Modems : Introduction

Many computer users need to be able to connect their computers to other computers located at remote sites. For example :

• Office workers want to communicate with computers at their workplaces from wherever they may be working in the world.
• Home computer users want to be able to access the Internet.

The Public Switched Telephone Network (PSTN) provides the most convenient communications channel for these users. This is because you can already access the telephone network from practically anywhere in the world.

Because the telephone network was set up before computers were in widespread use, telephone lines were not designed to carry the digital signals that computers use. They were designed to carry human voices in the form of analogue electrical waveforms. An analogue signal can take any value in a given range, e.g. -5 to +5 volts.

The binary digital signals that a computer generates (0s and 1s) which are often represented as 0 volts (off) for a 0 and 5 volts (on) for a 1 can not be transmitted directly down a telephone line.

A special device known as a modem must be used to enable a computer to communicate over a telephone line. The modem converts the binary digital data that the computer wants to transmit into an analogue signal for transmission down the telephone line. Another modem must be used at the other end of the telephone line to convert the analogue signal back into a digital one that the receiving computer can understand.

A modem (MODulator - DEModulator) is a device used to allow a computer to communicate over a telephone line. Modulation is the process of converting digital electrical signals generated by a computer into analogue electrical signals which can be transmitted over a telephone line. Demodulation is the process of converting received analogue signals back into digital signals which a computer can understand.

GCSE ICT Companion 04 - (C) P Meakin 2004