local area network (LAN), a computer network dedicated to sharing data among several single-user workstations or personal computers, each of which is called a node. A LAN can have from two to several hundred such nodes, each separated by distances of several feet to as much as a mile, and should be distinguished from connections among computers over public carriers, such as telephone circuits, that are used for other purposes. Because of the relatively small areas involved, the nodes in a LAN can be connected by special high-data-rate cables. A metropolitan area network (MAN) is defined as being restricted to a larger area (maximum distances of 50-60 miles) than a LAN but one still small enough so that dedicated links (such as microwave links) can be used. See also modem.

A local area network (LAN) is a computer network covering a small physical area, like a home, office, or small group of buildings, such as a school, or an airport. The defining characteristics of LANs, in contrast to wide-area networks (WANs), include their usually higher data-transfer rates, smaller geographic range, and lack of a need for leased telecommunication lines.

Ethernet over unshielded twisted pair cabling, and Wi-Fi are the two most common technologies currently, but ARCNET, Token Ring and many others have been used in the past.

History

As larger universities and research labs obtained more computers during the late 1960s, there was increasing pressure to provide high-speed interconnections. A report in 1970 from the Lawrence Radiation Laboratory detailing the growth of their "Octopus" network, gives a good indication of the situation.

Early systems

Cambridge University's Cambridge Ring was started in 1974 but was never developed into a successful commercial product.

Ethernet was developed at Xerox PARC in 1973–1975, and filed as . In 1976, after the system was deployed at PARC, Metcalfe and Boggs published their seminal paper - "Ethernet: Distributed Packet-Switching For Local Computer Networks"''

ARCNET was developed by Datapoint Corporation in 1976 and announced in 1977 - and had the first commercial installation in December 1977 at Chase Manhattan Bank in New York

The personal computer

In reality, the concept was marred by proliferation of incompatible physical layer and network protocol implementations, and confusion over how best to share 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 the 40 or so competing card/cable types, and a much more sophisticated operating system than most of its competitors. Netware dominated 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.

The development and proliferation of CP/M-based personal computers from the late 1970s and then DOS-based personal computers a from 1981 meant that a single site began to have dozens or even hundreds of computers. The initial attraction of networking these was generally to share disk space and laser printers, which were both very 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”.

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. None of these were particularly successful.

In this same timeframe, 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. A TCP/IP based LAN is also known as an Intranet.

Cabling

Early LAN cabling had always been based on various grades of co-axial cable, but IBM's Token Ring used shielded twisted pair cabling of their own design, and in 1984 StarLAN showed the potential of simple Cat3 unshielded twisted pair—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 LANs today.

Technical aspects

Although switched Ethernet is now the most common data link layer protocol and IP as a network layer protocol, many different options have been used, and some continue to be popular in niche areas. Smaller LANs generally consist of one or more switches linked to each other—often with one connected to a router, cable modem, or DSL 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 via VLANs. Larger LANS also contain a wide variety of network devices such as switches, firewalls, routers, load balancers, sensors and so on.

LANs may have connections with other LANs via leased lines, leased services, or by 'tunneling' across the Internet using VPN technologies. Depending on how the connections are made and secured, and the distance involved, they become a Metropolitan Area Network (MAN), a Wide Area Network (WAN), or a part of the internet.

See also

• Computer network - general overview
• Ethernet physical layer
• LAN messenger
• LAN party
• Metropolitan area network
• Network card
• Wide area network

References

External links

• LAN design and sizing
• Types of LAN Cables

• Going The Distance