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effect of noise , distance to be covered , services provided , requirement of digital communication , need ?
In this article, we shall compare the performance of analog communication systems with that of digital communication systems based on the following factors:
(i) effect of noise
(ii) distance to be covered
(iii) services provided.
1.11.1. Comparison of Analog and Digital Transmission : Effect of Noise
Figure 1.10 shows the transmitted and received signal in an analog communication system.
From figure 1.10, it is obvious that in analog communication, the shape of the transmitted signal gets distorted due to non-adequacies of the communication channel. Since, all the information is contained in the shape of the waveform, it is essential to preserve the shape of waveform. However, the noise distorts the waveform as shown in figure 1.10 (b) Therefore, the received information is also distorted.
Now, let us consider the transmission of digital signal as shown in figure 1.11. To transmit such a binary signal, the objective is to transmit either 0 or 1. The function of the receiver is to identify whether the received symbol is 0 or 1. Here, in case of digital communication, it may be observed that even though the
received waveform is distorted in figure 1.11 (b), the receiver is still able to make a correct decision.
NOTE: Therefore, it is obvious that digital transmission is more immune to noise as compared to analog communication.
1.11.2. Comparison of Analog and Digital Transmission : Based upon Distance to be Covered
As a matter of fact, the digital communication is suitable for long distances. In fact, the digital communication becomes cost effective over analog communication if the distance involved is large. Let us consider a long distance communication link as shown in figure 1.12.
FIGURE 1.12 A long distance communication link.
In digital transmission, as the signal from the source travels a distance from source, its amplitude reduces due to attenuation. Moreover, the signal becomes increasingly distorted. The noise from various sources gets added to the signal. In digital communication system, the repeaters are introduced between the source and destination as shown in figure 1.12.
Basically, the repeaters are re generators of the signals. In fact, the repeaters receive the (signal + noise) at their input, separate out the signal from noise and regenerate the signal which is free from noise. Because of the use of repeaters, the noise performance of digital communication systems is much better than that of the analog systems. However, it must be noted that repeaters cannot be used for the analog communication system. Figure 1.13 clears the operation of a digital repeater.
FIGURE 1.13 A digital repeater.
1.11.3. Comparison of Analog and Digital Transmission : Based upon Services Provided
As a matter of fact, the digital networks can handle many types of services. in fact, the networks based on digital transmission are capable of handling any type of information which can be represented in the digital form. Hence, we can say that such networks can handle many types of services.