Information Technology Notes

INFORMATION TECHNOLOGY NOTES

DATA AND INFORMATION

Data processing cycle Validation Checks Input device Storage devices- how updating is done on direct access and sequential (grand father Primary Storage Storage hierarchy pyramid Key to disk and key to diskette

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INFORMATION TECHNOLOGY NOTES Information Technology Definition – It is an organized combination and use of hardware, software, telecommunications, database management and other information processing technologies used in a computer-based information system. Information technology transforms data into a variety of useful information products specifically by the use of a computer. Computer - It is a device that has the ability to accept data, internally store and execute a program of instructions, perform mathematical, logical and manipulative operations on the data and reports on the results. Put simply, it is a machine that accepts data (input) and processes it into useful information (output). Computer System It is an interrelated system of input, processing, output, storage and control components Thus a computer system consist of input and output devices, primary and storage devices, the central processing unit, the control unit within the CPU and other peripherals Data and Information concepts The terms data and information are loosely used interchangeably in ordinary discussions. The terms, however, are different in their usage in the field of information systems. Data – it is the complete range of facts, events, transactions, and opinions, judgments that exist both within and outside the organization. Data are raw facts from which information is produced. Information – it is part of the total data available which is appropriate to the requirements of a particular user or group of users. It is processed data upon which a user may rely for decision. A computer performs the following basic functions: (a) Input – computer accepts and captures data from outside for processing (b) Processing – a computer performs operations on data it holds (c) Storage – a computer holds data before, during and after processing (d) Output – a computer produces data for use (information) 2

COMPONENTS AND CONFIGURATION OF A COMPUTER Computer Hardware- refers to the physical components of a computer both mechanical and electronic. A computer is made up of the following components. Input devices Central Processing Unit Memory Secondary Storage unit Output devices

The configuration of a basic computer system

INPUT DEVICES/METHODS These facilitate communication between the user and the computer. They facilitate the insertion of data into the computer for processing. (a) Keyboard A keyboard is laid out like a typewriter It allows data to be typed in by the pressing of the relevant keys The pressed key or instruction is displayed or executed This is the most commonly used input device (Study Key-to-disk systems, Key-to-diskette systems –Refer to Clifton) (b) Mouse It is a hand held pointing device electronically connected to the computer which is used to control the cursor or a pointer on the screen through the rolling motion of a ball on a flat surface. The cursor or pointer on the video screen moves in the same direction as the movement of the mouse. When the pointer is on the required menu item (icon) a button is clicked to select that item. 3

(c) Light Pen It is a pen-shaped device that uses photoelectric circuitry to enter data into the computer through a special video screen. A user can write on the video display ⇒ The high sensitive pen enables the computer to calculate the coordinates of the points on the screen touched by the light pen ⇒ The handwriting or graphic is digitalized, accepted as input and displayed on the VDU ⇒ Input therefore is directly onto the screen (d) Touch Screens Use an inlaid screen to accept input through the act of physically touching the screen The computer senses the selected position and execute the instruction accordingly This device works more suitably with menu driven applications (e) Optical Character Readers ⇒ These use photoelectric cells to measure the amount of light reflected from a particular set of characters in a particular font ⇒ It then transfers the details to a computer ⇒ The reader incorporates a light beam which follows the outline of the characters being read ⇒ It compares these with stored reference characters ⇒ The most common fonts are OCR-A (American)and OCR-B (European) (f) Optical Mark Readers (OMRs) ⇒ These are similar to OCRs but can detect marks, lines, perforations rather than characters ⇒ They work with specific programs to read and interpret a particular document (so are less flexible than OCRs) ⇒ OMRs are used to extract data from surveys, questionnaire forms or mark multiple choice exams ⇒ OMRs are also used in the processing of orders, processing of time sheets for workers, inputting diagrams into the computer. (g) Magnetic Ink Character Readers (MICR) ⇒ They use pre-printed human readable characters ⇒ The characters are printed in a special ink mixed with iron oxide ⇒ Before reading the document passes through a magnetic field and the characters get magnetized ⇒ The machine reader recognizes the character by its magnetic field pattern, codes it and provides input into the computer. 4

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This method is not in general use because the print has got to be made in a special way to be readable This method of input is widely used in the banking sector to process documents like cheques.

(h) Page Scanners ⇒ These devices read documents, digitalise them and sent them for processing by the computer ⇒ These are like photocopying machines that can read a wide variety of text and graphics including photographs. (i) Bar Code Readers ⇒ Bar coding is the representation of code numbers or other data in the form of optical or magnetic bars on a data carrier (badge) which may be tag, label, ticket, plastic card ⇒ A bar code reader reads the bar coding and allows the processing of information relating to the data ⇒ The bar code reader is usually hand held and uses a laser beam to read. ⇒ When a bar code is read and recognized, the scanner emits a short sound. ⇒ These are widely used on bank credit cards and in libraries to identify books and users

(j) Point of Sale (POS)/Electronic Point of Sale (EPOS) ⇒ These are used in supermarkets ⇒ A POS or EPOS incorporates a scanner (bar code reader), a printer and a keyboard built into a service point ⇒ An item is passed through a window through which the scanner can see the bar code and thereby identifies the item; alternatively a hand held scanner is used to do the same thing. ⇒ When a bar code is read and recognized, the scanner emits a short sound. ⇒ The linked central computer then automatically looks up details relating to the item. ⇒ The details are then printed on the customer’s receipt (k) Voice Data Entry (VDE) ⇒ Can also be called voice input or speech recognition 5

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A voice recognizing device analyses and classifies speech or vocal tract patterns The device converts the speech into digital codes for entry into the computer These are suitable where users do not wish to use their hands to input data e.g. sorting of luggage at an airport, recording of weight in a slaughterhouse or butchery. However, their use is limited by the limited nature of recognizable vocabulary The computer needs to be “trained” to recognize an individual’s voice. There are bound to be in accuracies in the data input so there is need for a careful proof reading by the user.

(l) Video Input ⇒ Video images can be entered into the computer from sources such as a TV receiver, VCR or camcorder. ⇒ The data is digitalized and compressed for storage on the computer’s memory sites. ⇒ This method is used to input and store information for a broadcaster of producer of optical disks Selection of the data collection method (Input device & method) The following are the main factors to be considered when deciding on the data capture systems: (a) Costs of the system – costs must be kept low (b) Accuracy – should have detection and correction procedures for errors (c) Time – turnaround time in capturing data should be short (d) Reliability – The system of capture should be free from breakdown (e) Flexibility – the system must cater for different types of data (f) Volume – a system should suit the volume of data to be captured (g) Existing equipment – a system that uses existing equipment would be most preferred. (h) User friendliness – a system should render itself easily to the user THE CENTRAL PROCESSING UNIT (CPU) It is the unit of the computer system that includes the circuits that control the interpretation and execution of instructions. It is the most important component of a computer system. Functions of the processor  To control the use of the memory to store data and instructions 6

 To control the sequence of operations  To give instructions to all parts of the computer to carry out processing The CPU can be subdivided into two major subunits; the control unit (CU) and the arithmetic logic unit (ALU). The primary (main) memory is an extension of the CPU and assists the CPU in its operations. The Control Unit (CU)  It is the nerve center of the computer which controls and co-ordinates all hardware operations i.e. those of peripheral units, main memory and the processor itself.  It also controls the sequence of the operations.  The unit fetches (obtains) the instruction from the main memory  It then interprets it  It then executes the instruction by sending a command signal to the appropriate hardware device in the computer system, ordering it to perform required operations.  The control unit is driven by a 'clock pulse'; the rate of the clock is called the speed.

The Arithmetic-Logic Unit (ALU) This has two major functions  Carries out arithmetic tasks e.g. division, addition etc  Performs logical operations e.g. comparing data items How the ALU operates: Data items to be processed are fetched from the main memory as directed by the CU into registers or accumulators where they are stored. The ALU then performs the operations as directed by the CU. The ALU leaves the result in a data register from where they are placed into the main memory again under the direction of the CU. The Main Memory It is an extension of the CPU which gives immediate access to data and programs. It is also called Immediate Access Memory (IAS). The main memory holds three types of contents which are: -program instructions to be executed or being executed -Data to be processed -Results of processing Operational features

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 The memory has uniquely addressable storage locations that are easily accessible to the CU.  Random Access - it is possible to fetch data from the locations in main storage in any order and time taken to access the location does not depend on the position of the location.  Volatility - the main memory can be volatile or non-volatile depending on the its physical characteristics  Details of single location - Each location consists of tiny devices that can take two states (on/off). The two states of each device are used to represent binary (0 - off, 1 - on). Each location in the main memory holds a unit of data called a word. Some computers had locations holding 8 binary digits and were therefore said to have an 8-bit word. Other computers have 16 bit storage locations, while others tend to have 32 bit locations. The Random Access Memory (RAM) This forms the largest part of the Main Memory and is often used to measure the power of a computer. It is used for temporary storage of data and programs during processing. RAM contains user data and application programs being processed. Data may be read from RAM and data can also be written onto and stored on RAM. RAM contents are volatile i.e. stored data is lost and the contents disappear if the power is interrupted or when computer is switched off. Storage capacity on RAM The number of storage locations in RAM dictates the storage capacity or size of the computer. Storage on computer is quoted in kilobytes (Kb) or megabytes (Mb) 8 bits = 1024 bytes = 1024 X 1024 bytes = 1024 X 1024 X 1024 bytes

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1 byte 1 kilobyte 1 megabyte 1 gigabyte

The Read Only Memory (ROM) It is used for permanent storage it holds the software programs such as operating systems, interpreters and compilers. The programs are written as part of the manufacturing process and the user cannot alter them. ROM cannot be written on it only allows its contents to be read. ROM contains data and instructions that the computer needs all the time from the moment it is switched on. ROM contents are said to be non-volatile i.e. they do not disappear or get lost when the computer is switched off or when power to the computer has been interrupted.

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OUTPUT DEVICES There are two forms of output devices, those that produce hardcopy (permanent) and those that produce softcopy. Printers are hardcopy devices while the VDU is a softcopy device. Hardcopies are needed when copies are to be taken away from the computer to be sent to a user of the information thereon, or to be filed away or even as legal documentation. Computers, therefore can produce a number of different document e.g. reports, sales invoices, payrolls, or graphics. Types of Printers Printers can be classified in a number of ways. (1) By print operation (a) Character printers (b) Line printers (c) Page printers (2) By method of producing print: (a) Impact printers (b) Non-Impact printers (3) By speed of printing

(a) Low speed printers (10cps to 300lpm) (b) High speed printers (over 300lpm)

The following is an illustration showing the types of printers and their sub types and examples of these.

Computer Printers

Line Printers

Character Printers

Page Printers

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Drum Chain Electrostatic Electrostatic Magnetic Ion Deposition (Barrel) (Train Band)

Impact

Dot Matrix Ink jet

Laser

Non-Impact

Daisy Wheel (Petal)

Thimble

Thermal

(a) Line Printers These produce a complete line of text in a single printing operation. These are suitable for bulk printing. i. Drum Printers. • They employ columns of complete characters embossed around the circumference of a rapidly rotating drum. • Every print position is capable of being occupied by any character • A print hammer situated at each print position forces the paper against the drum through the ribbon (interposed between the paper and the ribbon) when the appropriate character is in position. • These are expensive to buy and maintain • The print quality is poor (especially if there are mistiming of print hammers) • They do not allow for change of fonts. • They are also very noisy

ii. • • • • •

Chain (Train) Printers They consist of a chain of characters mounted on a chain that rotates in a horizontal plane. There is a hammer at each print position that strikes the appropriate character once it is in position. Character sets can be easily changed by replacing the chain. The principle of operation is similar to that of drum printers They are cheaper and faster that drum printers

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•

They produce better print quality than drum printers.

(b) Character printers These are also known as serial printers. These print one character at a time across. The method of printing necessitates one (or two) print head(s). There are two categories of character printers: impact and non-impact printers. Impact printers – they form characters and graphics on the paper by pressing a printing element (such as print wheel or cylinder) and an inked ribbon against paper e.g. a dot matrix printer. Multiple copies can be used through the use of a carbonized paper. Non-impact printers - do not use force and are quieter than impact printers. They use specially treated paper and can form characters by laser, thermal (heat) or electrochemical processes. They produce higher quality of print than impact printers. They, however, cannot produce multiple copies. They can print graphics and can support many fonts. Types of Impact printers (i) Dot Matrix – it consists of matrix tiny tubes containing needles in the print head. Each character in formed from the square or rectangle array of dots. The needles are fired onto the printer ribbon in a pattern corresponding to the shape of the character required. Each character is printed by the repeated horizontal movement of the print head. The quality of the print depends on the dots in the matrix (most common are the 7 rows by 9 columns matrices). These printers are cheap to purchase and maintain but do not produce good print quality. (ii) Daisy Wheel – Uses a rotatable wheel consisting a number of flexible metal or plastic spokes (usually 96) at the end of which is a mirror image of a character. During printing the wheel is rotated until the required character comes into line with the print hammer which then hits the character and the ribbon onto the paper and the paper is printed. They are cheap and offer better print quality than Dot Matrix Printers. However, they are slower and much noisier. Types of Non-Impact Printers (i) Thermal printer

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The print head of a thermal printer contains an array of heating elements that have electric currents switched through them so as to form the required character. When the print head comes into contact with a special paper, it burns away an aluminum coating to reveal a black core. They can produce colour printing and can produce high quality prints. However, they use only a special type of paper.

(ii) Ink – Jet Printers These spray dots of ink to form characters under electronic control. These are quieter and produce higher print quality even graphics; they can print in colour and can print many font types and sizes. However they are more expensive to purchase and maintain.

(c) Page Printers They print one complete page at any given time. Although they appear to produce a complete page, they actually produce the document by printing a line at a time but their speed of operation is such that the full effect is that of creating the whole page.

Types of page printers (i) Laser Printers They form characters by flashing a low-powered beam onto a rotating drum. A special ink (toner) is then attracted to the laser-exposed areas of the drum. The document's outline are created by flashing a photographic negative of the document onto the photo-conductive drum. The complete photo-conductive drum surface stores the resulting image. When the paper comes into contact with the drum the image is then transferred and then fused permanently onto it. The photo conductive drum is erased before receiving the next image - these operation occur while printing is in progress. They can print an infinite number of fonts and can even mix different fonts in the same line. They can produce high quality print, and can be used to print logos, illustrations and graphics. They are very quiet during printing. However, they are very expensive to purchase and maintain. (ii) Magnetic Printers

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They use a rotating drum or belt onto which tiny magnetic dots are recorded. The image formed is coated with toner against which heated paper is pressed to form the required image on paper. Typical printer speeds Printer Type Fonts Speed Daisy Wheel Dot matrix Ink jet Drum Chain Magnetic Laser (small) Laser (large)

Impact Impact Non-impact Impact Impact Non-impact Non-Impact Non-impact

Many Many Any Fixed Many Any Any Any

2000cpm 15000cpm 15000cpm 2500lpm 2000lpm 5000lpm 10ppm 500ppm

Factors influencing the choice of a printer  The cost of the printer  Purpose of the output  The volume of work to be printed  The environment in which the printer will work  Nature and desired quality of the document(s) to be produced

Other Forms of Computer Output Speech Output (Sound Output) Some computers are capable of producing speech. The computer can actually speak out from a stored digital representation of either words or other sounds. A person's voice can be stored and reproduced later. This method of output is of great use to the blind, or in poor light. Advanced forms of speech output are being used to answer telephones, answer some inquiries from an organization's callers etc. SECONDARY STORAGE DEVICES This is also called backing storage or auxiliary storage. It is used to supplement the computer's main memory. Main memory is volatile and expensive, therefore backing storage is used to hold programs and data files when they are not required in the main memory. The speed of accessing backing store is much slower than that for accessing in IAS. Backing Store Devices Floppy Disk Drive - These work in conjunction with floppy or magnetic diskettes. They have a narrow slot where the diskette is inserted. The slot has a push button or

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lever which must be closed when the diskette has been inserted. The process of closing engages a turn table which rotates the disk and so brings the read/write head into contact with the disk. Features of floppy diskettes  Consist of a polyester film with iron oxide compound  The disk is mounted and rotates freely inside a protective plastic jacket  The jacket has access openings to allow reading and writing to occur from the read/write head on the disk drive unit.  The disk is divided into sectors and data is stored on the tracks as magnetized bits (0,1)  Most disks are 3.5inch and capacities of 1.44Mb

Caring for diskettes  Use write protect security tag to prevent accidental erasure of data  Protect the disk from dust, rain, humidity, store in an envelop  Do not bend the disk  Do not touch the recording surface  Store diskette in a temperature of 10° C to 52° C  Do not place the diskette in a magnetic field Hard Disk  They are permanent fixed hard disks with lubricated surfaces to allow read/write heads to move close to the disk as it rotates.  They are built into the personal computer (PC) in an air tight enclosure  They are non-removable  They have greater storage capacity (from 20Mb to 1Gb)  They have faster data transfer rate and are reliable.  However, they need very careful security procedures  Modern computers (note books) have removable disk cartridge versions. Magnetic Tape  This is similar to the kind found for audio or video tapes  It is a film coated with iron oxide  Portions of the tape are magnetized to represent bits  It uses separate read write heads to transfer data from the tape to the main memory and to record.  Vacuum columns in the unit absorb the force of sudden starts and stops to prevent tape snatches. 14

 They store data in a sequence so data has to be moved over sequentially in order to read the required, this means access is slower.  Features of the Magnetic Tape - Common width of 1.27cm and length of 100m to 1100m - Data is stored on tracks which run along the tape - The beginning and end of the tape are marked by an aluminum strip - Recording densities range from 200 to 6000 bytes per inch (bpi) - Has storage capacity of 20Mb to 160Mb - The tape is reusable - to prevent accidental erasure, a write permit ring is attached when writing is required. The Exchangeable Disk Unit (Magnetic Disk)  They are coated with iron oxide  They can be steel platters (hard disks) or plastic (Floppy)  Each disk is divided into 200 concentric tracks or rings  Data is recorded on the track by moving a read/write head until it is over the required track.  Data is recorded in bits represented by magnetized areas  Disks are mounted into a unit in the form of an exchangeable pack of 6 to 11 disks that are inseparable.  The disk pack is fitted into a unit or drive which has a comb-type assembly containing one read/write head for each recording surface.  When reading or writing, the R/W head goes directly to the required piece of data, are also called a direct access storage medium.  Disk storage can also be operated in sequential mode.  Storage is costly and they are less portable  They have greater storage capacity (60Mb to 5Gb)  They have much quicker access time Optical Disk Drives  These are similar the audio compact discs (CDs)  It is rigid plastic disk onto which data is recorded by special lasers that physically create micro crevices in the disk.  Data is directly accessed from the disk by the use of a laser beam  The laser beam measures the difference in reflected light caused by the crevice (or lack therefore) on disk.  Each crevice represents binary digit 0, representing the lack of a crevice, or binary digit 1, representing the presence of a crevice.  So the presence or lack of a crevice determines the bit.  The disk cannot be erased or overwritten, this feature is known as WORM (write once read many times) 15

     

Thus the disk is called CD-ROM (Compact disk read only Memory). They have great storage capacities of 50Mb to 10Gb. Access to stored data is very fast They can be used to store text, graphical, audio or video data They are suitable for storing large amounts of data. However, they can not be edited

COMPUTER SYSTEMS Computers can be classified as general purpose or special purpose. General-purpose computers are used to perform a variety of applications and the most common in business while special purpose computers are used for specific or limited applications e.g. military or scientific research. Computers can be classified by size or by use Categories of Computer Systems (by size) (i) Microcomputers (ii) Minicomputers (iii) Mainframe computers (iv) Super computers Microcomputers  They are also called Personal Computers (PCs) or Desktop Computers.  These are relatively small and inexpensive.  They consist of a single processor or a chip  The system is normally made up of the microprocessor, keyboard, VDU one or two floppy disk drives, a printer and a hard disk drive.  It has a hard disk capacity of 20Mb to 520Mb  May use a colour or monochrome cathode ray tube (CRT)  Have capabilities for networking.  They are single user.  They occupy little space.  They are capable of multiprogramming.  They are compatible with a wide range of software.  PCs come in a variety of sizes: notebooks (handheld), laptops, desktops and floor standing.  Minicomputer System (Mid-range Computers)  They are systems the size of a small table that can accommodate a number of users at a time.  They have storage capability larger than personal computers.  They can support a variety of transaction processing activities 16

Hardware features - Support magnetic tape storage - They are multi-user more than 100 users at time for super minis - Have multiple hard disks, both fixed and exchangeable - Can be upgraded when necessary - They do not require a special environment to work so can allow for decentralisation. - They are less expensive than the mainframe systems - They have bigger capacities than microprocessors, some have 32 bit microprocessors. Mainframe computers  These are large, powerful computers with a lot of processing capabilities.  They are suitable for medium-size to large corporations.  They can also be linked together to form a powerful and flexible system. Hardware Features - Similar to minicomputers but have several large processors and high processing speeds of up to 200 million instructions per second (mips) - They have massive amounts of storage power. - They can use high speed line printers - They have a large number of magnetic disks and tape units with large capabilities - They are multi-user and multi processing - They have improved reliability - Their performance may be enhanced by slotting a smaller system, like a minicomputer between the terminal and the main processor - the front end processor (FEP) - Both processors run concurrently with the FEP passing on partially processed data to the main frame for further processing. - They, however, are expensive to buy and maintain, they need special and very expensive software and they also require a special environment. - They can be used for large corporations (such as large international banks) and government departments Supercomputer Systems These are extremely powerful mainframe computer systems. They are specifically designed for high-speed numeric calculations. These an process hundreds of millions instructions per second (mips) They can be used by government research agencies, national weather forecasting, spacecraft construction and navigation. 17

Considerations on classifications based on size The classification of computers based on the physical size of the computer has become blurred and a bit misleading. Some seemingly small systems can perform more powerfully than the larger ones. The power of the computer system depends on the microprocessor. A microprocessor is a type of an integrated circuit (ic). It has two distinct characteristics - word size and speed of operation. Word size - the number of bits dealt with at the same time, some processors are 8 bit, others even 32 bit. The larger the word size the more powerful a computer system is. So some physically bigger systems may have smaller word sizes and hence less power. Speed of the microprocessor - it is the clock rate or rate of data transfer, it is the rate at which data bits are moved around inside at bits per second (megahertz) this is also called the baud rate. Systems with higher speeds tend to be more powerful though they tend to be physically small. Computer classification by use 1. The Word Processor - it is a computer used to produce office documents usually in text. It has very limited memory and processing capabilities. They are cheap to buy 2. Home Computer - it is a cheap computer that is used for domestic purposes e.g. programmes for games and controlling home finances. 3. Personal Computer - it is a microcomputer that is usually for use by one person in an office or at home. 4. Desktop - any computer designed to be used on a desk in an office environment 5. Workstation - a computer terminal (PC or desktop) designed to support the work of one person. It can be high-powered or have other superior capabilities to PCs or ordinary desktops e.g. capacity to do calculations, graphics and other advanced logical operations. 6.

7.

Lap - top - it is a small computer with a flat screen that a user can place on his lap. It is portable and has an in-built rechargeable battery that can support it when there is no power from the mains. It can be carried in a briefcase. Embedded Computers - These are computers in other devices that cannot be accessed directly e.g. those in petrol pumps, ATMs, vending machines, cellphones and elevators.

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SOFTWARE Software refers to computer programs that control the workings of the computer hardware, along with the program documentation used to explain the programs to the user. Computer programs are sets of instructions or statements to the computer that direct the circuitry within the hardware to operate in a certain fashion. Software can be classified into Three different categories are 1. Systems Software 2. Application Software 3. Development Software SYSTEMS SOFTWARE It is a collection of programs that interact with the computer hardware and application software programs creating a layer of insulation between the two. Systems Software contains instructions which: (a) Manage a computer system’s hardware components to coordinate them so that they work efficiently (b) Schedule the computer’s time to make the best use of that time. Systems (i) (ii) (iii) (iv) (v)

Software includes a variety of programmes such as: Operating Systems Database Management Systems Communications Control Programs Service and Utility Programs Programming Language Translators

Operating Systems This forms the largest component of Systems Software. ⇒ It is a system of programs that run or control the execution of computer programs and act as an interface with application programs. ⇒ It is an integrated system of programs that manages the operations of the CPU, controls the input. Output and storage resources and activities of a computer system. ⇒ The primary purpose of the operating system is to maximise the productivity of a computer system. It minimises the amount of user intervention required during data entry and processing. It helps application programs perform common

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operations such as entering data, saving, retrieving files, printing and displaying output. Functions of an Operating System Generally the operating system is expected to perform five basic functions (a) User Interfacing – an Operating System allows a user to communicate with the computer in loading programs, accessing files and accomplishing tasks through command driven, menu driven or graphical user interfaces. In command driven interfaces, the user uses brief end commands, in menu driven interfaces the user selects choices from menus of options, in graphical user interface (GUI) the user selects icons, bars, buttons, boxes or other images to him get things done by the system.

(b) Operating Environment Management – Use of GUI enables the user to connect to other separate application packages so that they can communicate and work together and share data files. Operating environment packages provide icon displays and support the use of some input devices to allow the running and output of several programs to be displayed at the same time. The Operating System allows for multitasking – i.e. where several programs or tasks can be processed at the same time. (c) Resource Management – Resource management programs of the operating system manage the hardware resources of a computer system including the CPU, memory, secondary storage devices and input/output peripherals. For example a memory program keeps track of where data and programs are stored. They subdivide memory into sections and swap parts of programs and data between main memory and secondary storage devices. This operation then can provide virtual memory capability i.e. the real memory capacity in main memory is larger than the capacity of its normal memory circuits. (d) File Management – The file management programs in the operating system control the creation, deletion and access of data and programs. The programs also keep track of the physical location of files on secondary storage units. They maintain directories of information about the location characteristics of files stored on a computer system’ s secondary storage devices.

(e) Task Management – The task management programs of an operating system manage the accomplishment of computing tasks as needed by the user. They 20

give each task a slice of the CPU’s time and interrupt the CPU operations to substitute other tasks. Task management may involve multitasking – where several computing tasks can occur at the same time. Multitasking may be in the form of multiprogramming (several programs are running at the same time). The operating system allows for time sharing – where the computing tasks of several users can be processed at the same time. Multitasking depends on the computing power of the CPU if too many programs are running concurrently the system may be overloaded or processing slowed down. Example of multitasking: printing and typing at the same time, word processing and financial analysis, browsing the internet and word processing. Popular Operating Systems MS-DOS (Microsoft Disk Operating System) ⇒ It has been widely in use especially before 1995 when the new Windows 95 took over. ⇒ It is a single user operating system – only one user can be using the system at a given time. ⇒ It is a single tasking operating system – It allows for the processing of only one task at a time ⇒ Use of short prompt commands – the user interfaces with the computer system through the use of commands that are fed into the computer usually after the prompt sign (>) e.g. C:\>cls (Command to clear the screen) ⇒ Instructions are put in only through the keyboard ⇒ Once a file is deleted it can never be recalled ⇒ The operating system is not user-friendly Windows ⇒ A widows based operating system is more advanced than the MS-DOS based operating system. ⇒ It features graphical user interfaces (GUIs) – User can feed commands into the computer through the selection of relevant icons, bars, buttons, boxes or other forms of images (mostly resembling the intended device or operation). ⇒ True multitasking – the operating system allows for numerous tasks to be done at the same time. ⇒ Networking – the operating system is multi-usable – many users can use it at the same time. It is capable of holding a number of computer systems networked together. ⇒ Multimedia Presentations- the operating system allows for the presentation of information in a variety of media including text, graphic displays, voice and other audio, photographs and video. 21

Virtual Memory – It allows for the use of storage devices as extension of the main memory thus giving the appearance of a larger main memory than actually exists. ⇒ Deleted file go to recycle bin ⇒ Makes use of easier input methods e.g. mouse, joystick ⇒ Windows operating systems has MS-DOS option ⇒ Examples of Windows Operating Systems packages, Windows NT (new technology) (1993) and Windows 95. ⇒ In computer terms, a window is one section of a computer’s multiple section display screen that can have a different display. OS/2 ⇒ It is called the Operating System /2 ⇒ It is an operating system developed by IBM and was introduced in 1994 with the OS/2 Warp version as the latest one. ⇒ It provides graphical user interfaces (GUIs), multitasking, virtual memory and telecommunications. UNIX ⇒ It was originally developed by AT&T but now being offered by many other vendors. ⇒ It is a multitasking, multi-user and net-work managing operating system ⇒ Because of its portability, it can be used on mainframes, midrange computers and microcomputers. ⇒ It is a popular choice for network servers. ⇒

The Macintosh System ⇒ It is an operating system for Apple from Macintosh microcomputers ⇒ It is a popular GUI as well as multitasking and virtual memory operating system. It is made up of the following programs: Language translators Utility programs Control programs Communication programs

Language translators and Utility programs Non-machine languages must be converted into machine language to be executed by the CPU. This is one by systems software called language translators. A language translator converts a statement from a high-level programming language into machine language called source code whereas the machine language code is refereed to as the object code. The translator converts the command given in human language into the form the computer has been programmed to understand before executing the instruction. There are two types of language translators – Interpreter & Compiler 22

Interpreter This is a language translator that converts each statement in a program into machine language and executes the programme statement at a time Compiler This language translator translates a complete program into a complete machine language program. The result is a program in machine language that can be run in its entirety, with a compiler, program execution is a two-stage process. First, the compiler translates the program into a machine language; second the machine language program is executed. Compilers run faster than interpreters so are more preferred. Utility programs It is a standard set of routines that assist the operation of the computer system by performing some frequently required processes such as to sort and merge sets of data, copy and keep track of computer jobs being run. Control programs These are designed to manage the general functions of the processor, memory and terminal interface. The programs are arranged in a hierarchy at the top of which is a kernel or executive program that controls the running of the other programs. In microcomputers the supervisor is held in ROM while on larger computer systems it is held on backing store. When the computer is switched on the supervisor is loaded into main memory, the other programs are kept on disk and are transferred to main memory when they are needed. The kernel assists the following: The job scheduler selects, initiates, terminates, queues and sequences the jobs that demand the use of the processor and main memory. The file manager has the responsibility of achieving the interleaving effect of Multiprogramming. The Input/Output manager has the responsibility of managing the interface with terminals and backing store in response to the requirements of any applications program being executed. Communications Programs

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These support network computer systems by allowing different types of hardware to be linked and to communicate with each other. The programs may help to select the best transmission medium for the message, coding and sending the data.

Virtual Storage Operating System This refers to a system for extending the capacity of main memory for running large application or utility programs. The operating system separates programs into sections some of which are put into backing store. The locations of these sections (addresses) and the part of the program being executed are held in main memory; the sections are called in and processed When required and then returned to backing storage. The sections of the programs are called pages and are said to page in from backing store and page out when being replaced by other pages. The execution of virtual storage is therefore called paging. APPLICATION SOFTWARE ⇒ Applications are programs designed to help the user carry out specific tasks for example to manipulate figures or write texts. ⇒ This also consists of programs written to solve particular user-oriented problems. It applies the power of the computer to give individuals, groups and organisations the ability to solve problems and perform specific activities or tasks e.g. Accounts receivable, accounts payable, automatic teller machines, inventory control, library operations and Control, invoicing etc Categories of generic applications: Word Processing ⇒ A word processing package is designed to enable text to be typed and manipulated. ⇒ It has many advantages over the type writer, - the ability to edit parts of the text without rewriting the whole document - allows for spell checking form the computer's Dictionary File, if it assumes a word is misspelt if not found in the dictionary) it highlights it or sometimes even correct it automatically. - allows for centering text, bold, underline and use of variety of fonts (typefaces) - allows for the production of multiple original copies from one softcopy. ⇒ Popular Word-processing packages are Microsoft Word, Lotus Word, WordPerfect, Quattro Word. Spread sheets

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-

A spreadsheet package is used to perform calculations that have been entered onto a grid. Formulae are entered into the grid using the figures, if the figures change; the results of the formulae are updated automatically. It is also possible to filter (select only the required data), sort or perform other forms of data manipulations. It is possible to produce graphs, charts and other forms of comparison using the entered figures from the spreadsheet. Popular examples of spreadsheet packages are: Microsoft Excel, Lotus 1-2-3, Quattro Pro

Databases - A database is an organised store of information, for example an address book, list of employees, list of students, customers or items of assets. - A database package is used to store records. - Data can be sorted, filtered for separate viewing. - Calculations and comparisons between data items can be done. - Popular database packages are: Microsoft Excel, lotus Approach, Paradox, dBase IV, Data Ease. Graphics - These are applications designed solely for designing graphs and text charts/posters. - They are often used to produce slides for use on overhead projectors, or presentations on computer-projectors. - Various types of charts are also produced - Examples of graphics packages are: Microsoft PowerPoint, Lotus Freelance, Harvard Graphics, Corel Draw Desktop Publishing - Desktop Publishing (DTP) applications give users powerful and versatile page design capabilities. - The user can incorporate text and graphics on very exact page layouts. - These applications produce magazines, catalogues, invitation cards, business cards and other sophisticated documents. - The application links up well with other applications as the user can import text and graphics from the other applications. - Examples of DTP packages are: Microsoft Publisher, PageMaker, Ventura and Frame maker.

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Suites - Many applications are grouped together into suites which users can purchase as one product containing, for example, Word-Processing, a Spreadsheet, Graphics application, Desktop Publishing, Database application. The following is the criteria for selecting applications software: -Accuracy - it must be free from errors -Flexibility - must be able to adapt to changing environment. -Compatibility - it must be compatible with available hardware -Recency - it must be current -Cost - it must have reasonable cost -Originality - It must be original -Support - consider continued support from the supplier. -Ease of use - it must be user friendly. -Performance - it must have efficiency in both response time and run time. Sources of Applications Software There are three sources of applications software; In-house Off-the shelf From a contractor. In-house Developed Software This approach requires a development team from within the organisation. The team is usually comprised of programmers and analysts. The team members should be high calibre, highly trained and reliable Advantages ⇒ Internal professionals understand operations better and therefore can produce an accurate solution. ⇒ The Software usually meets user requirements. ⇒ Management are in total control of the development process ⇒ More flexibility - there is more flexibility in making modifications. ⇒ Problem specificity - in-house developed software can give an organisation software programs that are easily tailored to a unique problem or task. Disadvantages ⇒ Time and costs of developing the program may be greater than other options ⇒ In-house staff may lack the expertise needed for the project ⇒ Reliability of the resultant software is highly questionable. ⇒ In some cases the developers may attempt to gain fraudulently from the system

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Off-the Shelf This is software that can be purchased, leased, or rented from a software company that develops programs and sells them to many computer users and organisations. Applications such as financial accounting, business graphics and pay roll may be bought. Advantages ⇒ Cheaper - the software company is able to spread the software development cost over a large number of customers, hence reducing the cost any one customer must pay. ⇒ Less risky - the software is existing, hence you can analyse the features and performance of the package. ⇒ The program is a well-tried and tested product with few errors. ⇒ Less time - Off-the -shelf software is often installed quickly and easily. ⇒ The package is well documented ⇒ The packages require little maintenance ⇒ There is continued support from the supplier through upgrades. Disadvantages ⇒ The organisation might need to pay for the features that are not required and never used. ⇒ Tie package may be for general needs and therefore not ideal for the user. ⇒ The user has no direct control over the package, Software from a Contractor This involves contracting out software development to a software house - better known as outsourcing especially where off-the-shelf packages are not available. Advantages ⇒ Software houses employ professionals and this may benefit the organisation ⇒ Transfer of skills to the existing professionals in an organisation ⇒ Organisation can get support from the contractor. Disadvantages ⇒ There could be breach of confidentiality. ⇒ Contractor may go bankrupt and cease trading leaving the user with no support. ⇒ Contractor might not have sufficient experience or understanding of the problem thus the solution might not be satisfactory. ⇒ It takes time for the contractor to understand the user's business and operations this makes the approach expensive. DATA COMMUNICATION 27

Refers to the means and methods whereby data is transferred between processing locations through the use of communication systems. Communication systems are defined as systems for creating, delivering, and receiving electronic messages. The communication system comprises of the following: - a device to send the message - The channel or communication medium - A device to receive the message - A device to send the message Modes of Communication and Codes There are various modes and codes of data transmission signals from the sending to the receiving device. Analogue transmission Analogue signals are continuous sine waves that send a continuous 5-volt signal on a channel but the signal will vary continuously between +5 to -5 volts. The number of cycles per second is the frequency of the signal and is expressed in units called hertz (Hz). The human voice forms oscillating patterns of changes in air pressure. The vibrations act on the telephone microphone and are converted to electrical voltage patterns that reflect the characteristics of the speech pattern. Analogue transmission is used to transmit voice or data in analogue signals. It is used in telephone systems and radio transmission. Voltage

Analogue Signal

+5

-5

Time

Digital Transmission This is the sending of data with digital symbols of 0 and 1 representing the switching on (1) and switching off (0) pulses of electricity. Most computer systems use this to create bits that make up bytes. One cycle is made up of two pulses. The number of pulses per second is called the baud rate. Digital Signal

1

1

1 0

0

0

28

Time Digital Transmission Versus Analogue Transmission Digital transmission offers less errors and noise in transmission especially over long distances. There is little data fatigue as the data being send does not lose power over the transmission so much that the signal is weakened by the length of the transmission media. Digital transmission is compatible with digital computers so there is no need to convert the data messages from analogue to digital when computer systems are using digital transmission lines. However, the setting up of digital transmission lines especially over long distances is considerably expensive. Data Transmission Modes These are ways through which organisations can communicate via the channel or transmission media. Simplex transmission Transmission takes place only in one direction. These are not suitable for long distance transmission because there is need for acknowledgement or error messages. It is used in the computer - printer communication. This is also used in radio and television transmission. Sender

Receiver

Half Duplex Messages can be sent both ways but only one way at a time. The channel alternately sends and receives data but these are not done at the same time. The same device is used for both sending and receiving. This is used in two-way radio communication. Sender

Receiver

Full Duplex This permits simultaneous transmission of messages in both directions. Sending and receiving can be done at the same time using the same devices. This is the mode used in modern telephone/cellular transmission. Sender

Receiver

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Ways of Transmitting Data a. Asynchronous Transmission - Each character is sent down the channel separately that each transmission unit is one unit in length. The characters are sent character by character at irregular intervals. The character is headed by a start bit and ended by a stop bit. These bits tell the receiving device that a character is coming and that the character has been sent. There may be a parity bit to verify correctness of transmission. Even parity is used where the number of bits in the character is even and an odd parity system for the character with an odd number of bits. This form is suitable conversational interactions and used in standard telephone lines using low transmission rates. Synchronous transmission - Prepared sets of characters are transmitted together as blocks at fixed rates. The message is preceded by the parity bits to verify what has been received. The beginning and end bits represent small percentages of the total bits sent thus reducing overhead costs of communication. Synchronous Transmission is faster and less expensive as given in that character are blocked and sent down as one message allowing for the transmission of a fuller message.

Protocols - There is need for there to be a way of signalling the start and end of the message by the use of data transmission protocols. The use of the parity bits is one method, use of "roger", "over" are protocols in two-way radio communication. Switching Alternatives Circuit Switching When a call is made the communication channel is opened and kept open until the communication session is complete. Message Switching Each message is sent to the receiver if a route is available. The messages are sent in blocks one at a time. The message may be stored for later transmission if the route is not available, sometimes this is called store-and-forward transmission. The message is delivered when the route becomes available or upon demand from the receiver.

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Packet Switching This involves sub-dividing the message into groups called packets. Each packet is then sent to the destination separately via the fastest route. At the destination the packets are put in sequential order and delivered to the receiver. Sometimes when there is no route open, the packets are stored and then forwarded once the route is open, so these are also store-and-forward systems. These fully put the network to better utilisation. Communication Channels Sources and Media a. Telephone Systems - telephone lines are used with online systems. The lines can be divided into leased and dial service lines. (i) Leased Lines - These are telephone lines that are dedicated to the sole use by the user to support online systems within the organisation. The telephone service provider (e.g Tel One) then charges a flat monthly rate for the service. In calculating the rate the length of the line may be considered. (ii) Dial service - With this telephone line the user is charged each time the line is used for online processing. When all the lines connecting the system are busy a user has to wait and an appropriate message is given by the system. The rate charged depends on the distance, time of use (peak or off peak), time spent during transmission. This is usually used for long distance communication between systems e.g. the Internet A leased line service provides for high-level online activities. It is much cheaper to use and provides better quality transmission. The leased lines are much more flexible than dial service lines. Users in the system have greater access through the many lines provided by the common carrier. However, leased lines are suitable for use in one organisation or in one building/locality. The amount of data transmitted depends on the capabilities of the communication channel. The capabilities may be classified as bandwidth. Bandwidth is the frequency range of the channel, representing the maximum transmission rate of the channel. The bandwidth can be measured in bits per second (bps) A narrow band - the channel offers the lowest transfer rate and supports transmission through telegraph lines. Voice band - this is a low-speed analogue (offering a rate between 300 and 9600 bps)

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Wideband - these offer the highest transfer rates with data communication through coaxial cables. b. Integrated Services Digital Network (ISDN) - it is an international effort to replace the analogue technology with digital technology through a single set of standard interfaces. c. Microwave Satellite Transmission - These transmissions are sent through the air. The transmission uses a line of light that must be unobscured. Usually these transmissions are from space satellites placed in stationary orbit and earth satellites and are communicated from and to satellite dishes.

Satellite

Destination

Source

Earth

This form of transmission can transmit large amounts of data over wider areas. This is in much use in wide area television broadcasting. Although transmission is of high quality, setting up the system is very expensive. d. Radio Transmission - This form of transmission uses radio waves, transportation and taxi companies for easier communication use it. The police and army to communicate also use this. e. f. Cellular Transmission - The local area under the transmission is divided into cells. As the cellular transmission user moves from one cell to the other the connection is passed from one cell to the other. The signals from individual cells are integrated into one regular telephone system. g. Physical Cables - Transmission uses physical cables which are connected to the communication points. There are three types of these cables: (i) Twisted-Pair Wire cable - this consists of a pair of twisted wires. The wires are insulated and twisted to avoid the bleeding of signals from or into the each other. There are shielded wires (with a special conducting wire) and unshielded twisted-pair (UTP) that has no 32

insulation. It is the type used in home telephone connections. The twisted pair wire cable is inexpensive. However, it does not allow high rates of data transfer, and is prone to interference and too bulky. (ii) Coaxial cable - Consists of an inner conductor coated with insulation called a dielectric (a layer of foil) that is covered by nonconductive insulation called a jacket. This offers a cleaner and crisper transmission than the twisted pair wire cable and has a higher data transfer rate. However, it is more expensive. (iii) Fibre optic cable - This is made up of a glass or plastic core surrounded by a cladding material (paper, polyethylene, twine or other sheathing). They use light which is turned on and off rapidly creating the on off bit. The sheathing of the cable prevents loss of reflection. Advantages 1. High transmission capacities 2. Smaller cable size and lighter weight 3. Very little cross talk and spying 4. Little interference 5. Repeaters (to boast strength of the signal) are not required 6. Are ideal for hazardous conditions since there is no sparking. These are however quite expensive to purchase and set up.

COMPUTER NETWORK SYSTEMS A network is a number of computers connected through some channel so that they may share some of the resources and allow access into them by users from other points. Advantages of networking a. Resources can be shared e.g. printers, computer files and programmes. b. More even distribution of processing and other work by the computers and users. c. More economic and fuller use of computers. d. Allow for the provision of local facilities without loss of central control. e. Mutual support and a spirit of cooperation exist. Disadvantages of networking. a. There could be congestion at the shared resources. b. Control of information and confidentiality may be lost. c. The costs of the system may be considerable. d. Viruses and other forms of corruption to programmes spread faster 33

Wide Area Networks (WAN) These are networks that are spread over large geographical areas. The telephone system is one such WAN. It is a long distant network of interconnected computers on separate sites, cities or even different countries. Purposes of Implementation of WAN -To get data from its source. -Improve on the productivity of the system by improving the speed of processing - Allow for instant feedback and expansion of the system. WAN hardware Hosts - This provides users with processing software and access. The host is usually a mainframe computer with microcomputers connected to it. Front end Processors (FEP) & Back End Processors (BEP) - These are minicomputers that are placed in front of (FEP) or at the back (BEP) of the main system CPU. These assist the main system CPU with accepting input perform the operations on it before forwarding it to the mainframe CPU for further processing or after the main system CPU to assist with the output activities. They generally manage communications. The main system CPU concentrates on process work without having to handle input and output activities. Modems - This is short for Modulate Demodulate. Telephone lines that are common link media between computers use the analogue signal whereas computers use digital signals. So there is need to convert the signals. From the sending computer, the digital signal should be modulated to analogue signal for transmission over the telephone link. At the receiving end the analogue signal has to be demodulated (converted back) into the digital signal. Modulation can be done using a number of methods. Amplitude modulation (AM) where the amplitude is modified to represent binary digits 0 and 1. Frequency Modulation (FM) adjusts frequency to represent the bits 0 and 1. The Phase Shift Modulation adjusts a fixed amount so that 0 and 1 can correspond to different phase shifts. Terminals - These are the microcomputers connected to the system on which the users can work to sent data or access information. Terminals may be dump or intelligent. Dump terminals have limited memory and intelligence while intelligent terminals have processing and memory capabilities that they can process data themselves.

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Multiplexor -it is a device that can subdivide one big channel so that many people can use it at the same time. There are two types of multiplexors: time division and frequency division. Time Division Multiplexors (TDM) - It slices multiple incoming signals into small time intervals that are then transmitted over a channel and then split by another TDM at the receiving end.

Time Slots

Time

HOST

Frequency Division Multiplexor (FDM) - incoming signals on different frequency ranges are sent across a transmission medium at the same time. At the receiving end another FDM splits the frequencies into multiple signals again.

Multiplexor

Multiplexor

Terminal controllers/Concentrators- are used to connect many terminals to a single line, they do not communicate directly with the FEP. Protocol Converters - Because of the diversity of technology in communication, WANs are using various devices, channels, modes and codes. To allow for communication between and among all these protocol converters are used to translate different signals from one system to another. Typical protocol converters include HTTP (Hyper Text Transfer Protocol) used on Internet communications, TCP/IP.

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WAN Software In order for the WAN hardware to operate there should appropriate software which should include telecommunications access programmes that handle messages between the host's memory and the remote devices, this programme could be in the FEP. There is also the network control programme that has the work of running the network off the host by checking if terminals have messages to send, do editing of incoming messages. Network concerns For the Manager 1. Reliability of the network - there should be minimal breakdown and errors in the system. 2. Response time of the Network - there should be less time spent on picking the required signals when required. 3. Costs of the Network - costs of setting up and maintaining the system may be considerable, so the manager should reduce these. 4. Compatibility of Software and hardware - hardware and the software should be compatible (capable of working together) otherwise there should at least be some conversion of either the software or the hardware to suit the other. 5. Access and Security of the System - with WAN there is greater risk of data being accessed by unauthorised people, there should be measures to reduce this risk e.g. use of passwords, physical access protection or through encryption (coding of data so that in the event that it is intercepted it may not be understood) WAN Topology

Zimbabwe

South Africa

Modem

Multiplexor

Terminals

Modem

Multiplexor

Host

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LOCAL AREA NETWORKS (LAN) It is an interconnected group of minicomputers or other terminals within a small geographical area. The area could be a room, a number of offices or a building. LAN Hardware and Software Network Workstations - these are usually microcomputers from where a user can work to enter or receive data from the network. These could be dump terminals or diskless Workstations. File Servers - These are computer systems attached to a network that control access to and other wise manage hard disks to allow the workstations to share disk space, programmes and data and to control the network. Print Servers - these are computers that have the duty of managing the printer resources, that is allocating print time, and carrying out other print routines. Communication Servers - these manage communication devices in the network (modems, multiplexors). A microcomputer or even the file server could do this job provided that it is not too busy with its own functions. Cabling - these connect the different computer systems and devices in the network. Sometimes LANs use radio waves and thus are wireless. Network interface Cards- these provide the necessary translation of signals between the devices in the network. They are sometimes called adapters. The Network operating System - These are programmes that manage the operations of the network allowing the user to install the network, the application software, analyse and solve network problems, manage the sending, routing and delivery of messages and do other routine and housekeeping duties for the network. LAN Topologies Topologies are ways in which a network can be structured or arranged depending on the resources, the needs and the structure of the given organisation. 1. Star Topology - there is a central node - the file server (which could be a minicomputer or microcomputer) to which all other computers in the system are connected. All the processing and storage are done by the host (central computer). There is no direct interterminal communication. The network is suitable for use in offices and homes. Advantages a. It is easy to trouble shoot b. It is economic and offers intensive use of the terminals 37

c. Requires a single intelligent computers, terminals could be dump there by saving on the cost of the network. d. Each terminal has direct and quicker access to the host Disadvantages a. It is prone to failure - failure of the node mean loss and breakdown to the whole system. b. The cabling may be costly c. Viruses can be transmitted easily within the network Terminal

Host

2. Bus (Multidrop) Topology All the computers are connected to a linear transmission medium called a bus through the use of a tap. The tap is used to create a link up to the medium and to the network.

This network handles higher traffic. Advantages a. Costs of setting up and maintaining the network are low. b. Each terminal can communicate directly with any other on the network. Disadvantages a. Risk of network malfunction b. Are more troublesome than the star topology. c. Trouble shooting is more difficult. d. There is the risk of data collision. 2. Ring Topology It is made up of a series of nodes connected to each other to form a closed loop. Each loop can accept data transmitted to it from an immediate neighbour and 38

forward data not addressed to it to the next neighbour. Messages are transmitted in packets or frames. Each node has an address and packet is delivered to the node bearing the address required.

Advantages a. There is by-pass technology so that messages can be passed on from a failed workstation - breakdown of a workstation does not affect the network much. b. It is easy to install c. There is less risk of packet collision d. More accurate and reliable Disadvantages a. There is poor response time b. There is risk of data interception so there should be data encyption. c. Unwanted data may continue to circulate the network, thus reducing traffic carrying capacity of the network.

4. Mesh Topology This topology combines the ring and the star topologies. Bi-directional links are established.

This offers better communication and reduces the risk of data collision because of the existence of alternative routes. The network has quick response time and is very accurate. However, the costs of the cabling are quite considerable. 5. Hierarchical (Tree) Topology

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The nodes are connected to form a hierarchy. Messages are passed along the branches until they reach their destinations. These networks do not need a centralised computer that controls communications. These are also very easy to troubleshoot and repair because the branches can be separated and serviced separately.

6. Mixed Topologies Various topologies are combined into one network.

LAN concerns for the Manager Costs - of connection - cables and software Compatibility of the hardware and software Appointment of a network manager where necessary. THE INTERNET The Internet is a worldwide network of computer systems. Millions of computer networks in different parts of the world are connected by telephone lines, cables, radio and microwave links and modems. Because the telephone system is not yet fully digitalized, there is need to convert the predominantly digital computer signal to analogue and reconvert to digital. This is done by the Modem (short for Modulator demodulator) which sits between the computer and the telephone line. Modems may be external or internal to the PC. There are many organizations that offer internet services. These are called Internet Service Providers (ISPs) and they usually charge a monthly fee for the connection. Some ISPs do not charge a fee for the service. If you want to explore the Internet, a web browser is required. A web browser contains programmes that assist in the surfing of the internet. The most used web browser is Microsoft Internet Explorer. 40

Web site – Web address – Facilities on the Internet Electronic Mail It is commonly referred to as the e-mail. Every user of the e-mail has a unique address. E-mail addresses have an @, for example [email protected] One can send and receive messages the same way an ordinary letter is sent and received. E-mail messages are sent from user to user on a computer network with message being stored in the recipient’s mailbox or inbox. The next time the user logs on, he is told that there is a new message, these messages can be read, printed or replied. E-mail allows for the sending of attachments. These are files that contain greater detail and are based on a particular application package like Microsoft Word, Microfoft Excel. One can even send a CV, an assignment or other document using e-mail. E-mail makes use of an existing internet connection and software. However, there is a misconception in the minds of many students that e-mail and internet are one and the same thing. Internet provides many facilities and the e-mail is only one of them. Advantages of e-mail 1. Speed - Messages are received instantly - Provides certainty of delivery of mail - Reading, sending replies, redirecting massages is faster. 2. Cost - Generally cheaper than ordinary mail - Less or no paper used. 3. Time - Less time spent on the phone waiting to be put through, finding people unobtainable, holding the line because line is engaged and leaving messages and having to call again. 4. Flexibility - Anyone with a PC at home can send and receive messages out of office hours. - Messages can be sent and received at any computer that is linked to the network. Disadvantages of e-mail 1. Delay – if a recepient takes long to log on, the speed advantages is lost - If there is a problem with the recepient’s server, one may not know immediately if the message has been received or not. 2. Information Overload – Sometimes there may be too many messages - Sometimes important or urgent messages are overlooked. 3. Junk mail – There may lots of unwanted messages received. 41

4. Breakdowns – Often log on problems may result in messages not being sent or received. 5. Security – There is the concern about security of confidential information that is sent and/or received. Intranet It is the network of computers usually within a company that uses e-mail and browser software but is not part of the internet. Employees can use the Intranet to access information related to the company such as training, social activities, job opportunities and product information. It enables employees to share information. E-commerce It involves the internet transactions of goods and services to businesses and consumers and can include: - retail – you can buy virtually anything on the Internet, e.g books - banking – there are online banking facilities that allow the payment of bills and access of balances. - Travel arrangements – airline and rail tickets can be bought and sold on the internet and bookings made thereon. Consumers connect to the online service and can then order goods and pay for these using a credit or debit card. DATA PROCESSING It is the process of collecting data and converting it into information. It may be manual (where only human effort is used), semi-manual where human effort is aided by an electronic device or mechanical/electronic, where computers replace human effort. Types of Data Processing

1. Centralised data processing This is where data is sent to and processed at a central place. Advantages of centralised data processing a. All required information can be got from one place b. Little distortion of information c. Easy to manage and control d. Cheaper to implement e. Easier to enforce standards f. Is not complicated

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Disadvantages a. The whole system is disturbed when the computer is down. b. Users of the system have little control over the data even their own. c. Loss of data by the processing computer may mean loss of data at other points. d. Needs specialists to set up and maintain the system and its environment. e. Processing of data may be slow due to congestion at the central computer. 2. Distributed processing Data is processed at different points in the organisation. Branch or department may have a microcomputer or minicomputer that processes the information. The minicomputer in the branch then sends to a host computer after processing. HOST COMPUTER 1

2

3

4

5

6

Advantages a. Data lost at any point may be recovered from the host b. Faster processing of data c. Processing may occur even if the host computer is down d. User have some control over their data e. Problem can be solved at the different points Disadvantages a. The system needs expensive equipment and has high maintenance costs. b. Management and control difficult c. There is dada duplication at the different points d. No uniformity of data e. There is no overall control time

3. Batch processing A central computer system normally processes all transactions against a central data base and produces reports at intervals. In batch processing transactions are accumulated over a period of time and processed periodically. A batch system handles the inputing, processing, job queuing and transmission of the data from the remote sites with little or no user intervention. Batch processing involves. a. Data gathering from source documents at the remote site through the remote job entry (RJE) b. recording the transaction data onto some medium e.g. magnetic tape or disk. c. Sorting the transactions into transaction files d. Processing of the data 43

e. Updating the master file. Advantages a. It is economical when large amounts of data are being processed b. Suitable where reports and information is needed periodically. c. Reduced risk of error due to the time involved in validation and verification. Disadvantages a. Master files are often out of date b. Immediate updated responses cannot be made. c. Unsuitable where decisions and follow up transactions depend on the results of earlier transactions e.g. in bank withdrawals and deposits. d. Once the system fails all output is delayed.

4. On-line Processing It is the input of transactions while the input device is connected directly to the main CPU of the system. There is no necessity of a batch. Advantages a. Items can be input much easier and quicker. b. Many errors are dealt with by the operator at entry c. Time is saved because a batch does not have to be produced first. d. User can recognise anomalies in the data being entered Disadvantages a. The system may be more expensive than batch processing. b. Sometimes accuracy of data depends on the operator who might fail to detect or prevent some errors. c. Sometimes source document are not used e.g. in the case of telephone orders.

5. On-line Real time processing It is the continual receiving and rapid processing of data as to be able to feedback the result of that input to the source of that data. There is no delay between the input of a transaction and the updating of the master files for the transaction. The only delay is for the necessary validation and verification of the data. This allows for immediate processing of data and results generated to provide immediate output to users. Data does not have to wait, as is the case under batch processing. Data is fed directly into the computer system from on-line terminals without the storing, sorting (these are done on-line)

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Advantages  Processing is instantaneous  Files and databases are always up to date Disadvantages  The systems are expensive to and complex to develop and put up.  Data with errors may be processed with damaging effect.  Hardware costs are high, there is need for on-line terminals, more CPU power, large on-line storage and back up facilities.

6. Time Sharing Processing Processor time is shared between users to provide multi-access (many users accessing data at the same time). This can be done by minicomputers and mainframe computer systems. Advantages a. Each user is given a chance b. More than one person can use the computer at the same time. Disadvantages The user may not require a service at the time his slice is given - this results in too much excess capacity at some periods. THE DATA PROCESSING CYCLE

Data Acquisition (From source Documents)

Output Communication verification)

Input/

Storage

Capture(validation

checks,

Processing

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Stage 1 Data acquisition - this is the collection of data from source documents (receipts, delivery notes) for input into the computer system. Stage 2 Input/Capture - This is the putting of the acquired data into the system e.g. through typing, scanning, or other forms of input.

Verification - data is checked for mistakes in copying so that it is correct. There may be garbage in and garbage out (GIGO) meaning that information systems will produce erroneous output if provided with erroneous input data or instructions. To avoid GIGO common validation is done: a. Checking data reasonableness, e.g. pregnancy for a three year old would be unreasonable. Checking data consistency - e.g. it would be inconsistent to record a pregnant male. b. Checking range units e.g. it would be impossible to have 30hours worked by one worker in a day. Timeliness - that data is not out of date.

Validation - the quality of the data is checked before it is entered or processed and errors detected and eliminated. Validation methods Allowed character checks Checks that ascertain that only expected characters are present in a field. For example a numeric field may only allow the digits 0-9, the decimal point and perhaps a minus sign or commas. A text field such as a personal name might disallow characters such as < and >, as they could be evidence of a markup-based security attack. An e-mail address might require at least one @ sign and various other structural details. Regular expressions are effective ways of implementing such checks. (See also data type checks below) Batch totals Checks for missing records. Numerical fields may be added together for all records in a batch. The batch total is entered and the computer checks that the total is correct, e.g., add the 'Total Cost' field of a number of transactions together. Cardinality check

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Checks that record has a valid number of related records. For example if Contact record classified as a Customer it must have at least one associated Order (Cardinality > 0). If order does not exist for a "customer" record then it must be either changed to "seed" or the order must be created. This type of rule can be complicated by additional conditions. For example if contact record in Payroll database is marked as "former employee", then this record must not have any associated salary payments after the date on which employee left organization (Cardinality = 0). Check digits Used for numerical data. An extra digit is added to a number which is calculated from the digits. The computer checks this calculation when data are entered. For example the last digit of an ISBN for a book is a check digit calculated modulus 10. Consistency checks Checks fields to ensure data in these fields corresponds, e.g., If Title = "Mr.", then Gender = "M". Control totals This is a total done on one or more numeric fields which appears in every record. This is a meaningful total, e.g., add the total payment for a number of Customers. Cross-system consistency checks Compares data in different systems to ensure it is consistent, e.g., The address for the customer with the same id is the same in both systems. The data may be represented differently in different systems and may need to be transformed to a common format to be compared, e.g., one system may store customer name in a single Name field as 'Doe, John Q', while another in three different fields: First_Name (John), Last_Name (Doe) and Middle_Name (Quality); to compare the two, the validation engine would have to transform data from the second system to match the data from the first, for example, using SQL: Last_Name || ', ' || First_Name || substr(Middle_Name, 1, 1) would convert the data from the second system to look like the data from the first 'Doe, John Q' Data type checks Checks the data type of the input and give an error message if the input data does not match with the chosen data type, e.g., In an input box accepting numeric data, if the letter 'O' was typed instead of the number zero, an error message would appear. File existence check Checks that a file with a specified name exists. This check is essential for programs that use file handling. Format or picture check

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Checks that the data is in a specified format (template), e.g., dates have to be in the format DD/MM/YYYY. Regular expressions should be considered for this type of validation. Hash totals This is just a batch total done on one or more numeric fields which appears in every record. This is a meaningless total, e.g., add the Telephone Numbers together for a number of Customers. Limit check Unlike range checks, data are checked for one limit only, upper OR lower, e.g., data should not be greater than 2 (