-
qwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmrtyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjklzxcvbnmqwertyuiopasdfghjkl
Fundamental of Computer Science
B.Sc- IT AMITY eLEARNING
Amity University
-
2
1.1 WHAT IS A COMPUTER?
Computer is an electronic device. As mentioned in the
introduction it can do arithmetic
calculations faster. But as you will see later it does much more
than that. It can be compared to a
magic box, which serves different purpose to different people.
For a common man computer is
simply a calculator, which works automatic and quite fast. For a
person who knows much about
it, computer is a machine capable of solving problems and
manipulating data. It accepts data,
processes the data by doing some mathematical and logical
operations and gives us the desired
output.
Therefore, we may define computer as a device that transforms
data. Data can be anything like
marks obtained by you in various subjects. It can also be name,
age, sex, weight, height, etc. of
all the students in your class or income, savings, investments,
etc., of a country. Computer can be
defined in terms of its functions. It can
i) accept data
ii) store data,
iii) process data as desired,
iv) retrieve the stored data as and when required
v) print the result in desired format. You will know more about
these functions as you go
through the later lessons.
Fig. 1.1 below depicts a personal computer.
-
3
1.4 CHARACTERISTICS OF COMPUTER
Let us identify the major characteristics of computer. These can
be discussed under the headings
of speed, accuracy, diligence, versatility and memory.
1.4.1 Speed
As you know computer can work very fast. It takes only few
seconds for calculations that we take
hours to complete. Suppose you are asked to calculate the
average monthly income of one
thousand persons in your neighborhood. For this you have to add
income from all sources for all
persons on a day to day basis and find out the average for each
one of them. How long will it take
for you to do this? One day, two days or one week? Do you know
your small computer can finish
this work in few seconds? The weather forecasting that you see
every day on TV is the results of
compilation and analysis of huge amount of data on temperature,
humidity, pressure, etc. of
various places on computers. It takes few minutes for the
computer to process this huge amount of
data and give the result.
You will be surprised to know that computer can perform millions
(1,000,000) of instructions
and even more per second. Therefore, we determine the speed of
computer in terms of
microsecond (10-6
part of a second) or nano-second (10-9
part of a second). From this you can
imagine how fast your computer performs work.
1.4.2 Accuracy
Suppose some one calculates faster but commits a lot of errors
in computing. Such result is
useless. There is another aspect. Suppose you want to divide 15
by 7. You may work out up to 2
decimal places and say the dividend is 2.14. I may calculate up
to 4 decimal places and say that
the result is 2.1428. Some one else may go up to 9 decimal
places and say the result is
2.142857143. Hence, in addition to speed, the computer should
have accuracy or correctness in
computing.
The degree of accuracy of computer is very high and every
calculation is performed with the
same accuracy. The accuracy level is determined on the basis of
design of computer. The errors
in computer are due to human and inaccurate data.
-
4
1.4.3 Diligence
A computer is free from tiredness, lack of concentration,
fatigue, etc. It can work for hours
without creating any error. If millions of calculations are to
be performed, a computer will
perform every calculation with the same accuracy. Due to this
capability it overpowers human
being in routine type of work.
1.4.4 Versatility
It means the capacity to perform completely different type of
work. You may use your computer
to prepare payroll slips. Next moment you may use it for
inventory management or to prepare
electric bills.
1.4.5 Power of Remembering
Computer has the power of storing any amount of information or
data. Any information can be
stored and recalled as long as you require it, for any numbers
of years. It depends entirely upon
you how much data you want to store in a computer and when to
lose or retrieve these data.
1,4.6 No IQ
Computer is a dumb machine and it cannot do any work without
instruction from the user. It
performs the instructions at tremendous speed and with accuracy.
It is you to decide what you
want to do and in what sequence. So a computer cannot take its
own decision as you can.
1.4. 7 No Feeling
It does not have feelings or emotion, taste, knowledge and
experience. Thus it does not get tired
even after long hours of work. It does not distinguish between
users.
1.4.8 Storage
The Computer has an in-built memory where it can store a large
amount of data. You can also
store data in secondary storage devices such as floppies, which
can be kept outside your
computer and can be carried to other computers.
-
5
1.5 HISTORY OF COMPUTER
History of computer could be traced back to the effort of man to
count large numbers. This
process of counting of large numbers generated various systems
of numeration like Babylonian
system of numeration, Greek system of numeration, Roman system
of numeration and Indian
system of numeration. Out of these the Indian system of
numeration has been accepted
universally. It is the basis of modern decimal system of
numeration (0, 1, 2, 3, 4, 5, 6, 7, 8, 9).
Later you will know how the computer solves all calculations
based on decimal system. But you
will be surprised to know that the computer does not understand
the decimal system and uses
binary system of numeration for processing.
We will briefly discuss some of the path-breaking inventions in
the field of computing devices.
1.5 .1 Calculating Machines
It took over generations for early man to build mechanical
devices for counting large numbers.
The first calculating device called ABACUS was developed by the
Egyptian and Chinese people.
The word ABACUS means calculating board. It consisted of sticks
in horizontal positions on
which were inserted sets of pebbles.It has a number of
horizontal bars each having ten beads.
Horizontal bars represent units, tens, hundreds, etc.
1.5.2 Napiers bones
English mathematician John Napier built a mechanical device for
the purpose of multiplication
in 1617 A D. The device was known as Napiers bones.
1.5.3 Slide Rule
English mathematician Edmund Gunter developed the slide rule.
This machine could perform
operations like addition, subtraction, multiplication, and
division. It was widely used in Europe
in 16th
century.
1.5.4 Pascal's Adding and Subtractory Machine
You might have heard the name of Blaise Pascal. He developed a
machine at the age of 19 that
could add and subtract. The machine consisted of wheels, gears
and cylinders.
-
6
1.5.5 Leibnizs Multiplication and Dividing Machine
The German philosopher and mathematician Gottfried Leibniz built
around 1673 a mechanical
device that could both multiply and divide.
1.5.6 Babbages Analytical Engine
It was in the year 1823 that a famous English man Charles
Babbage built a mechanical machine
to do complex mathematical calculations. It was called
difference engine. Later he developed a
general-purpose calculating machine called analytical engine.
You should know that Charles
Babbage is called the father of computer.
1.5.7 Mechanical and Electrical Calculator
In the beginning of 19th
century the mechanical calculator was developed to perform all
sorts of
mathematical calculations. Up to the 1960s it was widely used.
Later the rotating part of
mechanical calculator was replaced by electric motor. So it was
called the electrical calculator.
1.5.8 Modern Electronic Calculator
The electronic calculator used in 1960 s was run with electron
tubes, which was quite bulky.
Later it was replaced with transistors and as a result the size
of calculators became too small.
The modern electronic calculator can compute all kinds of
mathematical computations and
mathematical functions. It can also be used to store some data
permanently. Some calculators
have in-built programs to perform some complicated
calculations.
Fig.: Vacuum tube, transistor, IC
-
7
1.6 COMPUTER GENERATIONS
You know that the evolution of computer started from 16th
century and resulted in the form that
we see today. The present day computer, however, has also
undergone rapid change during the
last fifty years. This period, during which the evolution of
computer took place, can be divided
into five distinct phases known as Generations of Computers.
Each phase is distinguished from
others on the basis of the type of switching circuits used.
1.6.1 First Generation Computers
First generation computers used Thermion valves. These computers
were large in size and
writing programs on them was difficult. Some of the computers of
this generation were:
ENIAC: It was the first electronic computer built in 1946 at
University of Pennsylvania, USA by
John Eckert and John Mauchy. It was named Electronic Numerical
Integrator and Calculator
(ENIAC). The ENIAC was 30 50 feet long, weighed 30 tons,
contained 18,000 vacuum tubes,
70,000 registers, 10,000 capacitors and required 150,000 watts
of electricity. Today your favorite
computer is many times as powerful as ENIAC, still size is very
small.
EDVAC: It stands for Electronic Discrete Variable Automatic
Computer and was developed in
1950. The concept of storing data and instructions inside the
computer was introduced here. This
allowed much faster operation since the computer had rapid
access to both data and instructions.
The other advantages of storing instruction was that computer
could do logical decision
internally.
Other Important Computers of First Generation
EDSAC: It stands for Electronic Delay Storage Automatic Computer
and was developed by
M.V. Wilkes at Cambridge University in 1949.
UNIVAC-1: Ecker and Mauchly produced it in 1951 by Universal
Accounting Computer setup.
-
8
Fig. 1.4: Univac Computer
Limitations of First Generation Computer
Followings are the major drawbacks of First generation
computers.
1. The operating speed was quite slow.
2. Power consumption was very high.
3. It required large space for installation.
4. The programming capability was quite low.
1.6.2 Second Generation Computers
Around 1955 a device called Transistor replaced the bulky
electric tubes in the first generation
computer. Transistors are smaller than electric tubes and have
higher operating speed. They have
no filament and require no heating. Manufacturing cost was also
very low. Thus the size of the
computer got reduced considerably.
It is in the second generation that the concept of Central
Processing Unit (CPU), memory,
programming language and input and output units were developed.
The programming languages
such as COBOL, FORTRAN were developed during this period. Some
of the computers of the
Second Generation were
1. IBM 1620: Its size was smaller as compared to First
Generation computers and mostly used
for scientific purpose.
2. IBM 1401: Its size was small to medium and used for business
applications.
3. CDC 3600: Its size was large and is used for scientific
purposes.
-
9
1.6.3 Third Generation Computers
The third generation computers were introduced in 1964. They
used Integrated Circuits (ICs).
These ICs are popularly known as Chips. A single IC has many
transistors, registers and
capacitors built on a single thin slice of silicon. So it is
quite obvious that the size of the
computer got further reduced. Some of the computers developed
during this period were IBM-
360, ICL-1900, IBM-370, and VAX-750. Higher level language such
as BASIC (Beginners All
purpose Symbolic Instruction Code) was developed during this
period.
Computers of this generations were small in size, low cost,
large memory and processing speed
is very high.
1.6.4 Fourth Generation Computers
The present day computers that you see today are the fourth
generation computers that started
around 1975. It uses large scale Integrated Circuits (LSIC)
built on a single silicon chip called
microprocessors. Due to the development of microprocessor it is
possible to place computers
central processing unit (CPU) on single chip. These computers
are called microcomputers. Later
very large scale Integrated Circuits (VLSIC) replaced LSICs.
Thus the computer which was occupying a very large room in
earlier days can now be placed on
a table. The personal computer (PC) that you see in your school
is a Fourth Generation
Computer.
1.6.5 Fifth Generation Computer
The computers of 1990s are said to be Fifth Generation
computers. The speed is extremely high
in fifth generation computer. Apart from this it can perform
parallel processing. The concept of
Artificial intelligence has been introduced to allow the
computer to take its own decision. It is
still in a developmental stage.
2.3 BASIC COMPUTER OPERATIONS
A computer as shown in Fig. 2.1 performs basically five major
operations or functions
irrespective of their size and make. These are
1) . Input Function : This is the process of entering data and
programs in to the computer
system.
-
10
2) Storing data : The process of saving data and instructions
permanently is known as storage.
3) Processing :The task of performing operations like arithmetic
and logical operations is called
processing it can process data as required by the user,
4) Outputting :This is the process of producing results from the
data for getting useful
information.
5) Controlling: it controls all operations inside a
computer.
2.4 FUNCTIONAL UNITS
In order to carry out the operations mentioned above the
computer allocates the task between its
various functional units. The computer system is divided into
three separate units for its
operation.
1. Input Unit :It includes the input interface and input
devices. You should know that computer
is an electronic machine like any other machine which takes as
inputs raw data and performs
some processing giving out processed data. Therefore, the input
unit takes data from us to the
computer in an organized manner for processing. all input
devices must provide a computer with
data that are transformed into the binary codes that the primary
memory of a computer is
designed a accept. Units called input interfaces accomplish this
transformation.Input interfaces
are designed to match the unique physical or electrical
characteristics of input device to the
requirements of the computer system. In short, the following
functions are performed by an input
unit:
1. It accept (or reads) the list of instructions and data from
the
outside world.
2. It converts these instructions and data in computer
acceptable form.
3. It supplies the converted instructions and data to the
computer system for further processing.
D. Output Unit
The job of an output unit is just the reverse of that of an
input unit. It supplies information and
results of computation to the outside world. Thus it links the
compute with the external
environment. As computers work with binary code, the results
produced are also in the binary
-
11
form. Hence, before supplying the result to the outside world,
it must be converted to human
acceptable (readable0 form. This task is accomplished by units
called output interfaces are
designed to match the unique physical or electrical
characteristics of output devices (terminals,
printers, etc.) to the requirements of the external environment.
In short, the following functions
are performed by an output unit :
1. It accepts the results produced by the computer, which are in
code from and hence cannot be
easily understood by us.
2. It converts these coded results to human acceptable
(readable) form.
3. It supplies the converted results to the outside world.
2. Storage Unit : Data has to be fed into the system before the
actual processing starts. It is
because the processing speed of Central Processing Unit (CPU) is
so fast that the data has to be
provided to CPU with the same speed. Therefore the data is first
stored in the storage unit for
faster access and processing. This storage unit or the primary
storage of the computer system is
designed to do the above functionality. It provides space for
storing data and instructions.
The storage unit performs the following major functions:
All data and instructions are stored here before and after
processing.
Intermediate results of processing are also stored here.
2.4.3 Central Processing Unit (CPU)
The ALU and the CU of a computer system are jointly known as the
central processing unit. You
may call CPU as the brain of any computer system. It is just
like brain that takes all major
decisions, makes all sorts of calculations and directs different
parts of the computer functions by
activating and controlling the operations.The Central Processing
Unit (CPU) takes data and
instructions from the storage unit and makes all sorts of
calculations based on the instructions
given and the type of data provided. It is then sent back to the
storage unit. It consists of
Arithmetic Logical Unit (ALU) and Control Unit (CU)
-
12
4.12 Arithmetic Logical Unit (ALU)
After you enter data through the input device it is stored in
the primary storage unit. The actual
processing of the data and instruction are performed by
Arithmetic Logical Unit. The major
operations performed by the ALU are addition, subtraction,
multiplication, division, logic and
comparison. Data is transferred to ALU from storage unit when
required. After processing the
output is returned back to storage unit for further processing
or getting stored.
Arithmetic Logic Unit (ALU)
does all of the mathematics in a computer
does all of the logic comparisons of values
some common logic comparison symbols
= equal to
< less than
> greater than
= greater than or equal to
not equal
2.4.2 Control Unit (CU)
The next component of computer is the Control Unit, which acts
like the supervisor seeing that
things are done in proper fashion. The control unit determines
the sequence in which computer
programs and instructions are executed. Things like processing
of programs stored in the main
memory, interpretation of the instructions and issuing of
signals for other units of the computer
to execute them. It also acts as a switch board operator when
several users access the computer
simultaneously. Thereby it coordinates the activities of
computers peripheral equipment as they
-
13
perform the input and output. Therefore it is the manager of all
operations mentioned in the
previous section..
The Control Unit
directs the flow of information into the CPU and/or memory or
storage
controls which instructions the CPU will do next
Personal Computer Configuration
Now let us identify the physical components that make the
computer work. These are
1. Central Processing Unit (CPU)
2. Computer Memory (RAM and ROM)
3. Data bus
4. Ports
5. Motherboard
6. Hard disk
7. Output Devices
8. Input Devices
All these components are inter-connected for the personal
computer to work.
PROBLEM SOLVING USING COMPUTERS
Computers are used to solve all the problems of todays world. It
is possible by using various
tools like
a. Algorithms b. Flowcharts c. Pseudocodes
Algorithm
An informal definition of an algorithm as "a set of instructions
for solving a problem" and we
illustrated this definition with a recipe, directions to a
friend's house, and instructions for
changing the oil in a car engine. You also created your own
algorithm for putting letters and
numbers in order. While these simple algorithms are fine for us,
they are much too ambiguous
-
14
for a computer. In order for an algorithm to be applicable to a
computer, it must have certain
characteristics. We will specify these characteristics in our
formal definition of an algorithm.
An algorithm is a well-ordered collection of unambiguous and
effectively
computable operations that when executed produces a result and
halts in a finite
amount of time [Schneider and Gersting 1995].
With this definition, we can identify five important
characteristics of algorithms.
1. Algorithms are well-ordered. 2. Algorithms have unambiguous
operations. 3. Algorithms have effectively computable operations.
4. Algorithms produce a result. 5. Algorithms halt in a finite
amount of time.
These characteristics need a little more explanation, so we will
look at each one in detail.
Algorithms are well-ordered
Since an algorithm is a collection of operations or
instructions, we must know the correct
order in which to execute the instructions. If the order is
unclear, we may perform the wrong
instruction or we may be uncertain which instruction should be
performed next. This
characteristic is especially important for computers. A computer
can only execute an
algorithm if it knows the exact order of steps to perform.
Algorithms have unambiguous operations
Each operation in an algorithm must be sufficiently clear so
that it does not need to be
simplified. Given a list of numbers, you can easily order them
from largest to smallest with
the simple instruction "Sort these numbers." A computer,
however, needs more detail to sort
numbers. It must be told to search for the smallest number, how
to find the smallest number,
how to compare numbers together, etc. The operation "Sort these
numbers" is ambiguous to a
computer because the computer has no basic operations for
sorting. Basic operations used for
writing algorithms are known as primitive operations or
primitives. When an algorithm is
written in computer primitives, then the algorithm is
unambiguous and the computer can
execute it.
-
15
Algorithms have effectively computable operations
Each operation in an algorithm must be doable, that is, the
operation must be something that
is possible to do. Suppose you were given an algorithm for
planting a garden where the first
step instructed you to remove all large stones from the soil.
This instruction may not be
doable if there is a four ton rock buried just below ground
level. For computers, many
mathematical operations such as division by zero or finding the
square root of a negative
number are also impossible. These operations are not effectively
computable so they cannot
be used in writing algorithms.
Algorithms produce a result
In our simple definition of an algorithm, we stated that an
algorithm is a set of instructions for
solving a problem. Unless an algorithm produces some result, we
can never be certain
whether our solution is correct. Have you ever given a command
to a computer and
discovered that nothing changed? What was your response? You
probably thought that the
computer was malfunctioning because your command did not produce
any type of result.
Without some visible change, you have no way of determining the
effect of your command.
The same is true with algorithms. Only algorithms which produce
results can be verified as
either right or wrong.
Algorithms halt in a finite amount of time
Algorithms should be composed of a finite number of operations
and they should complete
their execution in a finite amount of time. Suppose we wanted to
write an algorithm to print
all the integers greater than 1. Our steps might look something
like this:
1. Print the number 2. 2. Print the number 3. 3. Print the
number
While our algorithm seems to be pretty clear, we have two
problems. First, the algorithm must have
an infinite number of steps because there are an infinite number
of integers greater than one. Second,
the algorithm will run forever trying to count to infinity.
These problems violate our definition that an
algorithm must halt in a finite amount of time. Every algorithm
must reach some operation that tells it
to stop.
-
16
When writing algorithms, we have several choices of how we will
specify the operations in
our algorithm. One option is to write the algorithm using plain
English. An example of this
approach is the directions to John's house given in the
introduction lesson. Although plain
English may seem like a good way to write an algorithm, it has
some problems that make it a
poor choice. First, plain English is too wordy. When we write in
plain English, we must
include many words that contribute to correct grammar or style
but do nothing to help
communicate the algorithm. Second, plain English is too
ambiguous. Often an English
sentence can be interpreted in many different ways. Remember
that our definition of an
algorithm requires that each operation be unambiguous.
Another option for writing algorithms is using programming
languages. These languages are
collections of primitives (basic operations) that a computer
understands. While programming
languages avoid the problems of being wordy and ambiguous, they
have some other
disadvantages that make them undesirable for writing algorithms.
Consider the following
lines of code from the programming language C++.
a = 1;
b = 0;
while (a
-
17
lesson. Each operation in the algorithm is written on a separate
line so they are easily
distinguished from each other. We can easily see the advantage
of this organization by
comparing the structured English algorithm with the plain
English algorithm.
How to change your motor oil Plain English Structured
English
First, place the oil pan underneath the oil plug of your
car.
Next, unscrew the oil plug and drain the oil. Now, replace
the oil plug. Once the old oil is drained, remove the oil
cap
from the engine and pour in 4 quarts of oil. Finally,
replace
the oil cap on the engine.
1. Place the oil pan underneath the oil plug of your car.
2. Unscrew the oil plug. 3. Drain oil. 4. Replace the oil plug.
5. Remove the oil cap from the engine. 6. Pour in 4 quarts of oil.
7. Replace the oil cap.
FLOWCHARTING
INTRODUCTION
The flowchart is a means of visually presenting the flow of data
through an information
processing systems, the operations performed within the system
and the sequence in which
they are performed. In this lesson, we shall concern ourselves
with the program flowchart,
which describes what operations (and in what sequence) are
required to solve a given
problem. The program flowchart can be likened to the blueprint
of a building. As we know a
designer draws a blueprint before starting construction on a
building. Similarly, a
programmer prefers to draw a flowchart prior to writing a
computer program. As in the case
of the drawing of a blueprint, the flowchart is drawn according
to defined rules and using
standard flowchart symbols prescribed by the American National
Standard Institute, Inc.
OBJECTIVES
At the end of this lesson, you will be able to understand:
the meaning of flowchart
the basic parts of the flowchart such as flowchart symbols and
the flow lines connecting these
symbols.
the advantages and limitations of flowchart
-
18
MEANING OF A FLOWCHART
A flowchart is a diagrammatic representation that illustrates
the sequence of operations to be
performed to get the solution of a problem. Flowcharts are
generally drawn in the early stages
of formulating computer solutions. Flowcharts facilitate
communication between
programmers and business people. These flowcharts play a vital
role in the programming of a
problem and are quite helpful in understanding the logic of
complicated and lengthy
problems. Once the flowchart is drawn, it becomes easy to write
the program in any high
level language. Often we see how flowcharts are helpful in
explaining the program to others.
Hence, it is correct to say that a flowchart is a must for the
better documentation of a complex
program.
25.4 GUIDELINES FOR DRAWING A FLOWCHART
Flowcharts are usually drawn using some standard symbols;
however, some special symbols
can also be developed when required. Some standard symbols,
which are frequently required
for flowcharting many computer programs are shown in Fig.
25.1
Start or end of the program
Computational steps or processing function of a
Program
Input or output operation
Decision making and branching
Connector or joining of two parts of program
-
19
Magnetic Tape
Magnetic Disk
Off-page connector
Flow line
Annotation
Display
Fig. 25.1 Flowchart Symbols
The following are some guidelines in flowcharting:
a. In drawing a proper flowchart, all necessary requirements
should be listed out in logical order.
b. The flowchart should be clear, neat and easy to follow. There
should not be any room for ambiguity in understanding the
flowchart.
c. The usual direction of the flow of a procedure or system is
from left to right or top to bottom.
d. Only one flow line should come out from a process symbol.
or
e. Only one flow line should enter a decision symbol, but two or
three flow lines, one for each possible answer, should leave the
decision symbol.
-
20
f. Only one flow line is used in conjunction with terminal
symbol.
g. Write within standard symbols briefly. As necessary, you can
use the annotation symbol to describe data or computational steps
more clearly.
h. If the flowchart becomes complex, it is better to use
connector symbols to reduce the number of flow lines. Avoid the
intersection of flow lines if you want to make it more
effective and better way of communication.
i. Ensure that the flowchart has a logical start and finish. j.
It is useful to test the validity of the flowchart by passing
through it with a simple test data.
25.5 ADVANTAGES OF USING FLOWCHARTS
The benefits of flowcharts are as follows:
1. Communication: Flowcharts are better way of communicating the
logic of a system to all
concerned.
2. Effective analysis: With the help of flowchart, problem can
be analysed in more effective
way.
3. Proper documentation: Program flowcharts serve as a good
program documentation,
which is needed for various purposes.
4. Efficient Coding: The flowcharts act as a guide or blueprint
during the systems analysis
and program development phase.
5. Proper Debugging: The flowchart helps in debugging
process.
6. Efficient Program Maintenance: The maintenance of operating
program becomes easy
with the help of flowchart. It helps the programmer to put
efforts more efficiently on that
part
-
21
25.6 LIMITATIONS OF USING FLOWCHARTS
1. Complex logic: Sometimes, the program logic is quite
complicated. In that case, flowchart becomes complex and
clumsy.
2. Alterations and Modifications: If alterations are required
the flowchart may require re-drawing completely.
3. Reproduction: As the flowchart symbols cannot be typed,
reproduction of flowchart becomes a problem.
4. The essentials of what is done can easily be lost in the
technical details of how it is done.
IN-TEXT QUESTIONS
1. Fill in the blank.
i. A program flowchart indicates the_________ to be per formed
and the __________ in which they occur.
ii. A program flowchart is generally read from _____________ to
________________ iii. Flowcharting symbols are connected together
by means of ___________________ iv. A decision symbol may be used
in determining the ____________ or ___________ of two data
items.
v. __________ are used to join remote portions of a flowchart
vi. ____________ connectors are used when a flowchart ends on one
page and begins again on other
page
vii. A ________ symbol in used at the beginning and end of a
flowchart. viii. The flowchart is one of the best ways of ________
a program..
ix. To construct a flowchart, one must adhere to prescribed
symbols provided by the __________ . x. The programmed uses a
____________ to aid him in drawing flowchart symbols.
FEW EXAMPLES ON FLOWCHARTING
Now we shall present few examples on flowcharting for proper
understanding of this
technique. This will help in student in program development
process at a later stage.
Example 1
Draw a flowchart to find the sum of first 50 natural
numbers.
.
-
22
Fig. Sum of first 50 natural numbers
Fig Flowchart for computing the sum of first 50 natural
numbers.
Example 2
Draw a flowchart to find the largest of three numbers A,B, and
C.
Answer: The required flowchart is shown in Fig 25.3
-
23
Fig Flowchart for finding out the largest of three numbers
Example 3
Draw a flowchart for computing factorial N (N!)
Where N! = 1 2 3 N .
The required flowchart has been shown in fig 25.4
Answer:
-
24
Fig Flowchart for computing factorial N
Pseudocodes
Pseudocode is a compact and informal high-level description of a
computer programming
algorithm that uses the structural conventions of some
programming language, but is intended
for human reading rather than machine reading. Pseudo-code
typically omits details that are
not essential for human understanding of the algorithm, such as
variable declarations, system-
specific code and subroutines. The programming language is
augmented with natural
language descriptions of the details, where convenient, or with
compact mathematical
notation.
The purpose of using pseudocode is that it is easier for humans
to understand than
conventional programming language code, and that it is a compact
and environment-
independent description of the key principles of an algorithm.
It is commonly used in
-
25
textbooks and scientific publications that are documenting
various algorithms, and also in
planning of computer program development, for sketching out the
structure of the program
before the actual coding takes place.
No standard for pseudocode syntax exists, as a program in
pseudocode is not an executable
program. Pseudo code resembles, but should not be confused with,
skeleton programs
including dummy code, which can be compiled without errors.
Flowcharts can be thought of
as a graphical alternative to pseudocode.
As the name suggests, pseudocode generally does not actually
obey the syntax rules of any
particular language; there is no systematic standard form,
although any particular writer will
generally borrow style and syntax for example control structures
from some conventional
programming language. Popular syntax sources include Pascal,
BASIC, C, C++, Java, Lisp,
and ALGOL. Variable declarations are typically omitted. Function
calls and blocks of code,
for example code contained within a loop, is often replaced by a
one-line natural language
sentence.
Depending on the writer, pseudocode may therefore vary widely in
style, from a near-exact
imitation of a real programming language at one extreme, to a
description approaching
formatted prose at the other.
Example:
=
if
do stuff
else
do other stuff
while
do stuff
for from to by
do stuff with variable
function ()
do stuff with arguments
return something
() // Function call
-
26
Here are a few general guidelines for checking your
pseudocode:
1. Mimic good code and good English. Using aspects of both
systems means adhering to the
style rules of both to some degree. It is still important that
variable names be mnemonic,
comments be included where useful, and English phrases be
comprehensible (full sentences are
usually not necessary).
2. Ignore unnecessary details. If you are worrying about the
placement of commas, you are using
too much detail. It is a good idea to use some convention to
group statements (begin/end,
brackets, or whatever else is clear), but you shouldn't obsess
about syntax.
3. In many cases, the type of a variable is clear from context;
unless it is critical that it is
specified to be an integer or real, it is often unnecessary to
make it explicit.
4. Take advantage of programming shorthands. Using if-then-else
or looping structures is more
concise than writing out the equivalent in English; general
constructs that are not peculiar to a
small number of languages are good candidates for use in
pseudocode. Using parameters in
specifying procedures is concise, clear, and accurate, and hence
should not be omitted from
pseudocode.
5. Consider the context. If you are writing an algorithm for
quicksort, the statement use quicksort
to sort the values is hiding too much detail; if we have already
studied quicksort in class and later
use it as a subroutine in another algorithm, the statement would
be appropriate to use.
6. Don't lose sight of the underlying model. It should be
possible to see through" your
pseudocode to the model below; if not (that is, you are not able
to analyze the algorithm easily),
it is written at too high a level.
7. Check for balance. If the pseudocode is hard for a person to
read or di_cult to translate into
working code (or worse yet, both!), then something is wrong with
the level of detail you have
chosen to use.
-
27
Introduction to Computer and Communication
4.1 INTRODUCTION
Today computer is available in many offices and homes and
therefore there is a need to share
data and programs among various computers with the advancement
of data communication
facilities. The communication between computers has increased
and it thus it has extended the
power of computer beyond the computer room. Now a user sitting
at one place can communicate
computers of any remote sites through communication channel. The
aim of this chapter is to
introduce you the various aspects of computer network.
4.2 OBJECTIVES
After going through this lesson you will be in a position
to:
explain the concept of data communication
understand the use of computer network
identify different components of computer network
identify different types of network
explain communication protocols
understand what is internet and email and its uses in modern
communication
appreciate the use of satellite communication.
4.3 DATA COMMUNICATION
We all are acquainted with some sorts of communication in our
day to day life. For
communication of information and messages we use telephone and
postal communication
systems. Similarly data and information from one computer system
can be transmitted to other
systems across geographical areas. Thus data transmission is the
movement of information using
some standard methods. These methods include electrical signals
carried along a conductor,
optical signals along an optical fibers and electromagnetic
areas.
Suppose a manager has to write several letters to various
clients. First he has to use his PC and
Word Processing package to prepare his letter. If the PC is
connected to all the client's PCs
through networking, he can send the letters to all the clients
within minutes. Thus irrespective of
-
28
geographical areas, if PCs are connected through communication
channel, the data and
information, computer files and any other program can be
transmitted to other computer systems
within seconds. The modern form of communication like e-mail and
Internet is possible only
because of computer networking.
Basic Elements of a Communication System
The following are the basic requirements for working of a
communication system.
1. A sender (source) which creates the message to be
transmitted.
2. A medium that carries the message.
3. A receiver (sink) which receives the message.
In data communication four basic terms are frequently used. They
are
Data: A collection of facts in raw forms that become information
after processing.
Signals: Electric or electromagnetic encoding of data.
Signaling: Propagation of signals across a communication
medium.
Transmission: Communication of data achieved by the processing
of signals.
4.3.1 Communication Protocols
You may be wondering how do the computers send and receive data
across communication
links. The answer is data communication software. It is this
software that enables us to
communicate with other systems. The data communication software
instructs computer systems
and devices as to how exactly data is to be transferred from one
place to another. The procedure
of data transformation in the form of software is commonly
called protocol.
The data transmission software or protocols perform the
following functions for the efficient and
error free transmission of data.
Data sequencing: A long message to be transmitted is broken into
smaller packets of fixed size
for error free data transmission.
Data Routing: It is the process of finding the most efficient
route between source and
destination before sending the data.
Flow control: All machines are not equally efficient in terms of
speed. Hence the flow control
regulates the process of sending data between fast sender and
slow receiver.
-
29
Error Control: Error detecting and recovering is the one of the
main function of communication
software. It ensures that data are transmitted without any
error.
4.3.2 Data Transmission Modes
There are three ways for transmitting data from one point to
another
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 turn-around time required in
half-duplex arrangement is eliminated. Example of this mode of
transmission is the telephone
line.
A B
Simplex A to B only
A B
Half-Duplex A to B or B to A
A B
Full-Duplex A to B and B to A
Fig. 4.1
4.3.3 Digital and Analog Transmission
Data is transmitted from one point to another point by means of
electrical signals that may be in
-
30
digital and analog form. So one should know the fundamental
difference between analog and
digital signals. In 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 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
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 digital signal into analog, and the reverse, is known
as modem.
Time
Analog Signal
MODEMS :They translate data from digital to analog form at the
sending end of the communications path and
from analog to digital at the receiving end .Put simply, the
object of a modem is to change the characteristics of a
simple sine wave, referred to as a carrier signal. We know this
carrier signal has several properties that can be
altered to represent data. It has amplitude (height); it has
frequency (a unit of time); and it has phase (a relative
starting point). Modems are capable of altering one or more of
these characteristics to represent data. The job a
modem performs can be divided into two discrete parts or phases
at each end of the communications link.
At the sending end, it converts digital bit streams (strings of
0s and 1s) into analog sine waves. This is
the encoding process. Another component within the modem then
changes (modulates) the analog signal
so the data may be transmitted simultaneously with other data
and voice traffic that has also been modulated.
This process is basically reversed at the receiving end. There,
the analog signal is brought back to its basic level
(demodulated), and the analog sine waves are reconverted
(decoded) back into their corresponding bit
streams
-
31
Digital data as it is encoded, modulated, transmitted,
demodulated, and decoded. 2-3
4.3.4 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.
Following are the major communication devices used to day.
Wire Pairs: Wire pairs are commonly used in local telephone
communication and for short
distance digital data communication. They are usually made up of
copper and the pair of
wires is twisted together. Data transmission speed is normally
9600 bits per second in a
distance of 100 meter.
-
32
Some features are:
consist of two insulated copper wires arranged in a regular
spiral pattern to minimize the
electromagnetic interference between adjacent pairs
low frequency transmission medium
low cost, small size, and ease of installation
limited distance, usually less than 100 meters
the most popular and is generally the best option for school
networks
Categories of UTP Cable
Category 1 Voice Only (Telephone Wire)
Category 2 Data to 4 Mbps (LocalTalk)
Category 3 Data to 10 Mbps (Ethernet)
Category 4 Data to 20 Mbps (16 Mbps Token Ring)
Category 5 Data to 100 Mbps (Fast Ethernet)
The standard connector for UTP cable is an RJ-45 connector,
which looks like a large
telephone modular connector
Wireless
use high frequency radio signals or infrared light beams to
communicate between the
workstations and the server
need transciever/antenna to send and receive data
widely used for connecting laptop computers to the LAN
expensive
poor security
slower than LANs using cabling
susceptible to electrical interference from lights and
radios
Coaxial Cables: Coaxial cable is groups of specially wrapped and
insulted wires that are able to
-
33
transfer data at higher rate. They consist of a central copper
wire surrounded by an insulation
over which copper mesh is placed. They are used for long
distance telephone lines and local
area network for their noise immunity and faster data
transfer.
Coaxial (Coax) Cable
Some features are:
like the wire used to connect a TV or VCR
has an inner conductor surrounded by a braided mesh
both conductors share a common center axial, hence the term
"co-axial"
bandwidth of up to 400 Mhz
highly resistant to signal interference
used for long distance (300-600 meters)
quite bulky and sometimes difficult to install
the most common type of connector used with coaxial cables is
the BNC (Bayone-Neill-
Concelman) connector
has two types of coaxial cable:
Thin coaxial cable o refers to as thinnet
o 10Base2 is the IEEE standard for Ethernet running on thin
coaxial cable
o the 2 refers to the approximate maximum segment length being
200 meters
o is popular in school networks, especially linear bus
networks
Thick coaxial cable
refers to as thicknet
10Base5 is the IEEE standard for Ethernet running on thick
coaxial cable
the 5 refers to the approximate maximum segment length being 500
meters
has an extra protective plastic cover that helps keep moisture
away from the
center
conductor
difficult to bend and install
-
34
used for long distance linear bus networks
Fiber Optic
Fiber Optic Cable
Fiber optic cabling consists of a center glass core surrounded
by several layers of protective
materials (See fig. 5). It transmits light rather than
electronic signals eliminating the problem of
electrical interference. This makes it ideal for certain
environments that contain a large amount
of electrical interference. It has also made it the standard for
connecting networks between
buildings, due to its immunity to the effects of moisture and
lighting.
Fiber optic cable has the ability to transmit signals over much
longer distances than coaxial and
twisted pair. It also has the capability to carry information at
vastly greater speeds. This capacity
broadens communication possibilities to include services such as
video conferencing and
interactive services. The cost of fiber optic cabling is
comparable to copper cabling; however, it
is more difficult to install and modify. 10BaseF refers to the
specifications for fiber optic cable
carrying Ethernet signals.
consist of a center glass core surrounded by several layers of
protective materials
immunity to environmental interference
greater capacity (bandwidth of up to 2 Gbps)
used for distances up to 100 kilometers
carry information at vastly greater speeds
very expensive
small size and lighter weight
difficult to install and modify, require highly skilled
installers
-
35
adding additional nodes is difficult
10BaseF refers to the specifications for fiber optic cable
carrying Ethernet signals
Microwave: Microwave system uses very high frequency radio
signals to transmit data through
space. The transmitter and receiver of a microwave system should
be in line-of-sight because
the radio signal cannot bend. With microwave very long distance
transmission is not
possible. In order to overcome the problem of line of sight and
power amplification of weak
signal, repeaters are used at intervals of 25 to 30 kilometers
between the transmitting and
receiving end.
Communication Satellite: The problem of line-sight and repeaters
are overcome by using
satellites which are the most widely used data transmission
media in modern days. A
communication satellite is a microwave relay station placed in
outer space. INSAT-1B is
such a satellite that can be accessible from anywhere in India.
In satellite communication,
microwave signal is transmitted from a transmitter on earth to
the satellite at space. The
satellite amplifies the weak signal and transmits it back to the
receiver. The main advantage
of satellite communication is that it is a single microwave
relay station visible from any point
of a very large area. In microwave the data transmission rate is
16 giga bits per second. They
are mostly used to link big metropolitan cities.
4.4 COMPUTER NETWORK
A computer network is an interconnection of various computer
systems located at different
places. In computer network two or more computers are linked
together with a medium and data
communication devices for the purpose of communicating data and
sharing resources. The
computer that provides resources to other computers on a network
is known as server. In the
network the individual computers, which access shared network
resources, are known as
workstations or nodes.
NEED of COMPUTER NETWORK
Sharing information: the computer can help you centralize
the
information and maintain control over it if you select one
computer to
-
36
store the shared information and have all other computers
reference the
information on that computer over the network. Sharing
hardware
resources: a network allows anyone connected to the network to
use
printers, fax modem, scanners, tape backup units or almost any
other
device that can be attached to a computer.
Sharing software resources: administrator can centrally install
and
configure the software and also restrict access to the software.
It is easier
than doing it on every one of the computers in an
organization.
Preserving information: a network also allows for information to
be
backed up to a central location. It is difficult to maintain
regular backups
on a number of stand-alone computers so important information
can be
lost easily by mistake or by accident.
Protecting information: a network provides a more secure
environment
for a company's important information than stand-alone
computers.
Networks provide an additional layer of security by way of
passwords.
Electronic-mail (e-mail): the computer network can also help
people
communicate by e-mail. You can attach electronic documents to
mail
message like photo, sound and video clip.
Computer Networks may be classified on the basis of geographical
area in two broad categories.
1. Local Area Network (LAN)
2. Wide Area Network (WAN)
4.4.1 Local Area Network
Networks used to interconnect computers in a single room, rooms
within a building or buildings
on one site are called Local Area Network (LAN). LAN transmits
data with a speed of several
megabits per second (106 bits per second). The transmission
medium is normally coaxial cables.
LAN links computers, i.e., software and hardware, in the same
area for the purpose of sharing
information. Usually LAN links computers within a limited
geographical area because they must
be connected by a cable, which is quite expensive. People
working in LAN get more capabilities
-
37
in data processing, work processing and other information
exchange compared to stand-alone
computers. Because of this information exchange most of the
business and government
organisations are using LAN.
Major Characteristics of LAN
every computer has the potential to communicate with any other
computers of the network
high degree of interconnection between computers
easy physical connection of computers in a network
inexpensive medium of data transmission
high data transmission rate
Advantages
The reliability of network is high because the failure of one
computer in the network does not
effect the functioning for other computers.
Addition of new computer to network is easy.
High rate of data transmission is possible.
Peripheral devices like magnetic disk and printer can be shared
by other computers.
Disadvantages
If the communication line fails, the entire network system
breaks down.
Use of LAN
Followings are the major areas where LAN is normally used
File transfers and Access
Word and text processing
Electronic message handling
Remote database access
Personal computing
Digital voice transmission and storage
4.4.2 Wide Area Network
-
38
The term Wide Area Network (WAN) is used to describe a computer
network spanning a
regional, national or global area. For example, for a large
company the head quarters might be at
Delhi and regional branches at Bombay, Madras, Bangalore and
Calcutta. Here regional centers
are connected to head quarters through WAN. The distance between
computers connected to
WAN is larger. Therefore the transmission medium used are
normally telephone lines,
microwaves and satellite links.
4.4.3 Characteristics of WAN
Followings are the major characteristics of WAN.
1. Communication Facility: For a big company spanning over
different parts of the country
the employees can save long distance phone calls and it
overcomes the time lag in overseas
communications. Computer conferencing is another use of WAN
where users communicate
with each other through their computer system.
2. Remote Data Entry: Remote data entry is possible in WAN. It
means sitting at any location
you can enter data, update data and query other information of
any computer attached to the
WAN but located in other cities. For example, suppose you are
sitting at Madras and want to
see some data of a computer located at Delhi, you can do it
through WAN.
3. Centralised Information: In modern computerised environment
you will find that big
organisations go for centralised data storage. This means if the
organisation is spread over
many cities, they keep their important business data in a single
place. As the data are
generated at different sites, WAN permits collection of this
data from different sites and save
at a single site.
4.4.4 Examples of WAN
1. Ethernet: Ethernet developed by Xerox Corporation is a famous
example of WAN. This
network uses coaxial cables for data transmission. Special
integrated circuit chips called
controllers are used to connect equipment to the cable.
2. Aparnet: The Aparnet is another example of WAN. It was
developed at Advanced Research
Projects Agency of U. S. Department. This Network connects more
than 40 universities and
institutions throughout USA and Europe.
-
39
Difference between LAN and WAN
LAN is restricted to limited geographical area of few
kilometers. But WAN covers great
distance and operate nationwide or even worldwide.
In LAN, the computer terminals and peripheral devices are
connected with wires and coaxial
cables. In WAN there is no physical connection. Communication is
done through telephone
lines and satellite links.
Cost of data transmission in LAN is less because the
transmission medium is owned by a
single organisation. In case of WAN the cost of data
transmission is very high because the
transmission medium used are hired, either telephone lines or
satellite links.
The speed of data transmission is much higher in LAN than in
WAN. The transmission speed
in LAN varies from 0.1 to 100 megabits per second. In case of
WAN the speed ranges from
1800 to 9600 bits per second (bps).
Few data transmission errors occur in LAN compared to WAN. It is
because in LAN the
distance covered is negligible.
4.5 NETWORK TOPOLOGY
Network Topology
Network topology are the physical layout of the network that the
locations of the computers and
how the cable is run between them. It is important to use the
right topology. Each topology has
its own strengths and weakness.
Bus topology A bus topology connects computers along a single or
more cable to connect linearly as figure 1.
A network that uses a bus topology is referred to as a "bus
network" which was the original form
of Ethernet networks. Ethernet 10Base2 (also known as thinnet)
is used for bus topology.
-
40
Bus topology is the cheapest way of connecting computers to form
a workgroup or departmental
LAN, but it has the disadvantage that a single loose connection
or cable break can bring down
the entire LAN
Termination is important issue in bus networks. The electrical
signal from a transmitting
computer is free to travel the entire length of the cable.
Without the termination, when the signal
reaches the end of the wire, it bounces back and travels back up
the wire. When a signal echoes
back and forth along an unterminated bus, it is called ringing.
The terminators absorb the
electrical energy and stop the reflections.
Advantages of the bus are following.
Bus is easy to use and understand and inexpensive simple
network
It is easy to extend a network by adding cable with a repeater
that boosts the signal and allows it
to travel a longer distance.
Disadvantages are following.
A bus topology becomes slow by heavy network traffic with a lot
of computer because networks
do not coordinate with each other to reserve times to
transmit.
It is difficult to troubleshoot a bus because a cable break or
loose connector will cause reflections
and bring down the whole network.
Star topology A star topology links the computers by individual
cables to a central unit, usually a hub as in
figure 2. When a computer or other networking component
transmits a signal to the network, the
signal travels to the hub. Then, the hub forwards the signal
simultaneously to all other
components connected to the hub. Ethernet 10BaseT is a network
based on the star topology.
-
41
Star topology is the most popular way to connect computers in a
workgroup or departmental
network.
Figure 2: Star topology
Advantages of star topology are such as:\
The failure of a single computer or cable doesn't bring down the
entire network.
The centralized networking equipment can reduce costs in the
long run by making network
management much easier.
It allows several cable types in same network with a hub that
can accommodate multiple cable
types.
Disadvantages of star topology are such as:
Failure of the central hub causes the whole network failure.
It is slightly more expensive than using bus topology.
Ring topology A ring topology connects the computers along a
single path whose ends are joined to form a
circle as figure 3. The circle might be logical only but the
physical arrangement of the cabling
might be similar to star topology, with a hub or concentrator at
the center. The ring topology is
commonly used in token ring networks that the ring of a token
ring network is concentrated
-
42
inside a device called a Multistation Access Unit (MAU) and
fiber Distributed Data Interface
(FDDI) networks that the ring in this case is both a physical
and logical ring and usually runs
around a campus or collection of buildings to form a high-speed
backbone network.
Figure 3: Ring topology
Advantages are following:
One computer cannot monopolize the network.
It continue to function after capacity is exceeded but the speed
will be slow.
Disadvantages are following:
Failure of one computer can affect the whole network.
It is difficult to troubleshoot.
Adding and removing computers disrupts the network.
Mesh topology In a mesh topology, each computer on network has
redundant data paths as showing in figure 4.
The mesh topology provides fault tolerance-if a wire, hub,
switch, or other component fails, data
can travel along an alternate path. A diagram of a mesh network
looks like a fishing net. A mesh
topology is most often used in large backbone networks in which
failure of a single switch or
router can result in a large portion of the network going
down.
-
43
Figure 4: Mesh topology
Overview of the Internet
What is the Internet?
The Internet is a worldwide connection of millions of computers
connected to thousands of
different networks. These computers "communicate" that is,
share, exchange, and transmit data
to another computer on the same or another network.
-
44
Who runs it?
Nobody owns or controls the Internet. It is a global information
system similar in some respects
to telephone networks that allow anybody to call any other
number anytime, anywhere. Each
network is run by its own operating center subject to the laws
in their country and international
conventions. There are, instead, several "councils of elders,"
consortiums, societies, and
communities whereby representatives from various countries,
institutions, and organizations
collaborate occasionally to establish common standards for
hardware, software, and
telecommunications technologies.
How did it come to be?
The Internet was born in the era of the Cold War. At that time
the U.S. Department of Defense
was interested in establishing a communication system using
computer, radio and satellite
networks that will be able to work around power outtages in the
event of a nuclear attack. How
will the government communicate if the communication network is
destroyed? The answer was
to do away with a centralized communication network and come up
with separate networks
where each will be independently responsible for getting
messages across through any route. An
experimental network called the ARPAnet was set up in 1968 to
enable scientists and researchers
in universities to collaborate on this project. Some
universities were later permitted during the
1970s to connect their local area networks to ARPAnet. Demand
grew as networking spread
among schools. With the invention of the first e-mail program in
1972, academic resources
started using the network not only for long-distance computing
but more to exchange
information and gossip with their colleagues. Concerned by
security risks, the military broke off
and established a separate network in 1983. By the late 1980s
the National Science Foundation
commissioned its own network called NSFNET to share its
information resources for scholarly
research. Major universities were connected to five computer
centers using telephone lines. To
avoid the cost of laying down telephone cables for every
institution to connect directly to the
computer centers, regional networks were created. This resulted
in a chain that allowed schools
to connect to their nearest neighbor, thus permitting their
computers to forward messages from
one link to another.
-
45
How does it work?
This ability of computers and networks all over the world to
share information was made
possible by two important communication protocols - the
Transmission Control Protocol (TCP)
and the Internet Protocol (IP). Together they are often referred
to as TCP/IP, and together they
make the Internet known as a "packet-switched network.".
A good analogy will be the postal service. Your computer is both
your
address and post office. To send a message to another computer
you
need to put the address of your destination, too.When you
transmit, your
message travels through cable lines or signals, in the case of
wireless
technologies. However, your message does not travel in one
piece. It is
first broken down into numbered, bite-size chunks called
"packets" and
sent separately through different routes, and then recombined
again in
their original form at their destination by Transmission Control
Protocol.
The Internet Protocol handles how these packets are routed. The
packets
are placed in separate IP "envelopes" and are sent through a
series of
switches or routers.
Speed and efficiency are essential, as you will soon find out.
Many
breakthroughs in Internet and computer technologies are guided
by the
principle of sending and receiving data in the shortest time
possible. The
data is broken down into packets for faster transmission. As the
packets
travel in IP envelopes routers examine the addresses of these
envelopes
and determine the most efficient path for sending each packet to
the
nearest available router until these reach their final
destination. Since the
traffic load on the Internet constantly changes every second,
the packets
may travel in different routes and arrive in different order. At
the their
destination the TCP will reassemble the packets into their
original form
-
46
according to their numbered order. If a packet is missing the
TCP
determines that the file or message was corrupted in transit,
and will
request for retransmission.
Here's another analogy. You have a big family who are traveling
from one country to attend a
reunion in another country. There are not enough seats on the
plane to accommodate everyone so
the TCP breaks up everybody in different planes (IP). These
flights take different routes and
make different stops. It is likely that the passengers in each
flight have different destinations. At
each of these stops or airports, the router checks the
destination of every passenger's ticket and
directs each to the next available flight towards their final
destination. The members of the
convention group arrive at their final destination separately.
To regroup them the TCP will do a
headcount making sure everybody is accounted for, and then usher
them out of the airport in
proper order, that is, grandpa first, grandma second, eldest
child next, and so on.
What is an Internet address?
As you have learned how messages are transmitted from one
computer to another, every
computer in the Internet must have a unique and specific
address. An Internet or IP address, as it
is sometimes called, consists of four numbers separated by
periods. The smallest address would
be 0.0.0.0 while the biggest would be 1234.5678.9101.1121. Don't
worry, only computers are
expected to remember all these numbers without getting confused.
For humans, we use the
Domain Name System (DNS).
What are domains?
Like IP addresses which are numerical, the alphabetical domain
names are also separated by
periods or dots. Thus, the U.S. Library of Congress will have an
IP address of 140.147.248.7 and
the domain name will be www.loc.gov.
Domain names have the format:
hostname.subdomain.top-level-domain. My school's domain
name, for example, is ocean.otr.usm.edu. This naming structure
will give you clues about the
address. Ocean is the name of the host computer, otr stands for
"Office of Technology
Resources" which manages the system, usm is the abbreviation for
University of Southern
-
47
Mississippi, the sub-domain, and edu signifies that this is an
educational institution. The gov for
the Library of Congress indicates it is a government
institution.
Here are the existing top-level domains in the Internet:
com - commercial
.edu - educational
.net - network
.org - organization
.gov - government
.mil - military
Other countries sometimes add their codes at the end, such as
.au for Australia, .ph for
Philippines, and .fr for France.
Internet Services
Most of these Internet Services operate on the client/server
model or concept. A computer is a
client if it is receiving files, and is a server if it is
sending files. Another way of putting it is you,
your computer, and the corresponding software for each type of
service are all clients. A server is
a software that allows a computer to offer a service to another
computer. The computer on which
the server software runs is also called a server. To gain access
to the Internet most people open
an account with an Internet Service Provider (ISP) in their
area. To connect they "dial-up" to
their ISP's server and "log-in" with their account name and
password. If you noticed, these
services are loosely and interchangeably used as nouns and
verbs, i.e. "please e-mail me."
Electronic Mail
E-mail, so far, is still the most popular service of the
Internet. Most users get acquainted with the
Internet using this service. It is better than "snail mail" by
post because your message can be
transmitted in seconds. It is better than telephones because the
recipient doesn't have to be
around to receive the message. A person also has the option to
read and respond to the messages
at one's convenience. Cheaper than long distance phone calls,
some people are even able to send
-
48
and receive e-mail wherever they go when they travel as long as
they have access to a connected
computer. Better than fax, through e-mail people can send forms,
documents, colored pictures,
etc. as attachments.
Mailing List
Listserv, a popular type of mailing list, is short for "list
server" and is based on the e-mail
protocol. As an electronic mailing list it is very convenient
when somebody wants to send a
message or newsletter, for example, to many people at once.
There are thousands of listservs for
every imaginable purpose, interest, service, and information
like online newsletters, online sales
or services, and class listservs. To join a listserv means to be
included in the mailing list. You
send an e-mail saying you want to subscribe and you will
henceforth receive announcements,
notices, and information through your e-mail inbox.
Newsgroups
This is the Internet equivalent of a discussion group or an
electronic bulletin board. There are
newsgroups for every conceivable topic and more, from
educational technology and Brazil
culture to stamp collecting and mountaineering. Those with
specific hobbies or unusual interests
are quiet happy to have this service in the Internet when there
are so few people in their area to
discuss their interests with. You can usually tell a newsgroup
by its name. Computer newsgroups
start with comp as in comp.apps.spreadsheets. while recreational
newsgroups start with
rec as in rec.arts.cinema. The others are alt for alternate, soc
for social, sci for science,
and news for news.
Some newsgroups are moderated, others are not. If it is
moderated a
human moderator screens all messages and decides if these are
worth
posting. Discussions can be "threaded." For example, in a
newsgroup for
beer (alt.beer), one member might ask about good beer pubs in
England and
another will start discussing how to keep a beer fresh. More
than one
threaded discussion can occur at the same time with different
members
posting their opinions and reactions to either one or more
threads.
-
49
Chat
Another popular form of communication over the Internet. Unlike
e-mail, listservs, and
newsgroups, chat allows people to converse in "real time."
People may actually see you type
your questions and responses. Chats are also organized in chat
channels and chat rooms
according to different factors. There are chat rooms for
"techies," cat lovers, singles, etc.
FTP
File Transfer Protocol is the standard method for transferring
files, whether downloading or
uploading, to and from your computer with another computer on
the Internet. It is fairly simple
to use and is the most popular way to download software and
other files from the Internet. All
the files in this web site -- the HTML, graphics, etc. -- were
uploaded into the Web using
WS_FTP95. There are private FTP sites that require you to log-in
with your account or user
name and password before you are allowed to enter. There are
also anonymous FTP sites that
allow you to enter using "anonymous" as your user name and your
e-mail address as your
password.
Telnet
Telnet is short for "terminal emulation." It is one amazing
feature of the Internet that lets you use
the resources of another computer in another part of the world.
This is done by remotely logging
to the distant computer which is called the host. Once you are
connected you can operate a
remote computer using your keyboard. Some systems require you to
log in as a "guest" and
others require your name and address, or user name and password.
The telnet is most useful to
access the databases of public services like library catalogs
and government directories. Telnet
also allows you to access your e-mail account if you can
remotely log in to your ISP's server
computer.
The World Wide Web
The World Wide Web is one of the latest information service to
arrive on the Internet but is
arguably the technology that revolutionized the Internet. It is
the fastest growing and most
exciting feature. People who "surf" or "browse" are describing
activities on the WWW. Other
words pertaining to the Web are homepages, websites, URL, HTML,
and HTTP. This section
certainly deserves a page or two of its own so please click on
the "Next" button to proceed.
-
50
How internet is useful in Education
There are many advantages to using the services of the Internet,
particularly the tools and
resources on the Web, in education. Instruction materials such
as syllabi, lecture notes,
presentations assignments, and announcements could be made
available online.
The Internet puts the concept of "anytime, anywhere" into a
higher level as far as learning is
concerned. Students will be able to learn at their pace. The
primary beneficiaries would be
students and adult learners who are too far or too busy to
attend classes. Even traditional students
will benefit when instructional materials are available to
support or complement classroom
attendance. Students will be able to prepare for the next class
if they are reminded of the
assignments and topics of discussion. Those who missed class or
missed the teacher's point can
go over the lecture notes and presentation.
Both teacher and student will also benefit from being able to
research any topic from library
catalogs, topical databases, and the World Wide Web. Anonymous
FTP and Telnet will alow
them to access public databases maintained by government
institutions. Services like e-mail and
mailing lists can support communication between teacher and
student, and among peers (teacher
to teacher, student to student). Using a combination of e-mail
and the Web, teachers and students
can collaborate on research and creative projects even with
those who don't live in the same city
or country.
The World Wide Web
What is the difference between the Internet and the World Wide
Web?
Think of the Net as primarily hardware and software, you know,
technical stuff -- and the Web
as wholly content and information, as in creative and
intellectual in nature. The Net consists of
cables, computers, satellites, and networks. Computers are able
to communicate using different
software and protocols with each other, connected either by
cable or wireless technology. The
Web, on the other hand, is an abstract concept that exists in
cyberspace. When we speak of the
Web we talk of text, sound, graphics, and videos that are
presented in Web page formats. To
-
51
navigate, surf, or browse around the Web is to move from one
site to another, from one page to
another, one file to another using the hypertext links.
Who invented it?
The father of the World Wide Web is Tim Berners-Lee, a physicist
working at CERN, the
European Particle Physics Laboratory in Switzerland. In 1989 he
developed a network protocol
called Hypertext Transfer Protocol (HTTP) as a way for
physicists to send documents over the
internet to share research information. He is also credited as
the man who coined the words
"World Wide Web" and defined standards such as the Uniform
Resource Locator (URL) and
Hypertext Mark-up Language (HTML).
What is a Web page?
It is a document written in HTML code that contains text and
links to other pages, files, or parts
of the document. The earliest Web pages were all-text documents
and at present there are still
text-based browsers like Lynx. Although Tim Berners-Lee also
wrote the first multimedia
browser in 1990, graphical user-interface (GUI) browsers didn't
become popular until Mosaic
came along in 1993.
What are browsers?
Browsers are viewer programs that display Web pages. There are
also browsers to view e-mail ,
newsgroup discussions, etc. The most popular GUI browsers today
are Netscape and Internet
Explorer. Marc Anderssen, the founder of Netscape, is
also the brains of Mosaic.
Web browsers interpret HTML codes how
to display text, graphics, links, and
multimedia files in a page. When your
computer loads a web page, that is, an
HTML file, you don't see these codes
unless you give the "View at Page
Source" command. To see what the HTML
-
52
code for this page looks like press CtrlandUat the same time,
or
click on the word View in the toolbar above you, and select
Page
Source.
Not all files on the Web can be displayed by a browser. There
are, for
example, applications that perform specific tasks when you click
on the
appropriate link, like e-mail.
With their "point-and-click" features, GUI browsers dramatically
made the
Internet user-friendly to novice users and its popularity
exploded when
people were able to dial-in using a home computer and modem to
an
Internet Service Provider. Before this only large institutions
like
universities, corporations, and government organizations that
could afford
to install cables have access to the Internet. These two
innovations
opened up the Internet to a critical mass of individual users,
people who
don't have the time or inclination to learn technical stuff, and
people who
use computers at home for work and personal activities.
Next to the e-mail, the Web is the most popular service on the
Internet.
According to the Online Computer Literacy Center, as of June 28,
1999,
there are 4.8 million Web sites all over the world, a 71 percent
increase
from 1998.
Websites, Homepages, and URL
A website is a collection of pages in the Web. Home page, as the
name suggests, is the main or
opening page of a website. You will notice that several websites
use the word "Home" or an
image of a house to guide surfers back to the main page. The URL
or Unform Resource Locator
is the specific address of a webpage, like
http://geocities.com/toe6000/www1.html for this particular page.
If you look up
at your Location Toolbar above you will see this address
displayed. Sometimes, the URL of
another website is all you need to go to that site. Try this,
click and delete the URL of this page
-
53
and in its place, type http://cnn.com which is the URL for the
home page of "CNN
Interactive," and hit Enter. You can always click on the Back
button to return to this page.
URLs are also used in newsgroups, FTP, and telnet to access
other addresses and files.
What makes the Web so popular?
Ease of use, thanks to the graphical user interface and
point-and-click features. Navigation is
non-linear and non-hierarchical. The hypertext links allows you
to jump from one section, page,
file, or site to another in any direction as you please. The
variety of file types makes it interactive
and multimedia. Once again, the hypertext links to different
file types allows you to perform any
task such as read articles and documents; write an e-mail; view
pictures or computer-generated
animation; watch a video clip; listen to music and radio; take
an online test; fill out a survey
form; and download files and software.
This sec