Local area network (LAN), any communications network for connecting computers within a building or small group of buildings. A LAN can be configured as a bus (1), a main route to which secondary nodes or channels are connected in a branch frame, (2) a ring, where each computer is connected to two adjacent computers to make a closed circuit, or (3 ) a star, in which each computer is connected directly to a central computer and indirectly to each other. All of them have advantages, although bus installation is the most common.
Even if only two computers are connected, they must follow
rules or protocols to communicate. For example, one person might send a
"ready to send" signal and wait for the other to give a "ready
to send." When multiple computers share a network, the protocol may
include "talk only when it's your turn" or "don't talk while
someone else is talking". Protocols must also be designed to handle
network errors.
The most popular LAN design since the mid-1970s is
bus-connected Ethernet, originally developed by Xerox PARC. Every computer or
other device on Ethernet has a unique 48-bit address. Every computer that tries
to send a signal listens to a transmission signal indicating that a
transmission is in progress. If it receives nothing, it starts sending, it
sends the recipient's address at the beginning of its sending. Every system on
the Internet receives all messages but ignores those that are not targeted.
While a system is launching, it also listens and if it detects simultaneous
transmission, it stops, stops randomly, and tries again. Random delays before
retry reduce the likelihood of relapse. This system is called a Multi-Sense
Carrier with Collision Detection (CSMA / CD). It works very well until the
network is moderately loaded, and then slows down when collisions are more
frequent.
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