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Communications

Computers represent data as binary digits (0's and 1's). These are transmitted within a computer system as electronic pulses. Unfortunately it is physically impossible to transmit electronic pulses down a long wire since the pulse distorts.

If computers are linked by long lengths of wire (over say 1.5 km long) then the 0's and 1's need to be represented by two different tones or frequencies. Tone signals can be transmitted electrically along a wire. The process of converting the pulses to tones is called frequency modulation. At the other end of the wire the tones must be demodulated. That is turned back into the pulses that the receiving computer system can handle.

The device that modulates and demodulates the computer pulses into tones that can be sent electronically down a wire is called a modem. Incidentally a modem would not be required if the link used fibre optic since this is capable of transmitting optical pulses.

The most important characteristic of a modem is the number of bits it can transmit or receive per second. This is called the baud rate of the modem. This feature decides the rate at which data can be communicated. Other features include the ability to transmit dialling tones thus allowing the computer to auto dial the destination and the ability to accept incoming calls thus allowing the computer to handle incoming data links automatically.

Larger systems may have a number of modems. This would be the case where a mainframe is linked to a number of remote terminals. Each remote terminal would have a modem attached. The mainframe would have a modem for each of the remote terminals. The modems at the mainframe end would be connected to a multiplexer. This divide is an electronic multi-switch that allows the mainframe to link up to each of the modems in turn, thus servicing each of the remote terminals in turn.

Internal modems are available for PC's. These are fitted inside the computer so taking up less desk space. Internal modems are particularly useful for lap top computers.

   

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