Local Area Networks (LANs)

Related Terms: Wide Area Networks

In the modern office environment, each worker is equipped with a personal computer with its own processor and multiple disk drives. The computer may be free-standing (very much the exception these days) or it may be connected to a network, minimally to the Internet. In many small operations, like a doctor's office, a single computer may be used—but linked to the Internet. In most typical office situations, the computers of the organization are interconnected to each other as well by way of a local area network (LAN), typically by means of a single dedicated computer known as the "server," short for "file server." The linkage may be by wire or by a special radio frequency. The server used may also provide each "node" in the network with Internet service; and interoffice communications between computers are by e-mail. As the name suggests, such networks are local and shielded from external influences except as these are mediated by the network server, which is itself protected by so-called "firewalls" from unauthorized interference. In larger organizations local networks may be connected to one another. This extended arrangement is then referred to as a wide area network or WAN. Communications between LANs may be over proprietary communications lines (wired, wireless, or a combination) or may use the Internet.

One of the benefits of a LAN is that it may be installed simply and incrementally, upgraded or expanded with little difficulty, and moved or rearranged with little disruption. LANs are also useful because they can transmit data quickly. Use of such networks is becoming ever easier because new employees almost always bring computer skills and Internet experience easily adapted to the local customs.

HISTORY

The advent of personal computers (PCs) changed the type of information sent over office computer networks. Before their rapid spread in the 1970s, employees communicated with mainframe and mini-computers by means of so-called "dumb" terminals. All the processing took place on the main computer that all individuals used simultaneously. When use was heavy, the system's performance slowed. The PCs took over processing tasks at the desk and thus speeded things up substantially. With massive computing power no longer needed, smaller and simpler "file servers" could be substituted. Computerization thus opened up to even quite tiny operations.

LANs developed simultaneously to connect freestanding computers in offices that, until LANs came, exchanged data by passing diskettes around, and in operations using dumb terminals, such terminals first being replaced by PCs and, later, the connection to mainframes severed with the PCs now connected either to each other or to a server; using servers became by far the most common LAN configuration.

Developments in LANs proceeded along two fronts in the 1990s: competing networking software systems developed and changes in wiring took place to provide ever-faster communications speeds. Wireless transmission appeared in the mid-1990s and had become the leading edge of LAN technology by the mid-2000s using a new radio-communications standard known as 802.11, issued by the Institute of Electrical and Electronics Engineers, Inc. With the foundation of Wi-Fi Alliance in 1998 as a certification agency, "Wi-Fi" has come to mean wireless communications. The abbreviation stands for Wireless Fidelity. Wireless LANs are referred to as WLANs and sometimes as LAWNs.

During the 1990s, as well, global networking brought about by the explosive development of the Internet has played an enhancing role—enhancing the intimate local aspects of LANs by giving such networks national, indeed international, access too. LAN technology, in fact, has migrated from businesses to homes. In many residences multiple computers are linked by network connections, some connected by wire and some by radio links.

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.

Network cards may communicate with the network either by wire or by radio signal. Wiring remains the most common form in the mid-2000s but may change over time. Where wiring is used, it determines 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 provide 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 low cost and simplicity. The downside is a more limited bandwidth.

A yet more recent development in LAN wiring was 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 computers, 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.

Wireless communication is between radio devices which are themselves cards or specialized modems. Advantages are avoidance of wiring costs and hassle; disadvantages are distance limitations and interference. Unless a wireless system is properly configured to use signal encryption, the problem of the "evil twin" appears—a phrase used to label a device that appears to participate in communications because it inadvertently interferes with a poorly configured network.

WIRED LAN TOPOLOGIES

LANs are designed in several different physical arrangements of node computers, known as topologies. These patterns 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, it broadcasts its message to all the terminals at once. The message is picked up by the terminal for which it is intended—or multiples of these. 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.

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