Introduction:
A modem is a device that converts digital signals from a computer into analog signals that can be transmitted over telephone lines. At the receiving end, it converts the analog signals back into digital signals that can be understood by the computer. In other words, it acts as a digital-analog converter at the sender end and as an analog-digital converter at the receiver end.

Explanation:
A modem performs two main functions in the communication process:

1. Digital-analog conversion at the sender end:
- The modem receives digital signals from the computer, which are in the form of binary code (0s and 1s).
- It then converts these digital signals into analog signals that can be transmitted over telephone lines.
- This conversion is necessary because telephone lines are designed to carry analog signals, which represent voice or audio data.
- The modem uses modulation techniques such as amplitude modulation (AM) or frequency modulation (FM) to convert the digital signals into an analog waveform that can be transmitted.

2. Analog-digital conversion at the receiver end:
- Once the analog signals reach the receiving end, the modem performs the opposite process.
- It converts the analog signals back into digital signals that can be understood by the computer.
- This conversion is necessary because computers can only process and understand digital data.
- The modem uses demodulation techniques to extract the original digital signals from the received analog waveform.
- These demodulation techniques include amplitude demodulation (AM demodulation) or frequency demodulation (FM demodulation).

Conclusion:
In summary, a modem acts as a digital-analog converter at the sender end by converting digital signals from the computer into analog signals for transmission. At the receiver end, it acts as an analog-digital converter by converting the received analog signals back into digital signals that can be understood by the computer. This conversion process allows for the transmission of data over telephone lines, enabling communication between computers.