Modem is a very familiar word to everyone of us. It is used for connecting a computer to a telephone line. The telephone lines are designed to carry analog signals and their bandwidth also is limited so they cannot be used for digital data transmission.
A modem converts the digital data from computers to analog signal and put it on the telephone lines. It is bi-directional device which converts analog on the telephone lines into digital data when it is used for data reception.

Modem is a combination of two words MODulator and DEModulator. The standard modulation techniques used in modem are:

  • Amplitude shift keying
  • Frequency shift keying
  • Phase shift keying

The normal digital signal cannot be put directly on the telephone lines, this is due to the fact that the bandwidth of the telephone lines is not sufficient to transfer the digital data without distortion. Thats why the data is first converted and then transmitted to the computer. This conversion is done by this special communication device box called as MODEM.