The correct answer is: D. All of the above
A modem is a device that modulates and demodulates signals. It converts digital signals into analog signals and vice versa. This is necessary for sending digital data over analog communication channels, such as telephone lines.
When a modem is sending data, it converts the digital signal into an analog signal by modulating the carrier wave. This means that the modem changes some characteristic of the carrier wave, such as its amplitude, frequency, or phase, in accordance with the digital signal.
The modulated signal is then transmitted over the communication channel. At the receiving end, the modem demodulates the signal, converting it back into a digital signal. This is done by reversing the process of modulation.
In order to send multiple digital signals over the same communication channel, modems can use a technique called multiplexing. Multiplexing allows multiple signals to be transmitted on the same channel at the same time by sharing the available bandwidth.
There are two main types of multiplexing: frequency-division multiplexing (FDM) and time-division multiplexing (TDM). In FDM, each signal is assigned a different frequency band. In TDM, each signal is assigned a different time slot.
Modems also use amplification to increase the strength of the signal. This is necessary because the signal can become weak over long distances. Amplification is done using a power amplifier.
Therefore, the correct answer is: D. All of the above