Computer Fundamental

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BSIT 12 Computer Fundamentals
Chapter 1
Computer ConceptS
1.0 INTRODUCTION
T
here has been a rapid growth in computer technology than in any other area of science and
technology. Developments in electronics manufacturing process has made the computer affordable
to the common man. Computers which were thought to be used only by scientists and engineers
have become part and parcel of every man in daily life. We have computers in all walks of our life. The
statements “ By the year 2099 , there will be no clear distinction between man and machine - we will
have merged “ and “Within the next twenty years , computers will be powerful like the human brain “
make us think the dominant role the computers are likely to play in the future. It is inevitable to live with
computers and one may ask what would be our life without computers.
Computers neither have the intelligence nor the capabilities to learn as we human beings can , but they
have created an atmosphere that computers are better than human beings. The superiority is that it is
capable of performing repetitive tasks very fast and store large volumes of data. It appears more efficient
than human beings. The efficiency is because of the ability of computers to follow the set of instructions
given by human beings in the form of program without human intervention.
1.1 NEED FOR COMPUTER LITERACY
Computers help businesses, schools and individuals at home to operate more efficiently. This efficiency
can take the form of working more quickly, more accurately or even in ways not previously conceived
possible.
Computers are impacting our society greatly. Buying groceries at the supermarket and using an automatic
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banking machine require using computers. The ability to communicate from one computer to another is
changing the way people live, study and work.
Today, knowing how to use a personal computer is a basic skill that is becoming part of the literacy
picture. In an increasing way, more information is available to us now via computers, so it is important to
know how to access it. New applications are being developed every day for using computers. Technology
has a new success story in the “computer.”
1.2 COMPUTER APPLICATIONS
Computers are used to gather, analyze and reproduce information in organized formats. They have the
capacity to store vast amounts of data in small areas and reproduce that information when it is needed.
Almost fifty years ago, the few computers in existence were large and expensive machines used mostly
for scientific purposes. But now the times have changed! They sit in offices, factories, homes, schools,
hospitals, banks and retail department stores.
At home, computers can monitor and control security systems, record your personal banking and
financial matters and keep a daily agenda of your time schedule for appointments and reminders. Figure
1.1 shows a typical arrangement of a computer at home.
Figure 1.1 Computer arrangement
In the schools, computers are being introduced in the primary levels to familiarize the children with a
new visual aid that presents the materials in an attractive way. Having grown up watching a television
screen, many children adapt easily to viewing the computer monitor. New educational software is continually
being developed to supplement academic learning curriculum at various grade levels. High schools and
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colleges are training students in actual application programs such as word processing, database, spreadsheets
and desktop publishing.
In the workplace, computers are part of the automation that is replacing manpower because of their
speed in performance. Computers have moved aside the need of older office equipment and are being
implemented in industrial and manufacturing applications and also in drafting and design work. Computers
are employed in the fields of engineering, aviation, communications and health services and many other
areas. Qualified specialists are now working as computer programmers, computerized accounting clerks,
computer technicians and computer software analysts. The changing face of the workplace is demanding
new courses in computers for various needs as they arise.
The following are the areas of knowledge required for computer literacy:
1. Computers themselves: You should understand the organization, capabilities and limitations of
the various machines or hardware that make up a modern computer system.
2. What computers do: You should be familiar with some of the most common applications of
computers in today’s society.
3. How computers are put to work: You need to know the analysis, design and program preparation
procedures that must be carried out in order to produce software. Basic knowledge of hardware
is also very essential.
4. The social impact of computers at work: Finally you should know how individuals and organizations
may be affected by present and future computer applications.
1.3 COMPUTER CAPABILITIES
Most people are aware that a computer is a machine that can perform arithmetic operations, but it is
much more than just an arithmetic number crunching device. It is also a machine that can choose, copy,
move compare and perform various non-arithmetic operations on many alphabetic, numeric and other
symbols that humans use to represent things. The computer manipulates these symbols in the desired
way through a sequence of instructions, called a program.
A program is a detailed set of human instructions that directs the computer to function in a specific
way to produce a desired result. The programs form the software part of the computer. Electrical or
electromechanical components are known as hardware.
a. Speed and accuracy capabilities
A computer works one step at a time. It can add, subtract, compare digits and letters, move and copy
numbers and letters. There’s nothing profound in these operations. What is significant is, the computer
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speed. This speed is measured in milliseconds, microseconds, nanoseconds and pico seconds (Recent
study says computer speed is being doubled every six months).
The speed required for computers to execute a basic operation like addition varies from a few
microseconds for the smallest machines to 80 nano seconds or less for the larger ones.
Thus, the slowest computers can perform hundreds of thousands of additions in a second while the
largest systems can compute several million additions in the same period.
In addition to being very fast, computers are also very accurate. It is estimated that you or I would
make one error in every 500 to 1,000 operations with a calculator. But the circuits in a computer require
no human interaction between processing operations and have no mechanical parts to wear out and
malfunction. Thus these circuits can perform hundreds of thousands (or millions) or operations every
second and can run errorless for hours and days together at a time.
Beyond this, computers also have built-in self-checking capabilities that permit them to monitor the
accuracy of their internal operations.
If the input data are correct and relevant and if the program of processing the instructions is reliable,
then the computer can generally be expected to produce accurate output. The phrase “ Garbage in,
Garbage-out” or GIGO is used often by people who work with computers to illustrate the importance of
input data and correct instruction.
b. Data manipulating capability
The first computers were built to manipulate numbers in order to solve arithmetic problems. Along
with numbers, we also use alphabets and different symbols in our daily life. Luckily, early computer
experts made the important discovery that a machine that can accept, store and process numbers can also
be used to manipulate non-numeric symbols. Manipulating these familiar symbols is possible if an identifying
code number is assigned to the symbol to be stored and processed. Thus, the letter A can be represented
by a code, so can the letter B, the addition symbol and so on. Of course, one must give the computer
instructions for it to manipulate the coded and stored symbols in a desired way.
c. Data - versus - information
The word “ data” is the plural of datum, which means fact. Data then, are facts or raw material of
information. Data are represented by symbols.
Data arranged in ordered or useful form is called information. That is, information is relevant knowledge
produced as output after data processing operations and acquired by people to enhance understanding
and to achieve specific goals.
The following are the stages in processing of raw data to provide information:
1. Data Collection
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2. Raw data input
3. Data Processing
4. Information
5. End user decision and actions.
Figure 1.2 shows the pipelined stages in a typical data processing job.
Figure 1.2 Data Processing
d. Data Processing activities
Data processing consists of gathering the raw data input, evaluating and bringing order to it and placing
it in proper perspective so that useful information is produced. All data processing, whether done by hand
or computer system consists or three basic activities: capturing the input data, manipulating the data and
managing the output results.
Capturing the input data
Data must be originated in some form and verified for accuracy prior to further processing.
They may initially be recorded on paper called source documents and then converted into a
machine - usable form processing or they may be captured directly in paperless machine-readable
form.
Manipulating the data
One or more of the operation such as classifying, sorting, calculating may have to be performed
on the data that is collected.
Classifying
Organizing items with like characteristics into groups or classes is called classifying. Consider
the data taken from a stores sales bill for example, it may be classified by product sold, by the
department sales person or any other classification useful for store management. Classifying is
usually accomplished by assigning pre-determined abbreviation codes to the items being arranged.
The three types of codes used are numeric, alphabetic and alphanumeric.
Sorting
Usually, it is easier to work with data if they are arranged in a logical sequence. For example:

Data
Collection
Raw Data
Input
Data
Processing
Information
Processing
End User
decision and
Action
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include first to last, biggest to smallest, oldest to newest. Arranging classified data in such a
sequence is called sorting.
Calculating
Arithmetic manipulations of the data is called calculating. For example: In calculating a sales
person’s pay, the hours worked multiplied by the hourly wage rate gives the total earnings.
Payroll deductions such as taxes are then calculated and subtracted from total earnings to get
the sales person’s take-home pay.
Summarizing
Reducing masses of data to a more concise and usable form is called summarizing.
Example: The general manager of a retail store is interested only in a summary of the total sales
of each department. A summary report would give only total sales information. Department
managers may want more detailed information. The total sales of each department broken
down into sales made by each of the sales person and also sales of the product type.
Managing the output results
Once the data has been captured and manipulated, further operations may be needed.
Storing and retrieving
Retaining data for future reference is storing. Storage media such as paper ( in sheet, punched
card or punched tape form), microfilm, or magnetic disks and tapes are generally used. Recovering
stored data and / or information is the retrieving activity. One slow approach is for people to
search file cabinets. A much faster method is to use electronic enquiry devices that are connected
directly to a computer and mass storage unit containing the data.
Communicating and reproducing
Transferring data from one location to another for use or for further processing is Data
communication, process continues until information is in a usable form and reaches the final
user. Some times of course, its necessary to copy or duplicate data. This reproduction activity
may be done by hand or by machine.
1.4 COMPUTER CONCEPTS
There is nothing new about the data processing steps just stated above. They have been performed
down through the ages, first by hand, then by machine assisted manual and electromechanical punched
card methods and now by computers.
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The four functions which follow are all that a computer can perform, but they also include most data
processing steps:
1. Input / Output operation: A computer can accept data(input) from and supply processed
data(output) to a wide range of input/ output devices. Such devices as keyboards and display
screens make human - machine communication possible. Multiple output documents may be
reproduced by printers.
2. Calculation operations: The circuits in a computer are designed to permit addition, subtraction,
multiplication and division.
3. Logic/ Comparison operation: The computer also has the ability to perform certain logic
operations. For example, when two numbers represented by the symbols A and B are compared,
there are only 3 possible outcomes:
1) A is equal to B ( A=B);
2) A is greater than B (A>B) or
3) A is less than B (A<B).
Figure .1.3 Possible outcomes when A is compared with B
Figure 1.3 shows the decision box with the possible outcomes.
The computer is able to perform a simple comparison and then, depending on the result, follow
a predetermined branch or course of action. This comparison ability makes it possible to classify
item A as having or not having the characteristic of B. This simple ability to compare is an
important computer property, because more sophisticated questions can be answered by using
combinations of comparison decisions.

Compare A,B
A>B
A<B
A=B

Input A, B
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4. Storage and retrieval operations: Both data and program instructions are stored internally
in a computer. Storing both data and instructions in same memory is known as stored program
concept or John Von Neumann Concept. And once stored, it may be quickly called or
retrieved for use. The time required for data to be retrieved is measured in micro or nanoseconds.
Various Software Applications/ Different types of Software
The programs that are designed to carry out certain tasks for computers are called software. Software
is now available to deal with the different activities which a computer can perform. The programs are
written in special languages that use letters, numbers, or codes which the computer interprets. (e.g. many
computer programs are written in VISUAL BASIC, C++, FORTRAN)
The programs can be system software which control the actual operations of the computer itself,
such as DOS (Disk Operating System),Microsoft Windows, Linux etc., System software will tell the
computer how to load, store and execute the application programs it uses.
Application software constitutes the actual programs which a company or individual may require.
These application software programs tell the computer how to produce the information stored. Some
samples of application software are: word processing software, electronic spreadsheet software, computer
graphics software or database software.
Word Processing Software can be used to write letters, memos and documents. It provides the user
with easy ways to add, delete, sort or change text on screen until it is suitable. It saves or prints the
information. Word processing software prepares forms and printouts that typewriters formerly prepared.
The more elaborate programs can correct spelling, change the text appearance, change margins and even
relocate entire paragraphs in the editing stage. Word processing software is popular because of its quickness
in printing and its disk storage capabilities. Some examples of such software are: WordPerfect, Microsoft
Word, First Choice and WordStar.
Database Software had its origins in record-keeping systems of bygone years. The need for worksheets
used in classifying, calculating and summarizing has always been strong in the accounting fields of finance.
Manual systems were replaced by punch-card equipment, which have been superseded by computers.
Database software allow the user to enter, retrieve and update data in an efficient manner. Information
can be classified, sorted and produced as reports needed for managing businesses. Examples of such
softwares is: AccPac Plus, dBASE IV, Lotus Works.
Electronic Spreadsheet Software is used by people working with numbers, who can enter data and
formulas so that the program can calculate or project results. With spreadsheet programs, the user can
ask “what if” questions by changing data and recalculating. Spreadsheets are helpful for setting production
and sales reports. The data may be presented in rows, columns or tables. This kind of software is popular
because of its timesaving advantage over manual calculations. Spreadsheets have aided in on-the-spot
decision-making. Some examples are: Lotus 1-2-3, Excel .
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Computer Graphics Software produces professional looking documents containing both text and
graphics. It can transform series of number values into charts and graphs for easier analysis or interpretation.
Computer graphics software is used in the architectural, drafting and design industries. These programs
present the data in a graphic “pie”, to aid in understanding statistics, trends, relationships and survey
results. The use of clip art or graphs in line, bar, or circle format can provide useful charts which can even
be colour-enhanced. Some examples are: Corel Print House, Desktop Publishing, Ventura, Pagemaker.
In businesses, one may be faced with the question of what software to get? This depends on the
requirement and study advantages and disadvantages of available software.
Canned software is pre-written mass-market software, ready-to-use and available nationwide.
Custom software is software programmed to the users specifications by experienced programmers.
This method is usually undertaken only when it is determined that the necessary software does not exist.
Computer Virus: A computer virus is a program that copies itself into other programs and spreads
through multiple computer systems. Most viruses cause damage to files on the system where the virus is
present. Some computer “hackers” have been responsible for designing these viruses to attack existing
programs. Recent viruses like the “Melissa virus” have resulted in stiff punishments because of the
potential vandalism, expense and inconvenience such public mischief has caused. “Antivirus programs”
prevent, detect and remove viruses. With the onslaught of new viruses being developed every day, virus
protection programs must be updated frequently to handle recently created threats. Often, new downloads
are available via Internet websites for a minimal fee.
1.5 BASIC COMPUTER TERMINOLOGY
Computer
It is an electronic device, operating under the control of instructions stored in its own memory unit. It
can accept input data, process it logically and produce output results which can be stored for future use.
General terms
Hardware: These are the physical components or equipment which make up the computer system.
Software: The programs containing instructions that tell the computer how to operate/what to do.
Computer Component terms
CPU: The Central Processing Unit is the brain of the computer. This has an Arithmetic and Logic unit
(ALU) and Control Unit (CU) and also some memory. It is the control unit which executes program
instructions and is the primary storage unit for data.
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Monitor: The screen/video device producing display of information onscreen also called as VDU.
Keyboard: Input device used for entering data into computer’s memory for screen display.
Mouse: It is a pointing device used to select processing options or data displayed on the screen. The
pointer of the mouse is usually in the shape of an arrow on the screen. User can also change the way the
cursor looks on the screen.
Printer: It is the hardware that produces a permanent hard copy of data in a form that can be
understood by the user.
Speakers: Components used for audio output.
Microphone: A user can record audio signals using the microphone.
Modem: A device that allows your computer to talk to other computers over a phone line (often an
internal fax modem.) To connect the computer to phone line for Internet access a modem is needed.
Modem is acronym for Modulator Demodulator.
Scanners: It is a device that is used to scan printouts, photos, images and artwork for conversion into
a form that can be processed and reproduced by the computer.
System Description terms
Input Devices: The hardware components that are used to pass data into the computer.
Output Devices: the hardware that receives and displays information coming from the computer.
ROM(Read Only Memory): the permanent memory that is built into your computer by the manufacturer.
ROM memory retains its contents even when the power is turned off. Data or programs stored here can
be read and used, but not altered.
RAM(Random Accesses Memory): temporarily stores data and program instructions when they are
being processed. It is the computer’s main memory or working memory.
Megahertz(Mhz):The unit of measure determining the system clock speed of all computer operations.
(One megahertz equals one million electronic pulses per second.) It is the fixed rate at which the computer
operates. GHz is for GigaHertz.
Storage Terms
Disk drives: It is a secondary storage or auxiliary storage devices that store instructions and data
when they’re not being used by the system unit. They are input sources when floppy disks are inserted
with previously stored data, they are read into memory.
Floppy disks: It is a portable magnetic storage disk used for storing computer data. They allow users
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to randomly access information. Available in 5-1/4" size, which is now almost obsolete, and 3- 1/2" size is
used.
How to format a floppy disk: The formatting process not only prepares the disk for storage but also
erases any data that is on the disk. It analyses its recording surface for any defective spots and establishes
a directory that will be used for recording information about files stored on the disk
CD-ROM drive: Used to input source of previously stored data which gets read into memory. It uses
a low-powered laser light to read data from removable CD-ROM’s.
Compact Disc (CD’s): A disk on which a laser has digitally recorded information such as audio,
video, or computer data.
Hard Disk Drive(HDD): It is a secondary storage device that contains a high-capacity disk or disks
that provide greater storage capacities than floppy disks.
Megabyte(MB): It is approximately a million bytes (or characters), it is the measuring unit for disk
storage.
Gigabyte(GB): It is approximately a billion bytes (or 1,000 MB)
APPLICATIONS TERMINOLOGY TERMS :
User Friendly: It is a term used to indicate a program that is easy to use.
Graphical User Interfaces (GUI):It is the use of graphical symbols (pictorial images), instead of
text commands to control common computer functions in a simpler way. (e.g. function keys, screen
prompts, menus, icons)
Icons: It is a small picture or symbol representing a computer hardware function or component.
Function keys: These are shortcut keys on the computer keyboard for special commands or operations.
Menus: A special kind of screen prompt providing a list of processing options in pull-down format
(usually across the top of the screen)
Numeric keypad: The keys on Right Hand side of the keyboard which resemble adding machine or
calculator format.
Mouse pointer: The arrow displayed on the screen at the movement of the mouse.
Cursor: It is a symbol which appears on the screen as a “flashing line”( | ) or “flashing
underline”( _ ) character that indicates where user is working on the screen.
Windows: It is an integrated type of software operating program which allows various software
programs and applications to communicate or interact with one another by using a common set of data.
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(Windows gets its name from the rectangular boxes displayed on the screen which allow the user to see
other parts of a program through multiple windows at the same time.)
Computer Programming Languages/Codes
ASCII Code(pronounced ask-ee):American Standard Code for Information Interchange: the most
widely used coding system used to represent data. It uses the right 7 of 8 bits in a byte in a binary number
system using only 2 symbols: zero(0) and one(1).[ The “0” represents off in an open circle, and the “1”
represents on in a solid circle.]
EBCDIC Code (pronounced eb-see-dick): Extended Binary Coded Decimal Interchange Code:
coding system used primarily on mainframe computers using all 8 bits of a byte to represent its characters.
PRINTERS :
Daisy wheel printer:This is the slowest printer, but of higher quality than DMP because of fully
formed characters. It is easier for changing fonts, but has no graphic output.
Dot matrix printer (DMP): It is a bidirectional printer that is versatile and inexpensive, having 9 to
24 head pins which print a series of small dots to present data in printed format.
Ink-jet printer:In this type the printer forms letters on page by shooting tiny electrically charged
droplets of ink via a nozzle. It produces hi-quality print and graphics, but cannot use multipart paper. A
common problem is that the ink sometimes smears on soft, porous paper.
Laser printer: This type of printer uses laser and photographic technology to produce high quality
output.
1.6 CONNECTING THE COMPUTER COMPONENTS
Figure 1.4 Personal Computer
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Most computers will come with a manua which will guide the user through the connection process.
The personal computer is shown in Figure 1.4
The general rule is: If it fits, it probably goes there! In spite of this saying, Murphy’s Law
sometimes does apply in the sense that “if anything can go wrong, it usually will.” So, given this fact, here
are some basics that one has to know
Power Source
First, to help protect your computer system from sudden increases or decreases in voltage and unforeseen
electrical power cut off, you should always use a “surge protector” and an un-interruptable power supply
(UPS). This will prevent from losing important data or files from your computer memory when you are
using or working on a program.
Computer Location
The computer should be located in a place where the cables cannot be stepped on or tripped over.
Keep the computer away from heat sources and do not block cooling vents. Avoid placing loose papers
underneath your computer and do not place it in a closed-in wall unit. If possible, try to avoid direct
sunlight from hitting the monitor of your computer by placing it away from a window or by installing blinds
to block out the direct sun light.
Keyboard and Mouse Connections
The keyboard and mouse must plug in to the back of the CPU. The keyboard will often have a circular
female pin-type of connector. (In certain computer systems, the connection spots may be color-coded or
labeled appropriately.) The mouse may have either a circular or rectangular connector, depending on the
style and version used with the system.
Monitor Connection (2 cords)
On older versions, the power cable was permanently attached to the back of the monitor. In newer
models, cables are removable and replaceable. The power cable is connected from the monitor to the
power bar.(It is advised that you not connect this till ready to start the system.) When connecting the
monitor cable to the back of the CPU, be careful to insert the connector without bending the connector
pins. There will be tightening thumb-screws which will secure the cable in place on the back of the CPU.
Speaker Connections
It is generally recommended that the speakers be positioned on each side of the monitor, with the
speaker housing the controls being placed on the right side of the monitor. There should be a cable going
from the left speaker to the jack on the back of the right speaker. Then, a cable with a plug goes from the
right speaker to the jack on the back of the CPU. An AC adapter will be plugged into a jack on the back
of the right speaker before being plugged into the wall outlet or power bar.
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Microphone: Insert the microphone jack to the place marked ‘mic’ ( usually a symbol of microphone
will also be embossed) behind the CPU tower.
Head phones : Insert the head phones jack to the place marked head phones
1.7 SUMMARY
In this chapter we have discussed the need to learn about computers and the advantages of using the
computer ,and its application. The important role a computer plays in data processing has been made
known. The various terms associated with the computer system have been dealt .The differences between
hardware and software is also made clear. The care to be taken while connecting the system is also
highlighted.
1.8 SELF TEST
1) A _____________________ is a fast and accurate electronic(or data) manipulating system that will accept
and store input data, process them and produce output results.
2) _________________________ are facts or informational raw materials represented by symbols
3) Arranging classified data in a predetermined sequence to facilitate processing is called
______________________.
4) The word “ data” is the plural of datum which means __________________.
5) What is the difference between data and information.
6) List the various parts of the computer.
7) List the various applications of computer.
Answers
1. Computer 2. Data 3. Sorting 4. Fact
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Chapter 2
Computer 5yStem 0rganlzatlon
2.0 INTRODUCTION
A computer is a fast and accurate symbol manipulating system. It is organized to accept, store, process
and produce results under the direction of a stored program. The computer is a system, the knowledge of
how the components in a computer system are organized is interesting .In this chapter the Organization
of computers input devices, Keyboard, mouse and classification of Computers.
2.1 THE SYSTEM CONCEPT
The system is a group of integrated parts that have a common purpose of achieving some objectives.
The following characteristics are important for understanding a system.
1. A group of parts : A system has more than one element. The various elements combined
with other components make a system.
2. Integrated parts: A logical relationship must exist between the parts of a system, and the
components must work together.
3. Common purpose of achieving objectives: The system is designed to accomplish one or
more objectives. All system elements should be controlled so that the objectives are achieved.
A computer is a group of integrated parts that have a common purpose of performing the operations
called for in the program being executed.
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2.2 ORGANIZATION OF COMPUTER SYSTEM
Fig 2.1 Basic Computer System
Figure 2.1 shows a basic computer system . There are four major units shown . They are the input
unit, the Central processing unit, the memory unit and the output unit.
A Computer is a complex system consisting of many different components. The heart or the brain of
the computer is the CPU. It does the actual computing. CPU means Central Processing Unit. In a
modern desktop computer, the CPU is a single “chip” of the order of one square inch in size which is
called a microprocessor. The job of the CPU is to execute programs.
A program is simply a list of unambiguous instructions meant to be followed mechanically by a computer.
A computer is built to carry out instructions that are written in a very simple type of language called
machine language. Each type of computer has its own machine language, and it can directly execute a
program only if it is expressed in that language. (It can execute programs written in other languages if
they are first translated into machine language.)
When the CPU executes a program, that program is stored in the computer’s main memory (also
called the RAM or random access memory). In addition to the program, memory can also hold data that
is being used or processed by the program. Main memory consists of a sequence of locations. These
locations are numbered, and the sequence number of a location is called its address. An address provides
a way of picking out one particular piece of information from among the millions stored in memory. When
the CPU needs to access the program instruction or data in a particular location, it sends the address of
that information as a signal to the memory; the memory responds by sending back the data contained in
the specified location. The CPU can also store information in memory by specifying the information to be
stored and the address of the location where it is to be stored.
2.2.1 Central Processing Unit(CPU)
The heart of a computer system is the CPU. The Central Processing Unit consists of the Arithmetic
and Logic unit and the Control unit.
The arithmetic - logic section
All calculations are and comparisons (decisions) are made in the arithmetic – logic section. Data are

Input Unit
Memory
CPU
ALU & CU
Output Unit
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temporarily placed in a designated working storage area until needed at a later time. Data may thus move
from primary storage to the secondary storage many times before the processing is finished. Once
completed, the final results are released to an output storage section and from there to an output device.
Control Section
The control section maintains order and directs the operation of the entire system. Although it does not
perform any actual processing of the data, the control unit acts as a central nervous system for the other
components of the computer. At the beginning of processing, the first program instruction is selected and
fed into the control section from the program storage area. There it is interpreted, and from there signals
are sent to other components to execute the necessary actions.
The CPU spends all its time fetching instructions from memory and executing them. However, the
CPU and main memory are only two out of many components that play a important role in a computer
system.
A computer system consisting of many devices is typically organized by connecting those devices to
one or more busses. A bus is a set of wires that carry various sorts of information between the devices
connected to those wires. The wires carry data, addresses and control signals. An address directs the
data to a particular device and perhaps to a particular register or location within that device. Control
signals can be used, for example, by one device to alert another that data is available for it on the data bus.
A fairly simple computer system might be organized as shown in figure 2.2 below:
Figure 2.2 Computer System
Now, devices such as keyboard, mouse, and network interface can produce input that needs to be
processed by the CPU. How does the CPU know that the data is there? One simple idea, which turns out
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to be not very satisfactory, is for the CPU to keep checking for incoming data. Whenever it finds data, it
processes it. This method is called polling, since the CPU polls the input devices continually to see
whether they have any input data to report. Unfortunately, although polling is very simple, it is very
inefficient. The CPU can waste an awful lot of time just waiting for input.
To avoid this inefficiency, interrupts are often used instead of polling. An interrupt is a signal sent by
another device to the CPU. The CPU responds to an interrupt signal by putting aside whatever it is doing
in order to respond to the interrupt. Once it has handled the interrupt, it returns to what it was doing before
the interrupt occurred. For example, when you press a key on your computer keyboard, a keyboard
interrupt is sent to the CPU. The CPU responds to this signal by interrupting what it was doing, reads the
key that you pressed, processes it, and then returns to the task it was performing before you pressed the
key.
Again, you should understand that this is purely mechanical process. A device signals an interrupt
simply by turning on a wire. The CPU is built so that when that wire is turned on, it saves enough
information about what it is currently doing so that it can return to the same state later. This information
consists of the contents of important internal registers such as the program counter. Then the CPU jumps
to some predetermined memory location and begins executing the instructions stored there. Those
instructions make up an interrupt handler that does the processing necessary to respond to the interrupt.
(This interrupt handler is part of the device driver software for the device that signalled the interrupt.) At
the end of the interrupt handler is an instruction that tells the CPU to jump back to what it was doing; it
does that by restoring its previously saved state.
Interrupts allow the CPU to deal with asynchronous events. In the regular fetch-and-execute cycle,
things happen in a predetermined order; everything that happens is “synchronized” with everything else.
Interrupts make it possible for the CPU to deal efficiently with events that happen “asynchronously”, that
is, at unpredictable times.
As another example of how interrupts are used, consider what happens when the CPU needs to
access data that is stored on the hard disk. The CPU can only access data directly if it is in main memory.
Data on the disk has to be copied into memory before it can be accessed. Unfortunately, compared to the
speed at which the CPU operates, the disk drive is extremely slow. When the CPU needs data from the
disk, it sends a signal to the disk drive telling it to locate the data and get it ready. (This signal is sent
synchronously, under the control of a regular program.) Then, instead of just waiting the long and
unpredictable amount of time the disk drive will take to do this, the CPU goes on with some other task.
When the disk drive has the data ready, it sends an interrupt signal to the CPU. The interrupt handler can
then read the requested data.
All modern computers use multitasking to perform several tasks at once. Some computers can be
used by several people at once. Since the CPU is so fast, it can quickly switch its attention from one user
to another, devoting a fraction of a second to each user in turn. This application of multitasking is called
timesharing. But even modern personal computers with a single user use multitasking. For example, the
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user might be typing a paper while a clock is continuously displaying the time and a file is being downloaded
over the network.
Each of the individual tasks that the CPU is working on is called a thread. (Or a process; there are
technical differences between threads and processes, but they are not important here.) At any given time,
only one thread can actually be executed by a CPU. The CPU will continue running the same thread until
one of several things happens:
The thread might voluntarily yield control, to give other threads a chance to run.
The thread might have to wait for some asynchronous event to occur. For example, the thread
might request some data from the disk drive, or it might wait for the user to press a key. While
it is waiting, the thread is said to be blocked, and other threads have a chance to run. When the
event occurs, an interrupt will “wake up” the thread so that it can continue running.
The thread might use up its alloted slice of time and be suspended to allow other threads to run.
Not all computers can “forcibly” suspend a thread in this way; those that can are said to use
preemptive multitasking. To do preemptive multitasking, a computer needs a special timer
device that generates an interrupt at regular intervals, such as 100 times per second. When a
timer interrupt occurs, the CPU has a chance to switch from one thread to another, whether the
thread that is currently running likes it or not.
Ordinary users, and indeed ordinary programmers, have no need to deal with interrupts and interrupt
handlers. They can concentrate on the different tasks or threads that they want the computer to perform;
the details of how the computer manages to get all those tasks done are not relevant to them. In fact, most
users, and many programmers, can ignore threads and multitasking altogether. However, threads have
become increasingly important as computers have become more powerful and as they have begun to
make more use of multitasking.
While programmers don’t actually deal with interrupts directly, they do often find themselves writing
event handlers, which, like interrupt handlers, are called asynchronously when specified events occur.
Such “event-driven programming” has a very different feel from the more traditional, synchronous
programming.
The software that does all the interrupt handling and the communication with the user and with hardware
devices is called the operating system. The operating system is the basic, essential software without
which a computer would not be able to function. Other programs, such as word processors and World
Wide Web browsers, are dependent upon the operating system. Common operating systems include
UNIX, DOS, Windows, and the Macintosh OS.
Microprocessor History
A microprocessor is also known as a CPU or central processing unit .It is a complete computation
engine that is fabricated on a single chip. The first microprocessor was the Intel 4004, introduced in
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1971. The 4004 was not very powerful , all it could do was add and subtract, and it could only do that 4
bits at a time. But it was amazing that everything was on one chip. Prior to the 4004, engineers built
computers either from collections of chips or from discrete components (transistors wired one at a time).
The 4004 powered one of the first portable electronic calculators.
The first microprocessor to make it into a home computer was the Intel 8080, a complete 8-bit
computer on one chip, introduced in 1974. The first microprocessor to make a real splash in the
market was the Intel 8088, introduced in 1979 and incorporated into the IBM PC (which first appeared
around 1982). If you are familiar with the PC market and its history, you know that the PC market moved
from the 8088 to the 80286 to the 80386 to the 80486 to the Pentium to the Pentium II to the Pentium III
to the Pentium 4. All these microprocessors are made by Intel and all of them are improvements on the
basic design of the 8088. The Pentium 4 can execute any piece of code that ran on the original 8088, but
it does it about 5,000 times faster!
The following table helps you to understand the differences between the different processors that Intel
has introduced over the years.
2.2.2 Input Devices
Computer systems use many devices for input purposes. Some allow direct human/ machine
communication. The keyboard of a workstation connected directly to or online to a computer. Devices
that read data magnetically recorded on specially coated plastic tapes or floppy plastic disks are popular.
Regardless of the type of device used, all are components for interpretation and communication between
people and computer systems.
There are few important input devices that are most commonly used. The keyboard and the mouse
are described below.
Chapter 2 - Computer System Organization
Name Date Transistors Microns Clock speed Data width
8080 1974 6,000 6 2 MHz 8 bits
8088 1979 29,000 3 5 MHz 16 bits, 8-bit bus
80286 1982 134,000 1.5 6 MHz 16 bits
80386 1985 275,000 1.5 16 MHz 32 bits
80486 1989 1,200,000 1 25 MHz 32 bits
Pentium 1993 3,100,000 0.8 60 MHz 32 bits
Pentium II 1997 7,500,000 0.35 233 MHz 32 bits
Pentium III 1999 9,500,000 0.25 450 MHz 32 bits,
Pentium 4 2000 42,000,000 0.18 1.5 GHz - 4GHz 32 bits
Xeon 2005 1.8GHz –3GHz 64 bits
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Keyboard: This is the most friendly input device through which data programs are keyed in and
certain commands to software can be given from the keyboard.
Figure. 2.3 Key Board
The keyboard consists key switches there is one key switch for each letter, symbol number etc. much
like a typewriter when a key is pressed, the key switch is activated. The keyboard has an electronic
circuit to determine which key has been pressed. Then the 8-bit binary code is generated and sent to the
computers. The binary code may be an ASCII (American Standard Code for Information Interchange),
EBCDIC (Extended Binary Coded - Decimal Interchange Code) or Hex code.
Keyboards are classified into two types depending on the number of keys present in them.
1) Ordinary Keyboard: consists of 86 keys.
2) Extended Keyboards: consists of 101 keys ( Windows ready )
Now keyboards called multimedia keyboards with provision for direct connection to interenet on one
stroke of key, and with special additional keys for changing volume of sound output are available.
Keyboards are of two types based on the fashion of data transfer.
1) Serial Keyboard: It sends the data, bit by bit in a serial fashion and the computer converts the
data into a parallel type.
2) Parallel Keyboard: It sends the data as a byte in parallel form, the bits are sent simultaneously
on different lines( wires).
The most common input device is the keyboard. Most keyboards have alphabetic keys arranged like
those on a typewriter. The numeric keys are arranged in an adding machine or calculator format to
allow you to enter data rapidly and are found on the right-hand side of most keyboards. The insertion
point (or cursor) indicates where on the screen the next character entered will appear and is controlled
by the arrow keys (down arrow)(up arrow). If you press any of these cursor control keys, the cursor
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moves one space in the same direction as the arrow. The Home key, when pressed, moves the cursor to
a beginning position such as the upper left position of the screen or document. The Insert, Delete, and
Backspace (keys let you alter or edit the text shown on the screen.
Remember: With most keys, if you hold them down, they will start to repeat automatically.
Pressing the Caps Lock key capitalizes all the letters you type. It is an example of a toggle key
because it switches, or toggles, the keyboard between two different modes (in this example, between
lower and upper case.
Pressing the Num Lock key turns the numeric keypad on or off. When it is on, you can use the keys
to type numbers. When it is off, the same keys work like arrow keys and move the insertion point. There
are usually status lights that will light up when the Caps Lock, Num Lock, or scroll lock keys are enabled.
Function Keys are the keys located at the top of the keyboard that are programmed to give commands
and do certain tasks. They are labelled with the letter ?F’ followed by a digit. (Often, the F1 key is
programmed as a Help key in word processing programs.) These function keys can also save users time
because it may be faster to press the function key than to move the mouse to choose a command. (Note:
Many application software packages are written so that user can use a shortcut menu, a button, a menu,
or a function key to obtain the same result.)
The Escape Key (Esc) is often used by computer software to cancel an instruction or exit from a
situation.
Mouse: A mouse is a graphical input device the term mouse is used because of its shape. A mouse
may be classified on the basis of the number of buttons it has , the technology it uses and the kind of
interface it has with the computer.
Fig. 2.4. Mouse
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The mouse has two or three buttons on its top. The function of each button is defined by the software.
The mouse also has a small rotating ball at the bottom. When the mouse is moved on a rubber pad, the ball
rolls whose movement is converted into an electrical signal and sent to the CPU on the CPU this movement
is interpreted and displayed on the screen of the monitor (CRT) in the form of movement of the arrow
mark . This movement of arrow mark is used to highlight the graphical index on the screen. When a
desired command shown on the screen is approached by the arrow mark due to the movement of the ball
in the mouse the click button is pressed to highlight the same.
A mouse may be classified as a mechanical mouse or an optical mouse, on the basis of the technology.
In a mechanical mouse , the ball that projects through the bottom surface rotates as the mouse is moved
along the flat surface. The direction of rotation is detected and relayed to the computer by the switches
inside the mouse. Microsoft, IBM and Logitech are some well known makers of the mechanical mouse.
An optical mouse uses a light beam instead of a rotating ball to detect movement. Microsoft makes
optical mouse that uses LEDs(Light emitting diodes) and photo-detectors to track movement.
There are two methods by which a mouse is connected to a computer, serial interface and parallel
interface. A serial mouse is connected to the PC through a serial port. A mouse with parallel interface is
similar to a serial mouse except that it comes with a dedicated port and does not need a free serial port on
the computer.
How to Use a Mouse Correctly: The mouse is best operated on a mouse pad (a rectangular piece
of cushioned material which provides better traction than the desk top. When you move the mouse right
on the surface of the pad, the pointer arrow moves right on the screen (?). The mouse usually is attached
to the computer by a fine cable, but wireless mouse units are also available.
The top of the mouse contains one or more buttons. After moving the mouse pointer on the screen,
simply press, or click the mouse button. To press and release a mouse button twice without moving the
mouse is called double-clicking. This process is often used to start programs or to open a document. ( The
function of the buttons can be changed to accommodate right-and left-handed people.) The main advantage
of a mouse is that it is easy to use. With a little practice,user can use a mouse to point to locations on the
screen just as easily as using a finger. However, it has its disadvantages: it requires empty desk space
where it can be moved about, it requires the user to remove a hand from the keyboard whenever the
pointer is to be moved or a command given and the mouse must be cleaned to remove dust and dirt from
the ball mechanism
2.2.3 Output Devices
Output devices:- Like the input devices, output devices are instruments of interpretation and
communication between humans and the computer system. Video Display unit(VDU) the Monitor and
Printer are considered as output devices.
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All input/output and secondary storage units are sometimes called peripheral devices. The peripheral
devices are dealt in dealt in detail in other chapters.
2.3 COMPUTER LIMITATIONS
Computers although they are very useful, they have some limitations.
i) Programs must be reliable
As a machine, the computer does what it’s programmed to do and nothing else. This doesn’t mean
that it must be stupid. Clever programs can be written to direct the computers to store the results of
previous decisions. But a seemingly flawless program that has operated without a problem for months
can suddenly produce non-sense. A reliable program that’s supplied with incorrect data may also produce
non-sense.
ii) Application logic must be understood
The computer can only process applications which can be expressed in a finite number of steps
leading to a previously defined goal. Each step must be specific and clearly defined, if the steps in the
problem solution cannot be precisely stated, the job cannot be done. The computer may not be of much
help to people in areas where qualitative material or evaluation is important.
For example, The market decision may be of a qualitative nature because sales volume data may rest
on future social, political, technological and economical events. However, the computer can let the manager
know how the product will fare under assumed price, cost and sales volume condition.
iii) Applications must be suitable
Writing programs is a human task. It can be time consuming and expensive. Thus, non-recurring jobs
are often not efficient areas for business data processing applications. Rather as a general rule it is most
economical to prepare business programs for large-volume, repetitive applications such as payrolls that
will be used many times.
2.4 CLASSIFICATION OF COMPUTERS
2.4.1 Classification based on data type
Computers are classified by the type of data they are designed to process. Data may be obtained
either as a result of counting or through the use of some measuring instrument. Data that are obtained by
counting are called discrete data, examples of discrete data are total number of students in a classroom.
Data that must be obtained through measurement are called continuous data. Example of continuous data
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are, the speed of an automobile measured by speedometer or the temperature of a patient as measured by
a thermometer.
A digital computer is a counting device that operates on discrete data. It operates by directly counting
members ( or digits ) that represent numerals, letters or other special symbols. Just as digital watches
directly count off the seconds and minutes in an hour, digital processors also count discrete values to
achieve the desired output results.
In contrast to digital processors, however, there are also analog machines that do not compute directly
with numbers. They deal with variables that are measured along a continuous scale and are recorded to
some predetermined degree of accuracy. Temperature for example may be measured to the nearest
tenth of a degree on the Celsius scale, voltage may be measured to the nearest of hundredth of a volt. A
service station petrol pump may contain analog processor.
Analog computers may be accurate to within 0.1 percent of the correct value. But digital computers
can obtain whatever degree of accuracy is required simply by calculating additional places to the right of
the decimal point.
Desirable features of analog and digital machines are sometimes combined to create a hybrid computing
system. In a hospital intensive-care unit, for example analog devices may measure a patients heart
function, temperature and other vital signs. These measurements may then be converted into members
and supplied to a digital component in the system.
2.4.2 Classification of computers based on their size, cost &
configuration
In this category computers are classified as micro-computers, mini computers, main frame computers,
home computers and super computers.
Micro Computers
A Micro computer is the smallest general-purpose processing system that can execute program
instructions to perform a wide variety of tasks.The CPU (Central Processing Unit ) of micro-computers
is a micro-processor.
The important features of micro computers are:
Micro computers are known as home computers, personal computers, desktop computers and
small business computers.
These are small, low-cost digital computer with a micro processor as its CPU, a memory unit an
input device and an output device.
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The word length of a micro-computer lies in the range of 8-32 bits.
CPU’s are in a single chip
Storage capacity is small
Applications include general-purpose calculations, industrial control, office automation etc.
Personal Computers (PC`s)
Personal computers are micro-computers for general-purpose computations. These are classified as
PC, PC/XT, PC/AT and super AT (or super micro) based on their cost and configuration. All these types
contain a CPU,RAM,ROM,CRT display, Keyboard and secondary memory.
Features
PC is the simplest and cheapest type of computer and it uses an INTEL 8088 ( micro processor
chip) as CPU, 640 KB, RAM, 8KB ROM ( ROM can be extended upto 64 KB) and floppy disk
as secondary memory; processing speed of a PC lies in the range 1-5 MIPS ( micro-instructions
per second )
PC/XT ( Personal computer extended technology) contains hard disk, floppy disk drives ( 360/
1.2 MB), Intel 8088 CPU, 640 KB RAM and 8 KB ROM. This is costlier than a PC because
of the hard disk it contains. PC/XT is a single user system. Processing speed of a PC lies in the
range 1-5 MIPS and hard disk capacity lies in the range 20-80 MB.
PC/AT ( Personal computer advanced technology) contains. Intel 80286 as CPU, 640 KB
RAM (expandable to 4 MB), 64 KB ROM (expandable to 128 KB), hard disk drive and floppy
disk drive (360 KB/1.2 MB), its CPU is powerful than the CPU of PC/XT and four to five
terminals can be connected to it to make it multiuser. Processing speed of PC/AT lies in the
range 1-5 MIPS and hard disk capacity lies in the range 20-80 MB.
Super AT( or super micros) uses a 32-bit CPU, Intel 80386 & 80486. The RAM capacity of
super macros lies in the range 2-8 MB in 80386 CPU can be extended upto 32 MB and in case
of 80486 upto 64 MB. The hard disk capacity of super macro lies in the range 200 –800 MB.
The processor speed of 80386 CPU lies in the range 3-5 MIPS and that of 80486 CPU is 12
MIPS to 25 MHz clock UNIX is used as operating system.
The processing speed of home computers and microcomputers for industrial control is less than 1
MIPS. Home computers are used for entertainment, basic education and home management and income
tax calculation, investment analysis etc.
At present we have personal computers which have CPU’s of which operates at 2.4 GHz and above.
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Mini Computers
The salient features are:-
The word length is 32 bits and their processing speed lies in the range 10-30 MIPS.
RAM capacity lies in the range 8 MB – 96 MB ( in same it can be extended upto 128-256 MB)
The hard disk capacity lies in the range 380 MB – 2GB (giga bytes)
Most mini-computers use Motorola 68030 and 68040 CPU.
The Minicomputers which are faster and more powerful than microcomputers can support upto 64 or
even 100 terminals and some minicomputers are Uni-Processor and some others are multi processor
systems. These are used for payroll preparation and scientific computation, multi-user and interactive
applications in college universities research organizations, industries, sophisticated real time(industrial)
control, interactive engineering design work etc..
Examples of mini computers are:
1. IBMAS/400/B60
2. VAX8842(VAX stands for Virtual address)
3. Extension - VAX computers are manufactured by Digital Data Corporation)
4. WIPRO S – 68030V & S-6833V built around 68030 CPU
5. WIPRO LANDMARK 860 ( a super mini) built around Intel i860-CPU
6. HP 9000 series 800 ( super mini) built around Hewlett Pacard’s VLSI RISC processors
7. HCL magnum built around 68030 CPU and 68040 CPU.
Main-frame Computers
The mainframe computers are faster and more powerful than mini computers. These computers are
used where large amount of data are to be processed or very complex calculations are to be made and
these tasks are beyond the computing capacity of mini computers.
The main features are :
The word length may be 48, 60 or 64 bits.
Memory capacity range is 64-256 MB and hard disk capacity is 1000 MB – 10 GB or more.
Processing speed lies in the range 30-100 MIPS.
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The mainframe computers are used in research organizations, large industries, business organizations,
government organizations, banks, airline reservations etc..
Example: IBM 4300 series
IBM 308X series
Latest model IBM 3090 series
IBM 9000 series
HP 9000 series 8705/400
HP 9000 model 8705/300
Supercomputers
The salient features are :
1. Their word length is 64-96 bits; memory capacity is 256 MB & more; hard disk capacity 1000
MB and more
2. Processing speed lies in the range of 400 MIPS – 10000 MIPS. In a single machine cycle tow
64 – bit data can be added
Super computers are much faster and more powerful than mainframe computers. Super computers
are specially designed to maximize the number of FLOPS ( Floating Point Operations Per Second). Their
FLOPS rating is usually more than igiga flops per second. A super computer contains a number of CPU’s
which operate in parallel and make it faster. They are used for massive data processing & solving very
sophisticated problems.
They are used for weather forecasting, weapons research and development, rocket launching,
seismology, atomic, nuclear and plasnia physics.
Example:-
CRAY – X – MP/14, X-MP/24 & X-MP/48
CRAY – MP, CRAYZ, CRAY3, CRAY Y-MPC
ETA 10 ( Developed by control data Corporation )
SX – 2 (Developed by Nippon Electric Corporation, Japan)
SX – 3R ( 25.6 GIGA FLOPS )
HITACS – 300 ( 32 GIGA FLOPS, HITACHI MAKE )
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Lap Top Computers
Laptop computers have been now widely used by all professionals.They are no longer a priced
possession. They are helpful in giving out presentations and demonstrations.The major hardware and
software features of a Laptop computer are listed. Figure 2.5 and Figure 2.6 shows the front and back
view of the computer, respectively.
Figure 2.5. Front View of a Lap top Computer
1 Touch pad
2 Keyboard
3 Power button
4 Microphone
5 Display
6 Display latch
7 Status indicator panel
8 Air intake
9 AC adapter connector
10 Audio jacks (3)
11 Speaker
12 Modular bay
13 Touch pad buttons
14 Battery bay
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Figure 2.6. Back View of the Computer
1 Fan
2 Parallel connector
3 USB connector
4 Status indicator panel
5 Docking connector
6 Docking connector door
7 Serial connector
8 Monitor connector
9 PS/2 connector
10 Infrared port
11 PC card slot
12 Hard-disk drive
13 Security cable slot
14 Speaker
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Hardware Features
Usually all Lap top computers have the following features:
Full multimedia capability through the following standard features:
o A 12.1-inch super video graphics array (SVGA), thin film transistor (TFT), 800 x 600
active-matrix color display or a 13.3-inch extended graphics array (XGA), TFT 1024 x
768 active-matrix color display.
o A CD-ROM drive that can be used in the modular bay.
o 256-bit hardware-accelerated video support, with 2.5 megabytes (MB) of video memory.
o Support for a zoomed video (ZV)  PC Card in the upper PC Card connector.
o Software wavetable support, Sound Blaster emulation
o Three audio jacks for connecting external speakers or headphones, a microphone, and a
record/playback device.
o Built-in microphone and two stereo speakers.
o Accelerated graphics port (AGP) architecture that increases system performance,
particularly video performance.
A modular bay that supports a CD-ROM drive, diskette drive, second battery or hard-disk drive,
or LS-120 drive module.
A 64-MB synchronous dynamic random-access memory (SDRAM) module is standard. Memory
can be increased up to 256 MB by installing combinations of 32-, 64-, or 128-MB SDRAM
modules in the two memory module sockets on the system board.
Two power conservation modes—suspend mode and suspend-to-disk mode—that help to
conserve battery power. If the batteries run out of power, suspend-to-disk mode prevents data
loss by copying all system data to the hard-disk drive and turning off the computer.
Connectors for two 3.3-volt (V) or 5-V PC Cards. The upper PC Card connector supports ZV
PC Cards.
Hardware and software support for the C/Port Advanced Port Replicator (C/Port APR) and C/
Dock Expansion Station.
A touch-pad pointing device positioned for both left- and right-handed users. The left and right
touch-pad buttons mimic mouse buttons; you can also perform many pointing functions by tapping
the touch pad itself. Click-and-drag buttonless functions are supported.
32
A lithium ion battery in the battery bay, with support for a second battery in the modular bay.
Express Charge technology charges a single battery in approximately 1 hour (when the computer
is off or in suspend mode).
High-performance parallel and serial ports and a multipurpose Personal System/2 (PS/2) connector
for attaching external devices. There is also a monitor connector for attaching an external
monitor to your computer and a Universal Serial Bus (USB) connector that supports stand-
alone and hub devices.
An infrared port that permits file transfer without using cable connections. The port is compatible
with the Infrared Data Association (IrDA) Standard 1.1 (Fast IR) and Standard 1.0 (Slow IR)
for use with external devices.
An automatic thermal management system that uses a variable-speed fan and microprocessor
speed changes to keep the system running at the optimum temperature.
The following software is normally included with the laptop computer system:
The Microsoft Windows 95, Windows 98, or Windows NT 4.0 or higher operating system is
installed on the hard-disk drive.
The System Setup program lets user view and change the system configuration. The Program
Diskette Maker allows user to create program diskette sets of software.
Diagnostics for evaluating the computer’s components and devices.
Options to Upgrade
As the computing requirements change, computer’s capabilities can be increased with the C/Port
APR or C/Dock Expansion Station. Provision to install a hard-disk drive of larger capacity, increase
system memory, and add functionality with PC Cards. Laptops also offers additional modules that you can
install in the modular bay, including a second hard-disk drive, CD-ROM drives, and LS-120 drive modules.
The laptops can be upgraded depending on requirement with the following devices :
C/Port APR and C/Dock Expansion Station
Additional batteries
External keyboards and a numeric keypad
External monitors
External pointing devices
External speakers, headphones, and microphones
Printers
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Hard-disk drives
Second hard-disk drive for the modular bay
LS-120 drive modules
AC adapter
PC Cards
32-, 64-, and 128-MB memory modules
Carrying case
2.5 SUMMARY
In this chapter we have made known the block representation of the simple computer system. The
functions of the Central Processing Unit (CPU) which is the brain of the computer has been dealt with in
detail. The classification of computers from their computing capability are now widely used lap top
computers feature has been discussed.
2.6 SELF TEST
I State True or False
1. Computer Operates on continuous data
2. Random access memory is secondary storage memory
3. Classification of computers done depending on size, cost and Configuration.
4. CRAY XMP/14 is a minicomputer
II Answer the following questions
1. Write the block diagram of a basic computer system and list the function.
2. What are the limitations of computers
3. What is the function of CPU?
4. How are computers classified.
Answers
1. False 2. False 3. True 4. False
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Chapter 3 - Software Types
34
Chapter 3
5yStem 5oftware
3.0 INTRODUCTION
S
oftware is the set of instructions that drives the hardware to perform various tasks. Software tells
a computer what to do and how to do it and also permits a dialogue between the user and the
hardware. The hardware and the software together form the computer system. Without software
the computer becomes a dumb box. In this chapter the various software and languages used in programming
have been discussed.
3.1 SOFTWARE
Software in a computer system is the coded instructions or program created by programmers or users
to tell the computer what to do. A program is a sequence of instructions where each instruction is a
command to the computer. There are basically two types of software:
System Software
Application Software
The categories help in getting an idea of the different functions of software.
System Software
For the user to enter and run an application program, the computer must already have some software
contained in its memory. The system software lies in between the hardware and the application software.
It helps the computer to manage itself and carry out our work. It controls the machine’s resources, such
as input and output devices, schedules the work within the machines and enables the hardware to understand
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our commands that are given in an ‘English like’ language.
System software is a collection of programs that are executed as needed to perform functions such as
Receiving and interpreting user commands
Entering and editing application programs and storing them as files in secondary storage devices
Managing the storage and retrieval of files in secondary storage devices such as hard disks etc.,
Running standard application program such as word processors, spreadsheets, or games , with
data supplied by the users
Controlling Input / Output units to receive input information and produce output results
Translating programs from source form prepared by the user into object form consisting of
machine instructions
Linking and running user-written application programs with existing standard library routines,
such as numerical computation packages
The system software consists of operating system, assembler, compiler, interpreter, debugging programs,
text editors etc., which are commonly used by all computer users.
3.2 OPERATING SYSTEM (OS)
Computers are the most powerful tool that man has ever created. They are so powerful that we are
not able to use the tool directly by ourselves. Instead through an intermediary of an operating system, we
use the computer- tool to make it self manageable enough for us to work with. Operating system (OS) is
the most important of the system software. An operating system is an integrated set of special programs
that are used to manage the resources and overall operations of the computer. The operating system acts
as an interface between user and the hardware of the computer and it controls the way the other software
uses the hardware.
Essentially, an operating system has three broad categories of functions:
Managing devices (I/O management)
Processing commands ( Command interpreter)
Controlling programs( Supervisor, this is the most important of all)
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Figure 3.1
Figure 3.1 Operating System and its functional relationship with various hardware components are
shown and Hierarchical relationships between computer hardware and Software is represented.
Examples: Micro Soft Windows 95 /98 /2000 /XP, MS DOS,UNIX/LINUX are some operating systems
that are common.
Types of Operating system
There are a vast number of Operating systems, but the most widely used are listed below:
Disk Operating System (DOS): This is single user operating system that is command oriented.
Windows 95/98/2000: Windows is the most widely used and most of the new personal computers
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come preloaded with Windows OS. Windows OS has an easy to use Graphical User Interface
(GUI) and there are many applications that run on it .This is the reason for its popularity and
wide spread use.
Windows NT: Windows NT is another Micro Soft operating system and this has networking
capabilities.
Macintosh System 7/8: This OS is used by Apple Macintosh brand of computers.
* NIX : This includes the operating systems like UNIX, LINUX, XENIX etc., UNIX is a
command prompt OS, much like MS DOS operating system. Now LINUX is becoming popular
because of X-Windows which allow UNIX systems to have a GUI like MS Windows OS. Also
LINUX is available free and also the source code is available free unlike MS Windows OS for
which one has to pay.
3.3 PROGRAMMING LANGUAGES
Each machine has its own set of instructions based on the design of its central processing unit (CPU).The
notations used for precise description of the set of instructions is called programming language. Programs
are written to communicate with computers and instruct the computer as to what operation it should
perform. Programming languages are classified depending on whether they are closer to the language or
to the computer itself as:
Low level language
High level language
Language design was viewed as a challenging job because the needs of the user and also the efficiency
of the machine was to be considered. Since the origin of the programming languages it has evolved over
five different generations. These generations are:
Machine language - First generation
Assembly language - Second generation
Procedural language - Third generation
Problem-Oriented languages - Fourth generation
Natural language - Fifth generation
The first and the second generation languages come under low level languages, and the other generations
come under high level languages.
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3.4 LOW LEVEL LANGUAGE
Programming language in which instructions to machine is written using 0’s and 1’s or alphabets,
numerals and special characters are called Low level languages. Machine language and Assembly language
are Low level programming languages. Low level language is a medium of communication that is machine-
dependent or specific to a given computer. Programs written in these languages are not transferable to
different types of machines.
3.4.1 Machine language
The internal programming language for a particular chip (processor) is called its machine language.
Initially programmers had to write programs in machine language. The machine language consists of 0’s
and 1’s and was called binary language. Words are formed by combining a number of bits(binary digits
0’s and 1’s) for a given processor. Bit is an abbreviation for the term binary digit.The word length ranges
from four bits for small computers to 64 bits for high speed large computers. Another term commonly
used to express word length is byte. A byte is defined as a group of eight bits. The term nibble is used
to represent a group of 4 bits . Two nibbles make a byte. The number of bits in a word for a processor is
fixed, and words are formed through various combinations of these bits. For example, a processor with a
word length of eight bits can have 2
8
(that is 256) combinations of eight bits –thus a language of 256
words is possible. However, all these words need not be used in the processor. The design engineer
selects combinations of bit pattern and gives a specific meaning to each combination by using electronic
logic circuits; this is called instruction. Instructions are made up of one word or several words. The set
of instructions designed into the processor makes up its machine language. It is a binary language,
composed of 0’s and 1’s and that is specific to the processor. Writing programs in machine language is
extremely difficult and time consuming because the machine language instructions are specific to a processor.
Advantage
Very fast execution of program
Disadvantages
Binary number 0’s and 1’s are used
It is processor dependent
Difficult to make any correction or modifications
3.4.2 Assembly language
This is also called as Symbolic language. In order to overcome the disadvantages of machine language,
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early programmers developed Assembly language. Programs in assembly language are automatically
translated into machine language instructions by Assembler. Every machine has its own assembly language
that depends on the architecture of the processor. An assembly language is designed to use a symbolic
code for each instruction, called mnemonics. The mnemonic code consists of symbols that consists of
alphabets, numerals and special characters.
Eg: Some mnemonics related to a processor are of the type MOV, MUL, ADD etc.,
Advantages
It is easier write programs in assembly language compared to machine language
Used for systems programming
Memory addresses can also be represented by symbols
Disadvantages
It is machine dependent
Slower than machine language
The assembly language has to be converted to machine language and hence need a translator
program called Assembler
Machine language and Assembly language programming is called low level programming.
3.4.3 ASCII Code (American Standard Code for Information
Interchange)
A computer is a binary machine, to communicate with the computer using alphabets, numerals and
characters the translational codes are necessary. The commonly used code is known as ASCII - American
Standard Code for Information Interchange. It is a 7- bit code with 128 (2
7
) combinations. Each combination
is assigned to a letter, a decimal number, a symbol, or a machine command and is unique. Hexadecimal
numbers are used in forming the ASCII code and not our decimal numbers. The hexadecimal system uses
base 16. In hexadecimal numbers 0 through 9 are represented as in decimal system using the digits 0
through 9 but numbers 10 through 15 are represented using alphabets A through F. A represents 10(TEN),
B=11, C=12, D=13, E=14,F=15 and 10 in hexadecimal is 16 in decimal system. To distinguish the type of
system used H is written after writing the number in hexadecimal form. For example 11H means it’s a
number in Hexadecimal, you have to read it as ONE ONE in hexadecimal and not read as eleven. Its
decimal equivalent is seventeen (17). 1F H represents twenty five in decimal system.
In ASCII code hexadecimal numbers are used. Hexadecimal 30H to 39H represent 0 to 9 decimal
40
digits, 41H to 5A H represent the capital letters A through Z, 20H to 2FH represent various symbols , and
initial codes 00H to 1FH represent machine commands such as carriage return and line feeds. In computer
systems , keyboards ( called ASCII keyboards),video screens, and printers are typical examples of devices
that use ASCII codes. When the key “9” is pressed on an ASCII key board, the computer receives 39H
in binary, which is corresponding ASCII charactor for 9. The system program translates ASCII characters
to appropriate binary numbers.
However, recent computers use many more characters than the original 128 combinations; this is
called Extended ASCII. It is an 8 bit code that provides 256 (2
8
) combinations ; the additional 128
combinations are assigned to various graphics characters.
3.5 HIGH-LEVEL LANGUAGES
The time and cost involved is more in developing machine language and assembly language programs.
Hence the High level languages were defined. The high level languages were designed to be machine
independent. They are English like. These languages are also called as procedure oriented languages
since they permitt the programmer to concentrate more on the procedure or method of problem solving.
High level languages are characterized by rules.
Examples of high level languages
FORTRAN ( Formula Translation)
COBOL (COmmon Business Oriented Language)
BASIC ( Beginners All purpose Symbolic Instruction Code)
LISP( LISt Processing)
PROLOG( PROgramming in LOGic)
C,C++, JAVA,Visual BASIC, Visual C++
Advantages
High level languages are machine independent
Easy to learn
Easy to understand and debug because of good documentation
Portable. The program can be executed on any another machine
Helps in software development
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Disadvantages
Computers can understand instructions written in machine language, hence the higher level
language has to be converted to machine language. A translator is needed.
Slower than machine language
More memory space to store program
3.5.1 Fourth Generation languages
These are high level languages that require fewer instructions to accomplish a particular task faster
than a third generation language. These are also called as non-procedural languages. They are machine
independent. These are mostly used for office automation, business applications but not for scientific
programs.
Eg., SQL, INQUIRE, INTELLECT ( Query Languages)
INGRESS, ORACLE, CYBASE( Application Generators)
3.5.2 Fifth generation languages
These are high level languages used in areas of artificial intelligence and expert systems.
Eg., LISP , PROLOG
3.6 TRANSLATORS
Translators convert a program that is written in assembly language or high-level language to machine
language. The programs written in Assembly language or high-level language by a programmer is called
as SOURCE code. The translation to machine language is done by using the translator. This is called as
OBJECT code.
Figure 3.2 The Translation of Higher level language Program to machine code
Translator are of mainly two types:
ASSEMBLER
COMPILER/INTERPRETER

Compiler/
Interpreter
Source Code
High level
Object Code
Machine level
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An Assembler is a program that converts Assembly language program written in mnemonics into
machine language.
A compiler or a interpreter is a program that translates high level language program into machine
language program. There is difference in the way the compiler translates to machine code and the way
the interpreter translates.
Compiler scans the entire program first and then translates to machine code. When all the syntax
errors are removed execution takes place. Execution time is less . Debugging is slow. Eg., C, C++,
FORTRAN
A interpreter translates the program line by line. Each time the program is executed, every line is
checked for syntax error and then conversion takes place. Good for fast debugging. Execution time is
more. Eg., BASICA
3.7 APPLICATION SOFTWARE
To automate specific business process application softwares are used. An application program carries
out real world functions such as financial accounting and inventory management .An application program
in a sales department might include programming to forecast sales, controlling purchase transactions,
maintaining customer data and sending bills to the customers. The software may be written by the company
programmers or may be purchased from an application software vendor. Application software is the
collection of programs which perform tasks pertaining to the normal activities of business or other human
endeavor. There are ready made programs that check the spellings of what we have written (spell
checkers),comment on the grammar and style of any text you have written(style guides) and even programs
that teach us how to use computers( tutors).Other examples are the word processing packages for
processing and manipulating text, spread sheets for calculating finance statements and data analysis are
provided by MS Office(MS stands for Micro Soft , MS Word, MS Excel), Auto CAD 2000 for designing
and drafting (CAD stands for Computer Aided Design ), COREL DRAW etc.,
3.8 SOFTWARE PIRACY
As with new inventions, “copyrighting” is an issue with software applications. Often with the purchase
of a particular software program, the buyer must consent to a “license agreement” which dictates the
terms of using the program. A common inclusion in such agreements is that the buyer is permitted to use
the program on “one computer at-a-time” and is not to make copies of the program for use by others. To
do so, is illegal operation which is subject to criminal charges. Because of the infringement upon many
licensing agreements, there is an ongoing development of software “piracy” (the use of programs of
which you have no legal right to access.). The use of unlicensed software is actually widespread. With the
increased availability of software over the Internet, keeping track of what’s licensed and what’s not has
become a problem. Most organizations don’t even keep an accurate inventory of their software programs.
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While most organizations do not intentionally “pirate” software, they leave themselves vulnerable to
Software Associations which randomly audit organizations to ensure compliance with licensing rules.
Organizations that have unlicensed software can be subject to excessive fines and penalties.
There are certain software programs, called “Shareware”, which can be downloaded and tried for
free for a trial period, yet for which one must pay a license fee if one decides to keep them. Shareware
programs can be copied for distribution amongst the friends on the basis of hoping that they will pay the
license fee, if they continue to use it.
“Public domain” software are programs that are available to anyone interested in using them, for
which no one wants sole ownership rights or copyright agreements
3.9 SUMMARY
The software is very important without which the computer becomes useless. We have discussed the
importance of software and the different types of software. The programming languages used in software
development is also made known.
3.10 QUESTION/ ANSWER KEYS
I State True or False
1. Machine language is one which is expressed in terms of 0’s & 1’s.
2. Assembly Language is one which is expressed in terms of mnemonics
3. Compiler is a software which converts machine language to high level Language.
4. Fortran is a low level language.
II Answer the following questions
1. What is a low level language ?
2. What is a compiler ?
3. What is a interpreter ?
4. What are the advantages of high level language ?
5. What is an operating system ?
Answers
1. True 2. True 3. False 4. False.
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Chapter 4 - Peripheral Devices
44
Chapter 4
Perlpheral DevlceS
4.0. INTRODUCTION
P
eripheral devices in general are the devices that are attached to the computer. They can be divided
into either input or output devices (although some of them can act as both input and output devices).
In addition to the standard input/output devices like keyboard, mouse and video-display unit, a
number of other peripheral devices like the printer, plotter, scanner and digital camera are being increasingly
used with the computer. It is mainly because of the necessity and convenience that we use peripheral
devices. Hence, provision is to be made to connect a number of peripheral devices to the computer. The
printer is needed to get the hardcopy(printout) of the document saved in the computer, the plotter to obtain
good quality statistical plots and designs obtained by CAD systems, Scanner is needed to digitize the hard
copy, Digital cameras to have photos and save them in digital form in the computer. The peripheral
devices help in digitization of the available information which is easy to store and retrieve, unlike the hard
copies which are difficult to maintain. In this chapter, we discuss the different interface methods available
to connect the peripheral device to the computer, the different types of printers, plotters, scanners and
their working principle.
4.1 INTERFACE METHODS
The processor, main memory and the input output devices are all interconnected by a set of lines called
Bus .The primary function of the bus is to provide a communication path for the transfer of data and
control lines. The I/O device is connected to the computer bus by an I/O interface circuit. On one side of
the interface circuit we have the path to transfer data between the interface and I/O device. This is called
a port. The peripheral devices have to be connected to the computer via this connection called port .
A port is a ‘place’ for transferring data from computer to peripheral device or from peripheral device
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to the computer. The port allows the computer to communicate with peripheral devices. Along with data
transfer the ports are used to setup and control the hardware, and get information about the status of the
peripheral devices. The ports are generally referred to as I/O ports. Depending on the type of communication
the I/O ports are classified as Serial Port and Parallel Port
Serial Port : A serial port is one through which data is transmitted or received bit by bit. It is
also referred to as RS-232C port or communications port.The ports are built on to the motherboard.
Usually there are four serial ports in a computer.
Parallel Port: In a parallel port data is transmitted or received in parallel( ie say 8 bits at a time)
so as to achieve higher data transfer rate than serial interface.The parallel port is essentially
intended to be a one way path from computer to printer.Usually there is one parallel port in a
computer.
There are many types of designs of the interface circuitry. An I/O device designed with one interface
circuit suitable for one computer may not be usable with other computers. Hence there are standards.
The SCSI Bus is one of the widely used standards. The acronym SCSI stands for Small Computer
System Interface.
Until recently, the most common standard for connecting devices such as printers scanners ,external
hard drives, and so on, was the SCSI( Small Computer Standard Interface). This had a number of
disadvantages:
The cables were complex, expensive and could be damaged.
To prevent damage, the computer had to be closed down before a device was attached to or
removed from the computer.
Each device had to be given an unique ID.(identification number, say ,a address)
Only 6 devices could be connected together.
Echoes on the cable could disrupt data transmission.
SCSI has been recently replaced by two new connecting systems ( interface standards)
(i) USB ( Universal Serial Bus )
(ii)Fire wire.
Universal Serial Bus or USB
Universal Serial Bus or USB is found on virtually every new computer, this allows the communication
of devices such as mouse keyboard, scanners ,printers and digital cameras. The USB standard allows up
to 127 devices to be connected to the same computer, but this can be difficult when there are only one or
46
two ports. The solution is to attach a device called USB Hub. We can increase the number of devices to
be connected to the computer by using the USB Hub.
USB devices are hot- pluggable, which means that devices can be connected and disconnected without
having to switch off the computer. Also the user does not have to bother about things such as device ID
and so on.
Plug and Play: As computers have become a part of the daily life the computer has to be more user
friendly. The user should not be concerned with the peripherals that are to be connected to it before
switching it on , the user should be in a position to connect the peripheral devices to the computer depending
on the necessity. In earlier computer systems the user had to switch off the system before connecting or
disconnecting the peripheral devices. The plug and play feature means that, a new device such as an
additional speaker, printer or web-camera can be connected at any time while the system is operating.The
system would detect the existence of this new device automatically,identify the appropriate device -
drivers software and any other facilities needed to service that device, establish appropriate connections
to enable the device and the computer to communicate.
Firewire (iLink or IEEE-1394)
Firewire (iLink or IEEE) is a high speed serial bus which is rather similar to USB. However, while
USB has bandwidth of 1.2 Megabits per second(Mbps), Firewire has one of 400Mbps.When the computer
is switched on, it checks the Firewire and assigns an address to each device that it detects. Firewire
devices are hot pluggable i.e., they can be connected or disconnected without switching off (powering
down)the computer, they are Plug and Play.( ie. the computer detects when a new device is added and
assigns an address to it).
Firewire supports 63 devices( compared to 127 devices by USB) but the devices are daisy chained(
connected one to another ) whereas USB uses hubs. Like USB, devices can be either powered or
unpowered.
4.2 PRINTERS
One of the most convenient and useful methods by which the computer can provide a hard copy is by
means of printing. For the sake of convenience, the printer should have the ability to print alphabetic
character, decimal digits, and common punctuation marks etc..
Functions of a Printer
The printer receives data characters from the computer and prints the characters on the paper. In
addition, the printer also receives control characters from the computer. These control characters are not
printable characters. They convey some sort of control information to the printer ( bold, italic underline
etc.).
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Some of the control characters widely used are:
CR (Carriage Return) : Specifies that the pointer head carriage should return to the first print column.
Any subsequent data character received will be printed starting from the first column.
LF (Line Feed): informs the printer to skip one line on the paper.
FF (Form Feed): Instructs the printer to skip the paper to the beginning of the next page or form. In
addition to these the printer has to send acknowledging signal back to the computer indicating the status.
Basic characteristics of a printer
Speed: Specified as CPS ( Characters Per Second ) or LPM ( Lines Per Minute). It indicates how
fast a printer works.
Quality: Specified as DRAFT, NLQ ( Near Letter Quality ) or LQP ( Letter Quality Printer). This
gives how good the shape of the printed character is.
Character Set: Indicates the total number of data characters and control characters recognized by
the printer. The normal ASCII and EBCDIC.
Interface: Specifying whether the printer receiving characters from the computer in parallel form or
in serial form.
Buffer size: Indicates how many data characters can be stacked in the printer buffer memory before
printing.
Print size: Specified as character size and number of characters per line.
4.3. TYPES OF PRINTERS
There are several types of printers that are designed for different types of mechanisms, based on the
approach of printing and speed.
Printers may be classified as:
Character printers
Line printers
Page printers
Printers are also classified as:
Impact printers and
Non-impact printers
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Impact printers: The printers which use the familiar type writer approach of hammering a typeface
against paper and inked ribbon are called impact printers. The most commonly used impact printer is the
dot-matrix printer(DMP).
Non-impact printers: The printers which do not hit or impact a ribbon to print, are called non-impact
printers. Such printers use thermal, electrostatic, chemical and inkjet technologies.
4.4 CHARACTER PRINTERS
This category of printers generally function in the same way as a typewriter. There are many types
of character printers some of them are:
Daisy-wheel printers: In daisy–wheel printer the raised letters are attached at the ends of spokes of
a wheel. A motor spins the wheel at a rapid rate. The wheel is rotated until the desired letter is in position
over the paper and then solenoid-driven hammer hits the ‘spoke’ against the ribbon to print the letter. The
daisy-wheel printers are impact printers. The advantages of the daisy-wheel printers are high print
quality, interchangeable fonts and print speed up to 55 characters per second. However the print quality
is not as good as that produced by the spinning golf ball.
Fig 4.1 Daisy Wheel Printer
Spinning golf ball printers: This also function in the same way as a daisy-wheel printer. The main
difference in the mechanism is that the entire character set is present as raised type around a sphere. The
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bottom of the sphere is connected to the motor drive mechanism. The ball can be easily shifted up or
down, rotated and tilted until the character to be printed can be precisely positioned over the desired
location on the paper. For printing the letter on the paper, the spinning golf ball is hit against the ribbon.
The head is moved across the paper to print a string of characters.
The advantages of spinning golf ball printers are excellent print quality and easy interchangeability of
fonts. However, this type of printers posses some disadvantages too, that it is mechanically complicated
and mechanism is noisy. It has low speed of about 14 character per second.
4.5 LINE PRINTERS
Line printers are most suitable for printing massive data at high speed.
Fig. 4.2 Line Printer
Drum printers
A drum type printer consists of a rapidly spinning drum which has a complete raised character set
constructed around it for each character position across the paper.
50
Fig. 4.3 Printing Cylinder of a Drum Printer
Each band contains all the possible characters. Magnetically driven hammers are provided in each
character position to hit the paper and ribbon against the spinning drum for printing the desired characters.
An entire line of characters can be printed during each rotation of the drum.
With the help of the drum printer, upto 2000 lines/min can be printed. The quality of printing is not good
and there is also high noise pollution.
Drum printers are also called line printers.
Chain printers
A rapidly moving chain called print chain is used in chain printers.
Each link of the print chain is a character’s font. For printing a character, a print hammer is located
behind the paper. As the print chain rotates, the properly timed hammers strike the paper and the ribbon
together against the desired character set several times on the chain, the speed of chain printers can be
improved. Speed of chain printers reaches upto 2500 characters per minute(cpm).
Dot-Matrix Impact printers
Fig 4.4 Dot-Matrix Printer
In a dot-matrix printer, each character is a pattern of dots. The print head comprises of a matrix of tiny
needles having seven rows with nine needles in each row. The tiny needles hammer out the characters in
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the form of patterns of tiny dots. The print needles are arranged in a vertical column so that characters
are printed out one dot column at a time as the print head is moved across the line. The quality of the print
is not so good. However the quality of printing can be improved by printing a line twice with the dots for
the second printing offset slightly from those of the first. There are printers which use up to 24 vertical
pins to characters with a higher resolution. Print speed of dot matrix impact printers is a maximum of 350
characters per second ( cps).
4.6 PAGE PRINTERS
The page printers are non-impact printers having speeds over 20,000 lines per minute. The technique
called Electro-photographic technique is developed from the paper copier technology.
The important type of page printers are discussed below.
4.6.1 Electromagnetic printers
The magnetic recording technique is used in this type of printers. A magnetic image of the matter to
be printed is written on a drum surface. The surface of the drum is then passed through magnetic powder
which adheres only to the charged areas. This powdered surface is now pressed on to the paper to obtain
the hard copy of the data. Speeds of electromagnetic printers up to 250 cps (characters per second) may
be obtained.
4.6.2 Electrostatic printers
In this type of printers, a paper coated with a non-conducting electric material which holds the charges
of voltages applied with writing heads, is used. The heads( or nibs) write dots on the paper as it passes.
The paper is now passed through a toner which contains material with particles carrying an opposite
charge to that of the paper written by the nibs. The particles adhere to the magnetized areas and form
printed characters.
4.6.3 Ink-jet printers
Ink-jet printers spray tiny drops of ink onto the paper from a nozzle on the print head. The nozzle can
have anywhere from 50 to several hundred small holes. Ink can be propelled by heat or pressure through
any combination of nozzle holes to form a character or image on the paper (which is similar to the concept
of the pins of a dot matrix printer.) These printers offer high- quality print and graphics. Standard weight
paper is suitable, but heavier weight premium paper is recommended to prepare better looking documents
which prevent ink-bleeding. These can also print overhead projector transparency sheets. Graphics and
52
colour print take more time to print than text. Good-quality colour ink-jet printers are available. Printers
print text at varying rates of pages per minute.
4.6.4 Laser and xerographic printers
The principle of laser printers and xerographic printers is almost the same. The image of the page to
be printed is first formed on a photosensitive drum in the printer. Powdered ink or tones is applied to the
surface of the drum. Now, the image from the drum is electrostatically transferred to the sheet of paper.
The ink image on the paper is finally fused by heating process internally.
In case of a laser printer, the image on the photosensitive drum is produced by a laser beam. The laser
is swept back and forth across the drum to produce an image in the same way as produced on a raster
scale CRT. The rotating mirror sweeps the laser beam across the rotating drum, With the help of a
modulator controlled by a microcomputer, the laser beam is turned on or off to produce dots. After the
image is inked and transferred to the paper, the drum gets cleaned and gets ready for the next page. The
speeds of laser and xerographic printers are to the tune of 20,000 lines per minute. The quality of print is
also good.
Laser Printer
Fig 4.5 Laser Printer principle

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4.7 PLOTTERS
A printer is capable of producing output that contains printed words. It is also capable of printing
straight lines and simple figures like squares , rectanagles and circles.
But very often one, might require to create high-quality visuals on paper, which a printer is not capable
of doing. For this purpose, a plotter is used. A plotter is a printer that is used to create presentation visuals,
charts, graphs tables and diagrams prepared from statistical data and drafting.
Figure 4.6 Plotter
Figure 4.6 shows a plotter.A plotter consists of an arm that moves across the paper on which the
diagram or graph needs to be drawn. A pen moves along the arm and the arm itself moves relative to the
paper. A combination of the two thus provides movement along the horizontal and vertical axes.
In some plotters, the paper is held stationary while the arm and the pens move over it. This is called a
flat bed plotter. In the other type of plotter, the paper is wrapped around a drum and anchored at both
ends. The drum rotates while the pen moves laterally along a fixed rail. This is called a drum plotter.
To draw clear, high-quality diagrams, a plotter needs high-quality pens with special inks of different
colors.
A plotter can be connected to a computer through a parallel port. A plotter is more software-dependent
than any other peripheral and needs much more instruction than the printer for producing output.
Plotters are now increasingly being used in applications like CAD(Compute Aided Design) which
require high-quality graphics on paper. Many other plotters now available in the market are desktop
models that can be used with personal computers. Businesses typically use plotters to present an analysis
in visual terms ( bar charts ,graphs, diagrams, etc.) as well as for engineering drawings.
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4.8 SCANNERS
Scanners have become an important part of the computer system over the last few years. Scanner
technology is applied everywhere and used in many ways.
BASIC PRINCIPLE
The basic principle of a scanner is to analyze an image and process it in some way. Image and text
capture (optical character recognition or OCR) allow you to save information to a file on your computer.
You can then alter or enhance the image, print it out or use it on your Web page.
The different types of scanners are:
Flatbed scanners
Sheet-fed scanners
Handheld scanners
Drum scanners
Flatbed scanners, also called desktop scanners, are the most versatile and commonly used scanners,
and this section gives the details of the technology as it relates to flatbed scanners.
Figure 4.7 Scanners
Sheet-fed scanners are similar to flatbed scanners except the document is moved and the scan head
is immobile. A sheet-fed scanner looks a lot like a small portable printer.
Handheld scanners use the same basic technology as a flatbed scanner, but rely on the user to move
them instead of a motorized belt. This type of scanner does not provide good image quality. However, it
can be useful for quickly capturing text.
Drum scanners are used by the publishing industry to capture incredibly detailed images. They use a
technology called a photomultiplier tube (PMT). In PMT, the document to be scanned is mounted on a
glass cylinder. At the center of the cylinder is a sensor that splits light bounced from the document into
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BSIT 12 Computer Fundamentals
three beams. Each beam is sent through a color filter into a photomultiplier tube where the light is changed
into an electrical signal.
Main Parts of a Scanner
Parts of a typical flatbed scanner include:
1. Charge-coupled device (CCD) array
2. Mirrors
3. Scan head
4. Glass plate
5. Lamp
6. Lens
7. Cover
8. Filters
9. Stepper motor
10. Stabilizer bar
11. Belt
12. Power supply
13. Interface port(s)
14. Control circuitry
The core component of the scanner is the CCD array. CCD is the most common technology for
image capture in scanners. CCD is a collection of tiny light-sensitive diodes. which convert photons (light)
into electrons (electrical charge). These diodes are called photosites. In a nutshell, each photosite is
sensitive to light — the brighter the light that hits a single photosite, the greater the electrical charge that
will accumulate at that site.
The image of the document that is scanned reaches the CCD array through a series of mirrors, filters
and lenses. The exact configuration of these components will depend on the model of scanner, but the
basics are pretty much the same.
The way how all the parts of the scanner work together is discussed in the next paragraph.
56
The Scanning Process
The steps that a scanner goes through when it scans a document is given below:
The document is placed on the glass plate and the cover is closed. The inside of the cover in most
scanners is flat white, although a few are black. The cover provides a uniform background that the
scanner software can use as a reference point for determining the size of the document being scanned.
Most flatbed scanners allow the cover to be removed for scanning a bulky object, such as a page in a thick
book.
A lamp is used to illuminate the document. The lamp in newer scanners is either a cold cathode
fluorescent lamp (CCFL) or a Xenon lamp. while older scanners may have a standard fluorescent lamp.
The entire mechanism (mirrors, lens, filter and CCD array) make up the scan head. The scan head is
moved slowly across the document by a belt that is attached to a stepper motor. The scan head is
attached to a stabilizer bar to ensure that there is no wobble or deviation in the pass. Pass means that
the scan head has completed a single complete scan of the document.
The stabilizer bar is very durable and tightly secured to the body of the scanner.
The image of the document is reflected by an angled mirror to another mirror. In some scanners,
there are only two mirrors while others use a three mirror approach. Each mirror is slightly curved to
focus the image it reflects onto a smaller surface.
The last mirror reflects the image onto a lens. The lens focuses the image through a filter on the CCD
array.
The filter and lens arrangement vary based on the scanner. Some scanners use a three pass scanning
method. Each pass uses a different color filter (red, green or blue) between the lens and CCD array.
After the three passes are completed, the scanner software assembles the three filtered images into a
single full-color image.
Most scanners today use the single pass method. The lens splits the image into three smaller versions
of the original. Each smaller version passes through a color filter (either red, green or blue) onto a discrete
section of the CCD array. The scanner combines the data from the three parts of the CCD array into a
single full-color image.
Another imaging array technology that has become popular in inexpensive flatbed scanners is contact
image sensor (CIS). CIS replaces the CCD array, mirrors, filters, lamp and lens with rows of red, green
and blue light emitting diodes (LEDs). The image sensor mechanism, consisting of 300 to 600 sensors
spanning the width of the scan area, is placed very close to the glass plate that the document rests upon.
When the image is scanned, the LEDs combine to provide white light. The illuminated image is then
captured by the row of sensors. CIS scanners are cheaper, lighter and thinner, but do not provide the same
level of quality and resolution found in most CCD scanners.
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Resolution and Interpolation
Scanners vary in resolution and sharpness. Most flatbed scanners have a true hardware resolution
of at least 300x300 dots per inch (dpi). The scanner’s dpi is determined by the number of sensors in a
single row (x-direction sampling rate) of the CCD or CIS array by the precision of the stepper motor
(y-direction sampling rate).
For example, if the resolution is 300x300 dpi and the scanner is capable of scanning a letter-sized
document, then the CCD has 2,550 sensors arranged in each horizontal row. A single-pass scanner would
have three of these rows for a total of 7,650 sensors. The stepper motor in the example given is able to
move in increments equal to 1/300ths of an inch. Likewise, a scanner with a resolution of 600x300 has a
CCD array with 5,100 sensors in each horizontal row.
Sharpness depends mainly on the quality of the optics used to make the lens and the brightness of the
light source. A bright xenon lamp and high-quality lens will create a much clearer, and therefore sharper,
image than a standard fluorescent lamp and basic lens.
Of course, many scanners proclaim resolutions of 4,800x4,800 or even 9,600x9,600. To achieve a
hardware resolution with a x-direction sampling rate of 9,600 would require a CCD array of 81,600
sensors. A quick look at the specifications shows that, these high resolutions are usually labeled software-
enhanced, interpolated resolution or something similar.
Interpolation is a process that the scanning software uses to increase the perceived resolution of an
image. It does this by creating extra pixels in between the ones actually scanned by the CCD array. These
extra pixels are an average of the adjacent pixels. For example, if the hardware resolution is 300x300 and
the interpolated resolution is 600x300, then the software is adding a pixel between every one scanned by
a CCD sensor in each row.
Another term used when talking about scanners is bit depth, also called color depth. This simply
refers to the number of colors that the scanner is capable of reproducing. Each pixel requires 24 bits to
create standard true color and virtually all scanners on the market support this. Many of them offer bit
depths of 30 or 36 bits. They still only output in 24-bit color, but perform internal processing to select the
best possible choice out of the colors available in the increased palette. There are many opinions about
whether there is a noticeable difference in quality between 24-, 30- and 36-bit scanners.
Image Transfer
Scanning the document is only one part of the process. For the scanned image to be useful, it must be
transferred to your computer. There are three common connections used by scanners:
Parallel - Connecting through the parallel port is the slowest transfer method available.
Small Computer System Interface (SCSI) - SCSI requires a special SCSI connection. Most SCSI
58
scanners include a dedicated SCSI card to insert into your computer and connect the scanner to, but you
can use a standard SCSI controller instead.
Universal Serial Bus (USB) - USB scanners combine good speed, ease of use and affordability in a
single package.
FireWire - Usually found on higher-end scanners, FireWire connections are faster than USB and
SCSI. FireWire is ideal for scanning high-resolution images.
On your computer, you need software, called a driver, that knows how to communicate with the
scanner. Most scanners speak a common language, TWAIN. The TWAIN driver acts as an interpreter
between any application that supports the TWAIN standard and the scanner. This means that the application
does not need to know the specific details of the scanner in order to access it directly. For example, you
can choose to acquire an image from the scanner from within Adobe Photoshop because Photoshop
supports the TWAIN standard.
In addition to the driver, most scanners come with other software. Typically, a scanning utility and
some type of image editing applications are included. Most of the scanners include OCR software. OCR
allows you to scan in words from a document and convert them into computer-based text. It uses an
averaging process to determine what the shape of a character is and match it to the correct letter or
number.
4.10 SUMMARY
In this chapter we have studied the interface standards and the peripheral devices. The printers are
important in getting the hard copy from the softcopy. Different technologies like the dot-matrix, inkjet,
laser jet used in printers have been discussed. The plotters, scanners and principle used in scanners to
convert the hard copy to digital form is also discussed.
4.11 CHECK YOUR PROGRESS
I Multiple Choice type
1. Select the correct one
a) Typical speeds of drum printers may be in the range of 300 to 2000 lines per minute.
b) Typical speeds of chain printers may be in the range of 400 to 2500 characters per minute.
c) Typical speeds of page prints may be even more than 20,000 lines per minute.
d) All the above.
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2. Printers
a) are most commonly used o/p device
b) are used to prepare permanent documents
c) are of several types
d) all the above
3. The impact printers use:
a) Thermal technology
b) Chemical technology
c) Electrical technology
d) None of the these
4. Drum printer is a:
a) Line Printers
b) Letter Quality Printers
c) Dot-Matrix Printers
d) None- of the above
5. The primarily used computer device for obtaining the hard copy is:
a) Line Printers
b) CRT
c) Keyboard
d) None
II. State whether the following statements are TRUE or FALSE :
1. The daisy wheel printers are called non-impact printers
2. Line printers are impact printers
3. A printer produces a hard copy
4. Line printers print an entire line at a time
5. A Daisy wheel printer is character printer
III. Answer the following questions
1. What is an interface
60
2. What are the different types of interfaces
3. How are printers classified
4. What is a scanner
5. What are the advantages of USB port
6. What do you mean by plug and play device
7. What is meant by hot pluggable.
Answers
I 1(d) 2(d) 3(d) 4(a) 5(a)
II 1)False 2) True 3) True 4) True 5) True
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61
Chapter 5
Memory
5.0 INTRODUCTION
M
emory is the most vital part of the computer. The memory is used to store information that is
used by the computer. The information may be in the form of programs and data on which the
processor operates on, are stored in the memory. Execution speed of programs is dependent
on the speed with which instructions and data are transferred between processor and the memory. Large
memory is needed to execute programs that are large and process huge amounts of data. The three main
requirements of a memory are that it has to be fast, large and inexpensive. It is difficult to meet all the
requirements. Methods are being developed to improve apparent speed and size of the memory, keeping
the cost reasonable. Apparent speed of memory is increased by using Cache (pronounced as CASH)
memory. Virtual memory concept is used to increase apparent size of memory. Memory devices that
provide large storage capability and which help in porting data from one machine to another are also
needed. Devices known as drives are needed to retrieve information from these storage devices. In this
chapter we discuss memory classification based on the proximity of memory to the processor, its purpose
and based on the technology used. The different types of drives and the advantages of these storage
devices are also discussed.
5.1 TYPES OF MEMORY
Memory is used to store programs and data by the computer. Memories are classified into two groups
based on the type of data access from the processor:
Primary Memory
Secondary Memory
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Chapter 5 - Memory
The Read-Write memory(R/W memory) and Read-Only memory (ROM) are examples of primary
memory, this is the memory the processor uses in executing and storing programs. This memory should
be able to respond fast enough to keep up with the execution speed of the processor. Hence this should be
a random access type so that the processor should be able to access information from any location with
same speed independent of its location in the chip. The size of memory chips is specified in terms of
bits.For example ,in general 1K memory chip means it can store 1K(1024) bits(not bytes). On the other
hand , memory in a Personal Computer system is specified in bytes. For example, 4M memory in a PC
means it has 4 megabytes of memory.At present PC come with 256MB or 512MB RAM.
The storage memory ,such as magnetic disks and tapes are used to store programs and results after
the completion of program execution. These are called as Secondary storage devices.The information
stored in these memories is non-volatile, meaning information is present even when the power is turned
off. The processor cannot directly execute or process programs stored in these devices. The programs
are to be copied into R/W prime (main) memory first.Therefore ,the size of the main memory (say 512
MB or so) in a PC(Personal computer) determines how large a program the system can process. The size
of the storage memory is unlimited,when one disk or tape is full, a new one can be used.
Figure 5.1 Memory Classification
Memory
Pri mary Memory
Storage Memory
Seco nd ar y Backup
R/ W memor y(RAM) Read Only Memory(ROM)
Semi Random
Access

Serial Access
Disks
Floppy
Hard disk
CD- ROM
Zi p Disk
P en Drive
*Magneti cT ap e
*CCD
Static
R/WM
Dynamic
R/ WM
R\WM
Er asable P er manent
EP ROM
EE-PROM
Fl ash
Masked ROM
PROM
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BSIT 12 Computer Fundamentals
Figure 5.1 shows the two groups in storage memory: secondary storage and backup storage. The
secondary storage is similar to what you put on a shelf in your study and the backup is similar to what you
store in your attic assuming that you need it some day in case there is a problem with the secondary
storage device. The secondary storage and the backup storage devices like the such disks , magnetic
tapes, magnetic bubble memory , and charged-coupled devices as shown in Figure 5.1. The primary
feature of these devices are high capacity ,low cost, and slow access. A disk is similar to a record , the
access to the stored information in the disk is semi-random. Some devices are serial, meaning if information
stored in the middle of the tape, the information can be accessed only after running half the tape.
5.2 PRIMARY MEMORY (MAIN MEMORY)
The primary memory is also called the main memory or internal storage or semiconductor memory.
This part of the memory is directly accessed by the processor. The main memory is implemented using
semiconductor integrated circuits. Internally the memory chips are made up of small storage area called
memory cell. Figure 5.2 shows a memory cell.A memory cell is designed using transistors, diodes and
resistors. A flip flop is an electronic circuit that that is used to store a bit of information bit in a memory
unit. A memory cell can store either a 0 or 1. Hence it is called a bit (bit is Binary DigIT).Each location in
a memory contains a fixed number of memory cells .The number of cells can be 4, 8, 16 or 32 .This is
called as word length of that storage unit.
Figure 5.2 A ROM cell
A group of 4 bits is called a nibble. A group of 8 bits is called a Byte. The memory locations have a
unique number assigned to it and it is called the address of the location in the memory. The memory
location is identified by its address. The contents in the form of word can be stored or retrieved in one
basic operation. Retrieving the stored data from the memory is called READ operation and storing data in
the memory is called the WRITE operation. There can be many locations in a memory and the number of
memory locations is called the capacity of the memory. The memory capacity is expressed in terms of
KiloByte, and MegaByte.
Bit line
P
T
Word line
Connected to store a 0
Not connected to store a 1
64
1024 Bytes make a KiloByte and it is not 1000 as we normally use, it is 2
10
=1024
The primary memory can be classified into:
Read only memory (ROM)
Read and Write memory( Random Access Memory)
5.2.1 Read Only Memory (ROM)
The information stored in this can be only read and it cannot altered. This is a nonvolatile memory. The
information stored in the ROM is not erased even when the power is switched off.
The processor [Central Processing Unit (CPU)] operates in a sequential manner by executing instructions
resident in the memory one after the other. On Power-on, or a RESET, the CPU always starts executing
programs from a fixed address. The subsequent actions of the computer are dictated by ( dependent on)
the instructions which begin at this fixed address. These instructions are usually initialization sequences
which direct the CPU to initialize all the subsystems in the computer system, and which ,in general ,define
the operations of the computer. For example , the CPU may initialize the display driver to display outputs
in a particular manner, or initialize the keyboard to accept only certain commands, or it could be a self test
program to test the functioning of all the subsystems, followed up by a prompt on the display. And then
wait for a command to be entered through the keyboard or mouse or any other input device.
This initialization sequence, which has to be performed each time the computer begins its operation is
part of the MONITOR software. The monitor software is stored in the permanent or nonvolatile memory.
The words permanent or non-volatile mean that the program is not lost even when the power is switched
off and that it is available each time the power is switched on. This non-volatile memory is implemented
by using a special kind of memory called the Read Only Memory ( ROM). The data in the ROM cannot
be written over and can only be read. The ROM is used to store information that should not change.
ROM’s are available in four types. These are
Masked ROM
PROM
EPROM
EEPROM ( E
2
PROM)
Flash memory
(a) Masked ROM
The instruction in such ROM’s are permanently installed by the manufacturer as per the specifications
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BSIT 12 Computer Fundamentals
provided by the system programmer and cannot be altered. This ROM contains cell arrays in which 1’s
and 0’s are stored by means of metallization interconnect mask at the time of fabrication of the integrated
circuit (IC’s). It is generally manufactured in bulk as the set up cost is high.
(b) Programmable Read Only Memory (PROM)
This memory has nichrome or polysilicon wires as links arranged in a matrix; these wires can be
functionally viewed as diodes or fuses. This memory can be can be programmed by the user with a
special PROM programmer that selectively burns the fuses according to the bit pattern to be stored. The
process is known as “burning the PROM”. The PROM uses fusible links that can be burned or melted by
using a special ‘PROM burning circuit’. A fused link corresponds to a ‘0’.The information stored in the
PROM after burning is permanent.
(c) Erasable Programmable Read only memory(EPROM)
This is also programmable by the user. It uses Metal Oxide Semiconductor charge storage (MOS)
technology. Information is stored by using an EPROM programmer, which applies high voltages to store
information in the form of charges. The information stored in an EPROM can be erased by exposing the
memory to ultraviolet light which erases the data stored in the total memory area . The memory is
exposed to ultraviolet light through a window on the chip which is covered during normal operation . High
energy photons collide with the EPROM’s electrons which are scattered in a random manner thus erasing
the contents. The memory can be reprogrammed by using an EPROM burning circuit. EPROM’s are
generally used by system designers for development of prototype permanent programs.
Disadvantages of EPROM are
1) it must be taken out of the circuit to erase it
2) the entire chip must be erased, and
3) the erasing process takes 15 to 20 minutes.
(d) Electrically Erasable Programmable Read Only Memory (EEPROM)
This memory is functionally similar to EPROM except that information can be altered or erased by
using electrical signals instead of ultraviolet light, and that the data in memory locations can be selectively
erased, there is no need to erase all the information. This has an advantage in field and remote control
applications. The information stored in the entire chip can be erased in 10 milliseconds as against 15 to 20
minutes needed to erase in the case of EPROM. However , this memory is expensive compared to
EPROM or flash memory.
(e)Flash Memory
This is a variation of EE-PROM that is becoming very popular. The major difference between the
flash memory and EE-PROM is in the erasure procedure: The EE-PROM can be erased at register level
66
(i.e., say one memory location ) but the flash memory must be erased either completely or select a group
of memory locations called as block (also called as sector)level. These memory chips can be erased and
programmed at least a million times .The power supply requirement for programming these chips was
around 12 volts, but now chips are available that can be programmed using a power supply as low as 1.8
volts.
5.2.2 Random Access Memory (RAM)
During the course of computations, it may sometimes be necessary to store intermediate results for
later use. The processor can write into or read from whenever necessary. This is used as a scratch pad
memory .This memory is called a Read/Write Memory (R/W M) or, more commonly, a Random Access
Memory (RAM). The processor can access any memory location by simply specifying its address. It
should be noted that the term Random Access Memory is also applicable to ROM’s ; however, it is more
commonly used for R/W Memories . Unlike the ROM , the conventional RAM is a volatile memory, that
is , the contents of the RAM are lost when the power is switched off.
RAM’s are of two types:
Static RAMs
Dynamic RAMs
Static RAM(SRAM)
Figure 5.3 shows a Static RAM cell .Static RAMs uses flip-flops for storage elements. Once the data
is written into a memory location , the data remains unchanged unless the same memory location is
written into again. There is no need to refresh as it does not have capacitors. Static RAM ‘s consume
more power and are also costly to manufacture.
Figure 5.3 A Static RAM cell
T
1
T
2
b
X Y
b
1

Word line
Bit lines
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BSIT 12 Computer Fundamentals
Figure 5.3 and 5.4 shows a CMOS memory cell.The Transistor pairs (T3,T5) and (T4,T6) form the
inverters in the latch. The state of the cell can be read or written.
Figure 5.4 CMOS memory cell
Dynamic RAM(DRAM)
Figure 5.5 shows a dynamic memory cell .In Dynamic RAM the basic storage element is a capacitor
C, the element is said to contain a ‘1’or ‘0’ depending on the presence or absence of charge. Unlike the
static RAM, the contents of the Dynamic RAM may change with time due to leakage of charge. It is
therefore necessary to periodically refresh the storage elements in a Dynamic RAM.
Figure 5.5 A single –transistor dynamic memory cell
The advantages of the Dynamic RAM (DRAM) are that it consumes less power than the SRAM and
that it has about five times more storage elements per unit area. The disadvantages are that the DRAMs
have slower access times and need special circuitry to periodically refresh memory. RAMs are also

T5

T
6
T
3
T
4
V
SUPPLY

Bit lines
b
b
1
C
Bit line
T
Word line
68
implemented using other technologies. The most notable amongst them are iRAMs( Integrated RAM’s
which are Dynamic RAM devices in which the memory refresh circuitry is implemented within the
device)and Bubble memories.
RAM chips used to be sold as individual chips, but today several RAM chips are soldered together
onto a plug-in board called a module. This RAM module is called a SIMM (Single In-line Memory
Module). SIMMs come in three basic designs: an older design that has 30 connector pins, a newer design
that has 72 connector pins, and the newest design that has 168 connector pins (also called SDRAM).
Each computer is designed to use one or the other of these SIMM designs, but today most computers use
the 168 connector pin design.
SIMMs come in several difference speeds. The most common speed is 70 nanoseconds (ns). The
rule in RAM is lower (or smaller) the nanosecond number, the faster the RAM will operate. Therefore, a
60 ns SIMM is faster than a 70 ns SIMM.  The new SDRAM has a speed of 10ns, which is 6 times faster
than the fastest 72 pin SIMMS.  All Pentium computers incorporate SDRAM.
Several new memory technologies seek to close the gap between processor and RAM performance.
The goal is to increase the chips’s speed and widen the bandwidth with which they communicate with the
processor. The players include double data rate RAM, or DDRRAM (also known as SDRAM II),
SLDRAM, Direct RDRAM (aka Direct Rambus) and Concurrent RDRAM (aka Concurrent Rambus).
Of these, Direct Rambus, endorsed by Intel, offers the greatest speed improvements, moving the peak
bandwidth from SDRAM’s 125MBps to an astounding 2.7GBps
Double Data Rate (DDR) SDRAM:
DDR is rated by its speed or potential bandwidth
PCI1600/DDR200 - 1.6 Gbps throughput, 200 MHz bus
PCI12100/DDR266 - 2.1 Gbps throughput, 266 MHz bus
PCI2700/DDR333 - 2.7 Gbps throughput, 333 MHz bus
CACHE MEMORY
Pronounced as cash, it is a special high-speed storage mechanism. It can be either a reserved section
of main memory or an independent high-speed storage device.
It is a small and fast memory that is placed between the main memory and the CPU. It holds the
currently active and more frequently used segments of program and its data. This is shown in Figure 5.6.
In this approach , the CPU need not always fetch the program and data from the main memory, instead it
can get it from the cache memory. Hence, the total time of execution is considerably reduced and it is
more economical.
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Figure 5.6 Cache Memory
Operation of a cache memory
The cache memory is divided into number of blocks. When the CPU requests for an addressed word,
it is transferred from the main memory to the cache. The memory-access control circuitry in the cache,
checks whether the requested word currently exists in the cache.
If the requested word exists in cache, then the Read or Write operation is performed on the appropriate
location in the cache. When the operation is Read, the main memory is not involved. However, if the
operation is Write, then there are two techniques followed. In the first technique known as store-
through, the main memory and cache locations are updated simultaneously. In the second technique, only
the cache location is updated and it is marked as updated using dirty or modified bit. The main memory
will be updated during swapping.
If the requested word does not exists in cache, and the operation is Read, then the block of words that
contain the requested word is loaded from the main memory into the cache and the requested word is
forwarded to the CPU. This technique is known as load-through. When the operation is Write, and the
requested word is not in cache, then it is written directly into the main memory.
The correspondence between the main memory and cache memory blocks are specified through a
mapping function. And mainly there are three mapping functions :
Direct mapping technique
Associative mapping technique
Block set associative mapping technique
To bring a new block into the cache when the cache is full, the cache controller must decide which old
block to overwrite . This is done with the help of replacement algorithms.
Two types of caching are commonly used in personal computers: memory caching and disk caching. 
A memory cache, sometimes called a cache store or RAM cache, is a portion of memory made of


CPU
Processor

Cache
Memory

Main
Memory
Memory access control and data paths
70
high-speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for
main memory. Memory caching is effective because most programs access the same data or instructions
over and over. By keeping as much of this information as possible in SRAM, the computer avoids accessing
the slower DRAM. 
Some memory caches are built into the architecture of microprocessors. The Intel 80486 microprocessor,
for example, contains an 8K memory cache, and the Pentium has a 16K cache. Such internal caches are
often called Level 1 (L1) caches. Most modern PCs also come with external cache memory, called
Level 2 (L2) caches. These caches sit between the CPU and the DRAM. Like L1 caches, L2 caches
are composed of SRAM but they are much larger.
Disk caching works under the same principle as memory caching, but instead of using high-speed
SRAM, a disk cache uses conventional main memory. The most recently accessed data from the disk (as
well as adjacent sectors) is stored in a memory buffer. When a program needs to access data from the
disk, it first checks the disk cache to see if the data is there. Disk caching can dramatically improve the
performance of applications, because accessing a byte of data in RAM can be thousands of times faster
than accessing a byte on a hard disk. 
When data is found in the cache, it is called a cache hit, and the effectiveness of a cache is judged by
its hit rate. Many cache systems use a technique known as smart caching, in which the system can
recognize certain types of frequently used data. The strategies for determining which information should
be kept in the cache constitute some of the more interesting problems in computer science
VIRTUAL MEMORY
In any computer system, if programs and data does not fit into the physical main memory, then secondary
storage devices such as magnetic disks or tapes are used to hold them. This memory space problem is
handled by the operating system. It automatically swaps the program and data between the main memory
and the secondary storage, whenever they are required for execution. This technique is known as Virtual
memory technique. Thus, the programmer has an infinite size of main memory. The addresses of program
and data being generated by the CPU are independent of the main memory. These addresses are referred
to as logical or virtual addresses. These virtual addresses are translated into physical addresses by
what is known as Virtual-memory address translation mechanism.
Bubble Memory
This is a type of non-volatile memory composed of a thin layer of material that can be easily magnetized
in only one direction. When a magnetic field is applied to a circular area of this substance that it is not
magnetized in the same direction, the area is reduced to a smaller area circle, or bubble.
5.3 SECONDARY MEMORY
The computers internal memory is usually adequate for normal data processing requirements. But in
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BSIT 12 Computer Fundamentals
large data processing environments , where the volume of data is large, there arises the need for more
memory .But , this is not possible, as semiconductor memory is costly and we cannot keep increasing the
memory . The maximum capacity of primary memory is 2
n
locations where ‘n‘ is the number of CPU
address lines. So to handle more data than allowed by the main( primary ) memory , secondary memory
is used. Moreover, any information in the RAM is lost when power is switched off. And, we cannot store
anything in ROM. These limitations have been overcome with the help of certain storage media where
data is stored for later retrieval. These are termed as secondary storage media or permanent storage
media.
Though the storage is taken care of , the data and instructions have to be copied into the RAM , from
the secondary storage media prior to processing. Because the CPU (processor) can not directly access
the secondary memory. The most popular media for storage is the magnetic media.
The working principle behind the secondary storage is very simple- a computer can process or store
data in the binary format only. While storing on the magnetic storage material(medium) , the data is
represented as magnetized and non- magnetized spots(representing 0’s and 1’s). While retrieving, these
magnetic spots are translated to the corresponding electrical pulses. The equipments that take care of
translations and storage are called the secondary storage devices.
The secondary storage devices actually serve three purposes. Apart from storing data/information for
a retrieval at a later day , they can supply this data to the CPU( input operations ) and can also receive the
results from the CPU(output operations).
The manner in which data is stored and retrieved in a medium decides the speed with which data is
accessed. Primarily, there are two types of access mechanisms
Serial ( Sequential )
Direct ( Random)
In serial access the n
th
item can only be read after reading( or skipping over) the n-1 items before it.
This kind of access is useful when all the data is to be processed as in producing pay checks for
employees.
The Random access allows us to directly read the n
th
item.This kind of access is useful when only
parts of the data are to be processed as in sending appointment letters to the selected candidates.
The most common secondary storage devices are tape drives and disk drives. Disks can be accessed
either sequentially or randomly, while tapes can be accessed only in serial fashion.
5.3.1 Sequentially accessed devices
Magnetic tape
This is most popular storage device for large data that are sequentially accessed and processed magnetic
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tape is a half-inch (1.27-cm) wide plastic ribbon, which is coated on the side with an iron oxide material
which can be magnetized. The tape ribbon is housed in a cassette. The computer tape can be erased and
reused indefinitely. The stored data on the tape gets automatically erased as and when new data, is
recorded in the same area of the tape.
Recording of information
Information in binary, is recorded on the tape as invisible magnetized and non-magnetized spots on the
iron oxide side of the tape. 0 of the binary is represented by a non-magnetized spot and 1 is represented
by the magnetized spot on the tape. The tape is divided into horizontal rows called channels or tracks and
into vertical columns called frames. Each character of the information is recorded per frame based on
the code of the computer.
Data transfer rate
The product of tape density and tape speed is known as the data transfer rate of the magnetic tape.
The number of frames per inch length of the tape is called tape density. The tape density of a magnetic
tape is expressed in characters per inch (CPI) or bytes per inch. The tape speed is measured in inches
per sec. Data transfer rate of a magnetic tape may even exceed 1 million bytes per second.
Advantages of magnetic storage:
The storage capacity of a magnetic tape is unlimited.
It possesses high data density
Its cost is much less than other data storage devices
Its rate of data transfer is high
It is very easy to handle magnetic tapes
It is very easy to transfer information from one place to another.
Draw-backs of magnetic tapes
As magnetic tape is a sequential access devices, the data recorded on tape can not be addressed
directly. The data item at the end of the tape will require abnormal access time because all the characters
before the data item need by processed. This is why magnetic tape device is not found suitable in case
data is frequently required to be accessed at random.
It is not possible to interpret, the stored data to know the content of the information.
Dust free environment is needed for magnetic tapes and the reel containers.
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a) Tape Cassettes: A high quality digital cassette similar in appearance to the home cassettes.
b) Cartridges: The large cassette which contains long strips of magnetic tape is called cartridge.
It is very convenient to mount the tape there is proper protection against dirt and contamination.
Both cassettes and cartridges have low data transfer rate, i.e., less than 10,000 bytes/sec.
5.3.2 Random access devices
Magnetic Disk
It is a thin, circular metal coated on both sides with a magnetic material similar to a gramophone
record. A number of magnetic disks mounted about half-an-inch apart from each other on a central shaft
which may rotate at speed of 2400 or more revolutions per minute (RPM), consists a disk pack. Magnetic
disk packs are the most popular medium for direct access secondary storage.
Information is stored on both sides of the disks except the upper surface of the top disk and lower
surface of the lower disk. Each face of the disk is divided into a number of invisible concentric circles
called tracks. A set of corresponding tracks in all the surfaces is called a cylinder. Each track is sub
divided into sectors.
Information is recorded on the tracks of a disk surface in the form of minute magnetic spots. The
magnetized spot represents a 1-bit and non-magnetized spot represents a 0-bit.
The information stored on a disk can be read a number of items and reading operation is non-destructive.
To write a new data, the previously stored data has to be erased. The storage capacity of a disk pack
primarily depends on the number of disks it contains. The storage capacity also depends on the track per
inch of surface and the number of bits per inch of track.
Magnetic Drum
Magnetic drum is also a direct access storage device that can be used for both sequential and random
processing. It is basically a cylinder with a magnetizable outer surface. A motor rotates the cylinder on
its axis at a rapid speed but constant rate. The surface of the drum is divided onto a number of tracks
upon which data is stored as magnetized spots similar to that of a disk. Data is stored on the magnetizable
surface of the cylinder as minute magnetized spots arranged in binary form in the series of parallel
circular tracks. Recording of data on the rotating drum and reading data from the drum is achieved by a
set of secondary read/write heads positioned very near to the drum surface. Writing a new data on the
drum erases the previously stored data simultaneously at that location. Data once recorded on the drum
remains intact until it is erased.
The drum rotates at a speed of few hundred to few thousand rotations per minute and its access
times is fast. Random access is possible to select any particular track of the drum but once this track is
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identified, the data is recorded or retrieved sequentially. To reduce the access time, sometime heads are
divided into two sets. Heads are positioned diametrically opposite i.e for a drum of 20 track 40 heads are
provided.
As compared to magnetic disks, data transfer rates are faster. Due to limited storage capacity, use of
magnetic drum storage is on decline.
Winchester Disk
Winchester disk are also used as a storage device. In this unit, disks are permanently housed and
hermetically sealed in contamination free containers. The disks are coated with a special lubricant which
minimizes the friction between the read/write heads and the disk surface .Computers using these sealed
housings are said to employ Winchester technology. The Winchester technology enables greater precision
of alignment, an increase in the number of tracks on the disk surface and a higher storage density per
track.
Winchester disks are fast and highly valuable and low priced compared with conventional hard disk
drives. They are available in sizes varying form 5.25" to 14" in diameter and there capacities are 10,12
and 20 megabytes. Dual disk drive doubles the capacity. Winchester disks are used in large computers.
HARD DISK DRIVES (HDD)
Figure 5.7 Hard Disk Drive
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A hard disk drive contains rigid, disk-shaped platters usually constructed of aluminum or glass. Unlike
floppy disks, the platters cannot bend or flex—hence the term hard disk.
The basic structure of a hard disk is similar to that of a diskette, the major differences between the
hard disk and floppy disk being that the hard disk has more than two sides, can hold much more data and
is much more faster. Physically, the hard disk has several disks or platters mounted vertically on the
spindle. The number of platters depend on the type of the hard disk. Each platter has a two read/write
head. Each side of the platter is divided into concentric tracks. All of the same tracks on each platter are
collectively known as cylinder. Further, tracks are divided into sectors.
The Figure 5.7 shows the main components of HDD: platters, Read/Write heads, Head Actuators,
spindle motor.
Access speed :The access time for data stored on a disk is determined by the
The seek time – The time required to position a head over the proper track.
The search or latency time – The time required to spin the needed data under the head.
In addition to the above, the time taken to read block of word need to be considered, but normally it is
too small. The average access time for most hard disk storage system is usually between 10 to 100
milliseconds.
Floppy Disk
A floppy disk is a circular Mylar disk coated and magnetically sensitive film. Since the thickness of the
Mylar disk is only a few thousandth of an inch it is named as floppy. The floppy diskette is coated with a
magnetic material and enclosed in a protective jacket. The floppy disks are used for backup of files and to
transfer files from one computer to another computer. Figure 5.8 shows a 1.44 MB floppy disk which is
very commonly used.
The diskette surface is logically divided into a fixed number of tracks (concentric circles). Data can be
read and written on the specified tracks and not in between the tracks. Each track is divided into a
number of sectors. The number of sectors in a track depends on the size.
A read/write head contacts the disk through the head slot to read from or write to the disk. It is
designed such that it can move both forward and backward.
The outermost track is numbered as track 0. The subsequent tracks are numbered sequentially. To
protect data from being erased accidentally during formatting or other writing operations floppy disks
have a write-protect window, which is a small hole/opening in the corner of the disk. A piece of plastic in
the window can be moved to open or close the window. If it is closed, the drive can write on the floppy
disk. If the window is open the drive cannot write on the floppy disk. The 3½ inch floppy disk which is still
widely used ,can store 1.44 MB of data.( The equivalent of 700 pages containing 2000 characters each.
Graphic images will take more storage space than written text).
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Figure 5.8 1.44 MB Floppy disk
Advantages of floppy disks are
They are very cheap compared with other storage
They greatly enhance the on-line storage capacity
They are also convenient off-line storage medium
The floppy disks generally available in the market are of 3½ inch diameter size and 1.44MB
Optical disks
In the case of optical disks, storage is achieved on the principle of light. The advent of compact
disks(CDs) of digital audio system have now given to low cost storage technology .
Applications of optical disks
Fast access to reference materials such as catalogs,directories,manuals,abstracts, periodicals
and like
Useful for storing long-term archival images of historical files of documents for image processing.
Video games,educational videos, multimedia,encyclopedia and adveritising materials can be stored.
COMPACT DISK READ ONLY MEMORY (CD-ROM)
CD ROM is prepared by using a high power laser beam to burn a 1micron (10
-6
m ) hole in the master
disk.From this a mould is used to stamp out copies on plastic disks.A thin aluminium is then deposited on
the surface, followed by a transparent layer for protection.
CDROMs are read by devices known as detectors, by measuring the energy reflected of f the surface
when a low power laser beam is aimed at the surface. The holes are called pits and unburned areas

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between the pits are called lands, have different reflectivity making it possible to distinguish between pits
and lands.Further more, instead of using pits for 0’s and lands for 1’s, each pit-land and land-pit transition
represents a bit. Information on CD ROM is written a single continuous spiral unlike magnetic disks with
their discrete concentric circles and sector. The storage density of optical disk is 700 MB and storage cost
is low.
Advantages of CDROM
Enormous storage
Inexpensive
Portable
Life of CD is more as there is no physical contact between the head and the surface.
Write Once and Read Many times (WORM)
This type of disks are useful for users to record their own data on blank optical disk. A laser recording
device writes one bit by melting or deforming a thin sensitive layer of material on the disk
surface.Unmodified areas represent 0 bit. The deformed WORM disk cannot be resorted to original.Reading
of stored data occurs when the low power beam laser passes over the disk and deflects differences in the
reflections coming from 0 and 1 bit area.
Erasable optical disk
Unlike CDROM and WORM this is a true secondary storage device. These disks are prepared by
using magneto-optical technology. In such systems, a laser beam is used along with a magnetic field to
read or write the information on the disk,which is coated with a magnetic material.The laser beam is used
to burn the spot on which the magnetic field is applied , then the polarization of the spot can be changed
while recording the desired data. The process does not cause any physical change in the disks, these can
be repeated many times. The read operation is performed by detecting the degree of rotation of polarized
laser beam reflected from these surfaces.
Digital Versatile Disc (DVD)
A Digital Versatile Disc (DVD) too is an optical storage medium. It also has the same shape and size
as the CD-ROM, but has a much larger storage capacity. A DVD can store about seven times more data
than a CD-ROM. The prices of the DVD media and DVD drives have fallen in the last few years.
Nevertheless, they still cost more than CD-ROMs and their drives.
A DVD is also called as DVD-ROM. They can store audio and video in addition to other data. Many
movies today are distributed on DVD because of their low cost and large storage capacity. DVD movies
also have superior quality picture and sound.
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To view DVD movies on a TV, you require a DVD player.
A DVD stores data in a digital format (a pattern of 0s and 1s).
DVD-R and DVD-RW
DVD-R (DVD-Recordable) can be used only to write data once (and not re-write). Because of their
large storage capacity, DVD-R media are particularly suited to store multimedia content. In DVD, software
and multimedia developers have a storage medium that facilitates them to add features - without the
space restriction. End-users have the advantage that they can use just one or a few DVD, instead of
many CD-ROMs.
DVD-RW (DVD-Rewritable) facilitates you to write, erase, and re-write data. They cost more than
DVD-R. DVD-RW media are more suitable to take periodic backup of data.
Single-sided, double-layer DVDs are gaining popularity. The unique thing about these DVDs is that
they can store twice as much data than a conventional DVD. Whereas a conventional DVD can store
only about 4.7 GB of data, a double-layered DVD can store 9.5 GB. To write on a double-layered DVD
R media , special DVD writers like the nu DDW-163 or BenQ DW 1600 are required.
DVD Drives
To use a DVD in a computer, a DVD drive is needed. It looks similar to that of a CD-ROM drive. The
DVD logo placed on the front side of the drive helps distinguish the two types of drives. DVD drives can
read both DVD and CD-ROM media. These drives are
poised to replace CD-ROM drives in the near future.
Many of the components inside a DVD drive are similar to that found inside a CD-ROM drive. The
DVD drive has a more complex laser assembly because it has to read both DVDs and CD-ROMs.
When compared to CD-ROM drives, DVD drives spin the disc much slower. A DVD drive though
has much better throughput. It means that the drive can transfer data to the computer fast. That is, more
kilobytes per second (KBps). Higher throughput results because the data bytes are stored much closer to
each other in a DVD. In a CD-ROM the data is stored near the top layer; in a DVD the same is stored
in the middle layer.
The drive reads and collects the patterns of 0s and 1s and sends them to the computer. It decodes the
data to display it on the screen.
DVD-RW Drives
DVD-RW stands for DVD-ReWritable. DVD-RW drives are also called DVD writers. These devices
can read data from and write data to a DVD. Along with the DVD writer, you require software, such as
Nero, to record data to a DVD-R/RW media.
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Unlike a CD-RW drive, which uses an infrared write laser to write data, a DVD-RW drive uses a red
laser beam. The write process used by DVD-RW drives is quite similar to that of CD-RW drives. The 0s
are stored as dark spots, which are non-reflective areas of a data track. The 1s are stored as translucent
spots, which are reflective areas.
Combo Drives
Today many computer vendors supply PCs that have a combo drive in it. Such a drive facilitates to
read both CDs and DVDs and write CDs.
ZIP Disk
ZIP drives have now become popular as secondary storage devices. A single ZIP disk can store about
100MB. This is portable.
USB hard Drive /Flash drive/Pen drive
Figure 5.9 USB/Flash/Pen Drive
USB Flash Drive is a common term referring to the small size mobile data storage device built on the
flash memory technology. Figure 5.9 shows a USB drive. It is a new generation of mobile, external hard
drive that interface with the computer USB port. Designed as a tiny drive and disk combined, the USB
flash drive replaces the bulky ZIP drive/disk and traditional floppies. The USB flash drive contains a solid
state flash memory chip and not a traditional magnetic disk that spins during data transfer between the
drive and the computer. A new way to store and transport data quickly, safely and easily between computers,
a USB flash drive is lighter and more compact than either the ZIP drive or floppy disks. A USB drive with
2GB and above are available. The size of USB drive is around 3inches and 1 ounce in weight.USB drives
are to be connected to the USB ports of the computer. Insert the USB pendrive into the USB port gently
pushing it into the slot. When the drive is mounted an icon representing the USB pen drive will be displayed.
It can be used like any other storage device for storing information or data.
Before removing the USB pen drive, unmount the volume by selecting the eject option.
5.4 ADVANCES IN MEMORY TECHNOLOGY
Memory technology has advanced considerably in recent years. In addition to satic and dynamic R/W
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memory. Other options are also available in memory devices.Examples include Zero-power RAM,Non-
Volatile RAM ,and Integrated RAM.
The Zero-power RAM is a CMOS Read/Write memory with battery back up built internally . It
includes lithium cells and voltage-sensing circuitry.When the external power supply voltage falls below
3volts the power switching circuitry connects the lithium battery ,thus , this memory provides the advantages
of R/W and Read-Only memory .
The Non-Volatile RAM is a high speed static R/W memory array backed up,bit for bit ,by EE-PROM
array for non-volatile storage. When the power is about to go off, the contents of the R/W memory are
quickly stored in the EE-PROM by activating the Store –signal on the memory chip, and the stored data
can be read into R/W memory segment when the power is again turned on.This memory chip combines
the flexibility of static R/W memory with the non-volatility of EE-PROM.
The Integrated RAM (iRAM) is a dynamic memory with the refreshed circuitry built on the chip.For
the user , it is similar to the static R/W memory. The user can derive the advantages of the dynamic
memory without having to build the external refresh circuitry.
5.5 SUMMARY
Memory is an important unit in the computer system. The memory is classified into mainly two groups.
The main memory and the secondary memory.The main memory is one of the major components in any
computer. Its characteristics such as size and speed play an important role in determining the capabilities
of a given computer. Advancements in technology has made a great impact in lowering the cost of
semiconductor memories. Bipolar memories are very useful in the implementation of cache memories
because of its high speed, where as MOS memories are used in the main memory because of its relative
low cost. The use of the virtual memory systems are increasing rapidly because of its advantages. The
secondary memory or the storage devices are very much needed to have the programs and data .The
various storage devices and drives that are more commonly used are the floppy disk, the hard-disk,and the
CD-ROMs.At present DVDs are being used to store large amount of data .Pendrives or USB drives are
now popular, they can store data of up to 512MB and above.They are very compact and convenient.
There is lot of advancement and technology is changing fast in the main memory and storage technology.
5.6 SELF TEST
I Select the appropriate answer
1. The primary storage unit has:
a) Faster access time
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b) Smaller storage capacity
c) Higher cost per bit of storage
d) All the above
2. The performance of the CPU may be increased by providing a small memory known as:
a) speed buffer
b) cache memory
c) both and b
d) neither a and b
3. The disadvantage of magnetic tape as secondary storage device is :
a) no direct access to data
b) no direct verification of data
c) dirty atmosphere affects the tape recording errors
d) all the above
4. Pick up the hard disk from the following:
a) magnetic disk
b) magnetic tape
c) punched paper tape
d) all the above
5. Pick up the incorrect statement from the following magnetic disk is advantages compared to magnetic tape:
a) Less vulnerable to damage from dust.
b) Higher transfer rate of data
c) Sequential processing is faster
d) None of the above
II State whether the following statements are True or False:
1. The amount of data that can be stored in the storage unit is called the storage capacity.
2. The small storage areas in the storage are called location.
3. Primary storage is generally refereed to as RAM
4. The information stored inside a ROM gets lost when the power supply is switched off
82
5. In sequential access device, access time for all locations is same
6. Magnetic disk and magnetic drum are typical sequential access storage devices
7. Magnetic tape is the most suitable storage media for random access secondary storage
8. Floppy disks are popular external storage media used with small computers
9. Each character is represented on a column by a set of holes punched in various rows
10. Winchester disks are permanently housed in sealed, contamination-free containers.
III Answer the following questions
1.How are memories classified ?
2. What is the difference between main memory and secondary memory ?
3.What is the difference between RAM and ROM
4. What are the advantages of CD-ROM
Answers
I 1(a) 2(c) 3(d) 4(a) 5(c)
II 1)True 2)True 3)True 4)False 5)False
6)False 7)False 8)True 9)True 10)True
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Chapter 6
Computer Network
6.0 INTRODUCTION
T
he need for human beings to communicate with each other gave rise to various forms of
communication techniques. Networking is one of them. Today computers are being used to send
information across the globe. A stand-alone computer’s capability can be increased by connecting
it with other computers. Networking in the present day is playing a major role in computing applications
and communications in general.
6.1 NETWORK
A computer network is a communication system where a group of computers and other devices like
printers are connected by cables and other hardware. The data and information is shared between the
computers. A network, besides facilitating data communication, allows resources (like printers, hard-disk
etc.) to be shared among all the computers connected to the network. The concept of connected computers
sharing resources is called Networking.
Computers that are connected in a network can share data, messages, printers, hard-disks, CD-ROM’s,
MODEMS and other hardware resources.
The advantages of Networking are:
Networks allow efficient management of resources. Users whose computers are connected to
the network can share a single high-quality printer instead of having many individual printers.
This will lead to efficient usage of printers, reduce the unnecessary investment and also the
maintenance cost of printers.
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Chapter 6 - Computer Network
Networks help keep information reliable and up-to-date. A well managed, centralized data storage
system allows multiple users to access data from different locations.
Networks help speed up data sharing. Transferring files across a network is always faster than
other non-network means of sharing files. For example one has to copy the files or folder in a
floppy from one computer and then copy it to the desired computer. The amount of data one can
copy on a floppy again limits the number of files/ folders, this limitation is over come in a
network.
Services like electronic-mail being offered by networks, allow much more efficient communication
among individuals. Now voice messages are being communicated over network.
Types of Networks
Networks can be classified as follows:
Local Area Network (LAN)
If a network is confined to a single location, typically one building or complex, it is called a Local
Area Network (LAN).The maximum distance from one end of a network to another is usually
limited by the signal strength or the network system’s built-in time limit for sending and receiving
messages through a physical connection such as a cable.
Wide Area Network (WAN)
Advantages of LAN led to need for expanding networking area. LANs could not adequately
support the network needs of large business, with offices and operations spread over a wide
area. This led to the development of Wide Area Networks (WAN).
When a network is spread over wide areas, such as across cities, states or countries, it is called
Wide Area Network (WAN).Communication on a WAN takes place via telephone lines satellites
or microwave links, rather than through a physical cable.
Metropolitan Area Network(MAN)
In between LAN and WAN is the metropolitan Area Network (MAN). A MAN is a network
that covers an entire city, but uses LAN technology. Cable television networks are examples of
MANs distributing television signals. Networks of this nature, which carry information, in the
form of computer signals from one computer to another make up a MAN.
6.2 THE BASIC NETWORK
The basic network is made up of the following:
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The Workstations
The Servers
The Networking interface Card’s(NIC)
The Communication Channel
Network operating system Software and LAN application programs.
6.2.1 Workstations (Clients)
The term workstations refer to the computers that are connected to the LAN and are seeking to share
the resources of the LAN. These workstations can be any Personal computer. Computers which do not
have either hard-disk or floppy drives can also be connected to the network. Such PC’s without hard-disk
are called diskless workstations. Such workstations cannot, obviously, hold any data or software. They
access them directly from the LAN server.
A computer (regardless of whether it has disk drives or not) is said to be a NODE of the LAN once
it is connected (attached) to the LAN.
Workstations have good quality monitors and high quality keyboards.
6.2.2 Server
A server is a computer that provides the data, software and hardware resources (like printers) that are
shared on the LAN. It serves the workstations. All the files, data and software that are shared by the
users of the LAN are stored in the server’s hard-disk. A LAN can have more than one server. Each
server has an unique name on the network and all LAN users identify the server by its name.
Servers are high-quality, heavy duty systems, the hard-disk has to be durable, reliable and should be
capable of serving multiple users requests simultaneously. The CPU should be a high speed processor and
it should have a large RAM.
The servers normally operate unattended and are placed in a secured area. The Server is usually
maintained by an administrator. Administrator who looks after the creation of LAN user account, the
amount of storage space for each user and the permission to access the resources of the LAN, such as
sharing software, printer etc., for the LAN users.
The servers can be classified as
Dedicated server(SERVER BASED network)
Non-dedicated Server(PEER-PEER network)
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Server-Based Networks
Most of the networks have a dedicated server. A dedicated server is a computer on a network that
functions as a server and is not used as a client or a workstation. A dedicated server is optimized to
service requests from network clients.
Servers perform a wide variety of tasks. Usually, large networks have specialized servers for different
tasks. Examples of specialized servers are:
File and print servers - These servers manage user access and use of file and printer resources.
Application servers – These servers make the data on the server available to clients.
Mail Servers- These servers manage electronic messaging between network users.
Communication Servers- These servers handle data flow and E-mail messages.
Peer- to- Peer Networks
In a peer-to-peer network, there are no dedicated servers. All the computers are equal, therefore they
are termed as peers. Normally, each computer functions as both a client and a server.
A peer to peer network has the advantage of simplicity in design and maintenance. It is usually less
expensive to set, as compared to Server-based networks. Peer-to–Peer networks are also called as
workgroups. The term workgroup implies a small group of users. Peer- to- peer networks are suitable for
following environments where:
There are a limited number of users (usually 10 or less).
The users are located in the same area.
Security is not an important issue.
The organization and the network have limited growth.
Users need to freely access data and programs that reside on other computers across the
network.
In a peer-to-peer network, no one is assigned to be an administrator to be held responsible for maintaining
the network. Users administer their own computers. All users can share any of their resources.
6.2.3 Networking Interface Card (NIC)
The network interface card is a device that is attached (plugged) to each of the workstations and the
server. It helps the workstation to establish the all-important connection with the network. As soon as the
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NIC is plugged the PC is considered as a node in the network. The NIC will allow the node to connect to
the network using network protocol. The NIC is simply a translator in the PC. Networks transmit data in
Serial format to the workstation and then NIC will convert it into parallel format for the node. Each NIU
that is attached to a workstation has a unique number identifying it. This number is called the node
address. Thus, the node address of a workstation means the address of the NIU that is attached to the
workstation.
6.2.4 Communication Channel
In a LAN, the word communication channels mean the connecting cables. The connecting cables are
the lifelines of a LAN.
There are three types of cables that are commonly used
1. Twisted Pair Cables
2. Coaxial Cable
3. Fiber Optic Cable
Twisted Pair Cables
The twisted pair cables are the oldest and least expensive type of cables used for LAN’s. It consists
of insulated wires within a protective casing, with a specified number of twists per foot. Twisting the
wires reduces the effect of electromagnetic interference on the signals being transmitted. Figure 6.1
shows a Twisted pair cable. There are two types of twisted pair cables. They are
Unshielded Twisted Pair (UTP)
Shielded Twisted Pair (STP)
Figure 6.1 Twisted Pair Cable
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Unshielded Twisted Pair (UTP): This is the most popular form of cables in the network and
is the cheapest. The UTP has four pairs of wires and all are inside plastic sheathing. Each wire
is protected with a thin plastic sheath.
Shielded Twisted Pair (STP):This is more commonly used in high-speed networks. The
difference between UTP and STP is that STP’s use metallic shield wrapping to protect the wire
from interference.
Co-axial Cables
A Co-axial cable consists of one or more small cables in a protective covering. These are expensive
than twisted pair cables and perform better than the twisted pair cables. Figure 6.2 shows a Co-Axial
cable.
Figure 6.2 Co-Axial Cable
The Co-axial cable are of two types
1. Thick coaxial cable
2. Thin co-axial cable
Thick coaxial cable This cable is usually yellow in color and used in what is called as
thicknets and has two conductors. This coax can be used in 500 meter lengths. The cable
itself is made up of a solid center wire with a braided metal shield and plastic sheathing
protecting the rest of the wire.
Thin coaxial cable This is used in thin nets. This type of cable is called as RG-58.The
cable is a cheaper version of the thick cable
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Fiber Optic Cable
Figure 6.3 Fiber Optic Cable
Fiber optic cables are made of plastic or glass and are about as thin as a human hair. These cables are
highly durable and offer excellent performance. Figure 6.3 shows a Fiber Optic Cable. Their speed of
transmission is very high. There is no electromagnetic interference as pulses of light are used for data
transfer. There is very less loss of signal during transmission. Hence the OFCs are used for long distance
transmission at high speeds. The disadvantages of OFCs are it is expensive, needs skilled persons for
splicing the cable, and installing the connectors.
At one end of the system is a transmitter. This is the place of origin for information coming on to fiber-
optic lines. The transmitter accepts coded electronic pulse information coming from copper wire. It then
processes and translates that information into equivalently coded light pulses. A light-emitting diode (LED)
or an injection-laser diode (ILD) can be used for generating the light pulses. Using a lens, the light pulses
are funneled into the fiber-optic medium where they transmit themselves down the line
Light pulses move easily down the fiber-optic line because of a principle known as total internal
reflection. “This principle of total internal reflection states that when the angle of incidence exceeds a
critical value, light cannot get out of the glass; instead, the light bounces back in. When this principle is
applied to the construction of the fiber-optic strand, it is possible to transmit information down fiber lines
in the form of light pulses.
The light source is pulsed on and off, and a light-sensitive receiver on the other end of the cable
converts the pulses back into the digital ones and zeros of the original signal.
The cables are also defined in numbers like CAT3, CAT5 so on .The bigger the number the better the
protection from interference.
Connectors
Connectors are needed to connect the cables to the card. A cable and connector are a pair in the
communication channel. LAN cable connectors look similar to the phone jack connectors and the Television
(TV) cable connectors except it is slightly bigger. Figure 6.4 shows the connectors.The RJ-45 connector
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and BNC (Bayonet Naur Connector) are the two commonly used connectors. The RJ-45 connector is
used in smaller networks using UTP/STP cable and BNC (Bayonet Naur Connector) is used with Thicknets.
Connectors Plug and Jack
Figure 6.4 Connectors
6.2.5 Network Operating System
The network operating system (NOS) is much more powerful than a stand alone operating system. It
has to operate in a more complex environment. The tasks which NOS is to perform are:
Directing data traffic through the network
Allowing and preventing access to data based on security requirements
Preventing access to data files while they are being processed
Managing the flow of information between a variety of workstations
Managing requests for printer services
Managing communication and messages between network users
Managing connections between the network and remote sites
Make service as transparent as possible to the user.
Components of the network operating system
There are two main components of the network operating system. They are
Client Software – installed on Clients
Server Software – installed on Servers
Client Software
In a network environment when the user initiates a request to use a resource that exists on a server in
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another part of the network, the request has to be forwarded or redirected to the server with the requested
resource. The component of the client software that does this task is called the redirector.
Redirector
The redirector may also be referred to as shell or requester. It is a small part of the network operating
system that:
Intercepts the requests in the computer
Determines if the requests can be serviced by the local system itself or it needs to be forwarded to the
server.
Designators
A designator is an alphabet that is assigned to each network resource. The redirector keeps track of
which drive designators are associated with which network resources.
Server Software
Server software makes it possible for users working in other machines to share the server’s data and
peripherals including shared directories, printers plotters and disks.
Resource sharing
The server software allows sharing of resources, and also determines the degree of sharing. The
degree of sharing includes:
Allowing different users different levels of access to the resources. For example, a file server
could give read or write or read and write permissions to different users.
Co-Ordinating access to the resources to make sure that two users do not use the same resource
at the same time.
Managing Users
Network operating systems make it possible for a network administrator to determine who will be able
to use the network. The server software can be used to:
1. Create user privileges. Privilege indicates who gets the permission to use the various resources
on the network.
2. Validate user names and passwords at the time of logging on.
3. Grant or revoke user privileges on the network.
4. Remove users from the list of users having access to the server.
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6.3 NETWORK TOPOLOGY
The topology refers to the physical arrangement or physical layout of the computers, cables and other
components of the network.
The network can be configured in the following basic topologies:
Bus
Star
Ring
6.3.1 BUS TOPOLOGY
The bus topology consists of several computers which are connected to a common cable called the
Trunk line. The bus topology is also called as Linear Bus. Figure 6.5 shows the bus topology.
Figure 6.5 Bus Topology
In the bus topology, data on the network is sent to all the computers on the network. However, the data
is accepted only by the computer that has the address (recall that NIC has a unique number) matching the
address in the signal. Only one computer at a time can send messages. The bus topology is a passive
topology in which the computers on the bus are not responsible for moving data. In this, failure of one
computer does not affect the performance of the network. The performance on a bus network can be
affected by the following factors:
1. Number of computers on the network
2. Hardware capabilities of the computers on the network
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3. Frequency of data transmission
4. Types of cable used on the network
5. Distance between computers on the network
6.3.2 Star Topology
Figure 6.6 Star Topology
In star topology, the computers are connected by cable segments to a centralized device known as
HUB. Signals are transmitted from the sending computer through the controller on the central hub to the
receiving computer. Figure 6.6 shows the Star Topology. As each computer is connected to a central
point, this topology requires a great deal of cabling. Also, if the central controller fails, the entire network
is disabled. If one of the computer, or the cable that connects it to the hub fails, the rest of the network
continues to function normally.
HUB: A hub is a device used to centralize network traffic through a single connection point. Hubs can
be of two types: Active and Passive Hub. An active hub regenerates and transmits the signals. It requires
electrical power to operate. A passive hub acts as a connection point and does not amplify or regenerate
the signal. Passive hubs do not require electrical power to function. Signals after traveling some distance,
become weak. Regeneration is the process by which the signals are given more power to travel further.
6.3.3 Ring Topology
In ring topology, computers are connected through a single circle of cable. There are no terminated
ends. The signals travel round the loop in one direction and pass through each computer. The ring is an
active topology where each computer acts like a repeater to boost signal, and sends it on to the next

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computer. Failure of one computer in a ring network can affect the entire network. The ring topology is
shown in Figure 6.7
Figure 6.7 Ring Topology
There are other topologies which are combination of the above topologies, they are
1. Star Bus topology
2. Star Ring topology
Star bus topology
The star bus topology is a combination of the bus and star topologies. In a star bus topology, there are
several star topology networks linked together with linear bus trunks. The advantage in this scheme is, if
one computer develops a fault, it will not affect the rest of the network. If a hub develops a fault, all the
computers on that hub will be disabled.
Figure 6.8 Star bus network
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Star Ring topology
The star ring is a combination of ring and star topologies. The hubs in a star ring are connected in a star
pattern by the main hub.
Figure 6.9 Star Ring network
6.4 ACCESS METHODS
The set of rules defining how a computer puts data on to the network cable and retrieves data from the
cable is called access method. Access methods prevent simultaneous access to the cable. By ensuring
that only one computer can put the data on the network cable, access methods keep the transmission of
network data an orderly process.
The three main access methods are:
Carrier Sense Multiple Access (CSMA)
1. With Collision Detection
2. With Collision Avoidance
Token Passing
Demand Priority
6.4.1 Carrier Sense Multiple Access.
Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
In this method, each computer on the network senses any signal passing on the cable. The computer

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can only transmit if the cable is free. If two or more computers send data at exactly the same time, it will
result in data collision. When a collision occurs, the two computers involved stop transmitting and then
attempt to re-transmit after a certain period of time. The collision detection method is only effective up to
2,500 meters.
With more traffic, collisions tend to increase, this slows down the network. So, CSMA/CD can be a
slow access method.
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
In the Carrier Sense Multiple Access with collision avoidance method, each computer signals its intent
to transmit before it actually transmits the data. This way, the computer can avoid collisions. CSMA/CA
is a slower access method and is less popular than CSMA/CD.
6.4.2 Token Passing
In token passing, a special packet called a token circulates around the ring network in one direction.
When a computer on the network wants to send data, it takes hold of a free token .The computer can then
transmit data. Until the token has been released by the computer sending data, no other computer can
transmit. As only one computer can use the token at a time, the possibility of a collision is ruled out.
6.4.3 Demand Priority
Demand Priority is an access method designed for the 100Mbps Ethernet standard called 100VG-
AnyLAN.The 100VG-AnyLAN network comprises repeaters and end nodes. An end node could be a
computer, bridge, router or Switch. The function of the repeater is to search for requests from all nodes on
the network. In a demand priority network, computers can receive and send data (transmit) at the same
time. This is because four pairs of wires are used in the cabling scheme defined for this method. In
demand priority, communications is only between the sending computer, the hub or the repeater, and the
destination computer. This method is more efficient than the CSMA/CD, which sends transmission to the
entire network.
6.5 LOCAL AREA NETWORK TECHNOLOGIES
The main local area network technologies are:
Ethernet
Token ring
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Attached Resource Computer Network
Fiber Distributed-Data Interface (Fiber Distributed Data Interface)
Application programs can be kept on the LAN server, the users who need an application program
frequently can download it once and then run it from their local hard disk. Users can order printing and
other services as needed through applications run on the LAN server. A user can share files with others
at the LAN server. The read and write access is maintained by the LAN administrator.
6.5.1 Ethernet
Ethernet is the most widely-installed local area network (LAN) technology. Specified in a standard,
IEEE 802.3, Ethernet was originally developed by Xerox and then developed further by Xerox, DEC, and
Intel. An Ethernet LAN typically uses coaxial cable or special grades of twisted pair wires. The most
commonly installed Ethernet systems are called 10base-T and provided transmission speeds up to 10
MBPS.
Devices are connected to the cable and compete for access using a Carrier Sense Multiple Access
with Collision Detection (CSMA/CD) protocol.
6.5.2 Token ring
A token ring network is a local area network (LAN) in which all the computers are connected in a ring
or star topology. A binary digit-or token-passing scheme is used in order to prevent the collision of data
between two computers that want to send messages at the same time. The token ring protocol is the
second most widely used protocol on local area networks after Ethernet. The IBM Token Ring protocol
led to a standard version specified as IEEE 802.5. Both protocols are used and are very similar. The
IEEE 802.5 token ring technology provides for data transfer rates of either 4 or 16 Megabytes per
second.
The simple scheme on how the token ring system works is explained below:
Empty information frames are continuously circulated on the ring. When a computer has a message to
send, it inserts a token in an empty frame( this may consist of simply changing a 0 to a 1 in the token bit
part of the frame) and inserts a message and a destination identifier in the frame. The frame is then
examined by each successive workstation. If the workstation sees that it is the destination for the message,
it copies the message from the frame and changes the token back to 0. When the frame gets back to the
originator, it sees that the token has been changed to 0 and that the message has been copied and received.
It removes the message from the frame. The frame continues to circulate as an “empty” frame, ready to
be taken by a workstation when it has a message to be sent.
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The token scheme can also be used with bus topology LANs
The standard for the token ring protocol is Institute of Electrical and Electronics Engineers (IEEE)
802.5.
6.6 LAN EXPANSION
The LAN topologies have their own limitations on the number of computers that can be connected in
the network. Computers cannot be simply added to the network by simply connecting the cable. We need
to have certain components which can increase the size of the network within the existing network.
These components are:
Repeaters
Routers
Bridges
Brouters
Gateways
6.6.1 Repeaters
A repeater is a device that generates signals so that the signal can travel on additional cable segments.
They do not translate or filter data. For a repeater to work, both segments that repeater joins must have
the same media access scheme, protocol and transmission technique.
Repeaters can move packets from one medium to another medium. Some multi-port repeaters can
connect different types of media. Repeaters improve performance by dividing the network in to segments,
thus reducing the number of computers per segment.
6.6.2 Routers
A router is a device used to connect networks that use different architectures and protocols. They can
switch and transfer information packets across multiple networks. This process is called routing. They
can determine the best path for sending data and filter broadcast traffic to the local segment. Routers
cannot link to remote computers. They only read addressed network packets. Routers can link segments
that use different data packaging and media access schemes.
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6.6.3 Bridges
A bridge is a device that can join two LANs. However, a bridge can also divide a overloaded network
into separate networks, reducing the traffic on each segment and making each segment more efficient. A
bridge can link unlike media such as twisted-pair and co-axial Ethernet. It can also link unlike network
segments such as Ethernet and Token Ring.
They are also called as Media Access Control layer bridges. A Media Access Control layer bridge
performs the following tasks:
Checks the source and destination address of each packet
Creates a routing table
Forwards packets.
If the destination address is not listed in the routing table, the bridge forwards the packets to all the
segments.
Multiple bridges can be used to combine several small networks in to a large network. A bridge can be
installed internally or externally. Bridges are faster than routers because routers perform complex functions
on each packet.
The main differences between bridges and routers are:
The bridge only recognizes the addresses of network cards in its own segments. Routers recognize
network addresses.
A bridge can only recognize one path between networks. A router can search among multiple
active paths and determine the best path.
The router only works with routable protocols.
6.6.4 Brouters
A brouter combines the best features of both a bridge and a router:
Route a routable protocols
Bridge non-routable protocols.
6.6.5 Gateways
Gateways make communication possible between systems that use different communication protocols,
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data formatting structures, languages and architectures. Gateways repackage data going from one system
to another. Gateways are usually dedicated servers on a network and are task-specific.
6.7 MODEM
The word MODEM is an acronym for Modulator – Demodulator, Modems accept digital data supplied
by the local PC, and convert it to a modulated analog wave form that can be transmitted over a normal
analog phone line. And conversely, modems also accept a modulated analog wave from the telephone
line convert it to digital form, and pass it on to the local PC.
A 28,800 or 33,600 BPS modem can send data over a phone line up to fourteen times faster than a
2400 BPS modem, and a 56k modem over twenty times faster couple this with the built-in data compression
ability of modern high speed modems which will compress ordinary text data by about 2:1, and some types
of data by up to 4:1, and you can see that throughput can be 25 to 50 times greater (or more!) than for a
2400 BPS modem. This tremendous increase in speed can dramatically decrease your online time and
decrease your phone charges, and make it feasible to transfer graphics screens such as WWW pages in
time acceptable to most users.
6.8 WIDE AREA NETWORK TRANSMISSION
The networking components such as bridges and routers, the communication service providers and
local area networks can be connected to create WANs. Communications services make use of the
following types of transmission technologies:
Analog
Digital
Packet-switching
Analog Connectivity
Analog transmission refers to telephone lines used with modems. These can be:
Dial-Up lines – used for infrequent data transmissions
Dedicated Lines- used for frequent and continuous data transmissions. This method provides a ready
communication link.
Digital Connectivity
Digital lines provide faster and more secure transmission as compared to the telephone lines. Digital
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transmission does not require the use of modems. Following are the different forms of digital lines that are
used:
DDS (Digital Data Service)
T1
T3
T4
Switched 56
DDS sends data through a device called a Channel Service Unit/Data Service Unit( CSU/DSU).This
device converts digital signals into a form that is part of the synchronous communications.( In Synchronous
communication, bits are sent in a timed, controlled fashion. Transmission stops at the end of one frame
and starts again with a new one.).
T1 and T3 use point-to –point transmission that can transmit voice data and video signals. T1 offers
high data speeds and is most widely used type of digital line. T3 is the highest capacity leased line service
available.
Switched 56 is a digital dial-up service that requires CSU/DSU to be installed with each computer on
the network.
Packet - Switching Networks
Packet-switching is a fast and efficient way to transmit data over wide areas. With packet-switching,
data is divided into packets of small size and a destination address is attached to each packet. This way
each packet can be sent separately over the network. Packets are transmitted along the best route
available between the source and the destination.
Advanced technologies for sending data across a WAN are available. Depending on the requirement
one can consider the appropriate technology. Some of the technologies are:
X.25 Packet switching
Asynchronous Transfer Mode (ATM)
Integrated Services Digital Network (ISDN)
Fiber Distributed Data Interface (FDDI)
X.25 Packet switching
X.25 is a set of protocols included in a packet switching network. An X.25 network uses switches,
circuits and the available routes for routing at any time.
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Asynchronous Transfer Mode (ATM)
ATM is an advanced form of packet-switching .It is a broadband method that transmits data in 53-
byte cells. Each cell consists of 48-bytes of data and 5-bytes of header information. Therefore, the
packets that are generated are consistent and uniform. ATM can be used with any media.
ATM switches are multiport devices that can act as either hubs or routers. In architectures, such as
Ethernet and Token Ring only one computer can transmit data, ATM uses switches which are muliplexers
and permit several computers to transmit data on the network. In the asynchronous data transmission the
data is transmitted as a stream. Each character is converted into a string of bits. Each string is separated
from the other strings by a start bit. In this method, there is no device or method to co-ordinate transmission
between the sender and the receiver.
Integrated Services Digital Network (ISDN)
Basic Rate ISDN uses digital transmission that divides its bandwidth into three channels. Two of these
transmit data at 64 Kbps, and the third at 16 Kbps. The 64 Kbps channel can carry voice, data or images.
The slower 16 Kbps channel carries signaling data.
A computer connected to an ISDN service can use both the 64 Kbps channels to have a 128Kbps data
stream.
The Fiber Distributed –Data Interface (FDDI) This is a high speed token-passing ring network
this uses fiber-optic media and was developed for high end computers. FDDI uses dual-ring topology. In
this there are two data streams flowing in opposite directions around two counter-rotating rings. One ring
is called the primary ring and the other is called the secondary ring. The secondary ring acts as a back up.
Computers may connect to one or both FDDI cables in a ring.
6.9 OPEN SYSTEM INTERCONNECTION REFERENCE MODEL
The open system interconnection (OSI) reference model was developed by the International Standards
Organizations as a guideline for developing standards to enable the interconnection of dissimilar computing
devices. It describes how information from a software application in one computer moves through a
network medium to a software application in another computer. The OSI reference model is a conceptual
model composed of seven layers, each specifying particular network functions. This is the primary
architectural model for inter-computer communications. The OSI model divides the tasks involved with
moving information between networked computers into seven smaller more manageable task groups. A
task or group of tasks is then assigned to each layer and can be implemented independently. This enables
the solutions offered by one layer to be updated without adversely affecting the other layers.
Layers in the Network. There are seven layers in the network. It is necessary to get familiar with
the network layers. They are listed below:
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The first layer is the Physical Layer. Which looks after the standards for the physical connections
between the computers. The physical parts of the network such as wires, cables, and their media along
with the length. This layer controls the transmission of the raw bit stream over the transmission medium.
This layer takes note of the electrical signals that transmit data throughout the system. Standards for this
layer define such parameters as the amount of voltage swing, the duration of the voltages ( bits) and other
such things.
The second layer is the Data Link Layer. In this layer we actually assign meaning to the electrical
signals in the network. The layer also determines the size and format of data sent to printers, and other
devices(They are also called nodes in the network).This layer will allow and define the error detection
and correction schemes that insure that data was sent and received correctly. Second layer is responsible
for the reliability of the physical link established at application layer. (layer 1). Standards define how data
frames are recognized to provide necessary flow control and error handling at frame level.
The third layer is called as the Network Layer. This layer provides the definition for the connection
of two dissimilar networks. This is responsible for establishing ,maintaining and terminating network
connections. Among other functions these standards define how data routing and relaying are handled.
The fourth layer is the Transport Layer. This layer allows data to be broken into smaller packages
for data to be distributed and addressed to other nodes (workstations).It insulates the upper three
layers(5,6,&7) from having to deal with complexities of layer 1,2&3, by providing functions necessary to
guarantee a reliable network link. Other functions include error recovery and flow control between the
two end points of network connection.
The fifth layer is known as the Session Layer. This layer helps in the task to carry information
from one node (workstation) to another node (workstation). A session has to be made before we can
transport information to another computer. This layer establishes, manages and terminates user connections
and also manages the interactions between the end systems. Services include establishing communications
as full or half duplex and grouping data.
The sixth layer is the Presentation Layer, this layer is responsible to code and decode data sent to
the node. That is performs data transformations to provide a common interface for user applications,
including services such as reformatting, data compression and encryption.
The upper most layer, Seventh layer is called the Application Layer. This layer allows the user
Layer 7 Application layer
Layer 6 Presentation layer
Layer 5 Session layer
Layer 4 Transport Layer
Layer 3 Network layer
Layer 2 Data link layer
Layer 1 Physical layer
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to use an application that will communicate with the operating system of a server. A good example would
be an user using the web browser at his node to interact with the operating system on a server such as
Windows NT, which in turn gets the data user requested. This layer provides services to user applications,
this layer must provide a wealth of services. Establishing privacy mechanism, authenticating the intended
communication partners and determining if adequate resources are present is also the function of this
layer.
( To remember the seven layers of the OSI reference model memorize the sentence “ All People
Seem To Need Data Processing”. The beginning alphabet of each word corresponds to the significant
name of the layer )
6.10 INTERNET
The Internet literally means, “network of networks”. The Internet is a global network .It can be
described as an interconnection of millions of computers of different types belonging to various networks
from all over the globe. Information stored in these computers is available to millions of users across the
world. The variety of information available varies from the current research documents to the latest
sports and news feature. The information needed can be stored and retrieved via the Internet.
In the beginning of the Internet, complex commands and procedures were to be followed to connect to
the Internet. Tim Berners- Lee, a researcher at the European Laboratory for Particle Physics in Geneva,
invented the HTML (Hyper Text Markup Language). HTML is based on the concept of hyperlinks, a
point and click mechanism. The HTML concept allows you to link words/Graphics in one document with
other documents, no matter where the other document is stored. It may be in the same machine or on any
machine anywhere in the globe connected to the Internet. A user has to click on a phrase or a picture and
the Internet will fetch the appropriate documents and display it on the computer screen. HTML became
popular and all documents were developed in HTML. The collection of HTML documents became a
distinct domain on the Internet and is now known as the World Wide Web (WWW). Internet interface
software is known as browser. MOSAIC was the first web browser developed by Marc Anderson and
his colleagues at the University Of Illinois. With the browser the World Wide Web was born. The
browser encapsulated the complex series of commands that were needed to locate, browse and download
information from the Internet, thus leading the way for non-computer oriented people to use the net. At
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People Presentation layer
Seem Session layer
To Transport layer
Need Network layer
Data Data link layer
Processing Physical layer
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present we have many browsers available. Some of the popular ones are Internet Explorer, Netscape,
Mozilla.
The Internet is an information superhighway and has compressed the world into a cyber colony. The
Internet users has grown enormously, people browse the internet to get information on almost any topic
using the search engines like GOOGLE, Alta- Vista, YAHOO etc., it is used to find information about a
product, its cost and availability. Internet is used for banking, commerce, to purchase railway ticketing, to
purchase air ticket etc., e-banking, e-business, e-ticketing has now become very common. Communicating
through e-mail has become so popular and has made it a necessity for every one to log into the Internet.
The Internet has millions of computers attached to it. When want to communicate with a particular
computer it has to be addressed and keep track of the sender and the receiver. The Internet Protocol
addressing system does this. (IP addressing system). Each computer on the network is called a host and
has a name and a number. The IF addressing system uses the letter addressing system and the number
addressing system. The letter address is a series of words separated by dots. Eg:- firewall. try. com
The last three letters are important because they provide information about the kind of organization
that address belongs to. The table below gives some of the abbreviations and the information they provide.
Abbreviation Represents
edu Education
mil Military sites
org Professional Societies
gov Government Departments
com Commercial organization
The IP addressing system also uses country codes comprising of two letters. Some of the country
codes to name a few are: in for India, au for Australia, uk for United Kingdom, ca for Canada and so on.
This letter system of IP addressing is called the Domain Naming System (DNS).
Another method by which the IP addressing system functions is by making use of numeric addresses.
A numeric address has four parts and is referred to as Dotted Quad and is also called as OCTET. An
OCTET means eight bits. When all the eight bits are 1 it represents 255 in decimal system. The IP
address of the computer you normally use must be known as this will help in fast access. Eg: for numeric
address: 213.118.174.250
Internet is a packet switching network. The data to be transmitted is converted into small packets. The
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software that is responsible for making the Internet function efficiently is the TCP/IP. TCP stands for
Transmission Control Protocol and IP stands for Internet Protocol.
Transmission Control Protocol (TCP) breaks up the data into little packets before sending. It guarantees
that any data sent to the destination computer reaches it in tact. It makes the process appear as if one
computer is directly connected to the other, providing what appears to be a dedicated connection.
Internet Protocol (IP) is a set of conventions used to pass packets from one host computer to another.
It is responsible for routing the packets to a desired destination IP address. Packets are passed using
three kinds of mechanisms. They are Bridges, Routers and Gateways.
Bridges are used to connect two or more networks so that they appear as a single large network. The
bridge scans all the packets that come to it. When it sees a packet on one network that is destined for a
host on the other, it copies it over. Usually the host is not aware that a packet is being bridged. Bridges can
connect networks only of the same type.
Routers can connect two or more IF networks. The host on the IF networks is to be aware that router
is involved in the data transfer. Routers can attach physically different networks. Routers are slower than
bridges because they have to calculate and find how to route packets, especially if the networks operate
at different speeds.
Gateways splice together two different types of protocols to another.
The Internet Society governs the Internet. This is a voluntary membership organization. The purpose
of the organization is to promote global information exchange through the Internet technology. It appoints
a council of invited volunteers called the Internet Architecture Board (IAB). The IAB is responsible for
the technical management and direction of the Internet. The Internet works because there are standards
for computers and software applications to talk to each other. These standards allow computers from
different vendors to communicate with out problems. IAB meets regularly to formulate standards and
allocate resources like addresses. When a standard is required, it considers the problem, adopts a standard
and announces it via the Internet.
IAB also keeps track of the information that uniquely identifies every computer connected to the
Internet. For example each computer on the Internet has a unique 32-bit address. No two computers have
the same address. IAB ensures that certain rules are followed in naming each computer.
6.11 SUMMARY
The need for networking, networking topology and components used in network have been discussed.
Some basic terms in networking that are to be remembered:
Peer-to-Peer, this is commonly two PC connected together to share files or a printer.
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Local Area Network (LAN), this is probably the most popular in smaller applications. This
comes down to many computers interconnected in an office or home. This can be anywhere
from two computers or hundreds in one physical area.
Wide Area Network (WAN), boils down to computer networks that are far apart and connect
via phone, satellite, and so forth.
Node, this is just another name for the client computer or the computer using the network. A
personal computer connected to the internet is also a node.
Topology, is the geometric arrangement of the network. This is more or less how the network is
arranged.
Protocol, the protocol is a way of communication for the network. Think of it as trying to speak
French to an English computer. We need a protocol so they can understand one another.
Data Packets, messages of data that has been sent over your network.
Architecture’s, this comes down to peer to peer or the client server applications. In the peer to
peer we share resources of each other’s computers equally and the client server is one computer
with all the host files and data. Right now you are connecting to my server for this web page as
a client /server architecture.
Media, this amounts to how your network is physically connected. Most LAN’s connect with a
copper wire or cable coaxial. Some newer technologies also allow you to connect wireless using
your existing power outlets.
Note that WAN and LAN are not the opposite of each other but one of the same. A bunch of LAN’s
make a WAN. 
6.12 SELF TEST
1. A ____________ network does not have any dedicated servers
2. A _____________ is a computer on a network that functions as a server, and is not used as a work station.
3. In a _____________ topology ,data on the network is sent to all the computers on the network
4. A ______________ is a device used to centralize network traffic through a single connection point.
5. In a ___________ topology, computers are connected by cable segments to a centralized device .
6. Define access method
7. List three access method
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8. In a _______________ , a special packet called token circulates around the ring network in one direction.
9. List the functions of a network operating system.
10. Differentiate between client software and server software.
11. List the various components that allow you to expand the network.
12. List the differences between a router and a bridge.
13. State whether true or false
a. ATM switches are multiport devices that can act either as hubs or routers
b. Event log service records system, security, and program events under event log.
c. Server software makes it possible for users at other machines to share resources.
14.__________ move packets from one medium to another.
15. ________________ use dual- ring topology.
Solutions
1. Peer- to- Peer
2. dedicated Server
3. bus
4. hub
5. star
6. Access methods is a set of rules defining how computer puts data on to the network cable and retrieves data
from the cable
7. CSMA with CD and CA , token passing and demand priority.
8. Token passing method.
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Chapter 7
Multlmedla CommunlcatlonS
7.0 INTRODUCTION
M
ultimedia Communications is the field referring to the representation ,storage, retrieval and
dissemination of machine processable information expressed in multimedia, such as text ,image,
graphics, speech , audio, video, animation, handwriting and data files. With the advent of high-
capacity storage devices, powerful and yet economical workstations and high speed Integrated Services
Digital Network(ISDN), providing a variety of multimedia communication services is possible technically
and economically. In addition, the broadband ISDN (BISDN) has been given special attention as a next
generation communication network infrastructure. BISDN will be capable of transmitting full motion
pictures and high speed data at 150 and 600 MB/sec and voice, as well as data, throughout the world.
Multimedia best suits the human beings complex perceptions and communicating behavior, as well as
the way of acting. It provides communication capabilities and information sharing for people irrespective
of location and time, but it also provides easy and immediate access to widely distributed information bank
and information processing centers. Applications in medicine, education ,travel, real estate , banking,
insurance,administration and publishing are emerging at fast pace.These applications have large multimedia
documents that are to be communicated within very short delays. “Computer Controlled Co-operative
Work” is a new concept that is emerging now. In this a group of users can jointly view , create,edit and
discuss multimedia documents and this is the characteristic of many transactions.Some glamorous
applications in multimedia processing include distance learning, virtual library and living books. In distance
learning, we learn and interact with the instructors remotely across a broad band communications network.
Virtual library access means that a person can instantly have access to all of the published material in the
world, in its original form and format, and that we can browse ,display, print and even modify the material
instantaneously. Living books supplement the written words and the associated pictures with animations,
and hyperlinks.
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Applications are now enhanced or enabled by videos and hence there is a need for the development of
multimedia network. The trends towards multimedia communication are shown in figure. 7.1
Figure 7.1 Multimedia communication
7.1 ELEMENTS OF MULTIMEDIA SYSTEMS
There are two key communication modes used in multimedia communication they are : person- to-
person communication mode and person –to-machine communications Figure 7.2 below shows the key
elements of multimedia systems.
Figure 7.2 Person to Person mode
In the person-to-person mode shown in Figure 7.2 , there is a user interface that provides the
mechanisms for all users to interact with each other and there is a transport layer that moves the multimedia
signal from one user location to some or all other user locations associated with the communications. The
user interacts with the multimedia signal in an easy –to-use manner. The transport layer preserves the

Audio Communication
(telephony,sound broadcasting, etc.)









Video Communication
(Video Telephony TV/HDTV)

M
DATA,text,
image Communication
(data transfer, FAXetc.)
Multimedia
Communicatiotn
Multimedia Communications

User
Interface
Transport
User
Interface
Person- to-Person
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quality of the multimedia signals so that all users receive what they perceive to be of high-quality signals
at each user location.
Examples of person-to-person mode are teleconferencing ,video-phones, distance learning and shared
workspace scenarios.
The person –to- machine mode there is again a user-interface for interacting with the machine, along
with a transport layer for moving the multimedia signal from the storage location to the user. Figure 7.3
shows the Person-to-machine mode .There is also a mechanism for storage and retrieval of multimedia
signals that are either created by the user or requested by the user. The storage and retrieval mechanisms
involve browsing and searching to find existing multimedia data. Also, these mechanisms involve storage
and archiving in order to move user-created multimedia data to appropriate place of access by others.
Examples of such applications for person-to-machine mode include creation and access of business meeting
notes, access of broadcast video and document archives from digital library or other repositories.
Figure 7.3 Person-to-Machine mode
7.2 USER REQUIREMENTS
The user needs a multimedia communication system that prepares and presents information of interest,
allows for the dynamic control of applications and provides for natural interface. The requirements of
multimedia communications from user point of view are :
Fast preparation and presentation of the different information types of interest , taking into
account the capabilities of available terminals and services.
Dynamic control of multimedia applications with respect of connection interactions and quality
on demand combined with user-friendly human/machine interfaces
Intelligent support of users taking into consideration their individual capabilities
Standardization

Processing
Storing and
Retrieval
Transport
User
Interface
Person- to-Machine
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7.3 NETWORK REQUIREMENTS
The most important requirements of multimedia communications from the network point of view are
the following
High speed and changing bit rates
Several virtual connections using the same access
Synchronization of different information types
Suitable standardized services and supplementary service supporting multimedia applications.
7.4 PACKET TRANSFER CONCEPT
In Packet Switching the services video, voice and data can be dealt with a common format. Packet
Switching offers dynamic allocation of bandwidths and switching resources , as well as the elimination of
channel structure. Packet networks allow integrated service transport. They can carry voice ,video and
data using the same hardware protocols. Packet communications does not require users to allocate a
fixed channel or bandwidth before data transmission. Asynchronous Transfer Mode (ATM ) networks
use a fixed length packet and is suitable for high speed applications .Hence ATM networks are used in
multimedia communications.
7.5 MULTIMEDIA TERMINALS
Every major advance in networking has been preceded by an advance in the user interface . For
example, the invention of telephone preceded the growth of switch networks, invention of Television took
place before the growth of TV networks and cable television, the radio telephone led to the cellular
network, the PC led to the LAN /WAN network and the browser led to the growth of the internet and the
Web. For the multimedia , new smart terminals are to be created to facilitate displaying, accessing,
indexing , browsing and searching of multimedia content in a convenient and easy-to-use manner.
Multimedia terminals are needed to retrieve, analyze, store and broad cast the new forms of written
sound and visual content. The architecture of these systems can be defined based on the telecommunications
data processing and audiovisual technology. By incorporating voice and data as well as still and moving
pictures into their communications ,business has made functions increasingly sophisticated to improve
access to distributed resources and to save valuable time in the decision process. Remote dialog ,discussion,
information production, maintenance and inspection are also possible for new multimedia systems at less
operating costs.
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There are two types of terminals at present. Multifunction office or computer workstations and dedicated
equipment such as enhanced telephone terminals, video phones or shared teleconferencing systems.
Multimedia communications requires powerful terminals. Upgraded PC’s, desktop workstations or
video computers. PC’s can be upgraded by addition of telecommunication and video-audio capabilities
and the TV receivers can be upgraded by addition of intelligence and interactivity. High Definition TV
(HDTV) are being developed to have all digital TV. HDTV is a technology driver for memories, image/
video processors and flat Screens.
Multimedia terminal equipment also comprises suitable cameras, scanners, printers and mass storage.
Special equipments are necessary for editing multimedia information , that is, the creation , alteration and
deletion of contents and structures. Three dimensional devices and speech recognition systems will further
facilitate faster and easier human interaction with multimedia applications or editors.
7.6 SUMMARY
In this chapter we have discussed what multimedia is, its importance and the challenges ahead. The
multimedia communications is sure to change the way we live.
7.7 SELF TEST
1. What is multimedia communication ?
2. What are the elements in a multimedia system ?
3. What are the network requirements in multimedia ?
4. What are Multimedia terminals?
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REFERENCES
1. Digital Computer Fundamentals, VI Edition, Thomas C Bartee, Mc Graw Hill.
2 Computer Organisation, V. Carl Hamacher, Zvonko G Varnesic, Safwat G Zaky, Mc Graw Hill.
3. Computer Networks, IV Edition, Andrew S Tanenbaum, Prentice Hall.
4. www.howstuffworks.com