Inside a computer, bits rush around at speeds of a million or more bits a second.  To travel to another computer, the bits have to go along a telephone line.  An ordinary telephone line carries low-frequency sound signals and cannot handle bits at this rate.

Unless you install a high-speed line, you need a modem to send and receive bits.  The modem is an external or internal unit that connects the computer to a telephone socket, and from there in a network of links that can transfer bits quickly in several ways.

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The radio wave is converted back into bits, which are sent as on-off flashes of infrared laser light along a glass fiber-optic cable.
The bits are converted back into a sound signal, which travels along themetal core of a cable in the telephone line.The wavy line represents the beeps in the sound.
The modem converts the sound signal back into a digital signal containing bits in the form of rapid on-off electric pulses. These bits go to the computer.
A modem works by converting the computer’s fast digital on-off signal into a low-frequency sound signal. It splits up the bits into groups, and converts the groups into very short beeps of sound.  One kind of beep carries certain combination of on-bits and off-bits, another beep carries another combination, and so on.  The sending modem puts all the beeps together to form a sound signal carrying all the bits, and the receiving modem changes the beeps back into bits.




A communications link can carry many separate digital messages by multiplexing. The message bits are split up into small packets, and packets from different messages are interleaved, sent and the messages are then reassembled on arrival. Several multiplexed signals can be sent at different frequencies over a link to increase its capacity even more.


The glass core of this cable carries four light signals at different frequencies shown as different colors. Each signal is multiplexed and consists of interleaved eight-bit packets from three messages, so that the cable carries 12 messages; nine (A to I) are sown. Multiplexing enables a single link to carry thousands of messages at once.