# Computer Networks

Topics: Modulation, Digital, Wave Pages: 9 (2697 words) Published: August 25, 2013
Module 2
Data Communication Fundamentals

Version 2 CSE IIT, Kharagpur

Lesson 1
Data and Signal

Version 2 CSE IIT, Kharagpur

Specific Instructional Objectives
At the end of this lesson the students will be able to: • • • • • • • • • Explain what is data Distinguish between Analog and Digital signal Explain the difference between time and Frequency domain representation of signal Specify the bandwidth of a signal Specify the Sources of impairment Explain Attenuation and Unit of Attenuation Explain Data Rate Limits and Nyquist Bit Rate Distinguish between Bit Rate and Baud Rate Identify Noise Sources

2.1.1 Introduction
A simplified model of a data communication system is shown in Fig. 2.1.1. Here there are five basic components: • Source: Source is where the data is originated. Typically it is a computer, but it can be any other electronic equipment such as telephone handset, video camera, etc, which can generate data for transmission to some destination. The data to be sent is represented by x(t).

COMMUNICATION MEDIUM

Figure 2.1.1 Simplified model of a data communication system • Transmitter: As data cannot be sent in its native form, it is necessary to convert it into signal. This is performed with the help of a transmitter such as modem. The signal that is sent by the transmitter is represented by s(t).

Version 2 CSE IIT, Kharagpur

Communication Medium: The signal can be sent to the receiver through a communication medium, which could be a simple twisted-pair of wire, a coaxial cable, optical fiber or wireless communication system. It may be noted that the signal that comes out of the communication medium is s’(t), which is different from s(t) that was sent by the transmitter. This is due to various impairments that the signal suffers as it passes through the communication medium. Receiver: The receiver receives the signal s’(t) and converts it back to data d’(t) before forwarding to the destination. The data that the destination receives may not be identical to that of d(t), because of the corruption of data. Destination: Destination is where the data is absorbed. Again, it can be a computer system, a telephone handset, a television set and so on.

2.1.2 Data
Data refers to information that conveys some meaning based on some mutually agreed up rules or conventions between a sender and a receiver and today it comes in a variety of forms such as text, graphics, audio, video and animation. Data can be of two types; analog and digital. Analog data take on continuous values on some interval. Typical examples of analog data are voice and video. The data that are collected from the real world with the help of transducers are continuous-valued or analog in nature. On the contrary, digital data take on discrete values. Text or character strings can be considered as examples of digital data. Characters are represented by suitable codes, e.g. ASCII code, where each character is represented by a 7-bit code.

2.1.3 Signal
It is electrical, electronic or optical representation of data, which can be sent over a communication medium. Stated in mathematical terms, a signal is merely a function of the data. For example, a microphone converts voice data into voice signal, which can be sent over a pair of wire. Analog signals are continuous-valued; digital signals are discrete-valued. The independent variable of the signal could be time (speech, for example), space (images), or the integers (denoting the sequencing of letters and numbers in the football score). Figure 2.1.2 shows an analog signal.

Version 2 CSE IIT, Kharagpur

Figure 2.1.2 Analog signal Digital signal can have only a limited number of defined values, usually two values 0 and 1, as shown in Fig. 2.1.3.

Figure 2.1.3 Digital signal Signaling: It is an act of sending signal over communication medium Transmission: Communication of data by propagation and processing is known as transmission.

2.1.4 Signal...