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Network Communications Networking Essentials MIM0609

Chapter 1 - Introduction to Computer Networks

Basic Network ConceptsPeople communicate with other people in a variety of ways. For example, we talk to people face-to-face, or we write a letter and send it to someone and they write us a letter back. These arecommon forms of communication. When people use computers to communicate they use acomputer network. This course is about computer networks and how they are used to transmitinformation between computers, and ultimately between people. It is a fundamental course thatprovides a broad overview and foundation for understanding networks and working in thecomputer and networking industry.

A network is a group of connected devices. For people using computers to communicate with oneanother, a network must be used. Simply stated, the requirements of a network are to: Moveinformation from a source to a destination

Networks are an interconnection of computers. These computers can be linked together using awide variety of different cabling types, and for a wide variety of different purposes.

People use computers and networks for a wide variety of reasons. Three common reasons thatpeople use networks to send information from a source, such as a personal computer (PC), to a

destination, such as a printer, are:1. Communicate and collaborate (i.e., e-mail and newsgroups)2. Share information (i.e., document sharing)3. Share resources (i.e., printers and servers)

Take for example a typical office scenario where a number of users in a small business requireaccess to common information. As long as all user computers are connected via a network, theycan share their files, exchange mail, schedule meetings, send faxes and print documents all fromany point of the network.

It would not be necessary for users to transfer files via electronic mail or floppy disk, rather, eachuser could access all the information they require, thus leading to less wasted time and hencegreater productivity.

Imagine the benefits of a user being able to directly fax the Word document they are working on,rather than print it out, then feed it into the fax machine, dial the number etc.

Small networks are often called Local Area Networks (LAN). A LAN is a network allowing easyaccess to other computers or peripherals. The typical characteristics of a LAN are,

Physically limited (< 2km)

High bandwidth (> 1mbps)

Inexpensive cable media (coax or twisted pair)

Data and hardware sharing between users

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Owned by the user

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Examples of sharing resources are:

Sharing computer files and disk space

Sharing high-quality printers

Access to common fax machines

Access to common modems

Multiple accesses to the Internet

Network DevicesA source or destination can be any device capable of transferring information electronically fromone point (source) to another point (destination). There are many examples of devices thatcommunicate over a network. They take many forms and vary widely in capabilities. Theseinclude:

PCs

Macintosh computers

Workstations

Printers

Servers

Generically speaking, these devices are referred to as nodes. Nodes are the various endpoints

on a network, connected together to form a network. The connection between nodes is madeusing some type of connection medium. Examples of connection mediums include:

Copper cables

Fiber optic cables

Radio waves/wireless

Networks are used in a wide variety of ways to tie computers together so they can communicatewith one another and provide services to the user of a network.

Computer ComponentsComputers come in all shapes and sizes, and are manufactured to serve different purposes.Some computers are made for operation in a single-user environment. Other computers aremade to support a small number of users in a workgroup environment, while still others may

support thousands of users in a large corporation.

Computers attach to a network through a network interface card (NIC). Typically cables areattached to a NIC to connect to other computers or networks.Several aspects of computer technology to be considered are:

Video

Microprocessor

Memory

Storage

Input/Output

Application software

System software

Device driver

Whether the computer that attaches to the network is a small desktop computer or a powerfulmainframe, all computers contain the same basic structure and the components mentionedabove.

Computers are the endpoints, or nodes, in a network and come in a variety of shapes and sizes.It is important to understand common components found in most computer systems.

The CPU is the brain of any computer. The CPU executes instructions developed by a computer

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programmer, and directs the flow of information within the computer. The terms microprocessorand CPU are often used interchangeably. At the heart of a CPU is a microprocessor.

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A CPU runs computer programs, and directs the flow of information between differentcomponents of a computer. There are two basic characteristics that differentiate microprocessors:

Data bus sizethe number of bits that a CPU can send or receive in a single instruction.

Clock speedgiven in MHz, the clock speed determines how many instructions persecond the processor can execute.

In both the cases, the higher the value, the more powerful the CPU is. For example, a 64-bitmicroprocessor that runs at 450 MHz is more powerful than a 16-bit microprocessor that runs at100 MHz. The vast majority of all desktop PCs incorporate a single Intel architecture processor(such as Pentium). Although Intel is the worlds largest microprocessor manufacturer, it does nothave the entire PC processor market. For example, Advanced Micro Devices (AMD)manufactures processors comparable to the Intel Pentium.Key PointThe I/O of a computer travels over a bus, which is a collection of wires that transmit data fromone part of a computer to another. When used in reference to desktop computers, the term bususually refers to an internal bus. An internal bus connects all internal computer components to theCPU and main memory. Expansion slots connect plug-in expansion boards, such as internalmodems or NICs, to the I/O bus. All buses consist of two parts, an address bus and a data bus.The data bus transfers actual data, and the address bus transfers information about where the

data should go.

The size of a bus, known as its width, is important because it determines how much data can betransmitted at one time. For example, a 16-bit bus can transmit 16 bits of data, and a 32-bit buscan transmit 32 bits of data.Every bus has a clock speed measured in MHz; the faster the bus clock, the faster the bus datatransfer rate. A fast bus allows data to be transferred faster, making applications run faster. OnPCs, the ISA bus is being replaced by faster buses, such as the PCI bus. PCs made todayinclude a local bus for data that requires especially fast transfer speeds, such as video data. Thelocal bus is a high-speed pathway that connects directly to the processor.

A computers memory stores information currently being worked on by the CPU; it is a short-termstorage component of a computer. Memory differs from the long-term storage systems of acomputer, such as a hard drive or floppy, where information is stored for longer periods of time.When you run a program, it is typically read from a disk drive (such as a hard drive or CD-ROMdrive), and put in memory for execution. After the program has completed or is no longer needed,it is typically removed from memory. Some programs remain in memory after execution and arecalled Terminate and Stay Resident (TSR). The program waits for an event to occur that notifiesthem to begin processing. Memory is the systems internal, board-mounted storage area in thecomputer, also known as physical memory. The term memory identifies data storage that comesin the form of chips or plug-in modules, such as SIMMs or DIMMs. Most computers also usevirtual memory, which is physical memory, swapped to a hard disk. Virtual memory expands theamount of memory for an application beyond the actual physical, installed memory by moving olddata to the hard drive. Every computer comes with a certain amount of physical memory, usuallyreferred to as main memory or RAM. A computer that has 1 megabyte (Mb) of memory can holdapproximately 1 million bytes (or characters) of information.

There are several different types of memory, some of which are listed below: RAM RAM is the same as main memory. When used by itself, the term RAM refers to

read and write memory; that is, you can both write data into RAM and read data fromRAM. This is in contrast to read-only memory (ROM), which only permits you to readdata. Most RAM is volatile, which means it requires a steady flow of electricity to maintainits contents. As soon as the power is turned off, whatever data was in RAM is lost.

ROMComputers almost always contain a small amount of ROM that holds instructionsfor starting up the computer. Unlike RAM, ROM cannot be written to after it is initiallyprogrammed.

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Programmable read-only memory (PROM)A PROM is a memory chip on which youcan store a program. After the PROM has been programmed, you cannot wipe it cleanand reprogram it.

Erasable programmable read-only memory (EPROM)An EPROM is a special type ofPROM that can be erased by exposing it to ultraviolet light. EPROM can bereprogrammed and reused after it is has been erased.

A NIC (pronounced nick) is the hardware component inserted into the PC or workstation thatprovides connectivity to a network. The NIC provides the interface between the physicalnetworking cable and the software implementing the networking protocols. The NIC is responsiblefor transmitting and receiving information to and from a network.

The NIC fits into an expansion slot on the motherboards I/O bus. This bus connects adaptercards, such as NICs, to the main CPU and RAM. The speed at which data may be transferred toand from the NIC is determined by the I/O bus bandwidth, processor speed, NIC design andquality of components, operating system, and the network topology used.

New cards are software configurable, using a software program to configure the resources usedby the card. Other cards are PNP (Plug and Play), which automatically configure their resourceswhen installed in the computer, simplifying installation. With an operating system like Windows95, auto-detection of new hardware makes network connections simple and quick.

On power-up, the computer detects the new network card, assigns the correct resources to it, andthen installs the networking software required for connection to the network. All the users need toassign the network details like computer name.

For Ethernet, a 48-bit number identifies each card. This number uniquely identifies the computer.These network card numbers are used in the Medium Access (MAC) Layer to identify thedestination for the data. When talking to another computer, the data you send to that computer isprefixed with the number of the card you are sending the data to.

This allows intermediate devices in the network to decide in which direction the data should go, inorder to transport the data to its correct destination.

A typical adapter card looks like,

There are many ways to connect NICs to a network. The NIC attaches to a network via a twistedpair cable through a wall outlet. On the other side of the wall, the twisted pair cable goes to apunchdown block, a place where cables are often terminated to provide continuing connections toother devices. From the punchdown block, the twisted pair cable is sometimes connected to ahub or Multi-station Access Unit (MAU) that forms the central connecting point of the network. Ifthe network contains a dedicated server or other communicating device, it also contains a NIC.

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Basic Computer Terminology

AnalogAn analog signal, also referred to as an analog wave or carrier wave, is acontinuous electrical signal on a communication circuit. An analog signal in its normalform does not have intelligence. Modulation is used to add intelligence to an analogwave.

BandwidthBandwidth is the difference between the highest and lowest frequencies or

it is the maximum bit rate that can be transmitted across a transmission line or through anetwork. It is measured in Hz for analog networks, and bps for digital networks.

BinaryBinary is the base two number system that computers use to represent data. Itconsists of only two numbers: 0 and 1.

BitA bit is the smallest unit of data in a computer. A bit has a single binary value, either0 or 1. Although computers usually provide instructions that can test and manipulate bits,they generally are designed to store data and execute instructions in bit multiples calledbytes. In most computer systems, there are eight bits in a byte. The value of a bit isusually stored as either above or below a designated level of electrical charge in a singlecapacitor within a memory device.

BusA bus connects the central processor of a PC with the video controller, diskcontroller, hard drives, and memory. There are many types of buses, including internal

buses, external buses, and LANs that operate on bus topologies. Internal buses arebuses such as AT, ISA, EISA, and MCA that are internal to a PC.

ByteIn most computer systems, a byte is a unit of information that is 8 bits long. A byteis the unit most computers use to represent a character such as a letter, number, ortypographic symbol (e.g., g, 5, or ?). A byte can also hold a string of bits that need tobe used in some larger unit for application purposes (for example, the stream of bits thatconstitute a visual image within an application program).

Central Processing Unit (CPU)A CPU is the processor in a computer that processesthe code and associated data in a computer system.

Client/ServerClient server or client/server is a mode in computer networking whereindividual computers can access data or services from a common high-performancecomputer. For instance, when a PC needs data from a common database located on acomputer attached to a LAN, the PC is the client and the network computer where the

database resides is the server.

ClusteringClustering is a grouping of devices or other components, typically for theenhancement of performance. Clustering computers to execute a single applicationspeeds up the operation of the application.

Device DriverA device driver is a program that controls devices attached to acomputer, such as a printer or hard disk drive.

Digital DataDigital data is electrical information that represents digits (i.e., 1s and 0s).1s and 0s (bits) are combined to form bytes and characters, such as letters of thealphabet.

Dual Inline Memory Module (DIMM)DIMM is a small PC circuit board that holdsmemory chips. A DIMM has a 64-bit memory path to the CPU, which is compatible withthe Intel Pentium processor.

Electronic Mail (E-mail)E-mail is a widely used application for transferring messagesand files from one computer system to another. If the two computers sending messagesuse different types of e-mail packages, an e-mail gateway is required to convert from oneformat to another.

Extended Data Output (EDO)EDO is a type of RAM memory chip with fasterperformance than conventional memory. Unlike conventional RAM, EDO RAM retrieves ablock of memory as it sends the previous block to the CPU.

Extended Industry Standard Architecture (EISA)EISA is a 32-bit bus technology forPCs that supports multiprocessing. EISA was designed in response to IBMs MCA;

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however, both EISA and MCA were replaced by the PCI bus. See PCI and bus.

HardwareHardware is the physical part of a computer, which can include things suchas hard drives, circuit boards inside a computer, and other computer components.

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Industry Standard Architecture (ISA)ISA is an older, 8- or 16-bit PC bus technologyused in IBM XT and AT computers. See bus.

Input/Output (I/O)An I/O channel is the path from the main processor or CPU of acomputer to its peripheral devices.

Internet Protocol (IP)IP is the protocol responsible for getting packets of informationacross a network.

Local Area Network (LAN)A LAN is a grouping of computers via a network typicallyconfined to a single building or floor of a building.

MainframeA mainframe is a large-scale computer system. Mainframe computers arepowerful, and attach to networks and high-speed peripheral devices, such as tape drives,disk drives, and printers.

Megahertz (Mhz)One hertz is one cycle of a sine wave per second. One MHz is 1million cycles per second.

Micro Channel Architecture (MCA)MCA is IBMs 32-bit internal bus architecture forPCs. MCA was never widely accepted by the PC industry, and was replaced by the PCIbus architecture.

ModulationModulation is the process of modifying the form of a carrier wave (electricalsignal) so that it can carry intelligent information on some sort of communicationsmedium. Digital computer signals (baseband) are converted to analog signals fortransmission over analog facilities (such as the local loop). The opposite process,converting analog signals back into their original digital state, is referred to asdemodulation.

Network Interface Card (NIC)A NIC is any workstation or PC component (usually ahardware card) that allows the workstation or PC to communicate with a network. A NICaddress is another term for hardware address or MAC address. The NIC address is builtinto the network interface card of the destination node.

Peripheral Component Interconnect (PCI)PCI is a newer 32-bit and 64-bit local bustechnology for PCs. See bus. (Servers use 64-bit PCI, and PCs use 32-bit.)

PeripheralsPeripherals are parts of a computer that are not on the primary board(mother board) of a computer system. Peripherals include hard drives, floppy drives, and

modems. Personal Computer Memory Card International Association

(PCMCIA)The PCMCIA slot in a laptop was designed for PC memory expansion. NICsand modems can attach to a laptop through the PCMCIA slot.

Personal Digital Assistant (PDA)PDA devices are very small, and provide a subset ofthe operations of a typical computer (PC). They are used for scheduling, electronicnotepads, and small database applications.

RedirectorA redirector is a client software component in a client/server configuration.The redirector is responsible for deciding if a request for a computer service (i.e., read afile) is for the local computer or network server.

Random Access Memory (RAM)RAM is a computers main working memory.Applications use RAM to hold instructions and data during processing. Applications can

repeatedly write new data to the same RAM, but all data is erased from RAM when thecomputer loses power or is shut down.

RJ-45 ConnectorAn RJ-45 connector is a snap-in connector for UTP cable, similar tothe standard RJ-11 telephone cable connector.

ServerA server is a device attached to a network that provides one or more services tousers of the network.

Single Inline Memory Module (SIMM)SIMM is a small PC circuit board that holdsmemory chips. A SIMM has a 32-bit memory path to the CPU. SIMM capacities aremeasured in bytes.

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Synchronous Dynamic Random Access Memory (SDRAM)SDRAM is a type ofRAM and is often referred to as DIMM. It is replacing EDO RAM because it isapproximately twice as fast (up to 133 MHz).

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Synchronous Graphic Random Access Memory (SGRAM)SGRAM is a type ofdynamic RAM optimized for graphics-intensive operations. Like SDRAM, SGRAM cansynchronize itself with the CPU bus clock, up to speeds of 100 MHz.

Transmission Control Protocol (TCP)TCP is normally used in conjunction with IP ina TCP/IP-based network. The two protocols working together provide for connectivitybetween applications of networked computers.

UNIXUNIX is an operating system used in many workstations and mid-range computersystems. It is an alternative to PC and Macintosh computer operating systems. Linux is aUNIX-like clone.

WorkstationsWorkstations are a type of computer, typically more powerful than a PCbut still used by a single user.

There are many different types of computers used in organizations, most of which are tied to anetwork. Some of these computers are small and can only run a limited amount of applications.

Others are large and can run many applications and service many users at the same time. Thislesson looks at classifications of computers found in networks and the primary purpose of eachtype.Computer classifications include:

Desktop computers

Mid-range computers and servers

Mainframe computers

Others

A desktop computer is a computer, possibly attached to a network that is used by a singleindividual. Desktop computers are sometimes divided into two broad categories, personalcomputers (PCs) and workstations. The difference between PCs and workstations, although notalways clear, is generally in the operating system software used and the graphics capabilities.PCs typically run one of several types of Microsoft Windows operating systems, or MacintoshOperating Systems in the case of Apple products, while a workstation typically runs a version ofthe UNIX operating system. A workstation often features high-end hardware such as large, fastdisk drives, large amounts of Random Access Memory (RAM), advanced graphics capabilities,and high performance, multiprocessors. There is a great deal of overlap in features and functionsof desktop computers, and the distinctions between PCs and workstations are blurring.

The term mid-range covers a wide range of computer systems that support more than one user,and may support many users. It covers an extensive range of computer systems, overlappingwith desktop computers at one-end and mainframe computers at the other end. Midrangecomputers include:

High-end Reduced Instruction Set Computer (RISC) CPU-based

Servers (IBM AS/400)

Intel-based servers (Compaq, Dell, and Hewlett-Packard)

UNIX-based servers of all types

Mid-range and server systems are commonly used in small to medium organizations, such asdepartmental information processing. Typical applications include:

Finance and accounting (AS/400)

Database (Intel-based or UNIX-based)

Printer servers (Intel or UNIX-based)

Communications servers (Intel-based)

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Mainframe computers (also referred to as super computers) and associated client/server productscan manage huge organization-wide networks, store and ensure the integrity of massive amountsof crucial data, and drive data across an organization. The unique and inherent capabilities ofleading-edge mainframe systems include:

Constant availabilitymainframes are designed to be operated around the clock everyday of the year. This is sometimes referred to as a number of nines; a level of reliability

desired and measured in nines (99.999% reliability). Rigorous backup, recovery, and securitymainframes provide automatic and constant

backup, tracking, and safeguarding of data.

Huge economies of scalethe vast resources of mainframes reduce the hidden costsassociated with multiple LANs, such as administration and training, extra disk space, andprinters.

High bandwidth I/O facilitiesthe huge I/O bandwidth on mainframes allows rapid andeffective data transfer so that thousands of clients to be serviced simultaneously, andcaters to emerging applications like multimedia, including digitized video on demand.

Laptops, palmtops or PDAs, and thin client terminals have become mainstream devices in thepersonal computing world over the last five years. To satisfy a society that is increasingly mobile,laptops and palmtops have become prevalent. Performance in laptops is closing in on thedesktop computer as miniaturization compounds annually.

Palmtops are also catching up with performance. E-mail, calendering, low-end spreadsheets, andhandwriting recognition compatible with Windows operating systems have become a reality withthe newest generation palmtops. In the last few years, thin client terminals have staged acomeback, in that they are reminiscent of the terminal of mainframe days, but with graphicscapabilities. Thin client terminals allow for an even greater reduced cost at the desktop computerlevel by having a central server handle processing otherwise handled by a PC. The terminal onlydisplays the screen images, mouse movements, and keystrokes of the application running on theserver.

Software and Operating SystemTwo primary types of system software are discussed in this lesson, operating systems and devicedrivers. Operating systems perform the basic tasks computers do to perform useful work. It is a

group of software programs that control a computer. The tasks of an operating system include: Managing the operation of computer programs

Interpreting keyboard input

Displaying data to the computer screen

File I/O

Controlling peripheral devices, such as floppy disks, hard disks, and printers

Operating systems can be classified as follows:

MultitaskingAllows more than one program to run.

MultithreadingAllows different parts of a single program to run.

MultiuserAllows multiple users to run programs on the same computer at the sametime. Some operating systems running on large-scale computers permit hundreds or

even thousands of concurrent users. MultiprocessingSupports running a program on more than one CPU.

Key PointOperating systems provide a software platform that can run other programs, called applicationprograms. The application programs must be written to run on top of a particular operatingsystem. Your choice of operating system determines, to a great extent, the applications you canrun. For PCs, the most popular operating systems are Windows 95, Windows 98, Windows NT,and Windows 2000, but others are also available, such as the UNIX-based operating systems,including Linux.

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As a user, you normally interact with the operating system through a set of commands. Thecommands are accepted and executed by a part of the operating system called the commandprocessor or command line interpreter.

Graphical user interfaces (GUIs) allow you to enter commands by pointing and clicking at objectsthat appear on the screen. Microsoft has generally dominated the PC operating system marketsince its foundation, first with its command line-based disk operating system (DOS), and then withthe Windows user interface, a graphical overlay for DOS. The Macintosh operating system is acompetitor, which runs on Macintosh platforms and not PC-based platforms.

Popular OS

Microsoft Windows was the initial GUI that ran on top of DOS. It was first launched in response tothe need to make PCs easier to use. It is similar to the Apple Macintosh computer, known for itseasy-to-use operating system. Windows 3.1 was the most popular Windows software, and is stillused in many networks.

Windows for Workgroups 3.11 was Microsofts first peer-to-peer network operating system. It wasa combination of Windows 3.1 and the Microsoft Windows Network software (which provides thepeer-to-peer networking capability), with enhancements such as improved disk I/O performance,

connectivity, and remote access, and a range of features intended to appeal to the networkmanager.

Windows 95 is a true operating system, not just a GUI as found in standard Windows. Windows95 is the first operating system to support Plug and Play, which makes system setup,management, and upgrading considerably easier. Other enhancements include improvements tothe user interface, better support for NetWare servers, long file names, and video playback; betterfax and modem support; improved system administration; and remote network access.

Windows 98 is another version of Microsoft Windows. It has many of the same features asWindows 95, but includes a different user interface and Web-related features.

Windows NT 3.1, Microsofts 32-bit operating system, was first launched in July 1993. It runs on

Intel 486 or higher PCs and PC servers, and Digital Equipments Alpha chip family. There are twoWindows NT options: Windows NT Workstation and Windows NT Server.

Windows NT Workstation requires only 16 MB of RAM, making it far more accessible. For userswith a strong requirement for security, C2 versions of Windows NT are also available. WindowsNT release 4.0 replaced Windows NT 3.1, 3.5, and 3.51. Another product called Windows 2000has also been released.

The mid-range software market is characterized by competition between UNIX, which runs on avery wide range of hardware platforms from workstations to mid-range systems, and theproprietary operating environments, designed specifically for interactive multiuser applicationsdeveloped by the leading systems vendors. Some vendors only offer UNIX on their platforms(e.g., Sun with Solaris). Other vendors accept and support a multiplicity of operating

environments: Windows on PCs, Windows NT, Linux UNIX, and NetWare on LAN servers; UNIXon midrange servers; and proprietary environments on mid-range systems and mainframes thatneed higher-than-average reliability and security.

A device driver is special-purpose software used to control specific hardware devices in acomputer system. These specific pieces of hardware can be disk drives, floppy drives, or NICs.Device drivers for NICs control the operation of the NIC and provide an interface for thecomputers operating system. The operating system and associated applications on the computercan use the device driver to communicate with the NIC and send and receive information on anetwork.

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Application SoftwareApplications are computer programs that are used for productivity and automation of tasks.Networks are used to move application information from source to destination.

Applications are software programs used to perform work. Applications can be divided into twobasic categories: single-user applications, and networked or multiuser applications. Someapplications run primarily as a single-user application, others run almost exclusively as anetworked application. Some applications can run in both modes. Commonly used applicationsare described below.

Single-user applications include:

Word processorsA word processor is used to enter, edit, format, print, and savedocuments. These programs are designed to produce various documents fororganizations, such as letters, memos, and reports. An example of a word processor isMicrosoft Word.

Desktop publishingDesktop publishing goes beyond basic word processing. It is usedto design professional publications, such as magazines and newsletters. An example of adesktop publishing package is Adobe Framemaker.

GraphicsGraphics programs are used to create pictures and artwork that are used bythemselves or imported into other documents using programs such as desktop publishingpackages. Examples of graphics programs are Adobe Illustrator and Shapeware Visio.

DatabaseA database program provides the capability to input, store, analyze, andretrieve information on a computer system. The Information is stored in records and ismanaged by a database program. Microsoft Access is a common database program.

SpreadsheetsSpreadsheet applications are primarily used to create financial reportsand organize financial information. They provide an easy mechanism to manipulatenumbers and compute mathematical equations. Lotus 123 is a spreadsheet program thatis commonly used.

Web browsersWeb browsers are used to locate and retrieve information from theWorld Wide Web. They are primarily used to go directly to another Internet Web site orsearch the Internet for specific information. Netscape Navigator is an example of anInternet browser.

Network applications include:

Database accessDatabase requests from client to server are made to retrieve recordsfrom a single source. Oracle is an example of a client/server database.

Print servicesClients generate print requests that are serviced by a print server. Jobsare queued by the print server, and the client is notified when the print job has beencompleted. Novell NetWare provides print services to NetWare clients.

E-mailE-mail programs typically reside on both a client, with packages such asEudora, and on a server. When users log on to a network, the e-mail server downloads e-mail messages to the individual clients in the network.

Fax servicesClients generate fax requests that are serviced by the server similar toprint requests. Microsoft Windows NT Small Business Sever provides built-in faxservices.

Application-to-Application CommunicationApplications use the underlying operating system, such as Windows 98, to carry out the neededtasks of the application. This includes accepting keystrokes from a keyboard and displaying thetyped information on a computer screen. If you are using a word processor and want to store afile on a hard drive, such as a local hard drive, the application would rely on the operating systemto store the information. The operating system stores the information on the hard drive bycommunicating with the appropriate hard drive device driver to physically place the wordprocessors information on the drive. Perhaps you want to store the information on a hard drive

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located on the other side of the building, in other words, over the network. What must happen inthis case? The following three items must be installed on the local machine by the local operatingsystem to provide for communication across a network:

NIC and NIC device driver

Client software

Communication software

The appropriate accessory card, such as a NIC, must first be installed in the computer, along withthe corresponding device driver. If you install a 3Com Ethernet NIC, Ethernet device driversoftware must also be installed. Client software is also needed to provide an interface betweenthe local operating system and communication software. Some client software provides file andprint sharing capabilities for computers, while others provide the capability to connect to sharedresources on other computers. You must also have communication software loaded on themachine, such as a TCP/IP protocol stack. Key Point

Client software requests are placed inside protocol headers for delivery across a network. Thefollowing steps are typical

1. The user of the application requests that a file be stored on a drive other than a localdrive (a network drive).

2. Computer software on the client machine (also known as a redirector) determines the fileis not destined for a local disk drive, but is destined for a remote disk drive.

3. The redirector takes the store file request from the application and requests the servicesof the communication software.

4. The communication software adds the appropriate communication information on thestore file request.

5. The request is sent from the main CPU of the computer, across the local bus to the NIC.6. The NIC transmits the information across the networking cables to the final destination,

such as a file server on the network.

From the users perspective, we have the ability to store files at multiple locations, including ahard drive located in our computer, or other hard drives on the network. It is up to the localoperating system to make sure the file gets properly stored on the local computer hardware.When we want to store information across a network, the request must be redirected out of the

computer via a NIC to the appropriate machine located on the network.

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Chapter 2 Network Media, Cables and Connectors

Digital and Analog TransmissionThe telecommunications network can transmit a variety of information, in two basic forms, analogand digital. In this lesson we will examine both. This information may be transmitted over acircuit/channel or over a carrier system.

Where: A circuit/channel is a transmission path for a single type of transmission service (voice ordata) and is generally referred to as the smallest subdivision of the network. A carrier, on theother hand, is a transmission path in which one or more channels of information are processed,converted to a suitable format and transported to the proper destination.

The two types of carrier systems are:

FDM (Frequency Division Multiplexing) -- analog

TDM (Time Division Multiplexing) - digital

Digital TransmissionThis is the single fastest growing family of signals. It is discrete and well defined, digital signalsvary from dial tone pulses to complex computer and data signals.

Analog SignalsAn analog signal, or sine wave, is a continuously varying signal

Analog transmissions are continuous in both time and value. This makes them particularlysusceptible to errors. Digital transmissions are discrete in both time and value. The exact digitalvalue can be received at the destination with very few errors.

All types of media can be decomposed into bits, which suggest that digital networks can be usedfor integrated services (ISDN). This allows compression and encryption that is not possible withanalog signals.

LAN, CAN, MAN, WAN, GANLocal Area Networks - local to a single limited geographical area- floor of a building, department

within a floor, or one entire building.

Local area networks can be defined as privately owned networks that generally span an entirebuilding or up to a few kilometers in size. LAN can be effectively used to connect personalcomputers and other resources like printers.

LANs often use a single cable to connect the resources. Typically, LANs operated at 10 to 100Mbps, have low delays and make few errors. Broadcast LANs follow many topologies.

Campus Area Networks - covers a large campus that has might include several city blocks, thislevel allows for more refinement in our definitions, it will distinguish between say the Microsoft HQCAN and the MAN which covers the city that the CAN is in.

Metropolitan Area Networks - Group of multiple LANs and/or CANs that communicate over a cityand/or several city blocks, MANs are limited to within a single city.

MAN uses similar technology as that of LAN. The difference is that the area coverage of MANs ismore than LANs. They support both voice and data. This property enables them to be efficientlyused for cable television. Typically, a MAN has one or two cables and does not require aswitching element.

Wide Area Networks - Groups of MANs that communicate over larger geographical distances likecities or states, WANs span city to city and State to state but will stay within a country.

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Wide area networks, as the name suggests, spans a large geographical area. The typicalstructure of a WAN consists of a number of host machines (which run applications) connected bya communication subnet. The main function of the subnet is to transfer data from one host toanother. The subnet, in turn, consists of two components transmission lines and switchingelements, also referred to as routers. Data is actually transferred via the transmission lines. Theswitching elements connect two or more transmission lines and decide in which outgoing line toforward the data.

Typically in a WAN, there are a number of transmission lines connected to a router. Not allrouters are connected to each other. Thus, unconnected routers interact with each other indirectlyvia other routers. In such a situation, when a router receives a packet, it stores the packet untilthe required output line is free and then forwards it. Subnets employing such a technique arecalled point-to-point or store-and-forward subnets.

Global Area Networks - covers a country's geographical area or a larger area worldwide, GANsspan countries and are our International level of Networking. This level was added to differentiatea WAN from the larger Global Networks. The Internet is the most famous example of the GAN

Network Topologies (Ring, Star, Bus, Hybrid)

In networking, the term topologyrefers to the layout of connected devices on a network. Thisarticle introduces the standard topologies of computer networking.One can think of a topology as a network's "shape." This shape does not necessarily correspondto the actual physical layout of the devices on the network. For example, the computers on ahome LAN may be arranged in a circle, but it would be highly unlikely to find an actual ringtopology there.Network topologies are categorizedinto the following basic types:

Bus

Ring

Star

Tree

MeshMore complex networks can be builtas hybrids of two or more of theabove basic topologies.

BusBus networks (not to be confusedwith the system bus of a computer)use a common backbone to connectall devices. A single cable, the backbone functions as a shared communication medium, thosedevices attach or tap into with an interface connector. A device wanting to communicate withanother device on the network sends a broadcastmessage onto the wire that all other devicessee, but only the intended recipient actually accepts and processes the message.

Ethernet bus topologies are relatively easy to install and don't require much cabling compared tothe alternatives. 10Base-2 ("ThinNet") and 10Base-5 ("ThickNet") both were popular Ethernetcabling options years ago. However, bus networks work best with a limited number of devices. Ifmore than a few dozen computers are added to a bus, performance problems will likely result. Inaddition, if the backbone cable fails, the entire network effectively becomes unusable.

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RingIn a ring network, every device hasexactly two neighbors for communication purposes. Allmessages travel through a ring in thesame direction (effectively either"clockwise" or "counterclockwise"). Afailure in any cable or device breaks

the loop and can take down the entirenetwork.To implement a ring network, onetypically uses FDDI, SONET, orToken Ring technology. Rings arefound in some office buildings orschool campuses.

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StarMany home networks use the startopology. A star network features a centralconnection point called a "hub" that maybe an actual hub or a switch. Devicestypically connect to the hub withUnshielded Twisted Pair (UTP) Ethernet.

Compared to the bus topology, a starnetwork generally requires more cable,but a failure in any star network cable willonly take down one computer's networkaccess and not the entire LAN. (If the hubfails, however, the entire network also fails.)

TreeTree topologies integrate multiple star topologies together onto a bus. In its simplest form, onlyhub devices connect directly to the tree bus and each hub functions as the "root" of a tree ofdevices. This bus/star hybrid approach supports future expandability of the network much betterthan a bus (limited in the number of devices due to the broadcast traffic it generates) or a star(limited by the number of hub ports) alone.

Mesh

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Mesh topologies involve the concept ofroutes. Unlike each of the previous topologies, messagessent on a mesh network can take any of several possible paths from source to destination.(Recall that in a ring, although two cable paths exist, messages can only travel in one direction.)Some WANs, like the Internet, employ mesh routing.

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MediaThe actual transmission of information in a computer network takes place via a transmissionmedium. These mediums can be broadly classified into guidedand unguided medium. Guidedmedium can be termed as that medium where the packets are directed towards the destination.Good examples of guided medium can be copper wire and optical fibers. Unguided mediumrefers to that medium in which the information is transmitted irrespective of the location of thedestination machine.

Twisted PairThe twisted pair transmission medium consists of two insulated copper wires, typically about1mm thick. These two wires are twisted together in a helical form. This twisting reduces theelectrical interference from similar cables close by. The telephone system is an excellent exampleof a twisted pair network.Twisted pair can be used for both analog and digital transmission. The bandwidth that can beachieved with twisted pair depends on the thickness and the distance traveled. Typically, atransmission rate of several megabits per second can be achieved for a few kilometers.

Coaxial cable has better shielding than twisted pair. Thus, they have the advantage that they canspan longer distance at relatively higher speed.

A coaxial cable consists of a stiff copper wire as the core, which is surrounded by an insulatingmaterial. A cylindrical conductor in the form of a closely woven braided mesh surrounds theinsulator. A plastic coating then covers this entire setup. Two types of coaxial cables that arewidely used;

The baseband coaxial cable is a 50-ohm cable and is commonly used for digital

transmission. Due to the shielding structure, they give excellent noise immunity. Thebandwidth depends on the length of the cable. Typically, 1 to 2 Gbps1 is possible for a 1-km cable. Longer cables may also be used. They, however, provide lower data ratesunless used with amplifiers or repeaters.

The broadband coaxial cable is a 75-ohm cable mostly used for analog transmission. Thestandard cable television network is an excellent example where broadband coaxialcables are used. The broadband coaxial cables can give up to 450 MHz and can span fornearly 100 kms for analog transmission. Broadband systems can be subdivided into anumber of independent channels. Each channel can transmit analog or digital data.

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Optical fibers, as the name suggests, employ the medium of light to transmit information. Thus,information can be transmitted at very high speed - the speed of light and it eliminates problemslike heat dissipation. Optical fibers are typically used to provide a bandwidth of 1 Gbps althoughbandwidth in excess of 50,000 Gbps is possible. This limitation is due to the unavailability oftechnology that can convert optical signals to electrical signals and vice versa at such a fast rate.

The technology behind optical fibers employs three components: the light source, transmissionmedium and the detector. The light source, connected at one end of the transmission medium,generates a pulse of light that corresponds to 1 bit of data. The presence of no light is equivalentto 0 bit. The transmission medium used is an ultra-thin fiber of glass. The detector at the otherend of the transmission medium detects the presence of light pulses and generates an electricalsignal accordingly.

Wireless TransmissionThe transmission media described above provides a physical connection between twocomputers. This is quite often not feasible especially when the geographical distance between thetwo computers is very large. Communication in these types of setup is carried out by employingvarious other mediums such as microwaves, radio waves etc. Communication, employing thesetypes of mediums, is called wireless communication.

Radio TransmissionThe obvious advantage of using radio waves comes from the fact that radio waves are easilygenerated, can travel longer distances, can penetrate buildings and are omnidirectional.However, radio waves have the disadvantage that arises from the fact that radio waves arefrequency dependent. At low frequencies, the power of the radio waves deteriorates as thedistance traveled from the source increases. At high frequencies, the radio waves tend to travel ina straight line and bounces of obstacles. They are also absorbed by rain and are subjected tointerference from motors and other electrical equipment.

Microwave TransmissionMicrowave transmission offers a high signal to noise ratio. However, it necessitates thetransmitter and the receiver to be aligned in a straight line without interference. In addition,

because of the fact that microwaves travel in a straight line, it becomes necessary to providerepeaters for long distances since the curvature of the earth becomes an obstacle. Some wavesmay be refracted off low-lying atmospheric layers and thus may take slightly longer to arrive.They may also be out of phase with the direct wave thus creating a situation called multipathfading where the delayed wave tend to cancel out the direct wave. Microwaves have theadvantage that they are relatively inexpensive and require less space to setup antennas. Theycan also be used long distance transmission.

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Cabling

A LAN can be as simple as two computers, each havinga network interface card (NIC) or network adapter andrunning network software, connected together with acrossover cable.The next step up would be a network consisting of threeor more computers and a hub. Each of the computers isplugged into the hub with a straight-thru cable(the crossover function is performed by the hub).

Let's start with simple pin-out diagrams of the two types of UTP Ethernet cables and watch howcommittees can make a can of worms out of them. Here are the diagrams:

Note that the TX (transmitter) pins are connected to corresponding RX (receiver) pins, plus toplus and minus to minus. And that you must use a crossover cable to connect units with identicalinterfaces. If you use a straight-through cable, one of the two units must, in effect, perform thecross-over function.Two wire color-code standards apply: EIA/TIA 568A and EIA/TIA 568B.

A straight-thru cable has identical ends.

A crossover cable has different ends.

It makes no functional difference which standard you use for a straight-thru cable. You can starta crossover cable with either standard as long as the other end is the other standard. It makesno functional difference which end is which. Despite what you may have read elsewhere, a 568Apatch cable will work in a network with 568B wiring and 568B patch cable will work in a 568Anetwork. The electrons couldn't care less.

Modular Plug Crimp Tool. You will need amodular crimp tool. This one is very similar tothe one I have been using for many years for allkinds of telephone cable work and it works justfine for Ethernet cables. You don't need a lot ofbells and whistles, just a tool that will securelycrimp RJ-45 connectors.

Connectors and TerminatorsTwisted Pair (Shielded Twisted Pair and Unshielded Twisted Pair) is becoming the cable ofchoice for new installations. Twisted pair cable is readily accepted as the preferred solution to

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cabling. It provides support for a range of speeds and configurations, and is widely supported bydifferent vendors. Shielded twisted pair uses a special braided wire, which surrounds all the otherwires, which helps to reduce unwanted interference.

The features of twisted pair cable are,

Used in token ring (4 or 16mbps), 10baset (Ethernet 10mbps), 100baset (100Mbps)

Reasonably cheap

Reasonably easy to terminate [special crimp connector tools are necessary for reliableoperation

UTP often already installed in buildings

UTP is prone to interference, which limits speed and distances

Low to medium capacity

Medium to high loss

Category 2 = up to 1Mbps (Telephone wiring)

Category 3 = up to 10Mbps (Ethernet and 10baset)

Category 5 = 100mbps (supports 10baset and 100baset)Unshielded Twisted Pair cable used in Category 5 looks like

Category 5 cable uses 8 wires. The various jack connectors used in the wiring closet look like,

Distance limitations exist when cabling. For category 5 cabling at 100Mbps, the limitationseffectively limit a workstation to wall outlet of 3 meters, and wall outlet to wiring closet of 90meters.

All workstations are wired back to a central wiring closet, where they are then patchedaccordingly. Within an organization, the IT department either performs this work or sub-contractsit to a third party.

In 10BaseT, each PC is wired back to a central hub using its own cable. There are limits imposedon the length of drop cable from the PC network card to the wall outlet, the length of thehorizontal wiring, and from the wall outlet to the wiring closet.

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Patch CablesPatch cables come in two varieties, straight through or reversed. One application of patch cablesis for patching between modular patch panels in system centers. These are the straight throughvariety. Another application is to connect workstation equipment to the wall jack, and these couldbe either straight through or reversed depending upon the manufacturer. Reversed cables arenormally used for voice systems.

How to determine the type of patch cableAlign the ends of the cable side by side so that the contacts are facing you, and then compare thecolors from left to right.

If the colors are in the same order on both plugs, the cable is straight through. If the colors appearin the reverse order, the cable is reversed.

Coaxial CableCoaxial cable has traditionally been the cable of choice for low cost, small user networks. Thishas been mainly due to its ease of use and low cost. Persons with minimal networkunderstanding can readily build a LAN using coax components, which can often be purchased inkit ready format.The general features of coaxial cable are,

Medium capacity

Ethernet systems (10mbps)

Slighter dearer than utp

More difficult to terminate

Not as subject to interference as utp

Care when bending and installing is needed

10base2 uses rg-58au (also called thin-net or cheaper net)

10base5 uses a thicker solid core coaxial cable (also called Thick-Net)

Thin coaxial cable [RG-58AU rated at 50 ohms], as used in Ethernet LAN's, looks like

The connectors used in thin-net Ethernet LAN's are T connectors (used to join cables togetherand attach to workstations) and terminators (one at each end of the cable). The T-connectors andterminators look like

The straight through and cross-over patch cables discussedin this article are terminated with CAT 5 RJ-45 modularplugs. RJ-45 plugs are similar to those you'll see on theend of your telephone cable except they have eight versus

four or six contacts on the end of the plug and they areabout twice as big. Make sure they are rated for CAT 5wiring. (RJ means "Registered Jack"). Also, there are RJ-45 plugs designed for both solid core wire and strandedwire. Others are designed specifically for one kind of wireor the other.

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Chapter 3 - Communication Protocols and Services

ProtocolsComputer networks use protocols to communicate. These protocols define the procedures to usefor the systems involved in the communication process. A data communication protocol is a set ofrules that must be followed for the two electronic devices to communicate.

Many protocols are used to provide and support data communications. They form communicationarchitecture, sometimes referred as protocol stack such as the TCP/IP family protocols.

Protocols:

Define the procedures to be used by systems involved in the communication process

In data communications, are a set of rules that must be followed for devices tocommunicate

Are implemented in software/firmware

Each protocol provides for a function that is needed to make the data communication possible.Many protocols are used so that the problem can be broken into manageable pieces. Eachsoftware module that implements a protocol can be developed and updated independently ofother modules, as long as the interface between modules remains constant.

Recall that a protocol is a set of rules governing the exchange of data between two entities.These rules cover:

Syntax Data format and coding

Semantics Control information and error handling

Timing Speed matching and sequencing

Layered ApproachA networking model represents a common structure or protocol to accomplish communicationbetween systems. These models consist of layers. You can think of a layer as a step that must becompleted to go on to the next step and, ultimately, to communicate between systems.

As described previously, a protocol is a formal description of messages to be exchanged andrules to be followed for two or more systems to exchange information.Model = StructureLayer = FunctionProtocol = Rules

Some of the advantages of using a layered model are:

Allows changes or new features to be introduced in one layer leaving the others intact

Divides the complexity of networking into functions or sublayers which are moremanageable

Provides a standard that, if followed, allows interoperability between software andhardware vendors

Eases troubleshooting

Implementing a functional internetwork is no simple task. Many challenges must be faced,especially in the areas of connectivity, reliability, network management, and flexibility. Each areais the key in establishing an efficient and effective internetwork.

The challenge when connecting various systems is to support communication among disparatetechnologies. Different sites, for example, may use different types of media operating at varyingspeeds, or may even include different types of systems that need to communicate.

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Because companies rely heavily on data communication, internetworks must provide a certainlevel of reliability. This is an unpredictable world; so many large internetworks include redundancyto allow for communication even when problems occur.

An ordered approach to network communication is a way out. The layered approach to OSI offersseveral advantages to system implementers. By separating the job of networking into logicalsmaller pieces, vendors can more easily solve network "problems" through divide-and-conquer. A product from one vendor that implements Layer 2, for example, will be muchmore likely to interoperate with another vendor's Layer 3 product when both vendors followthis standard model. Finally, the OSI layers afford extensibility. New protocols and othernetwork services will generally be easier to add to a layered architecture than to a monolithicone. Protocol Layers

The communication between the nodes in a packet data network must be precisely defined toensure correct interpretation of the packets by the receiving intermediate and the end systems.The packets exchanged between nodes are defined by a protocol - or communications language.

There are many functions which may be need to be performed by a protocol. These range fromthe specification of connectors, addresses of the communications nodes, identification ofinterfaces, options, flow control, reliability, error reporting, synchronization, etc. In practice there

are so many different functions, that a set (also known as suite or stack) of protocols are usuallydefined. Each protocol in the suite handles one specific aspect of the communication.

The protocols are usually structured together to form a layered design (also known as a "protocolstack"). All major telecommunication network architectures currently used or being developed uselayered protocol architectures. The precise functions in each layer vary. In each case, however,there is a distinction between the functions of the lower (network) layers, which are primarilydesigned to provide a connection or path between users to hide details of underlyingcommunications facilities, and the upper (or higher) layers, which ensure data exchanged are incorrect and understandable form. The upper layers are sometimes known as "middleware"because they provide software in the computer which convert data between what the applicationsprograms expect, and what the network can transport. The transport layer provides theconnection between the upper (applications-oriented) layers and the lower (or network-oriented)

layers.

The basic idea of a layeredarchitecture is to divide the design intosmall pieces. Each layer adds to theservices provided by the lower layers insuch a manner that the highest layer isprovided a full set of services tomanage communications and rundistributed applications. A basicprinciple is to ensure independence oflayers by defining services provided byeach layer to the next higher layer

without defining how the services areto be performed. This permits changesin a layer without affecting other layers.Prior to the use of layered protocolarchitectures, simple changes such asadding one terminal type to the list ofthose supported by an architectureoften required changes to essentiallyall communications software at a site.

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Protocol StacksThe protocol stacks were once defined using proprietary documentation - each manufacturerwrote a comprehensive document describing the protocol. This approach was appropriate whenthe cost of computers was very high and communications software was "cheap" in comparison.Once computers became readily available at economic prices, users saw the need tointerconnect the computers from different manufacturers using computer networks. It was costlyto connect computers with different proprietary protocols, since for each pair of protocols aseparate "gateway" product had to be developed. This process was made more complicated insome cases, since variants of the protocol existed and not all variants were defined by publisheddocuments.

Network CommunicationNetwork Architectures

Peer to Peer

Client Server

Console Terminal

Peer To PeerA peer-to-peer type of network is one in which each workstation has equivalent capabilities andresponsibilities. This differs from client/server architectures, in which some computers arededicated to serving the others. Peer-to-peer networks are generally simpler, but they usually donot offer the same performance under heavy loads.

This definition captures the traditional meaning of peer-to-peer networking. Computers in aworkgroup, or home computers, are configured for the sharing of resources such as files andprinters. Although one computer may act as the file server or FAX server at any given time, allcomputers on the network generally could host those services on short notice. In particular, thecomputers will typically be situated near each other physically and will run the same networkingprotocols.

P2P systems involve seven key characteristics:

User interfaces load outside of a web browser

User computers can act as both clients and servers

The overall system is easy to use and well-integrated

The system includes tools to support users wanting to create content or add functionality

The system provides connections with other users

The system does something new or exciting

The system supports "cross-network" protocols like SOAP or XML-RPC

In this updated view of peer-to-peer computing, devices can now join the network from anywherewith little effort; instead of dedicated LANs, the Internet itself becomes the network of choice.Easier configuration and control over the application allows non networking-savvy people to jointhe user community. In effect, P2P signifies a shift in emphasis in peer networking from thehardware to the applications.

Client ServerAges ago (in Internet time), when mainframe dinosaurs roamed the Earth, a new approach tocomputer networking called "client/server" emerged. Client/server proved to be a more cost-effective way to build many types of networks, particularly PC-based LANs running end-userdatabase applications. Many types of client/server systems remain popular today.

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What Is Client/Server?The most basic definition of client/server

Client/server is a computational architecture that involves client processes requestingservice from server processes.

In general, client/server maintains a distinction between processes and network devices. Usuallya client computer and a server computer are two separate devices, each customized for theirdesigned purpose. For example, a Web server will often contain large amounts of memory anddisk space, whereas Web clients often include features to support the graphic user interface ofthe browser such as high-end video cards and large-screen displays.

Client/server networking, however, focuses primarily on the applications rather than thehardware. The same device may function as both client and server; for example, Web serverhardware functions as both client and server when local browser sessions are run there.Likewise, a device that is a server at one moment can reverse roles and become a client to adifferent server (either for the same application or for a different application).

Client/Server ApplicationsSome of the most popular applications on the Internet follow the client/server design:

Email clients FTP (File transfer) clients

Web browsers

Each of these programs presents a user interface (either graphic- or text-based) in a clientprocess that allows the user to connect to servers. In the case of email and FTP, the user entersa computer name (or sometimes an IP address) into the interface to set up future connections tothe server process.

When using a Web browser, the name or address of the server appears in the URL of eachrequest. Although a person may start a Web surfing session by entering a particular server name(such as www.about.com), the name regularly changes as they click links on the pages. In theWeb model, the HTML content developer encoded in the anchor tags provides server information.

Client/Server at HomeMany home networkers use client/server systems without even realizing it. Microsoft's InternetConnection Sharing (ICS), for example, relies on DHCP server and client functionality built intothe operating system. Cable modem and DSL routers also include a DHCP server with thehardware unit. Many home LAN gaming applications also use a single-server/multiple-clientconfiguration.

Console TerminalThe Console-Terminal architecture is one in which the Terminal has to access the Console forevery piece of information. The terminals are also some times referred to as thin clients.

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Chapter 4 - OSI Reference Model

The OSI Reference ModelModern computer networks are designed in a highly structured way. To reduce their designcomplexity, most networks are organized as a series of layers, each one built upon itspredecessor.

The OSI Reference Model is based on a proposal developed by the International Organization forStandardization (ISO). The model is called ISO OSI (Open Systems Interconnection) ReferenceModel because it deals with connecting open systems - that is, systems that are open forcommunication with other systems.

The OSI model has seven layers. The principles that were applied to arrive at the seven layersare as follows:

A layer should be created where a different level of abstraction is needed.

Each layer should perform a well defined function.

The function of each layer should be chosen with an eye toward defining internationallystandardized protocols.

The layer boundaries should be chosen to minimize the information flow across theinterfaces.

The number of layers should be large enough that distinct functions need not be throwntogether in the same layer out of necessity, and small enough that the architecture doesnot become unwieldy.

The Seven Layers ModelSeven layers are defined:Application: Provides different services to the applicationsPresentation : Converts the informationSession: Handles problems which are not communication issuesTransport : Provides end to end communication controlNetwork: Routes the information in the networkData Link : Provides error control between adjacent nodesPhysical : Connects the entity to the transmission media

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The Application LayerThe application layer contains a variety of protocols that are commonly needed. For example,there are hundreds of incompatible terminal types in the world. Consider the plight of a full screeneditor that is supposed to work over a network with many different terminal types, each withdifferent screen layouts, escape sequences for inserting and deleting text, moving the cursor, etc.

One way to solve this problem is to define an abstract network virtual terminal for which editorsand other programs can be written to deal with. To handle each terminal type, a piece of softwaremust be written to map the functions of the network virtual terminal onto the real terminal. Forexample, when the editor moves the virtual terminal's cursor to the upper left-hand corner of thescreen, this software must issue the proper command sequence to the real terminal to get itscursor there too. All the virtual terminal software is in the application layer.

Another application layer function is file transfer. Different file systems have different file namingconventions, different ways of representing text lines, and so on. Transferring a file between twodifferent systems requires handling these and other incompatibilities. This work, too, belongs tothe application layer, as do electronic mail, remote job entry, directory lookup, and various othergeneral-purpose and special-purpose facilities.

The Presentation Layer

The presentation layer performs certain functions that are requested sufficiently often to warrantfinding a general solution for them, rather than letting each user solve the problems. In particular,unlike all the lower layers, which are just interested in moving bits reliably from here to there, thepresentation layer is concerned with the syntax and semantics of the information transmitted.

A typical example of a presentation service is encoding data in a standard, agreed upon way.Most user programs do not exchange random binary bit strings. They exchange things such aspeople's names, dates, amounts of money, and invoices. These items are represented ascharacter strings, integers, floating point numbers, and data structures composed of severalsimpler items. Different computers have different codes for representing character strings,integers and so on. In order to make it possible for computers with different representation tocommunicate, the data structures to be exchanged can be defined in an abstract way, along witha standard encoding to be used "on the wire". The job of managing these abstract data structures

and converting from the representation used inside the computer to the network standardrepresentation is handled by the presentation layer.

The presentation layer is also concerned with other aspects of information representation. Forexample, data compression can be used here to reduce the number of bits that have to betransmitted and cryptography is frequently required for privacy and authentication.

The Session LayerThe session layer allows users on different machines to establish sessions between them. Asession allows ordinary data transport, as does the transport layer, but it also provides someenhanced services useful in a some applications. A session might be used to allow a user to loginto a remote time-sharing system or to transfer a file between two machines.

One of the services of the session layer is to manage dialogue control. Sessions can allow trafficto go in both directions at the same time, or in only one direction at a time. If traffic can only goone way at a time, the session layer can help keep track of whose turn it is.

A related session service is token management. For some protocols, it is essential that both sidesdo not attempt the same operation at the same time. To manage these activities, the sessionlayer provides tokens that can be exchanged. Only the side holding the token may perform thecritical operation.

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Another session service is synchronization. Consider the problems that might occur when tryingto do a two-hour file transfer between two machines on a network with a 1 hour mean timebetween crashes. After each transfer was aborted, the whole transfer would have to start overagain, and would probably fail again with the next network crash. To eliminate this problem, thesession layer provides a way to insert checkpoints into the data stream, so that after a crash, onlythe data after the last checkpoint has to be repeated.

The Transport LayerThe basic function of the transport layer, is to accept data from the session layer, split it up intosmaller units if need be, pass these to the network layer, and ensure that the pieces all arrivecorrectly at the other end. Furthermore, all this must be done efficiently, and in a way that isolatesthe session layer from the inevitable changes in the hardware technology.

Under normal conditions, the transport layer creates a distinct network connection for eachtransport connection required by the session layer. If the transport connection requires a highthroughput, however, the transport layer might create multiple network connections, dividing thedata among the network connections to improve throughput. On the other hand, if creating ormaintaining a network connection is expensive, the transport layer might multiplex severaltransport connections onto the same network connection to reduce the cost. In all cases, thetransport layer is required to make the multiplexing transparent to the session layer.

The transport layer also determines what type of service to provide to the session layer, andultimately, the users of the network. The most popular type of transport connection is an error-free point-to-point channel that delivers messages in the order in which they were sent. However,other possible kinds of transport, service and transport isolated messages with no guaranteeabout the order of delivery, and broadcasting of messages to multiple destinations. The type ofservice is determined when the connection is established.

The transport layer is a true source-to-destination or end-to-end layer. In other words, a programon the source machine carries on a conversation with a similar program on the destinationmachine, using the message headers and control messages.

Many hosts are multi-programmed, which implies that multiple connections will be entering and

leaving each host. There needs to be some way to tell which message belongs to whichconnection. The transport header is one place this information could be put.

In addition to multiplexing several message streams onto one channel, the transport layer musktakes care of establishing and deleting connections across the network. This requires some kindof naming mechanism, so that process on one machine has a way of describing with whom itwishes to converse. There must also be a mechanism to regulate the flow of information, so thata fast host cannot overrun a slow one. Flow control between hosts is distinct from flow controlbetween switches, although similar principles apply to both.

The Network LayerThe network layer is concerned with controlling the operation of the subnet. A key design issue isdetermining how packets are routed from source to destination. Routes could be based on static

tables that are "wired into" the network and rarely changed. They could also be determined at thestart of each conversation, for example a terminal session. Finally, they could be highly dynamic,being determined anew for each packet, to reflect the current network load.

If too many packets are present in the subnet at the same time, they will get in each other's way,forming bottlenecks. The control of such congestion also belongs to the network layer.

Since the operators of the subnet may well expect remuneration for their efforts, there is oftensome accounting function built into the network layer. At the very least, the software must counthow many packets or characters or bits are sent by each customer, to produce billing information.

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When a packet crosses a national border, with different rates on each side, the accounting canbecome complicated.

When a packet has to travel from one network to another to get to its destination, many problemscan arise. The addressing used by the second network may be different from the first one. Thesecond one may not accept the packet at all because it is too large. The protocols may differ, andso on. It is up to the network layer to overcome all these problems to allow heterogeneousnetworks to be interconnected.

In broadcast networks, the routing problem is simple, so the network layer is often thin or evennonexistent.

The Data Link LayerThe main task of the data link layer is to take a raw transmission facility and transform it into aline that appears free of transmission errors in the network layer. It accomplishes this task byhaving the sender break the input data up into data frames (typically a few hundred bytes),transmit the frames sequentially, and process the acknowledgment frames sent back by thereceiver. Since the physical layer merely accepts and transmits a stream of bits without anyregard to meaning of structure, it is up to the data link layer to create and recognize frameboundaries. This can be accomplished by attaching special bit patterns to the beginning and end

of the frame. If there is a chance that these bit patterns might occur in the data, special care mustbe taken to avoid confusion.

The data link layer should provide error control between adjacent nodes.

Another issue that arises in the data link layer (and most of the higher layers as well) is how tokeep a fast transmitter from drowning a slow receiver in data. Some traffic regulation mechanismmust be employed in order to let the transmitter know how much buffer space the receiver has atthe moment. Frequently, flow regulation and error handling are integrated, for convenience.If the line can be used to transmit data in both directions, this introduces a new complication thatthe data link layer software must deal with. The problem is that the acknowledgment frames for Ato B traffic compete for the use of the line with data frames for the B to A traffic.

The Data Link Layer: Error ControlA noise burst on the line can destroy a frame completely. In this case, the data link layer softwareon the source machine must retransmit the frame. However, multiple transmissions of the sameframe introduce the possibility of duplicate frames. A duplicate frame could be sent, for example,if the acknowledgment frame from the receiver back to the sender was destroyed. It is up to thislayer to solve the problems caused by damaged, list, and duplicate frames. The data link layermay offer several different service classes to the network layer, each of a different quality andwith a different price.

The Physical LayerThe physical later is concerned with transmitting raw bits over a communication channel. Thedesign issues have to do with making sure that when one side sends a 1 bit, it is received by theother side as a 1 bit, not as a 0 bit. Typical questions here are how many volts should be used to

represent a 1 and how many for a 0, how many microseconds a bit lasts, whether transmissionmay proceed simultaneously in both directions, how the initial connection is established and howit is torn down when both sides are finished, and how many pins the network connector has andwhat each pin is used for. The design issues here deal largely with mechanical, electrical, andprocedural interfaces, and the physical transmission medium, which lies below the physical layer.Physical layer design can properly be considered to be within the domain of the electricalengineer.

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Sending Data via the OSI ModelEach layer acts as though it is communicating with its corresponding layer on the other end.

In reality, data is passed from one layer down to the next lower layer at the sending computer, tillthe Physical Layer finally transmits the data onto the network cable. As the data it passed downto a lower layer, it is encapsulated into a larger unit (in effect, each layer adds its own layerinformation to that which it receives from a higher layer). At the receiving end, the message ispassed upwards to the desired layer, and as it passes upwards through each layer, theencapsulation information is stripped off.

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Chapter 5 LAN Protocols & Standards

Data Link Layers Protocols (LLC and MAC)The logical link layer deals with the transmission offrames. Each frame is a packet of data with asequence number that is used to ensure delivery and a checksum to track corrupted frames.Several algorithms are available for use to acknowledge delivery of packages. The basic protocolworks as follows.

The sending station1. The sending station sends one or more frames with a sequence numbers them.2. The sending station awaits acknowledgements of the sent frames before transmitting

further frames.3. If no acknowledgement for a particular frame arrives within a fixed time the frame is

retransmitted

The Receiving StationThe receiving station acknowledges uncorrupted frames as they are received. Note that theremust be a large enough range for the sequence numbers so that the receiving station candistinguish between resends (this will happen if an ACK gets lost) and new frames. Threeprotocols for the data link layer.

1. Stop and Wait. This requires only two sequence numbers2. Go back n3. Selective repeat

The last two allow several frames to be in transit.

The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer intotwo sublayers: Logical Link Control (LLC) and Media Access Control (MAC).

The Logical Link Control (LLC) sublayer of the data link layer manages communications betweendevices over a single link of a network. LLC is defined in the IEEE 802.2 specification andsupports both connectionless and connection-oriented services used by higher-layer protocols.IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layerprotocols to share a single physical data link. The Media Access Control (MAC) sublayer of thedata link layer manages protocol access to the physical network medium. The IEEE MACspecification defines MAC addresses, which enable multiple devices to uniquely identify oneanother at the data link layer.

Data Link Layer AddressesA data link layer address uniquely identifies each physical network connection of a networkdevice. Data-link addresses sometimes are referred to as physical or hardware addresses. Data-link addresses usually exist within a flat address space and have a pre-established and typicall

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