocols are carrier sense multiple access with collision detection (CSMA/CD) and token passing.

With the CSMA/CD protocol, a node that wants to transmit its data must first listen to the medium to hear if any other node is using the medium. If not, the node may transmit immediately. However, while the transmission is taking place, the transmitting node must continue listening to ascertain if anyone else has begun transmitting. If the transmitting node detects that someone else is also transmitting, the node aborts its own transmission, waits for a random amount of time, and then restarts the process until its data transmission succeeds.

With the token-passing protocol, the right to transmit is granted by a token, a predefined bit pattern that is recognized by each node. The token is passed for one node to another in a predetermined order.

(Local Area Network) A communications network that is typically confined to a building or premises. The "clients" are user workstations running the Windows, Mac or Linux operating systems, while the "servers" hold programs and data shared by the clients. Servers come in a wide range of sizes from PC-based servers to mainframes.

A LAN is a local network, whereas a WAN (wide area network) spans long distances. See WAN.

Thick and Thin Clients

In a company LAN, the client machines are mostly Windows-based PCs that contain a variety of installed applications. These "thick" clients are the norm; however, some organizations use "thin" clients, which are stripped-down machines. Windows terminals are also used, which are PCs that act like input/output terminals to a central server and perform no business processing on their own. See thin client and Remote Desktop Services.

The Network OS

The software that enables sharing is the network operating system in the servers, typically running Windows, Linux or Unix. A component part resides in each client operating system, which allows the application in the client to read and write data from the server as if it were on the local machine.

Client workstations can also function as a server, allowing users access to data on another user's machine. These peer-to-peer networks are often simpler to install and manage, but dedicated servers provide better performance and handle higher transaction volume. In large networks, multiple dedicated servers are used.

The Transport

Data transfer over the LAN is typically managed by the TCP/IP transport protocol, and the physical transmission is handled by Ethernet. The actual communications path is twisted pair wire or optical fiber, which physically interconnects each client, server and network device. Using Wi-Fi, the wireless counterpart of Ethernet, clients and servers can connect without cables. See WAN, TCP/IP, Ethernet and client/server.

Clients and Servers in a LAN

Software in a Network Client

Software in a Network Server

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In the modern office environment, each worker is equipped with a personal computer, containing its own disk drives and processor. Each of these computers can communicate with another by the way of a local area network (LAN), which is a computer network that covers a small area, usually a single building or group of buildings. In addition, the LAN may also connect the network of computers with a series of printers, a mainframe computer or file server with even greater processing power and memory storage, and with other devices that can send messages from the network over telephone lines to another location.

As the name suggests, a LAN is local, meaning that it is a proprietary system limited to a finite number of users. It generally serves an area of less than one mile. It is also a network, affording users both functional and communicative diversity through a distribution of resources. A LAN permits workers—isolated in separate offices—to operate off the same system, as if they were all sitting around a single computer.

One of the great attributes of a LAN is that it may be installed simply, upgraded or expanded with little difficulty, and moved or rearranged without disruption. LANs are also useful because they can transmit data quickly. Perhaps most importantly, anyone familiar with the use of a personal computer can be trained to communicate or perform work over a LAN. But despite their great potential and capabilities, LANs have yet to demonstrate an increase in office productivity. They have certainly eliminated paper and speeded the flow of information, but in many cases they have also created additional work in terms of organization, maintenance, and trouble-shooting.

The History of Lans

The advent of personal computers changed the type of information sent over office computer networks. Terminals were no longer "dumb," but contained the power to perform their own instructions and maintain their own memories. This took considerable pressure off mainframe devices, whose energies could now be devoted to more complex tasks.

LANs allowed for the transmission of data between workers. In turn, they enabled this shared data to be directed to a common printer, serving a larger group of users. This eliminated the need for each worker to have a printer and ensured that the one printer provided was not underutilized. In addition, LANs allowed data to be called up directly on other workers' computers, providing immediate communication and eliminating the need for paper. The most common application was in interoffice communications, or electronic mail (e-mail). Messages could be directed to one or several people and copied to several more over the LAN. As a result, an e-mail system became something of an official record of communications between workers. Addressees became obligated to respond to e-mail messages in a timely manner because their failure to answer could be easily documented for supervisors.

Personal computers transformed LANs from mere shared processors to fully integrated communication devices. With processing power distributed among several computers, the mainframe's main role was eclipsed and complex processing, administrative functions, and data file storage became the job of a new device, the file server. Today, there are many different types of LANs. For example, many Macintosh computers use Appletalk, while IBM computers commonly use Ethernets.

Physical Components of Lans

The physical properties of a LAN include network access units (or interfaces) that connect the personal computer to the network. These units are actually interface cards installed on computer motherboards. Their job is to provide a connection, monitor availability of access to the LAN, set or buffer the data transmission speed, ensure against transmission errors and collisions, and assemble data from the LAN into usable form for the computer.

The next part of a LAN is the wiring, which provides the physical connection from one computer to another, and to printers and file servers. The properties of the wiring determine transmission speeds. The first LANs were connected with coaxial cable, the same type used to deliver cable television. These facilities are relatively inexpensive and simple to attach. More importantly, they provided great bandwidth (the system's rate of data transfer), enabling transmission speeds initially up to 20 megabits per second.

Another type of wiring, developed in the 1980s, used ordinary twisted wire pair (commonly used for telephones). The primary advantages of twisted wire pair are that it is very cheap, simpler to splice than coaxial, and is already installed in many buildings. The downside of this simplicity is that its bandwidth is more limited.

A more recent development in LAN wiring is optical fiber cable. This type of wiring uses thin strands of glass to transmit pulses of light between terminals. It provides tremendous bandwidth, allowing very high transmission speeds and because it is optical rather than electronic, it is impervious to electromagnetic interference. Still, splicing it can be difficult and requires a high degree of skill. The primary application of fiber is not between terminals, but between LAN buses (terminals) located on different floors. As a result, fiber distributed data interface is used mainly in building risers. Within individual floors, LAN facilities remain coaxial or twisted wire pair.

When a physical connection cannot be made between two LANs, such as across a street or between buildings, microwave radio may be used. However, it is often difficult to secure frequencies for this medium. Another alternative in this application is light transceivers, which project a beam of light similar to fiber optic cable, but through the air rather than over cable. These systems do not have the frequency allocation or radiation problems associated with microwave, but they are susceptible to interference from fog and other natural obstructions.

Lan Topologies

LANs are designed in several different topologies, or physical patterns, connecting terminals. These shapes can range from straight lines to a ring. Each terminal on the LAN contends with other terminals for access to the system. When it has secured access to the system, it broadcasts its message to all the terminals at once. The message is picked up by the one or group of terminal stations for which it is intended. The branching tree topology is an extension of the bus, providing a link between two or more buses.

A third topology, the star network, also works like a bus in terms of contention and broadcast. But in the star, stations are connected to a single, central node (individual computer) that administers access. Several of these nodes may be connected to one another. For example, a bus serving six stations may be connected to another bus serving 10 stations and a third bus connecting 12 stations. The star topology is most often used where the connecting facilities are coaxial or twisted wire pair.

The ring topology connects each station to its own node, and these nodes are connected in a circular fashion. Node 1 is connected to node 2, which is connected to node 3, and so on, and the final node is connected back to node 1. Messages sent over the LAN are regenerated by each node, but retained only by the addressees. Eventually, the message circulates back to the sending node, which removes it from the stream.

Transmission Methods Used By Lans

LANs function because their transmission capacity is greater than any single terminal on the system. As a result, each station terminal can be offered a certain amount of time on the LAN, like a timesharing arrangement. To economize on this small window of opportunity, stations organize their messages into compact packets that can be quickly distributed. When two messages are sent simultaneously, they could collide on the LAN causing the system to be temporarily disrupted. Busier LANs usually utilize special software that virtually eliminates the problem of collisions by providing orderly, non-contention access.

The transmission methods used on LANs are either baseband or broadband. The baseband medium uses a high-speed digital signal consisting of square wave DC voltage. While it is fast, it can accommodate only one message at a time. As a result, it is suitable for smaller networks where contention is low. It also is very simple to use, requiring no tuning or frequency discretion circuits. This transmission medium may be connected directly to the network access unit and is suitable for use over twisted wire pair facilities.

By contrast, the broadband medium tunes signals to special frequencies, much like cable television. Stations are instructed by signaling information to tune to a specific channel to receive information. The information within each channel on a broadband medium may also be digital, but they are separated from other messages by frequency. As a result, the medium generally requires higher capacity facilities, such as coaxial cable. Suited for busier LANs, broadband systems require the use of tuning devices in the network access unit that can filter out all but the single channel it needs.

The File Server

The administrative software of the LAN resides either in a dedicated file server; in a smaller, less busy LAN; or in a personal computer that acts as a file server. In addition to performing as a kind of traffic controller, the file server holds files for shared use in its hard drives, administers applications such as the operating system, and allocates functions.

When a single computer is used as both a workstation and a file server, response times may lag because its processors are forced to perform several duties at once. This system will store certain files on different computers on the LAN. As a result, if one machine is down, the entire system may be crippled. If the system were to crash due to undercapacity, some data may be lost or corrupted.

The addition of a dedicated file server may be costly, but it provides several advantages over a distributed system. In addition to ensuring access even when some machines are down, its only duties are to hold files and provide access.

Other Lan Equipment

LANs are generally limited in size because of the physical properties of the network including distance, impedance, and load. Some equipment, such as repeaters, can extend the range of a LAN. Repeaters have no processing ability, but simply regenerate signals that are weakened by impedance. Other types of LAN equipment with processing ability include gateways, which enable LANs operating dissimilar protocols to pass information by translating it into a simpler code, such as ASCII. A bridge works like a gateway, but instead of using an intermediate code, it translates one protocol directly into another. A router performs essentially the same function as a bridge, except that it administers communications over alternate paths. Gateways, bridges, and routers can act as repeaters, boosting signals over greater distances. They also enable separate LANs located in different buildings to communicate with each other.

The connection of two or more LANs over any distance is referred to as a wide area network (WAN). WANs require the use of special software programs in the operating system to enable dial-up connections that may be performed by a telephone lines or radio waves. In some cases, separate LANs located in different cities—and even separate countries—may be linked over the public network.

Lan Difficulties

LANs are susceptible to many kinds of transmission errors. Electromagnetic interference from motors, power lines, and sources of static, as well as shorts from corrosion, can corrupt data. Software bugs and hardware failures can also introduce errors, as can irregularities in wiring and connections. LANs generally compensate for these errors by working off an uninterruptable power source, such as batteries, and using backup software to recall most recent activity and hold unsaved material. Some systems may be designed for redundancy, such as keeping two file servers and alternate wiring to route around failures.

Security problems can also be an issue with LANs. They can be difficult to manage and access because the data they use is often distributed between many different networked sources. In addition, many times this data is stored on several different workstations and servers. Most companies have specific LAN administrators who deal with these issues and are responsible for the use of LAN software. They also work to backup files and recover lost files.

Purchasing a Lan

When considering if a LAN is suitable for a business, several things must be considered. The costs involved and the administrative support needed often far exceed reasonable predictions. A complete accounting of potential costs should include such factors as purchase price of equipment, spare parts, and taxes, installation costs, labor and building modifications, and permits. Operating costs include forecasted public network traffic, diagnostics, and routine maintenance. In addition, the buyer should seek a schedule of potential costs associated with upgrades and expansion and engineering studies.

The vendor should agree to a contract expressly detailing the degree of support that will be provided in installing and turning up the system. In addition, the vendor should provide a maintenance contract that binds the company to make immediate, free repairs when performance of the system exceeds prescribed standards. All of these factors should be addressed in the buyer's request for proposal that is distributed to potential vendors.

LANs can also be purchased for home use. Initially, these kits were expensive and slow and transmitted data via the phone lines in the home. New products have emerged that is faster, more affordable and uses wireless technology such as radio waves to allow multiple computers to share printers and perform other LAN functions. This technology allows phone lines and LANs to be used simultaneously and is perfect for a small business owner that works out of his or her home.

Lans and Small Businesses

A 1996 Business Week article estimated that only 70 percent of small businesses own PCS, and that of those, only 20 percent operate computer networks. But today, many companies offer products designed specifically to help small businesses install and use LANs simply and affordably. Many major software and network providers are planning products tailored toward the small business market as well. As a result, many more small businesses should soon be able to gain access to the advantages of LANs.

Further Reading:

Arnst, Catherine, et al. "Wiring Small Business." Business Week. November 25, 1996.

Derfler, Frank J., and David Greenfield. "Beyond the LAN." PC Magazine. December 20, 1994.

Feltman, Charles. "A Reality Check on Virtual LANs." Business Communications Review. July 1996.

Green, Harry James. The Business One Irwin Handbook of Telecommunications. 2nd ed. Business One Irwin, 1991.

Seitel, Peter. "Realistic Expectations Are Linchpins of Decent LAN." Puget Sound Business Journal. September 16, 1994.

See also: Wide Area Networks

• General Technology - LAN: local area network; system for linking terminals, PCs, or work stations with each other or with mainframe to share data, peripherals, and programs, usu. in one building

"LAN" redirects here. For other uses, see LAN (disambiguation).

Computer network types by geographical scope
Near field (NFC) Body (BAN) Personal (PAN) Near-me (NAN) Local (LAN) Home (HAN) Storage (SAN) Campus (CAN) Backbone Metropolitan (MAN) Wide (WAN) Internet Interplanetary Internet
v t e

A local area network (LAN) is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building using network media.^[1] The defining characteristics of LANs, in contrast to wide area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased telecommunication lines.

ARCNET, Token Ring and other technology standards have been used in the past, but Ethernet over twisted pair cabling, and Wi-Fi are the two most common technologies currently used to build LANs.

Contents 1 History 1.1 Standards evolution 1.2 Cabling 2 Technical aspects 3 See also 4 References 5 External links

History

How a local area network using Ethernet might be connected

The increasing demand and use of computers in universities and research labs in the late 1960s generated the need to provide high-speed interconnections between computer systems. A 1970 report from the Lawrence Radiation Laboratory detailing the growth of their "Octopus" network^[2][3] gave a good indication of the situation.

Cambridge Ring was developed at Cambridge University in 1974^[4] but was never developed into a successful commercial product.

Ethernet was developed at Xerox PARC in 1973–1975,^[5] and filed as U.S. Patent 4,063,220. In 1976, after the system was deployed at PARC, Metcalfe and Boggs published a seminal paper, "Ethernet: Distributed Packet-Switching For Local Computer Networks."^[6]

ARCNET was developed by Datapoint Corporation in 1976 and announced in 1977.^[7] It had the first commercial installation in December 1977 at Chase Manhattan Bank in New York.^[8]

Standards evolution

The development and proliferation of personal computers using the CP/M operating system in the late 1970s, and later DOS-based systems starting in 1981, meant that many sites grew to dozens or even hundreds of computers. The initial driving force for networking was generally to share storage and printers, which were both expensive at the time. There was much enthusiasm for the concept and for several years, from about 1983 onward, computer industry pundits would regularly declare the coming year to be “the year of the LAN”.^[9][10][11]

In practice, the concept was marred by proliferation of incompatible physical layer and network protocol implementations, and a plethora of methods of sharing resources. Typically, each vendor would have its own type of network card, cabling, protocol, and network operating system. A solution appeared with the advent of Novell NetWare which provided even-handed support for dozens of competing card/cable types, and a much more sophisticated operating system than most of its competitors. Netware dominated^[12] the personal computer LAN business from early after its introduction in 1983 until the mid 1990s when Microsoft introduced Windows NT Advanced Server and Windows for Workgroups.

Of the competitors to NetWare, only Banyan Vines had comparable technical strengths, but Banyan never gained a secure base. Microsoft and 3Com worked together to create a simple network operating system which formed the base of 3Com's 3+Share, Microsoft's LAN Manager and IBM's LAN Server - but none of these were particularly successful.

During the same period, Unix computer workstations from vendors such as Sun Microsystems, Hewlett-Packard, Silicon Graphics, Intergraph, NeXT and Apollo were using TCP/IP based networking. Although this market segment is now much reduced, the technologies developed in this area continue to be influential on the Internet and in both Linux and Apple Mac OS X networking—and the TCP/IP protocol has now almost completely replaced IPX, AppleTalk, NBF, and other protocols used by the early PC LANs.

Cabling

Early LAN cabling had always been based on various grades of coaxial cable. However shielded twisted pair was used in IBM's Token Ring implementation, and in 1984 StarLAN showed the potential of simple unshielded twisted pair by using Cat3—the same simple cable used for telephone systems. This led to the development of 10Base-T (and its successors) and structured cabling which is still the basis of most commercial LANs today. In addition, fiber-optic cabling is increasingly used in commercial applications.

As cabling is not always possible, wireless Wi-Fi is now the most common technology in residential premises^{[citation needed]}, as the cabling required is minimal and it is well suited to mobile laptops and smartphones.

Technical aspects

Network topology describes the layout pattern of interconnections between devices and network segments. Switched Ethernet has been for some time the most common Data Link Layer and Physical Layer implementation for local area networks. At the higher layers, the Internet Protocol (TCP/IP) has become the standard. Smaller LANs generally consist of one or more switches linked to each other, often at least one is connected to a router, cable modem, or ADSL modem for Internet access.

Larger LANs are characterized by their use of redundant links with switches using the spanning tree protocol to prevent loops, their ability to manage differing traffic types via quality of service (QoS), and to segregate traffic with VLANs. Larger LANs also contain a wide variety of network devices such as switches, firewalls, routers, load balancers, and sensors.^[13]

LANs may have connections with other LANs via leased lines, leased services, or by tunneling across the Internet using virtual private network technologies. Depending on how the connections are established and secured in a LAN, and the distance involved, a LAN may also be classified as a metropolitan area network (MAN) or a wide area network (WAN).

See also

	Computer networking portal
	Computer Science portal

• IEEE 802 family of IEEE standards
• Ethernet physical layer
• LAN messenger
• LAN party
• Network card

References

1. ^ Gary A. Donahue (2007-06). Network Warrior. O'Reilly. p. 5. 
2. ^ Samuel F. Mendicino (1970-12-01). "Octopus: The Lawrence Radiation Laboratory Network". Rogerdmoore.ca. Archived from the original on 2010-10-11. http://www.webcitation.org/5tP07Xoec. 
3. ^ "THE LAWRENCE RADIATION LABORATORY OCTOPUS". Courant symposium series on networks (Osti.gov). 29 Nov 1970. OSTI 4045588. 
4. ^ "A brief informal history of the Computer Laboratory". University of Cambridge. 20 December 2001. Archived from the original on 2010-10-11. http://www.webcitation.org/5tP0nKIlL. 
5. ^ "Ethernet Prototype Circuit Board". Smithsonian National Museum of American History. http://americanhistory.si.edu/collections/object.cfm?key=35&objkey=96. Retrieved 2007-09-02. 
6. ^ "Ethernet: Distributed Packet-Switching For Local Computer Networks". Acm.org. http://www.acm.org/classics/apr96/. Retrieved 2010-10-11. 
7. ^ "ARCNET Timeline". ARCNETworks magazine. Fall 1998. Archived from the original on 2010-10-11. http://www.webcitation.org/5tP1JOSj5. 
8. ^ Lamont Wood (2008-01-31). "The LAN turns 30, but will it reach 40?". Computerworld.com. http://www.computerworld.com/action/article.do?command=viewArticleBasic&articleId=9060198. Retrieved 2010-10-11. 
9. ^ "'The Year of The LAN' is a long-standing joke, and I freely admit to being the comedian that first declared it in 1982...", Robert Metcalfe, InfoWorld Dec 27, 1993
10. ^ "...you will remember numerous computer magazines, over numerous years, announcing 'the year of the LAN.'", Quotes in 1999
11. ^ "...a bit like the Year of the LAN which computer industry pundits predicted for the good part of a decade...", Christopher Herot
12. ^ Wayne Spivak (2001-07-13). "Has Microsoft Ever Read the History Books?". VARBusiness. Archived from the original on 2010-10-11. http://www.webcitation.org/5tP23vwBy. 
13. ^ "A Review of the Basic Components of a Local Area Network (LAN)". NetworkBits.net. http://networkbits.net/lan-components/local-area-network-lan-basic-components/. Retrieved 2008-04-08. 

External links

Wikimedia Commons has media related to: Local area network

v t e Internet access Wired Dial-up DSL DOCSIS Ethernet G.hn HD-PLC HomePlug HomePNA IEEE 1901 ISDN MoCA PON Power line Cable modem Wireless LTE Bluetooth DECT EVDO GPRS HSPA iBurst Li-Fi MMDS Muni Wi-Fi Satellite UMTS-TDD WiBro/WiMAX Wi-Fi Wireless USB

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Dansk (Danish)
abbr. - Local Area Network, lokalt netværk

Nederlands (Dutch)
lokaal netwerk (computers)

Français (French)
abbr. - (abrév = Local Area Network) (Comput) réseau local

Deutsch (German)
abbr. - (Comp.) privates Ortsnetz

Ελληνική (Greek)
abbr. - τοπικό δίκτυο

Italiano (Italian)
LAN

Português (Portuguese)
abbr. - rede (m) local (Comp.)

Русский (Russian)
местная компьютерная сеть

Español (Spanish)
abbr. - red de área local
n. - red de área local

Svenska (Swedish)
abbr. - lokalt nätverk

中文（简体）(Chinese (Simplified))
局部区域网络

中文（繁體）(Chinese (Traditional))
abbr. - 局部區域網路
n. - 局部區域網路

한국어 (Korean)
abbr. - local area network(근거리 정보 통신망)
n. - 건물에 컴퓨터를 연결시키는 조직

日本語 (Japanese)
n. - (電算)区域内情報通信網

العربيه (Arabic)
‏(اختصار) شرفه, فرندة, وافر الصوف أو الوبر‏

עברית (Hebrew)
abbr. - ‮רשת תקשורת מקומית (מחשבים)‬

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