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Modem is a device that converts digital signal to analog signal as a modulator and analog signal to digital signal as a demodulator.


  • Enable computers to communicate over telephone lines.
  • Speed of modem is measured in bits per second and varies depending upon the type of modem. Higher the speed, the faster you can send and receive data over the network.
  • Used to connect computer to the internet.


  • Consider a communication between two computers A and B.
  • Computer A transmits the digital signals to its modem in the form of binary 0's and 1's.
  • Modem of computer A converts these digital signals it into analog signals and sends over the telephone line. This process is called as modulator.
  • While at the other end, modem of computer B receives the analog signals and converts back into digital signals. This process is called as demodulator.
  • Converted digital signals by the modem are sent to the computer B for processing.
  • In similar way computer B can communicate with computer A.

Types based on transmission media

Asynchronous Modem

  • Asynchronous modem uses start and stop bit for synchronization instead of clock.
  • Contents or data in each frame is placed between start and stop bit.
  • Data is grouped in very short blocks to prevent slipping of data, so usually character data is transmitted.
  • Transmission is simple and inexpensive.
  • Used in pc to pc communication

Synchronous Modem

  • Asynchronous modem uses clock on transmitter and receiver devices for synchronization.
  • Before transmitting data both devices will synchronize the clock with each other.
  • Data is grouped in blocks and can transmit wide variety of data types in a frame.
  • Transmission speed is faster than asynchronous modems.
  • Used in dedicated leased line and high speed broad bands.

Basic types

  • External Modem
  • Internal Modem
  • Wireless Modem
  • PC Card Modem

Tip Box

Did you know baseband and broadband


  • Entire capacity of the medium is devoted to one commmunication channel.


  • Allows two or more communication channels to share the bandwidth of the medium.


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