Computer communications:Modems

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Modems

The word modem is short for modulator/demodulator, terms used to describe a device which enables you to send and receive signals via a telephone carrier wave. If you want to communicate with other computers via the telephone line, you have to convert your computer's digital output to an analogue (i.e. wave-like) telephone signal, and convert the incoming analogue signal to digital computer input. Hence the need for a modem, linking your computer to the telephone socket.

As with other computer hardware, there are many models on the market, and a number of standards. However, the most common standard is that set by the Hayes modem, so it's a good idea to purchase either a Hayes or Hayes­ compatible modem. Prices vary, depending on the features. The cheapest and least satisfactory, are acoustic modems. These cost only about £50, but they require you to dial up manually and place the telephone receiver in an acoustic box. Not only is this awkward, it also means that any extraneous noises can interfere with the signal, corrupting the data being sent or received.

Nowadays, few modems use an acoustic link. Instead, electronics within the modem converts between computer and telephone signals. On cheaper models you may still have to dial manually, the procedure being to switch over to the computer and replace the receiver when the connection is made. Most modems, though, are fully automatic, allow­ing dial-up via the computer, either by keying in the number at the keyboard or by selecting from a library of numbers stored on disk.

As well as these differences, cheap modems offer a very restricted choice of communications speeds, perhaps only the most common speed of 300 baud. Mid-range modems generally offer not only 300 baud but 1200 baud as well (also widely used, and four times as fast), and the split speeds used by view data systems of 1200/75 (i.e. 1200 baud to receive data, 75 baud to transmit). More expensive modems offer higher speeds as well, allowing for very fast (and therefore inexpensive) data communications.

To use a modem, you have to connect it to the computer

using an RS-232 cable, unless it is an internal modem that sits inside the computer's casing. You also have to connect it to the telephone socket. The modem's telephone cable may be supplied with a two-way adaptor which fits into the wall socket, in which case both it and your phone can be plugged in at the same time.

With most modems, your communications software will enable you to set the baud rate and other handshaking protocols, and to dial up the number of the remote computer.

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