Question

Explain the working of FAX machine.

Hint:

To ensure that two FAX machines can communicate, they perform a handshake protocol at the start of the call. This allows them to agree on compatible transmission speeds (baud rates), compression standards, and resolution capabilities.

Answer 1

Step 1: Introduction to Facsimile (FAX) Technology:
A FAX (Facsimile) machine is a telecommunication device designed to scan, encode, modulate, transmit, demodulate, and print documents over standard analog telephone lines (Public Switched Telephone Network or PSTN). It serves as a combined scanner, modem, and printer.

Step 2: Step-by-Step Working Mechanism of a FAX Machine:
The entire operation of sending and receiving a document occurs in the following sequential stages:

• Document Illumination and Optical Scanning (Transmitting End): The document page is fed into the machine, where it is illuminated line-by-line by a high-intensity LED array. The reflected light from the page is focused onto an optical sensor array, typically a Charge-Coupled Device (CCD) or a Contact Image Sensor (CIS). The sensor converts the light variations into proportional analog electrical voltages, where high voltage represents white areas and low voltage represents black ink.
• Analog-to-Digital Conversion and Compression: The raw analog electrical signals are converted into a digital binary stream of 1s and 0s (pixels). To reduce transmission time over slow telephone channels, this digital data is compressed using standard binary compression algorithms (such as Modified Huffman or Modified Read coding).
• Modem Modulation: Because telephone lines are designed to carry voice frequencies ($300\text{ Hz}$ to $3.4\text{ kHz}$), a built-in modem modulates the digital binary data into analog audio tones using modulation schemes like Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) before sending them over the telephone line.
• Line Transmission: The modulated audio tones travel over the standard telephone network to the destination FAX number.
• Demodulation, Decompression, and Printing (Receiving End): The receiving FAX machine answers the call, and its internal modem demodulates the incoming audio tones back into compressed digital binary data. The microprocessor decompresses this data to reconstruct the original pixel map of the page. It then sends this pixel data to a printer engine (usually a thermal head or laser drum) to reproduce the document on paper.