Higher Computing: Computer Systems CONTENTS

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1 CONTENTS Topic 1 DATA REPRESENTATION Text Numbers... 4 Exercise Integers: positive and negative numbers Signed Bit Twos Complement... 5 Exercise Numbers real... 7 Exercise Bit-mapped graphics Calculating Bit Mapped Storage Requirements Data Compression Vector graphics... 9 Exercise... 9 Topic COMPUTER STRUCTURE Categories of computer The main categories A comparison of the different categories of computer Exercise Computer organisation The structure of a computer, a two-state machine Memory The links between the processor and memory The size and number of memory locations Buses Read from memory Exercise The structure of the CPU Clock Exercise Measures of processor speed Memory and system performance Cache Bus widths and system performance Exercise Extension exercise for Outcome BIT Department Deans Community High School Page

2 TOPIC 1 DATA REPRESENTATION You should already be familiar with the following terms and the part they play in data representation: bit, byte, Kilobyte, Megabyte, Gigabyte, Terabyte, word, ASCII, control characters, binary, integer and real numbers. These are all in your notes! After studying the following section you should have a deeper knowledge of data representation and be able to describe the internal representations of graphics, text and number. Information can be stored in a computer in many ways. Here are a few of the main ways: 1.1 Text Each character is stored in the computer as a unique binary code. To help different types of computer communicate, the ASCII system was introduced so that all computers would use the same codes to represent the same characters. Each character is normally stored in one byte. For example, the letter A is stored as 65 (Or in binary) and the character is stored as 35 (or ). The ASCII system uses 1 byte (8 bits) and so can store 56 different characters, which is more than enough for the upper and lower case English characters, punctuation and numeric symbols. Extended ASCII ( Bytes) would be used to store a character if foreign alphabets, special symbols such as mathematical ones etc. are required. Such a system would be capable of storing 56 * 56 = 65,536 characters. As well as the ASCII system a similar system, called Unicode exists. It exists because of the need to represent non-latin characters in computers e.g. Japanese and Chinese. Its characters are encoded using 16 bits and this allows it to represent up to 65,536 symbols. Many packages, including Microsoft Office store documents in Unicode. So what advantages does Unicode have over ASCII? Unicode has a much wider range of characters available compared to ASCII (65,536 v 56 characters) Unicode is designed to be used for languages other than Latin type languages. Unicode is an accepted world standard, used by most major computer manufacturers / software companies However, ASCII normally takes up less memory. BIT Department Deans Community High School Page 3

3 1. Numbers Number input is converted to binary, which is the base number system. Just as our decimal system is based on the number 10 with columns for or so binary is based on the number with columns for or Here is an example of how to convert the binary number to a decimal: = = 154. This is how to convert the decimal number 69 to binary: R 1 17 R 0 8 R 1 4 R 0 R 0 1 R 0 0 R 1 Giving A system using 1 byte for number storage can have 56 different numbers i.e but if bytes are used then different numbers can be used i.e. 0-65,535. Increasing the size of the storage for numeric data increases the range of numbers that can be stored. Exercise 1 (a) Find a source showing the ASCII table and write down the ranges representing A-Z, a-z and 0-9. (b) Try to find out what some of the codes that don t represent text are used for. Convert the following decimal numbers to binary 5, 1, 16, 3, 55 and Convert the following binary numbers to decimal , , , and What is the largest number that can be stored in 1 byte? 5 Calculate 8 1. The 8 is because a byte has 8 bits. This is a useful shortcut for finding the maximum. Use this method to find the largest number that can be stored in bytes and 4 bytes Your teacher may now provide you with a copy of the Binary program to help you learn more about binary. BIT Department Deans Community High School Page 4

4 1.3 Integers: positive and negative numbers If a computer system has only to deal with positive integers, these can be converted directly to their equivalent binary numbers. The size of number that can be handled depends on the number of bytes allocated to storing each number.. For example, if two bytes (16 bits) are used to store each integer number then this can handle numbers in the range from up to In decimal this is 0 to 16-1 or 0 to (see Q5 on previous page) Signed Bit If negative integers are to be included,, then you still get the same number of numbers but the range changes to 3768 to One bit is used to store whether a number is positive or negative and the maximum number from bytes becomes In other words, introducing negative numbers halves the size of the largest number that can be stored if only positive integers were used. The most significant bit (the leftmost bit) is normally used to store the sign of the number. This is called the Signed Bit. Positive is 0 and negative is 1. At first sight it might appear that the negative of 5 or would simply become but this approach doesn t work for two reasons: 1. Addition doesn t work properly. Adding 5 and 10 would give +15. This would be absurd [The extra 1 is ignored!] B) There are two zeroes and No number system works with two values for the same number. The following section shows how negative numbers are now handled inside a computer Twos Complement To convert 5 to 5, all 0s are converted to 1s and vice versa and then 1 is added. So Or 5 Becomes Or -5 Exactly the same procedure changes a negative back to a positive. Addition works And there is only 1 zero: BIT Department Deans Community High School Page 5

5 Increasing the size of data storage for twos complement has the same effect as for purely positive numbers - the range of numbers capable of being stored is increased. Exercise Convert 6, -17, -3 and 18 to binary using twos complement. Try changing 0 to 0 using twos complement. What happens? Again, your teacher may provide you with the Integer & Real program to give you more practice in twos complement. This is dealt with in the integer section of the program. BIT Department Deans Community High School Page 6

6 1.4 Numbers real Real numbers are stored in a computer as floating point numbers. These are not always stored exactly as it is not always possible to convert precisely from decimal (base 10) to binary and vice versa. It s like changing the fractions ½, ¼ and 1 / 3 to decimals. The first two can be done exactly but the 1 / 3 cannot. Floating point is like standard form or scientific notation in the decimal system. To convert to standard form involves moving the point so the number is between 1 and 10 and counting how many places you have moved the point. So becomes * The 4 is because the point has moved 4 places to the left. The 10 is because we are working in base 10. The is called the mantissa and the 4 is called the exponent. In binary, to change to floating point a similar procedure is followed except the point is moved in front of the first 1. So would be written as * 4 where the is because we are working in binary and the 4 is the number of places that the point has moved. The full answer is * 100 because the 4 is converted into binary as well. The computer would store the mantissa and exponent 100. The normal way is to use four bytes for the mantissa and one byte for the exponent. This gives a range for the exponent of 0 to 55, which is approximately 10 0 to and nine figure accuracy when in decimal (see Q below). Increasing the size of the mantissa increases the accuracy with which the real number can be stored. Increasing the size of the exponent increases the range of numbers that can be stored. Exercise 1) a) If an integer number is stored using one byte, what range of integers can be represented by this? b) If an integer number is stored using four bytes, what range of integers can be represented by this? ) A number format is said to have precision or accuracy if it can store the numbers from 0 to 99. Use your answers to question 1 and the text above to find out what precision can be stored in one byte, two bytes and four bytes. 3) Find the largest and smallest numbers that can be stored in a four byte mantissa and one byte exponent (Assume that all numbers are positive numbers). 4) A floating-point format uses four bytes for the mantissa and one byte for the exponent. If this is changed to three and two bytes respectively, what effect will this have on a) The accuracy and b) Range of the numbers stored? BIT Department Deans Community High School Page 7

7 1.5 Bit-mapped graphics For a graphic drawn in a painting package, the computer stores the necessary information as a two-dimensional array of pixels. Each pixel in a black and white image is represented by one bit in memory hence the term bit-mapped. Once drawn, each individual pixel can be edited i.e. the computer forgets the graphic as a whole and remembers only individual pixels. Each pixel making up the image has to be stored in memory. This graphic shows how the letter A, drawn in a painting package, would be stored in memory. When colour is introduced, the memory requirements to store the details of each pixel must be increased. For example, two bits can store four colours: 00 Black 01 Red 10 Yellow 11 White Similarly, eight bits can store 56 colours, 16 bits ( bytes) can represent 65,536 colours with 4 bits (3 bytes) allowing colours (Known as true colour) Calculating Bit Mapped Storage Requirements To do this we need to know 3 things, size of the image, resolution and bit depth. Example 1 Calculate the amount of storage required for a 1 square inch bit-mapped graphic with 640 x 480 pixels and 16 colours. The formula is Pixels Across multiplied by Pixels Down multiplied by Bit Depth (NOT number of colours) 1 x 640 x 1 x 480 x 4 bits = bits / 8 = bytes /104 = 150 Kb Example Calculate the amount of storage required for a true colour 600 dpi bit-mapped graphic which is 6 inches by 4 inches. Pixels Across multiplied by Pixels Down multiplied by Bit Depth 6 x 600 x x 4 x 600 x 4 bits = bits /8 = bytes /104 = 5315 Kb /104 = 4.7 Mb BIT Department Deans Community High School Page 8

8 1.5. Data Compression As we have just seen, Bit-mapped images can need a lot of storage. A colour bit mapped image with a high resolution and 4 bit colour needs over 50Mb for a smallish photo. To combat this, File Compression is used to reduce storage requirements. There are many Different techniques, including using complex mathematical equations, coding using an index of colours actually used and not coding differences indistinguishable to the human eye. The common factor is the need for bit mapped images to be reduced to save memory space, especially if being ed. 1.6 Vector graphics In a CAD or drawing package, the computer stores information about an object by its attributes i.e. a description of how it is to be drawn. For a rectangle it might be: start x and y position length, breadth and angle of rotation thickness and colour of the lines colour fill etc. The only data stored is that concerning the actual object drawn. This means that the rectangle can be selected at a later time and altered by changing its dimensions, dragging to a new position etc. Any editing is done at the level of the attributes e.g. it is possible to change the colour of the lines forming the rectangle and the interior fill. Editing at pixel level is not possible e.g. you can t change the colour of any individual part of the rectangle. Exercise 1. Bit-mapped and vector graphics (sometimes called object-orientated graphics) use vastly different amounts of storage depending on the graphic. Try using a paint and a drawing package to investigate this. You should save a blank file in each. Draw the same small picture (perhaps a circle, rectangle and triangle) in both, copy and paste this to different parts of the screen five times and save the two graphics with different file names. Repeat the pasting and saving procedure three or four times, being careful not to overwrite the previous files on each occasion. Now look at the file sizes generated and see what conclusions you can draw.. If a screen, 800 by 600 pixels in size, is showing a black and white bit-mapped image, calculate the memory requirements in bytes to store this graphic. 3. How many bits are needed to differentiate 56 colours for one pixel? If the same 800 by 600 pixel screen displays 56 colours, recalculate the memory requirements for Q. 4. What is the relationship between the bit depth and file size? BIT Department Deans Community High School Page 9

9 TOPIC COMPUTER STRUCTURE.1 Categories of computer After studying the following section you should be able to describe and distinguish between the hardware characteristics of single-user (desktop, laptop and palmtop), multi-user and networked computers in terms of input, process and output devices..1.1 The main categories Computers are normally classified into these categories: Mainframe Embedded Computer Desktop computer Laptop computer Palmtop computer Networked computer. The last four can be brought together under the heading of microcomputers i.e. a computer built around a microprocessor. Your class notes have a lot more detail on each of these. These categories are defined by size rather than by critical factors such as processor speed. For example, the processor speed of a new desktop computer is probably equivalent to that of a mainframe of a few years ago. A mainframe computer is a high-level computer designed for the most intensive computational tasks. Multiple users connected to the computer via terminals share the processors of mainframe computers. They perform highly complex and time-consuming computations and are used heavily in both pure and applied research by scientists, large businesses, and the military. A Palmtop Computer is a pocket sized computer, that tends to run applications that are useful to individuals, eg diary, address book, calculator, etc. An Embedded Computer is hardware and software combined to do a specific function, e.g. in a washing machine. They are always part of a larger system, and work in real-time A networked computer is just a computer (usually a micro) that has access to a network. A terminal on a mainframe would not be considered a networked computer as it has no processor. Networked computers each have their own processor and memory. They may or may not have their own local backing storage but will have access to a file server for programs and/or data. The server will probably be local but could even be on the Internet or an Intranet. Further details on networked computers are given in the notes for Topics 5 and 6 related to a networked operating system. BIT Department Deans Community High School Page 10

10 .1. A comparison of the different categories of computer Your task here is to try and differentiate between the various categories by copying and completing the following tables using information given in Web pages (your teacher will tell you how to access them), information available in your room from computer adverts etc., and your own knowledge: Computer Category No & Type of Processor Speed of Processor Memory Size Backing Storage Input/Output Devices No of Users Typical Uses Mainframe Micro Network File Server Computer Category Display Type Backing Storage Input/Output Devices Memory Size Processor Speed Portability Typical Uses Desktop Laptop Palmtop Networked Terminal After completing the above tables, discuss which of the headings are more suitable for categorising different types of computers. BIT Department Deans Community High School Page 11

11 Exercise 1) What three headings would you use to give the best comparison between a mainframe system with terminals and a networked system of microcomputers? Explain your choices. ) A company wants to equip its sales force with either laptop or palmtop computers. What two headings would be best for you to build a decision upon? Explain your choices. 3) For what kind of tasks would a mainframe system be better than a networked system and vice-versa? 4) Technical data on microcomputers is readily available but less so on more powerful computers. Use web pages to try to get data on mainframes and supercomputers. One name to try is the Cray T3E. BIT Department Deans Community High School Page 1

12 . Computer organisation You should already be familiar with the following terms and concepts: processor, ALU, control unit, registers, word, RAM, ROM and CPU. After studying the following section you should be able to explain that a computer is a twostate machine and explain the organisation of a computer in terms of CPU, memory, buses and the fetch-execute cycle...1 The structure of a computer, a two-state machine You will be familiar with this four box diagram of a computer and you will be extending this view in more detail so the fundamental way in which a processor and memory is used to execute a program becomes clear. All components used in a computer system and all data storage devices have only two states. For example a switch is off or on, transistors conduct or don t conduct, and a signal is a pulse of electricity or no pulse. Binary, using the numbers 0 and 1, can be represented by all these states. The main advantages of this are: Simplicity in only having to generate and detect two voltage levels Good tolerance because a degraded 1 is still recognisable as a 1 The calculations are also simpler as, for example, only four addition rules are needed for 0 and 1 against 100 rules for our numbers 0 to 9. [0 + 0 = 0; = 1; =1; = 10] Memory Memory can be divided into the following categories: RAM (or Random Access Memory) is memory that has the same access time for all locations. It is said to be volatile because it loses its contents when the power is switched off. RAM may be either static, which holds its contents as long as there is a power supply, or dynamic, which has to be refreshed by reading and rewriting its contents frequently (about every milliseconds). Dynamic RAM (or DRAM) is more widely used than static RAM (SRAM) because it needs less power and its circuitry is simpler. ROM (or Read Only Memory) is memory whose contents may be read but cannot be written to, that is, it is permanent or non-volatile memory. Software and data are fixed into the ROM during manufacture. PROM (Programmable Read Only Memory) is a type of ROM that is manufactured as an empty storage array and is later permanently programmed by the user. EPROM (Erasable PROM) is a type of ROM whose data can be erased by a special process, often exposure to ultraviolet radiation. BIT Department Deans Community High School Page 13

13 Memory maps are often included with the sale of the computer. These describe the way storage is organised in the computer. For example, parts of memory may be allocated to the screen display, program code, variables etc. This information can be useful for programmers to know. External memory, such as the hard disk, holds quantities of data too large to store in main memory. It is also used to keep a permanent copy of programs and data. Examples of external memory devices are: hard disk; floppy disk; zip disk; CD-R; magnetic tape; flash drive...1. The links between the processor and memory To execute a program you must first load from disc the program and any relevant data into the RAM section of computer s memory. The program and data are stored in RAM until needed by the processor. This is called the stored program concept. A program may contain thousands of instructions but the processor can only execute one instruction at a time. The first instruction is fetched from memory into the processor where it is decoded and executed. Then the second instruction is fetched, decoded and executed, and so on until the program ends. This is known as the FETCH EXECUTE CYCLE. To execute the cycle, the processor has to be able to pinpoint any memory location where instructions or data are stored. Each memory location is assigned an ADDRESS. This is a unique binary number from 0 up to the (number of locations 1) The size and number of memory locations The word length of a computer is the size of the data, in bits, which can be manipulated as a single unit by the processor. The size is determined by the width of the data pathways (or buses) within the computer. In an ideal computer the size of the data pathways and the size of memory locations will match. Thus, if the size of the memory locations and the word length is 16 bits, then every fetch will bring the contents of one memory location (two bytes) into the processor. However, sometimes the ideal situation does not always prevail. Early computers had a word size of one byte and a four byte integer would take four fetches before it could be used in a calculation. Most modern microcomputers have a word size of four bytes so the same integer can be brought into the processor with one fetch. A dramatic speed increase! BIT Department Deans Community High School Page 14

14 ..1.4 Buses Obviously information has to be transferred backwards and forwards between memory and the processor and this is done by buses. A bus is an electronic highway or a collection of cables where each cable can transmit one bit. Usually several wires are activated simultaneously to transmit whole byte(s). There are three buses: The address bus that is used by the processor to pinpoint the memory location needed. This is a one-way bus. Remember that every memory location has a unique address that is used to locate it, this is referred to as addressability. The data bus that is used to transfer the data. Its size will usually match the word size i.e. the size of the memory locations. This is a two-way bus. The control bus, which is used to initiate and control what is happening. A newer processor might have a 36-bit address bus and a 3-bit data bus whereas an older processor might have 16-bit address and 8-bit data buses. The control bus is not really a bus at all. Each wire is used for a separate purpose whereas the data and address buses consist of a collection of wires that are always used together. Some of the common lines in the control bus are reviewed here: Read: Write: Clock: A signal from the processor that initiates a memory read operation once the address bus has been set up with the appropriate memory address. A signal from the processor that initiates a memory write operation once the address bus has been set up with the address. This signal synchronises related components by generating pulses at a constant rate. A typical clock might be 400 MHz (400,000,000 ticks per second). Clock pulses are used to trigger components to take their next step, so keeping all components in time with each other. Interrupt: This is a signal into the processor that informs the processor that some external activity has taken place like a key-press. The current state of processing is saved and the processor then deals with the device that made the interrupt returning to the previous state when the interrupt has been dealt with. If the processor is doing something important, it may ignore the interrupt for a short time. NMI: Reset: The Non-Maskable Interrupt operates like an interrupt but it cannot be ignored. It must be dealt with immediately. This would be used to signal serious situations such as a fall-off in power in the computer or situations like a meltdown in a nuclear reactor. This is a signal into the processor which, when activated, clears all internal registers, aborts the current program and starts fetching instructions from a predefined place to give control back to the operating system. BIT Department Deans Community High School Page 15

15 ..1.5 Read from memory A processor needs an instruction from memory or requires some data to perform a calculation. Here are the steps and buses needed to fetch that information from memory. STEPS Processor sets up address bus with the appropriate address EFFECT This pinpoints the desired location Processor activates the read line on the control bus This tells the location that it is to be read from Data from the memory is copied onto the data bus Data is transferred to the processor Your teacher may give you the Fetch-Execute program to see some of these features in action. Exercise 1. Create a table similar to that above to show the steps needed to write data to a memory location.. Name the three types of bus in a CPU and describe the purpose of each bus. 3. Why is the address bus a one-way bus? 4. Why are the address and data buses true buses while the control bus not really a bus? 5. Explain the concept of addressability when referring to memory... The structure of the CPU The CPU can be considered to be made up of three components: Arithmetic and Logic Unit (ALU) Control Unit Registers. The ALU is the part of the CPU where data is processed and manipulated. This generally involves arithmetic operations or logical comparisons that allow a program to make decisions. The Control Unit is the part of the CPU that manages the execution of instructions. The control unit is responsible for initiating the fetching, decoding and execution of instructions. It does this by sending control signals to other parts of the computer. The other main component of a CPU is the registers. A register is a storage location inside the CPU itself. A register can be used for tasks like holding data for a calculation, storing the address of the next instruction to be executed or holding the instruction as it is being decoded and executed. Modern processors are so fast that it makes sense to keep as much data on the processor as possible. This means that it can be accessed immediately rather than waiting while it is being fetched from memory. BIT Department Deans Community High School Page 16

16 There are many registers including the: Memory address register (MAR), Memory data register (MDR), Program counter (PC), Stack pointer (SP), Accumulator (A), General purpose registers and the Status register. Older computers might only have those registers mentioned above (or similar) but modern processors will have many more registers than that. Some have over 100 general-purpose registers. The diagram shows how the buses and the three components making up the CPU link together. Note that there are also internal CPU buses so that data can be transferred inside the processor....1 Clock Every processor has a clock that ticks continuously and is used by the control unit to tell all the components when certain tasks have to be done. A current (1998) processor might have a clock of 00MHz (megahertz) that means that the clock ticks 00,000,000 times a second. So a processor with a 00MHz clock will be faster than a similar processor with a 150MHz clock. Exercise 1. Static RAM is generally faster than dynamic RAM and doesn t need to be continually refreshed. So why is most of the RAM in a computer dynamic?. Define the terms ALU, control unit and register. 3. How do the ALU, control unit and registers communicate with each other? 4. Describe how the control unit communicates with the other parts of the CPU...3 Measures of processor speed When comparing one computer s performance against another, one of the main criteria is the clock speed of the processor. It takes little intelligence to figure out that a faster clock gives you faster performance. This is certainly true if you are comparing a Pentium Pro 150MHz against a Pentium Pro 00MHz. However you have to watch if you compare a 00MHz Pentium against a 180MHz Cyrix or a 0MHz PowerPC. There is nothing in the last sentence to tell you which processor is the fastest. BIT Department Deans Community High School Page 17

17 Another measure that is often used is the number of instructions that the processor executes per second (often called Mips millions of instructions per sec). This sounds a better measure of system performance but some manufacturers will just time the simplest and fastest instructions only. It is also very difficult to compare the speed of processing instructions from a Pentium against totally different instructions from an Alpha processor. A better measure is Flops (floating point operations per sec). The procedures involved in doing a floating-point multiplication are basically the same for every processor and these kinds of operations are used in most software so they provide the basis of a reasonable comparison of system performance. Probably the only true measure is to try the software you want to use on the various processors in your normal working situation to see which is best for your work but that is rarely possible. This is simulated by using Benchmark Tests. These are well defined standardised routines to test the performance of a computer. The two most common are: Dhrystone tests string and frequently used functions Whetstone test using arithmetic functions..4 Memory and system performance There are various aspects of memory that affect system performance, for example speed of access, word size, amount of memory and cache memory. If you look at computer adverts you will find many kinds of computer memory with differing access speeds normally in the 15 to 90 nanosecond range. Unfortunately you can t just take the fastest memory and put it in your computer. The motherboard and processor will dictate what memory type must be used. The main way to improve system performance is to increase the amount of memory that you have in your computer. If your computer is struggling to run some software at an appropriate speed or you can t load all the software that you want at the same time, then adding extra memory improves your system s speed especially if you are involved in graphics or multimedia. Adding memory beyond that however doesn t give you any advantages apart from future proofing. The way modern software is increasing in size means that you normally can t have enough memory...5 Cache Most modern systems employ a cache. This is a section of memory between the processor and the main memory. It has very fast access, so sections of a program and associated data are copied there to take advantage of its shorter fetch cycle. Some processors have a cache on the processor itself which makes for even faster access. Caches are also used between a disk and memory (a disk cache). They help speed up the transfer of large amounts of data from disk. BIT Department Deans Community High School Page 18

18 ..6 Bus widths and system performance The width of the data bus defines how much data can be accessed in one fetch. Obviously a 3-bit data bus can carry twice as much as a 16-bit data bus but this does not mean that it can go twice as fast. Other factors come into consideration. However, a 3-bit processor will be distinctly faster than a 16-bit processor of the same system type. The width of the address bus defines the maximum amount of memory that the processor has the potential to access. (A computer might not be supplied with that size of memory for reasons such as the cost of memory or because the applications normally used don t need that much). A single line can address locations with addresses 0 and 1 i.e. two locations while two lines can generate the four addresses 00, 01, 10 and 11. Every extra line means you double the number of potential memory addresses. To find the maximum memory size that a 4-bit address bus can support, assuming a location size of two bytes, you can use one of the following methods: Method 1 4 = * bytes Because every location is two bytes long = / 104 K = 3768 / 104 M = 3 M Method The second method is to realise that 10 =1K. 4 bits has two sets of 10 bits i.e. 1K each and 4 bits left over which means you have to double 4 times. 4 = 1K * 1K * * * * locations Memory size = 1M * 16 locations = 16M * bytes = 3 M Exercise 1. Calculate the maximum size of memory for a computer with a 16-bit address bus and 8-bit memory locations.. Calculate the maximum size of memory for a computer with a 36-bit address bus and 3-bit memory locations. 3. If a processor has a 3-bit address bus, how many memory locations can it access? 4. Many micros have a 3-bit address bus and 3-bit memory locations but only have 3Mb or 64Mb of memory. Why do you think this is? 5. Use a computer magazine or web pages to find out typical memory and cache sizes for today s microcomputers. 6. Computer magazines often use benchmarks to compare the performance of similar computers. Try to find out what a typical benchmark is. 7. Investigate what is meant by the benchmarks SPECint95 and SPECfp95. Use web pages to find the results reasonably quickly. BIT Department Deans Community High School Page 19

19 EXTENSION EXERCISE FOR OUTCOME 1 These questions are for revision and preparation for the external examination. Your teacher will advise you if and when you should tackle these. 1) Give three reasons why digital computers make use of the binary system. 3 ) a) Originally, different computers used different codes to represent keyboard characters. Identify a problem that would arise from this? 1 b) IBM mainframes still use EBCDIC as opposed to the ASCII code used by most other computers. How might the problem you identified in (a) be resolved in this case? 1 3) Describe how a negative number is stored in binary. You may use an example to illustrate your explanation. 4) Why is twos complement necessary? Try to give two reasons. 5) a) A microcomputer uses 4 bytes to represent an integer. Use an example to describe how the bit pattern used in this representation would be understood by the microcomputer. b) A microcomputer uses 5 bytes to represent a real number. Use an example to describe how the bit pattern used in this representation would be understood by the microcomputer. c) The representation of real numbers is a compromise between the precision and the range of the numbers that can be stored. Given that 5 bytes are used to represent a real number, describe how the precision of a real number can be increased and state how this would affect the range of numbers that can be stored. 6) What characteristic would you expect a palmtop computer to have that a desktop computer would not have? 1 7) Describe two methods that could be used to identify if a graphic was created in a bitmapped or object-orientated graphics package. 8) If two otherwise similar computers have a 16-bit and a 4-bit data bus respectively, explain clearly why one will be faster than the other. 1 9) State three advantages of a computer with a lot of memory. 3 10) If your task were to select a computer for use in desktop publishing in a school, what characteristics would you be looking for in the computer? Give five with reasons. 5 BIT Department Deans Community High School Page 0

20 11) State three ways in which the speed of a processor can be measured. 1) a) What is a benchmark? b) A few years ago, benchmarks were often the timing of how long specific programs took to run. Nowadays, they test things like how long a large spreadsheet takes to recalculate, a complex database search etc. Which do you think gives the better test and why? 13) Why doesn t adding more memory to a computer system automatically speed up the system? 14) A lot of memory is one way of future proofing a computer. What does this mean and why is it true for memory? 15) Explain what a cache is and how it speeds up processor throughput. 16) Explain the effect each of the following has on processor throughput: a) Increasing the width of the data bus; b) Increasing the clock speed; c) Increasing the width of the address bus BIT Department Deans Community High School Page 1

21 CONTENTS Topic 3 PERIPHERALS Peripherals and System Performance Factors which affect System Performance Interfaces and Input-Output devices More on Interfaces...5 Keywords Input, Output & Backing Storage...8 MODIFIED KEYBOARD...8 Keywords...8 SCANNER...9 Keywords...10 SOUND CARD...11 Keywords...13 GRAPHICS CARD...14 Keywords...16 VIDEO CAPTURE BOARD...17 Keywords...18 DIGITAL CAMERA...19 Keywords...0 INK JET PRINTER...1 Keywords... LASER PRINTER...3 Keywords...4 FILM PRINTER...5 Keywords...5 MULTI-SCAN MONITOR...6 Keywords...7 DIGITAL AUDIO TAPE...8 Keywords...9 MAGNETIC DISK...30 Keywords...31 MAGNETIC TAPE...3 Keywords...3 MAGNETO-OPTICAL DISK...33 Keywords...33 OPTICAL DISK...34 Keywords...35 SOLID STATE DEVICES Selecting Hardware to Match Operational Requirements...37 BIT Department Deans Community High School Page 1

22 TOPIC 3 PERIPHERALS 3.1 Peripherals and System Performance Peripherals generally perform at much slower speeds than the CPU and, unless steps are taken, these will have the effect of reducing system performance drastically. Buffers and spooling to disc are two examples of the steps that can be taken to alleviate this situation. Peripheral manufacturers are addressing the problem by increasing the power of the interface. For example, a sound card controlling the speakers will have its own processor, RAM and ROM. After studying the following section you should be able to explain the different concepts of topologies, resource sharing and data integrity and how they affect network system performance. 3.1 Factors which affect System Performance Interfaces and Input-Output devices Computer peripherals such as disk drives, printers, mice etc. all work in totally different ways and linking them to the processor is a difficult task. They all work at different speeds, use different codes, transfer different amounts of data at a time, and even work at different voltages. An interface must be used to make the link between the processor and a peripheral. It is the hardware and software needed for communication between a processor and a peripheral device, to compensate for the differences in their operating characteristics. Its task is to transfer data so that the processor is delayed as little as possible. To do this, the interface has buffers that store blocks of data in transfer between the CPU and its peripheral. It also has a register that stores the status of the peripheral so that the processor knows when the peripheral is able to send or receive data and when problems have occurred. It also has to convert processor commands into commands that the peripheral can understand. For example printing might involve the following steps: The processor checking with the interface to see if the printer is ready to print The interface sending back an OK The processor sending data to the interface until the interface says its buffer is full The interface sending the data to printer The interface signalling that it is ready to accept more data. Most peripherals also have buffers of their own. Interfaces and peripherals often have their own processors and memory to control their operation. Some laser printers have a better processor and more memory than the computer it is connected to. BIT Department Deans Community High School Page

23 Most processors use part of their main memory to access the interfaces. The processor has 64K of memory dedicated to I/O and the interfaces link to these memory locations. If the processor wants to send data to a peripheral it writes data to the locations specific to that peripheral. It also reads data from the same locations if it is being sent data or if it is checking the status of the peripheral. This is known as memory mapped I/O. (as shown in diagram) Another approach to ensuring the processor is not held up by slow peripherals is to send large quantities of data to a spool file on disk. This is especially useful while dealing with a slow peripheral like a printer. The data is kept on disk and transferred to the printer gradually as the processor has time to deal with it. This is called background printing. Some processors have special buses and commands for I/O but this is generally more expensive to implement. Exercise 1. Some interfaces can handle more than one device such as two hard drives. How will the interface be able to choose between them?. If the interface has no buffer or a small one, what effect will this have on the processor s performance? BIT Department Deans Community High School Page 3

24 3.1. More on Interfaces As we have just seen, in order to connect input, output and backing storage devices to the computer an interface is required. The interface acts as the boundary between the device and the CPU and provides data conversion, data storage, status information, control signals, and device selection. A number of standards are used in the computer industry. Some of these are outlined briefly here. Parallel Interface Parallel data transmission involves transferring data between the peripheral device and the CPU one word at a time. A series of lines, one for each bit in the word are used to do this. Data can only be transferred using parallel communication over a short distance due to the electrical differences between different lines. Once data has travelled more than a few metres skewing tends to occur which means that bits arrive at the destination unevenly. Parallel Port - Centronics Most computers will have a parallel port which allows direct connection to a scanner or printer using a standard 36 pin connector. IDE The hard disc and CD-ROM (or CD-R / DVD) in a typical computer may well be connected to the motherboard using an IDE (Integrated Drive Electronics) interface. This form of parallel interface allows data to be transferred quickly between the hard disk and the central processing unit. There have been a number of improvements to the IDE standard in order to increase throughput and data transfer rates. Typically, a computer will be able to support up to 4 devices using an IDE interface. SCSI SCSI (Small Computer System Interface) allows up to eight peripheral devices to be connected to the computer in a daisy chain fashion. Each device is given a different identification number to allow data to be addressed correctly. This is particularly useful where a combination of scanners, CD-ROM drives, printers, etc are connected to one computer. PCI Expansion cards can often be fitted inside a computer. A common standard for this is PCI which allows graphics, sound, Ethernet and modem cards to be connected directly to the motherboard. This direct connection to the motherboard allows the attached cards to communicate via a very fast parallel port. BIT Department Deans Community High School Page 4

25 Serial Serial data transmission involves data being transferred one bit after another down a single line. This is a slow way to transfer data but it is possible to use this method over far grater distances. RS3 External modems, keyboards and mice are often connected via this form of interface. Data is transmitted one bit at a time in groups of 8 bits with extra padding and check digits to ensure data integrity. USB This is a relatively new form of interface which allows very fast serial communication between devices. It has a number of advantages over other forms of interface: It is possible to connect a large number of devices (up to 17) to the computer using USB. A hub can be connected which allows a large number of USB devices to be connected to a single socket in the computer. The USB cable carries a 5V power supply so devices requiring a low consumption power supply can be connected to the computer without the need for another 13A plug. It allows serial data transmission at up to 1 M/bps. It is a plug-and-play technology in that devices can be plugged in and unplugged without the need to restart the computer. The device is detected as soon as it is connected to the PC. Keywords Interface, parallel, serial, word, bit, IDE, SCSI, PCI, ISA, RS3, USB. BIT Department Deans Community High School Page 5

26 Exercise When decisions are made on purchasing a peripheral device comparisons have to be made. Use an example from the range you have studied to highlight these comparisons. When a family purchases a new computer system they will often wish to purchase a printer as well. What will influence their choice? Magnetic tape has been used for a great many years as a medium for backing up. Based on the knowledge you have gained in this unit are there any situations where this may not be the best choice? You are considering purchasing a new digital camera. What factors will affect your choice? The cost will often be an important factor when considering buying a new scanner. Why must care be taken when you are on a tight budget. You should now be aware that the pace of technological change is very fast. How can this be taken into account when making a decision about purchasing a new CD-ROM drive? BIT Department Deans Community High School Page 6

27 3. Input, Output & Backing Storage MODIFIED KEYBOARD General Description The keyboard is the most widely used input device for the computer. When a key is pressed, an 8-bit binary word representing the character is generated. This binary word is usually in ASCII (American Standard Code for Information Interchange) format. The binary data is then transferred to the computer using serial data transmission. The standard keyboard uses the QWERTY layout which was originally developed to slow down typists using early typewriters. These typewriters jammed if too many keys were pressed quickly so the layout that we are all familiar with came about as a result. This layout has drawbacks for some applications so modified keyboards have been developed. Modifications can be made for a number of reasons. Here are some: Ergonomic design Reducing the number of keys Disabled users Specialist applications The ergonomic design of the keyboard can make it easier for a typist to enter text quickly and can reduce the chance of health and safety issues such as Repetitive Strain Injury (RSI). Ergonomically designed keyboards produced by companies such as Apple and Microsoft split the keyboard into two halves and angle them slightly to allow the users elbows to rest in a more natural position. The two halves will also have wrist supports to reduce the strain on the wrist. Some will have a detachable numeric keypad to allow the mouse to be moved closer to the keyboard. Experiments have been carried out to reduce the number of keys on the keyboard. Any ASCII character can then be created by pressing a combination of keys. The main drawback to this form of device is the length of time it takes to learn the key combinations required. Some keyboards have been designed to allow disabled users to use the computer more effectively. Modifications include increasing the size of the keys, grouping them in different ways or increasing the size of the lettering on the keys for users with poor eyesight. Some keyboards are modified to add extra keys or to limit the range of keys for that application. Examples include CNC lathes and specific applications within call centres. Keywords ASCII, ergonomic, RSI, serial data transmission. See also Interfaces BIT Department Deans Community High School Page 7

28 SCANNER General Description A scanner is an input device which allows graphical data to be captured by the computer. This data can be in the form of photographs, line drawings or even text. To capture the data, the scanner reflects light off a paper image which is placed on a sheet of glass. The reflected light is gathered by photosensitive cells. The electrical current representing the reflected light intensity passes through an analogue-to-digital converter to create a bit map of the original image. This digital image is then sent through an interface to the computer. A typical flat bed scanner comprises an A4 glass plate underneath which a scan head is moved down the length of the glass in small steps. The scan head includes both a light and an array of photosensitive cells. The light is reflected off the image and picked up by the sensors a strip at a time. The control software for the scanner usually allows a number of image characteristics to be altered. It is important to be aware of these as the file size created for a single A4 page can be as high as 0M. Resolution The resolution of a scanner is a measurement of how close together the scanned pixels are located. It is usually measured in dots per inch (dpi). Images scanned for display purposes on a monitor (e.g. for use in a multimedia presentation) should normally be scanned at 7 dpi as this will match the resolution of most monitors. Most flat bed scanners can give a resolution of up to 100 dpi which is appropriate for most laser printers. If the images being scanned are for use in commercial publishing applications then a higher resolution may be required. Bit depth The number of bits allocated to each pixel indicates the number of colours available to create the image. 8 bits will give 56 colours, 16 bits will give and so on. BIT Department Deans Community High School Page 8

29 Type of picture The type of scan used will depend on the source image and the actual use to be made of the scanned image. Line art Grey scale True colour Line drawings can be scanned using this mode. The resultant file size is small as each pixel is either a 1 or a 0 representing black and white dots in the image. Black and white photographs can be scanned without the extra data required for colours that are not used. If an 8 bit colour depth is used then 56 different shades of grey are possible. This use of a specialised colour palette will reduce the file size whilst retaining a very high quality image. To scan colour photographs true colour is required. This means that 4 bits are used for each pixel. The resultant file size is large but gives good photo-realistic images for desk top publishing and similar applications. Image size An A4 image from a scanner will generate a very large file so it usually preferable to select only part of the A4 area for storage. This cropping will drastically reduce the file size as only the required part of the image is captured. Compression Once the image has been scanned it has to be stored on a backing storage medium such as a hard disk. To reduce the file size it is possible to compress the image. A number of standard data formats have been developed for compressing image files. These include JPEG (Joint Photographic Expert Group) and GIF (Graphic Interchange Format). OCR It is also possible to use Optical Character Recognition (OCR) software to take the text from an image and convert it into text which can be editing in a word processor. This software compares each letter which has been scanned with a library of shapes of all letters in a variety of different fonts and styles. It extracts appropriate letters from this library to create text which can be edited. Keywords Analogue to digital converter, bit map, compression, grey scale, line art, OCR, resolution, true colour. See also Interfaces Monitors Laser printer Ink jet printer BIT Department Deans Community High School Page 9

30 SOUND CARD General Description The sounds we hear are generated by air particles vibrating backwards and forwards. These are continually changing with the pitch and volume of the sound. Sound is an analogue quantity and so must be converted to a digital form before it can be processed or stored in the computer. Analogue Signal Digital Signal The sound card is a hardware component which allows sound to be input to, and output from, the computer. Joystick / MIDI connection Wave Table Synthesiser FM Synthesiser CD audio connection Line in CD Interface Speaker out Microphone in amplifier DAC / ADC Connection to PC motherboard Memory This diagram shows the main components of a sound card. Many multimedia computers (including the Apple Macintosh series) incorporate the functions of the sound card into the motherboard. Sound Input The sound card uses an analogue-to-digital converter (ADC) to turn the analogue signals from a microphone or other audio source into a digital form that the computer can understand and store. This device samples the size of the electrical signal generated by the microphone and represents this by a number. The number of times this sound is sampled per second is known as the sampling frequency (or sample rate). The sampling frequency will typically be in the range of 11kHz to 44kHz. In other words, the sound will be sampled times per second. BIT Department Deans Community High School Page 10

31 As each sample is represented by a binary number in the computer s memory, the number of bits used to represent this number will determine the quality of the sample. Sound cards will often use 8 or 16 bits per sample. 8 bits gives a range of 56 tones, 16 bits gives The larger the range of numbers possible, the more accurate the sample will be. Khz sampling 44 Khz sampling Original audio source As can be seen from this diagram increasing the sampling frequency increases the quality of the audio. The example which has been sampled at 44KHz is much closer to the original sound. The sampling frequency and sampling resolution of the sound sample will affect the amount of memory required to store the sound. If the sound sampled is in stereo then the amount of memory required will be doubled. Let us look at the amount of data that a sound card has to process for a 5 second sample of sound to be stored on the computer. The sampling frequency is 44khz and the sampling resolution of each sample is 16 bits. The sound is to be recorded in stereo. 1 second of audio at 44KHz and 16 bits per sample = bytes = 86K The sound is sampled in stereo = 86K x = 17K 5 seconds = 17K x 5 = 860K Compression Audio data files tend to be very large so compression techniques are often used when the data is sampled. Whilst these techniques will reduce the overall size of the sampled data they will also cause a reduction in the final quality. Sound Output The output from the sound card is normally fed into an amplifier and speakers. The quality of the amplifier and speakers will have a large impact on the quality of the sound produced. BIT Department Deans Community High School Page 11

32 The output may be from a digital sample which is converted back into an analogue signal in the analogue-to-digital converter (ADC) or it may be synthesised using frequency modulation or wave table synthesis. MIDI (Musical Instrument Digital Interface) devices will use these synthesisers to create sound output, whereas the playback of samples will use the ADC. Frequency Modulation This method of generating a sound involves creating two sine waves. One acts as the carrier wave and the other is the modulator. When the two waveforms are close together in frequency a complex waveform is produced which creates a particular type of sound. Wave Table Synthesis This form of sound generation involves using sound samples of real instruments. The quality of the sound will depend on the quality of the original recording. When a sound sample is played at a different rate from which it was recorded then the pitch will be altered. This can be done for a number of different octaves. Many samples have to be recorded for each instrument to recreate a full range of sounds so a lot of data will have to be stored in ROM on the sound card. Typically a sound card may use as much as 4M of ROM to store the data required. Surround Sound Many modern sound cards have added features to give a more life like audio experience. This is particularly the case in multimedia systems and those designed specifically for games. It is possible to use software such as Microsoft DirectX to creates sounds as though they appear anywhere in 3D space. This is known as positional audio. The audio is split up into a number of different components and fed to up to six loud speakers. A great deal of processing power is required to be able to handle this form of audio output. Keywords Sample, sampling frequency, sampling resolution, Analogue to Digital Converter, Digital to Analogue Converter, wave table synthesis, frequency modulation, MIDI, compression. See also Interfaces BIT Department Deans Community High School Page 1

33 GRAPHICS CARD General Description A graphics card is an output device which is responsible for creating the picture you see on a computer screen. The operating system of most modern computers incorporates a graphical user interface (G.U.I.). This requires vast amounts of data processing which could slow down the running of the programs. Therefore the central processing unit allocates instructions relating to screen display to a graphics card to be processed and sent to the monitor. The card will have the following main parts: processor to interpret the graphics instructions, video RAM to store the current screen image, and digital-to-analogue converter to turn the digital information into an analogue signal for the monitor. Digital-to-analogue converter video RAM connection to monitor graphics processing chip connection to PC motherboard This diagram shows an example of a typical graphics card as found in many modern computers. However, some computers, for example Apple Macintosh, include the functions of the graphics card on the motherboard. Whilst the quality of the image is primarily a factor of the monitor itself, the features of the graphics card play an essential role in processing the data required for screen images. Let us look more closely at this data. Resolution The image on the screen is made up of a two dimensional array of pixels. The total image is made up of thousands of pixels. The resolution of the image on the screen defines the quality or sharpness of the image and is related to the number of pixels and their closeness together. To get an accurate understanding of what goes into making a screen image, we must consider the dot pitch and the bit depth. The dot pitch is a measure of the distance between the centres of two adjacent pixels. The dot pitch affects the number of pixels which can be displayed on the screen. The smaller the dot pitch the sharper the image. If one screen has more pixels in the space than another then the resolution of the image on the first screen will be greater. Typical examples are as follows: BIT Department Deans Community High School Page 13

34 Type of screen Pixels (breadth x depth) VGA 640 x 480 SVGA 800 x 600 XGA 104 x 768 SXGA 180 x 104 UXGA 1600 x 100 The bit depth used in a screen image also affects the quality of the screen image but it also makes a significant impact on the amount of data to be stored and processed by the graphics card. For every pixel in the image there must be a corresponding area of memory in the graphics card. The number of colours which can be displayed on the screen is dependent on the amount of memory used to store the information about each pixel. The more bits of memory used for each pixel the more colours that can be displayed. For example, if 8 bits are used for each pixel then up to 56 different colours can be displayed Each binary pattern from 0 to 55 can be used to represent a different colour. This table shows the number of colours which can be displayed for a range of bit depths. Bit depth Number of colours 1 (black and white) , ,777,16 A bit depth of 4 bits per pixel is often referred to as true colour as this amount of data is required to convince the human eye that the image is truly accurate. The colour of each pixel is determined by a combination of red, green and blue elements. In a bit depth of 4 bits per pixel, 8 bits are required for the reds, 8 for the greens and 8 for the blues in each pixel. Memory Requirements If we take resolution and bit depth together we can calculate the amount of memory required to display an image on the screen. BIT Department Deans Community High School Page 14

35 For example a screen image whose resolution gives 640 x 480 pixels and using a bit depth of 8 bits will require 300K of memory. The following table give some examples of the possible outputs from the graphics card depending on the amount of memory on the graphics card. Video memory Resolution Colour depth No. of colours 1Mb 104 x x bit 16-bit 56 65,536 Mb 104 x x x bit 16-bit 4-bit 56 65, million 4Mb 104 x bit 16.7 million 8Mb 1600 x bit 16.7 million Graphics cards are essential to modern graphical screen images. High resolution screens showing true colours require the support of graphics cards with lots of on-board memory. A large monitor will require more video memory to be able to display a high resolution image. Keywords Bit depth, memory map, resolution, video RAM. See also Multi-scan Monitor Video Capture Card BIT Department Deans Community High School Page 15

36 VIDEO CAPTURE BOARD General Description Digital video is becoming more and more widely used in a number of different application areas. To be able to transfer moving images from analogue video to the computer, a video capture card is required. This card converts each frame of the video into a bit mapped image. In the PAL system (Phase Alternating Line) each horizontal line is split into 768 sections. This results in 768 pixels per line. At each section a red, green and blue component is calculated. The standard TV has 65 lines but some of these contain information about teletext so only 576 lines are digitised. Digital video requires huge amounts of storage space: A full frame is made up of 768 x 576 pixels Each pixel requires 3 bytes for true colour. The PAL system takes two scans to make up a single picture so 1 second of video will be 768 x 576 x 3 x 5fps x = 65 M. Factors affecting system performance A number of factors need to be considered when capturing digital video: Frame rate Frame Size Resolution Colour depth Compression The frame rate is a measure of the number of times the picture changes every second. Television and cinema films typically use a frame rate of 5fps. The frame rate can be reduced without a noticeable reduction in viewing quality but if it drops below 15fps, the image becomes jerky. The size of the image on the screen is an important factor in determining the overall size of the file created. A frame size of 30 x 40 gives an image that is a reasonable size on the screen without requiring a huge amount of storage space. Reducing the image size will reduce the overall file size. The resolution of the image is a measure of the quality of the image. The closer together the pixels are the higher the resolution and the sharper the image will appear on the screen. The number of bits used to represent each pixel is an important factor. The more bits used the more colours can be displayed by each pixel. 8 bits per pixel will give 56 different colours. For some applications this may be acceptable but 16 bits per pixel are used to give realistic tones. Because of the huge file sizes created by digital video it is vital that the file is compressed. The video capture card will normally incorporate hardware and software which compresses the video data. BIT Department Deans Community High School Page 16

37 With the advent of digital video cameras and webcams the video capture card is not as vital as it once was. These devices which produce digital data can usually be connected to the computer directly and only require software to deal with the characteristics of the video file. Applications such as video-conferencing are possible using relatively cheap digital webcams connected directly to the computer via a USB interface. Keywords Compression, colour depth, frame rate, resolution. See also Interfaces Monitors Graphics cards Questions What effect does reducing the frame rate have on the quality of the video? Why does increasing the image size impact on the quality of video reproduction? What problems can you see with using the delta compression technique? What amount of data is saved during the compression? What visual effect can you observe as a result of the compression being used? Is the effect of the compression more or less important than the frame refresh rate? What effect does changing the size of the image have on the overall video replay? What hardware factors affect the effectiveness of video on the computer? BIT Department Deans Community High School Page 17

38 DIGITAL CAMERA General Description The digital camera is an input device which looks similar to a conventional camera in that there is a lens and a viewfinder. An image is created of the object being photographed. The lens causes this to be generated upside down on the back plate of the camera where the film is situated. The main difference in a digital camera is that the film has been replaced with an array of image sensors and a storage device. The image sensor is made from a two dimensional array of photosensitive cells. These cells generally use CMOS (complimentary metal-oxide semiconductor) technology. This is the same technology used to manufacture processors and memory chips. As CMOS technology is widely used these photosensitive arrays can be made fairly cheaply. Each cell converts the light which strikes it into an electrical signal which is proportional to the intensity of the light. -D array of photo-sensitive cells This electrical signal from the photosensitive cell is passed through an analogue-to-digital converter to a signal processor which adjusts the contrast and detail of the image. The digital signal is then compressed and sent to the storage device. Picture Quality The picture quality is dependent on a number of factors: Optical quality of the lens Image capture chip resolution As with any camera the lens quality has a huge impact on the quality of the image. Distortion of the image and focussing are improved by improving the quality of the lens. The number of pixels in the array of photosensitive cells can vary from 640 x 480 pixels in some of the cheapest devices, through 104 x 768, 180 x 960 and up to resolutions as high as 1800 x 100 pixels in professional quality cameras. Bit depth The bit depth of each element of the array will affect the quality of the image. The more bits allocated to each pixel, the more colours that can be used to make up the image. Compression algorithm If the image capture chip provides a high resolution image then a large amount of memory will be required to store the image. To reduce the amount of memory required, compression algorithms are used. Some manufacturer s use a proprietary compression format which can only be accessed by that manufacturers software but most will use standard formats such as JPEG which are readable in most graphics packages. BIT Department Deans Community High School Page 18

39 Storing the image The storage device in the camera can either be in the form of internal RAM or a small magnetic disk. Once the device is full the data must be transferred to a computer by means of a serial cable before the camera can be used again. Alternatively, removable storage can be used. This has the advantage that it can be replaced once it is full. Removable media is typically in the form of a Flash card. This is a form of non-volatile RAM. Once the data is written to it the card can be removed from the device without the data being wiped. The capacity of these cards varies from 18M to 300M. Adjusting the image quality The image quality can often be set by the user. This will allow the user to adjust the resolution and the bit depth. If high quality images are required then the resolution and bit depth can be increased. This will mean that far fewer images can be stored. If it is more important that a larger number of images are stored, then the resolution and bit depth can be reduced. Connecting to a Computer The digital camera is a device which does not require to be connected to a computer to use it. However, the images created by the camera must be downloaded to a computer before they can be manipulated using graphics software. Typically the data is downloaded using a serial connection. Software in the computer communicates with the camera and controls the flow of data into the computer. Keywords Analogue to digital converter, bit depth, CMOS, compression, resolution, serial. See also Interfaces Magnetic disk BIT Department Deans Community High School Page 19

40 INK JET PRINTER General Description Ink jet (sometimes referred to as bubble jet) printers are very popular output devices with home computer users due to the fact that coloured output can be produced relatively cheaply. High quality colour output is good for printing greetings cards, party invitations, banners and photo-realistic images. The ink jet printer works by squirting small amounts of ink through tiny nozzles. The print head scans across the page in horizontal bands using a stepper motor. With each pass of the print head a strip of the image is printed. To print colour, the process is slightly different from displaying colour on a monitor. A monitor uses three light sources to create the colours (the primary colours: red, green and blue (RGB)), the different combinations of these three colours fools the human eye into believing a new colour has been created. A printer, however, uses the subtractive primary colours (cyan, magenta and yellow (CMY)) to create the appropriate mix. From a distance the different amounts of each colour merge to create the illusion of a single colour. This process is known as dithering. Black Cyan Magenta Yellow The print head has a line of nozzles for each colour Most ink jet printers are able to print in colour and black and white. These printers have four ink cartridges corresponding to cyan, magenta, yellow and black (known as CMYK). These printers can swap between printing in colour and printing in black and white on the same page without any problem. Resolution The quality of the printed image is measured by the number and spacing of the dots of ink on the page. The image resolution is generally measured in dots per inch (dpi). The higher the dpi, the better the quality or sharpness of the printed image. As already stated, the print head moves across the page delivering a band of dots horizontally. The vertical and horizontal resolutions may, therefore, be different dependent on the number of nozzles on the print head and the distance moved across the page by the stepper motor each time. Typical examples of printer resolution used for home computers range from 360 x 360 dpi up to about 1440 x 70 dpi. BIT Department Deans Community High School Page 0

41 Number of Colours The number of colours which can be printed is the other important factor in the quality of the printed output. The simplest form of colour printer is one in which the cyan, magenta, yellow and black dots are either on or off and only eight colours can be printed. The data from the computer will require four bits per dot to achieve this. To improve the number of colours a technique known as halftoning can be used. This divides the dot resolution into a grid of halftone cells and then turns on or off a varying number of dots within these cells in order to mimic a variable dot size. Some manufacturers have increased the number of colours of ink cartridges to six thus increasing the number of colours which can be reproduced. These printers tend to produce a higher quality of photographic image. Speed Printer speed is measured in pages per minute (ppm). Ink jet printers must repeatedly move the print head across the page in horizontal bands. The print head must take a very small amount of time to decide whether a dot is to be printed or not (about 1/5000 th of a second). For this reason ink jet printers appear to be quite slow. Typically they range from about 4 ppm up to 1 ppm. Running Costs Although an ink-jet printer is fairly cheap to purchase, the running costs are far higher than laser printers. Ink cartridges need to be replaced more frequently than laser toner cartridges and special paper is required for high quality images. Statistics suggest that ink jet printers are about ten times more expensive to run than laser printers. Keywords CMYK, dithering, dots per inch, halftoning, pages per minute, resolution, RGB. See Also Interfaces Laser printers Monitors BIT Department Deans Community High School Page 1

42 LASER PRINTER General Description Laser printers are output devices which are predominantly used in office environments where fast printing of high quality documents is required. Although most laser printers print black and white images, it is possible to adapt the technology to print colour images. The laser printer works as follows: A page description language such as Postscript is used to communicate the data from the computer to the printer. A laser scans the surface of a specially coated drum which can hold electrostatic charge. The laser is switched on and off corresponding to whether the point in the image is black or white. Where the laser hits the drum points of positive charge are left. These points of charge represent a copy of the final image. As the drum rotates it picks up toner (a black powder) in the areas where there is a point of positive charge. A sheet of paper is pulled across another charged area making it negatively charged. As the paper comes in contact with the drum the toner is attracted to the negatively charged paper. This results in the image being transferred to the paper. The powder is designed to melt quickly and the final stage involves the toner being fused to the paper under pressure. Finally the drum must then be cleaned before the next page is printed. Mirror Laser Fuser Drum Paper Laser printers can be connected to a stand-alone computer using a parallel interface or can be connected using a printer server on a network. Some of the more expensive printers will have a combination of different interface options to allow flexibility in their use. In a colour laser printer, four passes are made through the electrostatic process corresponding to the colours cyan, magenta, yellow and black. Colour laser printers are still very expensive devices to purchase and to run and are usually only used for specialised applications. Resolution The resolution of a printer is generally measured in dots per inch (dpi). The higher the resolution, the more dots there are in every inch and the sharper the image will appear. Most laser printers will have a resolution of 600 or 100 dpi. Speed The data for the page is all sent to the printer before the printing is started. This means that it appears as though the printer is doing nothing for a period of time before the paper starts to BIT Department Deans Community High School Page

43 be passed through. The printer speed is measured in pages per minute (ppm). Speeds vary between 6 and 0 ppm Buffering Laser printers are often used to print large documents and system performance can be improved dramatically by incorporating a buffer. This is an area of RAM in the printer which allows data to be downloaded from the computer to the printer and frees the processor to resume other tasks. Buffer sizes from 4M to 0M of RAM are common. Cost Although expensive to buy, laser printers are relatively cheap to run. The toner cartridge will produce thousands of pages, and as they are mostly used for textual data which has a lot of white space, the toner can go a long way. Keywords Buffer, dots per inch, pages per minute, resolution, toner. See also interfaces ink jet printers BIT Department Deans Community High School Page 3

44 FILM PRINTER General Description Video projectors are a very common form of output device for presentations by business people and lecturers. Unfortunately, video projectors are not powerful enough or give high enough resolution in large lecture theatres. In situations such as this, slide projectors are often used to display high resolution images. A film printer (sometimes referred to as a film recorder) is a specialised output device which allows data from a presentation package such as PowerPoint or a graphics package such as PhotoShop to be printed on photographic film. This film is often in the form of 35mm slides which are widely available and used for presentation purposes. A camera is set up so that it is focussed on a cathode ray tube (CRT) monitor. To create the image on film the output from the computer is separated into its red, green and blue components before being displayed on the monitor as three separate images, one after another. A filter is placed in front of the screen in the appropriate colour and an exposure taken. This process is repeated three times to make up the full picture. The film must then be developed using traditional photographic techniques. Resolution Film recorders are available with various resolutions. The most common ones are known as 4K and 8K. A 4K film recorder is capable of creating 4,096 pixels along the long axis of the film being exposed. Since 35mm format has a 1:1.5 aspect ratio, this means that the image would be,731 pixels x 4,096 pixels. An 8K film recorder is capable of creating 8,19 pixels along the long axis. In order to attain the full available resolution of a film recorder, it is necessary to send it a file with enough information. The optimum file size for an 8K film recorder creating 35 mm slides is calculated as follows: 8,19 (horizontal) x 5,461 (vertical) x 3 (colours - RGB) = 18M This means that high capacity backing storage will be required if a number of images are to be transferred to film. Keywords Cathode ray tube, pixels, resolution. See Also Interfaces Ink jet printers Laser printers Monitors BIT Department Deans Community High School Page 4

45 MULTI-SCAN MONITOR General Description Monitors can be split into two main categories: Cathode Ray Tube (CRT) and flat panel displays. CRT monitors have been around since before the invention of television. They comprise a sealed glass tube which has no air inside it. An electron gun at one end fires a stream of tiny electrons at the screen located at the other end. The inside surface of the glass screen has a special coating of phosphor which emits light when the electrons hit it. The phosphor on the screen is grouped into small blobs called triads. Each triad is made up of three dots; one emits red light, one green and one blue. The dots are very close together so appear to the eye as a single dot. The image is formed when the electron gun is fired and electrons strike different areas of the screen. The electron beam is moved across the screen in lines by magnetic fields. The beam starts in the top left of the screen and moves across each row (or raster) in turn. This is repeated for every row on the screen many times a second. The image which is built up fools the eye into believing that a picture is displayed on the screen all at once. Frame Scan line Return line To ensure the electron beam is focussed properly before it hits the screen it passes through a perforated sheet called a shadow mask. The main characteristics which need to be considered in specifying a monitor are: The maximum resolution which can be displayed The refresh rate Interlacing BIT Department Deans Community High School Page 5

46 Resolution The resolution is defined as the number of pixels on the screen horizontally and vertically. The more pixels in the same space the higher the resolution. A standard VGA resolution is 640 x 480 pixels. The actual resolution is set by the graphics card but cannot exceed the maximum allowable by the screen. The output from a graphics card is fed to a monitor to actually display the image. The quality of the graphics card has a major impact on the quality of the image displayed on the monitor. For many applications, such as Computer Aided Design or Desk Top Publishing, the overall size of the screen is important. A larger screen allows more of the data file to be displayed at once. The resolution on a large screen follows the same rules as for smaller ones but the number of pixels required to give a reasonable quality image is far greater. Refresh Rate The refresh rate is a measure of how many times the screen is redrawn every second. Refresh rate is measured in Hertz. To obtain a flicker free screen, a refresh rate of 70Hz or higher is required. The electrical signal from the graphics card is based on the resolution and refresh rate. This signal is known as the horizontal scanning frequency. Increasing the resolution or the refresh rate will increase the horizontal scanning frequency. A monitor which can handle different horizontal scanning frequencies is known as multiscanning. Multimedia applications such as viewing digital video will require a multi-scan monitor in order to reduce flicker. Interlacing An interlacing monitor is one which draws every other row at one time and then returns to the top and fills in the missing ones. An interlaced monitor with a refresh rate of 100Hz will only completely refresh the screen 50 times a second. Keywords Electron gun, interlacing, multi-scan, raster, refresh rate, triad. See Also Graphics cards Interfaces BIT Department Deans Community High School Page 6

47 DIGITAL AUDIO TAPE General Description Magnetic tape is a very cheap form of backing storage. It is widely used to backup up systems such as network servers and mainframe computers. One form of magnetic tape is known as Digital Audio Tape (DAT). It was originally developed for recording high quality audio, but as the format is digital it is ideal for recording data from a computer system. read / write heads Tape A 4mm tape is read using a technology called helical scan. Helical scan recorders write data onto the tape in narrow tracks at an angle to the edge of the tape. Short diagonal tracks are written across the width of the tape so that they are about eight times longer than the width of the tape. Each track contains about 18K of data. The read heads verify the data written by the write heads. The cylindrical head is tilted slightly in relation to the tape, and spins at a rate of 000 revolutions per minute (rpm). The tape moves in the opposite direction to the cylindrical spin, at less than one inch per second but because it is recording more than one line at a time, it has an effective speed of 150inches per second. Diagonal tracks on the tape Capacity The capacity of DAT ranges from Gb up to 40Gb and is dependent on the physical length of tape which can be fitted in a cartridge. Manufacturers such as Fuji, Sony and TDK produce cartridges of varying capacity. The capacity of a typical network hard disk will be about 0G so this is one of the few forms of backing storage which can back up the hold hard disk without having to use multiple cartridges. BIT Department Deans Community High School Page 7

48 Sequential Access Access to data on DAT is sequential. This means that the data blocks have to be accessed in order. Access times are therefore slow in comparison to random access devices such as magnetic disks and unsuitable for data which requires to be accessed in a non-linear or random fashion such as records in a database. To be able to access the data on DAT there must be some form of directory of files. There is normally a catalogue file at the start of the tape which details the contents of that tape. To find a particular file on DAT, the read heads read the address of the file in the directory, read through all the files preceding the required file and only begin to pass data to the processor when they reach the specified file. Keywords Capacity, helical scan, random access, sequential access. See Also Magnetic Tape Interfaces BIT Department Deans Community High School Page 8

49 MAGNETIC DISK General Description Magnetic disks are the most commonly used form of backing storage. They allow random access and range in size from removable floppy disks which can store 1.4M up to large hard disk packs able to store 8G to 100G and more. The hard disk is normally made up of a series of platters which are covered, on both sides, in a magnetic medium. Typically two or three platters are stacked on top of each other with a spindle that turns them all at the same time. A read / write head sits very close to the surface of each platter and is moved in or out to read data from the disk surface. The heads are so close to the platter that a small particle of dust could cause the magnetic surface to be scraped. Spindle actuator Read / write heads (one per surface) When the disk stops spinning the heads are designed to touch down on an area called the landing zone. Before the disk can be used it must be formatted. This involves dividing the magnetic surface into a series of tracks and sectors. The intersection between a track and a sector is known as a block. Each block on the disc stores the same amount of data. sector block track Tracks physically above one another on different platters are grouped together and known as cylinders. The data blocks in a cylinder can all be accessed without moving the read / write heads thus speeding up data recording and retrieval. Hard disks are usually connected to the motherboard using an IDE or SCSI interface to allow a fast transfer of large amounts of data between the disk and the computer s memory. BIT Department Deans Community High School Page 9

50 Speed The time taken to access data on a disk is known as the access time. This figure is calculated by adding the time taken to move the read / write head to the required track on the disk (known as the seek time) to the time for the disk to rotate so that the correct sector is under the head (known as the latency and normally taken as the time taken for half a revolution of the disk). When choosing a hard disk the technical specifications will usually give details of the spindle speed (the number of times the disk rotates per minute (rpm)) and the seek time in milliseconds. Example Spindle speed = 700 rpm Seek time = 6 ms (Note that units must be changed before any calculations are carried out!) Capacity As already mentioned, the capacity of hard disks can be as high as many hundreds of Gigabytes. The main limiting factor in the use of many hard disks is the operating system in the attached computer. Many older operating systems have a limit to the amount of data which can be accessed. To be able to address data on the whole disk in these cases, the disk must be partitioned. This process involves formatting the disk so that the operating system views it as a number of separate logical disks. Keywords Blocks, capacity, cylinders, IDE, latency, partition, random access, SCSI, sectors, seek time, tracks. See also interfaces magnetic tape optical disk BIT Department Deans Community High School Page 30

51 MAGNETIC TAPE General Description Magnetic tape is a common form of backing storage for making copies of large scale systems such as the hard disks in network servers. Access is sequential so data is normally sorted before being archived. The magnetic tape is passed over the read / write heads at a constant speed. When the tape is not being read from or written to, it is moved away from the heads to allow faster winding. Data is read from the tape into a buffer which is either in the computer or in the tape unit itself. The amount of data read from the tape in one operation is known as a block. Blocks vary in size from 10 to bytes. Each block of data incorporates error checking to ensure accurate transfer of data. Gaps are left between each block to allow the tape unit to speed up and slow down. Capacity Early tape systems used on mainframe computers used large reels of tape. More recently the tape has been placed in a cartridge to make it easier to handle. A typical format using a tape of 0.5 width (a format known as Quarter Inch Cartridge or QIC) gives a capacity of about 80M up to 4G. Keywords Block, buffer, QIC, read write head, sequential access. See also DAT Magnetic disk Interfaces BIT Department Deans Community High School Page 31

52 MAGNETO-OPTICAL DISK General Description Magneto-optical devices allow storage of data using a combination of magnetic and optical techniques. The surface of the disk is covered in a magnetic medium which can be magnetised using the heat from a laser. The magnetic material is stable at room temperature. Its state can only be changed when the temperature is raised above the Curie Point (the temperature at which the molecules of a material can be altered when subjected to a magnetic field). When writing data to the disk the area to be magnetised is heated by a laser with a very fine focus. Once the particles reach the Curie Point a magnetic field can be used to change their polarity. This means that more data can be squeezed into the same area as on conventional magnetic disks Data is read from the disk using a less powerful laser which does not change the data. When the laser light is reflected from the magnetic particles it is polarised depending on the orientation of the magnetic particles on the disk. This polarisation of the laser light as it is reflected is known as the Kerr effect. Due to the fact that magneto-optical disks are written to and read from using a laser and because they have to be heated before the data can be changed they tend to be a very robust form of media. They are not susceptible to being damaged by magnetic fields in the same way as magnetic disks or tapes. Capacity A 3.5 magneto-optical disk has a capacity of 640M, although higher capacity devices are available. The 5.5 format has a capacity of.6g. Most manufacturers follow the ISO standard so that disks from different manufacturers can be shared. Keywords Curie point, Kerr effect, ISO, random access. See also Interfaces Magnetic disks Optical disks BIT Department Deans Community High School Page 3

53 OPTICAL DISK General Description An optical disk is a form of backing storage device which uses optical technology to read and write data. The main types of optical device are CD-ROM, CD-Recordable and DVD. CD-ROM The principles involved in reading data from a CD-ROM apply to reading data from any optical disk. A laser beam is focussed on a spot on the surface of the disk. The surface of the disk is made up of pits and lands. The light is all reflected from a land and scattered from a pit. The reflected beam is sensed by a photo-sensitive cell. This information is turned into an electrical signal. disc motor protective coating information surface plastic coating photodetector lens glass prism laser The data on a CD-ROM is organised as a sequence of equal length blocks formed into a single spiral starting at the centre. Each block contains 048 bytes of data, plus address and error detection bytes. A single speed CD-ROM reads data at 75 blocks per second. This means that the data transfer rate is 150KBps The speed of modern CD-ROM drives is normally faster than this. It is usually given as a number of times faster than a single speed drive. Examples include 16x, 3x and 48x. CD-ROM drives are typically connected to the computer via an IDE or SCSI interface. The International Standards Organisation (ISO) has set a standard data format for reading data from a CD-ROM. The ISO9660 format allows a CD-ROM created for one platform to be read by another platform. In this format, filenames are restricted to uppercase letters, the digits 0 to 9 and the _ character. BIT Department Deans Community High School Page 33

54 A CD-ROM is a read-only device as there is no way that the laser can be used to change the pits and lands on the CD surface. To allow the CD-ROM to be written to, the material of which the disk is made had to be changed as well as the way the read/write mechanism operates. CD-Recordable and CD-Rewritable The CD-R (Compact Disk Recordable) and CD-RW (Compact Disk Rewritable) formats allows small businesses, multimedia designers and home recording artists to write data to CDs which are usable in standard CD-ROM drives. Write Once / Read Many storage (WORM) has been used widely since the 1980s and is a type of optical disk drive that can be written to and read from. CD-R uses the same technology as WORM where a layer of dye replaces the reflective sheet in a standard CD- ROM. This dye is photo-sensitive and when the laser hits the surface it burns pits in the dye. When heated beyond a certain temperature the burned area becomes opaque and thus reflects less light than areas that have not been heated by the laser. Recently CD-Recordable devices have become much cheaper and as a result are now provided as standard in many desktop PCs specified for home or small business use. CD-RW allows the user to record over old data on a CD. The disks are usually slightly different in colour from CD-R. The CD-RW uses a crystalline compound which has special properties. When it is heated by the laser the crystalline substance changes its properties to become opaque so that light is not reflected. DVD The capacity of a CD-ROM is about 650M. This means that it is not suitable for some of the very high capacity applications now becoming commonplace in multimedia and digital video applications. DVD (Digital Versatile Disk, originally known as Digital Video Disk) brought in a new standard which can provide up to 17G of storage space. DVD looks like CD-ROM and contains a spiral track with pits and lands which are read by a laser. To achieve the increase in capacity, the tracks are closer together and the pits are much smaller than a CD-ROM. Data can also be scanned from more than one layer of DVD by changing the focus of the laser. It is also possible to create a double sided disk which uses two lasers to read the data from the two layers. To allow a whole feature film to be stored on a DVD, video compression techniques such as MPEG are used. Keywords Block, CD-ROM, CD-R, CD-RW, compression, DVD, IDE, lands, MPEG, pits, SCSI. See also Interfaces Magnetic Disk Magnetic Tape BIT Department Deans Community High School Page 34

55 SOLID STATE DEVICES There are many devices on the market today that allow data to be saved on small, portable backing storage devices. These can be take the form of a pen, key ring or even a watch. USB Flash Memory A USB flash memory a memory chip with a USB plug, you plug it into your computer, copy the file onto it, then simply plug it into the USB socket of the other computer and access the file. They are small enough to attach to your keyring and are difficult to damage. Remove the cap and the USB plug is revealed. Plug it in and the computer will automatically recognise it. Files are stored on it simply by dragging and dropping them onto the icon. Storage Capacities start from 64 Megabytes upwards. This type of memory uses Erasable Programmable Read Only Memory (EPROM) technology. USB flash drives have less storage capacity than an external hard drive, but they are smaller and more durable because they do not contain any internal moving parts. Flash drives also are called pen drives, key drives or simply USB drives. Memory Card Reader/Writer These can be used with most of the memory card formats to allow the cards to be used as a file transfer medium similar to the USB Flash Memory. The reader/writer is connected to a USB port and once a memory card has been connected is identified by the system as an external drive. Files can simply be dragged and dropped to transfer data. Once full/empty the card is simply removed and another fitted. This is useful as it allows memory cards from digital cameras etc to be removed and used without the camera being connected to the computer. Cost Drives with faster transfer rates and larger storage capacities tend to be more costly. The main factors are: transfer rates interfaces; capacity; type of storage. Compatibility Solid state storage devices can be connected to computer systems depending upon the interface and operating system used. BIT Department Deans Community High School Page 35

56 3.3 - Selecting Hardware to Match Operational Requirements Obviously there is a wide range of devices that can be used with computers. So which ones do you choose? Basically it is down to the task that needs to be solved. Typical questions that could be asked are: Are graphics and/or photography required? How much backing storage? What type of print outs are necessary? How will the data be displayed? What are the RAM requirements? Is processor performance important? Are there specific peripherals that are required? As the cost of tackling problems using computer hardware can be a costly one, it is necessary to be able to justify why choices have been made. Here are typical tasks that a computer system with specific peripherals could solve. Production of a Multimedia Catalogue Multimedia involves the use of media elements, such as text, graphics, sound, video and animation. The computer system itself must be of a good specification to create, edit and run these elements. So typical minimum hardware could be: Hardware Justification Computer System with: 56 RAM To open media elements and run software 1 GHz Processor To run applications quickly and view video at an appropriate speed 0 Gb Hard disk drive To store programs and media elements CD-RW drive To load software and save catalogue onto Firewire interface To attach camera and capture video Video card To capture and playback video elements Sound card To capture and playback audio High resolution monitor To view media elements Digital video camera To record video frames Microphone To record audio Mouse and keyboard To input characters and control selection of elements Scanner To capture still images High resolution colour laser printer To print out multimedia catalogue Speakers To playback audio BIT Department Deans Community High School Page 36

57 Setting Up a Local Area Network (LAN) In School Setting up a network would require many desktop computers to be linked together. Possible hardware for this task could be: Hardware Justification 50 Computer Systems with: 64 RAM To open media elements and run software 500 MHz Processor To run software at an appropriate speed 10 Gb Hard disk drive To store programs CD-ROM drive To load networking software Network Interface Card (NIC) To allow computer to be connected to the network Monitor To view data Cables To allow computer to be connected to the network Mouse and keyboard To input characters and control selection of elements Hubs To connect devices on the network Switches To filter and forward packets Network laser printer To print documents across the network Network server To store users files across the network BIT Department Deans Community High School Page 37

58 Development Of A School Website Creating a school website could possibly involve using multimedia elements, so a higher specification computer system is required. Also possible Internet access may be a requirement. Possible hardware for this task could be: Hardware Justification 50 Computer Systems with: 56 RAM To open media elements and run software 1 GHz Processor To run software at an appropriate speed 0 Gb Hard disk drive To store programs CD-ROM drive To load networking software Network Interface Card (NIC) To allow computer to be connected to the network Video card To capture and playback video elements Sound card To capture and playback audio High resolution monitor To view data Cables To allow computer to be connected to the network Mouse and keyboard To input characters and control selection of elements Digital video camera To connect devices on the network Scanner To capture still images Ink jet printer To print out completed pages for checking Digital camera To take digital photographs BIT Department Deans Community High School Page 38

59 CONTENTS Topic 4 Networks What is a Network? Local Area Network (LAN) Wide Area Network (WAN) Intranet Internet Advantages of using networks Disadvantages of using networks Factors affecting Choice of Network Transmission Media Bandwidth Geographical Spread Functions Mainframes and Servers Mainframe With terminals Network of Computers File Server Web Server Print server CD-ROM server Mail server Application server Questions Network Types Peer-to-peer Networks Client Server Network Questions Topologies Characteristics of Different Network Topologies Questions Comparing types of Networks Network Hardware Questions Widespread use of Networks Advances in Hardware Advances in Software The need for higher bandwidth Wireless communications Questions Misuse of Networks Breaching copyright Hacking Planting Viruses Computer Misuse Act Copyright Designs and Patents Act Data Protection Act Questions... 7 Appendix 1: The Acts in More Detail... 8 BIT Department Deans Community High School Page

60 DATA PROTECTION ACT Basic Principles... 8 COMPUTER MISUSE ACT Areas Covered... 9 COPYRIGHT, DESIGNS AND PATENTS ACT, Areas Covered... 9 BIT Department Deans Community High School Page 3

61 TOPIC 4 NETWORKS 4.1 What is a Network? A computer network is formed when a group of computers are connected or linked together. Networks are usually set up to allow users to communicate electronically, share data, share peripherals and provide security and up to date information. Computers that are not networked together are called computers. stand-alone There are many different types of networks Local Area Network (LAN) A Local Area Network is a network of computers that are all situated close together for example in a room or building. Since the distance between computers is relatively small wire cables are frequently used to connect the computers together. LANs are usually set up to allow users to share data, communicate and share expensive peripherals. A fileserver is used within a LAN to store data centrally. The fileserver is a powerful computer with a large amount of internal memory (RAM), fast processor and large amount of backing storage. Each authorised user of the network can use the fileserver to save data. The network manager or administrator (the person responsible for the running of the network) has to set up a username and password for each user of the network. This allows each user to log on. Each user of the network is given a folder which only they can use to store their files Wide Area Network (WAN) This is a network of computers connected across a distance. Telephone lines, satellites or other communication technology is required for WANs to work. Most major companies need a WAN in order to communicate with other branches either nationally or internationally Intranet This is a network that allows a private connection access to Internet services. This usually happens within a particular company or organisation. Authorised users only are allowed to access these services and this makes the network relatively secure. It allows secure communication and the distribution of information similar to the World Wide Web. It can also allow companies to advertise on the Internet Internet This is the most common example of a WAN. It uses telecommunications to transfer data between computers and distribute information. The World Wide Web (WWW) is a collection of information held on the Internet. The WWW is made up of millions of BIT Department Deans Community High School Page 4

62 documents called web pages and these pages are available to any user of the Internet. The Internet can also be used to communicate via (webmail), chat rooms, mailing lists etc. Many e-commerce companies have set up websites so customers can bank, shop, pay for travel tickets etc. on-line Advantages of using networks Sharing of peripherals (printers etc.) Sharing of storage Sharing of files Communication between computers Disadvantages of using networks Too many users can slow the network down Server crashes and data isn't available No central storage Cabling and transmission problems The type of network used depends upon several factors; we will look at these over the next few pages: BIT Department Deans Community High School Page 5

63 4. Factors affecting Choice of Network 4..1 Transmission Media This allows the transmission of the electronic signals from one computer to another. There are many different types of transmission media: Unshielded Twisted Pair (UTP) - this is a type of cable that consists of two unshielded wires twisted around each other. UTP cabling does not offer as high bandwidth or as good protection from interference, but it is cheap and easier to work with; Shielded Twisted Pair (STP) this is a type of copper telephone wiring in which each of the two copper wires that are twisted together are coated with an insulating coating that functions as a ground for the wires. The extra covering in shielded twisted pair wiring protects the transmission line from electromagnetic interference leaking into or out of the cable. STP cabling often is used in Ethernet networks, especially fast data rate Ethernets; Coaxial this is a type of wire that consists of a center wire surrounded by insulation and then a grounded shield of braided wire. The shield minimizes electrical and radio frequency interference; Fibre Optic this is technology that uses glass (or plastic) threads (fibres) to transmit data. A fibre optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves; Leased Line this is when a dedicated phone line allows network connection. Usually leased lines are used by businesses to connect geographically distant offices. A leased line is always active and as the connection doesn't carry anybody else's communications, the quality of service is usually very good; Wireless Connection this is when no physical cabling is used. Instead signals are usually sent by infra-red communication, laser, radio waves, microwave transmission or satellite links. BIT Department Deans Community High School Page 6

64 4.. Bandwidth This is the amount of data that can be transmitted in a fixed amount of time or the range of frequencies that a channel can handle. Bandwidth can be given as a transmission rate. This is usually in megabits per second (Mbps) which is 1 million bits per second. Bandwidth may also be given as a frequency and this is usually measured in kilohertz (khz) Geographical Spread This is the distance that the network has to cover Functions These are the tasks that the network has to perform. The table below defines typical factors for the different types of networks. Network Type LAN Transmission Media Coaxial cable Shielded Twisted Pair (STP) Unshielded Twisted Pair (UTP) Bandwidth Typically 10Mbps 16 Mbps up to 500 Mbps 10 Mbps up to 100 Mbps Geographical Spread Up to 500 metres 100 metres 100 metres Functions share data share peripherals communication centrally store data Fibre optic Typically 100Mbps Kilometres WAN Wireless Fibre optic Wireless Leased line Up to 54Mbps Typically 100Mbps Up to 54 Mbps Between 10Mbps and 100 Mbps 100 metres Kilometres Kilometres Kilometres share data communication centrally store data BIT Department Deans Community High School Page 7

65 Intranet Fibre optic Wireless Leased line Typically 100Mbps Up to 54 Mbps Between 10Mbps and 100 Mbps Kilometres Kilometres Kilometres share data share peripherals shop, bank, travel etc. communication centrally store data advertise Internet Fibre optic Wireless Leased line Typically 100Mbps Up to 54 Mbps Between 10Mbps and 100 Mbps Kilometres Kilometres Kilometres share data shop, bank, travel etc. communication centrally store data advertise 4.3 Mainframes and Servers Mainframe With terminals Mainframes are very large computers which can be accessed by other computers called terminals. Mainframes are capable of supporting hundreds or even thousands of users simultaneously. Terminals consist of usually a keyboard and monitor only and rely on the mainframe to carry out all the processing and backing storage required. Mainframes are generally used by huge organisations that have massive amounts of data to be stored and processed. These companies generally require central database management, for example, banks, insurance companies, government offices etc. Mainframes are usually kept in separate rooms due to security and their physical size. A mainframe computer s functions is not so much defined by their CPU speed as by their massive internal memory, high capacity external storage and reliability. These machines can and do run successfully for years without interruption, with repairs taking place whilst they continue to run. Terminals usually consist of just a keyboard and a monitor. The mainframe is responsible for doing all the work that the terminal needs processed. Advantages cheap to buy Disadvantages have no processing power or backing storage of their own if mainframe crashes or goes down the terminal is deemed useless BIT Department Deans Community High School Page 8

66 4.3. Network of Computers Most computer networks have stations that can work efficiently on their own without the aid of a mainframe. These networks usually consist of desktop computers being linked together. Servers are used to carry out various tasks across the network. There are many different types of server: File Server This provides a central disk storage area for any users across the network. The file server stores users files separately. Users can then access their files from any workstation on the network; Web Server This computer deals with web pages across a network. Every web server has an IP address and possibly a domain name. For example, if you enter the URL in your browser, this sends a request to the server whose domain name is yahoo.com. The server then fetches the page named index.html and sends it to your browser; Print server This computer allows the management of printing across the network. It uses a spooler to store users' files and can provide a queuing facility with prioritising if necessary; CD-ROM server This server allows all workstations across the network to obtain data from CD-ROM disks; Mail server Mail servers move and store mail over networks and across the Internet. When you send an it is sent to the server that then processes it and sends it to the recipient; Application server This server runs one or more applications that can be shared by workstations. BIT Department Deans Community High School Page 9

67 4.3.9 Questions 1. Describe what a mainframe is.. Describe what a terminal is. 3. Describe two hardware differences between a mainframe with terminals and a network of computers. 4. Give two disadvantages of using a mainframe with terminals compared with a network of computers. 5. Name two tasks suited for a mainframe computer. 6. What is a server? 7. Describe two different types of server. 8. Draw a table similar to the one below in your jotter and then fill in the boxes indicating five different types of servers and at least two uses for them. You may use the Internet or magazines to help you Server Applications File Server 9 Total = Network Types Peer-to-peer Networks This network is generally easy to set up, operate and maintain. This is a type of network which each workstation has equivalent capabilities and responsibilities. Each computer on the network has equal status. Any station can make its resources available to the rest of the stations on the network. The decision to share resources and what they are is taken by the user currently working on that station. There is no central control over resources. Usernames and passwords and access rights of shared files can be set up, but it is not a secure network. BIT Department Deans Community High School Page 10

68 4.4. Client Server Network This network is does not have stations with equal status. Basically a workstation is either a client or a server. A server is a computer that carries out various jobs across the network. A client is a computer that is connected to the network. A user works on this computer in order to carry out tasks on the network and access the servers. A user is allocated privileges that permit access to some of the resources on the servers and deny access to others. A user logs onto the server using a username and password. The server then authenticates (checks) that the user is authorised to use the network and sets appropriate access privileges. A network administrator or manager is usually responsible for setting up users and allocating their access rights. Network Type Advantages Disadvantages Peer-to-peer No need to pay for servers No or little network congestion due to smaller number of users Network can still function if any clients fail to work on the network No central storage of files Security across the network is poor since it is not centrally controlled Backups of network data is difficult since there is no central storage Each client on the network has equal rights Client server Backups of data stored on servers can be carried out regularly All shared files are stored on servers which means data is up to date and correct Security is controlled more easily due to the central storage of data and setting up of users access rights Servers can fail and hence stop data, applications etc. being shared across the network Servers are expensive to buy Due to the number of users network congestion is likely BIT Department Deans Community High School Page 11

69 4.4.3 Questions 1. Describe what a peer-to-peer network is.. Describe what a client server network is. 3. Who is responsible for setting up resources on: i) a peer-to-peer network; ii) a client server network? 4. On a client server network, what is: i) a client; ii) a server? 5. Explain how a server knows that a user is authorised to access the network. 6. Give two disadvantages of using a peer-to-peer network compared with client server network. 7. Give two advantages of using a peer-to-peer network compared with client server network Total = 11 BIT Department Deans Community High School Page 1

70 4.5 Topologies Networks can be arranged in different ways. Such arrangements are called topologies. There are 5 main types of networking design. These are bus, star, ring, tree and mesh. How their topologies are arranged are shown below: Star Topology Tree Topology Ring Topology Bus Topology node link Mesh Topology Mesh (Fully Connected) Node (computers within the network) links or channels to connect nodes together BIT Department Deans Community High School Page 13

71 4.5.1 Characteristics of Different Network Topologies The table below highlights some of the characteristics of different network topologies. Type Of Network Advantage Disadvantage Affect On Performance Mesh (WAN) Fault in one cable doesn t affect whole network. Data is rerouted. Lots of cabling required. Expensive. Excellent performance. Tree (WAN) Multiple transmissions can occur at the same time. Local central nodes support cluster of outer nodes. This reduces the number of links to the central node. Lots of cabling required. Expensive. Excellent performance. Bus (LAN) Fault in one node does not affect the rest of the network. Very easy to expand. All nodes use the same line (backbone) and hence contention occurs. Instant access but high rate of data crashing. Cheap. Ring (LAN) Control system in charge of transmission. Additional expense for control software and control system. Network rarely fails to data crashes. Nodes guaranteed access to transmission. Nodes may have to wait their turn to transmit data. Star (LAN) Short path between two nodes. Fault in central node means whole network is unusable. Central node gives more robust network, but slows down communications between nodes as data always has to pass through central node. BIT Department Deans Community High School Page 14

72 4.5. Questions 1. Name five different types of network topology.. Name two topologies that could be used for: i) a LAN; ii) a WAN. 3. What is meant by: i) a node; ii) a channel? 4. Draw two different network topologies in your jotter. You must name your chosen topology and label your diagram. 5. Explain why a network with a ring topology rarely fails due to data crashes. 6. Using a table similar to the one below fill in the boxes explaining what would happen if a node or channel failure occurred within the given topology. 4 1 Topology Node Failure Channel Failure Star Ring Bus Mesh Tree 7. What type of topology would you recommend for: 10 i) a peer-to-peer network; ii) a mainframe with terminals? Justify your choices. 4 Total = 5 BIT Department Deans Community High School Page 15

73 4.6 Comparing types of Networks These tables will help you compare LAN s, WAN s, Intranets and Internetworks. Functions Geographical spread Transmission media Bandwidth LOCAL AREA NETWORK Sharing of data files, of peripherals, enabling communications via s. Limited to one building such as a school, an office block or a factory. The limit is usually up to kilometres. Twisted-pair copper cable, fibre-optic cable, and co-axial cable. The bandwidth available depends on the transmission media: Twisted-pair copper cable: Mbps; Fibre-optic cable; Co-axial cable. Functions Geographical spread Transmission media Bandwidth WIDE AREA NETWORK Supports transfer of files, communication via , shared use of multi-user databases, conferencing. There is no physical limitation to a WAN. It could cover a city, a country or stretch around the world. Telecommunication systems covering large distances. The bandwidth available depends on the nature of the telecommunications link. If you are connected to a WAN using a dial-up modem you are limited to 56 kbps. An ISDN line could support up to 18 kbps and using a leased telecom T3 line would support a 44.7 Mbps transmission. BIT Department Deans Community High School Page 16

74 Functions Geographical spread Transmission media Bandwidth INTRANET Internal communication within an organisation. Supports internal , sharing of data files, web pages. Examples are: using web pages to advertise internal jobs or training courses, using the intranet to share data files on costs and prices. There is no physical limitation to an intranet. It could cover a city, a country or stretch around the world. Intranets use the same telecommunications technologies as WAN s to cover large distances. The bandwidth available depends on the nature of the telecommunications links used. Dial-up modems, limited to 56 kbps, ISDN lines could support up to 18 kbps and using a leased telecom T3 line would support a 44.7 Mbps transmission. Broadband connections could also be used Functions Geographical spread Transmission media Bandwidth INTERNETWORK An internetwork consists of several networks joined by devices such as routers or switches. The functions available to the users are those available on LANs, see table above. An internetwork can vary in its geographical spread. It could be used to link several LANs in the same complex or, using the telecom system, it could link networks spread across a city or a country. This depends on the configuration of the internetwork. If it is linking several LANs in the same geographical location it will probably use a high-speed backbone medium such as fibre-optic cable. If it is linking over a wide geographical area it will use a form of broadband connection over the telecommunication system. Using a fibre-optic cable to link LANs would support a bandwidth of up to 100 Gbps. Linking networks across a wide area would involve using telecom communications such as those used by a WAN. (See previous table). BIT Department Deans Community High School Page 17

75 4.7 Network Hardware Certain hardware devices are necessary to enable a network to function properly. an Ethernet network. Network Interface Card (NIC) This is an expansion board that can be inserted into a computer so the computer can be connected to a network. Most NICs are designed for a particular type of network although some can serve multiple networks. A common NIC is an Ethernet card that allows a computer to be connected to Hub This is a common connection point for devices in a network. Hubs are commonly used to connect segments of a LAN. A hub contains multiple ports. The function of a hub is simply to extend the distance between nodes on the network. Switch (switched hub) This device filters and forwards packets (parts of data) between LAN segments. A switch splits the network up into a number of different parts, usually to help avoid collisions within the network. These are called collision domains. A collision in one domain will have no effect on network traffic in another collision domain. Router This device forwards data packets along networks. A router is connected to at least two networks, commonly two LANs or WANs or a LAN and its ISP s network. Routers are located at gateways, the places where two or more networks connect. A router creates or maintains a table of all the available routes around the network and uses this table to determine the best route for any given packet of data. Bridge This device connects two local area networks, or two segments of the same LAN that use the same protocol (an agreed format for transmitting data between two devices), such as Ethernet. Sometimes LANs are divided up into smaller networks to reduce the traffic of data and avoid collisions. These separate networks can be connected using bridges. This enables the traffic of data to be localised to a smaller network and reduces the number of collisions. Repeater When data is send across a large distance within a network, the signal can deteriorate as the distance increases. This may cause data to be incomplete or corrupt when it arrives at the receiving node. Therefore the data is sent to an intermediate node (a repeater) which boosts the signal. Every packet that arrives at a repeater is retransmitted across the network. BIT Department Deans Community High School Page 18

76 4.7.1 Questions 1. Describe what a Network Interface Card is and describe one of its functions.. Describe what a hub is and describe one of its functions. 3. What is the main difference between a hub and a switch? 4. Describe what a router is and describe one of its functions. 5. What is meant by a collision within a network? 6. Describe what a switch is and describe one of its functions. 7. What is meant by the term packet? Total = 11 BIT Department Deans Community High School Page 19

77 4.8 Widespread use of Networks The advantages of users having their computer systems networked, in particular, to the Internet, have encouraged a computer sales boom. Users can now invest and shop on-line, keep in touch with friends and relatives cheaply and frequently, enjoy multiplayer games, surf the vast resources of the Internet and so on. The main reasons for having networked computers are: simultaneous high speed Internet access using a single ISP account peripheral sharing sharing files and applications entertainment Due to user demand for on the spot information, most computer systems are now manufactured with network capabilities. It does not matter what the computer system may be, whether it be desktops, laptops, palmtops etc. The development of networks can be explained by many different factors including economic and social, however the most crucial is the technical factors that are involved. Some of the results of these technical developments are: advances in computer technology new data transmission media and techniques improved quality of networking software establishment of communications standards adoption of Internet technology. There have been many advances in computer hardware and software over the years that have allowed networking to be increasingly widespread within computer systems Advances in Hardware Hardware prices have dropped, available network speeds have accelerated, and signal attenuation and noise problems have been addressed using low cost, high performance signal processing. The majority of computer systems have a typical system specification that can cope with most network functions. Computer systems can now process, store and deal with large amounts data very quickly. Even a low specification computer can now work as a server across a network. This is mainly due to advances in: processors (typically GHz and beyond) RAM (typically 1Gb and beyond) backing storage (hard disk capacity typically 40Gb) data transfer rates When networking first started only text data was processed across the network and the bandwidth didn t need to be very high. Today networks have to deal with all media types BIT Department Deans Community High School Page 0

78 such as text, video, graphics, sound and animation. The bandwidth of a network has to be high in order to deal with this memory intense data. The table below shows three different networks and the typical bandwidth of each. Dial up Ethernet Wireless Speed 100 Kbps - 10 Mbps 10 Mbps Mbps 700 Kbps - 11 Mbps 4.8. Advances in Software Software prices have generally risen dramatically over the last 10 years. However the majority of computer systems are now equipped with system software that supports networking and have a variety of browsers installed. These items of software have improved dramatically over the years and support networking as standard. There have also been improvements in communications software and network operating systems. Typical networking software is: browsers (Internet Explorer and Netscape Communicator) These allow users to surf the Internet i.e. to locate and display Web pages. operating systems with networking functions (Windows XP, Mac OS X, UNIX) These stand alone operating systems can allow networking functions to be carried out. network operating systems (Windows Server, Mac OS X Server, Novell Netware) Typical tasks involve encryption of data, authenticating users, setting privileges and access rights etc. communications software This software controls networking cards, encodes and manages the flow of data. Although hardware and software capabilities have improved other aspects of networking need to be catered for The need for higher bandwidth When networks were first introduced much of the information that had to be dealt with was textual or binary. Therefore the bandwidth required for file transfer was quite low. Nowadays most networks need to deal with larger non text files and so the file sizes become larger. Users really want to work with multimedia data so networks have to cope with not only text but still images, audio and video clips as well. The addition of more users on any network, for example on the Internet, causes the traffic within the network to increase substantially. Another example is if users are sending , these small packets add up quickly and require more bandwidth or the mail will be delayed. Also if the users are clients accessing servers, as on the World Wide Web, that also adds more traffic, especially if much of the traffic is graphics, video and audio. BIT Department Deans Community High School Page 1

79 4.8.4 Wireless communications Wireless networks use electromagnetic airwaves, either infrared or radio to communicate information from one point to another without relying on any physical connection. Some wireless networks can be implemented using PCMCIA cards in notebook computers, PCI cards in desktop computers, or integrated within hand held computers. Adapters provide an interface between the client network operating system and the airwaves (via an antenna). In 1999, Apple sparked the wireless revolution with the introduction of AirPort, the first affordable and easy to use solution for accessing the Internet without restrictive cables. Apple s AirPort Express continues to advance wireless technology, delivering the first device to pack wireless networking, audio, printing and bridging capabilities into a single affordable, portable unit. Bluetooth is also a widely used technology to connect devices without wires. It provides short range connections between mobile devices and to the Internet via bridging devices to different networks (wired and wireless) that provide Internet capability. Wireless technologies allow more flexible use of applications and devices. With the number of mobile users continuing to increase, this flexibility is even important than ever. Due to the nature of using large quantities of data across a network, it is important that this data is not abused. There are various ways that networks can be misused for illegal purposes Questions 1. Describe two reasons for having computers networked.. Describe two technical developments of networking. 3. Describe two advances in computer hardware that have helped the expansion of networks. 4. Describe two advances in software that have helped the expansion of networks. 5. Explain why most desktop computers could be used as a server across a network. 6. Give two reasons why high bandwidth is required for networks today. 7. Explain what is meant by a wireless network and describe how it works. 8. Give two reasons why wireless technology is becoming a more popular choice when networking computers. Total = 16 BIT Department Deans Community High School Page

80 on the hard drive, some will format the hard drive wiping all data. A logic bomb is a virus waiting to start executing given a certain trigger, or a failure to cancel. Higher Computing: Computer Systems BIT Department Deans Community High School Page 3

81 A computer system can catch a virus by launching an infected application or starting up your computer from a disk that has infected system files. Once a virus is in memory, it usually infects any application that is run, including network applications (if the user have write access to network folders or disks). For example, a virus may be designed to conceal itself until a predetermined date, and then flash a message on all network computers. However, a properly configured network is less susceptible to viruses than a stand alone computer. There are various laws that have been passed to help prevent illegal use of networks. BIT Department Deans Community High School Page 4

82 4.9.4 Computer Misuse Act Three specific computer misuses covered by the Computer Misuse Act of 1990 are: unauthorised access to computer programs or data, unauthorised access with a further criminal intent, unauthorised modification of computer material i.e. programs or data. Some of the crimes are: deliberately planting viruses into a computer system to cause damage to program files and data; using computer time to carry out unauthorised work; copying computer programs; hacking into someone s system with a view to seeing or altering the information.; using a computer to commit various electronic frauds Copyright Designs and Patents Act The Copyright, Designs and Patents Act of 1988, makes it illegal to: copy software; run pirated software; transmit copyrighted software over a telecommunications line. Licences are bought from a software company in order to legally run software. There are limitations for multiple users and special licences have to be bought for running the software on a network. BIT Department Deans Community High School Page 5

83 4.9.6 Data Protection Act The aim of the Data Protection Act 1998 is to protect the rights of the individual about whom data is obtained, stored, processed or supplied rather than those of the people or organisations who control and use personal data. The Act applies to both computerised and paper records. Data users have to notify the Office of the Information Commissioner of the information held and its uses. Across a network with many users, data users must ensure that they provide proper security. This could be done using: Secure rooms Making sure that servers containing sensitive data are under locked away in a secure area. A room that has no windows, secure doors, a burglar alarm and limited access to keys should be suitable. Usernames and passwords If an unauthorised person does break into the building and any secure area it would be useful if the network will only accept authorised users. i.e. users who are known to the system and given limited access. This can be done by giving authorised users identification names and passwords. BIT Department Deans Community High School Page 6

84 4.9.7 Questions 1. Describe two ways of breaching copyright.. Why is breach of copyright becoming more serious when using the Internet? 3. What is meant by the term hacking? 4. Describe two methods of hacking into a network. 5. Describe two methods of deliberately planting a virus across a network. 6. Describe two reasons for copyrighting work. 7. Why was the Computer Misuse Act passed? 8. State two computer misuses covered by the Computer Misuse Act. 9. Why was the Copyright Designs and Patents Act passed? 10. State two computer misuses covered by the Copyright Designs and Patents Act. 11. Why is software licensing an issue when dealing with networks? 1. Why was the Copyright Designs and Patents Act passed? 13. State two aims which must be complied with when implementing the Data Protection Act. 14. Using the Internet or otherwise, describe four areas of good practise which would be useful when dealing with users and data across a network. Total = BIT Department Deans Community High School Page 7

85 APPENDIX 1: THE ACTS IN MORE DETAIL DATA PROTECTION ACT 1998 Introduction The 'right to privacy' is a right we all expect. We do not expect personal details such as our age, medical records, personal family details, political and religious beliefs to be freely available to everybody. With the growth of ICT, large databases are able to hold huge quantities of information and global networks are able to share and distribute this information around the world in seconds. In order to control this development and to protect people s right to privacy, the Data Protection Act was introduced. The act works in two ways: It gives individuals (Data Subjects) rights concerning data held about them. o To check if any organisation keeps information about you on computer o To see a copy of this personal data It requires those who record and use the data about people (Data Users) to be registered and to follow certain rules or principles. If any person, organisation, company or business wishes to hold personal information about people, they are called Data Controllers, and they must register with the Office of the Data Protection Commissioner. The Data Protection Act contains eight basic principles. A summary of these is shown below: Basic Principles Obtained and processed fairly and lawfully Held only for the reasons described in the data user s register entry Adequate, relevant and not excessive for the reason held Accurate and up-to-date Not held longer than is necessary Accessible to the person it is about and they have the right to have the data corrected or erased Be kept secure against loss, damage and unauthorised and unlawful processing Not to be transferred to countries outside the European Economic Area BIT Department Deans Community High School Page 8

86 COMPUTER MISUSE ACT 1990 Introduction The Computer Misuse Act is intended to protect all types of information stored on computer systems. Like the Data Protection Act, it has been drawn up because of the growth in the use of computers and the need to change the law to stop people abusing them. Under the law the following offences can be dealt with. Areas Covered Hacking this is when someone gains unauthorised access to computer files and interferes with the data. The penalty for this is a maximum fine of 000 and a six-month prison sentence. Computer fraud and Blackmail - The penalty for this is an unlimited fine and a maximum 5-year jail sentence! Making unauthorised modifications to computer materials this includes o Making unauthorised changes to software or data. o Preventing authorised access by changing or deleting passwords o Computer viruses -this is the writing and distribution of computer viruses and other programs that destroy data and cause computers to crash unpredictably. The penalty for this is an unlimited fine and a maximum 5-year jail sentence! To prevent viruses spreading: Don t copy software Check your computer discs regularly with an anti-virus program Don t share discs COPYRIGHT, DESIGNS AND PATENTS ACT, 1989 Introduction Copying computer software, or software piracy, is now a criminal offence under this act. All software copyright is protected by law for 50 years after it is published. The Act makes it illegal to: Areas Covered Make unauthorised copies of software that is copyright protected Run unauthorised copies of software Transmit software over networks and copy it without licensed permission. BIT Department Deans Community High School Page 9

87 CONTENTS Topic 5 SOFTWARE What is System Software? Operating System Disk Based ROM Based Bootstrap Loader Human Computer Interface Command Driven Examples Single User Operating System File management Memory management Interpret commands Input/Output (I/O) system Resource allocation Process management Questions Utility Programs Types Of Utilities Virus Checker Defragmenter Disk Editor Disk Repair or First Aid Advantages of using Utilities Questions Standard File Formats Creating and manipulating text Working with standard file formats for graphics Types and Formats of Images Questions Software Compatibility Virus Info Viruses that affect files File Virus Boot Sector Virus Macro Virus virus Worm Trojan horse Questions Different actions of viruses Replication Camouflage Watching Delivery Anti-Virus Software Detection Techniques Use of Checksum Searching for virus signatures BIT Department Deans Community High School Page

88 Heuristic detection Memory resident monitoring Dealing with Viruses Questions Selecting Software Production Of A Multimedia Catalogue Setting Up A Local Area Network (LAN) In School Development Of A School Website Questions... 0 BIT Department Deans Community High School Page 3

89 TOPIC 5 SOFTWARE 5.1 What is System Software? System software is the set of programs used to operate and maintain a computer system. It includes the operating system and utilities. The systems software provides a layer of software that lets users operate the computer without having to know about the underlying processes that are going on all the time Operating System The operating system is responsible for managing the hardware and communicating with the user. It can be broken down in to various layers. At the top layer the user communicates with the computer with no knowledge of the internal workings of the computer, at the bottom layer is the hardware communications. The advantage of putting software in to layers is that, if a part of your system changes, only the software dealing with that layer needs to be rewritten Disk Based Most modern computers keep their operating system on disk and load it into RAM on startup. The main advantage of this is that it is very simple to upgrade the operating system. It also means that only those parts of the operating system that are really needed have to be loaded. Other parts can be left on disk until needed. The main disadvantage is that other programs can corrupt the operating system. Examples of operating systems like this are Windows XP and the Mac OS X ROM Based Older computers often had their operating system in ROM. This meant that it was ready as soon as the computer was switched on. This meant that the operating system took very little time to be up and running unlike disc based systems. It is impossible to corrupt the system but very difficult to upgrade it. The BBC Microcomputer uses a ROM based operating system Bootstrap Loader Before a computer is switched on the operating system is not up and running. The bootstrap loader is the program that the processor runs on startup and whose task it is to load in the rest of the operating system from the hard disk. BIT Department Deans Community High School Page 4

90 Human Computer Interface The way in which the operating system has communicated between the human user and the computer has altered over the years and become much more user friendly. Operating systems like Windows XP and Mac OS X use a WIMP environment that makes the interface easier to use for beginners. Many experienced users still prefer the command driven operating systems such as MS-DOS and UNIX which allow the user very fast interaction with the operating system but which require a great deal of time to learn, and a manual at your side Command Driven Examples MS-DOS cd\myprogs - make myprogs the current directory UNIX Is > mydir - list the contents of the current directory to the file mydir 5.1. Single User Operating System The task of a single user operating system is to handle efficiently all the resources of a computer system, hiding their complexity from the user and the rest of the system. The main resources are memory, backing storage, peripherals, processor and the user File management The operating system supervises the creation, deletion and updating of files. A directory has to be kept which keeps track of where files are stored on the various types of backing storage. It has to maintain the hierarchical directory structure. This is when files are placed in directories (or folders) and subdirectories. BIT Department Deans Community High School Page 5

91 5.1.. Memory management The operating system has to decide where programs and data are to be placed in memory and, in a multi-tasking environment, keep track of what stage each program is at and ensure that no process tries to access the memory of a different process. This applies especially to the operating system. If it is corrupted in some way it could cause the whole computer system to crash Interpret commands The operating system has to take user commands from the keyboard, mouse etc., interpret the commands and pass them to the appropriate part of the Operating System which deals with that command. If the command cannot be understood, a suitable error message should be given. This is often known as the Command Language Interpreter Input/Output (I/O) system The main problem is that all the peripherals work at different speeds and have different characteristics. The I/O system hides these differences and makes them all appear to operate in a similar manner. To an application all peripherals behave the same way (device independence) that makes them much easier to use. It is the task of the I/O system to do all the actual data transfers and issue the appropriate control signals to the peripherals. If extra hardware is added then the manufacturer will supply the appropriate software (driver) to handle that peripheral Resource allocation In many operating systems resource could mean data or routines that are available to programs. It can also relate to tasks that are carried out by certain peripherals. This part of the operating system organises what resource is dealt with at any one time Process management The operating system has to decide which of the tasks in a multi-tasking environment (many programs running at once) gets to use the processor. In a single user operating system it only has to choose between the operating system itself and the package being run but on larger systems this becomes much more complex. This is usually called the kernel as it is right at the core of the operating system. BIT Department Deans Community High School Page 6

92 Questions 1. Why is an operating system required?. Describe two advantages of a ROM based operating system over a disk based operating system. 3. Explain the need for a bootstrap loader. 4. Explain two file management tasks that an operating system could carry out. 5. Describe two tasks the memory management part of the operating system may need to carr out when loading a file from disk into memory. 6. Describe two tasks the file management part of the operating system may need to carry out when opening a file from disk 7. Explain the need for a command language interpreter within an operating system Total = 11 BIT Department Deans Community High School Page 7

93 5. Utility Programs Utility programs are programs that aid the maintenance of the computer. They are not normally essential for the running of the computer and are normally kept on disk until needed. Utilities are programs that perform a variety of functions directly related to managing computer resources. They are supplied with the system software but can be bought as extras e.g. Norton Utilities Types Of Utilities Virus Checker These are programs to detect and remove computer viruses. The simplest kind scans executable files and boot blocks for a list of known viruses. Others are constantly active, attempting to detect the actions of general classes of viruses Defragmenter Once disks have been used many times (i.e. files saved and deleted from them), the deleted files cause the disk to have gaps in it (like Swiss cheese). This program move files around a disk until they are all stored in consecutive sectors. Disk defragmentation is the process of reassembling files and folders in one location on a volume. The process, which works only on local volumes, consolidates files and folders in one contiguous place. Defragmentation results in improved disk access because it consolidates most, but not all, of the volumes free space. The time it takes to defragment a volume depends on several factors, including the size of the volume, the amount of fragmentation, the number of files and folders, and the available system resources Disk Editor This is used for editing the contents of a disk. It includes tools for disk repairs, finding corrupt files etc Disk Repair or First Aid A lot of unused data collects on your hard disk during normal usage. A disk repair utility scans your hard disk for these unnecessary files and removes them, e.g. wrong aliases etc Advantages of using Utilities frees the computer system of unnecessary clutter improves the performance of the computer system can speed up Internet connection frees up system memory recovers accidentally deleted files can prevent virus infection BIT Department Deans Community High School Page 8

94 5.. Questions 1. Explain the need for utility programs as part of the system software.. Describe two tasks that a virus checker may need to carry out. 3. Why are defragmenters required? 4. Describe what a defragmenter does. 5. Describe what a disk editor does. 6. A floppy disk has been inserted into a floppy disk drive and the operating system has responded to the user stating that the disk is faulty. Describe a utility program that may solve this problem 7. Describe two benefits of using utility programs Total = Standard File Formats Sometimes it can be really frustrating for users when they want to use another computer to load a file. The problems that can occur are: The backing storage drive is different from the media the file is saved on. For example - the file is saved on a floppy that has been formatted for a Mac and the disc drive will only read PC formatted discs. Differing operating systems and the files won't be recognised. An error message from the operating system will be given to the user. For example - a AppleWorks file was saved on a computer with MacOSX and you try to load it on a PC which has Windows XP as its operating system. The PC will look for the filename to have a specific suffix e.g. cwk. The computer does not have the appropriate application software. For example, the file was created on Microsoft Word and the other computer only has AppleWorks. The first two problems could be resolved by using either a different computer or ensuring that your media is formatted for the correct standard or installing different operating system software. BIT Department Deans Community High School Page 9

95 The third problem is typical as each application saves files in its own way and other packages find it difficult to read these files. However, standards have been set which allow the user to save data in a format that can be read by other packages. Examples are: Application Spreadsheet Text Graphics Standard SYLK RTF (Rich Text Format) JPEG (Joint Photographic Expert Group) Sometimes it is possible to save a file in one package as another application's file e.g. a AppleWorks file as a MicroSoft Word file. However, the disadvantage of saving files using these standards is that special formatting created within the original file may be lost e.g. different styles, fonts and sizes. These different formats can usually be chosen in the Save As... menu option in each package Creating and manipulating text Text can be captured using almost any word processor, (AppleWorks, Word etc.), and each word processor tends to save files under its own format. To transfer files between different word processors, and between a word processor and other applications, there are two standards. The ASCII standard transfers just raw text and all formatting data (such as bold, italic and different fonts) is lost in transfer. RTF (Rich Text Format) is a file transfer standard in which the data formatting is preserved. When saving text for transfer, it is usual to have to specify the format in which you wish to save it. One of the benefits of an integrated package or a family set of applications is that files can usually be transferred between the different applications with the formatting preserved and the user is protected from having to know about file transfer standards. Text can also be created in using a desk top publishing (DTP) program. Common features between word processing and DTP packages include text handling (similar fonts, sizes and styles), drawing tools and page layout tools which will divide pages into columns. It is very important to be able to save text documents in a common format as using text between these two packages is a very popular process, especially in the magazine and newspaper industry Working with standard file formats for graphics Graphics is a term used to cover the following formats: clip art, graphs and charts, drawings and diagrams, digital images from scanners or cameras, animations and video Types and Formats of Images When considering graphical images as opposed to animation and video it is important to look at the formats in which they can be stored. There are two different methods of storing images, bit-mapped (or pixel) graphics and vector (or object-orientated) graphics. On the next page are a few standard file formats that are commonly used for representing graphics. BIT Department Deans Community High School Page 10

96 TIFF A common method of capturing real pictures is by scanning. The software that operates modern colour scanners tends to save the image in a Tag Image File Format (TIFF). TIFF format is a standard file format, developed jointly by Aldus and Microsoft, which can handle monochrome, grey scale, 8-bit or 4-bit colour images. TIFF is a standard for pixel graphics and is popular because graphics can be edited right down to pixel level. However, TIFF files tend to be large and need to be compressed. TIFF files tend to be used for DTP rather than Multimedia. JPEG If editing at that level is not required then a large TIFF file is often converted into a JPEG format and saved as a JPEG file. JPEG is a standard developed by the Joint Photographic Experts Group and provides lossy compression. This follows an algorithm that cuts out certain pixels and then saves the resulting image in a vector format. This means that the image is not as sharp as the original. Currently JPEG is the most popular standard for several reasons. It can handle a large number of colours. A large number of applications recognise it. It is a cross-platform standard. GIF The Graphic Interchange Format (GIF) is an older graphics format which operates in a similar way to JPEG files. The GIF standard was originally devised by CompuServe to make graphic files smaller for downloading on the Internet and hence provide faster access. GIF files can have a colour range up to 56 colours ( to the power of 8). This 56-colour limitation limits its ability to store photo images effectively. Some other types that are worth knowing are: - PICT PICTURE (PICT) files follow a standard original devised for Apple computers that also stores images in a vector format. BMP Bit-Mapped Picture is the Windows standard bitmap graphics file. It is uncompressed and hence takes up unnecessary file space. EPS Encapsulated Postscript uses the standard Postscript language to communicate with Postscript printers. The output is high quality and the make of printer is not important as long as it has Postscript printing capability. There are many other different formats for storing graphics such as PNG, PCX TGA etc. However JPEG, GIF and TIFF are the most commonly used format and allow graphic files to be exchanged between software packages. BIT Department Deans Community High School Page 11

97 5.3.4 Questions 1. Explain the need for standard graphic file formats.. Describe how the TIFF file format works. 3. Describe how the JPEG file format works. 4. Describe how the GIF file format works. 5. Describe two advantages of using JPEG over GIF. 6. What is the main disadvantage of saving a graphic as a TIFF file instead of any other file format? 7. Name one task that would benefit from graphics being saved in a GIF format Total = 11 BIT Department Deans Community High School Page 1

98 5.4 Software Compatibility When purchasing application software it is vitally important that the software can be run on your computer. If it does not then you won't be able to use your software. Each piece of software includes details of the minimum system specification required to run the application. This will include: Operating System Minimum RAM Processor Hard disc space Peripherals required Here are two typical examples of the type of specification given: Software Excel 97 Type Spreadsheet Operating System Windows 95 or Windows NT Processor Intel 486 or higher Memory 1Mb for Windows 95 16Mb for Windows NT Disk Space Peripherals 8-60 Mb with 40Mb Recommended CD-ROM drive VGA Display Software Type Operating System Processor Memory Disk Space Peripherals PageMaker 6.5 Plus Desk Top Publisher Mac OS or later Power PC 1 Mb 56 Mb CD-ROM drive Video display card (4-bit screen display) PostScript printer BIT Department Deans Community High School Page 13

99 5.5 Virus Info A virus is a program that has been designed to attach itself to other programs and make a computer system unreliable. A virus is a piece of software that latches on to other programs. For example, a virus might attach itself to a program such as a word processor. Each time the word processing program runs, the virus runs, too, and it has the chance to reproduce by attaching to other programs. Even a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. More dangerous types of virus are capable of transmitting across networks and bypassing security systems. Viruses can: damage data stored in memory or on disk affect the computer s usual operations copy itself automatically onto a disk or network. Most viruses only affect particular operating systems usually Windows based ones. All computer viruses are manmade. Recent viruses include the Mydoom worm, ILOVEYOU virus and Melissa virus Viruses that affect files There are many classification of virus that can affect different file types File Virus As viruses can replicate themselves, they can attach themselves to files. More common are files that are attached to messages. Since virus code must be executed to have any effect, files that the computer treats as pure data are safe. This includes graphics and sound files such as.gif,.jpg,.mp3,.wav,.txt etc. The virus code has to be in a form, such as an.exe program file or a Word.doc file, that the computer will actually try to execute Boot Sector Virus This virus has the ability to infect the boot sector (bootstrap loader) on floppy disks and hard disks. The boot sector contains a tiny program that tells the computer how to load the rest of the operating system. By putting its code in the boot sector, a virus can guarantee it gets executed. It can load itself into memory immediately, and it is able to run whenever the computer is on. Most boot sector viruses are not very common any more as most operating systems can now protect the boot sector Macro Virus This virus is encoded as a macro that is embedded in a document. Many applications support macro languages. These applications allow you to embed a macro in a document, and have the macro execute each time the document is opened. According to some estimates, 75% of all viruses today are macro viruses. Once a macro virus gets onto your machine, it can embed itself in all future documents you create with that application. BIT Department Deans Community High School Page 14

100 virus An virus attaches itself to messages, and usually replicates itself by automatically mailing itself to dozens of people in a user s address book Worm A worm is a program or algorithm that replicates itself over a computer network and usually performs actions, such as using up the computer's resources and possibly shutting the system down. At approximately 5pm on November 1988 the Morris Worm was created in the MIT AI laboratory in Cambridge, Massachusetts. It quickly spread to Cornell, Stanford, and then on to other sites. By the next morning, almost the entire Internet was infected Trojan horse Worm.net This program pretends to be a normal working program, however they also perform harmful processes at the same time. Unlike viruses, Trojan horses do not replicate themselves but they can be just as destructive. One of the most dangerous types of Trojan horse is a program that claims to rid your computer of viruses but instead introduces viruses onto your computer Questions 1. What is a virus?. Describe three problems a virus can cause for computer systems. 3. Name two types of files that are deemed to be safe from viruses. 4. Explain the difference between a virus and a worm. 5. Explain the difference between a worm and a trojan horse. 6. Explain why a boot sector virus is potentially dangerous. 7. Explain how a macro virus works Different actions of viruses There are various virus codes that can be used to try and create actions of different forms Replication Most viruses use replication to infect computer systems. Basically the virus has the ability to copy itself by attaching to other files Camouflage Some viruses can avoid detection by taking on characteristics that cannot be found or monitored. Total = 14 BIT Department Deans Community High School Page 15

101 Watching Other viruses can wait until a particular event happens before it attaches itself to a program or file. Usually some action or condition has to be met before the virus will attach itself Delivery Sometimes viruses can be attached to external backing storage media such as floppy disks. Once the disk has loaded its contents into memory the virus can attach itself to executable programs Anti-Virus Software Detection Techniques Anti-virus programs are used to detect and remove viruses. A virus scanner is a type of antivirus program that searches a system for virus signatures that have attached to executable programs and applications such as clients. A virus scanner can either search all executables when a system is booted or scan a file only when a change is made to that file. However, detection and removal of viruses are often difficult. Techniques that can be used by anti-virus software are: Use of Checksum Adding up all the instructions used within that file can form a checksum for a file. This is then added to the file. When the file is about to be run the checksum is recalculated and if there is an error then it is assumed that the file could be infected and a warning is given Searching for virus signatures A virus signature is a unique string of bits of a virus. The virus signature is like a fingerprint in that it can be used to detect and identify specific viruses. Anti-virus software uses the virus signature to scan for the presence of malicious code and remove (disinfect) it Heuristic detection This type of detection is based upon learning through previous knowledge. Using a virus signature heuristics are used by looking at characteristics of viruses that are already known. This is useful for finding unknown viruses. By using previous knowledge the heuristic can measure how likely it is that the computer system has been infected Memory resident monitoring Executable programs are divided into memory resident and non resident ones. A memory resident program leaves its code or part of if in RAM after ending and the operating system allocates a memory area necessary for program operation. After that, the memory resident program operates in parallel with other programs. A non resident program does not leave its code in memory after its termination, and the memory area occupied by it is cleared. Some anti-virus software can be memory resident and hence it monitors the computer system by checking any program that is run once the computer is switched on. The downside of this type of anti-virus software is it takes up RAM and hence can slow down the usual function of the computer as checks are being consistently carried out as programs are being loaded and run. BIT Department Deans Community High School Page 16

102 5.5.7 Dealing with Viruses When dealing with the various types of viruses as well as using anti-virus software, there are various ways of either preventing viruses or making the virus infection less costly and painful. Install anti-virus software from a well-known, reputable company, Update it regularly, and use it. In addition to scanning for viruses on a regular basis, install an 'on access' scanner and configure it to start automatically each time you boot your system. This will protect your system by checking for viruses each time your computer accesses an executable file. Virus scan any new programs or other files that may contain executable code before you run or open them, no matter where they come from. There have been cases of commercially distributed floppy disks and CD-ROMs spreading virus infections. Anti-virus programs aren't very good at detecting Trojan horse programs, so be extremely careful about opening binary files and Word/Excel documents from unknown sources. This includes posts in binary newsgroups, downloads from web/ftp sites that aren't well known or don't have a good reputation, and executable files unexpectedly received as attachments to or during an on-line chat session. Be extremely careful about accepting programs or other files during on-line chat sessions. This is one of the more common means that users end up with a virus or Trojan horse problems. Do regular backups. Some viruses and Trojan horse programs will erase or corrupt files on your hard drive, and a recent backup may be the only way to recover your data. Ideally, you should back up your entire system on a regular basis. If this isn't practical, at least backup file that you can't afford to lose or that would be difficult to replace: documents, bookmark files, address books, important , etc. BIT Department Deans Community High School Page 17

103 5.5.8 Questions 1. Describe two actions viruses can carry out.. What are anti-virus programs? 3. Explain what a virus scanner does. 4. What is a virus signature? 5. Describe two anti-virus software detection techniques. 6. What type of virus is anti-virus software not good at detecting? Justify your answer. 7. Explain why regular back ups are good practise when dealing with viruses. 1 1 Total = Selecting Software When using a computer system, software must be used in order to work the computer properly and carry out any tasks. There is a wide range of software on the market, whether it be system software or application software, that can be used with computers. Unfortunately the cost of software has risen considerably in the last decade. So which ones do you choose? Basically it is down to the task that needs to be solved and the computer system and peripherals that being used. (We already looked at this in the Peripherals topic, booklet.) Typical questions that need to be considered are: How much backing storage does the software require? How data types are being used? What is the task that needs to be solved? What are the RAM requirements for the software? Does the software require a high processor performance? Are there specific peripherals that are required for the software to operate? As the cost of tackling problems using a computer system with a range of software can be a costly one, it is necessary to be able to justify why choices have been made. Here are typical tasks that a computer system could solve: Production of a multimedia catalogue Setting up a LAN in a school Development of a school website Creating an audio CD Creating a DVD movie with added extras BIT Department Deans Community High School Page 18

104 Here are possible solutions to three of these typical tasks: Production Of A Multimedia Catalogue Multimedia involves the use of media elements, such as text, graphics, sound, video and animation. The computer system itself must be of a good specification to create, edit and run these elements. In order to put the catalogue together these media elements need particular software to create them. Software Computer System with: System software Video editing software Graphics manipulation software Sound manipulation software Word processor Animation software Multimedia authoring software Photo editing software Justification Operating system and utilities that can support multimedia To create and edit video frames To create and edit graphics To create and edit audio To create and edit text To create and edit animations To create and edit multimedia catalogue To create and edit digital photographs 5.6. Setting Up A Local Area Network (LAN) In School Setting up a network would require many desktop computers to be linked together. Possible software for this task could be: Hardware 50 Computer Systems with: System software Communication software Networking card drivers Browsers Network operating system Justification Operating system and utilities that can support networking To allow computers to operate across the network To allow networking cards to operate To be able to search and use the network facilities To manage and administer the network BIT Department Deans Community High School Page 19