Modems

A modem is a device that connects a computer to a telephone line, or other external communication link such as television cables (coaxial cables or fibre optics), or microwaves.

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You will be expected to know the differences including advantages and disadvantages of modems and ISDN.

The name modem is short for modulator–demodulator and its function is to encode data so that it can be transmitted, both from and to the computer.

KEY POINT -  The digital signals from one computer are converted by the modem into analogue signals for transmission, and when they reach the modem at the destination computer, the signals are converted back into digital form.

Important features of modems are:

• Data transmission rate. The speed at which data is transferred is very important. This is because the user is connected to a telephone line, and the use of this line costs money. You should understand how faster modems have a higher baud rate.
• Users also do not want to sit in front of a screen and have to wait ages for files to be downloaded.
• Speed is either measured in baud (a unit that measures the speed with which data is transferred) or in bits per second. Do not confuse bit rate with baud rate.

Some modems are able to compress data before sending it, which reduces transmission time and cost.

When a connection is made, the modem at each end negotiates with the other modem – they check each other’s capabilities and settle on a suitable data transmission rate, and procedure for correcting any errors

• Automatic dialling. Almost all modems now perform automatic dialling and can accept other communications commands from the computer.
• Fax compatibility. Some modems are compatible with fax machines and, with suitable software, can send and receive fax messages.
• Internal or external location. An internal modem fits inside the computer. An external modem connects to the computer by means of a cable to the computer’s serial port.