There are three ways of transmitting data from one point to another as shown in Figure.
1. Simplex : In simplex mode, the communication can take place in one direction. The receiver receives the signal from the transmitting device. In this mode the flow of information is Uni-directional. Hence it is rarely used for data communication.
2. Half-duplex: In half-duplex mode, the communication channel is used in both directions, but only in one direction at a time. Thus a half-duplex line can alternately send and receive data.
3. Full-duplex: In full duplex, the communication channel is used in both directions at the same time. Use of full-duplex line improves the efficiency as the line turnaround time required in a half-duplex arrangement is eliminated. Example of this mode of transmission is the telephone line.
Digital and Analog Signal
Data is transmitted from one point to another point by means of electrical signals that may be in digital and analog form. So one should know the fundamental difference between analog and digital signals. In an analog signal, the transmission power varies over a continuous range with respect to sound, light and radio waves.
On the other hand, a digital signal may assume the only discrete set of values within a given range. Examples are computer and computer-related equipment. Analog signal is measured in Volts and its frequency is in Hertz (Hz). A digital signal is a sequence of voltage represented in binary form. When digital data are to be sent over an analog form the digital signal must be converted to analog form. So the technique by which a digital signal is converted to analog form is known as modulation. And the reverse process, that is the conversion of analog signal to its digital form, is known as demodulation. The device, which converts a digital signal into analog, and the reverse, is known as a modem.
Asynchronous and Synchronous Transmission
Data transmission through a medium can be either asynchronous or synchronous. In asynchronous transmission data is transmitted character by character as you go on typing on a keyboard. Hence there is irregular gaps between characters. However, it is cheaper to implement, as you do not have to save the data before sending. On the other hand, in the synchronous mode, the saved data is transmitted block by block. Each block can contain many characters. Synchronous transmission is well suited for remote communication between a computer and related devices like card reader and printers.