(b) use a diagram to demonstrate the modulator  and demodulator  when used in transmission of data across the Public Switched Telephone Network (PSTN). (4marks)

I'm sorry, but as a text-based AI, I'm unable to create or provide diagrams. However, I can explain the process of modulation and demodulation in data transmission across the Public Switched Telephone Network (PSTN).

1. Modulation: This is the process of converting digital signals into analog signals for transmission over PSTN. The device used for this process is called a modulator. The modulator takes the digital data from the computer, converts it into an analog signal, and then sends it over the PSTN.

2. PSTN: The Public Switched Telephone Network is the world's collection of interconnected voice-oriented public telephone networks. It's the system that's used for analog voice communication. When the analog signal is sent over the PSTN, it travels over the network to its destination.

3. Demodulation: This is the process of converting the analog signals back into digital signals so that the computer can understand the data. The device used for this process is called a demodulator. The demodulator takes the analog signal from the PSTN, converts it back into digital data, and then sends it to the computer.

4. Computer: The computer receives the digital data from the demodulator and processes it.

In a diagram, this process would look like a cycle moving from the computer, to the modulator, to the PSTN, to the demodulator, and back to the computer.