Data communications and computer networks curt white pdf

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S E V E N T H E D I T I O N

Data Communications and Computer Networks A Business User’s Approach

Curt M. White DePaul University

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Data Communications and Computer Networks: A Business User’s Approach, Seventh Edition Curt M. White

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To Kathleen, Hannah Colleen, and Samuel Memphis—it’s never boring

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Brief Contents

PREFACE xv

1 Introduction to Computer Networks and Data Communications 1

2 Fundamentals of Data and Signals 29 3 Conducted and Wireless Media 63 4 Making Connections 103 5 Making Connections Efficient:

Multiplexing and Compression 121 6 Errors, Error Detection, and Error Control 149 7 Local Area Networks: Part 1 175 8 Local Area Networks: Part II 207 9 Introduction to Metropolitan Area

Networks and Wide Area Networks 241 10 The Internet 269 11 Voice and Data Delivery Networks 307 12 Network Security 339 13 Network Design and Management 373

GLOSSARY 401

INDEX 415

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Contents

PREFACE xv

1 Introduction to Computer Networks and Data Communications 1 The Language of Computer Networks 3 The Big Picture of Networks 4 Communications Networks—Basic Layouts 5

Microcomputer-to-local area network layouts 6 Microcomputer-to-Internet layouts 7 Local area network-to-local area network layouts 7 Personal area network-to-workstation layouts 8 Local area network-to-metropolitan area network layouts 9 Local area network-to-wide area network layouts 9 Wide area network-to-wide area network layouts 10 Sensor-to-local area network layouts 10 Satellite and microwave layouts 11 Cell phone layouts 11 Terminal/microcomputer-to-mainframe computer layouts 12

Convergence 13 Network Architectures 14

The TCP/IP protocol suite 15 The OSI model 18 Logical and physical connections 20

Network Layouts in Action 22 The TCP/IP Protocol Suite in Action 23 Summary 24 Key Terms 26 Review Questions 26 Exercises 26 Thinking Outside the Box 27 Hands-On Projects 27

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2 Fundamentals of Data and Signals 29 Data and Signals 31

Analog vs. digital 32 Fundamentals of signals 35

Converting Data into Signals 39 Transmitting analog data with analog signals 40 Transmitting digital data with digital signals: digital encoding schemes 40 Transmitting digital data with discrete analog signals 45 Transmitting analog data with digital signals 48

Data Codes 51 EBCDIC 53 ASCII 54 Unicode 55

Data and Signal Conversions in Action: Two Examples 56 Summary 58 Key Terms 58 Review Questions 59 Exercises 59 Thinking Outside the Box 60 Hands-On Projects 61

3 Conducted and Wireless Media 63 Conducted Media 64

Twisted pair wire 64 Coaxial cable 69 Fiber-optic cable 70

Wireless Media 74 Media Selection Criteria 91 Conducted Media in Action: Two Examples 94 Wireless Media in Action: Three Examples 96 Summary 99 Key Terms 99 Review Questions 100 Exercises 100 Thinking Outside the Box 101 Hands-On Projects 102

4 Making Connections 103 Interfacing a Computer to Peripheral Devices 104

Characteristics of interface standards 105 An early interface standard 106 Universal Serial Bus (USB) 106 Other interface standards 108

Data Link Connections 110 Asynchronous connections 110 Synchronous connections 112 Isochronous connections 113

Terminal-to-Mainframe Computer Connections 113 Making Computer Connections in Action 115 Summary 116 Key Terms 117 Review Questions 117 Exercises 118 Thinking Outside the Box 118 Hands-On Projects 119

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5 Making Connections Efficient: Multiplexing and Compression 121 Frequency Division Multiplexing 122 Time Division Multiplexing 125

Synchronous time division multiplexing 125 Statistical time division multiplexing 130

Wavelength Division Multiplexing 131 Discrete Multitone 133 Code Division Multiplexing 134 Comparison of Multiplexing Techniques 136 Compression—Lossless vs. Lossy 137

Lossless compression 138 Lossy compression 140

Business Multiplexing in Action 144 Summary 145 Key Terms 146 Review Questions 146 Exercises 146 Thinking Outside the Box 147 Hands-On Projects 148

6 Errors, Error Detection, and Error Control 149 Noise and Errors 151

White noise 151 Impulse noise 152 Crosstalk 152 Echo 153 Jitter 153 Attenuation 154

Error Prevention 154 Error Detection 155

Parity checks 156 Arithmetic checksum 158 Cyclic redundancy checksum 159

Error Control 161 Toss the frame/packet 162 Return a message 162 Correct the error 168

Error Detection in Action 170 Summary 171 Key Terms 172 Review Questions 172 Exercises 173 Thinking Outside the Box 173 Hands-On Projects 174

7 Local Area Networks: Part 1 175 Primary Function of Local Area Networks 176 Advantages and Disadvantages of Local Area Networks 178 The First Local Area Network: The Bus/Tree 180 A More Modern LAN 182

Contention-based protocols 184

Contents ix

Switches 186 Isolating traffic patterns and providing multiple access 190 Full-duplex switches 190 Virtual LANs 191 Link aggregation 192 Spanning tree algorithm 192 Quality of service 194

Wired Ethernet 194 Wired Ethernet Frame Format 197 LANs in Action: A Small Office Solution 198 Summary 201 Key Terms 202 Review Questions 203 Exercises 203 Thinking Outside the Box 204 Hands-On Projects 205

8 Local Area Networks: Part II 207 Wireless Ethernet 209

Wireless LAN standards 211 Wireless CSMA/CA 212 CSMA/CA frame format 214

Network Operating Systems 215 Network Operating Systems Past and Present 216

Novell NetWare 217 Microsoft Windows NT and Windows Server 2000, 2003, and 2008 220 UNIX 223 Linux 223 Mac OS X Server 224

Servers 225 Client/server networks vs. peer-to-peer networks 227

Network Support Software 227 Utilities 228 Internet software 230

Software Licensing Agreements 230 LAN Support Devices 232 Lan Software in Action: A Small Company Makes a Choice 234

Primary uses of current system 234 Network maintenance and support 234 Cost of the NOS 235 Any unique hardware choices affecting NOS decision 235 Single location or multiple locations 235 Political pressures affecting decision 236 Final decision 236

Wireless Networking in Action: Creating a Wireless LAN for Home 236 Summary 237 Key Terms 238 Review Questions 239 Exercises 239 Thinking Outside the Box 240 Hands-On Projects 240

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9 Introduction to Metropolitan Area Networks and Wide Area Networks 241 Metropolitan Area Network Basics 242

SONET vs. Ethernet 244

Wide Area Network Basics 247 Types of network clouds 249 Connection-oriented vs. connectionless network applications 252

Routing 254 Dijkstra’s least-cost algorithm 256 Flooding 257 Centralized vs. distributed routing 258 Adaptive vs. fixed routing 260 Routing examples 261

Network Congestion 261 The problems associated with network congestion 262 Possible solutions to congestion 262

WANs in Action: The Smartphone 264 Summary 265 Key Terms 266 Review Questions 266 Exercises 267 Thinking Outside the Box 268 Hands-On Projects 268

10 The Internet 269 Internet Protocols 271

The Internet Protocol 272 Internet Protocol version 6 277 The Transmission Control Protocol 280 The Internet Control Message Protocol 282 User Datagram Protocol 282 The Address Resolution Protocol 283 The Dynamic Host Configuration Protocol 284 Network Address Translation 284 Tunneling protocols and virtual private networks 285

The World Wide Web 286 Locating a document on the Internet 287

Internet Services 289 Electronic mail (e-mail) 289 The File Transfer Protocol 290 Remote login (Telnet) 292 Voice over IP 292 Listservs 295 Streaming audio and video 295 Instant messages, tweets, and blogs 295

The Internet and Business 296 Cookies and state information 297 Intranets and extranets 297

The Future of the Internet 298 The Internet in Action: A Company Creates a VPN 299 Summary 301 Key Terms 302 Review Questions 303

Contents xi

Exercises 303 Thinking Outside the Box 304 Hands-On Projects 304

11 Voice and Data Delivery Networks 307 The Basic Telephone System 308

Telephone lines and trunks 308 The telephone network before and after 1984 310 Telephone networks after 1996 311 Limitations of telephone signals 312 Dial-up Internet service 313

Digital Subscriber Line 314 DSL basics 315 DSL formats 316

Cable Modems 317 T-1 Leased Line Service 318 Frame Relay 319

Committed information rate or service level agreements 321

Asynchronous Transfer Mode 322 ATM classes of service 323 Advantages and disadvantages of ATM 325

MPLS and VPN 325 Summary of the Data Delivery Services 326 Convergence 327

Computer-telephony integration 328 Unified communications 330

Telecommunications Systems in Action: A Company Makes a Service Choice 330

Prices 330 Making the choice 330

Summary 333 Key Terms 335 Review Questions 335 Exercises 336 Thinking Outside the Box 337 Hands-On Projects 337

12 Network Security 339 Standard System Attacks 340 Physical Protection 343 Controlling Access 344

Passwords and ID systems 346 Access rights 347 Auditing 349

Securing Data 350 Basic encryption and decryption techniques 350

Securing Communications 359 Spread spectrum technology 359 Guarding against viruses 361 Firewalls 362 Wireless security 365

Security Policy Design Issues 365 Network Security in Action: Making Wireless LANs Secure 367

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Summary 368 Key Terms 370 Review Questions 370 Exercises 371 Thinking Outside the Box 371 Hands-On Projects 372

13 Network Design and Management 373 Systems Development Life Cycle 374 Network Modeling 376

Wide area connectivity map 377 Metropolitan area connectivity map 378 Local area connectivity map 378

Feasibility Studies 379 Capacity Planning 382 Creating a Baseline 385 Network Administrator Skills 388 Generating Usable Statistics 389 Network Diagnostic Tools 390

Tools that test and debug network hardware 390 Network sniffers 391 Managing operations 391 Simple network management protocol 392

Capacity Planning and Network Design in Action: Better Box Corporation 394 Summary 396 Key Terms 397 Review Questions 398 Exercises 398 Thinking Outside the Box 399 Hands-On Projects 399

GLOSSARY 401

INDEX 415

Contents xiii

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Preface

Today’s business world could not function without data communications and computer networks. Most people cannot make it through an average day without coming in contact with or using some form of computer network. In the past, this field of study occupied the time of only engineers and technicians, but it now in- volves business managers, end users, programmers, and just about anyone who might use a telephone or computer! Because of this, Data Communications and Computer Networks: A Business User’s Approach, Seventh Edition maintains a business user’s perspective on this vast and increasingly significant subject.

In a generic sense, this book serves as an owner’s manual for the individual computer user. In a world in which computer networks are involved in nearly every facet of business and personal life, it is paramount that each of us under- stands the basic features, operations, and limitations of different types of com- puter networks. This understanding will make us better managers, better employees, and simply better computer users. As a computer network user, you will probably not be the one who designs, installs, and maintains the network. Instead, you will have interactions—either direct or indirect—with the indivi- duals who do. Reading this book should give you a strong foundation in com- puter networks, which will enable you to work effectively with network administrators, network installers, and network designers.

Here are some of the many scenarios in which the knowledge contained in this book would be particularly useful:

■ You work for a company and must deal directly with a network specialist. To better understand the specialist and be able to conduct a meaningful dia- log with him or her, you need a basic understanding of the many aspects of computer networks.

■ You are a manager within a company and depend on a number of network specialists to provide you with recommendations for the company’s network. You do not want to find yourself in a situation in which you must blindly accept the recommendations of network professionals. To ensure that you can make intelligent decisions regarding network resources, you need to know the basic concepts of data communications and computer networks.

■ You work in a small company, in which each employee wears many hats. Thus, you may need to perform some level of network assessment, administration, or support.

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■ You have your own business and need to fully understand the advantages of using computer networks to support your operations. To optimize those advantages, you should have a good grasp of the basic characteristics of a computer network.

■ You have a computer at home or at work, and you simply wish to learn more about computer networks.

■ You have realized that to keep your job skills current and remain a key player in the information technology arena, you must understand how different computer networks work and become familiar with their advantages and shortcomings.

Audience Data Communications and Computer Networks: A Business User’s Approach, Seventh Edition is intended for a one-semester course in business data commu- nications for students majoring in business, information systems, management information systems, and other applied fields of computer science. Even computer science departments will find the book valuable, particularly if the students read the Details sections accompanying most chapters. It is a readable resource for computer network users that draws on examples from business environments.

In a university setting, this book can be used at practically any level above the first year. Instructors who wish to use this book at the graduate level can draw on the many advanced projects provided at the end of each chapter to cre- ate a more challenging environment for the advanced student.

Defining Characteristics of This Book The major goal of this seventh edition is the same as that of the first edition: to go beyond simply providing readers with a handful of new definitions, and in- stead introduce them to the next level of details found within the fields of com- puter networks and data communications. This higher level of detail includes the network technologies and standards necessary to support computer network systems and their applications. This book is more than just an introduction to advanced terminology. It involves introducing concepts that will help the reader achieve a more in-depth understanding of the often complex topic of data com- munications. It is hoped that once readers attain this in-depth understanding, the topic of networks and data communications will be less intimidating to them. To facilitate this understanding, the book strives to maintain high stan- dards in three major areas: readability, a balance between the technical and the practical, and currency.

Readability Great care has been taken to provide the technical material in as readable a fashion as possible. Each new edition has received a complete rewrite, in which every sentence has been re-examined in an attempt to convey the concepts as clearly as possible. Given the nature of this book’s subject matter, the use of ter- minology is unavoidable. However, every effort has been made to present terms in a clear fashion, with minimal use of acronyms and even less use of computer jargon.

Balance Between the Technical and the Practical As in the very successful first edition, a major objective in writing Data Communications and Computer Networks, Seventh Edition was to achieve a good balance between the more technical aspects of data communications and its everyday practical aspects. Throughout each chapter, there are sections entitled

xvi Preface

“Details,” which delve into the more specialized aspects of the topic at hand. Should readers not have time to explore this technical information, they can skip these Details sections without missing out on the basic concepts of the topic.

Current Technology Because of the fast pace of change in virtually all computer-related fields, every attempt has been made to present the most current trends in data communica- tions and computer networks. Some of these topics include:

■ Introduction to Thunderbolt interface ■ Updated information on IPv6 ■ Spanning tree algorithm, link aggregation, and quality of service for LANs ■ Latest wireless technologies ■ Updated examples on multiplexing techniques ■ Greater emphasis on switching in local area networks ■ Advanced encryption standards ■ Compression techniques ■ Current LAN network operating systems (Windows Server®, UNIX®and

Linux®) ■ Introduction to cloud computing

It is also important to remember the many older technologies still in preva- lent use today. Discussions of these older technologies can be found, when appropriate, in each chapter of this book.

Organization The organization of Data Communications and Computer Networks, Seventh Edition roughly follows that of the TCP/IP protocol suite, from the physical layer to the upper layers. In addition, the book has been carefully designed to consist of 13 chapters in order to fit well into a typical 15- or 16-week semester (along with any required exams). Although some chapters may not require an entire week of study, other chapters may require more than one week. The in- tent was to design a balanced introduction to the study of computer networks by creating a set of chapters that is cohesive but at the same time allows for flexibility in the week-to-week curriculum.

Thus, instructors may choose to emphasize or de-emphasize certain topics, depending on the focus of their curriculums. If all 13 chapters cannot be covered during one term, it is possible for the instructor to concentrate on certain chapters. For example, if the curriculum’s focus is information systems, the instructor might concentrate on Chapters 1, 3, 4, 6–8, 10, 12, and 13. If the focus is on the more technical aspects of computer networks, the instructor might concentrate on Chapters 1–11. It is the author’s recommendation, however, that all chapters be covered in some level of detail.

Features To assist readers in better understanding the technical nature of data communi- cations and computer networks, each chapter contains a number of significant features. These features are based on older, well-tested pedagogical techniques as well as some newer techniques.

Opening Case Each chapter begins with a short case or vignette that emphasizes the main con- cept of the chapter and sets the stage for exploration. These cases are designed to spark readers’ interest and create a desire to learn more about the chapter’s concepts.

Preface xvii

Learning Objectives Following the opening case is a list of learning objectives that should be accom- plished by the end of the chapter. Each objective is tied to the main sections of the chapter. Readers can use the objectives to grasp the scope and intent of the chapter. The objectives also work in conjunction with the end-of-chapter summary and review questions, so that readers can assess whether they have adequately mastered the material.

Details Many chapters contain one or more Details sections, which dig deeper into a particular topic. Readers who are interested in more technical details will find these sections valuable. Since the Details sections are physically separate from the main text, they can be skipped if the reader does not have time to explore this level of technical detail. Skipping these sections will not affect the reader’s overall understanding of a chapter’s material.

In Action At the end of each chapter’s main content presentation is an In Action example that demonstrates an application of the chapter’s key topic in a realistic environ- ment. Although a number of In Action examples include imaginary people and organizations, every attempt was made to make the hypothetical scenarios as representative as possible of situations and issues found in real-world business and home environments. Thus, the In Action examples help the reader visualize the concepts presented in the chapter.

End-of-Chapter Material The end-of-chapter material is designed to help readers review the content of the chapter and assess whether they have adequately mastered the concepts. It includes:

■ A bulleted summary that readers can use as a review of the key topics of the chapter and as a study guide.

■ A list of the key terms used within the chapter. ■ A list of review questions that readers can use to quickly check whether or

not they understand the chapter’s key concepts. ■ A set of exercises that draw on the material presented in the chapter. ■ A set of Thinking Outside the Box exercises, which are more in-depth in na-

ture and require readers to consider various possible alternative solutions by comparing their advantages and disadvantages.

■ A set of Hands-On Projects that require readers to reach beyond the material found within the text and use outside resources to compose a response. Many of these projects lend themselves nicely to writing assignments. Thus, they can serve as valuable tools for instructors, especially at a time when more and more colleges and universities are seeking to implement “writing across the curriculum” strategies.

Glossary At the end of the book, you will find a glossary that includes the key terms from each chapter.

Student Online Companion The student online companion for this book can be found at www. cengagebrain.com, and search by title, author name, or ISBN. It contains a number of features, including:

■ Hands-on labs that allow students to practice one or more of the chapter concepts ■ A set of more in-depth discussions on older topics such as X.21, dial-up

modems, ISDN, Dijkstra’s algorithm, SDLC, and BISYNC ■ Suggestions for further readings on numerous topics within the book

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www.cengagebrain.com
www.cengagebrain.com
This Web site also presents visual demonstrations of many key data commu- nications and networking concepts introduced in this text. A visual demonstration accompanies the following concepts:

■ Chapter One: Introduction to Computer Networks and Data Communications— Layer encapsulation example

■ Chapter Four: Making Connections—RS-232 example of two modems establishing a connection

■ Chapter Five: Making Connections Efficient: Multiplexing and Compression— Example of packets from multiple sources coming together for synchronous TDM, and a second example demonstrating statistical TDM

■ Chapter Six: Errors, Error Detection, and Error Control—Sliding window example using ARQ error control

■ Chapter Seven: Local Area Networks: Part One—CSMA/CD example with workstations sending packets and collisions happening

■ Chapter Seven: Local Area Networks: Part One—Two LANs with a bridge showing how bridge tables are created and packets routed; a second example shows one LAN with a switch in place of a hub

■ Chapter Nine: Introduction to Metropolitan Area Networks and Wide Area Networks—Datagram network sending individual packets; and virtual circuit network first creating a connection and then sending packets down a prescribed path

■ Chapter Ten: The Internet—Domain Name System as it tries to find the dotted decimal notation for a given URL

Changes to the Seventh Edition In order to keep abreast of the changes in computer networks and data commu- nications, this Seventh Edition has incorporated many updates and additions in every chapter, as well as some reorganization of sections within chapters. Here’s a summary of the major changes that can be found in each of the following chapters:

Chapter One, Introduction to Computer Networks and Data Communications, introduces an update on the many types of computer network connections, along with many of the major concepts that will be discussed in the following chapters, with an emphasis on the TCP/IP protocol suite followed by the OSI models. The topic of convergence has been introduced in this first chapter and will be revisited as needed in subsequent chapters.

Chapter Two, Fundamentals of Data and Signals, covers basic concepts that are critical to the proper understanding of all computer networks and data communications.

Chapter Three, Conducted and Wireless Media, introduces the different types of media for transmitting data. The section on cellular telephones was updated to include the latest cell phone technologies.

Chapter Four, Making Connections, discusses how a connection or interface is created between a computer and a peripheral device, with a stronger emphasis on the USB interface.

Chapter Five, Making Connections Efficient: Multiplexing and Compression, introduces the topic of compression. Lossless compression techniques such as run-length encoding are discussed, as well as lossy compression techniques such as MP3 and JPEG. Examples of multiplexing have been updated.

Chapter Six, Errors, Error Detection, and Error Control, explains the actions that can take place when a data transmission produces an error. The concept of arithmetic checksum, as it is used on the Internet, is included.

Chapter Seven, Local Area Networks: Part One, is devoted to the basic con- cepts of local area networks. These two chapters on local area networks have been reorganized. The topics of minimum spanning tree, link aggregation, and

Preface xix

quality of service have been introduced. The local area network switch has been given more prominence, to reflect its current importance in the industry.

Chapter Eight, Local Area Networks: Part Two, introduces wireless local area networks and discusses the various network operating systems and other network software, with updated material on Microsoft, Linux, Unix, and the MAC OS X Server.

Chapter Nine, Introduction to Metropolitan Area Networks and Wide Area Networks, introduces the basic terminology and concepts of both metropolitan area networks and wide area networks. Cloud computing is also introduced.

Chapter Ten, The Internet, delves into the details of the Internet, including TCP/IP, DNS, and the World Wide Web. Additional information on IP addresses and IPv6 has been included. A discussion on the topic of Voice over IP is included, as well as the material on MPLS, service level agreements, and convergence.

Chapter Eleven, Voice and Data Delivery Networks, provides a detailed introduction to the area of telecommunications—in particular, networks that specialize in local and long-distance delivery of data. The topic of basic dial-up telephone service was reduced to better reflect its diminishing importance in today’s technology markets.

Chapter Twelve, Network Security, covers the current trends in network security. The topic of firewalls was updated.

Chapter Thirteen, Network Design and Management, introduces the systems development life cycle, feasibility studies, capacity planning, and base- line studies, and shows how these concepts apply to the analysis and design of computer networks.

Teaching Tools The following supplemental materials are available when this book is used in a classroom setting. All of the teaching tools available with this book are provided to the instructor on a single CD-ROM. Many can also be found at the Cengage Web site (login.cengage.com/sso).

Electronic Instructor’s Manual—The Instructor’s Manual that accompanies this textbook includes additional instructional material to assist in class preparation, including Sample Syllabi, Chapter Outlines, Technical Notes, Lecture Notes, Quick Quizzes, Teaching Tips, Discussion Topics, and Key Terms.

ExamView®—This textbook is accompanied by ExamView, a powerful testing software package that allows instructors to create and administer printed, computer (LAN-based), and Internet exams. ExamView includes hundreds of questions that correspond to the topics covered in this text, enabling students to generate detailed study guides that include page references for further review. The computer-based and Internet testing components allow students to take exams at their computers and also save the instructor time by grading each exam automatically.

PowerPoint Presentations—This book comes with Microsoft PowerPoint slides for each chapter. These are included as a teaching aid for classroom presentation, to make available to students on the network for chapter review, or to be printed for classroom distribution. Instructors can add their own slides for additional topics they introduce to the class.

Acknowledgments Producing a textbook requires the skills and dedication of many people. Unfortu- nately, the final product displays only the author’s name on the cover and not the names of those who provided countless hours of input and professional advice. I would first like to thank the people at Course Technology for being so vitally

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login.cengage.com/sso
supportive and one of the best teams an author could hope to work with: Charles McCormick, Jr., Senior Acquisitions Editor; Kate Mason, Senior Product Manager; and Divya Divakaran, Content Product Manager.

I must also thank my colleagues at DePaul University who listened to my problems, provided ideas for exercises, proofread some of the technical chapters, and provided many fresh ideas when I could think of none myself.

Finally, I thank my family: my wife Kathleen, my daughter Hannah, and my son Samuel. It was your love and support (again!) that kept me going, day after day, week after week, and month after month.

Curt M. White

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Chapter 1 Introduction to Computer Networks and Data Communications

MAKING PREDICTIONS is a difficult task, and predicting the future of computing is no exception. History is filled with computer-related predictions that were so inaccurate that today they are amusing. For example, consider the following predictions:

“I think there is a world market for maybe five computers.” Thomas Watson, chairman of IBM, 1943

“I have traveled the length and breadth of this country, and talked with the best people, and I can assure you that data processing is a fad that won’t last out the year.” Editor in charge of business books for Prentice Hall, 1957

“There is no reason anyone would want a computer in their home.” Ken Olsen, president and founder of Digital Equipment Corporation, 1977

“640K ought to be enough for anybody.” Bill Gates, 1981

“We believe the arrival of the PC’s little brother [PCjr] is as significant and lasting a develop- ment in the history of computing as IBM’s initial foray into microcomputing has proven to be.” PC Magazine, December 1983 (The PCjr lasted less than one year.)

Apparently, no matter how famous you are or how influential your position, it is very easy to make very bad predictions. Nevertheless, it is hard to imagine that anyone can make a prediction worse than any of those above. Buoyed by this false sense of optimism, let us make a few forecasts of our own:

Someday before you head out the door, you will reach for your umbrella, and it will tell you what kind of weather to expect outside. A radio signal will connect the umbrella to a local weather service that will download the latest weather conditions for your convenience.

Someday you will be driving a car, and if you go faster than some predetermined speed, the car will send a text message to your parents informing them of your “driving habits.”

Someday we will wear a computer—like a suit of clothes—and when we shake hands with a person, data will transfer down our skin, across the shaking hands, and into the other person’s “computer.”

Sometime in the not too distant future, you will place some hot dogs and hamburgers on the grill and then go inside to watch the ball game. Suddenly, you will get a message on your cell phone: “Your food is done cooking.”

OBJECTIVES After reading this chapter, you should be able to:

Ê Define the basic terminology of computer networks

Ê Recognize the individual components of the big picture of computer networks

Ê Recognize the basic network layouts

Ê Define the term “convergence” and describe how it applies to computer networks

Ê Cite the reasons for using a network architecture and explain how they apply to current network systems

Ê List the layers of the TCP/IP protocol suite and describe the duties of each layer

Ê List the layers of the OSI model and describe the duties of each layer

Ê Compare the TCP/IP protocol suite and OSI model, and list their differences and similarities

1

Someday you will have a car battery that, when the power in the battery gets too weak to start the car, will call you on your cell phone to inform you that you need a replacement or a charge.

One day you will be in a big city and place a call on your cell phone to request a taxi. The voice on the other end will simply say, “Stay right where you are. Do you see the taxi coming down the street? When it stops in front of you, hop in.”

Someday you will be driving in a big city and your phone or Global Positioning System (GPS) device will tell you where the nearest empty parking spot on the street is.

Do these predictions sound far-fetched and filled with mysterious technologies that only scientists and engineers can understand? They shouldn’t, because they are not predictions. They are scenar- ios happening today with technologies that already exist. What’s more, none of these advances would be possible today were it not for computer networks and data communications.

INTRODUCTION

The world of computer networks and data communications is a surprisingly vast and increasingly significant field of study. Once considered primarily the domain of network engineers and technicians, computer networks now involve business managers, computer programmers, system designers, office managers, home computer users, and everyday citizens. It is virtually impossible for the average person on the street to spend 24 hours without directly or indirectly using some form of computer network.

Ask any group, “Has anyone used a computer network today?,” and more than one-half of the people might answer, “Yes.” Then ask the others, “How did you get to work, school, or the store today if you did not use a computer net- work?” Most transportation systems use extensive communication networks to monitor the flow of vehicles and trains. Expressways and highways have computer- ized systems for controlling traffic signals and limiting access during peak traffic times. Some major cities are placing the appropriate hardware inside city buses and trains so that the precise location of each bus and train is known. This information enables the transportation systems to keep the buses evenly spaced and more punc- tual, and allows the riders to know when the next bus or train will arrive.

In addition, more and more people are using satellite-based GPS devices in their cars to provide driving directions and avoid traffic hotspots. Similar systems can unlock your car doors if you leave your keys in the ignition and can locate your car in a crowded parking lot—beeping the horn and flashing the headlights if you cannot remember where you parked.

But even if you didn’t use mass transit or a GPS device in your car today, there are many other ways to use a computer network. Businesses can order parts and inventory on demand and build products to customer-designed specifi- cations electronically, without the need for paper. Online retail outlets can track every item you look at or purchase. Using this data, they can make recommen- dations of similar products and inform you in the future when a new product becomes available. Twenty-four-hour banking machines can verify the user’s identity by taking the user’s thumbprint.

In addition, cable television continues to expand, offering extensive pro- gramming, pay-per-view options, video recording, digital television and music, and multi-megabit connectivity to the Internet. The telephone system, the oldest and most extensive network of communicating devices, continues to become more of a computer network every day. The most recent “telephone” networks can now deliver voice, Internet, and television over a single connection. Cellular telephone systems cover virtually the entire North American continent and allow

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users to upload and download data to and from the Internet, send and receive images, and download streaming video such as television programs. That hand- held device you are holding can play music, make phone calls, take pictures, surf the Web, and let you play games while you wait for the next train.

Welcome to the amazing world of computer networks! Unless you have spent the last 24 hours in complete isolation, it is nearly impossible to not have used some form of computer network and data communications. Because of this growing integration of computer networks and data communications into business and life, we cannot leave this area of study to technicians. All of us— particularly information systems, business, and computer science students—need to understand the basic concepts. Armed with this knowledge, we not only will be better at communicating with network specialists and engineers but also will become better students, managers, and employees.

THE LANGUAGE OF COMPUTER NETWORKS

Over the years, numerous terms and definitions relating to computer networks and data communications have emerged. To gain insight into the many subfields of study, and to become familiar with the emphasis of this textbook, let us examine the more common terms and their definitions.

A computer network is an interconnection of computers and computing equipment using either wires or radio waves and can share data and computing resources. Computer networks that use radio waves are termed wireless and can involve broadcast radio, microwaves, or satellite transmissions. Networks span- ning an area of several meters around an individual are called personal area networks (PANs). Personal area networks include devices such as laptop com- puters, personal digital assistants, and wireless connections. Networks that are a little larger in geographic size—spanning a room, a floor within a building, a building, or a campus—are local area networks (LANs). Networks that serve an area up to roughly 50 kilometers—approximately the area of a typical city— are called metropolitan area networks (MANs). Metropolitan area networks are high-speed networks that interconnect businesses with other businesses and the Internet. Large networks encompassing parts of states, multiple states, countries, and the world are wide area networks (WANs). Chapters Seven and Eight concentrate on local area networks, and Chapters Nine, Ten, and Eleven concentrate on metropolitan area networks and wide area networks.

The study of computer networks usually begins with the introduction of two important building blocks: data and signals. Data is information that has been translated into a form more conducive to storage, transmission, and calculation. As we shall see in Chapter Two, a signal is used to transmit data. We define data com- munications as the transfer of digital or analog data using digital or analog signals. Once created, these analog and digital signals then are transmitted over conducted media or wireless media (both of which are discussed in Chapter Three).

Connecting devices to a computer, or a computer to a network, requires in- terfacing, a topic covered in Chapter Four. Because sending only one signal over a medium at one time can be an inefficient way to use the transmission medium, many systems perform multiplexing. Multiplexing is the transmission of multiple signals on one medium. For a medium to transmit multiple signals simulta- neously, the signals must be altered so that they do not interfere with one an- other. Compression is another technique that can maximize the amount of data sent over a medium. Compression involves squeezing data into a smaller package, thus reducing the amount of time (as well as storage space) needed to transmit the data. Multiplexing and compression are covered in detail in Chapter Five.

When the signals transmitted between computing devices are corrupted and errors result, error detection and error control are necessary. These topics are discussed in detail in Chapter Six.

Introduction to Computer Networks and Data Communications 3

Once upon a time, a voice network transmitted telephone signals, and a data network transmitted computer data. Eventually, however, the differences between voice networks and data networks disappeared. The merging of voice and data networks is one example of convergence, an important topic that will be presented later in this chapter and further developed in subsequent chapters.

Computer security (covered in Chapter Twelve) is a growing concern of both professional computer support personnel and home computer users with Internet connections. Network management is the design, installation, and sup- port of a network and its hardware and software. Chapter Thirteen discusses many of the basic concepts necessary to support properly the design and improvement of network hardware and software, as well as the more common management techniques used to support a network.

THE BIG PICTURE OF NETWORKS

If you could create one picture that tries to give an overview of a typical computer network, what might this picture include? Figure 1-1 shows such a picture, and it includes examples of local, personal, and wide area networks. Note that this picture shows two different types of local area networks (LAN 1 and LAN 2). Although a full description of the different components constitut- ing a local area network is not necessary at this time, it is important to note that most LANs include the following hardware:

■ Workstations, which are personal computers/microcomputers (desktops, laptops, netbooks, handhelds, etc.) where users reside

■ Servers, which are the computers that store network software and shared or private user files

■ Switches, which are the collection points for the wires that interconnect the workstations

■ Routers, which are the connecting devices between local area networks and wide area networks

Figure 1-1 An overall view of the interconnection between different types of networks

User A

WAN 1 Modem

Microwave Tower

LAN 1 LAN 2

PAN 1

Web Server

PDA Workstations

Routers

Switch

WAN 2

Routers

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There are also many types of wide area networks. Although many different technologies are used to support wide area networks, all wide area networks include the following components:

■ Nodes, which are the computing devices that allow workstations to connect to the network and that make the decisions about where to route a piece of data

■ Some type of high-speed transmission line, which runs from one node to another

■ A subnetwork, or cloud, which consists of the nodes and transmission lines, collected into a cohesive unit

To see how the local area networks and wide area networks work together, consider User A (in the upper-left corner of Figure 1-1), who wishes to retrieve a Web page from the Web server shown in the lower-right corner. To do this, User A’s computer must have both the necessary hardware and software required to communicate with the first wide area network it encoun- ters, WAN 1—User A’s Internet service provider. Assuming that User A’s com- puter is connected to this wide area network through a DSL telephone line, User A needs some type of modem. Furthermore, if this wide area network is part of the Internet, User A’s computer requires software that talks the talk of the Internet: TCP/IP (Transmission Control Protocol/Internet Protocol).

Notice that no direct connection exists between WAN 1, where User A resides, and LAN 2, where the Web server resides. To ensure that User A’s Web page request reaches its intended receiver (the Web server), User A’s soft- ware attaches the appropriate address information that WAN 1 uses to route User A’s request to the router that connects WAN 1 to LAN 1. Once the request is on LAN 1, the switch-like device connecting LAN 1 and LAN 2 uses address information to pass the request to LAN 2. Additional address information then routes User A’s Web page request to the Web server, whose software accepts the request.

Under normal traffic and conditions, this procedure might take only a frac- tion of a second. When you begin to understand all the steps involved and the great number of transformations that a simple Web page request must undergo, the fact that it takes only a fraction of a second to deliver is amazing.

COMMUNICATIONS NETWORKS—BASIC LAYOUTS

The beginning of this chapter described a few applications of computer net- works and data communications that you encounter in everyday life. From that sampling, you can see that setting out all the different types of jobs and services that use some sort of computer network and data communications would gener- ate an enormous list. Instead, let us examine basic network systems and their layouts to see how extensive the uses of data communications and computer networks are. The basic layouts that we will examine include:

■ Microcomputer-to-local area network ■ Microcomputer-to-Internet ■ Local area network-to-local area network ■ Personal area network-to-workstation ■ Local area network-to-metropolitan area network ■ Local area network-to-wide area network ■ Wide area network-to-wide area network ■ Sensor-to-local area network ■ Satellite and microwave ■ Cell phones ■ Terminal/microcomputer-to-mainframe computer

Introduction to Computer Networks and Data Communications 5

Microcomputer-to-local area network layouts Perhaps the most common network layout today, the microcomputer-to- local area network layout is found in virtually every business and academic environment—and even in many homes. The microcomputer—which also is com- monly known as the personal computer, PC, desktop computer, laptop computer, notebook, netbook, or workstation—began to emerge in the late 1970s and early 1980s. (For the sake of consistency, we will use the older term “microcomputer” to signify any type of computer based on a microprocessor, disk drive, and memory.) The LAN, as we shall see in Chapter Seven, is an excellent tool for pro- viding a gateway to other networks, software, and peripherals. In some LANs, the data set that accompanies application software resides on a central computer called a server. Using microcomputers connected to a LAN, end users can request and download the data set, then execute the application on their computers. If users wish to print documents on a high-quality network printer, the LAN con- tains the network software necessary to route their print requests to the appropri- ate printer. If users wish to access their e-mail from the corporate e-mail server, the local area network provides a fast, stable connection between user worksta- tions and the e-mail server. If a user wishes to access the Internet, the local area network provides an effective gateway to the outside world. Figure 1-2 shows a diagram of this type of microcomputer-to-local area network layout.

Figure 1-2 A microcomputer lab, showing the cabling that exits from the back of a workstation and runs to a LAN collection point

Metal Conduit

Cables

LAN Collection Point

Workstations

Cables

One common form of microcomputer-to-local area network layout in the business world is the client/server system. In a client/server system, a user at a microcomputer, or client machine, issues a request for some form of data or service. This could be a request for a database record from a database server or a request to retrieve an e-mail message from an e-mail server. This request travels across the system to a server that contains a large repository of data and/or programs. The server fills the request and returns the results to the client, displaying the results on the client’s monitor.

A type of microcomputer-to-local area network layout that continues to grow in popularity is the wireless layout. A user sitting at a workstation or laptop uses wireless communications to send and receive data to and from a wireless access point. This access point is connected to the local area network and basically serves as the “bridge” between the wireless user device and the wired network. Although this setup uses radio frequency transmissions, we still consider it a microcomputer-to-local area network layout.

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Microcomputer-to-Internet layouts With the explosive growth of the Internet and the desire of users to connect to the Internet from home (either for pleasure or work-related reasons), the microcomputer-to-Internet layout continues to grow steadily. Originally, most home users connected to the Internet via a dial-up telephone line and a modem. This arrangement allowed for a maximum data transfer rate of roughly 56,000 bits per second (56 kbps). (The connections do not actually achieve 56 kbps, but that is a discussion we will have in Chapter Eleven.) No longer is the dial- up modem the most often used layout. Today, a majority of home users either connect to the Internet using digital subscriber line (DSL) or access the Internet through a cable modem service. All of these telecommunications services will be examined in more detail in Chapter Eleven. (In comparing the various data transfer rates of services and devices, we will use the convention in which lower- case k equals 1000. Also as part of the convention, lowercase b will refer to bits, while uppercase B refers to bytes.)

To communicate with the Internet using a dial-up, DSL, or cable modem connection, a user’s computer must connect to another computer already com- municating with the Internet. The easiest way to establish this connection is through the services of an Internet service provider (ISP). In this case, the user’s computer needs to have the necessary software to communicate with the Inter- net. The Internet “talks” only TCP/IP, so users must use software that supports the TCP and IP protocols. Once the user’s computer is talking TCP/IP, a connection to the Internet can be established. Figure 1-3 shows a typical microcomputer-to-Internet layout.

Figure 1-3 A microcomputer/ workstation sending data over a DSL line to an Internet service provider and onto the Internet Modem

Internet Service Provider

High-Speed Line

Internet

Router

Modems

Local area network-to-local area network layouts Because the local area network is a standard in business and academic environ- ments, it should come as no surprise that many organizations need the services of multiple local area networks and that it may be necessary for these LANs to communicate with each other. For example, a company may want the local area network that supports its research department to share an expensive color

Introduction to Computer Networks and Data Communications 7

laser printer with its marketing department’s local area network. Fortunately, it is possible to connect two local area networks so that they can share peripherals as well as software. The devices that usually connect two or more LANs are the switch and router.

In some cases, it may be more important to prevent data from flowing between local area networks than to allow data to flow from one network to another. For instance, some businesses have political reasons for supporting multiple networks—each division may want its own network to run as it wishes. Additionally, there may be security reasons for limiting traffic flow between networks; or allowing data destined for a particular network to traverse other networks simply may generate too much network traffic. Devices that connect local area networks can help manage these types of services as well. For example, the switch can filter out traffic not intended for the neighboring net- work, thus minimizing the overall amount of traffic flow. Figure 1-4 provides an example of two LANs connected by a switch.

Figure 1-4 Two local area networks connected by a switch

Hub

LAN A

Switch

LAN B Hub

Personal area network-to-workstation layouts The personal area network was created in the late 1990s and is one of the newer forms of computer networks. Using wireless transmissions with devices such as personal digital assistants (PDAs), laptop computers, and portable music players, an individual can transfer voice, data, and music from handheld devices to other devices such as microcomputer workstations (see Figure 1-5). Likewise, a user can download data from a workstation to one of these portable devices. For example, a user might use a PDA to record notes during a meeting. Once the meeting is over, the user can transmit the notes over a wireless connection from the PDA to his or her workstation. The workstation then runs a word processor to clean up the notes, and the formatted notes are uploaded to a local area network for corporate dissemination. Another example is the hands-free Bluetooth-enabled connection that people hang on their ear so they can converse with their cell phone without placing the cell phone up to their ear. It is also very common now to transfer digital photos and videos from cameras to micro- computers using short-range, wireless signals.

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Figure 1-5 A user transferring data from a personal digital assistant via a personal area network to a workstation attached to a local area network

Local Area

Network

WorkstationPersonal Area

Network

Local area network-to-metropolitan area network layouts Toward the end of the twentieth century, a new form of network appeared that interconnected businesses within a metropolitan area. Typically, this intercon- nection uses only fiber-optic links at extremely high speeds. These new networks are labeled metropolitan area networks. A metropolitan area network is a high- speed network that interconnects multiple sites within a close geographic region, such as a large urban area. For example, businesses that require a high-speed connection to their Internet service providers might use a metropolitan area net- work for interconnection (see Figure 1-6). As we shall see in more detail in Chapter Nine, metropolitan area networks are a cross between local area networks and wide area networks. They can transfer data at fast LAN speeds but over larger geographic regions than typically associated with a local area network.

Figure 1-6 Businesses interconnected within a large metropolitan area via a metropolitan area network

Fiber-Optic System

Businesses

Internet Service Provider

Local area network-to-wide area network layouts You have already seen that the local area network is commonly found in business and academic environments. If a user working at a microcomputer connected to a local area network wishes to access the Internet (a wide area

Introduction to Computer Networks and Data Communications 9

network), the user’s local area network must have a connection to the Internet. A device called a router is employed to connect these two networks. A router converts the local area network data into wide area network data. It also performs security functions and must be properly programmed to accept or reject certain types of incoming and outgoing data packets. Figure 1-7 shows a local area network connected to a wide area network via a router.

Figure 1-7 Local area network- to-wide area network layout

Wide Area

Network

Workstations

Hub or Switch Router

Wide area network-to-wide area network layouts The Internet is not a single network but a collection of thousands of networks. In order to travel any distance across the Internet, a data packet undoubtedly will pass through multiple wide area networks. Connecting a wide area network to a wide area network requires special devices that can route data traffic quickly and efficiently. These devices are high-speed routers. After the data packet enters the high-speed router, an address in the network layer (the IP address) is extracted, a routing decision is made, and the data packet is for- warded on to the next wide area network segment. As the data packet travels across the Internet, router after router makes a routing decision, moving the data toward its final destination. We will examine the Internet in more detail in Chapter Ten, then follow up with a discussion of several other types of wide area network technologies in Chapter Eleven.

Sensor-to-local area network layouts Another common layout found in everyday life is the sensor-to-local area network layout. In this type of layout, the action of a person or object triggers a sensor—for example, a left-turn light at a traffic intersection—that is connected to a network. In many left-turn lanes, a separate left-turn signal will appear if and only if one or more vehicles are in the left-turn lane. A sensor embedded in the roadway detects the movement of an automobile in the lane above and triggers the left-turn mechanism in the traffic signal control box at the side of the road. If this traffic signal control box is connected to a larger traffic control system, the sensor is connected to a local area network.

Another example of sensor-to-local area network layout is found within manufacturing environments. Assembly lines, robotic control devices, oven tem- perature controls, and chemical analysis equipment often use sensors connected to data-gathering computers that control movements and operations, sound alarms, and compute experimental or quality control results. Figure 1-8 shows a diagram of a typical sensor-to-local area network layout in a manufacturing environment.

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Figure 1-8 An automobile moves down an assembly line and triggers a sensor

Wiring to LAN Sensors

Wiring to LAN

Robot Arm

Satellite and microwave layouts Satellite and microwave layouts are continuously evolving technologies used in a variety of applications. If the distance between two networks is great and running a wire between them would be difficult (if not impossible), satellite and microwave transmission systems can be an extremely effective way to connect the two networks or computer systems. Examples of these applications include digital satellite TV, meteorology, intelligence operations, mobile maritime telephony, GPS navigation systems, wireless e-mail, worldwide mobile telephone systems, and video conferencing. Figure 1-9 shows a diagram of a typical satellite system.

Figure 1-9 Example of a television company using a satellite system to broadcast television services into homes and businesses

Homes

Satellite Dish

Television Company

Home

Business

Satellite

Cell phone layouts One of the most explosive areas of growth in recent years has been cell phone networks. Figure 1-10 shows an example of a handheld PDA that, along with making telephone calls, can transmit and receive data. The PDA has a modem installed, which transmits the PDA’s data across the cell phone network to the

Introduction to Computer Networks and Data Communications 11

cell phone switching center. The switching center then transfers the PDA’s data over the public telephone network or through a connection onto the Internet. Many newer handheld devices have combined data accessing capabilities with a cell phone and can transfer data over cell phone connections.

Figure 1-10 An example of a user with a smart cell phone transmitting and receiving data

Telephone Company

Land-Based Telephone Line

Wireless Transmission

Tower

Smart Cell Phone

Terminal/microcomputer-to-mainframe computer layouts Today, many businesses still employ a terminal-to-mainframe layout, although the number of these systems in use is not what it used to be. During the 1960s and 1970s, the terminal-to-mainframe layout was in virtually every office, manufacturing, and academic environment. These types of systems are still being used for inquiry/response applications, interactive applications, and data-entry applications, such as you might find when applying for a new driver’s license at the Department of Motor Vehicles (Figure 1-11).

Figure 1-11 Using a terminal (or thin client workstation) to perform a text-based input transaction

Computer Terminal

Next!

Cable Connecting to Mainframe

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The terminal-to-mainframe layouts of the 1960s and 1970s used “dumb” terminals because the end user was doing relatively simple data-entry and retrieval operations, and a workstation with a lot of computing power and storage was not necessary. A computer terminal was a device that was essen- tially a keyboard and screen with no long-term storage capabilities and little, if any, processing power. Computer terminals were used for entering data into a system, such as a mainframe computer, and then displaying results from the mainframe. Because the terminal did not possess a lot of computing power, the mainframe computer controlled the sending and receiving of data to and from each terminal. This required special types of protocols (sets of rules used by communication devices), and the data was usually transmitted at relatively slow speeds, such as 9600 or 19,200 bits per second (bps).

During this period, many of the same end users who had terminals on their desks also now found a microcomputer there (and thus had very little room for anything else). In time, terminal-emulation cards were developed, which allowed a microcomputer to imitate the abilities of a computer terminal. As terminal emulation cards were added to microcomputers, terminals were removed from end users’ desks, and microcomputers began to serve both functions. Now, if users wished, they could download information from the mainframe computer to their microcomputers, perform operations on the data, and then upload the information to the mainframe. Today, one rarely sees dumb computer terminals. Instead, most users use microcomputers and access the mainframe using either a terminal emulation card, a Web browser and Web interface, Telnet software (more on this in Chapter Ten), or a thin client. A thin client workstation is similar to a microcomputer but has no hard drive storage.

CONVERGENCE

A dictionary might define “convergence” as the process of coming together toward a single point. With respect to computer networks and communications systems, this definition is fairly relevant. Over the years, the communications industry has seen and continues to see different network applications and the technologies that support them converge into a single technology capable of supporting various applications. In particular, we can define three different types of convergence: technological convergence, protocol convergence, and industrial convergence. For example, one of the earliest and most common examples of technological convergence was the use of computers and modems to transmit data over the telephone system. This was an example of voice trans- mission systems converging with data transmission systems and yielding one system capable of supporting both data and voice. By the 1990s, telephone systems carried more computer data than voice. At about the same time, local area networks began to transfer telephone calls. Because local area networks originally were designed for data applications, this was another example of voice and data systems converging. Now we are seeing substantial growth in the Voice over Internet Protocol (VoIP) field. VoIP involves converting voice signals to packets and then sending those packets over packet-driven networks such as local area networks and the Internet.

Today we see many more examples of technological convergence, particu- larly in the wireless markets. For example, it is now quite common to snap a photo using a cell phone and then transfer the image over the cell phone network to another cell phone. Shortly after the introduction of photo-enabled cell phones, cell phones also became capable of sending and receiving instant messages. Then in 2005, cell phone providers started offering services that allow a user to transmit high-speed data over a cell phone connection. These all are examples of the convergence of two different applications (for example, digital

Introduction to Computer Networks and Data Communications 13

photography and cell phones in the case of photo-enabled cell phones) into a single technology. As we will see in a later chapter, many of the telephone companies that provide local and long-distance telephone service have con- verged into fewer companies. These are examples of industrial convergence. Also in a later chapter, we will see how older network protocols have given way or merged with other protocols, thus demonstrating protocol convergence.

Throughout the rest of this book, we will examine other examples of convergence within the communications industry. In addition to introducing the technologies involved, we also will examine the effects a given convergence of technologies might have on individual users and businesses.

NETWORK ARCHITECTURES

Now that you know the different types of networks and layouts, we need a framework to understand how all the various components of a network interop- erate. When someone uses a computer network to perform an application, many pieces come together to assist in the operation. A network architecture, or com- munications model, places the appropriate network pieces in layers. The layers define a model for the functions or services that need to be performed. Each layer in the model defines what services either the hardware or software (or both) provides. The two most common architectures known today are the TCP/IP protocol suite and the Open Systems Interconnection (OSI) model. The TCP/IP protocol suite is a working model (currently used on the Internet), while the OSI model (originally designed to be a working model) has been relegated to being a theoretical model. We will discuss these two architectures in more detail in the following pages. But first you should know a bit more about the components of a network and how a network architecture helps orga- nize those components.

Consider that a typical computer network within a business contains the following components that must interact in various ways:

■ Wires ■ Printed circuit boards ■ Wiring connectors and jacks ■ Computers ■ Centrally located wiring concentrators ■ Disk and tape drives ■ Computer applications such as word processors, e-mail programs, and

accounting, marketing, and electronic commerce software ■ Computer programs that support the transfer of data, check for errors when

the data is transferred, allow access to the network, and protect user transac- tions from unauthorized viewing

This large number of network components and their possible interactions inspires two questions. First, how do all of these pieces work together harmo- niously? You do not want two pieces performing the same function, or no pieces performing a necessary function. Like the elements of a well-oiled machine, all components of a computer network must work together to produce a product.

Second, does the choice of one piece depend on the choice of another piece? To make the pieces as modular as possible, you do not want the selection of one piece to constrain the choice of another piece. For example, if you create a network and originally plan to use one type of wiring but later change your mind and use a different type of wiring, will that change affect your choice of word processor? Such an interaction would seem highly unlikely. Alternately, can the choice of wiring affect the choice of the software program that

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checks for errors in the data sent over the wires? The answer to this question is not as obvious.

To keep the pieces of a computer network working together harmoniously and to allow modularity between the pieces, national and international organi- zations developed network architectures, which are cohesive layers of protocols defining a set of communication services. Consider the following non-computer example. Most organizations that produce some type of product or perform a service have a division of labor. Office assistants do the paperwork; accountants keep the books; laborers perform the manual duties; scientists design products; engineers test the products; managers control operations. Rarely is one person capable of performing all these duties. Large software applications operate the same way. Different procedures perform different tasks, and the whole would not function without the proper operation of each of its parts. Computer net- work applications are no exception. As the size of the applications grows, the need for a division of labor becomes increasingly important. Computer network applications also have a similar delineation of job functions. This delineation is the network architecture. Let’s examine our two network architectures or models: the TCP/IP protocol suite, followed by the OSI model.

The TCP/IP protocol suite The TCP/IP protocol suite was created by a group of computer scientists in order to support a new type of network (the ARPANET) being installed across the United States in the 1960s and 1970s. The goal was to create an open archi- tecture that would allow virtually all networks to inter-communicate. The design was based on a number of layers, in which the user would connect at the uppermost layer and would be isolated from the details of the electrical signals found at the lower layer.

The number of layers in the suite is not static. In fact, some books present the TCP/IP protocol suite as four layers, while others present it as five. Even then, different sources use different names for each of the layers. For this book, we will define five layers, as shown in Figure 1-12: application, transport, network, network access, and physical. Note that the layers do not specify pre- cise protocols or exact services. In other words, the TCP/IP protocol suite does not tell us, for example, what kind of wire or what kind of connector to use to connect the pieces of a network. That choice is left to the designer or implemen- ter of the system. Instead, the suite simply says that if you specify a type of wire or a specific connector, you do that in a particular layer. In addition, each layer of the TCP/IP protocol suite provides a service for the next layer. For example, the transport layer makes sure the data received at the very end of a transmis- sion is exactly the same as the data originally transmitted, but it relies upon the network layer to find the best path for the data to take from one point to the next within the network. With each layer performing its designated function, the layers work together to allow an application to send its data over a network of computers. Let us look at a simple e-mail application example (Figure 1-13) to understand how the layers of the TCP/IP protocol suite work together.

Figure 1-12 The five layers of the TCP/IP protocol suite Application

Transport

Physical

Network

Network Access

Introduction to Computer Networks and Data Communications 15

A common network application is e-mail. An e-mail program that accepts and sends the message, “Andy, how about lunch? Sharon,” has many steps. Using the TCP/IP protocol suite, the steps might look like the following. To begin, the e-mail “application worker” prompts the user to enter a message and specify an intended receiver. The application worker would create the appropriate data package with message contents and addresses, and send it to a “transport worker,” which is responsible for providing overall transport integ- rity. The transport worker might establish a connection with the intended receiver, monitor the flow between sender and receiver, and perform the neces- sary operations to recover lost data in case some data disappears or becomes unreadable.

The “network worker” would then take the data package from the trans- port worker and might add routing information so that the data package can find its way through the network. Next to get the data package would be the “network access worker,” which would insert error-checking information and prepare the data package for transmission. The final worker would be the “physical worker,” which would transmit the data package over some form of wire or through the air using radio waves.

Each worker has its own job function. Figure 1-13 shows how these workers work together to create a single package for transmission.

Figure 1-13 “Network workers” perform their job duties at each layer in the model

User

Message

Application Worker

Transport Worker

Message with

Transport Information

Network Worker

Message with

Transport Information and Network Address Network Access

Worker

Message with Transport

Information, Network Address,

and Error- checking Data

Physical Worker

10110111...

Let’s examine each layer in more detail. The top layer of the TCP/IP proto- col suite, the application layer, supports the network applications and might in some cases include additional services such as encryption or compression. The TCP/IP application layer includes several frequently used applications:

■ Hypertext Transfer Protocol (HTTP) to allow Web browsers and servers to send and receive World Wide Web pages

■ Simple Mail Transfer Protocol (SMTP) to allow users to send and receive electronic mail

■ File Transfer Protocol (FTP) to transfer files from one computer system to another

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■ Telnet to allow a remote user to log in to another computer system ■ Simple Network Management Protocol (SNMP) to allow the numerous

elements within a computer network to be managed from a single point

The next layer in the TCP/IP protocol suite is the transport layer. The TCP/IP transport layer commonly uses TCP to maintain an error-free end-to-end connection. To maintain this connection, TCP includes error control information in case one packet from a sequence of packets does not arrive at the final destina- tion, and packet sequencing information so that all the packets stay in the proper order. We say that the transport layer performs end-to-end error control and end-to-end flow control. This means the transport layer is not in use while the data packet is hopping from point to point within the network—it is used only at the two endpoints of the connection. TCP is not the only possible protocol found at the TCP/IP transport layer. User Datagram Protocol (UDP) is an alternative also used, though less frequently, in the TCP/IP protocol suite.

The two layers described so far are called end-to-end layers. They are responsible for the data transmitted between the endpoints of a network connec- tion. In other words, these layers perform their operations only at the beginning point and ending point of the network connection. The remaining three layers of the TCP/IP protocol suite—the network, network access, and physical layers—are not end-to-end layers. They perform their operations at each node (or device) along the network path, not just at the endpoints.

TCP/IP’s network layer, sometimes called the Internet layer or IP layer, is used to transfer data within and between networks. The Internet Protocol (IP) is the software that prepares a packet of data so that it can move from one network to another on the Internet or within a set of corporate networks. As this layer sends the packet from node to node, it generates the network addres- sing necessary for the system to recognize the next intended receiver. To choose a path through the network, the network layer determines routing information and applies it to each packet or group of packets.

The next lower layer of the TCP/IP protocol suite is the network access layer. If the network layer deals with passing packets through the Internet, then the network access layer is the layer that gets the data from the user workstation to the Internet. In a majority of cases, the connection that gets the data from the user workstation to the Internet is a local area network. Thus, the network access layer prepares a data packet (called a frame at this layer) for transmission from the user workstation to a router sitting between the local area network and the Internet. This frame contains an identifier that signals the beginning and end of the frame, as well as spaces for control information and address information. In addition, the network access layer can incorporate some form of error detection software. If an error occurs during transmission, the network access layer is responsible for error control, which it does by informing the sender of the error. The network access layer might also perform flow control. In a large network where the data hops from node to node as it makes its way across the network, flow control ensures that one node does not overwhelm the next node with too much data. Note that these network access operations are quite similar to some of the transport layer operations. The primary difference is that the transport layer might perform its operations only at the endpoints, while the network access layer performs its operations at every stop (node) along the path. This is also the last layer before the data is handed off for transmission across the medium. The network access layer is often called the data link layer.

The bottom-most layer in the TCP/IP protocol suite (or at least according to the way we are doing it) is the physical layer. The physical layer is the layer in which the actual transmission of data occurs. As noted earlier, this transmission

Introduction to Computer Networks and Data Communications 17

can be over a physical wire, or it can be a radio signal transmitted through the air. To perform this transmission of bits, the physical layer handles voltage levels, plug and connector dimensions, pin configurations, and other electrical and mechanical issues. Furthermore, because the digital or analog data is encoded or modulated onto a digital or analog signal at this point in the process, the physical layer also determines the encoding or modulation tech- nique to be used in the network. Note that some people combine the network access layer and physical layer into one layer.

Having distinctly defined layers enables you to “pull” out one layer and insert an equivalent layer without affecting the other layers. For example, let us assume a network was designed for copper-based wire. Later, the system owners decided to replace the copper-based wire with fiber-optic cable. Even though a change is being made at the physical layer, it should not be necessary to make any changes at any other layers. In reality, however, a few relationships exist be- tween the layers of a communication system that cannot be ignored. For example, if the physical organization of a local area network is changed (say from a wired layout to a wireless layout), it is likely that the frame description at the network access layer also will need to be changed. (We will examine this phenomenon in Chapter Seven.) The TCP/IP protocol suite recognizes these relationships and merges many of the services of the physical and data link layers into one layer.

The OSI model Although the TCP/IP protocol suite is the model of choice for almost all installed networks, it is important to study both this architecture and the OSI model. Many books and articles, when describing a product or a protocol, often refer to the OSI model with a statement such as, “This product is compliant with OSI layer x.” If you do not become familiar with the various layers of the OSI model and the TCP/IP protocol suite, this lack of important basic knowledge might impede your understanding of more advanced concepts in the future.

The OSI model was designed with seven layers, as shown in Figure 1-14. Note further the relationship between the five layers of the TCP/IP protocol suite and the seven layers of the OSI model. The top layer in the OSI model is the application layer, where the application using the network resides. This OSI layer is similar to the application layer in the TCP/IP protocol suite. The next layer in the OSI model, the presentation layer, performs a series of miscella- neous functions necessary for presenting the data package properly to the sender or receiver. For example, the presentation layer might perform ASCII- to-non-ASCII character conversions, encryption and decryption of secure documents, and the compression of data into smaller units. There is no separate presentation layer in the TCP/IP protocol suite.

Figure 1-14 The seven layers of the OSI model compared to the five layers of the TCP/IP protocol suite

OSI TCP/IP Protocol Suite

Application

Transport

Network

Network Access

Physical

Application

Presentation

Session

Transport

Network

Data Link

Physical

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DETAILS

The Internet’s Request for Comments (RFC) Network models, like communications protocols, computer hardware, and application software, continue to evolve daily. The TCP/IP protocol suite is a good example of a large set of protocols and standards constantly being revised and improved. An Internet standard is a tested specification that is both useful and adhered to by users who work with the Internet. Let us examine the path a proposal must follow on the way to becoming an Internet standard.

All Internet standards start as an Internet draft, which is a preliminary work in progress. One or more internal Internet committees work on a draft, improving it until it is in an acceptable form. When the Internet authorities feel the draft is ready for the public, it is published as a Request for Comments (RFC), a document open to all interested parties. The RFC is assigned a number, and it enters its first phase: proposed standard. A proposed standard is a proposal that is stable, of interest to the Internet community, and fairly well understood. The specification is tested and implemented by a number of different groups, and the results are published. If the proposal passes at least two independent and interoperable implementations, the proposed standard is elevated to draft standard. If, after feedback from test implementations is taken into account, the draft standard experiences no further problems, the proposal is finally elevated to Internet standard.

If, however, the proposed standard is deemed inappropriate at any point along the way, it becomes a historic RFC and is kept for historical perspective. (Internet standards that are replaced or superseded also become historic.) An RFC also can be categorized as experimental or informational. In these cases, the RFC in question probably was not meant to be an Internet standard, but was created either for experimental reasons or to provide infor- mation. Figure 1-15 shows the levels of progression for an RFC.

Internet Standard

Historic

Internet Draft

Defeated Standard Proposed Standard

Draft Standard

Defeated Standard

Figure 1-15 Levels of progression as an RFC moves toward becoming a standard

It is possible to obtain a printed listing of each RFC. See the Internet Engineering Task Force’s Web page at www.ietf.org/rfc.html for the best way to access RFCs.

The Internet is managed by the work of several committees. The topmost committee is the Internet Society (ISOC). ISOC is a nonprofit, international committee that provides support for the entire Internet standards-making process. Associated with ISOC is the Internet Architecture Board (IAB), which is the technical advisor to ISOC. Under the IAB are two major committees: the Internet Engineering Task Force (IETF) and the Internet Research Task Force (IRTF). The IETF manages the working groups that create and support functions such as Internet protocols, security, user services, operations, routing, and network management. The IRTF manages the working groups that focus on the long-range goals of the Internet, such as architecture, technology, applications, and protocols.

Internet committees are not the only groups that create pro- tocols or approve standards for computer networks, data com- munications, and telecommunications. Another organization that creates and approves network standards is the International Organization for Standardization (ISO), which is a multinational group composed of volunteers from the standards- making committees of various governments throughout the world. ISO is involved in developing standards in the field of information technology and created the Open Systems Interconnection (OSI) model for a network architecture.

Other standards-making organizations include:

■ American National Standards Institute (ANSI)— A private, nonprofit organization not associated with the U.S. government, ANSI strives to support the U.S. economy and protect the interests of the public by encouraging the adoption of various standards.

■ International Telecommunication Union- Telecommunication Standardization Sector (ITU-T)—Formerly the International Telegraph and Telephone Consultative Committee (CCITT), ITU-T is devoted to the research and creation of standards for telecommunications in general, and telephone and data systems in particular.

■ Institute of Electrical and Electronics Engineers (IEEE)—The largest professional engineering society in the world, IEEE strives to promote the standardization of the fields of electrical engineering, electronics, and radio. Of particular interest is the work IEEE has performed on standardizing local area networks.

■ Electronic Industries Alliance (EIA)—Aligned with ANSI, EIA is a nonprofit organization devoted to the standardization of electronics products. Of particular interest is the work EIA has performed on standardizing the interfaces between computers and modems.

Introduction to Computer Networks and Data Communications 19

www.ietf.org/rfc.html
The session layer is another layer that does not exist in the TCP/IP protocol suite and is responsible for establishing sessions between users. It also can support token management, a service that controls which user’s computer talks during the current session by passing a software token back and forth. Additionally, the session layer establishes synchronization points, which are backup points used in case of errors or failures. For example, while transmitting a large document such as an electronic book, the session layer might insert a synchronization point at the end of each chapter. If an error occurs during transmission, both sender and receiver can back up to the last synchronization point (to the beginning of a previously transmitted chapter) and start retrans- mission from there. Many network applications do not include a specific session layer and do not use tokens to manage a conversation. If they do, the “token” is inserted by the application layer, or possibly the transport layer, instead of the session layer. Likewise, if network applications use synchronization points, these points often are inserted by the application layer.

The fourth layer in the OSI model, the transport layer, operates in the same way as the transport layer of the TCP/IP protocol suite. It ensures that the data packet that arrives at the final destination is identical to the data packet that left the originating station.

The network layer of the OSI model is similar to the network layer of the TCP/IP protocol suite and is responsible for getting the data packets from router to router through the network. The data link layer, similar to TCP/IP’s network access layer, is responsible for taking data from the network layer and transforming it into a frame. The bottom layer in the OSI model—the physical layer—handles the transmission of bits over a communications channel. This layer is essentially identical to the physical layer of the TCP/IP protocol suite.

Logical and physical connections An important concept to understand with regard to the layers of a communica- tion model is the lines of communication between a sender and a receiver. Consider Figure 1-16, which shows the sender and receiver using a network application designed on the TCP/IP protocol suite.

Figure 1-16 Sender and receiver communicating using the TCP/IP protocol suite

Application

Transport

Network

Network Access

Physical

Receiver

Application

Transport

Network

Network Access

Physical

Sender Physical

Connection

Logical Connections

Notice the dashed lines between the sender’s and receiver’s application layers, transport layers, network layers, and network access layers. No data flows over these dashed lines. Each dashed line indicates a logical connection. A logical connection is a nonphysical connection between sender and receiver that allows an exchange of commands and responses. The sender’s and receiver’s transport layers, for example, share a set of commands used to perform transport-type functions, but the actual information or data must be passed through the physical layers of the sender and receiver, as there is no direct

20 Chapter 1

connection between the two transport layers. Without a logical connection, the sender and receiver would not be able to coordinate their functions. The physical connection is the only direct connection between sender and receiver, and is at the physical layer, where actual 1s and 0s—the digital content of the message—are transmitted over wires or airwaves.

For an example of logical and physical connections, consider an imaginary scenario in which the dean of arts and sciences wants to create a new joint degree with the school of business. In particular, the dean would like to create a degree that is a cross between computer science and marketing. The dean of arts and sciences could call the dean of business to create the degree, but deans are not necessarily experts at assembling all the details involved in a new degree. Instead, the dean of arts and sciences starts the process by issuing a request for a new degree from the dean of business. Before this request gets to the dean of business, however, the request must pass through several layers. First, the request goes to the chairperson of the computer science department. The chairperson will examine the request for a new degree and add the necessary information to staff the program. The chairperson will then send the request to the computer science curriculum committee, which will design several new courses. The curriculum committee will send the request to its department secre- tary, who will type all the memos and create a readable package. This package is then placed in the intercampus mail and sent to the marketing department in the school of business.