Module 5. Principles of communication

Lesson 11

COMMUNICATION PRINCIPLES

11.1 Introduction

This lesson will introduce the need and concept of communication systems. Students will learn about the capabilities, elements and limitations of communication systems, representation of signals, analog and digital signals. These topics will be useful for students to develop an understanding about the basics of communication process and intricacies involved in transmitting data.

Communication is the transfer of data/ information from source to destination. Basically through communication information/ views are being shared among interested user groups. Communication has always been the requirement of human being since the origin. The sharing can be local or remote. Local communication takes place between two persons face to face while remote communication takes place over distance. In today’s dynamic world the tools and methods of communication are changing very fast. Computer networks, Internet and World Wide Web (WWW) are the most commonly used communication tools to interact people at global level. These tools have changed the life style of people and business environment. The development of computers had brought tremendous changes for business, industry, science and education. Computers gave the new processing power to the scientific and business world but still an efficient way of sharing information was missing. Development of computer networks during 1970s proved the revolutionary point in communication era. This laid the foundation of foot prints of Internet. Finally Internet became a commercial success in the 1990s and now a day it is household name. Internet is one of the most interesting and exciting phenomena of computer networking. Internet was the key technology of the 20th century and will also play a lead role in 21st century.

The success of Internet is based on the technological innovations made in the networking of computers and the communication facilities. Reduction in the prices of computing and networking devices has also played a significant role in bringing Internet and computing devices to common man. Networking of computers was only the matter of interest of defense and academic personnel twenty years ago. But latter on it gain popularity among the business houses and now it has become the requirement of each and every household. Computer networks especially the Internet has changed the lifestyle of a common man. Networks and Internet is being used in all sphere of life for example research, business, banking transactions, governance, military, advertisements, etc. therefore, it is beyond the imagination to survive without Internet in the new century. Communication networks are being used for diversified applications. Various applications and use of network can be broadly grouped in the following groups:

·         Access to remote information: Applications under this group involves interaction between a user and a remote database such as file sharing, payment of bills, banking and financial services, investments, shopping, electronic data interchange (EDI), news papers, information services, marketing and sales, WWW, etc. 

·         Person to person communication: Applications of this group are email, chat, voice mail, voice conferencing, video conferencing, newsgroups, cellular phone etc.

·         Interactive entertainment: Applications in this group are cable TV network, games playing, 3-dimensional real time images, photographic quality moving images etc.

11.2  Communication Process

Communication process is the way of conveying data or messages (voice/ video/ numbers/ text etc.) between two points and this information or messages may be verbal or non-verbal. Data refers to facts, concepts and instructions presented in whatever the form is agreed upon by the concerned parties involved in creating and using data. Data is represented by binary information units or bits in the form of 0 and 1. Data communication can be defined as exchange of data (in the form of 0’s and 1’s) electronically between two devices via some form of transmission medium (wire cable or wireless). The effectiveness of data communication depends on: Delivery, Accuracy, and Timeliness. A communication process requires a sender, a message, receiver, communication channel or medium through which information travels between two points and protocols that govern the communication. The communication process is completed once the receiver completely understood the message sent by sender.

11.3 Communication Principles

Operations of any communication system are governed by the following principles:

                          I.            Infrastructure to transfer information from one point to another point. All services supported by the network are designed to facilitate the exchange of information.

                       II.            The receiver must understand the message. Without understanding, no meaningful communication takes place.

Any communication system is useful and effective when a system is able to accept source data; structure the source data so that it can be sent quickly and accurately; transmit the data to specific destination; able to detect the error during transmission (if any); and once the data is received, reconvert it to a form understandable by the destination.

11.4 Components (Elements) of Communication

A data communication system is made up of the following components (see figure 11.1):

Fig. 11.1 Components of a data communication system

·         Data source: A communication system serves to communicate data between two locations.  A data source generates the data to be transmitted.  

·         Message: A message is the information (data) to be communicated. A message can contain text, numbers, voice, video, images or any combination of these.

·         Transmitter: The main function of the transmitter is to convert the message obtained from the data source into electrical signal with the help of transducer if it is not in the electrical form. It convert low frequency signal to high frequency signal for long distance transmission.

·         Medium or channel: Channel means the medium through which message signals travels from transmitter to receiver. In other words, the function of the channel is to provide a physical connection (wired or wireless) between the sender and destination.

·         Receiver: Accept signal from transmission system and converts (reproduce) it into a form that can be handled by destination. This reproduction is accomplished by a process known as demodulation. Demodulation is the process of extracting back the original signal from the modulated signal.

·         Destination: Destination is the final stage where an electrical signal is converted into original form for further use.

·         Protocols: A protocol is a set of rules that govern all aspects of information communication. A protocol defines what is communicated, how it is communicated, and when it is communicated.

11.5 Limitation on Communication System

A communication system consists of a number of components and devices. Therefore, one may face some constraints or limitations mainly due to imperfection of transmission medium used in communication system. Some of the important limitations are:

·         Transmission impairments

·         Limited bandwidth

11.5.1 Transmission impairments

Generally transmission mediums are not perfect and this cause impairment in the signal sent through the medium. This means the signal received at the end is not the same as sent originally. There are three types of impairments in the signals:

  1. Attenuation: This is loss of energy. After travelling through the transmission medium a signal loses some of its energy to overcome the resistance of the medium.  To compensate this loss, signal is amplified using amplifiers.
  2. Distortion: Distortion means that the signal changes its form or shape. Generally distortion occurs in composite signal made up of different frequencies. Each signal component has its own propagation speed through a medium and therefore, its own delay in arriving at the final destination. At the destination, received signal is found different from the original signal. 
  3. Noise: Noise is an unwanted signal which interferes with the transmitted signals in a communication system. Noise limits the ability to identify the desired message correctly.  Noise is random in nature it cannot be eliminated completely but its effect can be minimized using several techniques. Several type of noise may corrupt the signal. Few are described below:

·         Thermal noise: It is the random motion of electrons in a wire that creates an extra signal not originally sent by the transmitter.

·         Crosstalk: It is effect of one wire on the other. One wire acts as sending antenna and other as the receiving antenna.

·         Impulse noise: It is a spike (a signal with high energy in a very short period of time) that comes from power lines, lightening etc.

·         Induces noise: It comes from sources such as motors and appliances. These devices act as sending antenna and the transmission medium acts as the receiving antenna.

11.5.2  Limited bandwidth

Bandwidth for a particular transmission medium is allocated by International regulatory agency to avoid interference among the signals having same frequency. But, for a given transmission medium this bandwidth may not be sufficient to transmit entire information. Thus puts a major impact on communication system.

11.6  Representation of Signal

In general term, anything which carries information is a signal. Signals may be verbal or nonverbal. Daily we receive number of signals and process them to extract information for taking appropriate action. For example human voice, vocalization of cattle, chirping of birds, lighting, body gestures, ring tones etc. can be treated as signals since they convey something.

In electronic communication, a signal is an electric current or electromagnetic field used to convey data trough transmission medium from source to destination. Data can be any message that is readable and understandable to the sender and receiver such as voice, image, text, numbers, or code. As such this information (say image) cannot be transmitted over a transmission medium. This image has to be encoded in digital form using 0 and 1 bit. This encoding must be understandable to the destination to decode the data. Further, even the string of 0s and 1s cannot be sent across links because transmission media as such do understand 0 and 1. These codes must be converted in to a form that transmission media can accept. Transmission media work by conducting energy along a physical path. Therefore the string of 0s and 1s must be converted into energy in the form of electrical signals. 

11.6.1 Analog

Analog refers to something that is continuous - a set of specific points of data and all possible points in between. Analog data refers to continuous data e.g. human voice – when someone speaks a continuous wave is created in the air, atmospheric temperature, real numbers etc. Analog describes any fluctuating, evolving, or continually changing process.

11.6.2 Digital

Digital refers to something that is discrete - a set of specific points of data and no other points in between. Digital data refers to discrete data e.g. data stored in computer memory, integer numbers, etc.

11.6.3 Signal

An electromagnetic wave that is propagated along a transmission media. Data is transmitted through signals in a media. A signal can take either analog or digital form. Both analog and digital signals can be of two forms periodic and aperiodic.

Periodic Signal: A Periodic signal consists of a continuously repeated pattern. It completes a pattern within a measurable time frame called a period and repeats the identical pattern over subsequent periods as shown in figure 11.2. The completion of full pattern is called as cycle.

Aperiodic signal: An aperiodic signal has no repetitive pattern. It changes constantly without exhibiting a pattern or cycle that repeats over time.

11.6.4 Analog signal

Analog signal refers to a continuous wave form which changes smoothly over a time. As the wave moves from value A to value B, it passes through and includes an infinite number of values along its path. In other word, an analog signal will have all possible (infinites) points within a given set of specific points of data. An analog signal can be represented as a series of sine waves. Analog signal can be classified as simple or composite. A simple analog signal or a sine wave cannot be decomposed into simpler signals. A composite analog signal is composed of multiple sine waves.

11.6.5 Simple analog signal

A simple analog signal is visualized as a simple oscillating curve, which is continuous, smooth and consistent. A simple analog signal can be represented by a sine wave (composed of single frequency). Three characteristics: amplitude, period or frequency, and phase describing a sine wave are as shown in figure 11.2 given below.

Fig. 11.2 A sine wave with different phases

11.6.6 Amplitude

It is a value of the signal at any point of time on the wave. It is equal to the vertical distance from a given point on the wave from the x-axis. The minimum and maximum amplitude of a sine wave are same. Amplitude is measured in volts, amperes, or watts depending on the type of signals. This also represents the strength of a signal.

11.6.7 Period and frequency of a signal

Period refers to the amount of time, in seconds, a signal needs to complete one cycle. Frequency refers to the number of periods in one second i.e. number of cycles per second. In other words frequency is rate of change with respect to time. Frequency is expressed in hertz (Hz). Relationship between period and frequency (f): f = 1/ Period. Let T be a period and f be frequency then T = 1/f.

Period is expressed in seconds:

Seconds(s)

=

1s

Milliseconds (ms)

=

10-3s

Microseconds (ms)

=

10-6s

Nanoseconds (ns)

=

10-9s

Picoseconds (ps)

=

10-12s

 Example: A sine wave has a frequency of 8KHz, then its period will be

               

               

11.6.8 Wavelength of a signal

Wavelength (λ) refers to the distance covered in one period. It binds the period or frequency of signal to the propagation speed of the medium. Wavelength can be calculated if the propagation speed (c) and the period of the signal are given. Wavelength = C x Period. If c is propagation speed of a signal in the medium and f is frequency then, Wavelength can be express as

11.6.9 Composite analog signal

In real world life an electromagnetic wave is a periodic composite signal that contains more than one sine wave of different amplitude, frequency and phase. Elements of a composite signal can be decomposed into simple analog signals through Fourier analysis. A composite signal is shown in figure 11.3 where two simple signal (a) frequency 5 and amplitude 6 and (b) DC current of frequency 0 and amplitude of 9 are forming a composite signal (c). The resultant signal is similar to a sine wave but time axis is shifted downward and amplitude is 15.

11.6.10 Frequency spectrum and bandwidth of a signal

Frequency spectrum of a signal is a collection of all component frequencies it contains. Bandwidth is a path required for transmission of signals and is expressed in frequencies. Bandwidth is a width of the frequency spectrum. In other words bandwidth refers to the range of component frequencies. And frequency spectrum refers to the elements within that range.

Example: A composite periodic signal having three sine waves with frequencies 200, 500, and 800MHz then bandwidth of this signal is 800-200MHz = 600MHz

Fig. 11.3 A composite analog signal

11.6.11 Digital signal

Digital signal refers to discrete change of the wave form into pre-defined levels. Data is represented in two states as 1 and 0. In digital signal 1 can be coded as positive voltage and 0 as zero voltage. Digital signals are generally aperiodic and therefore, period and frequency is not appropriate terms for digital signals. In digital signals, the terms bit interval (instead of period) and bit rate (instead of frequency) are used to describe digital signal as shown in figure 11.4. The bit interval is the time required to send one single bit and bit rate is the number of bits sent in one second expressed in bits per second (bps).

Fig. 11.4 A digital signal with bit rate and bit interval

11.7 Fourier Analysis

Mathematically, equation for a basic sine-wave with frequency f (frequency measured Hertz, i.e. cycles/second) can be defined as:

               

where:            

             y(t) is the signal at time the

                A is the maximum amplitude of the signal

                f is the number of cycle per second

                q is the phase of the signal

If the phase shift is 90 degree (p/2 radians), the same signal can be expressed as cosine wave instead of sine wave:

               

Example: The human voice is a composite signal i.e. summation of number of sine waves, each having different amplitude, frequency and phase. The bandwidth is normally between 300Hz and 3300Hz. The general equation for this signal will be like:

               

11.7.1 Fourier series

Fourier series were introduced by Joseph Fourier (1768–1830). A Fourier series decomposes any periodic signal into the sum of a (possibly infinite) set of simple oscillating sine and cosine waves each having different frequency and phase. A periodic composite signal can be decomposed as follows:

               

The coefficients c0, c1, …, cn are the amplitude of the simple signals. c0 is the amplitude of the signal with frequency 0 (i.e. DC component).

11.8 Fourier Transformation

The Fourier transformation decompose a composite a periodic signal into the sum of a (possibly infinite) set of simple oscillating sine and cosine waves each having different frequency and phase.

Note: Mathematical explanation of Fourier series and transformation is beyond the scope of this lesson.