Local-area networks

Local-area networks

Computer networks that usually cover a limited range, say, within the boundary of a building. A computer network is two or more computers that communicate with each other through some medium. The primary usage of local-area networks (LANs) is the sharing of hardware, software, or information, such as data files, multimedia files, or electronic mail. Resource sharing provided by local-area networks improves efficiency and reduces overhead. See Digital computer, Electronic mail, Multimedia technology

Four basic types of media are used in local-area networks: coaxial cable, twisted-pair wires, fiber-optic cable, and wireless. Each medium has its advantages and disadvantages relative to cost, speed, and expandability. Coaxial cables provide high speed and low error rates. Twisted-pair wires are cheaper than coaxial cables, can sustain the speeds common to most personal computers, and are easy to install. Fiber-optic cable is the medium of choice for high-speed local-area networks. Wireless local-area networks have the advantage of expandability. See Coaxial cable, Communications cable, Fiber-optic circuit, Optical communications

The topology of a local-area networks is the physical layout of the network. For wired local-area networks, there are four basic topologies: bus, ring, star, and mesh. The most widely used local-area network topology is the bus, where the medium consists of a single wire or cable to which nodes are attached. A message transmitted over a bus propagates in both directions along the bus, passing each tap until it is finally absorbed at the ends.

There are a number of ways in which nodes can communicate over a network. The simplest is to establish a dedicated link between the transmitting and receiving stations. This technique is known as circuit switching. A better way of communicating is to use a technique known as packet switching, in which a dedicated path is not reserved between the source and the destination. Data are wrapped up in a packet and launched into the network. In this way, a node only has exclusive access to the medium while it is sending a packet. During its inactive period, other nodes can transmit. A typical packet is divided into preamble, address, control, data, and error-check fields. See Packet switching

An access protocol is a set of rules observed by all the nodes in a local-area network so that one node can get the attention of another and its data packet can be transferred. Two common protocols are carrier sense multiple access with collision detection (CSMA/CD) and token passing.

With the CSMA/CD protocol, a node that wants to transmit its data must first listen to the medium to hear if any other node is using the medium. If not, the node may transmit immediately. However, while the transmission is taking place, the transmitting node must continue listening to ascertain if anyone else has begun transmitting. If the transmitting node detects that someone else is also transmitting, the node aborts its own transmission, waits for a random amount of time, and then restarts the process until its data transmission succeeds.

With the token-passing protocol, the right to transmit is granted by a token, a predefined bit pattern that is recognized by each node. The token is passed for one node to another in a predetermined order.