LANs connect many computers in a
relatively small geographical area such as a home, an office, a
building, or a campus.
 The
network could connect each computer to each of the others by using a
separate communications channel from each computer to each of the
others. A direct connection from one computer to another is called a
link. If the network was
designed using links, the number of links would grow
rapidly as new computers were added to the network. For each new
computer, the network would need a new separate connection to each of
the other computers. This approach would be very costly and difficult
to manage.
Starting in the late 1960s and early 1970s, network engineers designed a form of network that enabled many computers in a small area to share a single communications channel by taking turns using it. These LANs now connect more computers than any other type of network. By allowing the computers to share a communications channel, LANs greatly reduce the cost of the network. For economic and technical reasons, links over longer distances are then used to connect computers and networks in separate towns, cities, or even across continents. The general shape or layout of a LAN is called its topology.
When all the computers connect to a central point, the network
takes on a star topology. An alternative topology connects the
computers in a closed loop. Here, the cable is run from one
computer to the next and then from the second to its neighbor
until the last one is connected back to the first. This forms a
ring topology. A third topology, called a bus, attaches each
computer into a single, long cable. Each topology has its benefits
and drawbacks. Today, most LANs are designed using some form of
star topology, although ring and bus layouts are still used in
some installations.
Whatever the layout, or topology, of the network, all LANs require networked computers to share the communications channel that connects them. The communications channel that they all share is called the medium, and it is typically a cable that carries electrical signals through copper, or it may be a fiber optic cable that carries light signals through purified glass or plastic. In the case of wireless networks, the computers may use antennas to broadcast radio signals to each other. In all cases, the computers on a LAN share the medium by taking turns using it. On a LAN, the rules for coordinating the use of the medium are called media access control. Since there are many computers on the network but only one of them can use the medium at a time, there must be some rules for deciding how they will take turns in sharing the network. The media access control rules allow each computer to have its turn in using the medium so that there is a fair and efficient way to share the network. If there are conflicts when more than one computer is contending for the media, the rules ensure that there is an agreed method for resolving the conflict. In later sections of this chapter, the major types of LANs will be reviewed, including the rules for sharing the medium.
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