Networks were defined as sets of computers connected together so they can share data. A company can make its current data, such as customer account information, available to all employees at any time. Networks allow resource sharing, file sharing, and enable different users to communicate with each other.
Concerning resource sharing, networks can allow one resource, such as a printer, to be used by multiple users. Without a network, a printer would have to be installed for every user who needs one.
Files can be shared without a network, but that means putting them on disk and walking them from one user to another (sometimes jokingly referred to as a sneakernet). But with a network, files can be shared without anyone leaving a chair. Users can then easily collaborate from their computers.
Networks allow communication. They allow users in different locations to communicate, either in real time (video conferencing) or not (e-mail). In general, a network allows a group of computers to act in concert, to act as if they were one computer. From a user's point of view, if the network is fast and robust, the locations of data and other users are irrelevant; it's as if all the data on the network is on each user's computer.
A network, then, is a mechanism for transmitting data from one computer to another. When data is transmitted across a network, it is referred to as a message. These messages may be fixed in size (a fixed number of bits of data) or of variable size, but there's always some upper limit. Because of this limitation, most blocks of data to be transferred are broken up into multiple messages.
In any single transmission, the computer that transmits the message is the sender, and the computer to which the message is transmitted is the receiver. This terminology refers to a single transmission only. In most cases, each computer is both sending and receiving.
Two computers both sending to each other at the same time is known as a full-duplex transmission. In some situations, both computers can send to each other, but not at the same time, which is a half-duplex transmission. A good example of this is a "push to talk" cell phone. When the operator of the phone pushes the "talk" button, the speaker on the phone cuts off as long as the button is pressed. When the phone is used this way, the operator can listen or talk but not do both at the same time.
If only one computer can send, it is known as a simplex transmission. If you have a dish on your roof for receiving television signals, you have a simplex communications link. The television data comes from your service provider, which beams it up to a satellite orbiting the Earth, which in turn beams it down to the ground to all the rooftop dishes. However, no mechanism sends data from the dish back up to the satellite; it's a one-way street.
Each computer connected to a network is known as a node. Some nodes are users' computers, some are devices (like a printer shared by users of a network), and some exist just to direct traffic to other nodes.
The network medium is the physical connection the message crosses to get from sender to receiver. Some networks use multiple media (the plural of the word "medium"). With satellite television, for example, the satellite beams signals through the air to the dish, which is one medium, but then the dish is connected to the television through wires, which is another medium.
Networks come in different sizes. Some networks involve only a few machines that are all in the same building. Others involve thousands of computers or more, spread across the globe.
The smallest network, a point-to-point connection, connects just two computers. A simple example of this would be someone who has two computers at home and wants to be able to easily share files between them. A local area network, or LAN, connects computers in a single building or in adjacent buildings. A wide area network, or WAN, connects computers that are widely separated, either in different parts of a city or in different countries.
The number of nodes does not determine whether a network is a LAN or WAN. While the average WAN has more nodes than the average LAN, either could have just a few nodes or many. What makes the difference is the physical separation between the computers.
In both a LAN and WAN, the same organization owns or controls the use of all the computers on the network. In a LAN, the organization also controls the other network components, such as the medium. In a WAN, the organization cannot physically string cable to the other end of a city or continent. Instead it has to connect to some general carrier to provide the connection.
The situation is analogous to a company phone system. If an employee wants to call someone in the same office, the call can be placed only using equipment that resides in the office. If that employee wants to make a call to someone in the branch office in another state, then some long distance company like AT&T, Sprint, or MCI provides the connection.
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