MODEM

Modem:

A modem is combination of modulator and demodulator. A modem is a device or program that enables a computer to transmit data over, for example, telephone or cable lines.
DSL: Digital Subscriber Line

Figure: DSL Modem

Modem Connection:

Figure: Modem

History:

Let us say, the computer name A want to communicate with computer name B, then computer A is connected with telephone landline serially and to the another computer via another telephone landline.

But, the computer can transmit the message in the form of binary language that telephone doesn't understand binary language, eventually, the telephone can understand only analog signals. So for this purpose, they used modulator to convert binary language to analog signals.

And again to convert analog signal to binary language they used demodulator,

For Being simple they added both to get Modem,