JEE Main & Advanced Physics Communication System / संचार तंत्र Modem and Fax

(1) Modem : Modems are used to interface two digital sources/receivers.

(i) Word modem has been obtained from the words modulator and demodulator. As the name implies both the functions (modulation) and demodulation) are included in a signal unit.

(ii) Modems are placed at both ends of the communication circuit as shown.

(iii) The modem at the transmitting station changes the digital output from a computer (or any other business machine) to a from (analog signal) which can be easily sent via a communication channel (Telephone line etc.). While the receiving modem reverses the process.

(iv) There are three modes of operation of a modem.

(a) Simplex mode : In this mode data is transmitted in only one direction.

(b) Half duplex : In this mode data is transmitted between the transmitter and the receiver in both direction, but only in one direction at a time.

(c) Full duplex : In this mode, the data are transmitted between the transmitter and receiver in both directions at the same time.  

Modem data transmission speed

(2) Fax (Facsimile transmission) : The electronic reproduction of a document at a distance place is known as facsimile transmission (FAX).

The original written document is converted into transmittable codes at the sending end. These codes are converted back into a copy of the original document at the receiving end.

The original written document is put into the machine. A scanner scans the whole document.

The scanned written document is then moved on a glass plate. A beam of light from a given source is projected through the glass and is reflected from the surface of the document.

The reflected light is focussed on a device known as photo detector which converts the optical signals (carrying the information regarding the patterns/writings/signatures etc.) in to electrical signals.

These electrical signals are then modulated and transmitted on to the telephone lines to the receiving end.