From the early 1900s until now, long distance communication has primarily been in the form of the telephone line. Now telephone lines are being used for more than the transmission of voice; they are also used for the transmission of computer data. There are many reasons why someone might want to transfer data between one PC and another using phone lines.
For example, communications software makes it possible to share data with a PC that is some distance away (maybe even around the world), to connect to a BBS (bulletin board service) in order to get help with a PC problem, or to connect to a pay for use on-line service (such as CompuServe or Prodigy) in order to leave messages for other users, buy and sell stocks, download programs, read articles, and more.
The telephone system transmits sounds and tones known as analog communications with a continuous electronic current that varies in frequency and amplitude (volume). The PC can't understand the tones and pulses transmitted over a phone line directly, so a device called a modem is used. The purpose of a modem is to change the tones and pulses heard through a telephone line into data that the computer can understand. So it's the modem that receives the transmission and changes the tones into binary codes that the computer can understand. Likewise, before computer data can be transmitted over telephone lines, it must first be changed from binary codes (data) into analog signals (tones And pulses) by the modem. Binary Codes involve only two signals, 1's (on) or 0's (off). By changing either the frequency or the amplitude (volume) of an analog signal, the 1's and 0's of computer data can be sent through telephone lines.
There is another way that computers can communicate: through a local area network. A local area network is a collection of computers (usually located within a single building) connected to each other through a series of cables or wires.
LANs (local area networks) are popular in educational settings, such as universities, and in large corporations. Small businesses usually can't afford to connect everyone to a LAN, so when they do invest in a network, it's usually limited to only certain departments such as payroll and personnel. LANs are made up of several components. A file server is a centrally located PC with a large storage capacity that holds data and programs shared by all the users on the network. Workstations are user PCs connected through a cable (or telephone wires) to the network. The network operating system handles the transmitting of data throughout the network. In addition to sharing data and programs, LANs can be used to share printers. A printer server is a printer connected to the network so it can be shared by all the users on the LAN. Through modems, two or more LANs can be connected into larger groups called WANs (Wide Area Networks).
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