Computers types
I, Computer: Definition A computer is a machine that can be programmed to manipulate symbols. Its principal characteristics are: It responds to a specific set of instructions in a well-defined manner. It can execute a prerecorded list of instructions (a program). It can quickly store and retrieve large amounts of data. Therefore computers can perform complex and repetitive procedures quickly, precisely and reliably. Modern computers are electronic and digital. The actual machinery (wires, transistors, and circuits) is called hardware; the instructions and data are called software. All general-purpose computers require the following hardware components: Central processing unit (CPU): The heart of the computer, this is the component that actually executes instructions organized in programs ("software") which tell the computer what to do. Memory (fast, expensive, short-term memory): Enables a computer to store, at least temporarily, data, programs, and intermediate results. Mass storage device (slower, cheaper, long-term memory): Allows a computer to permanently retain large amounts of data and programs between jobs. Common mass storage devices include disk drives and tape drives. Input device: Usually a keyboard and mouse, the input device is the conduit through which data and instructions enter a computer. Output device: A display screen, printer, or other device that lets you see what the computer has accomplished. In addition to these components, many others make it possible for the basic components to work together efficiently. For example, every computer requires a bus that transmits data from one part of the computer to another.
II, Computer sizes and power Personal Computers Least powerful
Workstations
Minicomputer s
Mainframes
Supercomputers Most powerful
Computers can be generally classified by size and power as follows, though there is considerable overlap: Personal computer: A small, single-user computer based on a microprocessor. Workstation: A powerful, single-user computer. A workstation is like a personal computer, but it has a more powerful microprocessor and, in general, a higher-quality monitor. Minicomputer: A multi-user computer capable of supporting up to hundreds of users simultaneously. Mainframe: A powerful multi-user computer capable of supporting many hundreds or thousands of users simultaneously. Supercomputer: An extremely fast computer that can perform hundreds of millions of instructions per second.
Supercomputer and Mainframe Supercomputer is a broad term for one of the fastest computers currently available. Supercomputers are very expensive and are employed for specialized applications that require immense amounts of mathematical calculations (number crunching). For example, weather forecasting requires a supercomputer. Other uses of supercomputers scientific simulations, (animated) graphics, fluid dynamic calculations, nuclear energy research, electronic design, and analysis of geological data (e.g. in petrochemical prospecting). Perhaps the best known supercomputer manufacturer is Cray Research.
Mainframe was a term originally referring to the cabinet containing the central processor unit or "main frame" of a room-filling Stone Age batch machine. After the emergence of smaller "minicomputer" designs in the early 1970s, the traditional big iron machines were described as "mainframe computers" and eventually just as mainframes. Nowadays a Mainframe is a very large and expensive computer capable of supporting hundreds, or even thousands, of users simultaneously. The chief difference between a supercomputer and a mainframe is that a supercomputer channels all its power into executing a few programs as fast as possible, whereas a mainframe uses its power to execute many programs concurrently. In some ways, mainframes are more powerful than supercomputers because they support more simultaneous programs. But supercomputers can execute a single program faster than a mainframe. The distinction between small mainframes and minicomputers is vague, depending really on how the manufacturer wants to market its machines.
Minicomputer It is a midsize computer. In the past decade, the distinction between large minicomputers and small mainframes has blurred, however, as has the distinction between small minicomputers and workstations. But in general, a minicomputer is a multiprocessing system capable of supporting from up to 200 users simultaneously. Workstation It is a type of computer used for engineering applications (CAD/CAM), desktop publishing, software development, and other types of applications that require a moderate amount of computing power and relatively high quality graphics capabilities. Workstations generally come with a large, high-resolution graphics screen, at large amount of RAM, built-in network support, and a graphical user interface. Most workstations also have a mass storage device such as a disk drive, but a special type of workstation, called a diskless workstation, comes without a disk drive. The most common operating systems for workstations are UNIX and Windows NT. Like personal computers, most workstations are single-user computers. However, workstations are typically linked together to form a local-area network, although they can also be used as stand-alone systems.
N.B.: In networking, workstation refers to any computer connected to a local-area network. It could be a workstation or a personal computer.
Personal or micro Computers for personal use come in all shapes and sizes, from tiny PDAs (personal digital assistant) to hefty PC (personal computer) towers. More specialized models are announced each week - trip planners, expense account pads, language translators...
Hand-held (HPC) PDA
Tablet PC
Laptop/Notebook
Types of computer according to technology Hybrid computers are computers that comprise features of analog computers and digital computers. The digital component normally serves as the controller and provides logical operations, while the analog component normally serves as a solver of differential equations. An analog computer (spelled analogue in British English) is a form of computer that uses electrical[1], mechanical or hydraulic phenomena to model the problem being solved. More generally an analog computer uses one kind of physical quantity to represent the behavior of another physical system, or mathematical function. Modeling a real physical system in a computer is called simulation. A digital system is one that uses discrete words such as electrical voltages, representing numbers or nonnumeric symbols such as letters or icons, for input, processing, transmission, storage, or display, rather than a continuous range of values (ie, as in an analog system). The distinction between "digital" and "analog" can refer to method of input, data storage and transfer, or the internal working of a device. How computers work Main articles: Central processing unit and Microprocessor A general purpose computer has four main sections: the arithmetic and logic unit (ALU), the control unit, the memory, and the input and output devices (collectively termed I/O). These parts are interconnected by busses, often made of groups of wires. The control unit, ALU, registers, and basic I/O (and often other hardware closely linked with these) are collectively known as a central processing unit (CPU). Early CPUs were composed of many separate components but since the mid-1970s CPUs have typically been constructed on a single integrated circuit called a microprocessor. Control unit Main articles: CPU design and Control unit The control unit (often called a control system or central controller) directs the various components of a computer. It reads and interprets (decodes) instructions in the program one by one. The control system decodes each instruction and turns it into a series of control signals that operate the other parts of the
computer.[11] Control systems in advanced computers may change the order of some instructions so as to improve performance. A key component common to all CPUs is the program counter, a special memory cell (a register) that keeps track of which location in memory the next instruction is to be read from.[12] Diagram showing how a particular MIPS architecture instruction would be decoded by the control system. The control system's function is as follows—note that this is a simplified description and some of these steps may be performed concurrently or in a different order depending on the type of CPU: 1. Read the code for the next instruction from the cell indicated by the program counter. 2. Decode the numerical code for the instruction into a set of commands or signals for each of the other systems. 3. Increment the program counter so it points to the next instruction. 4. Read whatever data the instruction requires from cells in memory (or perhaps from an input device). The location of this required data is typically stored within the instruction code. 5. Provide the necessary data to an ALU or register. 6. If the instruction requires an ALU or specialized hardware to complete, instruct the hardware to perform the requested operation. 7. Write the result from the ALU back to a memory location or to a register or perhaps an output device. 8. Jump back to step (1). Since the program counter is (conceptually) just another set of memory cells, it can be changed by calculations done in the ALU. Adding 100 to the program counter would cause the next instruction to be read from a place 100 locations further down the program. Instructions that modify the program counter are often known as "jumps" and allow for loops (instructions that are repeated by the computer) and often conditional instruction execution (both examples of control flow). It is noticeable that the sequence of operations that the control unit goes through to process an instruction is in itself like a short computer program - and indeed, in some more complex CPU designs, there is another yet smaller computer called a microsequencer that runs a microcode program that causes all of these events to happen. Arithmetic/logic unit (ALU) Main article: Arithmetic logic unit The ALU is capable of performing two classes of operations: arithmetic and logic. The set of arithmetic operations that a particular ALU supports may be limited to adding and subtracting or might include multiplying or dividing, trigonometry functions (sine, cosine, etc) and square roots. Some can only operate on whole numbers (integers) whilst others use floating point to represent real numbers—albeit with limited precision. However, any computer that is capable of performing just the simplest operations can be programmed to break down the more complex operations into simple steps that it can perform. Therefore, any computer can be programmed to perform any arithmetic operation—although it will take more time to do so if its ALU does not directly support the operation. An ALU may also compare numbers and return boolean truth values (true or false) depending on whether one is equal to, greater than or less than the other ("is 64 greater than 65?"). Logic operations involve Boolean logic: AND, OR, XOR and NOT. These can be useful both for creating complicated conditional statements and processing boolean logic. Superscalar computers contain multiple ALUs so that they can process several instructions at the same time. Graphics processors and computers with SIMD and MIMD features often provide ALUs that can perform arithmetic on vectors and matrices. Memory Main article: Computer storage
Magnetic core memory was popular main memory for computers through the 1960s until it was completely replaced by semiconductor memory. A computer's memory can be viewed as a list of cells into which numbers can be placed or read. Each cell has a numbered "address" and can store a single number. The computer can be instructed to "put the number 123 into the cell numbered 1357" or to "add the number that is in cell 1357 to the number that is in cell 2468 and put the answer into cell 1595". The information stored in memory may represent practically anything. Letters, numbers, even computer instructions can be placed into memory with equal ease. Since the CPU does not differentiate between different types of information, it is up to the software to give significance to what the memory sees as nothing but a series of numbers. In almost all modern computers, each memory cell is set up to store binary numbers in groups of eight bits (called a byte). Each byte is able to represent 256 different numbers; either from 0 to 255 or -128 to +127. To store larger numbers, several consecutive bytes may be used (typically, two, four or eight). When negative numbers are required, they are usually stored in two's complement notation. Other arrangements are possible, but are usually not seen outside of specialized applications or historical contexts. A computer can store any kind of information in memory as long as it can be somehow represented in numerical form. Modern computers have billions or even trillions of bytes of memory. The CPU contains a special set of memory cells called registers that can be read and written to much more rapidly than the main memory area. There are typically between two and one hundred registers depending on the type of CPU. Registers are used for the most frequently needed data items to avoid having to access main memory every time data is needed. Since data is constantly being worked on, reducing the need to access main memory (which is often slow compared to the ALU and control units) greatly increases the computer's speed. Computer main memory comes in two principal varieties: random access memory or RAM and read-only memory or ROM. RAM can be read and written to anytime the CPU commands it, but ROM is pre-loaded with data and software that never changes, so the CPU can only read from it. ROM is typically used to store the computer's initial start-up instructions. In general, the contents of RAM is erased when the power to the computer is turned off while ROM retains its data indefinitely. In a PC, the ROM contains a specialized program called the BIOS that orchestrates loading the computer's operating system from the hard disk drive into RAM whenever the computer is turned on or reset. In embedded computers, which frequently do not have disk drives, all of the software required to perform the task may be stored in ROM. Software that is stored in ROM is often called firmware because it is notionally more like hardware than software. Flash memory blurs the distinction between ROM and RAM by retaining data when turned off but being rewritable like RAM. However, flash memory is typically much slower than conventional ROM and RAM so its use is restricted to applications where high speeds are not required.[13] In more sophisticated computers there may be one or more RAM cache memories which are slower than registers but faster than main memory. Generally computers with this sort of cache are designed to move frequently needed data into the cache automatically, often without the need for any intervention on the programmer's part. Input/output (I/O) Main article: Input/output
Hard disks are common I/O devices used with computers. I/O is the means by which a computer receives information from the outside world and sends results back. Devices that provide input or output to the computer are called peripherals. On a typical personal computer, peripherals include input devices like the keyboard and mouse, and output devices such as the display and printer. Hard disk drives, floppy disk drives and optical disc drives serve as both input and output devices. Computer networking is another form of I/O. Often, I/O devices are complex computers in their own right with their own CPU and memory. A graphics processing unit might contain fifty or more tiny computers that perform the calculations necessary to display 3D graphics. Modern desktop computers contain many smaller computers that assist the main CPU in performing I/O. Registers are fast memory, almost always connected to circuitry that allows various arithmetic, logical, control, and other manipulations, as well as possibly setting internal flags. Most early computers had only one data register that could be used for arithmetic and logic instructions. Data registers are used for temporary scratch storage of data, as well as for data manipulations (arithmetic, logic, etc.). In some processors, all data registers act in the same manner, while in other processors different operations are performed are specific registers. Address registers store the addresses of specific memory locations. Often many integer and logic operations can be performed on address registers directly (to allow for computation of addresses). Personal computer: It can be defined as a small, relatively inexpensive computer designed for an individual user. In price, personal computers range anywhere from a few hundred pounds to over five thousand pounds. All are based on the microprocessor technology that enables manufacturers to put an entire CPU on one chip. Businesses use personal computers for word processing, accounting, desktop publishing, and for running spreadsheet and database management applications. At home, the most popular use for personal computers is for playing games and recently for surfing the Internet. Personal computers first appeared in the late 1970s. One of the first and most popular personal computers was the Apple II, introduced in 1977 by Apple Computer. During the late 1970s and early 1980s, new models and competing operating systems seemed to appear daily. Then, in 1981, IBM entered the fray with its first personal computer, known as the IBM PC. The IBM PC quickly became the personal computer of choice, and most other personal computer manufacturers fell by the wayside. P.C. is short for personal computer or IBM PC. One of the few companies to survive IBM' s onslaught was Apple Computer, which remains a major player in the personal computer marketplace. Other companies adjusted to IBM' s dominance by building IBM clones, computers that were internally almost the same as the IBM PC, but that cost less. Because IBM clones used the same microprocessors as IBM PCs, they were capable of running the same software. Over the years, IBM has lost much of its influence in directing the evolution of PCs. Therefore after the release of the first PC by IBM the term PC increasingly came to mean IBM or IBMcompatible personal computers, to the exclusion of other types of personal computers, such as Macintoshes. In recent years, the term PC has become more and more difficult to pin down. In general, though, it applies to any personal computer based on an Intel microprocessor, or on an Intel-compatible microprocessor. For nearly every other component, including the operating system, there are several options, all of which fall under the rubric of PC
Today, the world of personal computers is basically divided between Apple Macintoshes and PCs. The principal characteristics of personal computers are that they are single-user systems and are based on microprocessors. However, although personal computers are designed as single-user systems, it is common to link them together to form a network. In terms of power, there is great variety. At the high end, the distinction between personal computers and workstations has faded. High-end models of the Macintosh and PC offer the same computing power and graphics capability as low-end workstations by Sun Microsystems, Hewlett-Packard, and DEC.
III, Personal Computer Types Actual personal computers can be generally classified by size and chassis / case. The chassis or case is the metal frame that serves as the structural support for electronic components. Every computer system requires at least one chassis to house the circuit boards and wiring. The chassis also contains slots for expansion boards. If you want to insert more boards than there are slots, you will need an expansion chassis, which provides additional slots. There are two basic flavors of chassis designs–desktop models and tower models– but there are many variations on these two basic types. Then come the portable computers that are computers small enough to carry. Portable computers include notebook and subnotebook computers, hand-held computers, palmtops, and PDAs. Tower model The term refers to a computer in which the power supply, motherboard, and mass storage devices are stacked on top of each other in a cabinet. This is in contrast to desktop models, in which these components are housed in a more compact box. The main advantage of tower models is that there are fewer space constraints, which makes installation of additional storage devices easier. Desktop model A computer designed to fit comfortably on top of a desk, typically with the monitor sitting on top of the computer. Desktop model computers are broad and low, whereas tower model computers are narrow and tall. Because of their shape, desktop model computers are generally limited to three internal mass storage devices. Desktop models designed to be very small are sometimes referred to as slimline models. Notebook computer An extremely lightweight personal computer. Notebook computers typically weigh less than 6 pounds and are small enough to fit easily in a briefcase. Aside from size, the principal difference between a notebook computer and a personal computer is the display screen. Notebook computers use a variety of techniques, known as flat-panel technologies, to produce a lightweight and non-bulky display screen. The quality of notebook display screens varies considerably. In terms of computing power, modern notebook computers are nearly equivalent to personal computers. They have the same CPUs, memory capacity, and disk drives. However, all this power in a small package is expensive. Notebook computers cost about twice as much as equivalent regular-sized computers. Notebook computers come with battery packs that enable you to run them without plugging them in. However, the batteries need to be recharged every few hours. Laptop computer A small, portable computer -- small enough that it can sit on your lap. Nowadays, laptop computers are more frequently called notebook computers. The Five Generations of Computers The history of computer development is often referred to in reference to the different generations of computing devices. Each generation of computer is characterized by a major technological development that fundamentally changed the way computers operate, resulting in increasingly smaller, cheaper, more powerful and more efficient and reliable devices. Read about each generation and the developments that led to the current devices that we use today.
First Generation 1940-1956: Vacuum Tubes The first computers used vacuum tubes for circuitry and magnetic drums for memory, and were often enormous, taking up entire rooms. They were very expensive to operate and in addition to using a great deal of electricity, generated a lot of heat, which was often the cause of malfunctions. First generation computers relied on machine language to perform operations, and they could only solve one problem at a time. Input was based on punched cards and paper tape, and output was displayed on printouts. The UNIVAC and ENIAC computers are examples of first-generation computing devices. The UNIVAC was the first commercial computer delivered to a business client, the U.S. Census Bureau in 1951. Second Generation 1956-1963: Transistors Transistors replaced vacuum tubes and ushered in the second generation of computers. The transistor was invented in 1947 but did not see widespread use in computers until the late 50s. The transistor was far superior to the vacuum tube, allowing computers to become smaller, faster, cheaper, more energy-efficient and more reliable than their first-generation predecessors. Though the transistor still generated a great deal of heat that subjected the computer to damage, it was a vast improvement over the vacuum tube. Secondgeneration computers still relied on punched cards for input and printouts for output. Second-generation computers moved from cryptic binary machine language to symbolic, or assembly, languages, which allowed programmers to specify instructions in words. High-level programming languages were also being developed at this time, such as early versions of COBOL and FORTRAN. These were also the first computers that stored their instructions in their memory, which moved from a magnetic drum to magnetic core technology. The first computers of this generation were developed for the atomic energy industry. Third Generation 1964-1971: Integrated Circuits The development of the integrated circuit was the hallmark of the third generation of computers. Transistors were miniaturized and placed on silicon chips, called semiconductors, which drastically increased the speed and efficiency of computers. Instead of punched cards and printouts, users interacted with third generation computers through keyboards and monitors and interfaced with an operating system, which allowed the device to run many different applications at one time with a central program that monitored the memory. Computers for the first time became accessible to a mass audience because they were smaller and cheaper than their predecessors. Fourth Generation 1971-Present: Microprocessors The microprocessor brought the fourth generation of computers, as thousands of integrated circuits were built onto a single silicon chip. What in the first generation filled an entire room could now fit in the palm of the hand. The Intel 4004 chip, developed in 1971, located all the components of the computer - from the central processing unit and memory to input/output controls - on a single chip. In 1981 IBM introduced its first computer for the home user, and in 1984 Apple introduced the Macintosh. Microprocessors also moved out of the realm of desktop computers and into many areas of life as more and more everyday products began to use microprocessors. As these small computers became more powerful, they could be linked together to form networks, which eventually led to the development of the Internet. Fourth generation computers also saw the development of GUIs, the mouse and handheld devices. Fifth Generation Present and Beyond: Artificial Intelligence Fifth generation computing devices, based on artificial intelligence, are still in development, though there are some applications, such as voice recognition, that are being used today. The use of parallel processing and superconductors is helping to make artificial intelligence a reality. Quantum computation and molecular and nanotechnology will radically change the face of computers in years to come. The goal of fifth-generation computing is to develop devices that respond to natural language input and are capable of learning and selforganization. An Introduction to DOS
Introduction
With the advent of graphical user interfaces (GUIs) like Microsoft Windows, the idea of actually using a command line prompt and typing in commands at the keyboard strikes many people as quite old-fashioned. Why go to the trouble to memorise commands and their syntax when you can just select them from menus, fill in dialog boxes with parameters, or drag files onto application icons? In fact, there are many things that are easier to do from a command prompt than in a GUI; how easy is it in a GUI to rename all files with a ".cxx" extension so that they have a ".cpp" extension instead? Anyway, let' s begin at the beginning. DOS stands for "Disk Operating System", and is a generic name for the basic IBM PC operating system. Several variants of DOS are available, including Microsoft' s version of DOS (MS-DOS), IBM' s version (PC-DOS), and several others. There' s even a free version of DOS called OpenDOS included on this CD. There are actually several levels to DOS. At the lowest level is the BIOS (Basic Input/Output System) which is responsible for managing devices like the keyboards and disk drives at the simplest possible level (e.g. the BIOS lets you say things like "get me sector 5 of track 3 from disk drive 1"). This is done partly by software in ROM (read-only memory) and partly by BIOS extensions which are loaded when the system first starts up (with MS-DOS, these are in a file called IO.SYS; on PC-DOS they' re in IBMBIO.COM). The second layer provides a set of higher level services implemented using the low-level BIOS services; you can now refer to disk drive 1 as "drive A:" and instead of referring to specific sectors or tracks you can refer to files by name (e.g. LETTER.TXT). You can also treat devices as if they were named files, so that for example you can use the name PRN: to refer to the printer. In other words, this level provides a file system (you can refer to files by name and let DOS worry about translating the name into a physical location) as well as some device independence (you don' t have to differentiate storing text in a file from sending it to the printer). This layer of the system is implemented by another file which is loaded when the system first starts up (the file is called MSDOS.SYS on MS-DOS systems, and IBMDOS.COM on PC-DOS systems). The third layer is the command interpreter (or shell), which is what most people think of as DOS (it' s not, but it' s what you interact with when you use the computer, so it' s an understandable confusion). This is contained in another file called COMMAND.COM, which is just an ordinary program that is started automatically (and you can replace it with something better, for example 4DOS, a shareware shell included on this CD). The shell' s job is to display a command prompt on the screen to let you know you' re supposed to type something, then to read a line of text that you type, and to interpret it as a command, which usually involves loading another program into memory and running it. When it' s finished doing this, it displays another prompt and waits for you to type in another command. The file system Before we go any further, it would be a good idea to look at the DOS file system. The file system lets us store information in named files. You can call a file anything you like which might help you remember what it contains as long as you follow certain basic rules: 1. File names can be up to 8 characters long. You can use letters and digits but only a few punctuation marks (! $ % # ~ @ - ( ) _ { }). You can' t exceed 8 characters or use spaces or characters like * or ? or +. Names are case-insensitive, i.e. it doesn' t matter whether you use capitals or lowercase letters; "A" and "a" are treated as the same thing. 2. File names can also have an extension of up to three characters which describes the type of file. There are some standard extensions, but you don' t have to use them. Examples include COM and EXE for executable programs, TXT for text files, BAK for backup copies of files, or CPP for C++ program files. The extension is separated by a dot from the rest of the filename. For example, a file called FILENAME.EXT has an 8-character name (FILENAME) followed by a threecharacter extension (.EXT). You could also refer to it as filename.txt since case doesn' t matter, but I' m going to use names in capitals for emphasis throughout this document. Files are stored in directories; a directory is actually just a special type of file which holds a list of the files within it. Since a directory is a file, you can have directories within directories. Directory names also follow the same naming rules as other files, but although they can have an extension they aren' t normally given one (just an 8-character name). The system keeps track of your current directory, and if you just refer to a file using a name like FILENAME.EXT it' s assumed you mean a file of that name in the current directory. You can specify a pathname to identify a file which includes the directory name as well; the directory is separated from the
rest of the name by a backslash ("\"). For example, a file called LETTER1.TXT in a directory called LETTERS can be referred to as LETTERS\LETTER1.TXT (assuming that the current directory contains the LETTERS directory as a subdirectory). If LETTERS contains a subdirectory called PERSONAL, which in turn contains a file called DEARJOHN.TXT, you would refer to this file as LETTERS\PERSONAL\DEARJOHN.TXT (i.e. look in the LETTERS directory for PERSONAL\DEARJOHN.TXT, which in turn involves looking in the PERSONAL subdirectory for the file DEARJOHN.TXT). Every disk has a root directory which is the main directory that everything else is part of. The root directory is called "\", so you can use absolute pathnames which don' t depend on what your current directory is. A name like \LETTERS\LETTER1.TXT always refers to the same file regardless of which directory you happen to be working in at the time; the "\" at the beginning means "start looking in the root directory", so \LETTERS\LETTER1.TXT means "look in the root directory of the disk for a subdirectory called LETTERS, then look in this subdirectory for a file called LETTER1.TXT". Leaving out the "\" at the beginning makes this a relative pathname whose meaning is relative to the current directory at the time. If you want to refer to a file on another disk, you can put a letter identifying the disk at the beginning of the name separated from the rest of the name by a colon (":"). For example, A:\LETTER1.TXT refers to a file called LETTER1.TXT in the root directory of drive A. DOS keeps track of the current directory on each disk separately, so a relative pathname like A:LETTER1.TXT refers to a file called LETTER1.TXT in the currently-selected directory on drive A. For convenience, all directories (except root directories) contain two special names: "." refers to the directory itself, and ".." refers to the parent directory (i.e. the directory that contains this one). For example, if the current directory is \LETTERS\PERSONAL, the name ".." refers to the directory \LETTERS, "..\BUSINESS" refers to \LETTERS\BUSINESS, and "..\.." refers to the root directory "\". Simple commands If you start up a computer running DOS (or select the "MS-DOS Prompt" icon in Windows), after a bit of initialisation you will end up with a prompt to let you know that the command interpreter is waiting for you to type in a command. The prompt might look like this: C> This prompt consists of the name of the current disk that you' re using (A for the main floppy disk drive, B for the secondary floppy disk, C for the primary hard disk drive, and so on) followed by ">". You can get a list of the files in the current directory on that disk by typing the command DIR (short for "directory"): C>DIR (The text in bold above indicates what you type in.) When you press the ENTER key, a list of files will be displayed on the screen, together with their sizes and the dates when they were last modified. If you want to see a list of the files on disk in the main floppy drive (drive A), you can do it by first selecting A as the current drive: C>A: Just type the letter of the drive you want to select followed by a colon, and then press ENTER. The prompt will then appear like this: A> and you can then type DIR as before: A>DIR Alternatively, you can specify the directory you want to list as a parameter separated by one or more spaces from the command name: C>DIR A: which lists the contents of the current directory on drive A, or C>DIR A:\ which lists the root directory of drive A, or C>DIR .. which lists the directory above the one you' re currently in. Note that for convenience you can arrange matters so the prompt tells you the current directory as well as the currently-selected disk drive, so that it might appear like this: C:\WINDOWS>
meaning that C:\WINDOWS is the current directory. Now imagine that you type "DIR ..": C:\WINDOWS>DIR .. You should be able to see that this will list the contents of C:\, i.e. the root directory on drive C. (From now on I won' t show the prompt in my examples, only what you need to type in.) The trouble with a command like DIR is that the list of files scrolls off the screen if there are more than a few files in the directory. Most commands have a set of options that you can use to modify their behaviour, which are specified after the command name and separated from it by a slash "/"; the DIR command has a /P (pause) option which makes it pause after each full screen of output, and waits for you to press a key before displaying the next screenful. Most commands support a "help" option called "/?"; typing DIR/? will give you a brief list of the options that DIR recognises, for instance. Some common commands Here' s a list of some common commands and what they do: X: -- select X: as the current drive DIR -- list the current directory DIR directory -- list the specified directory CLS -- clear the screen CD directory -- change current directory to directory TYPE file -- display specified file on the screen COPY file1 file2 -- copy the first file to the second COPY file directory -- copy the file to the specified directory DEL file -- delete the specified file EDIT file -- edit the specified file REN file newname -- rename file to newname FORMAT drive -- format the disk in the specified drive MD directory -- make a new directory called directory RD directory -- remove the specified directory (which must be empty) In the list above, file (or file1 or file2) is the pathname of a file, directory is the pathname of a directory and drive is the name of a disk drive (e.g. A: or C:). In the case of the REN (rename) command, newname is the new file name (not a path name because this just renames the file, it can' t move the file to a new directory). Wildcards In some situations it' s useful to be able to specify sets of files to be operated on by a single command. For example, you might want to delete all files with the extension ".TXT". You can do this like so: DEL *.TXT The star "*" matches any name at all. You can read it as "anything .TXT", although if I had to read this command out loud I' d probably say "del star-dot-text". Anyway, what it means is that any file whose name matches the pattern *.TXT (any file with a .TXT extension) will be deleted. You can do more sophisticated things: COPY NOTE*.TXT NOTES will copy any file whose name begins with NOTE (i.e. NOTE followed by anything) and has a .TXT extension to the NOTES subdirectory. The star is referred to as a wildcard by analogy with the role of a Joker in some card games, where the Joker can be used as any other card. The command DEL *.* will delete all the files in the current directory (all files with any name and any extension). Since this is obviously risky, you' ll be prompted with a message that says "Are you sure (Y/N)?". If you are sure, type Y for "yes"; if not type N for "no". A non-obvious use of this technique is allowed by the REN (rename) command: REN *.TXT *.LST will rename all files with a .TXT extension to files with the same name but a .LST extension instead. A limitation of this technique is that you can only use a star as a wildcard at the end of the name or extension part of a filename; you might think that you could delete all files whose name ends with X like this: DEL *X.*
Unfortunately this has the same effect as DEL *.* since the star matches everything up to the end of the name, and the X after that is just ignored. You can use a question mark to match a single character: DEL ?X.* This will match any file whose name consists of any single character followed by an X with any extension. To delete any files whose name ends with X would actually require a whole sequence of commands: DEL X.* DEL ?X.* DEL ??X.* DEL ???X.* DEL ????X.* DEL ?????X.* DEL ??????X.* DEL ???????X.* This deletes all files whose name is X, then all files whose name is any one character followed by X, then all files whose name is any two characters followed by X, and so on up to any seven characters followed by X. Redirecting input and output One nice thing about using a command prompt rather than a GUI is that it' s much simpler to capture the output of a program in a file, or take the input for a program from somewhere other than the keyboard. All programs have a number of standard input/output channels, of which the most important are the standard input (normally the keyboard) and the standard output (normally the screen). There is also a standard error output which is also associated with the screen (and is intended for displaying error messages). A few programs write their output directly to the screen (or elsewhere) but in many cases they will write results to the standard output. If you don' t do anything unusual, this will be the screen. However, you can associate the standard output with a file instead of the screen so that when the program writes anything to the standard input, it' ll end in the file you' ve specified rather than being displayed on the screen. All you have to do is specify "> filename" on the command line and the standard output will be routed to the file filename instead of the screen. For example, DIR displays a directory listing on the screen, but DIR >DIRLIST.TXT puts the directory listing in a file called DIRLIST.TXT, and doesn' t display anything on the screen. (Read the ">" out loud as "to"; "dir to dirlist-dot-text".) If the file already exists, doing this will overwrite the existing contents. If you don' t want this to happen, use ">>" instead of ">". This will append the program' s output to the existing contents of the file rather than destroying it: DIR >>DIRLIST.TXT This has the same effect as before, except that if the file DIRLIST.TXT already exists, the program' s output will be tacked on to the end of it rather than replacing it. The standard input can be redirected using "<": MYPROG
MORE which displays what it reads from the standard input onto the screen a screenful at a time, and then waits for you to press a key before continuing. So you could redirect the output of DIR into a temporary file and then use MORE to display it a screen at a time, like this: DIR >DIRLIST.TMP MORE PRN: The first of these copies the file FILE to the printer; the second one displays it to the standard output but redirects the standard output to the printer. The null device NUL: is for situations where you want an empty input file or you want program output to be ignored. If you read from it, it looks like a file containing zero bytes of data; if you write to it, everything you write is cheerfully ignored and is just discarded. This is useful for throwing away unwanted output: COPY RESULT*.TXT PRN: >NUL: ECHO Y | DEL *.* >NUL: The first example copies all files whose names begin with RESULT to the printer, and throws away all the messages that the COPY command normally displays on the screen ("1 file copied" or whatever). The second is a completely silent way of deleting all files in the current directory; the "Are you sure (Y/N)?" prompt is thrown away, and the answer to this unasked question is then read from the output produced by the ECHO command. Batch files
Sometimes you will want to execute the same sequence of commands over and over again. This is easy to do; just create a file containing those commands (one command per line) and give it a name with a .BAT extension (e.g. DO_IT.BAT). A file like this is known as a batch file. Once you' ve done this you can just type DO_IT at the command prompt, and (assuming that DO_IT.BAT is the first file that the command interpreter finds when it' s looking for the command, as described above) the command interpreter will read this file and execute each line as a separate command. For example, assume that DO_IT.BAT contained the following lines: CLS ECHO HELLO, WORLD Typing DO_IT would clear the screen and the display the message "HELLO, WORLD". By default, each line of the file is displayed on the screen ("echoed") before it' s executed. This is useful for debugging but it ruins the effect in the batch file above. You can disable the echoing by prefixing the command with "@": @CLS @ECHO HELLO, WORLD Another method is to disable echoing at the start of the batch file with the command "ECHO OFF" (which needs to be prefixed by "@", otherwise it will be echoed to the screen): @ECHO OFF CLS ECHO HELLO, WORLD Batch files can be quite sophisticated. For example, you can get at the command line parameters by referring to them as %1, %2, %3 and so on up to %9. The following batch file displays the second command line parameter followed by the first one: @ECHO OFF ECHO %2 %1 If this is saved as DO_IT.BAT, typing DO_IT HELLO WORLD will display the message WORLD HELLO There are also commands which give you the basics of a simple programming language: IF statements to choose between alternative courses of action, FOR loops to repeat a sequence of actions, tests to determine if a command executed successfully, and so on. One nasty trap to watch out for is when you try to execute a batch file from inside another batch file. Imagine you' ve got this in DO_IT.BAT: DO_OTHER.BAT ECHO Finished! The ECHO command on the second line won' t be executed; if you refer to one batch file inside the other, the effect is a GOTO rather than a CALL (i.e. you go to the inner batch file but you don' t remember where you came from, so at the end of the inner batch file you end up back at the command prompt). The solution is to use the internal CALL command instead: CALL DO_OTHER.BAT ECHO Finished! This will work as expected; it' ll execute DO_OTHER.BAT and then display the message "Finished!". The AUTOEXEC.BAT file The file C:\AUTOEXEC.BAT is a special one, because when the system is first started the command interpreter will automatically execute this file as a batch file (hence the name). This is where to put all your system startup commands; for example, setting the path, loading your keyboard and mouse driver, running a virus checker, or whatever. Whenever you install any new DOS software, you' ll need to check whether you need to update this file (and if you do, you' ll either need to restart the system or type C:\AUTOEXEC.BAT to re-execute it before any of the changes will take effect). You' ll often need to update your path to include the directory where your new software was installed. Some software will do this automatically, and amazingly often it will get it wrong or do it in such a way that something else no longer works. What I always do is to have a separate file (C:\BOOT\AUTOEXEC.BAT) where all the "real" work is done, and then I have a one-line C:\AUTOEXEC.BAT file which says this:
@C:\BOOT\AUTOEXEC.BAT Remember, C:\BOOT\AUTOEXEC.BAT will never go back to C:\AUTOEXEC.BAT when it' s finished (I deliberately didn' t use a CALL command). This means that anything that an auto-installation utility adds to the end of C:\AUTOEXEC.BAT will be ignored, and I can inspect it and decide whether it' s made a sensible change or not. If so, I can then update my real AUTOEXEC.BAT file to include the changes. Data processing Data processing is any computer process that converts data into information or knowledge 1. Modes of data processing a) Batch processing: it requires the data to be initially grouped before it is processed serially, and the result obtained periodically. batch processing System for processing data with little or no operator intervention. Batches of data are prepared in advance to be processed during regular ‘runs’ b) On line – processing: the results of data processing transaction is available immediately. The use of computers to undertake tasks where transactions are initiated and data entered from terminals located in the users'offices. Common examples are the booking of airline tickets, holidays, hotels, and car hire, and transactions in building societies and some banks. c) Real time processing: a data processing system in which the time interval required to process and respond to inputs is so small that the response itself is useful for controlling the physical activity of a process. d) Distributed data processing : it involves a computer system linked by a communication network where processing is performed by separate computers. Computers are dispersed throughout organization. Allows greater flexibility in structure 2. Basic function of data processing a) Origination: it involves the nature, type, and origin of the source documents must be determined. b) Data capture: data must be recorded in some form and important data should be collected and processed upon the organization and the system. c) Sorting: arranged in the sequential manner d) Merging: allows multiple files to be put together in sequence, provided the files are already. e) Calculating: The arithmetic manipulation of data to create meaningful results. f) Summarizing: reducing masses of data to a more concise and usable form. g) Output: the delivery or communication of the information ar results must be carried out by reporting, issuance of document, retrieval, analysis, communicating and reproducing. h) Storage: the results of the processing of data must be retained for future use of references. 3. Data Hierarchy a) Bit: value 0 and 1 b) Byte: basic unit of information c) Field: one or more bytes that contain data attributed of an entity. d) Record: collection of file. e) Database: contains list of files.
Software Development Life Cycle Once upon a time, software development consisted of a programmer writing code to solve a problem or automate a procedure. Nowadays, systems are so big and complex that teams of architects, analysts, programmers, testers and users must work together to create the millions of lines of customwritten code that drive our enterprises. To manage this, a number of system development life cycle (SDLC) models have been created: waterfall, fountain, spiral, build and fix, rapid prototyping, incremental, and synchronize and stabilize.
• • • • • • •
The oldest of these, and the best known, is the waterfall: a sequence of stages in which the output of each stage becomes the input for the next. These stages can be characterized and divided up in different ways, including the following: Project planning, feasibility study: Establishes a high-level view of the intended project and determines its goals. Systems analysis, requirements definition: Refines project goals into defined functions and operation of the intended application. Analyzes end-user information needs. Systems design: Describes desired features and operations in detail, including screen layouts, business rules, process diagrams, pseudocode and other documentation. Implementation: The real code is written here. Integration and testing: Brings all the pieces together into a special testing environment, then checks for errors, bugs and interoperability. Acceptance, installation, deployment: The final stage of initial development, where the software is put into production and runs actual business. Maintenance: What happens during the rest of the software' s life: changes, correction, additions, moves to a different computing platform and more. This, the least glamorous and perhaps most important step of all, goes on seemingly forever.
Introduction to Flowcharts and Algorithms A flowchart is a graphical representation of an algorithm. These flowcharts play a vital role in the programming of a problem and are quite helpful in understanding the logic of complicated and lengthy problems. Once the flowchart is drawn, it becomes easy to write the program in any high level language. Often we see how flowcharts are helpful in explaining the program to others. Hence, it is correct to say that a flowchart is a must for the better documentation of a complex program. Flowcharts are usually drawn using some standard symbols; however, Start or end of the program Computational steps or processing function of a program Input or output operation Decision making and branching Connector or joining of two parts of program The following are some guidelines in flowcharting: a. In drawing a proper flowchart, all necessary requirements should be listed out in logical order. b. The flowchart should be clear, neat and easy to follow. There should not be any room for ambiguity in understanding the flowchart. c. The usual direction of the flow of a procedure or system is from left to right or top to bottom. d. Only one flow line should come out from a process symbol.
or
e. Only one flow line should enter a decision symbol, but two or three flow lines, one for each possible answer, should leave the decision symbol.
f. Only one flow line is used in conjunction with terminal symbol.
h. If the flowchart becomes complex, it is better to use connector symbols to reduce the number of flow lines. Avoid the intersection of flow lines if you want to make it more effective and better way of communication. i. Ensure that the flowchart has a logical start and finish. j. It is useful to test the validity of the flowchart by passing through it with a simple test data. PART II: Example of a flowchart: Problem 1: Write an algorithm and draw the flowchart for finding the average of two numbers Algorithm: Input: two numbers x and y START Output: the average of x and y Steps: 1. input x Input x 2. input y 3. sum = x + y 4. average = sum /2 Input y 5. output average Sum = x + y
Average = sum/2
Output Average
END
Programming concept. When I first time learn program, I thought that programming is equal to coding! I only need to sit in front of the computer and start typing programming code. Is that what you are thinking? Actually in a programming project, we spend most of our time to design the program, instead of coding. You may wonder why? The answer is simple, because NO ONE can just sit in front of the computer and write a program of any complexity!! However, I will talk about "Software Engineering" later when you get start. In this section, I will just give you some basic idea of programming methodology. Top-Down programming design method
During the 1970s and 80s, the primary software engineering methodology is called structured programming approach. When software engineer design a program, they will try to break down the problem into smaller pieces and work on each smaller piece separately, continues this process until every smaller pieces of problem are easy to work with without further decomposition. We call this designing method ' Top-Down' programming method. This approach has its value and popular approach at that time. However, it has its own weakness. For example, when one piece of code is develop in previous project, that piece of source code is depend on the upper level of code and unique to each project due to the top-down approach. So every time you need to redevelop some code you already develop. In other other, the cost of producing a highquality program will become very high. Bottom-up programming design method The bottom-up design approach, you can tell by the name, is start the design process at the bottom, it is possible when you already has some idea of the problem, or you already have source code which can fit into the project This Bottom-up approach usually use together with Top-down approach to solve problems. Object-oriented programming (OOPs) You may heard a lot about Object-oriented programming in the past several years. Everybody is talking about OOPs. You may feel the power of OOPs, even you don' t know any programming. Anyway, let' s talk about OOPs. Data communication • • • • • • • • • • • • • • • • • • • • •
Data communication is the transfer of data from one device to another via some form of transmission medium. A data communications system must transmit data to the correct destination in an accurate and timely manner. The five components that make up a data communications system are the message, sender, receiver, medium, and protocol. Text, numbers, images, audio, and video are different forms of information. Data flow between two devices can occur in one of three ways: simplex, half-duplex, or full-duplex. A network is a set of communication devices connected by media links. In a point-to-point connection, two and only two devices are connected by a dedicated link. In a multipoint connection, three or more devices share a link. Topology refers to the physical or logical arrangement of a network. Devices may be arranged in a mesh, star, bus, or ring topology. A network can be categorized as a local area network (LAN), a metropolitan-area network (MAN), or a wide area network (WAN). A LAN is a data communication system within a building, plant, or campus, or between nearby buildings. A MAN is a data communication system covering an area the size of a town or city. A WAN is a data communication system spanning states, countries, or the whole world. An internet is a network of networks. The Internet is a collection of many separate networks. TCP/IP is the protocol suite for the Internet. There are local, regional, national, and international Internet service providers (ISPs). A protocol is a set of rules that governs data communication; the key elements of a protocol are syntax, semantics, and timing. Standards are necessary to ensure that products from different manufacturers can work together as expected. The ISO, ITU-T, ANSI, IEEE, and EIA are some of the organizations involved in standards creation. Forums are special-interest groups that quickly evaluate and standardize new technologies. A Request for Comment (RFC) is an idea or concept that is a precursor to an Internet standard.
Network Topology
Designing a Network Topology The term topology, or more specifically, network topology, refers to the arrangement or physical layout of computers, cables, and other components on the network. "Topology" is the standard term that most network professionals use when they refer to the network' s basic design. In addition to the term "topology," you will find several other terms that are used to define a network' s design: • Physical layout • Design • Diagram • Map A network' s topology affects its capabilities. The choice of one topology over another will have an impact on the: • Type of equipment the network needs. • Capabilities of the equipment. • Growth of the network. • Way the network is managed. Developing a sense of how to use the different topologies is a key to understanding the capabilities of the different types of networks. Before computers can share resources or perform other communication tasks they must be connected. Most networks use cable to connect one computer to another. Standard Topologies All network designs stem from four basic topologies: • Bus • Star • Ring • Mesh A bus topology consists of devices connected to a common, shared cable. Connecting computers to cable segments that branch out from a single point, or hub, is referred to as setting up a star topology. Connecting computers to a cable that forms a loop is referred to as setting up a ring topology. A mesh topology connects all computers in a network to each other with separate cables. These four topologies can be combined in a variety of more complex hybrid topologies. Bus The bus topology is often referred to as a "linear bus" because the computers are connected in a straight line. This is the simplest and most common method of networking computers. Figure 1.15 shows a typical bus topology. It consists of a single cable called a trunk (also called a backbone or segment) that connects all of the computers in the network in a single line.
Figure 1.15 Bus topology network
Run the c01dem01 video located in the Demos folder on the compact disc accompanying this book to view a demonstration of a bus-topology connection.
Communication on the Bus Computers on a bus topology network communicate by addressing data to a particular computer and sending out that data on the cable as electronic signals. To understand how computers communicate on a bus, you need to be familiar with three concepts: • Sending the signal • Signal bounce • Terminator Sending the Signal Network data in the form of electronic signals is sent to all the computers on the network. Only the computer whose address matches the address encoded in the original signal accepts the information. All other computers reject the data. Figure 1.16 shows a message being sent from 0020af151d8b to 02608c133456. Only one computer at a time can send messages.
Run the c01dem02 video located in the Demos folder on the compact disc accompanying this book to view a demonstration of how data is transferred in a bus topology.
Figure 1.16 Data is sent to all computers, but only the destination computer accepts it Because only one computer at a time can send data on a bus network, the number of computers attached to the bus will affect network performance. The more computers there are on a bus, the more computers will be waiting to put data on the bus and, consequently, the slower the network will be. There is no standard way to measure the impact of a given number of computers on the speed of any given network. The effect on performance is not related solely to the number of computers. The following is a list of factors that— in addition to the number of networked computers— will affect the performance of a network: • Hardware capabilities of computers on the network • Total number of queued commands waiting to be executed • Types of applications (client-server or file system sharing, for example) being run on the network • Types of cable used on the network • Distances between computers on the network Computers on a bus either transmit data to other computers on the network or listen for data from other computers on the network. They are not responsible for moving data from one computer to the next. Consequently, if one computer fails, it does not affect the rest of the network.
Star In the star topology, cable segments from each computer are connected to a centralized component called a hub. Figure 1.21 shows four computers and a hub connected in a star topology. Signals are transmitted from the sending computer through the hub to all computers on the network. This topology originated in the early days of computing when computers were connected to a centralized mainframe computer.
Figure 1.21 Simple star network The star network offers the advantage of centralized resources and management. However, because each computer is connected to a central point, this topology requires a great deal of cable in a large network installation. Also, if the central point fails, the entire network goes down. If one computer— or the cable that connects it to the hub— fails on a star network, only the failed computer will not be able to send or receive network data. The rest of the network continues to function normally.
Run the c01dem11 video located in the Demos folder on the CD accompanying this book to view a demonstration of what happens when a computer on a star topology network goes down. Ring The ring topology connects computers on a single circle of cable. Unlike the bus topology, there are no terminated ends. The signals travel around the loop in one direction and pass through each computer, which can act as a repeater to boost the signal and send it on to the next computer. Figure 1.22 shows a typical ring topology with one server and four workstations. The failure of one computer can have an impact on the entire network. NOTE A network' s physical topology is the wire itself. A network' s logical topology is the way it carries signals on the wire.
Figure 1.22 Simple ring network showing logical ring
Run the c01dem12 and c01dem13 videos located in the Demos folder on the CD accompanying this book to view demonstrations of logical and actual flows of data on a ring-topology network. Variations on the Standard Topologies Many working topologies are hybrid combinations of the bus, star, ring, and mesh topologies. Star Bus The star bus is a combination of the bus and star topologies. In a star-bus topology, several star topology networks are linked together with linear bus trunks. Figure 1.28 shows a typical star-bus topology. If one computer goes down, it will not affect the rest of the network. The other computers can continue to communicate. If a hub goes down, all computers on that hub are unable to communicate. If a hub is linked to other hubs, those connections will be broken as well.
Figure 1.28 Star-bus network
Run the c01dem018, c01dem19, and c01dem20 videos located in the Demos folder on the CD accompanying this book to view demonstrations of what happens when computers and hubs in a star-bus topology go down. Star Ring The star ring (sometimes called a star-wired ring) appears similar to the star bus. Both the star ring and the star bus are centered in a hub that contains the actual ring or bus. Figure 1.29 shows a star-ring network. Linear-bus trunks connect the hubs in a star bus, while the hubs in a star ring are connected in a star pattern by the main hub.
Figure 1.29 Star-ring network Peer-to-Peer Many small offices use a peer-to-peer network as described earlier in this chapter in Lesson 2: Network Configuration. Such a network can be configured as either a physical star or a bus topology. However, because all computers on the network are equal (each can be both client and server), the logical topology looks somewhat different. Figure 1.30 shows the logical topology of a peer-to-peer network.
Figure 1.30 Logical peer-to-peer topology Selecting a Topology
There are many factors to consider when deciding which topology best suits the needs of an organization. Table 1.2 provides some guidelines for selecting a topology. Table 1.2 Topology Advantages and Disadvantages Topology Advantages Use
of
Disadvantages
Bus
cable
is
economical. Network can slow down in heavy traffic. Media is inexpensive and easy to work with. Problems are difficult to isolate. System is simple and reliable. Bus is easy to extend. Cable break can affect many users.
Ring
System provides equal access for all computers. Performance is even despite many users.
Star
Modifying system and adding new computers is If the centralized point fails, the easy. network fails. Centralized monitoring and management are possible. Failure of one computer does not affect the rest of the network.
Mesh
System provides increased redundancy reliability as well as ease of troubleshooting.
Failure of one computer can impact the rest of the network. Problems are hard to isolate. Network reconfiguration disrupts operation.
and System is expensive to install because it uses a lot of cabling.
Bus Topology A few situations will cause a bus network' s termination to fail and thereby take the network down. Possible scenarios include the following: • A cable on the network breaks, causing each end of the cable on either side of the break to lose its termination. Signals will bounce, and this will take the network down. • A cable becomes loose or is disconnected, thereby separating the computer from the network. It will also create an end that is not terminated, which in turn will cause signals to bounce and the network to go down. • A terminator becomes loose; thereby creating an end that is not terminated. Signals will start to bounce and the network will go down. Ring Topology A ring network is usually very reliable, but problems can occur. Possible scenarios include the following: • One of the cables in the ring breaks, causing the network to stop functioning temporarily. In tokenring networks, restoring the cable will immediately restore the network. • One of the cables in the ring becomes disconnected, causing the network to temporarily stop functioning. In token-ring networks, restoring the cable will immediately restore the network. What is the Open System Interconnect (OSI) model? The Open Systems Interconnection (OSI) reference model was an architectural model for open networking systems that was developed by the International Organization for Standardization (ISO) in Europe in 1974. The OSI reference model was intended as a basis for developing universally accepted networking protocols. So, the OSI reference model is an idealized model of the logical connections that must occur in order for network communication to take place. Most protocol suites, such as TCP/IP, DECnet, and Systems Network Architecture (SNA), map loosely to the OSI reference model. The OSI model is not a protocol but it is good for understanding how various protocols within a protocol suite function and interact. The OSI reference model has seven logical layers, as shown in the following table.
The OSI Reference Model 7 Application layer Interfaces user applications with network functionality, controls how applications access the network, and generates error messages. Protocols at this level include HTTP, FTP, SMTP, and NFS. 6 Presentation layer Translates data to be transmitted by applications into a format suitable for transport over the network. Redirector software, such as the Workstation service for Microsoft Windows NT, is located at this level. Network shells are also defined at this layer. 5 Session layer Defines how connections can be established, maintained, and terminated. Also performs name resolution functions. This layer enables applications running at two workstations to coordinate their communications into a single session. 4 Transport layer Sequences packets so that they can be reassembled at the destination in the proper order. Generates acknowledgments and retransmits packets. Assembles packets after they are received. If a duplicate packet arrives, the transport layer recognizes it as a duplicate and discards it. TCP and SPX are transport-layer protocols. 3 Network layer Defines logical host addresses, creates packet headers, and routes packets across an internetwork using routers and Layer 3 switches. Strips the headers from the packets at the receiving end. This layer is responsible for the entire route of a packet, from source to destination. IP and IPX are examples of networklayer protocols. 2 Data-link layer Specifies how data bits are grouped into frames, and specifies frame formats. Responsible for error correction, flow control, hardware addressing (such as MAC addresses), and how devices such as hubs, bridges, repeaters, and Layer 2 switches operate. The Project 802 specifications divide this layer into two sublayers, the logical link control (LLC) layer and the media access control (MAC) layer. The MAC layer deals with network access and network control. The LLC layer, operating just above the MAC layer, is concerned with sending and receiving the user massages. 1 Physical layer Defines network transmission media, signaling methods, bit synchronization, architecture (such as Ethernet or Token Ring), and cabling topologies. Defines how network interface cards (NICs) interact with the media (cabling). You can think this layer as the hardware layer. A local area network (LAN) is a computer network covering a small geographic area, like a home, office, or group of buildings Wide Area Network (WAN) is a computer network that covers a broad area Asynchronous Transfer Mode (ATM) is a cell relay, packet switching network and data link layer protocol which encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixedsized cells. ATM provides data link layer services that run over Layer 1 links Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media Access Control (MAC)/Data Link Layer, and a common addressing format. ARCNET (also CamelCased as ARCnet, an acronym from Attached Resource Computer NETwork) is a local area network (LAN) protocol, similar in purpose to Ethernet or Token Ring. ARCNET was the first widely available networking system for microcomputers and became popular in the 1980s for office
automation tasks. It has since gained a following in the embedded systems market, where certain features of the protocol are especially useful. Stations on a token ring LAN are logically organized in a ring topology with data being transmitted sequentially from one ring station to the next with a control token circulating around the ring controlling access. Physically, a token ring network is wired as a star, with ' hubs'and arms out to each station and the loop going out-and-back through each. Network switches are capable of inspecting data packets as they are received, determining the source and destination device of that packet, and forwarding it appropriately. By delivering each message only to the connected device it was intended for, a network switch conserves network bandwidth and offers generally better performance than a hub. A fast Ethernet adapter can be logically divided into a media access controller (MAC) which deals with the higher level issues of medium availability and a physical layer interface (PHY) Fiber distributed data interface (FDDI) provides a standard for data transmission in a local area network that can extend in range up to 200 kilometers (124 miles).
