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A group of computers & other devices connected together is called a network & the concept of the connected computers sharing resources is called networking. Computers that are part of a network can share the following: -

& other hardware resources This list is constantly growing as new ways are found to share & communicate by means of computer.

Networking allows people to share resources such as files & to use interactive applications such as scheduling & e-mail. At its most eliminate level, a network consist of two computers connected to eachother by a cable so that they can share data. There are many benefits of networking:-

Organization implement networks primarily to share resources & enable online communication. Resources include data, applications & peripherals. A peripheral is a device such as an external disc drive, printers, mouse, modem or joystick. Online communication includes sending messages back & forth, or e-mail.

LAN (Local Area Network):-A local area network consists of several computers & the peripherals cabled together in a limited area, such as a department of company or a single building. Network started out small with perhaps ten computers connected together with a printer, the technology limited the size of the network including number of computers connected as well as the physical distance that could be covered by the network. This type of network within a limited area is known as LAN (Local Area Network)

The bus topology is also known as leaner BUS, this is the simplest & the most common method of networking. It consists of a single cable called a trunk (also backbone) or segment that connects all of the computers in the network in a single line. Computers or a bus topology communicate by addressing data or a particular computers & putting the data on cable in the form of electronic signals to understand how computers communicate on a bus you need to be familiar with three concepts:- 1 Sending the signals 2 Signal bounce 3 The terminator

In star topology computers are connected by cable segment to a centralized component called a HUB. Signals are transmitted from the sending computer through the HUB to all computers on the network, the star network offers a centralized resources & management, however each computer is connected to a central point called a HUB. If the central point fails the entire network goes down. The star network only the failed computer will not be able to send or receive data. The rest of the network continuous to function normally. HUB One network component that's becoming standard equipment in more and more networks is the hub. A hub is the central component in the star topology.

ACTIVE HUBS:- Most hubs are active in that they regenerate and retransmit the signals the same way a repeater does. Active hubs require electrical power to run.

PASSIVE HUBS:- Some types of hubs are passive, for example, wiring panels or punchdown blocks. Passive hubs don't require electrical power to run. HYBRID HUBS:- Advanced hubs that will accommodate several different types of cables are called hybrid hubs. Connecting more than one hub can expand a hub-based network.

The vast majority of networks today are connected by some sort of cable wires or cabling, which acts as the network transmission medium carrying signals b/w computer. There's a variety of cables that can meet the varying needs & sizes of network from small to large. There are more than 2200 types of cables, fortunately only three major groups of cabling connect the majority of networks:-

At one type Coaxil cable was the most widel used network cabling. Coaxil was relatively inexpensive & it was light, flexible & easy to work with. It was so popular that to become a safe & easy supported installation. In its simplest form Coaxil consist of a core made of solid copper surrounded by isolation, a braided metal shielding & an outer cover. One layer of foil insolation & one layer of braided metal shielding is referred to as dual shielding. However quad shielding is available for environments that are subject to higher interference. Quad shielding consist of two layers of foil insolation & two layers of braided metal shielding. Shielding refers to the woven or stranded metal mesh or other material that surrounds some types of cabling. The core of coaxial cable carries the electronic signals, which make up the data. This core wire can be either solid or stranded. If the core is solid its usually copper. The core surrounded by an insulating layer, which separates it from the wire mesh. The braided wire mesh as a ground & protects the core from electrical noise & cross talk. Cross talk signals are signal overflow from an adjacent wire.

1 THIN (thinnet):-Thinnet is a flexible coaxial cable about 0.25 inch thick. Because this type of a coaxial cable is flexible & easy to work with it can be used in almost any type of network installation. Thinnet coaxial cable can carry a signal up to approximately 185 meters (607 feet). Before the signal starts to suffer from attenuation. Thinnet is included in a group referred to as the RG-58 family & has 50-ohm impedance.

2 THICKNET:-Thicknet is relatively rigid coaxial cable about 0.5 inch in diameter. The copper core is thicker then the thinnet core. The thicker the copper core the farther the cable can carry signals. Thicknet can carry a signal for 500 meters (1640 feet) THINNET v/s THICKNET:- As a general rule the thicker the cable the more difficult its to work with. Thin cable is flexible easy to install & relatively inexpensive. Thick cable doesn't bend easily & is therefore hard to install. Thick cable is more expensive then the thin cable but will carry a signal farther.

In 1984 the ISO (International Standard Organization) released a version of the model & called it the Open System Interconnection (OSI) reference model. This model is the best known & most widely used guide to describe networking environments. Vendors design network products based on the specifications of the OSI model. A LAYERED ARCHITECTURE The OSI model is an architecture that divides network communication into seven layer covers different network activities, equipment or protocols. Each OSI layer has well defined networking function & the function of each layer communicate & work with the function of the layer immediately above & below it. The lowest layers 1 (physical) & 2 (data link) defines the networks physical media & related task such as putting data bits onto the network adapter cards & cables. The highest layers 7 (application) & 6 (presentation) defines how applications access communication services each layer provides some service or action that prepares the data for delivery over the network to another computer. The layers are separated from eachother boundaries called interface.

Layer 7 the top most layer of the OSI model is the application layer. It serves as the window for application process to access network services. This layer represents the services that directly supports user application such as software for file transfer, for database access & for e-mail. The application layer handles general network access, flow control & error recover.

2 PRESENTATION:-Layer 6 the presentation layer determine the format used to exchange the data among network computer. It can be called the network translator the presentation layer is responsible for protocol conversion, translating the data, encrypting the data changing or converting the character set & expanding graphics commands.

3 SESSION:-Layer 5 the session layer allows two applications on different computers to establish use & end collection called session. This layer performs more recognition & the function such as security, needed to allow two applications to communicate over the network. This layer also implements dialogue control between communication processes, regulating which side transmits when, for how long & so on.

Layer 4 the transport layer provides an additional connection level beneath the session layer. The transport layer in sure that packets are delivered error free, in sequence & with no loose or duplications. The transport layer provides flow control, error handling & is involved in solving problems concerned with the transmission & reception of packets.

Layer 3 the network layer is responsible for addressing messages & translating logical addresses & names into the physical addresses. This layer also determines the route from the source to destination computer. It also manages traffic problems on the network, such as packet switching, routing, and controlling the congestion of data.

Layer 2, the data link layer, sends data frames from the network layer to the physical layer. The data link layer is responsible for providing the error-free transfer of frames from one computer to another through the physical layer. This allows the network layer to assume virtually error-free transmission over the network connection.

Layer 3, the bottommost layer of the OSI model, is the physical layer. The physical layer relates the electrical, optical, mechanical and functional interfaces to the cable. The physical layer also carries the signals that transmit the data generated by all of the higher layers. The physical layer is responsible for transmitting bits (zeros and ones) from one computer to another.

In the late 1970s, when LANs first began to emerge as a potential business tool, the IEEE realized that there was a need to define certain LAN standards. To accomplish this task, the IEEE launched what became known as Project 802, named for the year and month it began (1980, Feb.) Project 802 defined network standards for the physical component of network-the interface card and cabling-which are connected for in the physical and data link layer of the OSI model. These standards, called the 802 specifications, have several areas of responsibility including: � Network adapter cards. � Wide area network components. � Components used to create twisted-pair and coaxial cable networks. IEEE 802 CATEGORIES:- The LAN standards the 802 committees defined fall into 12 categories, which can be identified by their 802 number as follows:

802.3 Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) LAN (Ethernet)

Protocols are rules and procedures for communicating. When several computers are networked, the rules and technical procedures governing their communication and interaction are called protocols. There are three points to keep on mind when thinking about protocols in a network environment: 1 There are many protocols. While each protocol allows basic communication, they have different purposes and accomplish different tasks. Each protocol Have its own advantages and restrictions. 2 Some protocols work at various OSI layers. The layer at which a protocol works describes its function. 3 Several protocols may work together in what is known as a protocol stack, or suite.

The entire technical operation of transmitting data over the network has to be broken down into discrete systematic steps. At each step, certain actions take place, which can not take place at any other step. Each step has its own rules and procedures, or protocol. In the sending computer, these steps must be carried out from the top down. In the receiving machine, these steps must be carried out the bottom up.

At the sending computer, the protocol: � Breaks the data into smaller sections, called packets, that the protocol can handle � Adds addressing information to the packets so the destination computer on the network will know the data belongs to it. � Prepares the data for actual transmission through the network adapter card and out onto the network cable.

At the receiving computer, a protocol carries out the same series of steps in reverse order. The receiving computer: � Takes the data packets off the cable. � Brings the data packets into the computer through the network adapter card. � Strips the data packets of all the transmitting information added by sending computer. � Copies the data from the packets to a buffer for reassemble. � Passes the reassembled data to the application in a usable form. Both the sending and receiving computers need to perform each step the same way so that the data will look the same when its received as it did when it was sent. X.25 X.25 is a set of protocols incorporated in a packet switching network made up of switching services. The switching services were originally established to connect remote terminals to main frame host systems.

A modem is a device that makes it possible for computers to communicate over a telephone line. When computers are too far apart to be joined by a standard computer cable, a modem can enable communicating b/w them. In a network environment, modems serve as a means of communicating b/w networks or connecting to the world beyond the local network.

Computers can not simply connect over a telephone line b/c the computer communicates in digital electronic in digital pulses (electronic signals) and a telephone line can only send analog pulses (sound). A digital signal is synonymous with binary, and the signal can only have a value of 0 or 1. An analog signal is smooth curve, which can represent an infinite range of values. Modems at the sending end MOdulates digital signals into analog signals, and receiving modem DEModulates analog signals back into digital signals.

Asynchronous, or async, may be the most widespread form of connectivity in the world. This is b/c async was developed so it could use common telephone lines. Communication is not synchronized. There is no clocking device or method to coordinate transmission b/w the sender and the receiver. The sending computer just sends data, and the receiving computer just receives the data. The receiving computer then checks to make sure that the received data matches what was sent. Twenty-five percent of the data traffic in async communications consists of data traffic control and coordination.

Synchronous communication relies on a timing scheme coordinated b/w two devices to separate groups of bits and transmit them in blocks known as frames. Synchronous protocols perform a number of jobs that asynchronous protocols do not. Synchronous protocols: � Format data into blocks. � Add control information. � Check information to provide error control. Synchronous communications are used in almost all digital and network communications.

HOW REPEATERS WORK:- A repeater works at the OSI physical layer to regenerate the network's signals and resend them out on other segments. A repeater takes a weak signal from one segment, regenerates it, and passes it to the next segment. Repeaters don't translate or filter anything. For a repeater to work, both segments that the repeater joins must have the same access method. A repeater: � Connects two segments of similar or dissimilar media. � Regenerates the signal to increase the distance transmitted. � Functions in the physical layer of the OSI model. � Passes all traffic in both directions. Use a repeater when you want to connect two segments in the most cost-effective manner. Repeaters improve performance by dividing the network into segments, thus reducing the number of computers per segment.

Like a repeater, a bridge can join segments or workgroup LANs. However, a bridge can also divide a network to isolate traffic or problems. Bridges can be used to: � Expand the distance of a segment. � Provide for an increased number of computers on the network. � Reduce traffic bottlenecks resulting from an excessive number of attached computers. � Link unlike physical media such as twisted-pair and coaxial Ethernet. � Link unlike network segments such as Ethernet and token ring, and froward packets b/w them.

In an environment consisting of several network segments with differing protocols and architectures, a bridge may not be adequate for ensuring fast communication among all of the segments. A network this complex needs a device, which not only knows the address of each segment, but can also determine the best path for sending data and filtering broadcast traffic to the local segment. Such a device is called router. Routers can provide the following functions of a bridge: � Filtering and isolating traffic. � Connecting network segments.

HOW ROUTERS WORK:- The routing table found in routers contains network addresses. However, host addresses may be kept depending on the protocol network is running. A router uses table to determine the destination address for incoming data. The table lists the following information: � All known network addresses � How to connect to other networks � The possible paths b/w those routers � The costs of sending data over those paths The router selects the best route for the data based on costs and available paths. When routers receive packets destined for a remote network, they send them to the router that manages the destination network. In some ways this is an advantage b/c it means routers can: � Segment large networks into smaller ones. � Act as a safety barrier b/w segments. � Prohibit broadcast storms, b/c broadcast are not forwarded.

ROUTABLE PROTOCOLS:- Not all protocols work with routers. The ones that are routable include: � DECnet � IP � IPX � OSI � XNS � DDP (Apple TalK) Protocols, which are not routable, include: � LAT (local area transport, a protocol from Digital Equipment Corporation.) � NetBEUI

TYPES OF ROUTERS:-The two major types of routers are: � STATIC: Static routers require an administrator to manually set up and configuration the routing table and to specify each route. � DYNAMIC Dynamic routers do an automatic discovery of routes and therefore have a minimal amount of set up configuration. They are more sophisticated in that they examine information from other routers and make packet-by-packet decisions about how to send data across the network.

Gateways communication possible b/w different architectures and environments. A gateway links two systems that do not use the same: � Communication protocols � Data formatting structures � Languages � Architecture Gateway performs protocol and conversion. Some limitations of gateways are: � They are task-specific � They can be slow � They are expensive

The world wide web (the web) is the internet's multimedia service that contains a vast storehouse of hypertext documents written using Hypertext Markup Language (HTML). The hypertext format allows the user to browse through topics in any order. There are tools and protocols that help you explore the internet. These tools help you locate and transport resources b/w computers.

FTP support is one method of supporting remote networks. It's a protocol which allows simple file transfers of documents. There are FTP servers which provide vast amounts of information stored as files. It's a file transfer program for TCP/IP environments and is implemented at the application layer of the OSI model.

Importance of Computer

Computer is important in this contemporary world of human efforts. The frequent use of computer rapidly changing the nature of work. Its impact is being felt in industries, banks, education etc. to solve the complicated and time consuming problems. Computer can improve the efficiency of an organization. The business person employing a computer can trim unnecessary over time reducing dependency upon unreliable person or outside agencies.

The importance and capabilities of computer can be discussed under three heads.

Speed
Accuracy
Reliability

SPEED

Computer is an electronic data processing machine which can perform arithmetic and logical operations with very much speed. It can perform million instructions per second. That would take a person year to complete. To classify the speed of different computers the manufacturer has the criteria of (MIPS) million instructions per second.

ACCURACY

The computer must process the data accurately. Accuracy is the prime consideration of installing computer. Computers are universally accused of making mistakes on bills, statements and etc. The probability of computer errors are quite often and traceable.

RELIABILITY

Computer system are widely accept because of there exceptional reliability. Computer constantly provide the same accurate results under all operating conditions.

Types of Computer

ANALOG COMPUTERS

Analog computers are used to process continuous data. Analog computers represent variables by physical quantities. Thus any computer which solve problem by translating physical conditions such as flow, temperature, pressure, angular position or voltage into related mechanical or electrical related circuits as an analog for the physical phenomenon being investigated in general it is a computer which uses an analog quantity and produces analog values as output. Thus an analog computer measures continuously. Analog computers are very much speedy. They produce their results very fast. But their results are approximately correct. All the analog computers are special purpose computers.

DIGITAL COMPUTERS

A computer which process information represented by combination of *** or discontinuous data. It is a device for performing sequence of arithmetic and logical operations, not only on data but on its ***. Digital computer that is capable of performing sequences of internally stored instructions for calculation. Digital computers are not very much fast but their results are very much accurate. All the digital computers are general purpose computers.

HYBRID COMPUTERS

Various specifically designed computers are with both digital and analog characteristics combining the advantages of analog and digital computers when working as a system. Hybrid computers are being used extensively in process control system where it is necessary to have a close representation with the physical world.

The hybrid system provides the good precision that can be attained with analog computers and the greater control that is possible with digital computers, plus the ability to accept the input data in either form.

CLASSIFICATION OF COMPUTERS

MAINFRAME COMPUTERS

The most expensive, largest and the most quickest or speedy computer are called mainframe computers. These computers are used in large companies, factories, organizations etc. the mainframe computers are the most expensive computers, they cost more than 20 million rupees. In this computers 150 users are able to work on one C.P.U. The mainframes are able to process 1 to 8 bits at a time. They have several hundreds of megabytes of primary storage and operate at a speed measured in nano second.

MINI COMPUTERS

Mini computers are smaller than mainframes, both in size and other facilities such as speed, storage capacity and other services. They are versatile that they can be fitted where ever they are needed. Their speeds are rated between one and fifty million instructions per second (MIPS). They have primary storage in hundred to three hundred megabytes range with direct access storage device.

MICRO COMPUTERS

These are the smallest range of computers. They were introduced in the early 70’s having less storing space and processing speed. Micro computers of todays are equivalent to the mini computers of yesterday in terms of performing and processing. They are also called "computer of a chip" because its entire circuitry is contained in one tiny chip. The micro computers have a wide range of applications including uses as portable computer that can be plugged into any wall.

DATA PROCESSING

Data processing often referred as "D.P" is the process collecting data together and converting the data into information. The method used for collecting data may be manual, semi manual, mechanical or electronic.

Data processing is a term mostly associated with business and commercial words. Since computer are being used in the processing of data. The term E.D.P (Electronic Data Processing) may also be used.

ELECTRONIC DATA PROCESSING

Data processing means transformation of data for producing meaningful results for carrying out scientific, business activities. The result of data processing is called "INFORMATION".

The transformation of data consist of a sequence of operations. The sequence is called "PROCEDURE". A data processing is a system which takes data as an input, carries out the required processing on data and produces the information. The system is called "MANUAL" when processing is performed by human beings and "AUTOMATIC" when machines are used. When computers are used for data processing.

Compiler

Since the computer hardware is capable of understanding only machine level instructions so it is necessary to convert the instruction of a programme written in high level language to machine instruction before the programme can be executed by the computer. This job is carried out by compiler.

A compiler is translating programme that translates the instruction of a high level language into machine language. A high level programme is called a source programme. After the source programme has to be converted into machine language by a compiler. It is referred to as an object programme.

A compiler can translate only those programme which has been written in the language for which the computer is meant e.g., FORTRAN compiler is only capable of translating source programme which have been written in FORTRAN. Each machine requires a separate compiler for each high level language.

The compiler analysis each statement in the source programme and generate a sequence of machine instruction. As the compiler analysis each statement it uncovers certain types of errors. Compiler can diagnose the following kinds of errors in a source programme.

Illegal characters.
Improper sequence of instruction in a programme.

A source programme containing an error diagnosed by compiler will not be compiled into an object programme. The compiler will print out (display) a suitable message.

Micro Computers

The electronic logic of micro computer is based on a single printed circuit board which is stored in a case and provided with a keyboard for data entry and a monitor. The I/O logic may support a range of peripherals such as printer, disk, mouse etc. Microcomputer have operating system and peripheral connectivity to support microcomputers.

Compiler

Illegal characters.
Improper sequence of instruction in a programme.

A source programme containing an error diagnosed by compiler will not be compiled into an object programme. The compiler will print out (display) a suitable message.

Computer Languages

Language is a system for representation and communication of information or data. Like human beings, a language or signal is required to communicate between two persons. Similarly, we cannot obtain any result by computer without langtage. Computer does not understand directly what we are communicating with computer as English or Arabic, it understands only machine language (binary codes 0-1). Computer translates English language into machine codes through interpreter then process instructions and give us the results.

The computer languages can be divided into two main levels.

Machine language (0-1)
Symbolic language (A-Z)

Symbolic languages are further divide into two main levels

High-level language
Low-level language

MACHINE LANGUAGE

Although computers can be programmed to understand many different computer language. There is only one language understood by the computer without using a translation program. This language is called the machine language or the machine codes. Machine codes are the fundamental language of the computer and is normally written as strings of binary 0-1.

ADVANTAGES AND LIMITATIONS OF MACHINE LANGUAGE

Programs written in machine language can be executed very fast by the computer. This is mainly because machine instructions are directly understood by the CPU and no translation of program is required.

However, writing a program in machine language has several disadvantage.

MACHINE DEPENDENT

Because the internal design of every type of computer is different from every other type of computer and needs different electrical signals to operate. The machine language also is different from computer to computer.

DIFFICULT TO PROGRAM

Although easily used by the computer, machine language is difficult to program. It is necessary for the programmer either to memorize the dozens of code number for the commands in the machine’s instruction set or to constantly refer to a reference card.

DIFFICULT TO MODIFY

It is difficult to correct or modify machine language programs. Checking machine instructions to locate errors is difficult as writing them initially.

In short, writing a program in machine language is so difficult and time consuming.

SYMBOLIC LANGUAGES

In symbolic languages, alphabets are used (a-z). symbolic languages are further divide into two main levels.

High level languages
Low level languages

LOW LEVEL LANGUAGE

A language which is one step higher than machine language in human readability is called Assembly Language or a low-level language. In an assembly language binary numbers are replaced by human readable symbols called mnemonics. Thus a low-level language is better in understanding than a machine language for humans and almost has the same efficiency as machine language for computer operation. An assembly language is a combination of mnemonic, operation codes and symbolic codes for addresses. Each computer uses and has a mnemonic code for each instruction, which may vary from computer to computer. Some of the commonly used codes are given in the following table.

COMMAND NAME MNEMONIC

Add ADD

Subtract SUB

Multiply MUL

Compare Registry CR

Compare COMP

Branch Condition BC

Code Register LR

Move Characters MVE

Store Characters STC

Store Accumulator STA

An assembly language is very efficient but it is difficult to work with and it requires good skills for programming. A program written in an assembly language is translated into a machine language before execution. A computer program which translates any assembly language into its equivalent machine code is known as an assembler.

HIGH – LEVEL LANGUAGE

A language is one step higher than low-level languages in human readability is called high-level language. High – level languages are easy to understand. They are also called English oriented languages in which instruction are given using words. Such as add, subtract, input, print, etc. high level language are very easy for programming, programmer prefer them for software designing that’s why these languages are also called user’s friendly languages. Every high level language must be converted into machine language before execution, therefore every high level language has its own separate translating program called compiler or interpreter. That’s why some time these languages are called compiler langauges. COBOL, BASIC, PASCAL, RPG, FORTRAN are some high level languages.

INTERPRETER

An interpreter is a set of programs which translates the high-level language into machine acceptable form. The interpreters are slow in speed as compared to compilers. The interpreter takes a single line of the source code, translates that line into object code and carries it out immediately. The process is repeated line by line until the whole program has been translated and run. If the program loops back to earlier statements, they will be translated afresh each time round. This means that both the source program and the interpreter must remain in the main memory together which may limit the space available for data. Perhaps the biggest drawback of an interpreter is the time it takes to translate and run a program including all the repetition which can be involved.

Input Devices

Input devices are used to provide data or information to the computer. The computer follows the instructions given to it by and input device. A variety of input devices are used with the computer depending on the type and purpose of input information. For example, a keyboard is commonly used to transfer data or information from human readable form to machine readable form. Other examples of input devices are: mouse, joystick, trackball, light pens, digitizers, scanners, optical character reader (OCR), touch window, etc.

KEYBOARD

A keyboard is the most commonly used input device which helps us in simply keying in required information in a computer. This information is subsequently stored in the computer’s memory. A keyboard can be used effectively to communicate with the computer but considered to be relatively slow as compared to other input devices. The keyboard is divided into following divisions:

ALPHABETIC KEYPAD

These keys are similar to a standard typewriter and is used to type general information.

NUMBERIC KEYPAD

These keys are used to input numeric data only. These are very useful in case of large numeric data input because all numeric keys can be accessed by one hand only. These keys can also be used as an alternative to the screen navigation and editing keys.

FUNCTION KEYS

These are keys marked as F1 - F12, located normally at the top of the keyboard. These are special keys provided to a programmer which allow him to attach special functions to each key. Each of these function keys are also given some special function in different packages.

SCREEN NAVIGATION AND EDITING KEYS

These keys are provided to move around in the screen. May programs use these keys to let the user move around the screen display. In some keyboards these keys are also provided inside the numeric keypad as alternate keys.

A computer peripheral that puts text or a computer generated image on paper or on another medium, such as a transparency. Printer can be categorized in several ways the most common distinction is IMPACT and NON-IMPACT.

IMPACT PRINTING

Is the method used by the conventional type writers. In some type of impact printing a metal "hammer" embossed with a character strikes a print ribbon, which presses the characters image into paper. In other types the hammer strikes the paper and presses it into the ribbon characters created through impact printing can be formed by either a solid font or dot matrix printing mechanism.

NON – IMPACT PRINTING

Does not depend on the impact of metal on paper. In fact no physical contact at all occurs between the printing mechanism and the paper. The most popular non-impact methods today utilize thermal transfer, ink-jet.

DOT MATRIX PRINTER

Any printer that produces character made up of dots using a wire pin printed head. The quality of output from a dot matrix printer depends largely on the number of dots in the matrix, which might be low enough to show individual dots or might be high enough to approach the look of fully formed characters. Dot matrix printers are often categorized by the number of pins in the printer head typically, 9 or 24.

LINE PRINTERS

Any printer that prints one line at one time, as opposed to one character at a time or one page at a time. Line printer typically produce the 11 by 17 inch "computer" printout. They are high speed devices and are often used with mainframes, minicomputers, or networked machines rather than with single user system. Types of line printers include chain printers and band printer.

LASER PRINTERS

An electrophotographic printer that is based on the technology used by photocopiers. A focussed laser beam and a rotating mirror are used to draw an image of the desired page on a photosensitive drum. This image is converted on the drum into an electrostatic charge, which attracts and holds toner. A piece of electrostatically charged paper is rolled against the drum, which pulls the toner away from the drum and onto the paper. Heat is then applied to fuse the toner to the paper. Finally, the electrified charge is removed from the drum and the excess toner is collected. By omitting the final step and repeating only the toner application and paper handling steps, the printer can make multiple copies.

DAISY WHEEL PRINTER

Daisy wheel printer are some times called letter quality printer because they are often used to produce attractive correspondence. The D.W.P is a flat circular device made of metal with character embossed on it. As this wheel spins at a very high speed the hammer hits the specific character against the ribbon which presses against the paper.

THERMAL TRANSFER PRINTER

It is a kind of non-impact printer. In electrothermal printing, characters are burned on to a special paper by heated rods on a print heat. They transfer ink from a wax-based ribbon onto plain paper. These printer can support high quality graphic.

INK – JET PRINTER

It is a kind of non-impact spray small dots of electrically charged ink onto a paper to form images. Ink jet printer are flexible enough to be used as plotters.

Secondary Storage Devices

Secondary storage devices are also called backup storage because it is used to store data. Volume of data on permanent basis which can be partially transferred to the primary storage, when required for data processing. Afterwards these devices are comparatively cheap and provide greater space to store the data /instructions are stored on secondary storage devices in the same binary codes as in primary storage.

RANDOM ACCESS DEVICES

Random Access Devices are those devices on which we can directly access the data. These devices are comparatively provide the fast communication.

For example, hard disk, floppy disk, optical disk.

SEQUENTIAL ACCESS DEVICES

Sequential Access Devices are those in which we can access the data one by one in a sequence. These devices provide slow communication as compared to Random Access Device.

NEEDS OF SECONDARY STORAGE DEVICE

The storage capacity of the primary storage of today’s computer is not sufficient. To store a large volume of data as a result additional memory called secondary storage is needed with most of the computer system.

These devices also provides the fast communication than I/O devices. The internal memory of a computer is a volatile memory. Therefore, we cannot save the data permanently. In that case we require secondary storage device which provide the facility to store the data for future use.

FLOPPY DISK

A floppy disk, also called simply a diskette or disk, is a small flexible Mylar disk coated with iron oxide on which data are stored. The floppy disk has been around since early 1970s, today it is available in three 3� inch, 5� inch and 8 inch sizes. The 5� and 8 inch diskettes are covered by stiff protective jacket with different holes. The central big hole called hub ring which is used to hold by disk drive during rotation. The elongated read write window is used to read and write data through read/write head. The small hole next to the hub ring is called index hole which is used to locating data through computer. The cut out on the side of the floppy disk is called write protect notch. If we cover this opening with a piece of paper then we can’t write data on to disk.

In small diskette a hard plastic cover and protective metal is used to protect disk. Before using a disk we have to format a disk in which disk is divided into tracks and sectors for storing the data. Diskettes may be double sided and single sided while the storage capacity become less or more.

Floppy diskettes are more convenient to use with microcomputers. A floppy disk which is a random access device can access data fast than magnetic tape.

MAGNETIC TAPE

Magnetic tape is a sequential access device about one half or one fourth inch in size and made of Mylar (a plastic material) coated with a thin layer of iron oxide. Data can be read and write through a device which is called tape drive. The read/write head of tape drive which is an electromagnetic component read, write and erase data from magnetic tape. Magnetic tape is divided into nine separate strips or tracks in which eight tracks are used to store data and ninth track is used for error checking bit.

Magnetic tape can store large quantities of data therefore they are erasable, usable and durable secondary storage device. But it can use with large computers.

Hard disk

Hard disk is a thin circular metal plate coated both side with a magnetic material. A hard disk pack consist of a number of disk mounted on central shaft which rotate at a speed of 2400 rpm or more. In a hard disk information is stored on both the surfaces of each disk plate except the upper and the lower surfaces of the bottom plate which are not used. Information is recorded on the track of the disk surfaces in the form of invisible tiny magnetic spot. The presence of a magnetic spot represent 1 bit and its absence represents 0 bit.

Hard disk are potentially very high capacity storage devices typically in the range of 20 megabyte to 1 gigabyte. Data are recorded on the tracks of a spinning disk surface and read from the surface by one or more read/write heads. There are two basic types of disk system.

Moving head
Fixed head

MOVING HEAD

The moving head consist of one read/write head for each disk surface mounted on an axis and which can be moved in and out. In this system each read/write head moves horizontally across the surface of the disk. So that it is able to access each track individually. Information stored on the tracks which constitute a cylindrical shape through the disk pack are therefore accessed simultaneously.

FIXED HEAD

In the fixed head system the axis are non-movable. A large number of read/write heads are distributed over the disk surfaces. One head for each track as a result no head movement is required and therefore information is accessed more quickly.

Flow Chart

Flow chart is a symbolic representation of flow of programme. It can graphically represent data processing procedure Flow chart serves two purposes, i.e., the planning of the program structure as an aid in writing the computer program. Secondly the documentation of the program logic and work flow for the purpose of the communication with other person and to recall a program at a later time. If the program needs to be changed. The second purpose is very important because it is very difficult to check the logic of the program.

TERMINAL

The terminal symbol is used to indicate the beginning, ending of the program logic flow. It is the first and last symbol of the flow chart.

INPUT / OUTPUT BOX

The input/output box is used to denote any function of an input and output in the program. If there is a program instruction to input or output the data from any I/O device that step will be indicated on this symbol.

PROCESS BOX

A processing symbol is used in a flow chart to represent arithmetical and data movement instructions.

FLOW LINES

Flow lines are used to indicate the flow of operations. The exact sequence in which the instructions are to be executed. The flow of flow chart is normally from top to bottom and left to right.

DECISION BOX

The decision box is used in a flow chart to indicate a point at which a decision has to be made and to branch to one or more alternative points that is possible during execution. The appropriate path followed, depends on the result of the decision.

CONNECTOR

A flow chart becomes complex, when the number and direction of flow lines is confusing or it spread over more than one page. It is useful to utilize the connector symbol as a substitute for flow lines.

SYSTEM FLOW CHART

System Flow Chart is used to describe complete data processing cycle including the hardware devices and media used. A System Flow Chart shows flow of data in a system where and in what form it is received by the system, how it is input to the computer and what storage media are used to hold it. The symbol used in system flow chart indicates the peripherals to be used in data processing procedure, where output will be produced. System flow chart do not explain the logic of the programs.