Question

explain modem also include modulator and demodulator

Answer 1

 a Modem is a hardware device that allows a computer to send and receive information over telephone lines by converting digital data into an analog signal used on phone lines.

Modulation:
the process of transforming digital information (1's an 0's) into analog (perceived as sound) signals. In the case of modems, capable of being transmitted over telephone lines.

Demodulation:
the process of transforming analog signals, previously modulated, back into digital information.

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Answer 2

A modem is a hardware device that allows a computer to send and receive data over a telephone line or a cable or satellite connection. In the case of transmission over an analog telephone line, which was once the most popular way to access the internet, the modem converts data between analog and digital formats in real time for two-way network communication. In the case of the high-speed digital modems popular today, the signal is much simpler and doesn't require the analog-to-digital conversion.
Modulator is a device that modulates the the message signal (i.e the signal which you want to transmit to another place) with a carrier signal of very high frequency. 
for demodulator , it is the device that separates the message signal from the carrier signal in the receiver.