Communications networks differ in geographical size. Networks may be constructed within a building or across several buildings. Networks may also be citywide and even international, using both cable and air connections. There are three major network types: LAN (local area networks), MAN (metropolitan area networks), and WAN (wide area networks).

Local Area Networks (LANs)

A computer communications network contained in a small area such as a commercial building is known as a local area network (LAN). A local area network's linkages usually are accomplished with either telephone, coaxial, or fiber-optic cables. Often, LANs use a bus configuration. The major benefit of a local area network is that it can help to reduce costs by allowing people and microcomputers to share expensive resources. Also, local area networks enable end users to participate in office automation technology. In addition, LANs may be connected to other networks by means of a network gateway. Network designers can choose the method in which messages are controlled in a local area network. Two basic methodologies are the token-passing approach and the contention-based approach. The token- passing approach allows the designers of a network to achieve a degree of centralized control. A group of data bits, or "token," is passed from one network node to another. A node can only send a message when it is in possession of the token.

In the contention-based approach, a node that wishes to send a message first listens to determine if another node is currently sending a message. If not, the node attempts to send its message. However, the lack of centralized control can result in a collision - two nodes attempting to send messages simultaneously. Consequently, contention-based approach is usually not suitable for networks with a large amount of communications activity.

Why Use an LAN?

There are two basic reasons for developing an LAN: information sharing and resource sharing.

• Information sharing: This refers to having users who access the same data files, exchange information via electronic mail, or search the Internet for information. The main benefit of information sharing is improved decision making, which makes it generally more important than resource sharing.

• Resource sharing: It refers to one computer sharing a hardware device (e.g., a printer) or a software package with other computers on the network. The main benefit of resource sharing is cost savings.