A Local Area Network (LAN) is a high-speed communications system designed to link computers and other data processing devices together within a small geographic area, such as a workgroup, department, or building. Several LANs can also be interconnected within a campus of buildings to extend connectivity (also called a Wide Area Network or WAN). This allows users to electronically share vital computing resources, such as expensive hardware (e.g. printers and CD-ROM drives), application programs, and information.
Local Area Networks implement shared access technology. This means that all of the devices attached to the LAN share a single communications medium, usually a coaxial, twisted-pair, or fiber-optic cable.
A physical connection to the network is made by putting a network interface card (NIC) inside the computer and connecting it to the network cable. Once the physical connection is in place, the network software manages communications between stations on the network.
To send messages to and from computers, the network software puts the message information in a packet. (If the message to be sent is too big to fit into one packet, it will be sent in a series of packets.) In addition to the message data, the packet contains a header and a trailer that carry special information to the destination. One piece of information in the header is the address of the destination.
The NIC transmits the packet onto the LAN as a stream of data represented by changes in electrical signals. As it travels along the shared cable, each NIC checks its destination address to determine if the packet is addressed to it. When the packet arrives at the proper address, the NIC copies it and gives its data to the computer. Since each individual packet is small, it takes very little time to travel to the ends of the cable. After a packet carrying one message passes along the cable, another station can send its packet. In this way, many devices can share the same LAN medium.
Each LAN has its own unique topology, or geometric arrangement. There are three basic topologies: bus, ring, and star. Most LANs are a combination of these arrangements.
In a bus topology all of the devices are connected to a central cable or backbone.
In a ring topology the devices are connected in a closed loop so that each device is connected to two others, one on either side. This kind of topology is robust; that is, one device's failure will probably not cause total network failure.
In a star topology the devices are all connected to a central hub, which forwards data towards its final destination. The NCI-Frederick LAN infrastructure is standardized on the star topology. If the data's destination is within the local star segment, the hub will forward data directly to the destination device; if the data's destination is outside the local star segment, the hub forwards the data to a router.
Depending on the topology and media that are used, as well as the protocols (formats for transmitting data) that are implemented, a LAN can permit data transfer rates of up to 100 Million bps.