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CERTIFICATE IN INFORMATION TECHNOLOGY Module Materials Computer Architecture and Systems Level 5 C20012

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Page 1: Module Materials Computer Architecture and Systems …assambc/computer_architecture_and_systems.pdf · Module Materials Computer Architecture and Systems ... list the main components

CERTIFICATE IN INFORMATION TECHNOLOGY

Module Materials

Computer Architecture and Systems

Level 5 C20012

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Specific Learning Outcomes Unit 1 Introduction to Computer Systems 1.1 list the main components of a computer system 1.2 state the function of each of these components 1.3 distinguish between hardware and software 1.4 list common examples of the use of a computing system 1.5 list the main types of computer 1.6 list uses of each type of computer 1.7 know the main elements of an information system (a typical business system) 1.8 list two examples of information systems 1.9 explain how the computer can be used as a means of communication 1.10 outline the historical development of computers. Unit 2 Computer Architecture 2.1 explain the role of the CPU in a computer 2.2 list the component parts in the CPU e.g. ALU, registers, decoder 2.3 explain the role of each component 2.4 explain the purpose of the instruction pointer 2.5 outline the steps involved in processing an instruction 2.6 explain the fetch – execute cycle. 2.7 describe the purpose of a memory cell 2.8 distinguish between random access memory (RAM) and read only memory (ROM) 2.9 list the different types of read only memory 2.10 distinguish between bit, byte, word, kilobyte and megabyte 2.11 distinguish between primary and secondary memory 2.12 explain the term cache memory 2.13 explain how cache memory can be used to improve the performance of the CPU 2.14 define a bus 2.15 distinguish between internal buses and external buses 2.16 list the different buses, which connect the CPU to the computer's main memory chips. 2.17 distinguish between different types of character recognition devices 2.18 list applications of character recognition devices 2.19 explain bar codes and list their uses 2.20 give examples of different types of user interface devices 2.21 explain the purpose of a magnetic tape 2.22 list advantages and disadvantages of using tapes 2.23 explain the purpose of a magnetic disk 2.24 explain the terms: track and sector 2.25 distinguish between hard disks and floppy disks 2.26 outline the basic disk structure 2.27 draw a diagram of a floppy disk, outlining its main components 2.28 draw a diagram of a hard disk, outlining its main components 2.29 explain the terms: access time, seek time and latency 2.30 list the advantages of hard disks over floppy disks 2.31 explain how direct memory access (DMA) improves the transfer of data from disks to

main memory 2.32 explain the purpose of optical disks 2.33 explain how data is stored on optical disks 2.34 explain how data compression works 2.35 explain what voice recognition software does 2.36 list the uses of voice recognition software 2.37 explain how voice recognition makes computer systems accessible to those with a

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disability 2.38 list devices used to produce computer output 2.39 list the different classifications of printers 2.40 describe how images are displayed on a visual display unit 2.41 describe specialised types of input/output devices, e.g. scanners, digital cameras, etc. 2.42 describe how special purpose storage devices such as smart cards can be used. Unit 3 Data Communications 3.1 define the term communications 3.2 list examples of how communications technology is used to-day 3.3 list the components that make up a communications system 3.4 describe the different types of transmission media used for communications channels 3.5 describe the ways in which the transmission media are connected 3.6 explain how data is transmitted 3.7 describe the communications equipment used in a communications system 3.8 list the functions performed by communications software 3.9 list the categories of network 3.10 describe the most common network layouts 3.11 explain the use of communications protocols 3.12 describe the Internet and how it works 3.13 list services provided by the Internet (email, ftp, etc.) 3.14 explain how to connect to the Internet and WWW. Unit 4 Operating Systems 4.1 list the functions of an operating system 4.2 explain how an operating system makes the computer hardware usable 4.3 explain the different types of operating system architecture: single-user, multi-tasking,

multi-user, networks 4.4 name and describe the major operating systems in use to-day 4.5 list some services provided by an operating system to a user 4.6 use the operating system user interface 4.7 give examples of different user interfaces 4.8 list the advantages and disadvantages of different types of user interface. Unit 5 System Applications 5.1 outline the main features of the Data Protection Act 5.2 discuss the role of computing systems in modern society 5.3 list the advantages of computing technology 5.4 identify examples of this technology in the local environment 5.5 list the advantages of electronic mail

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Unit 1: Introduction to Computer Systems Probably nothing has influenced our lives more in the past 50 years than the invention of the electronic computer. Today computers are nearly omnipresent. They are in our homes, our cars, our microwaves, our cellular phones, and even in our toys. Tiny computers called microprocessors are responsible for controlling many of the common appliances that we use every day from the automatic coffee maker to the hi-fi VCR. What exactly are computers and how do they look like? Where do they come from? And what do we use them for?

1.1 What is a computer?

It would be possible to build a specific machine to carry out a specific task - but this would be expensive and inconvenient. A general purpose machine can be given new instructions to perform new tasks, offering a more flexible, economical solution. A computer can be defined as a general purpose programmable digital machine which function is to run instructions. A computer system comprises hardware and software. The hardware components of a computer system are the electronic and mechanical parts while the software components of a computer system are the intangible parts: the data and the computer programs. The major hardware components of a computer system are the processor, main memory, secondary memory, peripheral interfaces and buses.

• The processor or Central Processing Unit (CPU) is the most significant component of a computer: it is the heart and brain of the system and contains all the circuitry that the computer needs to manipulate data and execute instructions.

• The main memory is used to store and retrieve data and programs being accessed by the processor. Two of the most commonly used types of memory are RAM (Random Access Memory) and ROM (Read Only Memory). RAM stores the data and general-purpose programs that the machine executes. RAM is temporary: its content can be changed at any time and it is erased when power to the computer is turned off. ROM is permanent; it can retain its content even when the machine is powered-off and is used to store programs for the initial boot-up of the machine.

• Secondary memory is a slower and cheaper form of memory and is used to store programs and data that are not currently used. The processor does not access the secondary memory directly. The content in it must first be copied into the main memory for the CPU to process. Secondary memory devices include hard drives, floppy disks, CDs and CDROMs etc.

• Peripheral interfaces mediate between the outside world and the interior world of the computer. They allow the computer to communicate to the user and to Input/Output devices. Input devices allow entering information into a computer while output devices display information that has been held of generated within a computer. Input devices include keyboards, joysticks, mice, light pens, touch-sensitive screens, scanners, graphics tablets, speech-recognition devices, and vision systems. Some examples of output devices are: monitors, speakers, printers.

• Buses are bundles of cables that carry information across the different units.

A typical desktop computer has its microprocessor, main memory, and other essential components on a motherboard (fig 1.1). Other components such as external storage, controllers for video display and sound, and peripheral devices may be attached to the motherboard as plug-in cards or via cables, although in modern computers it is increasingly common to integrate some of these peripherals into the motherboard itself.

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Fig1.1: A computer motherboard showing the location of the CPU, memory and I/O card sockets.

1.2 Where do computers come from? Modern computing can probably be traced back to the 'Harvard Mk I' and Colossus (both of 1943). Colossus was an electronic computer built in Britain at the end 1943 and designed to crack the German coding system - Lorenz cipher. The 'Harvard Mk I' was a more general purpose electro-mechanical programmable computer built at Harvard University with backing from IBM. These computers were among the first of the 'first generation' computers. First generation computers were normally based around wired circuits containing vacuum valves and used punched cards as the main (non-volatile) storage medium. Another general purpose computer of this era was 'ENIAC' (Electronic Numerical Integrator and Computer) which was completed in 1946. It was typical of first generation computers, it weighed 30 tonnes contained 18,000 electronic valves and consumed around 25KW of electrical power. It was, however, capable of an amazing 100,000 calculations a second. The next major step in the history of computing was the invention of the transistor in 1947. This replaced the inefficient valves with a much smaller and more reliable component. Transistorised computers are normally referred to as 'Second Generation' and dominated the late 1950s and

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early 1960s. Despite using transistors and printed circuits these computers were still bulky and strictly the domain of Universities and governments. The explosion in the use of computers began with 'Third Generation' computers. These relied Jack St. Claire Kilby's invention - the integrated circuit or microchip; the first integrated circuit was produced in September 1958 but computers using them didn't begin to appear until 1963. While large 'mainframes' such as the I.B.M. 360 increased storage and processing capabilities further, the integrated circuit allowed the development of Minicomputers that began to bring computing into many smaller businesses. Large scale integration of circuits led to the development of very small processing units, an early example of this is the processor used for analyising flight data in the US Navy's F14A `TomCat' fighter jet. This processor was developed by Steve Geller, Ray Holt and a team from AiResearch and American Microsystems. On November 15th, 1971, Intel released the world's first commercial microprocessor, the 4004. Fourth generation computers were developed, using a microprocessor to locate much of the computer's processing abilities on a single (small) chip. Coupled with one of Intel's inventions - the RAM chip (Kilobits of memory on a single chip) - the microprocessor allowed fourth generation computers to be even smaller and faster than ever before. The 4004 was only capable of 60,000 instructions per second, but later processors (such as the 8086 that all of Intel's processors for the IBM PC and compatibles is based) brought ever increasing speed and power to the computers. Supercomputers of the era were immensely powerful, like the Cray-1 which could calculate 150 million floating point operations per second. The microprocessor allowed the development of microcomputers, personal computers that were small and cheap enough to be available to ordinary people. The first such personal computer was the MITS Altair 8800, released at the end of 1974, but it was followed by computers such as the Apple I & II, Commodore PET and eventually the original IBM PC in 1981. Although processing power and storage capacities have increased beyond all recognition since the 1970s the underlying technology of LSI (large scale integration) or VLSI (very large scale integration) microchips has remained basically the same, so it is widely regarded that most of today's computers still belong to the fourth generation.

1.3 Common uses for computers

Computers can be instructed to perform a variety of individual functions. A set of instructions that tells a computer what to do is called a program. Today, more than 10,000 application programs are available for use on personal computers. They include such popular programs as word processing programs, spreadsheet programs, database programs, and communication programs.

• Word processing programs are used to type, correct, rearrange, or delete text in letters, memos, reports, and school assignments.

• Spreadsheet programs enable individuals to prepare tables easily. The users of such programs establish rules for handling large groups of numbers. For example, using a spreadsheet program, a person can enter some numbers into a table and the program will calculate and fill in the rest of the table. When the user changes one number in the table, the other numbers will change according to the rules established by that user. Spreadsheets may be used for preparing budgets and financial plans, balancing a chequebook, or keeping track of personal investments.

• Database programs allow a computer to store large amounts of data (information) in a systematic way. Such data might include the name, address, telephone number, salary, and starting date of every employee in a company. The computer could then be asked to produce a list of all employees who receive a certain salary.

• Communication programs connect a personal computer to other computers. People can thereby exchange information with one another via their personal computers. In addition,

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communication programs enable people to link their personal computers with databanks. Databanks are huge collections of information stored in large centralized computers. News, financial and travel information, and other data of interest to many users can be obtained from a databank.

• Other programs include recreational and educational programs for playing games, composing and hearing music, and learning a variety of subjects. Programs have also been written that turn household appliances on and off. Some people develop their own programs to meet needs not covered by commercially prepared programs. Others buy personal computers mainly to learn about computers and how to program them.

1.4 Main types of computers The main types of computer include Personal Computer, Workstation, Mainframe and Microcontroller.

Fig. 1.2: Examples of computer systems

• Personal Computers (PC) are usually designed to be used by one person at the time for general data processing, multimedia and communication. Desktop PC are fixed and usually required a constant supply of current from the main for power supply. Laptops are portable PC take can operate from a rechargeable battery.

• A workstation station is a high-performance computer designed for technical or scientific applications. Workstations are commonly used as servers connected to a local area network and running multi-user applications.

• Mainframe computers are computers used mainly by large organisations for critical applications, typically bulk data processing such as census, industry and consumer statistics, Enterprise Resource Planning and financial transaction processing.

• A computer wholly contained within a machine, e.g. mobile phone, is said to be ‘embedded.’ Examples of embedded computers are mobile phones, PDA, mp3 players, etc… The market for embedded computers is larger than for ‘ordinary’ computers.

• A Microcontroller is a small computer on a integrated circuit package, consisting of a relatively of a CPU, memory, and peripheral interfacing. Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, remote controls, office machines, appliances, power tools, and toys.

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1.5 Computers in Information Systems and communication Computer systems can be integrated in business and organisation to increase productivity, effectiveness and competivity. Information’s Systems relate to the combination of hardware, software, data, network infrastructure and trained personnel organized to facilitate planning, control, coordination, and decision making. The main elements of an Information Systems in a typical business are: Major types of Information systems include structural databases and information management software that can include the following:

• Transaction Processing Systems (TPS) -- collect, store, modifie and retrieve the data transactions of an enterprise;

• Management Information Systems (MIS) -- collect, process, store and disseminate data in the form of information needed to carry out the functions of management;

• Decision-Support Systems (DSS) -- help decision makers compile useful information from raw data, documents, personal knowledge, and/or business models to identify and solve problems and make decisions;

• Executive Support Systems (ESS): produce useful summarized reports and provide analysis tools that predict a series of performance outcomes over time using the input data. Collect information from all company levels and departments such as billing, cost accounting, staffing, scheduling, etc… .

Computers can be used as a means of communication by being part of a network. Computer networks allow multiple computers to be interconnected in a single building (e.g., the home) and cable services permit a high-speed connection to the Internet. Unlike with conventional telecommunications, the Internet is very affordable and it costs little to transmit information across the world.

Conclusion 1. The main hardware components of a computer system are the processor, main memory,

secondary memory, peripheral interfaces and buses. 2. The processor manipulates data and executes instructions. Main memory is used to store

and retrieve data and programs being accessed by the processor. Secondary memory is used to store programs and data that are not currently used. Peripheral interfaces mediate between the outside world and the interior world of the computer. They are all connected via buses that carry information.

3. The hardware components of a computer system are the electronic and mechanical parts while the software components of a computer system are the intangible parts: the data and the computer programs.

4. Common examples of the use of a computing system include such popular programs as word processing programs, spreadsheet programs, database programs, and communication programs.

5. The main types of computer include Personal Computer, Workstation, Mainframe, Microprocessor and Microcontroller.

6. Personal Computers (PC) are usually designed to be used by one person at the time for general data processing, multimedia and communication. Workstations are commonly used as servers connected to a local area network and running multi-user applications. Mainframes are used mainly by large organisations for critical applications. Microcontrollers are used in automatically controlled products and devices.

7. The main elements of an information system hardware, software, data, networks and people.

8. Examples of information systems include Transaction Processing Systems, Management

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Information Systems, Decision-Support Systems, and Executive Support Systems. 9. A computer can be used as a means of communication by being interconnected with other

computers as part of a network. 10. The historical development of computers: The first generation computers were introduced

in 1943, based on vacuum valves and punched cards. The invention of the transistor in 1947 allows the design of a second generation of computers that dominated until the early 1960s. The introduction of integrated circuits accelerated the development of a third generation in the 1960s. In 1971, Intel released the first commercial microprocessor used to develop the fourth generation. Although processing power and storage capacities have dramatically increased since the 1970s, it is regarded that most of today's computers belong to the fourth generation.

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Unit 2 Computer Architecture A computer can be seen as a machine that manipulates coded data and responds to events occurring in the external world. This is called the stored-program or Von Neumann machine architecture. In this unit, our primary goal is to develop an understanding of this machine architecture. 2.1 Central Processing Unit (CPU) A simple representation of a computer consists of a processor connected to some memory and some input and output (I/O) devices by a bus or data highway. This design principle separates special functions into special purpose devices.

Fig. 2.1 Representation of the Von Neumann machine architecture

The Concept of Stored Program: The core of the Von Neumann architecture states that the memory is used to store both data and programs. In order for a CPU to accomplish meaningful work, it must have two inputs: instructions and data. Instructions tell the CPU what actions need to be performed on the data. Instructions are represented with binary codes just like data. In fact, the CPU makes no distinction about the whether it is storing instructions or data in memory. This concept is called the stored-program concept. Early computing devices were not known for their flexibility, as the program that each device executed tended to be built into the control unit as a part of the machine. One approach used to gain flexibility in early electronic computers was to design the control units so they could be conveniently rewired. A breakthrough came with the realization that the program, just like data, can be coded and stored in main memory. If the control unit is designed to extract the program from memory, decode the instructions, and execute them, a computer's program can be changed merely by changing the contents of the computer's memory instead of rewiring the control unit. This stored-program concept has become the standard approach used today. To apply it, a machine is designed to recognize certain bit patterns as representing certain instructions.

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The Central Processing Unit (CPU) is where all the work is done. It contains the hardware resources required to execute instructions. The CPU reads the sequence of commands that make up a program one by one from memory and executes them. The CPU functions by following a cycle of fetching an instruction, decoding it, and executing it. This process is known as the fetch-decode-execute cycle. The cycle begins when an instruction is transferred from memory to the CPU along the data bus. Then the unique bit patterns that make up the machine-language are extracted and decoded. A decoder recognises which operation the bit pattern represents and activates the correct circuitry to perform the operation. Once the operation is performed, the cycle repeats after the CPU takes the time to put away results to off-processor locations. Fig. 2.2 illustrates the CPU’s functional units that make possible the instruction execution cycle. The CPU registers and units involved in the process are: MAR The Memory Address Register holds the address of the next location in the store to be

accessed. The contents of the MAR point to the location of information in memory. For example, if the MAR contains the value $4000, the CPU is going to access address number $4000 in the memory.

PC The Program Counter contains the address of the next instruction to be executed. MBR The Memory Buffer Register holds the data just read from the main store, or data to be

written into the main store. All information that flows into or out of memory passes through the MBR.

IR The Instruction Register holds the most recently read instruction from the main store. ALU The Arithmetic and Logic Unit calculates a function of one or two inputs. The actual

function performed by the ALU is determined by the bit pattern of the instruction in the IR.

CU The Control Unit interprets the instruction in the IR. That is, the CU is responsible for converting the bit pattern of an instruction into the sequence of actions necessary to execute the instruction.

D0 Data Register D0 can be used as an accumulator to hold temporary data in the CPU during arithmetic and logical operations.

CCR The Condition Code Register is used to implement conditional branches.

• Fetching an instruction begins with the contents of the Program Counter being moved to the Memory Address Register ([MAR] <- [PC]). The MAR is now pointing at the next instruction to be executed. Once the PC has done its job, it is automatically incremented to point the next instruction in sequence. After the increment, the PC points at the next instruction while the current instruction is being executed.

• The next step is to read the contents of the memory location pointed at by the MAR. The data read from memory is first deposited in a temporary holding register, the Memory Buffer Register ([MBR]<-[MAR]).

• In the final step of the fletch cycle the contents of the MBR are copied the Instruction Register ([IR]<-[MBR]). The IR holds the instruction while it is decoded by the Control Unit. One field of the IR contains the operation code (op-code) that tells the CPU what operation is to be carried out. A second field, called the operand field, contains the address of the data to be used by the instruction.

• The fletch phase is followed by an execution phase in which the Control Unit generates all signals necessary to execute the instruction. The CU controls all parts of the CPU including all programmable registers and the Arithmetic and Logic Unit.

• A path between the CCR and the CU is used by the CU to decide whether to continue with the next instruction in series, or to jump to the address field specified by the branch field of a conditional instruction.

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Fig. 2.2: Structure of a typical CPU

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2.2 MEMORY Computer storage is classified into primary (main) memory and secondary (peripheral) storage. Primary storage is usually RAM (~10ns access time) while secondary storage is usually hard disk drives (~10ms access time). Secondary storage is a lot cheaper than primary and secondary storage is persistent when RAM is volatile. This section will give an overview of the memory hierarchy from cache to secondary storage.

2.2.1 Why defining a Memory Hierarchy? A memory hierarchy is required because no single technology provides all the necessary requirements of ideal memory; that is, low-cost, fast access time, non-volatile. By combining various memory technologies, it is possible to create a system with a memory system that has, collectively, the required requirements. Fig. 1 illustrates the nature of a memory hierarchy.

Fig. 2.3 Memory hierarchy

2.2.2 RAM and ROMs The RAM (random access memory) is the place in a computer where the OS, application programs, and data in current use is kept so that they can be quickly reached by the processor. The RAM is volatile. The ROM (Read-Only Memory) is computer memory containing data that normally can only be read, not written to. The ROM contains the programs that allow a computer to be "booted up"; it is a non-volatile type of memory. Programmable read-only memory (PROM) is a form of digital memory used to store programs permanently. The key difference from a strict ROM is that the programming is applied after the device is constructed. They are frequently seen in video game consoles, or such products as electronic dictionaries, where PROMs for different languages can be substituted. FLASH memory is a semiconductor read-mostly memory that uses EPROM (Electronically Programmable ROM) technology to store data. Data can be read electronically and written electronically into data cells. It has a fast read access time but a relatively slow write time. FLASH memory can be used to store data (e.g. BIOS). FLASH memory is now the basis of digital cameras and MP3 memory systems. FLASH memory is important because an entire branch of computing (personal, mobile, pervasive) depends on it. EEPROM (electrically erasable programmable read-only memory) is user-modifiable read-only memory (ROM) that can be erased and reprogrammed (written to) repeatedly through the application of higher than normal electrical voltage. Unlike EPROM chips, EEPROMs do not need to be removed from the computer to be modified.

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Memory cells are the building blocks of computer data storage. The bit is the smallest unit of digital information; however data is usually represented as groups of multiples bits to form nybbles, bytes, words, longwords, kilobytes, megabyte, gigabytes and terabytes:

• 1nybble = 4 bits • 1byte= 8 bits = 2 nybbles • 1word=2 bytes • 1longword=2 words = 4 bytes • 1quadword=4words= 8bytes • 1kilobyte (KB) = 1024 bytes • 1 megabyte (MB) = 1024 KB • 1 gigabyte (GB) = 1024 MB • 1 terabyte (TB) = 1024 GB

Symbols: k = 103=1000, K =210= 1024, b = bit, B = byte

2.2.3 The system bus All units of a computer system are connected via a bus that carries information: it is a bundle of cables, like a highway where information travels. The bus has a limited speed which determines its ability to carry information.

• An internal bus links units within the computer: CPU, main memory and peripheral interfaces.

• An external bus connects external Input/Output devices to the peripheral interfaces. Three types of information can travel via a bus: data, address and control signals. Then some computer systems have three separate buses for each type of signal.

2.2.4 Cache memory There is a high time cost in getting information from the memory to the processor via the bus, which is why programmers try to keep as much information in the CPU as much as possible. The processor is a special purpose device dedicated in executing instructions, not storing them. But what you can do is to have the processor store a tiny piece of a program, perhaps the piece of a program that is frequently used, that you can easily access. This can dramatically speed up program execution. That is called the cache. Cache memory is a special high-speed storage mechanism. A memory, sometimes called a cache store or RAM cache, is a portion of memory made of high-speed static RAM (SRAM) instead of the slower and cheaper dynamic RAM (DRAM) used for main memory. Memory caching is effective because most programs access the same data or instructions over and over. By keeping as much of this information as possible in SRAM, the computer avoids accessing the slower DRAM.

Transistors per bit

Access time Persistant ? Sensitive ? Cost Applications

SRAM 6 1X Yes No 100x cache memories

DRAM 1 10X No Yes 1X Main memories, frame buffers

Disk caching works under the same principle as memory caching, but instead of using high-speed SRAM, a disk cache uses conventional main memory. The most recently accessed data from the disk is stored in a memory buffer. When a program needs to access data from the disk, it first checks the disk cache to see if the data is there. Disk caching can dramatically improve the performance of applications, because accessing a byte of data in RAM can be thousands of times faster than accessing a byte on a hard disk.

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2.3 COMPUTER PERIPHERALS So far we've examined the internal structure and operation of the computer's central processing unit. However, if a computer is to be of any real value to people it must have some way of communicating with them. After all, there's no point in creating a supercomputer the size of a paperback that can solve the ultimate question of life, the universe, and everything, if it can't tell us the answer. In this chapter we look at the ways and means by which information gets into and out of a computer and the devices or peripherals that are connected to the computer (e.g. the printer and the display).

2.3.1 Structure and operation of magnetic storage devices We will look at the operation of three magnetic storage devices: hard disks, floppy disks and tapes. Disk drives are flat, circular, rigid sheet of aluminium coated with a thin layer of magnetic material. Hard disk drives contain multiple rotating platters in which a head (one per disk surface) records data along a concentric track. The track is divided into basic units called sectors (a sector is the smallest unit of data that can be read from or written to a disk). The read write head hovers above the surface (but not touching it) at a tiny fraction of an inch (the head to surface distance is the order of a wavelength of light).

Platters

Read/write headsSpindle

Boom

Track

Fig. 2.4 typical hard disk drive layout There are three stages in reading/writing to a disk drive:

• Seek time - head movement to the correct track: ~6ms; • Rotational latency - wait for the desired sector to arrive under the head: ~4ms; • Data transfer - relatively small.

Therefore, reading from physically adjacent blocks is relatively fast.The performance of disks has increased dramatically with 1 TB now available. However, data access times have improved little. Disk typically rotate at 7,200 rpm and the fastest disks rotate at 15,000 rpm.

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The floppy disk is a removable secondary storage medium that can be transported from one system to another. The original floppy disk, created in 1971 by IBM, was made of plastic coated with a magnetic material enclosed in an eight inch square protective envelope. In operation, the drive grabs the floppy's centre and spins it inside its housing. The read/write head contacts the surface through an opening in the plastic shell or envelope. Floppies rotate at 300 RPM, which is from 10 to 30 times slower than a hard disk. They are also at rest until a data transfer is requested. Following are the three types developed, from newest to oldest, and their raw, uncompressed storage capacity: Housing Max. Storage

Capacity Creator

3.5" rigid 1.44MB Sony 3.5" rigid 2.88MB IBM 5.25" flexible

1.2MB Shugart

8" flexible 500KB IBM

Fig. 2.5 structure of a floppy disk

Floppy disks have long access times and much lower capacity than hard disks. Woefully undersized for today's use, the floppy disk is no longer standard equipment on computers.

Fig. 2.6 examples of removable tapes

Tape-based secondary systems were popular in the 1970s and 1980s and were largely associated with mainframe computers. Today, tape is mainly found in small cartridge-based systems where it provides backup and archival storage. Tapes provide an easy media method for preserving multiple full-system backups. In general, tape drives are used where high capacity and high reliability are paramount. They can be expensive initially but are extremely inexpensive when you factor in the low cost of the media over time. However, tape-base backup systems present non-negligible drawbacks: • Creating a tape backup copy of files or of a drive

requires the use of a special backup program in most cases.

• Retrieving data from most tape backup drives requires that the data files be restored to the hard disk.

• Tape backups store and retrieve data sequentially. The last file backed up can't be accessed until the rest of the tape is read.

The increase in the capacity and decrease in the cost of hard drives together with the growth of optical storage systems make tape much less attractive than it once was.

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2.3.2 Structure and operation of optical disks An optical memory disk is a rigid plastic disk whose surface is covered with a long spiral track. The track is laid down on a substrate inside the disk and is covered with a transparent plastic protective layer.

Fig.2.7: structure & operation of a CD-ROM

Like a magnetic disk, information is stored along a track in binary form. Unlike the magnetic disk, the track in an optical is in a continuous spiral. The spiral on an optical disk begins at the innermost track and spirals outwards. An optical disk stores information by means of reflecting or non-reflective metallic dots along the track. A beam of light produced by a semiconductor laser is focused on the surface of the track and a photosensitive transistor or diode detects the light reflected back from the surface. The functional problems of optical storage are reliability, detecting the presence of tiny dents called pits on the surface of the disk, optically tracking the reflective elements, and encoding/decoding the data. Compact Disk (CD) are Digital Versatile Disks (DVD) are the most common forms of optical stage disks and can be found in multiple declinations (single/double side, single/double layers, read-only memory, rewritable, etc…). The CD has a capacity of about 600 Mbytes. The DVD is a replacement for the CD which uses virtually identical technology. However, modern manufacturing processes have allowed the system to be scaled down to fit more bits on the disk (typically 4.7 GBytes). Sophisticated focusing mechanisms have allowed two planes of data (dual layer) within a DVD which has doubled the capacity to over 9 GBytes. Hard drive vs DVD: The capacity of hard drives has increased more rapidly than that of optical storage. The standard DVD has a capacity of 4.7 Gbytes which is 1% of that of a large hard drive. However, the DVD provides very low cost removable storage and can be used for backup and archival purposes. The access time of DVDs is mot lower than that of the hard disk because the DVD rotates more slowly (it is not in a sealed enclosure). Moreover, the DVD’s track is not concentric and is a spiral. Locating a specific point on a DVD is therefore slower than locating a sector on a disk. The head assembly in a DVD is far heavier than that in a hard disk and that means that the head cannot step rapidly from track to track. This reduces the access time of the DVD. At the moment, two new optical storage mechanisms are becoming available (both are modified DVDs). Blu-ray has a capacity of 50 GByte/disk and high density DVD, HD-DVD, has a capacity of 15-45 GBytes. Unfortunately, these mechanisms are mutually incompatible. Such large storage mechanisms are required for both high definition video and backing up the increasingly low-cost large magnetic disk drives.

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2.3.3 Direct Memory Access (DMA) Consider the way in which a block of data is written to a disk drive. It's often impractical to use programmed data transfers for disk I/O because programmed transfers are too slow. The output strategy most frequently used is direct memory access (DMA), in which the data is transferred from the computer's memory to a peripheral, or vice versa, without passing through the CPU's registers. The CPU simply tells the DMA hardware to move a block of data and the DMA hardware gets on with the task, allowing the CPU to continue its main function of information processing. This strategy (i.e. DMA) requires special hardware to implement it. Fig. 2.8 illustrates the DMA mechanism. An interface chip called a DMA controller (DMAC) is responsible for moving the data between the memory and the peripheral.

Fig. 2.8: description of DMA mechanism

The DMAC must provide addresses for the source or destination of data in memory, and signal to the peripheral that data is needed or is ready. Furthermore, the DMAC must collect the computer's internal data and address buses for the duration of a data transfer. Data transfer by DMA must be done while avoiding a conflict with the CPU for the possession of the buses. The bus switches connect the CPU, DMA controller, and peripheral to the data bus. In normal operation the CPU accesses the buses. The DMA controller requests the CPU to give up the bus and turns off the CPU‘s bus switch and turns on the DMA controller bus switch.

2.3.4 Data Compression Data compression consists of recombining the bits and bytes that make up your data into a smaller, more compact form. The actual information doesn't change (in case of loseless compression), but the internal representation of it does. Without data compression, it would be virtually impossible to implement technologies such as streaming media, large database applications, or any other application that handles lots and lots of information. There is simply too much data to handle easily. Data compression is a coding/decoding process. The coding side of it uses a particular algorithm to rearrange the internal representation of your data. Generally, an application handles that transparently and in many cases you don't need to know how or why it works. When it's time to actually use the information it has to be decoded, using the reverse process of the method used in the coding process. An example of this is the popular WinZip program, which compresses data files. To add files to a WinZip archive, you use the available menu option to add a file. When you want a copy of a file

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within an archive, you choose to extract it. In both cases, a mouse click is all that's required of you. But behind the scenes, the WinZip program is using a particular algorithm to compress and uncompress the information in your files.

2.3.5 Voice recognition software Speech recognition (also known as automatic speech recognition or computer speech recognition) converts spoken words to machine-readable input (for example, to key presses, using the binary code for a string of character codes). The term "voice recognition" is sometimes incorrectly used to refer to speech recognition, when actually referring to speaker recognition, which attempts to identify the person speaking, as opposed to what is being said. Confusingly, journalists and manufacturers of devices that use speech recognition for control commonly use the term Voice Recognition when they mean Speech Recognition. Speech recognition applications include voice dialing (e.g., "Call home"), call routing (e.g., "I would like to make a collect call"), domotic appliance control and content-based spoken audio search (e.g., find a podcast where particular words were spoken), simple data entry (e.g., entering a credit card number), preparation of structured documents (e.g., a radiology report), speech-to-text processing (e.g., word processors or emails), and in aircraft cockpits (usually termed Direct Voice Input). People with disabilities are another part of the population that benefit from using speech recognition programs. It is especially useful for people who have difficulty with or are unable to use their hands, from mild repetitive stress injuries to involved disabilities that require alternative input for support with accessing the computer. In fact, people who used the keyboard a lot and developed RSI became an urgent early market for speech recognition. Speech recognition is used in deaf telephony, such as spinvox voice-to-text voicemail, relay services, and captioned telephone. Individuals with learning disabilities who have problems with thought to paper communication (essentially they think of an idea but it is processed incorrectly causing it to end up differently on paper) can benefit from the software as it helps to overlap that weakness.

2.3.6 Character recognition devices Optical character recognition (OCR) is a process of capturing an image of a document and then extracting the text from that image. In recent years, it has become possible to scan in paper copies of documents so as to form computerized images of such documents, analyze images in the text areas of the documents so as to recognize individual characters in the text data, and form computer readable files of character codes corresponding to the recognized characters. With the advent of optical imaging capabilities and optical character recognition (OCR) software, data placed on a form can be digitized by such instruments as a scanner or fax machine and the digitized data can be interpreted as text by the OCR software. A number of optical character recognition (OCR) systems are known. Typically, such systems comprise apparatus for scanning a page of printed text and performing a character recognition process on a bit-mapped image of the text, which is a pixel-by-pixel representation of the overall image in a binary form. The recognition system reads characters of a character code line by framing and recognizing the characters within the image data. During the recognition process, the document is analyzed for several key factors such as layout, fonts, text and graphics. The document is then converted into an electronic format that can be edited with application software. The output image is then supplied to a computer or other processing device, which performs an OCR algorithm on the scanned image. The document can be of many different languages, forms and features. The purpose of the OCR algorithm is to produce an electronic document comprising a collection of recognized words that are capable of being edited. In general, electronic reading machines using computer-based optical character recognition (OCR) comprise personal computers outfitted with computer scanners, optical character recognition software, and computerized text-to-voice

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hardware or software. An optical scanner is a device that can read text or illustrations printed on paper and translate the information into a form the computer can use. A scanner works by digitising an image which consists of dividing it into a grid of boxes and representing each box with either a zero or a one, depending on whether the box is filled in. For colour and grey scaling, the same principle applies, but each box is then represented by up to 24 bits. The resulting matrix of bits, called a bit map, can then be stored in a file, displayed on a screen, and manipulated by programs. Optical scanners do not distinguish text from illustrations; they represent all images as bit maps. Therefore, it is not possible to directly edit text that has been scanned. To edit text read by an optical scanner, one needs an optical character recognition (OCR) system to translate the image into ASCII characters. Most optical scanners sold today come with OCR packages. A range of scanners are available: flat bed scanners or hand held scanners. The latter scanners are commonly used in the retail trade (shops and supermarkets) or libraries to read barcodes. A barcode is an optical machine-readable representation of data. Originally, bar codes represented data in the widths (lines) and the spacings of parallel lines and may be referred to as linear or 1D (1 dimensional) barcodes or symbologies. But they also come in patterns of squares, dots, hexagons and other geometric patterns within images termed 2D (2 dimensional) matrix codes or symbologies. In spite of there being no bars, 2D systems are generally referred to as barcodes as well. The first use of barcodes was to automate grocery checkout systems, a task where they have become almost universal today. Their use has spread to many other roles as well, tasks that are generically referred to as Auto ID Data Capture (AIDC). Magnetic-Ink Character Recognition, MICR, is a type of font capable of recognition using magnetically charged ink. Computers equipped with the right hardware and software can print and/or read the character printed in such ink. MICR font is commonly used to print checks, deposit slips, mortgage coupons, etc. There are various types of MICR fonts, the MICR E-13B font is used in the Canada, Panama, Puerto Rico, UK, and the United States. The MICR CRC-7 was created according to the ISO standards and is a font used in France, Mexico, Spain, and most other Spanish speaking countries.

2.3.7 User interface devices The user interface (also known as Human Computer Interface or Man-Machine Interface (MMI)) is the aggregate of means by which users interact with the system—a particular machine, device, computer program or other complex tool. The user interface provides means of input, allowing the users to manipulate a system, and Output, allowing the system to indicate the effects of the users' manipulation. Currently the following types of user interface are the most common:

• Graphical user interfaces (GUI) accept input via devices such as computer keyboard and mouse and provide articulated graphical output on the computer monitor. There are at least two different principles widely used in GUI design: Object-oriented user interfaces (OOUIs) and application oriented interfaces[verification needed].

• Web-based user interfaces or web user interfaces (WUI) accept input and provide output by generating web pages which are transmitted via the Internet and viewed by the user using a web browser program. Newer implementations utilize Java, AJAX, Adobe Flex, Microsoft .NET, or similar technologies to provide real-time control in a separate program, eliminating the need to refresh a traditional HTML based web browser. Administrative web interfaces for web-servers, servers and networked computers are often called Control panels.

• Command line interfaces, where the user provides the input by typing a command string with the computer keyboard and the system provides output by printing text on the computer monitor. Used for system administration tasks etc.

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• Tactile interfaces supplement or replace other forms of output with haptic feedback methods. Used in computerized simulators etc.

• Touch interfaces are graphical user interfaces using a touchscreen display as a combined input and output device. Used in many types of point of sale, industrial processes and machines, self-service machines etc.

• Attentive user interfaces manage the user attention deciding when to interrupt the user, the kind of warnings, and the level of detail of the messages presented to the user.

• Crossing-based interfaces are graphical user interfaces in which the primary task consists in crossing boundaries instead of pointing.

• Gesture interfaces are graphical user interfaces which accept input in a form of hand gestures, or mouse gestures sketched with a computer mouse or a stylus.

• Multi-screen interfaces, employ multiple displays to provide a more flexible interaction. This is often employed in computer game interaction in both the commercial arcades and more recently the handheld markets.

• Noncommand user interfaces, which observe the user to infer his / her needs and intentions, without requiring that he / she formulate explicit commands.

• Voice user interfaces, which accept input and provide output by generating voice prompts. The user input is made by pressing keys or buttons, or responding verbally to the interface.

2.3.8 Common output devices An output device is any piece of computer hardware equipment used to communicate the results of data processing carried out by an information processing system (such as a computer) to the outside world. The most common outputs are visual display units, speakers and printers. A visual display unit (also called VDU, monitor, or screen) offers a two-dimensional visual presentation of information. A speaker can be used for various sounds mean to alert the user, as well as music and spoken word. Printers produce a permanent hard copy of the information on paper. The most common technologies rely on inkjet or laser. Inkjet printers use a print head that squirts a tiny drop of ink onto the paper. The drop is ejected from the print head either by heating the ink and momentarily boiling it, or by flexing a quartz crystal to send a shockwave through the ink to eject a drop. A print head has multiple nozzles. Laser printers use a drum coated with a photoconductive material. First a high voltage charge is placed on the drum. An image is projected on the drum by a laser (or other means) which causes the charge to leak away in light areas. The drum is then covered with a toner (a fine powder) which sticks to the charged (dark) areas. The drum is then rotated against the paper and the toner transferred to the paper where it is set (fused) by heating. Both ink jet and laser printers are fairly reliable. However, the inkjet printer is normally used for domestic and small office applications with small print runs. Inkjet printers operate a line at a time and scan the paper by moving the print head along a line. This means that inkjet printers are ultimately slower than laser printers that print an image at a time. Inkjet printers are remarkably cheap. This is partially because they are mass produced and partially because the complexity of the printer is in the print head which is thrown away when the ink is used. This approach means that the life of the most critical part of the printer need be measured only in terms of hundreds of sheets of paper rather than tens of thousands. Moreover, one problem of inkjet printing is closing œ the nozzles become clogged by dried ink and fail. By replacing the print head when the ink is used, no maintenance is required.

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2.3.9 Example of special purpose storage devices: smart cards A smart card, chip card, or integrated circuit card (ICC), is in any pocket-sized card with embedded integrated circuits which can process data. This implies that it can receive input which is processed — by way of the ICC applications — and delivered as an output. There are two broad categories of ICCs. Memory cards contain only non-volatile memory storage components, and perhaps some specific security logic. Microprocessor cards contain volatile memory and microprocessor components. The card is made of plastic, generally PVC, but sometimes ABS. The card may embed a hologram to avoid counterfeiting. Using smartcards also is a form of strong security authentication for single sign-on within large companies and organizations.

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Unit 3: Data Communications

3.1 Network communication The primary purpose of any network is to provide a method to communicate information. From the very earliest primitive humans to the most advanced scientists of today, sharing information with others is crucial for human advancement. All communication begins with a message, or information, that must be sent from one individual or device to another. The methods used to send, receive and interpret messages change over time as technology advances. All communication methods have three elements in common. The first of these elements is the message source, or sender. Message sources are people, or electronic devices, that need to communicate a message to other individuals or devices. The second element of communication is the destination, or receiver, of the message. The destination receives the message and interprets it. A third element, called a channel, provides the pathway over which the message can travel from source to destination.

3.2 Components of a communications system There are many components that can be part of a network, for example personal computers, servers, networking devices, and cabling. These components can be grouped into four main categories:

• Hosts • Shared peripherals • Networking devices • Networking media

The network components that people are most familiar with are hosts and shared peripherals. Hosts are devices that send and receive messages directly across the network.

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Shared peripherals are not directly connected to the network, but instead are connected to hosts. The host is then responsible for sharing the peripheral across the network. Hosts have computer software configured to enable people on the network to use the attached peripheral devices. The network devices, as well as networking media, are used to interconnect hosts. Some devices can play more than one role, depending on how they are connected. For example, a printer directly connected to a host (local printer) is a peripheral. A printer directly connected to a network device and participates directly in network communications is a host. All computers connected to a network that participate directly in network communication are classified as hosts. Hosts can send and receive messages on the network. In modern networks, computer hosts can act as a client, a server, or both. The software installed on the computer determines which role the computer plays. Servers are hosts that have software installed that enable them to provide information, like email or web pages, to other hosts on the network. Each service requires separate server software. For example, a host requires web server software in order to provide web services to the network. Clients are computer hosts that have software installed that enable them to request and display the information obtained from the server. An example of client software is a web browser, like Internet Explorer. A computer with server software can provide services simultaneously to one or many clients. Additionally, a single computer can run multiple types of server software. In a home or small business, it may be necessary for one computer to act as a file server, a web server, and an email server. A single computer can also run multiple types of client software. There must be client software for every service required. With multiple clients installed, a host can connect to multiple servers at the same time. For example, a user can check email and view a web page while instant messaging and listening to Internet radio.

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The role of computers in a network

3.3 Types of transmission media used for communications channels In order for communication to occur a source, destination, and some sort of channel must be present. A channel, or medium, provides a path over which the information is sent. In the networked world, the medium is usually some sort of physical cable. It may also be electromagnetic radiation, in the case of wireless networking. The connection between the source and destination may either be direct or indirect, and may span multiple media types. Many different types of cables exist to interconnect the various devices in a NOC or local network. There are two kinds of physical cable. Metal cables, usually copper, have electrical impulses applied to them to convey information. Fiber optic cables, made of glass or plastic, use flashes of light to convey information.

Twisted Pair: Modern Ethernet technology generally uses a type of copper cable known as

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twisted pair (TP) to interconnect devices. Because Ethernet is the foundation for most local networks, TP is the most commonly encountered type of network cabling. Coaxial cable is usually constructed of either copper or aluminum, and is used by cable television companies to provide service. It is also used for connecting the various components which make up satellite communication systems. Fiber optic cables are made of glass or plastic. They have a very high bandwidth, which enables them to carry very large amounts of data. Fiber is used in backbone networks, large enterprise environments and large data centers. It is also used extensively by telephone companies.

In addition to the wired network, various technologies exist that allow the transmission of information between hosts without cables. These are known as wireless technologies. Wireless technologies use electromagnetic waves to carry information between devices. An electromagnetic wave is the same medium that carries radio signals through the air. The electromagnetic spectrum includes such things as radio and television broadcast bands, visible light, x-rays and gamma-rays. Each of these has a specific range of wavelengths and associated energies as shown in the diagram. Some types of electromagnetic waves are not suitable for carrying data. Other parts of the spectrum are regulated by governments and licensed to various organizations for specific applications. Certain areas of the spectrum have been set aside to allow public use without the restriction of having to apply for special permits. The most common wavelengths used for public wireless communications include the Infrared and part of the Radio Frequency (RF) band. Infrared (IR) is relatively low energy and cannot penetrate through walls or other obstacles. However, it is commonly used to connect and move data between devices such as Personal Digital Assistants (PDAs) and PCs. A specialized communication port known as an Infrared Direct Access (IrDA) port uses IR to exchange information between devices. IR only allows a one-to-one type of connection. IR is also used for remote control devices, wireless mice, and wireless keyboards. It is generally used for short-range, line-of-sight, communications. However, it is possible to reflect the IR signal off objects to extend the range. For greater ranges, higher frequencies of electromagnetic waves are required. Radio Frequency (RF) waves can penetrate through walls and other obstacles, allowing a much greater range than IR. Certain areas of the RF bands have been set aside for use by unlicensed devices such as wireless LANs, cordless phones and computer peripherals. This includes the 900 MHz, 2.4 GHz, and the 5 GHz frequency ranges. These ranges are known as the Industrial Scientific and Medical (ISM) bands and can be used with very few restrictions. Bluetooth is a technology that makes use of the 2.4 GHz band. It is limited to low-speed, short-range communications, but has the advantage of communicating with many devices at the same time. This one-to-many communications has made Bluetooth technology the preferred method

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over IR for connecting computer peripherals such as mice, keyboards and printers. Other technologies that make use of the 2.4 GHz and 5 GHz bands are the modern wireless LAN technologies that conform to the various IEEE 802.11 standards. They are unlike Bluetooth technology in that they transmit at a much higher power level, which gives them a greater range.

3.4 Network categories There are many types of networks that provide us with different kinds of services. In the course of a day, a person might make a phone call, watch a television show, listen to the radio, look up something on the Internet, or even play a video game with someone in another country. All of these activities depend on robust, reliable networks. Networks provide the ability to connect people and equipment no matter where they are in the world. People use networks without ever thinking about how they work or what it would be like if the networks did not exist. Communication technology in the 1990s, and before, required separate, dedicated networks for voice, video and computer data communications. Each of these networks required a different type of device in order to access the network. Telephones, televisions, and computers used specific technologies and different dedicated network structures, to communicate. But what if people want to access all of these network services at the same time, possibly using a single device? New technologies create a new kind of network that delivers more than a single type of service. Unlike dedicated networks, these new converged networks are capable of delivering voice, video and data services over the same communication channel or network structure. New products are coming to market that take advantage of the capabilities of converged information networks. People can now watch live video broadcasts on their computers, make a telephone call over the Internet, or search the Internet using a television. Converged networks make this possible. Networks come in all sizes. They can range from simple networks consisting of two computers, to networks connecting millions of devices. Networks installed in small offices, or homes and home offices, are referred to as SOHO networks. SOHO networks enable sharing of resources, such as printers, documents, pictures and music between a few local computers. In business, large networks can be used to advertise and sell products, order supplies, and communicate with customers. Communication over a network is usually more efficient and less expensive than traditional forms of communication, such as regular mail or long distance phone calls. Networks allow for rapid communication such as email and instant messaging, and provide consolidation, storage, and access to information on network servers. Business and SOHO networks usually provide a shared connection to the Internet. The Internet is considered a "network of networks" because it is literally made up of thousands of networks that are connected to each other.

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3.5 Network topology In a simple network consisting of a few computers, it is easy to visualize how all of the various components connect. As networks grow, it is more difficult to keep track of the location of each component, and how each is connected to the network. Wired networks require lots of cabling and network devices to provide connectivity for all network hosts. When networks are installed, a physical topology map is created to record where each host is located and how it is connected to the network. The physical topology map also shows where the wiring is installed and the locations of the networking devices that connect the hosts. Icons are used to represent the actual physical devices within the topology map. It is very important to maintain and update physical topology maps to aid future installation and troubleshooting efforts. In addition to the physical topology map, it is sometimes necessary to also have a logical view of the network topology. A logical topology map groups hosts by how they use the network, no matter where they are physically located. Host names, addresses, group information and applications can be recorded on the logical topology map. The graphics illustrate the difference between logical and physical topology maps.

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3.6 Communication protocols Protocols are specific to the characteristics of the source, channel and destination of the message. The rules used to communicate over one medium, like a telephone call, are not necessarily the same as communication using another medium, such as a letter. Protocols define the details of how the message is transmitted, and delivered. This includes issues of: Message format, message size, timing, encapsulation, encoding and standard message pattern. Many of the concepts and rules that make human communication reliable and understandable also apply to computer communication. Computers, just like humans, use rules, or protocols, in order to communicate. Protocols are especially important on a local network. In a wired environment, a local network is defined as an area where all hosts must "speak the same language" or in computer terms "share a common protocol". If everyone in the same room spoke a different language they would not be able to communicate. Likewise, if devices in a local network did not use the same protocols they would not be able to communicate.

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The most common set of protocols used on local wired networks is Ethernet. The Ethernet protocol defines many aspects of communication over the local network, including: message format, message size, timing, encoding, and message patterns. IP traffic is managed based on the characteristics and devices associated with each of the three layers: Access, Distribution and Core. The IP address is used to determine if traffic should remain local or be moved up through the layers of the hierarchical network. The Access Layer provides a connection point for end user devices to the network and allows multiple hosts to connect to other hosts through a network device, usually a hub or switch. Typically, all devices within a single Access Layer will have the same network portion of the IP address. If a message is destined for a local host, based on the network portion of the IP address, the message remains local. If it is destined for a different network, it is passed up to the Distribution Layer. Hubs and switches provide the connection to the Distribution Layer devices, usually a router. The Distribution Layer provides a connection point for separate networks and controls the flow of information between the networks. It typically contains more powerful switches than the Access Layer as well as routers for routing between networks. Distribution Layer devices control the type and amount of traffic that flows from the Access Layer to the Core Layer. The Core Layer is a high-speed backbone layer with redundant (backup) connections. It is responsible for transporting large amounts of data between multiple end networks. Core Layer devices typically include very powerful, high-speed switches and routers. The main goal of the Core Layer is to transport data quickly.

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3.7 The Internet Every day millions of people exchange information through the Internet - but what exactly is the Internet? The Internet is a worldwide collection of computer networks, cooperating with each other to exchange information using common standards. Through telephone wires, fiber optic cables, wireless transmissions and satellite links, Internet users can exchange information in a variety of forms. The Internet is a network of networks that connects users in every country in the world. There are currently over one billion Internet users worldwide. Up to now the networks we have discussed have been controlled by one individual or organization. The Internet is a conglomerate of networks and is owned by no one individual or group. There are, however, several major International organizations that help manage the Internet so that everyone uses the same rules.

Any home, business or organization that wants to connect to the Internet must use an Internet Service Provider (ISP). An ISP is a company that provides the connections and support to access the Internet. It can also provide additional services such as Email and web hosting. ISPs are essential to gaining access to the Internet. No one gets on the Internet without a host computer, and no one gets on the Internet without going through an ISP. ISPs range in size from small to very large and differ in terms of the area they service. ISPs may provide limited services to a small geographical area or can have a wide variety of services and support entire countries with millions of customers. ISPs also differ in the types of connection technologies and speeds they offer. Examples of well known ISPs include AOL, EarthLink, and Roadrunner. Individual computers and local networks connect to the ISP at a Point of Presence (POP). A POP is the connection point between the ISP's network and the particular geographical region that the POP is servicing.

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An ISP may have many POPs depending on its size and the area it services. Within an ISP, a network of high-speed routers and switches move data between the various POPs. Multiple links interconnect the POPs to provide alternate routes for the data should one link fail or become overloaded with traffic and congested. ISPs connect to other ISPs in order to send information beyond the boundaries of their own network. The Internet is made up of very high-speed data links that interconnect ISP POPs and ISPs to each other. These interconnections are part of the very large, high capacity network known as the Internet Backbone. Connecting to the ISP at the POP provides users with access to the ISP's services and the Internet. ISPs provide a variety of ways to connect to the Internet, depending on location and desired connection speed. In a major city there are typically more choices for ISPs and more connection options than in a rural area. For example, cable Internet access is only available in certain metropolitan areas where cable TV service is available. Remote areas may only have access via dial-up or satellite. Each Internet access technology uses a network access device, such as a modem, in order to connect to the ISP. It may be built in to your computer or may be provided by the ISP. The simplest arrangement is a modem that provides a direct connection between a computer and the ISP. However, if multiple computers connect through a single ISP connection, you will need additional networking devices. This includes a switch to connect multiple hosts on a local network, and a router to move packets from your local network to the ISP network. A home networking device, such as an integrated router, can provide these functions, as well as wireless capability, in a single package.

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The choice of Internet access technologies depends on availability, cost, access device used, media used and the speed of the connection. Most of the technologies shown are used for both home and small business. Leased lines are typically used for business and large organizations, but can be used to provide high speed connectivity in areas where cable or DSL are not available.

Conclusion The term Local Area Network (LAN) refers to a local network, or a group of interconnected local networks that are under the same administrative control. In the early days of networking, LANs were defined as small networks that existed in a single physical location. While LANs can be a single local network installed in a home or small office, the definition of LAN has evolved to include interconnected local networks consisting of many hundreds of hosts, installed in multiple buildings and locations. The important thing to remember is that all of the local networks within a LAN are under one administrative control. Other common characteristics of LANs are that they typically use Ethernet or wireless protocols, and they support high data rates. The term Intranet is often used to refer to a private LAN that belongs to an organization, and is designed to be accessible only by the organization's members, employees, or others with authorization.

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Unit 4 Operating Systems

4.1 Functions of an operating system System components and peripherals, by themselves, are nothing more than a collection of electronics and mechanical parts. To get these parts to work together to perform a specific task, a special type of computer program, known as an operating system (OS), is required. Suppose that a user wants to write a report and print it out on an attached printer. A word processing application is required to accomplish this task. Information is entered from the keyboard, displayed on the monitor, saved on the disk drive and then finally sent to the printer. In order for the word processing program to accomplish all of this, it must work with the OS, which controls input and output functions. In addition, the entered data is manipulated inside of the computer, stored in RAM and processed by the CPU. This internal manipulation and processing is also controlled by the OS. All computerized devices, such as servers, desktops, laptops or handhelds, require an OS in order to function.

4.2 An operating system makes the computer hardware usable The OS acts like a translator between user applications and the hardware. A user interacts with the computer system through an application, such as a word processor, spreadsheet, computer game or instant messaging program. Application programs are designed for a specific purpose, such as word processing, and know nothing of the underlying electronics. For example, the application is not concerned with how information is entered into the application from the keyboard. The operating system is responsible for the communication between the application and the hardware. When a computer is powered on, it loads the OS, normally from a disk drive, into RAM. The portion of the OS code that interacts directly with the computer hardware is known as the kernel. The portion that interfaces with the applications and user, is known as the shell. The user can interact with the shell using either the command line interface (CLI) or graphical user interface (GUI). When using the CLI, the user interacts directly with the system in a text-based environment by entering commands on the keyboard at a command prompt. The system executes the command, often providing textual output. The GUI interface allows the user to interact with the system in an environment that uses graphical images, multimedia, and text. Actions are performed by interacting with the images on screen. GUI is more user friendly and requires less knowledge than CLI of the command structure to utilize the system. For this reason, many individuals rely on the GUI environments. Most operating systems offer both GUI and CLI. Operating systems have complete control of local hardware resources. They are designed to work with one user at a time. They enable the user to multitask. The operating system keeps track of which resources are used by which application. In order to work with resources that are not directly connected to the computer system, a special piece of software must be added that allows a device to send and receive data from the network. This software, known as a redirector, may either be an integral part of the OS or may need to be installed separately as a network client. When installed, the operating system becomes a network operating system (NOS). A NOS offers complex scheduling and user management software that allow a device to share resources between many users and treat networked resources as though they are directly connected.

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4.3 Types of operating system architecture As computers have progressed and developed so have the types of operating systems. Below is a basic list of the different types of operating systems and a few examples of operating systems that fall into each of the categories. Many computer operating systems will fall into more than one of the below categories. GUI - Short for Graphical User Interface, a GUI Operating System contains graphics and icons and is commonly navigated by using a computer mouse. See our GUI dictionary definition for a complete definition. Below are some examples of GUI Operating Systems. E.g.: System 7.x, Windows 98 and Windows CE. Multi-user - A multi-user operating system allows for multiple users to use the same computer at the same time and/or different times. See our multi-user dictionary definition for a complete definition for a complete definition. Below are some examples of multi-user operating systems. E.g.: Linux, Unix and Windows 2000. Multiprocessing - An operating system capable of supporting and utilizing more than one computer processor. Below are some examples of multiprocessing operating systems. E.g.: Linux, Unix and Windows 2000. Multitasking - An operating system that is capable of allowing multiple software processes to run at the same time. Below are some examples of multitasking operating systems. E.g.: Unix and Windows 2000. Multithreading - Operating systems that allow different parts of a software program to run concurrently. Operating systems that would fall into this category are: Linux, Unix and Windows 2000.

4.4 Major operating systems in use today There are many different operating systems available. The major groupings are listed here with some examples. Microsoft Windows: XP, Vista, 2003 Server UNIX-Based: IBM AIX, Hewlett Packard HPUX, and Sun Solaris BSD - Free BSD Linux-Based (Many varieties) Macintosh OS X Non-Unix Proprietary: IBM OS/400, z/OS While most of these operating systems require the user to purchase and agree to a commercial license, there are several operating systems released under a different type of licensing scheme known as the GNU Public License (GPL). Commercial licenses usually deny end-users the ability to modify the program in any way. Windows XP, Mac OS X and UNIX are all examples of commercial OS software. In contrast, the GPL allows end-users to modify and enhance the code, if they desire, to better suit their environment. Some common operating systems, which are released under the GPL, include Linux and BSD.

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Operating systems require a certain amount of hardware resources. These resources are specified by the manufacturer and include such things as: Amount of RAM Hard disk space required Processor type and speed Video resolution Manufacturers often specify both a minimum and recommended level of hardware resources. System performance at the minimum specified hardware configuration is usually poor and only sufficient to support the OS and no other functionality. The recommended configuration is usually the better option and is more likely to support standard additional applications and resources. To take advantage of all of the features provided by an operating system, additional hardware resources such as sound cards, NICs, modems, microphones, and speakers are generally required. Many of the OS developers test various hardware devices and certify that they are compatible with the operating system. Always confirm that the hardware has been certified to work with the operating system before purchasing and installing it. Choosing an appropriate OS requires many factors to be considered before deciding which one to use in a given environment. The first step in selecting an OS is to ensure that the OS being considered fully supports the requirements of the end user. Does the OS support the applications that will be run? Is the security and functionality sufficient for the needs of the users? Next, conduct research to make sure that sufficient hardware resources are available to support the OS. This includes such basic items as memory, processors, and disk space, as well as peripheral devices such as scanners, sound cards, NICs and removable storage. Another consideration is the level of human resources needed to support the OS. In a business environment, a company may limit support to one or two operating systems and discourage, or even disallow, the installation of any other OS. In the home environment, the ready availability of technical support for an OS may be a determining factor. When considering implementing an OS, it is the total cost of ownership (TCO) of the OS that must be considered in the decision making process. This not only includes the costs of obtaining and installing the OS, but also all costs associated with supporting it. Another factor that may come into play in the decision making process is the availability of the operating system. Some countries and/or businesses have made decisions to support a specific type of OS or may have restrictions barring individuals from obtaining certain types of technologies. In this type of environment, it may not be possible to consider a particular OS regardless of its suitability to the task. The process for selecting an operating system must take all of these factors into account.

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Unit 5 System Applications Technology has played a massive role in solving social ills. Every technology has been created for a purpose. Technological innovation has certainly had a major impact on our modern day society. One of the technologies that have been used worldwide are computers. The computer's era has provided society with a variety of facilities. The computer era began after the industrial revolution, when people started thinking about their problems critically. Society has taken advantage of the features provided by computer technologies. This has made the computer revolution have a greater impact on modern society than the industrial revolution; without computer technology nothing would be possible today. The Industrial Revolution took place during the 18th and 19th centuries. Countries such as Britain and France were involved in machine-based manufacturing. Heavy use of machines was made in order to produce certain products. Innovations were made such as textiles, steam power and iron founding. The Industrial revolution transformed the agricultural environment to an industrial environment. Other innovations included glass making, roads, railways, etc. The Industrial Revolution has thus had enormous impact on society. There was a major shift from agriculture to industry. However, the computer revolution didn't lag behind in many fields. 5.1 Principles of Data Protection Act Personal data should: Only be obtained and processed lawfully and fairly Only be held for specified, lawful and registered purposes Only be used for registered purposes and only disclosed to registered recipients Shall be adequate, relevant and not excessive for their registered purposes Shall be accurate and, where necessary, kept up to date Shall not be kept for longer than is necessary for their stated purpose Individuals shall have the right of access to any data held about them that is capable of being processed automatically Appropriate security measures shall be taken against unauthorised access to, or alteration, disclosure or destruction of, or accidental loss of, personal data Governmental exemptions

Police Secret service MI5 MI6 GCHQ Ministry of Defence

Conditional exemptions

Only for the production of documents (implicitly meant to be word processing) Only for the calculation of payments in accounting, payroll and pensions Only for the distribution of information or articles to the data subjects (e.g. lists of their names and addresses)

Unconditional exemption for management of personal, family or household affairs.

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5.2 Advantages of electronic mail Electronic mail or e-mail, the electronic transmission of messages, letters, and documents. In its broadest sense electronic mail includes point-to-point services such as telegraph and facsimile (fax) systems. It is commonly thought of, however, in terms of computer-based message systems where the electronic text file that is received can be edited, replied to, excerpted, or even pasted into another electronic document that can be used or manipulated by a word processor , desktop publishing system, or other computer program . Advantages of electronic mail include convenience, cost effectiveness, improvement of communication skills, records of all correspondence, student participation, and rapidity of response time by professor

5.3 Computing technology at the workplace For Western European society, debates about the changes computers have brought to the workplace present many conflicting images. Critics insist on an information age marked by job losses and dehumanized work whilst optimistics praise the creation of a post-industrial economy of skilled workers. Advantages of computer and automated technology in the workplace include:

• Employment: technological innovation is associated with further employment prospects and not an overall reduction in the employment numbers. e.g.: the number of employees engaging in research and development, information processing and servicing increases whilst the number of workers engaging production and office work decreases.

• Productivity: gains are obtained in terms of administrative effectiveness. e.g. the use of electronic mail, calendars and filling all make for swifter and more informed decisions.

• Skills: many information workers are IT-competent and poised to negotiate higher wages and receive more status from what has been traditionally regarded as low-status work. e.g.: secretaries can assess the various technologies on the market, be in charge of recommending purchases and they need high degree of computer literacy.

• Social: the office become mobile since processing can be done by distance from portable terminal. e.g.: with developments such as email and teleconferencing home working can be implemented.

5.4 Computing technology in business The use of information technology to monitor a business’s performance can also enable the business to highlight areas where they are not making the most use of their resources. The use of information technologies can also increase the businesses income through advertising in the various available forums. Advances in information technology over the last thirty years have lead to the television for example being more widely used today than thirty years ago (e.g. the introduction of transistor based televisions reduced costs while increasing reliability). An e-commerce approach would provide a world wild market for products advertising and sales. According to analysts from the online consulting firm Strategic eCommerce Solution (SeS), benefits of e-commerce for a new starting up business include the expansion of geographical reach and costumer base as well as an increased visibility. Besides, using Internet as a market platform could help providing “customers valuable information about the business”, “building customer loyalty” and “collecting customer data”. In addition, marketing and advertising expenses can be greatly reduced and sales are likely to grow since the business is continuously open. According to the business analyst Evan I Schwartz, the essential principles for growing the new businesses on the World Wide Web include:

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• Prioritize the quality of the experience over the quantity of people visiting the web site; • Valorize results instead of exposure on the web; • Compensate costumers for disclosing data about themselves; • Be aware that consumers will shop online only of information-rich products; • Consider that self-service provides for the highest level of customer comfort; • "Value-Based Currencies" Enable You to Create Your Own Monetary System; • Trusted Brand Names Matter Even More on the Web • Even the Smallest Business Can Compete in the Web's Global "Marketspace" • Agility Rules - Web Sites Must Continually Adapt to the Market

5.5 Computing technology for News & Media organisations How has the development of information technology influenced the way news and media organisations deliver content? Due to the nature of news and media organisations, the information technologies have particular relevance to them. As noted earlier "Information technology is the technology used to store, manipulate, distribute or create information". News and media organisations are intimately acquainted with each of these elements of information technology. However this report will focus on the distribution and creation of information. Developments such as the Internet and satellite television have created new medium and audiences through which and to which these organisations can disseminate their information. Given the situation thirty years ago the developments that we have seen have enabled the news and media organisations access to more people, they have a wider audience. The audience however now has a wider, global choice. News reports can be received which highlight many different sides of an international conflict for example than was possible before. The relative cheapness of being able to publish information on the Internet for example means that virtually anyone can publish information accessible anywhere in the world.

5.6 Computing technology in educational organisations How has the development of information technology influenced the way educational resources are provided? As information technology has developed over the last thirty years, educational establishments have been influenced in various ways. The most obvious example has been the introduction of information technology related courses. These courses are introduced to try to satisfy the demand that society has for qualified people to develop these information technologies. The developments that have occurred in information technology have also had other influences on educational establishments. As was discussed earlier about news and media organisations, educational organisations also have a goal to distribute information from a source (lecturer, books, on-line resources etc.) to the student. The processes by which educational establishments distribute information have become increasingly diverse, and the effectiveness of the process has also improved. The distribution of information is not the only concern of educational establishments. For example one of the aims of Universities is to create information. This "creation" is done by research. Information technologies have enabled researchers to access a wider source of

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information than previously available through such technologies as the Internet (the original ArpaNet being set up primarily to assist research). The Internet and other related technologies such as electronic mail, also enable collaborative projects to be undertaken between geographically distant groups. Collective Activities and Interests One of the aims of the report is to analyse the influence of "information technology" on the collective activities and interests of a broad range of people. The phrase "collective activities and institutions" has a wide scope. This report divides these into two main areas:

Conclusion It has been shown that the developments in information technology have had an impact on general society perception of information. Without going into specific detail about individual situations, it has been shown that that impact has been fourfold: Storage, manipulation, distribution and creation are four areas dealing with information are the four areas in which societies perception of information has changed. As communication and information technologies have been developed, the various elements that makeup society, whether they be individuals or organisations, expect to be able to use information in ways that were not possible thirty years ago. Society expects to be able to store more than was previously conceived. Society expects to be able to manipulate the information they have for their benefit, to increase understanding and discover new relationships. Society expects to be able to distribute information quickly, efficiently and cheaply. Society now expects the creation of new information to be facilitated by these new technologies.