DATA AND COMPUTER
COMMUNICATIONS
Eighth Edition
William Stallings
Upper Saddle River, New Jersey 07458
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©2007 Pearson Education, Inc.
Pearson Prentice Hall
Pearson Education, Inc.
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All rights reserved. No part of this book may be reproduced in any form or by any means, without permission
in writing from the publisher.
Pearson Prentice Hall™ is a trademark of Pearson Education, Inc.
All other tradmarks or product names are the property of their respective owners.
The author and publisher of this book have used their best efforts in preparing this book.These efforts include the
development, research, and testing of the theories and programs to determine their effectiveness.The author and
publisher make no warranty of any kind, expressed or implied, with regard to these programs or the documentation
contained in this book.The author and publisher shall not be liable in any event for incidental or consequential
damages in connection with, or arising out of, the furnishing, performance, or use of these programs.
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ISBN: 0-13-243310-9
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For my scintillating wife
ATS
WEB SITE FOR DATA AND COMPUTER
COMMUNICATIONS, EIGHTH EDITION
The Web site at WilliamStallings.com/DCC/DCC8e.html provides support for instructors and
students using the book. It includes the following elements.
Course Support Materials
The course support materials include
• Copies of figures from the book in PDF format
• A detailed set of course notes in PDF format suitable for student handout or
for use as viewgraphs
• A set of PowerPoint slides for use as lecture aids
• Computer Science Student Support Site: contains a number of links and
documents that the student may find useful in his/her ongoing computer
science education. The site includes a review of basic, relevant mathematics;
advice on research, writing, and doing homework problems; links to
computer science research resources, such as report repositories and
bibliographies; and other useful links.
• An errata sheet for the book, updated at most monthly
T
DCC Courses
The DCC8e Web site includes links to Web sites for courses taught using the book. These
sites can provide useful ideas about scheduling and topic ordering, as well as a number of
useful handouts and other materials.
Useful Web Sites
The DCC8e Web site includes links to relevant Web sites, organized by chapter. The links
cover a broad spectrum of topics and will enable students to explore timely issues in greater
depth.
iv
WEB SITE FOR DATA AND COMPUTER COMMUNICATIONS, EIGHTH EDITION
v
Supplemental Documents
The DCC8e Web site includes a number of documents that expand on the treatment in the
book. Topics include standards organizations, Sockets, TCP/IP checksum, ASCII, and the
sampling theorem.
Internet Mailing List
An Internet mailing list is maintained so that instructors using this book can exchange information, suggestions, and questions with each other and the author. Subscription information
is provided at the book’s Web site.
Simulation and Modeling Tools
The Web site includes links to the cnet Web site and the modeling tools Web site. These packages can be used to analyze and experiment with protocol and network design issues. Each
site includes downloadable software and background information. The instructor’s manual
includes more information on loading and using the software and suggested student projects.
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CONTENTS
Web Site for Data and Computer Communications
Preface
iv
xv
Chapter 0 Reader’s and Instructor’s Guide
0.1
Outline of the Book 2
0.2
Roadmap 3
0.3
Internet and Web Resources 5
0.4
Standards 6
1
PART ONE OVERVIEW 9
Chapter 1 Data Communications, Data Networking, and the Internet 10
1.1
Data Communications and Networking for Today’s Enterprise 12
1.2
A Communications Model 16
1.3
Data Communications 19
1.4
Networks 22
1.5
The Internet 25
1.6
An Example Configuration 29
Chapter 2 Protocol Architecture, TCP/IP, and Internet-Based Applications
2.1
The Need for a Protocol Architecture 33
2.2
The TCP/IP Protocol Architecture 34
2.3
The OSI Model 42
2.4
Standardization within a Protocol Architecture 44
2.5
Traditional Internet-Based Applications 48
2.6
Multimedia 48
2.7
Recommended Reading and Web Sites 53
2.8
Key Terms, Review Questions, and Problems 54
Appendix 2A The Trivial File Transfer Protocol 57
PART TWO DATA COMMUNICATIONS 62
Chapter 3 Data Transmission 65
3.1
Concepts and Terminology 67
3.2
Analog and Digital Data Transmission 78
3.3
Transmission Impairments 86
3.4
Channel Capacity 91
3.5
Recommended Reading and Web Site 96
3.6
Key Terms, Review Questions, and Problems
Appendix 3A Decibels and Signal Strength 99
32
96
Chapter 4 Transmission Media 102
4.1
Guided Transmission Media 104
4.2
Wireless Transmission 117
4.3
Wireless Propagation 125
vii
viii
CONTENTS
4.4
4.5
4.6
Line-of-Sight Transmission 129
Recommended Reading and Web Sites 133
Key Terms, Review Questions, and Problems 134
Chapter 5 Signal Encoding Techniques 138
5.1
Digital Data, Digital Signals 141
5.2
Digital Data, Analog Signals 151
5.3
Analog Data, Digital Signals 162
5.4
Analog Data, Analog Signals 168
5.5
Recommended Reading 175
5.6
Key Terms, Review Questions, and Problems
175
Chapter 6 Digital Data Communication Techniques 180
6.1
Asynchronous and Synchronous Transmission 182
6.2
Types of Errors 186
6.3
Error Detection 186
6.4
Error Correction 196
6.5
Line Configurations 201
6.6
Recommended Reading 203
6.7
Key Terms, Review Questions, and Problems 204
Chapter 7 Data Link Control Protocols 207
7.1
Flow Control 209
7.2
Error Control 216
7.3
High-Level Data Link Control (HDLC) 222
7.4
Recommended Reading 228
7.5
Key Terms, Review Questions, and Problems 229
Appendix 7A Performance Issues 232
Chapter 8 Multiplexing 239
8.1
Frequency-Division Multiplexing 242
8.2
Synchronous Time-Division Multiplexing 248
8.3
Statistical Time-Division Multiplexing 258
8.4
Asymmetric Digital Subscriber Line 265
8.5
xDSL 268
8.6
Recommended Reading and Web Sites 269
8.7
Key Terms, Review Questions, and Problems 270
Chapter 9 Spread Spectrum 274
9.1
The Concept of Spread Spectrum 276
9.2
Frequency Hopping Spread Spectrum 277
9.3
Direct Sequence Spread Spectrum 282
9.4
Code-Division Multiple Access 287
9.5
Recommended Reading and Web Site 290
9.6
Key Terms, Review Questions, and Problems
291
CONTENTS
PART THREE WIDE AREA NETWORKS 295
Chapter 10 Circuit Switching and Packet Switching 297
10.1
Switched Communications Networks 299
10.2
Circuit Switching Networks 301
10.3
Circuit Switching Concepts 304
10.4
Softswitch Architecture 307
10.5
Packet-Switching Principles 309
10.6
X.25 317
10.7
Frame Relay 319
10.8
Recommended Reading and Web Sites 324
10.9
Key Terms, Review Questions, and Problems 325
Chapter 11 Asynchronous Transfer Mode 328
11.1
Protocol Architecture 329
11.2
ATM Logical Connections 331
11.3
ATM Cells 335
11.4
Transmission of ATM Cells 340
11.5
ATM Service Categories 345
11.6
Recommended Reading and Web Sites 348
11.7
Key Terms, Review Questions, and Problems 349
Chapter 12 Routing in Switched Networks 351
12.1
Routing in Packet-Switching Networks 352
12.2
Examples: Routing in ARPANET 362
12.3
Least-Cost Algorithms 367
12.4
Recommended Reading 372
12.5
Key Terms, Review Questions, and Problems 373
Chapter 13 Congestion Control in Data Networks 377
13.1
Effects of Congestion 379
13.2
Congestion Control 383
13.3
Traffic Management 386
13.4
Congestion Control in Packet-Switching Networks
13.5
Frame Relay Congestion Control 388
13.6
ATM Traffic Management 394
13.7
ATM-GFR Traffic Management 406
13.8
Recommended Reading 409
13.9
Key Terms, Review Questions, and Problems 410
Chapter 14 Cellular Wireless Networks 413
14.1
Principles of Cellular Networks 415
14.2
First Generation Analog 427
14.3
Second Generation CDMA 429
14.4
Third Generation Systems 437
14.5
Recommended Reading and Web Sites 440
14.6
Key Terms, Review Questions, and Problems 441
387
ix
x
CONTENTS
PART FOUR LOCAL AREA NETWORKS 444
Chapter 15 Local Area Network Overview 446
15.1
Background 448
15.2
Topologies and Transmission Media 451
15.3
LAN Protocol Architecture 457
15.4
Bridges 465
15.5
Layer 2 and Layer 3 Switches 473
15.6
Recommended Reading and Web Site 478
15.7
Key Terms, Review Questions, and Problems 479
Chapter 16 High-Speed LANs 482
16.1
The Emergence of High-Speed LANs 483
16.2
Ethernet 485
16.3
Fibre Channel 500
16.4
Recommended Reading and Web Sites 504
16.5
Key Terms, Review Questions, and Problems 506
Appendix 16A Digital Signal Encoding for LANs 508
Appendix 16B Performance Issues 514
Appendix 16C Scrambling 518
Chapter 17 Wireless LANs 522
17.1
Overview 523
17.2
Wireless LAN Technology 528
17.3
IEEE 802.11 Architecture and Services 531
17.4
IEEE 802.11 Medium Access Control 535
17.5
IEEE 802.11Physical Layer 543
17.6
IEEE 802.11 Security Considerations 549
17.7
Recommended Reading and Web Sites 550
17.8
Key Terms, Review Questions, and Problems 551
PART FIVE INTERNET AND TRANSPORT PROTOCOLS
Chapter 18 Internetwork Protocols 556
18.1
Basic Protocol Functions 558
18.2
Principles of Internetworking 566
18.3
Internet Protocol Operation 569
18.4
Internet Protocol 576
18.5
IPv6 586
18.6
Virtual Private Networks and IP Security 596
18.7
Recommended Reading and Web Sites 599
18.8
Key Terms, Review Questions, and Problems 600
Chapter 19 Internetwork Operation 603
19.1
Multicasting 605
19.2
Routing Protocols 614
19.3
Integrated Services Architecture 625
19.4
Differentiated Services 636
554
CONTENTS
19.5
19.6
19.7
19.8
Service Level Agreements 645
IP Performance Metrics 646
Recommended Reading and Web Sites 649
Key Terms, Review Questions, and Problems 651
Chapter 20 Transport Protocols 655
20.1
Connection-Oriented Transport Protocol Mechanisms
20.2
TCP 674
20.3
TCP Congestion Control 683
20.4
UDP 693
20.5
Recommended Reading and Web Sites 695
20.6
Key Terms, Review Questions, and Problems 695
PART SIX
Chapter 21
21.1
21.2
21.3
21.4
21.5
21.6
21.7
21.8
21.9
xi
657
INTERNET APPLICATIONS 699
Network Security 701
Security Requirements and Attacks 703
Confidentiality with Conventional Encryption 705
Message Authentication and Hash Functions 713
Public-Key Encryption and Digital Signatures 720
Secure Socket Layer and Transport Layer Security 727
IPv4 and IPv6 Security 732
Wi-Fi Protected Access 737
Recommended Reading and Web Sites 739
Key Terms, Review Questions, and Problems 740
Chapter 22 Internet Applications—Electronic Mail and Network Management
22.1
Electronic Mail: SMTP and MIME 745
22.2
Network Management: SNMP 760
22.3
Recommended Reading and Web Sites 770
22.4
Key Terms, Review Questions, and Problems 771
743
Chapter 23 Internet Applications—Internet Directory Service and World Wide Web
23.1
Internet Directory Service: DNS 774
23.2
Web Access: HTTP 784
23.3
Recommended Reading and Web Sites 795
23.4
Key Terms, Review Questions, and Problems 796
Chapter 24 Internet Applications—Multimedia 799
24.1
Audio and Video Compression 800
24.2
Real-Time Traffic 808
24.3
Voice Over IP and Multimedia Support—SIP 811
24.4
Real-Time Transport Protocol (RTP) 820
24.5
Recommended Reading and Web Sites 831
24.6
Key Terms, Review Questions, and Problems 832
773
xii
CONTENTS
APPENDICES 835
Appendix A Fourier Analysis 835
A.1
Fourier Series Representation of Periodic Signals 836
A.2
Fourier Transform Representation of Aperiodic Signals 837
A.3
Recommended Reading 840
Appendix B Projects for Teaching Data and Computer Communications
B.1
Practical Exercises 842
B.2
Sockets Projects 843
B.3
Ethereal Projects 843
B.4
Simulation and Modeling Projects 844
B.5
Performance Modeling 844
B.6
Research Projects 845
B.7
Reading/Report Assignments 845
B.8
Writing Assignments 845
B.9
Discussion Topics 846
References
847
Index 858
ONLINE APPENDICES
WilliamStallings.com/DCC
Appendix C Sockets: A Programmer’s Introduction
C.1
Versions of Sockets
C.2
Sockets, Socket Descriptors, Ports, and Connections
C.3
The Client/Server Model of Communication
C.4
Sockets Elements
C.5
Stream and Datagram Sockets
C.6
Run-Time Program Control
C.7
Remote Execution of a Windows Console Application
Appendix D Standards Organizations
D.1
The Importance of Standards
D.2
Standards and Regulation
D.3
Standards-Setting Organizations
Appendix E
The International Reference Alphabet
Appendix F
Proof of the Sampling Theorem
Appendix G Physical-Layer Interfacing
G.1
V.24/EIA-232-F
G.2
ISDN Physical Interface
Appendix H The OSI Model
H.1
The Model
H.2
The OSI Layers
841
CONTENTS
Appendix I Queuing Effects
I.1
Queuing Models
I.2
Queuing Results
Appendix J Orthogonality, Correlation, and Autocorrelation
J.1
Correlation and Autocorrelation
J.2
Orthogonal Codes
Appendix K The TCP/IP Checksum
K.1
Ones-Complement Addition
K.2
Use in TCP and IP
Appendix L
TCP/IP Example
Appendix M Uniform Resource Locators (URLs) and Uniform Resource
Identifiers (URIs)
M.1
Uniform Resource Locator
M.2
Uniform Resource Identifier
M.3
To Learn More
Appendix N
Glossary
Augmented Backus-Naur Form
xiii
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PREFACE
Begin at the beginning and go on till you come to the end; then stop.
—Alice in Wonderland, Lewis Carroll
OBJECTIVES
This book attempts to provide a unified overview of the broad field of data and computer communications. The organization of the book reflects an attempt to break this massive subject
into comprehensible parts and to build, piece by piece, a survey of the state of the art.The book
emphasizes basic principles and topics of fundamental importance concerning the technology
and architecture of this field and provides a detailed discussion of leading-edge topics.
The following basic themes serve to unify the discussion:
• Principles: Although the scope of this book is broad, there are a number of
basic principles that appear repeatedly as themes and that unify this field.
Examples are multiplexing, flow control, and error control. The book highlights
these principles and contrasts their application in specific areas of technology.
• Design approaches: The book examines alternative approaches to meeting
specific communication requirements.
• Standards: Standards have come to assume an increasingly important, indeed
dominant, role in this field. An understanding of the current status and future
direction of technology requires a comprehensive discussion of the related
standards.
INTENDED AUDIENCE
The book is intended for both an academic and a professional audience. For the professional
interested in this field, the book serves as a basic reference volume and is suitable for self-study.
As a textbook, it can be used for a one-semester or two-semester course. It covers the material
in Networking (NET), a core area in the Information Technology body of knowledge, which
is part of the Draft ACM/IEEE/AIS Computing Curricula 2005. The book also covers the
material in Computer Networks (CE-NWK), a core area in Computer Engineering 2004
Curriculum Guidelines from the ACM/IEEE Joint Task Force on Computing Curricula.
PLAN OF THE TEXT
The book is divided into six parts (see Chapter 0):
• Overview
• Data Communications
• Wide Area Networks
xv
xvi
PREFACE
• Local Area Networks
• Internet and Transport Protocols
• Internet Applications
In addition, the book includes an extensive glossary, a list of frequently used acronyms,
and a bibliography. Each chapter includes problems and suggestions for further reading.
The chapters and parts of the book are sufficiently modular to provide a great deal of flexibility in the design of courses. See Chapter 0 for a number of detailed suggestions for both
top-down and bottom-up course strategies.
INSTRUCTIONAL SUPPORT MATERIALS
To support instructors, the following materials are provided:
• Solutions Manual: Solutions to all end-of-chapter Review Questions and
Problems.
• PowerPoint Slides: A set of slides covering all chapters, suitable for use in
lecturing.
• PDF files: Reproductions of all figures and tables from the book.
• Projects Manual: Suggested project assignments for all of the project categories listed below.
Instructors may contact their Pearson Education or Prentice Hall representative for
access to these materials.
In addition, the book’s Web site supports instructors with:
• Links to Webs sites for other courses being taught using this book
• Sign up information for an Internet mailing list for instructors
INTERNET SERVICES FOR INSTRUCTORS AND STUDENTS
There is a Web site for this book that provides support for students and instructors.
The site includes links to other relevant sites, transparency masters of figures in the book,
and sign-up information for the book’s Internet mailing list. The Web page is at
WilliamStallings.com/DCC/DCC8e.html; see the section, Web Site for Data and Computer
Communications, preceding the Table of Contents, for more information. An Internet mailing list has been set up so that instructors using this book can exchange information, suggestions, and questions with each other and with the author. As soon as typos or other errors
are discovered, an errata list for this book will be available at WilliamStallings.com.
PROJECTS AND OTHER STUDENT EXERCISES
For many instructors, an important component of a data communications or networking
course is a project or set of projects by which the student gets hands-on experience to reinforce concepts from the text. This book provides an unparalleled degree of support for
including a projects component in the course. The instructor’s supplement not only includes
guidance on how to assign and structure the projects but also includes a set of User’s
PREFACE
xvii
Manuals for various project types plus specific assignments, all written especially for this
book. Instructors can assign work in the following areas:
• Practical exercises: Using network commands, the student gains experience in
network connectivity.
• Sockets programming projects: The book is supported by a detailed description of Sockets available at the book’s Web site. The Instructors supplement
includes a set of programming projects. Sockets programming is an “easy”
topic and one that can result in very satisfying hands-on projects for students.
• Ethereal projects: Ethereal is a protocol analyzer that enables students to
study the behavior of protocols.
• Simulation projects: The student can use the simulation package cnet to
analyze network behavior.
• Performance modeling projects: Two performance modeling techniques are
provided a tools package and OPNET.
• Research projects: The instructor’s supplement includes a list of suggested
research projects that would involve Web and literature searches.
• Reading/report assignments: The instructor’s supplement includes a list of
papers that can be assigned for reading and writing a report, plus suggested
assignment wording.
• Writing assignments: The instructor’s supplement includes a list of writing
assignments to facilitate learning the material.
• Discussion topics: These topics can be used in a classroom, chat room, or
message board environment to explore certain areas in greater depth and to
foster student collaboration.
This diverse set of projects and other student exercises enables the instructor to use the
book as one component in a rich and varied learning experience and to tailor a course plan
to meet the specific needs of the instructor and students. See Appendix B for details.
WHAT’S NEW IN THE EIGHTH EDITION
This eighth edition is seeing the light of day less than four years after the publication of the
seventh edition. During that time, the pace of change in this field continues unabated. In this
new edition, I try to capture these changes while maintaining a broad and comprehensive
coverage of the entire field. To begin the process of revision, the seventh edition of this book
was extensively reviewed by a number of professors who teach the subject. The result is that,
in many places, the narrative has been clarified and tightened, and illustrations have been
improved. Also, a number of new “field-tested” problems have been added.
Beyond these refinements to improve pedagogy and user friendliness, there have been
major substantive changes throughout the book. Every chapter has been revised, new
chapters have been added, and the overall organization of the book has changed.
Highlights include:
• Updated coverage of Gigabit Ethernet and 10-Gbps Ethernet: New details of
these standards are provided.
• Updated coverage of WiFi/IEEE 802.11 wireless LANs: IEEE 802.11 and the
related WiFi specifications have continued to evolve.
xviii
PREFACE
• New coverage of IP performance metrics and service level agreements
(SLAs): These aspects of Quality of Service (QoS) and performance monitoring are increasingly important.
• Address Resolution Protocol (ARP): This important protocol is now covered.
• New coverage of TCP Tahoe, Reno, and NewReno: These congestion control
algorithms are now common in most commercial implementations.
• Expanded coverage of security: Chapter 21 is more detailed; other chapters
provide overview of security for the relevant topic. Among the new topics are
Wi-Fi Protected Access (WPA) and the secure hash algorithm SHA-512.
• Domain Name System (DNS): This important scheme is now covered.
• New coverage of multimedia: Introductory section in Chapter 2; detailed coverage in Chapter 24. Topics covered include video compression, SIP, and RTP.
• Online appendices: Fourteen online appendices provide additional detail on
important topics in the text, including Sockets programming, queuing models,
the Internet checksum, a detailed example of TCP/IP operation, and the BNF
grammar.
In addition, throughout the book, virtually every topic has been updated to reflect the
developments in standards and technology that have occurred since the publication of the
seventh edition.
ACKNOWLEDGMENTS
This new edition has benefited from review by a number of people, who gave generously of
their time and expertise. The following people reviewed all or a large part of the manuscript:
Xin Liu- (UC, Davis), Jorge Cobb, Andras Farago, Dr. Prasant Mohapatra (UC Davis), Dr.
Jingxian Wu (Sonoma State University), G. R. Dattareya (UT Dallas), Guanling Chen
(Umass, Lowell), Bob Roohaprvar (Cal State East Bay), Ahmed Banafa (Cal State East
Bay), Ching-Chen Lee (CSU Hayward), and Daji Qaio (Iowa State).
Thanks also to the many people who provided detailed technical reviews of a single chapter: Dave Tweed, Bruce Lane, Denis McMahon, Charles Freund, Paul Hoadley, Stephen Ma,
Sandeep Subramaniam, Dragan Cvetkovic, Fernando Gont, Neil Giles, Rajesh Thundil, and
Rick Jones. In addition, Larry Owens of California State University and Katia Obraczka of
the University of Southern California provided some homework problems.
Thanks also to the following contributors. Zornitza Prodanoff of the University of North
Florida prepared the appendix on Sockets programming. Michael Harris of the University
of South Florida is responsible for the Ethereal exercises and user’s guide. Lawrie Brown of
the Australian Defence Force Academy of the University of New South Wales produced the
PPT lecture slides.
Finally, I would like to thank the many people responsible for the publication of the book,
all of whom did their usual excellent job. This includes the staff at Prentice Hall, particularly
my editor Tracy Dunkelberger, her assistants Christianna Lee and Carole Snyder, and production manager Rose Kernan. Also, Patricia M. Daly did the copy editing.
CHAPTER
0
READER’S AND INSTRUCTOR’S
GUIDE
0.1
Outline of the Book
0.2
Roadmap
0.3
Internet and Web Resources
0.4
Standards
1
2
CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE
“In the meanwhile, then,” demanded Li-loe, “relate to me the story to which reference
has been made, thereby proving the truth of your assertion, and at the same time
affording an entertainment of a somewhat exceptional kind.”
“The shadows lengthen,” replied Kai Lung, “but as the narrative in
question is of an inconspicuous span I will raise no barrier against your flattering
request, especially as it indicates an awakening taste
hitherto unexpected.”
—Kai Lung’s Golden Hours, Earnest Bramah
This book, with its accompanying Web site, covers a lot of material. Here we give
the reader some basic background information.
0.1 OUTLINE OF THE BOOK
The book is organized into five parts:
Part One. Overview: Provides an introduction to the range of topics covered in
the book. This part includes a general overview of data communications and networking and a discussion of protocols, OSI, and the TCP/IP protocol suite.
Part Two. Data Communications: Concerned primarily with the exchange of
data between two directly connected devices.Within this restricted scope, the key
aspects of transmission, interfacing, link control, and multiplexing are examined.
Part Three. Wide Area Networks: Examines the internal mechanisms and
user-network interfaces that have been developed to support voice, data, and
multimedia communications over long-distance networks. The traditional technologies of packet switching and circuit switching are examined, as well as the
more recent ATM and wireless WANs. Separate chapters are devoted to routing
and congestion control issues that are relevant both to switched data networks
and to the Internet.
Part Four. Local Area Networks: Explores the technologies and architectures
that have been developed for networking over shorter distances. The transmission media, topologies, and medium access control protocols that are the key
ingredients of a LAN design are explored and specific standardized LAN systems examined.
Part Five. Networking Protocols: Explores both the architectural principles and
the mechanisms required for the exchange of data among computers, workstations, servers, and other data processing devices. Much of the material in this part
relates to the TCP/IP protocol suite.
Part Six. Internet Applications: Looks at a range of applications that operate
over the Internet.
A more detailed, chapter-by-chapter summary of each part appears at the
beginning of that part.
0.2 / ROADMAP
3
0.2 ROADMAP
Course Emphasis
The material in this book is organized into four broad categories: data transmission
and communication; communications networks; network protocols; and applications and security. The chapters and parts of the book are sufficiently modular to
provide a great deal of flexibility in the design of courses. The following are
suggestions for three different course designs:
• Fundamentals of Data Communications: Parts One (overview) and Two (data
communications) and Chapters 10 and 11 (circuit switching, packet switching,
and ATM).
• Communications Networks: If the student has a basic background in data
communications, then this course could cover Parts One (overview), Three
(WAN), and Four (LAN).
• Computer Networks: If the student has a basic background in data communications, then this course could cover Part One (overview), Chapters 6 and 7
(data communication techniques and data link control), Part Five (protocols),
and part or all of Part Six (applications).
In addition, a more streamlined course that covers the entire book is possible
by eliminating certain chapters that are not essential on a first reading. Chapters
that could be optional are Chapters 3 (data transmission) and 4 (transmission
media), if the student has a basic understanding of these topics; Chapter 8 (multiplexing); Chapter 9 (spread spectrum); Chapters 12 through 14 (routing, congestion
control, cellular networks); Chapter 18 (internetworking); and Chapter 21 (network
security).
Bottom-Up versus Top-Down
The book is organized in a modular fashion. After reading Part One, the other parts
can be read in a number of possible sequences. Figure 0.1a shows the bottom-up
approach provided by reading the book from front to back. With this approach, each
part builds on the material in the previous part, so that it is always clear how a given
layer of functionality is supported from below. There is more material than can be
comfortably covered in a single semester, but the book’s organization makes it easy
to eliminate some chapters and maintain the bottom-up sequence. Figure 0.1b
suggests one approach to a survey course.
Some readers, and some instructors, are more comfortable with a top-down
approach. After the background material (Part One), the reader continues at the
application level and works down through the protocol layers. This has the advantage of immediately focusing on the most visible part of the material, the applications, and then seeing, progressively, how each layer is supported by the next layer
down. Figure 0.1c is an example of a comprehensive treatment and Figure 0.1d is an
example of a survey treatment.
4
CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE
Part One
Overview
Part One
Overview (1, 2)
Part Two
Data Communications
Part Two
Data Communications (3, 6, 7, 8)
Part Three
Wide Area Networks
Part Three
WANs (10, 12)
Part Four
Local Area Networks
Part Four
LANs (15)
Part Five
Internet and Transport Protocols
Part Five
TCP/IP (18, 20)
Part Six
Internet Applications
(a) A bottom-up approach
(b) Another bottom-up approach
Part One
Overview
Part One
Overview
Chapter 18
The Internet Protocol
Chapter 18
The Internet Protocol
Part Six
Internet Applications
Part Six
Internet Applications
Part Five
TCP/IP
Part Five
TCP/IP
Part Three
WANs
Part Three
WANs (10, 12)
Part Four
LANs
Part Four
LANs (15)
Part Two
Data Communications
(c) A top-down approach
(d) Another top-down approach
Figure 0.1 Suggested Reading Orders
Finally, it is possible to select chapters to reflect specific teaching objectives by
not sticking to a strict chapter ordering. We give two examples used in courses
taught with the seventh edition. One course used the sequence Part One
(Overview); Chapter 3 (Data Transmission); Chapter 6 (Digital Data Communications Techniques); Chapter 7 (Data Link Control); Chapter 15 (LAN Overview);
Chapter 16 (High-Speed LANs); Chapter 10 (Circuit and Packet Switching);
Chapter 12 (Routing); Chapter 18 (Internet Protocols); and Chapter 19 (Internet
Operation). The other course used the sequence Part One (Overview); Chapter 3
(Data Transmission); Chapter 4 (Guided and Wireless Transmission); Chapter 5
(Signal Encoding Techniques); Chapter 8 (Multiplexing); Chapter 15 (LAN
0.3 / INTERNET AND WEB RESOURCES
5
Overview); Chapter 16 (High-Speed LANs); Chapter 10 (Circuit and Packet
Switching); Chapter 20 (Transport Protocols); Chapter 18 (Internet Protocols); and
Chapter 19 (Internet Operation).
0.3 INTERNET AND WEB RESOURCES
There are a number of resources available on the Internet and the Web to support
this book and to help one keep up with developments in this field.
Web Sites for This Book
A special Web page has been set up for this book at WilliamStallings.com/DCC/
DCC8e.html. See the two-page layout at the beginning of this book for a detailed
description of that site.
As soon as any typos or other errors are discovered, an errata list for this book
will be available at the Web site. Please report any errors that you spot. Errata
sheets for my other books are at WilliamStallings.com.
I also maintain the Computer Science Student Resource Site, at
WilliamStallings.com/StudentSupport.html. The purpose of this site is to provide documents, information, and links for computer science students and professionals. Links
and documents are organized into four categories:
• Math: Includes a basic math refresher, a queuing analysis primer, a number
system primer, and links to numerous math sites
• How-to: Advice and guidance for solving homework problems, writing technical reports, and preparing technical presentations
• Research resources: Links to important collections of papers, technical
reports, and bibliographies
• Miscellaneous: A variety of useful documents and links
Other Web Sites
There are numerous Web sites that provide information related to the topics of this
book. In subsequent chapters, pointers to specific Web sites can be found in the
Recommended Reading and Web Sites section. Because the addresses for Web sites
tend to change frequently, I have not included URLs in the book. For all of the Web
sites listed in the book, the appropriate link can be found at this book’s Web site.
Other links not mentioned in this book will be added to the Web site over time.
The following are Web sites of general interest related to data and computer
communications:
• Network World: Information and links to resources about data communications and networking.
• IETF: Maintains archives that relate to the Internet and IETF activities.
Includes keyword-indexed library of RFCs and draft documents as well as
many other documents related to the Internet and related protocols.
6
CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE
• Vendors: Links to thousands of hardware and software vendors who currently
have Web sites, as well as a list of thousands of computer and networking companies in a phone directory.
• IEEE Communications Society: Good way to keep up on conferences, publications, and so on.
• ACM Special Interest Group on Communications (SIGCOMM): Good way
to keep up on conferences, publications, and so on.
• International Telecommunications Union: Contains a listing of ITU-T recommendations, plus information on obtaining ITU-T documents in hard copy or
on DVD.
• International Organization for Standardization: Contains a listing of ISO
standards, plus information on obtaining ISO documents in hard copy or on
CD-ROM.
• CommWeb: Links to vendors, tutorials, and other useful information.
• CommsDesign: Lot of useful articles, tutorials, and product information. A bit
hard to navigate, but worthwhile.
USENET Newsgroups
A number of USENET newsgroups are devoted to some aspect of data communications, networks, and protocols. As with virtually all USENET groups, there is a
high noise-to-signal ratio, but it is worth experimenting to see if any meet your
needs. The most relevant are as follows:
• comp.dcom.lans, comp.dcom.lans.misc: General discussions of LANs
• comp.dcom.lans.ethernet: Covers Ethernet, Ethernet-like systems, and the IEEE
802.3 CSMA/CD standards
• comp.std.wireless: General discussion of wireless networks, including wireless
LANs
• comp.security.misc: Computer security and encryption
• comp.dcom.cell-relay: Covers ATM and ATM LANs
• comp.dcom.frame-relay: Covers frame relay networks
• comp.dcom.net-management: Discussion of network management applications,
protocols, and standards
• comp.protocols.tcp-ip: The TCP/IP protocol suite
0.4 STANDARDS
It has long been accepted in the telecommunications industry that standards are
required to govern the physical, electrical, and procedural characteristics of communication equipment. In the past, this view has not been embraced by the computer industry. Whereas communication equipment vendors recognize that their
0.4 / STANDARDS
7
equipment will generally interface to and communicate with other vendors’ equipment, computer vendors have traditionally attempted to monopolize their customers. The proliferation of computers and distributed processing has made that an
untenable position. Computers from different vendors must communicate with
each other and, with the ongoing evolution of protocol standards, customers will no
longer accept special-purpose protocol conversion software development. The
result is that standards now permeate all of the areas of technology discussed in
this book.
There are a number of advantages and disadvantages to the standards-making
process. We list here the most striking ones. The principal advantages of standards
are as follows:
• A standard assures that there will be a large market for a particular piece of
equipment or software. This encourages mass production and, in some cases,
the use of large-scale-integration (LSI) or very-large-scale-integration (VLSI)
techniques, resulting in lower costs.
• A standard allows products from multiple vendors to communicate, giving the
purchaser more flexibility in equipment selection and use.
The principal disadvantages are as follows:
• A standard tends to freeze the technology. By the time a standard is developed, subjected to review and compromise, and promulgated, more efficient
techniques are possible.
• There are multiple standards for the same thing. This is not a disadvantage of
standards per se, but of the current way things are done. Fortunately, in recent
years the various standards-making organizations have begun to cooperate
more closely. Nevertheless, there are still areas where multiple conflicting
standards exist.
Throughout this book, we describe the most important standards in use or
being developed for various aspects of data and computer communications. Various
organizations have been involved in the development or promotion of these standards. The following are the most important (in the current context) of these organizations:
• Internet Society: The Internet SOCiety (ISOC) is a professional membership society with more than 150 organizational and 6000 individual members in over 100 countries. It provides leadership in addressing issues that
confront the future of the Internet and is the organization home for the
groups responsible for Internet infrastructure standards, including the
Internet Engineering Task Force (IETF) and the Internet Architecture
Board (IAB). All of the RFCs and Internet standards are developed
through these organizations.
• IEEE 802: The IEEE (Institute of Electrical and Electronics Engineers) 802
LAN/MAN Standards Committee develops local area network standards and
metropolitan area network standards. The most widely used standards are for
the Ethernet family, wireless LAN, bridging, and virtual bridged LANs. An
individual working group provides the focus for each area.
8
CHAPTER 0 / READER’S AND INSTRUCTOR’S GUIDE
• ITU-T: The International Telecommunication Union (ITU) is an international organization within the United Nations System where governments and
the private sector coordinate global telecom networks and services. The ITU
Telecommunication Standardization Sector (ITU-T) is one of the three sectors of the ITU. ITU-T’s mission is the production of standards covering all
fields of telecommunications.
• ATM Forum: The ATM Forum is an international nonprofit organization
formed with the objective of accelerating the use of ATM (asynchronous
transfer mode) products and services through a rapid convergence of interoperability specifications. In addition, the Forum promotes industry cooperation
and awareness.
• ISO: The International Organization for Standardization (ISO)1 is a worldwide federation of national standards bodies from more than 140 countries,
one from each country. ISO is a nongovernmental organization that promotes
the development of standardization and related activities with a view to facilitating the international exchange of goods and services, and to developing
cooperation in the spheres of intellectual, scientific, technological, and economic activity. ISO’s work results in international agreements that are published as International Standards.
A more detailed discussion of these organizations is contained in Appendix D.
1
ISO is not an acronym (in which case it would be IOS), but a word, derived from the Greek, meaning
equal.
PART ONE
Overview
The purpose of Part One is to provide a background and context for the
remainder of this book. The broad range of topics that are encompassed in the
field of data and computer communications is introduced, and the fundamental
concepts of protocols and protocol architectures are examined.
ROAD MAP FOR PART ONE
Chapter 1 Data Communications, Data Networks, and
The Internet
Chapter 1 provides an overview of Parts Two through Four of the book, giving the
“big picture.” In essence, the book deals with four topics: data communications
over a transmission link; wide area networks; local area networks; and protocols
and the TCP/IP protocol architecture. Chapter 1 provides a preview of the first
three of these topics.
Chapter 2 Protocol Architecture, TCP/IP, and
Internet-Based Applications
Chapter 2 discusses the concept protocol architectures. This chapter can be read
immediately following Chapter 1 or deferred until the beginning of Part Three,
Four, or Five. After a general introduction, the chapter deals with the two most
important protocol architectures: the Open Systems Interconnection (OSI) model
and TCP/IP. Although the OSI model is often used as the framework for discourse in
this area, it is the TCP/IP protocol suite that is the basis for most commercially available interoperable products and that is the focus of Parts Five and Six of this book.
9
CHAPTER
1
DATA COMMUNICATIONS, DATA
NETWORKS, AND THE INTERNET
10
1.1
Data Communications and Networking for Today’s Enterprise
1.2
A Communications Model
1.3
Data Communications
1.4
Networks
1.5
The Internet
1.6
An Example Configuration
The fundamental problem of communication is that of reproducing at one
point either exactly or approximately a message selected at another point.
—The Mathematical Theory of Communication, Claude Shannon
KEY POINTS
•
•
•
The scope of this book is broad, covering three general areas: data
communications, networking, and protocols; the first two are introduced in this chapter.
Data communications deals with the transmission of signals in a reliable and efficient manner. Topics covered include signal transmission,
transmission media, signal encoding, interfacing, data link control, and
multiplexing.
Networking deals with the technology and architecture of the communications networks used to interconnect communicating devices.
This field is generally divided into the topics of local area networks
(LANs) and wide area networks (WANs).
The 1970s and 1980s saw a merger of the fields of computer science and data
communications that profoundly changed the technology, products, and companies of the now combined computer-communications industry. The computercommunications revolution has produced several remarkable facts:
• There is no fundamental difference between data processing (computers)
and data communications (transmission and switching equipment).
• There are no fundamental differences among data, voice, and video communications.
• The distinction among single-processor computer, multiprocessor computer,
local network, metropolitan network, and long-haul network has blurred.
One effect of these trends has been a growing overlap of the computer and
communications industries, from component fabrication to system integration.
Another result is the development of integrated systems that transmit and process
all types of data and information. Both the technology and the technical standards
organizations are driving toward integrated public systems that make virtually all
data and information sources around the world easily and uniformly accessible.
This book aims to provide a unified view of the broad field of data and
computer communications. The organization of the book reflects an attempt to
break this massive subject into comprehensible parts and to build, piece by
piece, a survey of the state of the art. This introductory chapter begins with a
general model of communications. Then a brief discussion introduces each of
the Parts Two through Four of this book. Chapter 2 provides an overview to
Parts Five and Six
11
12
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
1.1 DATA COMMUNICATIONS AND NETWORKING FOR
TODAY’S ENTERPRISE
Effective and efficient data communication and networking facilities are vital to any
enterprise. In this section, we first look at trends that are increasing the challenge for
the business manager in planning and managing such facilities. Then we look specifically at the requirement for ever-greater transmission speeds and network capacity.
Trends
Three different forces have consistently driven the architecture and evolution of
data communications and networking facilities: traffic growth, development of new
services, and advances in technology.
Communication traffic, both local (within a building or building complex) and
long distance, both voice and data, has been growing at a high and steady rate for
decades. The increasing emphasis on office automation, remote access, online
transactions, and other productivity measures means that this trend is likely to continue. Thus, managers are constantly struggling to maximize capacity and minimize
transmission costs.
As businesses rely more and more on information technology, the range of
services expands. This increases the demand for high-capacity networking and transmission facilities. In turn, the continuing growth in high-speed network offerings
with the continuing drop in prices encourages the expansion of services. Thus,
growth in services and growth in traffic capacity go hand in hand. Figure 1.1 gives
some examples of information-based services and the data rates needed to support
them [ELSA02].
Finally, trends in technology enable the provision of increasing traffic capacity
and the support of a wide range of services. Four technology trends are particularly
notable:
1. The trend toward faster and cheaper, both in computing and communications,
continues. In terms of computing, this means more powerful computers and
clusters of computers capable of supporting more demanding applications,
such as multimedia applications. In terms of communications, the increasing
use of optical fiber has brought transmission prices down and greatly
increased capacity. For example, for long-distance telecommunication and
data network links, recent offerings of dense wavelength division multiplexing
(DWDM) enable capacities of many terabits per second. For local area networks (LANs) many enterprises now have Gigabit Ethernet backbone networks and some are beginning to deploy 10-Gbps Ethernet.
2. Both voice-oriented telecommunications networks, such as the public switched
telephone network (PSTN), and data networks, including the Internet, are more
“intelligent” than ever. Two areas of intelligence are noteworthy. First, today’s
networks can offer differing levels of quality of service (QoS), which include
specifications for maximum delay, minimum throughput, and so on. Second,
today’s networks provide a variety of customizable services in the areas of network management and security.
1.1 / DATA COMMUNICATIONS AND NETWORKING FOR TODAY’S ENTERPRISE
Speed (kbps)
9.6
14.4
28
64
144
384
13
2000
Transaction processing
Messaging/text apps
Voice
Location services
Still image transfers
Internet/VPN access
Database access
Enhanced Web surfing
Low-quality video
Hifi audio
Large file transfer
Moderate video
Interactive entertainment
High-quality video
VPN: virtual private
network
Performance:
Poor
Adequate
Good
Figure 1.1 Services versus Throughput Rates
3. The Internet, the Web, and associated applications have emerged as dominant
features of both the business and personal world, opening up many opportunities
and challenges for managers. In addition to exploiting the Internet and the Web
to reach customers, suppliers, and partners, enterprises have formed intranets and
extranets1 to isolate their proprietary information free from unwanted access.
4. There has been a trend toward ever-increasing mobility for decades, liberating
workers from the confines of the physical enterprise. Innovations include
voice mail, remote data access, pagers, fax, e-mail, cordless phones, cell phones
and cellular networks, and Internet portals. The result is the ability of employees to take their business context with them as they move about. We are now
seeing the growth of high-speed wireless access, which further enhances the
ability to use enterprise information resources and services anywhere.
1
Briefly, an intranet uses Internet and Web technology in an isolated facility internal to an enterprise; an
extranet extends a company’s intranet out onto the Internet to allow selected customers, suppliers, and
mobile workers to access the company’s private data and applications.
14
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
Data Transmission and Network Capacity Requirements
Momentous changes in the way organizations do business and process information
have been driven by changes in networking technology and at the same time have
driven those changes. It is hard to separate chicken and egg in this field. Similarly,
the use of the Internet by both businesses and individuals reflects this cyclic dependency: the availability of new image-based services on the Internet (i.e., the Web)
has resulted in an increase in the total number of users and the traffic volume generated by each user. This, in turn, has resulted in a need to increase the speed and
efficiency of the Internet. On the other hand, it is only such increased speed that
makes the use of Web-based applications palatable to the end user.
In this section, we survey some of the end-user factors that fit into this equation. We begin with the need for high-speed LANs in the business environment,
because this need has appeared first and has forced the pace of networking development. Then we look at business WAN requirements. Finally we offer a few words
about the effect of changes in commercial electronics on network requirements.
The Emergence of High-Speed LANs Personal computers and microcomputer workstations began to achieve widespread acceptance in business computing
in the early 1980s and have now achieved virtually the status of the telephone: an
essential tool for office workers. Until relatively recently, office LANs provided
basic connectivity services—connecting personal computers and terminals to mainframes and midrange systems that ran corporate applications, and providing workgroup connectivity at the departmental or divisional level. In both cases, traffic
patterns were relatively light, with an emphasis on file transfer and electronic mail.
The LANs that were available for this type of workload, primarily Ethernet and
token ring, are well suited to this environment.
In the 1990s, two significant trends altered the role of the personal computer
and therefore the requirements on the LAN:
1. The speed and computing power of personal computers continued to enjoy explosive growth. These more powerful platforms support graphics-intensive applications and ever more elaborate graphical user interfaces to the operating system.
2. MIS (management information systems) organizations have recognized the LAN
as a viable and essential computing platform, resulting in the focus on network
computing. This trend began with client/server computing, which has become a
dominant architecture in the business environment and the more recent Webfocused intranet trend. Both of these approaches involve the frequent transfer of
potentially large volumes of data in a transaction-oriented environment.
The effect of these trends has been to increase the volume of data to be handled over LANs and, because applications are more interactive, to reduce the
acceptable delay on data transfers. The earlier generation of 10-Mbps Ethernets and
16-Mbps token rings was simply not up to the job of supporting these requirements.
The following are examples of requirements that call for higher-speed LANs:
• Centralized server farms: In many applications, there is a need for user, or
client, systems to be able to draw huge amounts of data from multiple centralized servers, called server farms. An example is a color publishing operation, in
1.1 / DATA COMMUNICATIONS AND NETWORKING FOR TODAY’S ENTERPRISE
15
which servers typically contain tens of gigabytes of image data that must be
downloaded to imaging workstations. As the performance of the servers themselves has increased, the bottleneck has shifted to the network.
• Power workgroups: These groups typically consist of a small number of cooperating users who need to draw massive data files across the network. Examples
are a software development group that runs tests on a new software version, or
a computer-aided design (CAD) company that regularly runs simulations of
new designs. In such cases, large amounts of data are distributed to several
workstations, processed, and updated at very high speed for multiple iterations.
• High-speed local backbone: As processing demand grows, LANs proliferate at
a site, and high-speed interconnection is necessary.
Corporate Wide Area Networking Needs As recently as the early 1990s, there
was an emphasis in many organizations on a centralized data processing model. In a
typical environment, there might be significant computing facilities at a few regional
offices, consisting of mainframes or well-equipped midrange systems. These centralized
facilities could handle most corporate applications, including basic finance, accounting,
and personnel programs, as well as many of the business-specific applications. Smaller,
outlying offices (e.g., a bank branch) could be equipped with terminals or basic personal
computers linked to one of the regional centers in a transaction-oriented environment.
This model began to change in the early 1990s, and the change accelerated
through the mid-1990s. Many organizations have dispersed their employees into multiple smaller offices. There is a growing use of telecommuting. Most significant, the
nature of the application structure has changed. First client/server computing and,
more recently, intranet computing have fundamentally restructured the organizational
data processing environment.There is now much more reliance on personal computers,
workstations, and servers and much less use of centralized mainframe and midrange
systems. Furthermore, the virtually universal deployment of graphical user interfaces to
the desktop enables the end user to exploit graphic applications, multimedia, and other
data-intensive applications. In addition, most organizations require access to the Internet. When a few clicks of the mouse can trigger huge volumes of data, traffic patterns
have become more unpredictable while the average load has risen.
All of these trends means that more data must be transported off premises and
into the wide area. It has long been accepted that in the typical business environment, about 80% of the traffic remains local and about 20% traverses wide area
links. But this rule no longer applies to most companies, with a greater percentage of
the traffic going into the WAN environment [COHE96]. This traffic flow shift places
a greater burden on LAN backbones and, of course, on the WAN facilities used by a
corporation. Thus, just as in the local area, changes in corporate data traffic patterns
are driving the creation of high-speed WANs.
Digital Electronics The rapid conversion of consumer electronics to digital
technology is having an impact on both the Internet and corporate intranets. As
these new gadgets come into view and proliferate, they dramatically increase the
amount of image and video traffic carried by networks.
Two noteworthy examples of this trend are digital versatile disks (DVDs) and
digital still cameras. With the capacious DVD, the electronics industry has at last
16
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
found an acceptable replacement for the analog VHS videotape. The DVD has
replaced the videotape used in videocassette recorders (VCRs) and replaced the
CD-ROM in personal computers and servers. The DVD takes video into the digital
age. It delivers movies with picture quality that outshines laser disks, and it can be
randomly accessed like audio CDs, which DVD machines can also play. Vast volumes of data can be crammed onto the disk, currently seven times as much as a CDROM. With DVD’s huge storage capacity and vivid quality, PC games have become
more realistic and educational software incorporates more video. Following in the
wake of these developments is a new crest of traffic over the Internet and corporate
intranets, as this material is incorporated into Web sites.
A related product development is the digital camcorder. This product has
made it easier for individuals and companies to make digital video files to be placed
on corporate and Internet Web sites, again adding to the traffic burden.
1.2 A COMMUNICATIONS MODEL
This section introduces a simple model of communications, illustrated by the block
diagram in Figure 1.2a.
The fundamental purpose of a communications system is the exchange of data
between two parties. Figure 1.2b presents one particular example, which is communication between a workstation and a server over a public telephone network.
Another example is the exchange of voice signals between two telephones over the
same network. The key elements of the model are as follows:
• Source. This device generates the data to be transmitted; examples are telephones and personal computers.
Source system
Source
Destination system
Transmitter
Transmission
System
Receiver
Destination
(a) General block diagram
Workstation
Modem
Public telephone network
(b) Example
Figure 1.2 Simplified Communications Model
Modem
Server
1.2 / A COMMUNICATIONS MODEL
17
• Transmitter: Usually, the data generated by a source system are not transmitted directly in the form in which they were generated. Rather, a transmitter
transforms and encodes the information in such a way as to produce electromagnetic signals that can be transmitted across some sort of transmission system. For example, a modem takes a digital bit stream from an attached device
such as a personal computer and transforms that bit stream into an analog signal that can be handled by the telephone network.
• Transmission system: This can be a single transmission line or a complex network connecting source and destination.
• Receiver: The receiver accepts the signal from the transmission system and
converts it into a form that can be handled by the destination device. For
example, a modem will accept an analog signal coming from a network or
transmission line and convert it into a digital bit stream.
• Destination: Takes the incoming data from the receiver.
This simple narrative conceals a wealth of technical complexity. To get some
idea of the scope of this complexity, Table 1.1 lists some of the key tasks that must be
performed in a data communications system. The list is somewhat arbitrary: Elements could be added; items on the list could be merged; and some items represent
several tasks that are performed at different “levels” of the system. However, the list
as it stands is suggestive of the scope of this book.
The first item, transmission system utilization, refers to the need to make
efficient use of transmission facilities that are typically shared among a number of
communicating devices. Various techniques (referred to as multiplexing) are used to
allocate the total capacity of a transmission medium among a number of users.
Congestion control techniques may be required to assure that the system is not
overwhelmed by excessive demand for transmission services.
To communicate, a device must interface with the transmission system. All the
forms of communication discussed in this book depend on the use of electromagnetic
signals propagated over a transmission medium. Thus, once an interface is established, signal generation is required for communication. The properties of the signal,
such as form and intensity, must be such that the signal is (1) capable of being propagated through the transmission system, and (2) interpretable as data at the receiver.
Not only must the signals be generated to conform to the requirements of the
transmission system and receiver, but also there must be some form of synchronization
Table 1.1 Communications Tasks
Transmission system utilization
Addressing
Interfacing
Routing
Signal generation
Recovery
Synchronization
Message formatting
Exchange management
Security
Error detection and correction
Network management
Flow control
18
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
between transmitter and receiver.The receiver must be able to determine when a signal
begins to arrive and when it ends. It must also know the duration of each signal element.
Beyond the basic matter of deciding on the nature and timing of signals, there is
a variety of requirements for communication between two parties that might be collected under the term exchange management. If data are to be exchanged in both
directions over a period of time, the two parties must cooperate. For example, for two
parties to engage in a telephone conversation, one party must dial the number of the
other, causing signals to be generated that result in the ringing of the called phone.The
called party completes a connection by lifting the receiver. For data processing
devices, more will be needed than simply establishing a connection; certain conventions must be decided on. These conventions may include whether both devices may
transmit simultaneously or must take turns, the amount of data to be sent at one time,
the format of the data, and what to do if certain contingencies such as an error arise.
The next two items might have been included under exchange management,
but they seem important enough to list separately. In all communications systems,
there is a potential for error; transmitted signals are distorted to some extent before
reaching their destination. Error detection and correction are required in circumstances where errors cannot be tolerated. This is usually the case with data processing systems. For example, in transferring a file from one computer to another, it is
simply not acceptable for the contents of the file to be accidentally altered. Flow
control is required to assure that the source does not overwhelm the destination by
sending data faster than they can be processed and absorbed.
Next are the related but distinct concepts of addressing and routing. When
more than two devices share a transmission facility, a source system must indicate
the identity of the intended destination. The transmission system must assure that
the destination system, and only that system, receives the data. Further, the transmission system may itself be a network through which various paths may be taken.
A specific route through this network must be chosen.
Recovery is a concept distinct from that of error correction. Recovery techniques
are needed in situations in which an information exchange, such as a database transaction or file transfer, is interrupted due to a fault somewhere in the system.The objective
is either to be able to resume activity at the point of interruption or at least to restore
the state of the systems involved to the condition prior to the beginning of the exchange.
Message formatting has to do with an agreement between two parties as to the
form of the data to be exchanged or transmitted, such as the binary code for characters.
Frequently, it is important to provide some measure of security in a data communications system. The sender of data may wish to be assured that only the
intended receiver actually receives the data. And the receiver of data may wish to be
assured that the received data have not been altered in transit and that the data
actually come from the purported sender.
Finally, a data communications facility is a complex system that cannot create or
run itself. Network management capabilities are needed to configure the system, monitor its status, react to failures and overloads, and plan intelligently for future growth.
Thus, we have gone from the simple idea of data communication between
source and destination to a rather formidable list of data communications tasks. In
this book, we elaborate this list of tasks to describe and encompass the entire set of
activities that can be classified under data and computer communications.
1.3 / DATA COMMUNICATIONS
19
1.3 DATA COMMUNICATIONS
Following Part One, this book is organized into five parts. Part Two deals with the
most fundamental aspects of the communications function, focusing on the transmission of signals in a reliable and efficient manner. For want of a better name, we
have given Part Two the title “Data Communications,” although that term arguably
encompasses some or even all of the topics of Parts Three through Six.
A Data Communications Model
To get some flavor for the focus of Part Two, Figure 1.3 provides a new perspective
on the communications model of Figure 1.2a. We trace the details of this figure using
electronic mail as an example.
Suppose that the input device and transmitter are components of a personal
computer. The user of the PC wishes to send a message m to another user. The user
activates the electronic mail package on the PC and enters the message via the keyboard (input device). The character string is briefly buffered in main memory. We
can view it as a sequence of bits (g) in memory. The personal computer is connected
to some transmission medium, such as a local network or a telephone line, by an I/O
device (transmitter), such as a local network transceiver or a modem. The input data
are transferred to the transmitter as a sequence of voltage shifts [g(t)] representing
bits on some communications bus or cable. The transmitter is connected directly to
the medium and converts the incoming stream [g(t)] into a signal [s(t)] suitable for
transmission; specific alternatives will be described in Chapter 5.
The transmitted signal s(t) presented to the medium is subject to a number
of impairments, discussed in Chapter 3, before it reaches the receiver. Thus, the
received signal r(t) may differ from s(t). The receiver will attempt to estimate
the original s(t), based on r(t) and its knowledge of the medium, producing a
sequence of bits g¿1t2. These bits are sent to the output personal computer, where
they are briefly buffered in memory as a block of bits 1g¿2. In many cases, the
destination system will attempt to determine if an error has occurred and, if so,
cooperate with the source system to eventually obtain a complete, error-free block
of data. These data are then presented to the user via an output device, such as a
Digital bit
stream
Analog
signal
Analog
signal
Digital bit
stream
Text
Text
Transmitter
Source
1
Input
information
m
2
Input data
g(t)
Transmission
System
3
Transmitted
signal
s(t)
Figure 1.3 Simplified Data Communications Model
Receiver
4
Received
signal
r(t)
Destination
5
Output data
g'(t)
6
Output
information
m'
20
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
printer or screen. The message 1m¿2 as viewed by the user will usually be an exact
copy of the original message (m).
Now consider a telephone conversation. In this case the input to the telephone
is a message (m) in the form of sound waves. The sound waves are converted by the
telephone into electrical signals of the same frequency. These signals are transmitted
without modification over the telephone line. Hence the input signal g(t) and the
transmitted signal s(t) are identical. The signals (t) will suffer some distortion over
the medium, so that r(t) will not be identical to s(t). Nevertheless, the signal r(t) is
converted back into a sound wave with no attempt at correction or improvement of
signal quality. Thus, m¿ is not an exact replica of m. However, the received sound
message is generally comprehensible to the listener.
The discussion so far does not touch on other key aspects of data communications, including data link control techniques for controlling the flow of data and detecting and correcting errors, and multiplexing techniques for transmission efficiency.
The Transmission of Information
The basic building block of any communications facility is the transmission line.
Much of the technical detail of how information is encoded and transmitted across a
line is of no real interest to the business manager. The manager is concerned with
whether the particular facility provides the required capacity, with acceptable reliability, at minimum cost. However, there are certain aspects of transmission technology that a manager must understand to be able to ask the right questions and make
informed decisions.
One of the basic choices facing a business user is the transmission medium. For
use within the business premises, this choice is generally completely up to the business. For long-distance communications, the choice is generally but not always made
by the long-distance carrier. In either case, changes in technology are rapidly changing the mix of media used. Of particular note are fiber optic transmission and
wireless transmission (e.g., satellite and radio). These two media are now driving the
evolution of data communications transmission.
The ever-increasing capacity of fiber optic channels is making channel capacity a virtually free resource. The growth of the market for optical fiber transmission
systems since the beginning of the 1980s is without precedent. During the past
10 years, the cost of fiber optic transmission has dropped by more than an order of
magnitude, and the capacity of such systems has grown at almost as rapid a rate.
Long-distance telephone communications trunks within the United States will soon
consist almost completely of fiber optic cable. Because of its high capacity and
because of its security characteristics—fiber is almost impossible to tap—it is
becoming increasingly used within office buildings to carry the growing load of business information. However, switching is now becoming the bottleneck. This problem
is causing radical changes in communications architecture, including asynchronous
transfer mode (ATM) switching, highly parallel processing in switches, and integrated network management schemes.
The second medium—wireless transmission—is a result of the trend toward
universal personal telecommunications and universal access to communications.
The first concept refers to the ability of a person to identify himself or herself easily
1.3 / DATA COMMUNICATIONS
21
and to use conveniently any communication system in a large area (e.g., globally,
over a continent, or in an entire country) in terms of a single account. The second
refers to the capability of using one’s terminal in a wide variety of environments to
connect to information services (e.g., to have a portable terminal that will work in
the office, on the street, and on airplanes equally well). This revolution in personal
computing obviously involves wireless communication in a fundamental way.
Despite the growth in the capacity and the drop in cost of transmission facilities, transmission services remain the most costly component of a communications
budget for most businesses. Thus, the manager needs to be aware of techniques that
increase the efficiency of the use of these facilities. The two major approaches to
greater efficiency are multiplexing and compression. Multiplexing refers to the ability of a number of devices to share a transmission facility. If each device needs the
facility only a fraction of the time, then a sharing arrangement allows the cost of the
facility to be spread over many users. Compression, as the name indicates, involves
squeezing the data down so that a lower-capacity, cheaper transmission facility can
be used to meet a given demand. These two techniques show up separately and in
combination in a number of types of communications equipment. The manager
needs to understand these technologies to be able to assess the appropriateness and
cost-effectiveness of the various products on the market.
Transmission and Transmission Media Information can be communicated
by converting it into an electromagnetic signal and transmitting that signal over some
medium, such as a twisted-pair telephone line. The most commonly used transmission media are twisted-pair lines, coaxial cable, optical fiber cable, and terrestrial and
satellite microwave. The data rates that can be achieved and the rate at which errors
can occur depend on the nature of the signal and the type of medium. Chapters 3 and
4 examine the significant properties of electromagnetic signals and compare the various transmission media in terms of cost, performance, and applications.
Communication Techniques The transmission of information across a transmission medium involves more than simply inserting a signal on the medium. The
technique used to encode the information into an electromagnetic signal must be
determined. There are various ways in which the encoding can be done, and the
choice affects performance and reliability. Furthermore, the successful transmission
of information involves a high degree of cooperation between the various components. The interface between a device and the transmission medium must be agreed
on. Some means of controlling the flow of information and recovering from its loss
or corruption must be used. These latter functions are performed by a data link control protocol. All these issues are examined in Chapters 5 through 7.
Transmission Efficiency A major cost in any computer/communications facility
is transmission cost. Because of this, it is important to maximize the amount of information that can be carried over a given resource or, alternatively, to minimize the
transmission capacity needed to satisfy a given information communications requirement.Two ways of achieving this objective are multiplexing and compression.The two
techniques can be used separately or in combination. Chapter 8 examines the three
most common multiplexing techniques—frequency division, synchronous time division, and statistical time division—as well as the important compression techniques.
22
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
1.4 NETWORKS
The number of computers in use worldwide is in the hundreds of millions. Moreover, the expanding memory and processing power of these computers means that
users can put the machines to work on new kinds of applications and functions.
Accordingly, the pressure from the users of these systems for ways to communicate
among all these machines is irresistible. It is changing the way vendors think and the
way all automation products and services are sold. This demand for connectivity is
manifested in two specific requirements: the need for communications software,
which is previewed in the next section, and the need for networks.
One type of network that has become ubiquitous is the local area network
(LAN). Indeed, the LAN is to be found in virtually all medium- and large-size office
buildings. As the number and power of computing devices have grown, so have the
number and capacity of LANs to be found in an office. Although standards have
been developed that reduce somewhat the number of types of LANs, there are still
half a dozen general types of local area networks to choose from. Furthermore, many
offices need more than one such network, with the attendant problems of interconnecting and managing a diverse collection of networks, computers, and terminals.
Beyond the confines of a single office building, networks for voice, data, image,
and video are equally important to business. Here, too, there are rapid changes.
Advances in technology have led to greatly increased capacity and the concept of
integration. Integration means that the customer equipment and networks can deal
simultaneously with voice, data, image, and even video. Thus, a memo or report can
be accompanied by voice commentary, presentation graphics, and perhaps even a
short video introduction or summary. Image and video services impose large
demands on wide area network transmission. Moreover, as LANs become ubiquitous and as their transmission rates increase, the demands on the wide area networks
to support LAN interconnection have increased the demands on wide area network
capacity and switching. On the other hand, fortunately, the enormous and everincreasing capacity of fiber optic transmission provides ample resources to meet
these demands. However, developing switching systems with the capacity and rapid
response to support these increased requirements is a challenge not yet conquered.
The opportunities for using networks as an aggressive competitive tool and as
a means of enhancing productivity and slashing costs are great. The manager who
understands the technology and can deal effectively with vendors of service and
equipment is able to enhance a company’s competitive position.
In the remainder of this section, we provide a brief overview of various networks. Parts Three and Four cover these topics in depth.
Wide Area Networks
Wide area networks generally cover a large geographical area, require the crossing
of public right-of-ways, and rely at least in part on circuits provided by a common
carrier. Typically, a WAN consists of a number of interconnected switching nodes. A
transmission from any one device is routed through these internal nodes to the
specified destination device. These nodes (including the boundary nodes) are not
1.4 / NETWORKS
23
concerned with the content of the data; rather, their purpose is to provide a
switching facility that will move the data from node to node until they reach their
destination.
Traditionally, WANs have been implemented using one of two technologies:
circuit switching and packet switching. More recently, frame relay and ATM networks have assumed major roles.
Circuit Switching In a circuit-switching network, a dedicated communications
path is established between two stations through the nodes of the network. That
path is a connected sequence of physical links between nodes. On each link, a logical channel is dedicated to the connection. Data generated by the source station are
transmitted along the dedicated path as rapidly as possible. At each node, incoming
data are routed or switched to the appropriate outgoing channel without delay. The
most common example of circuit switching is the telephone network.
Packet Switching A quite different approach is used in a packet-switching network. In this case, it is not necessary to dedicate transmission capacity along a path
through the network. Rather, data are sent out in a sequence of small chunks,
called packets. Each packet is passed through the network from node to node along
some path leading from source to destination. At each node, the entire packet is
received, stored briefly, and then transmitted to the next node. Packet-switching
networks are commonly used for terminal-to-computer and computer-to-computer
communications.
Frame Relay Packet switching was developed at a time when digital longdistance transmission facilities exhibited a relatively high error rate compared to
today’s facilities. As a result, there is a considerable amount of overhead built into
packet-switching schemes to compensate for errors. The overhead includes additional bits added to each packet to introduce redundancy and additional processing
at the end stations and the intermediate switching nodes to detect and recover from
errors.
With modern high-speed telecommunications systems, this overhead is unnecessary and counterproductive. It is unnecessary because the rate of errors has been
dramatically lowered and any remaining errors can easily be caught in the end systems by logic that operates above the level of the packet-switching logic. It is counterproductive because the overhead involved soaks up a significant fraction of the
high capacity provided by the network.
Frame relay was developed to take advantage of these high data rates and low
error rates. Whereas the original packet-switching networks were designed with a
data rate to the end user of about 64 kbps, frame relay networks are designed to
operate efficiently at user data rates of up to 2 Mbps. The key to achieving these
high data rates is to strip out most of the overhead involved with error control.
ATM Asynchronous transfer mode (ATM), sometimes referred to as cell relay,
is a culmination of developments in circuit switching and packet switching. ATM
can be viewed as an evolution from frame relay. The most obvious difference
between frame relay and ATM is that frame relay uses variable-length packets,
called frames, and ATM uses fixed-length packets, called cells. As with frame
relay, ATM provides little overhead for error control, depending on the inherent
24
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
reliability of the transmission system and on higher layers of logic in the end systems to catch and correct errors. By using a fixed packet length, the processing
overhead is reduced even further for ATM compared to frame relay. The result is
that ATM is designed to work in the range of 10s and 100s of Mbps, and in the
Gbps range.
ATM can also be viewed as an evolution from circuit switching. With circuit
switching, only fixed-data-rate circuits are available to the end system. ATM
allows the definition of multiple virtual channels with data rates that are dynamically defined at the time the virtual channel is created. By using small, fixed-size
cells, ATM is so efficient that it can offer a constant-data-rate channel even
though it is using a packet-switching technique. Thus, ATM extends circuit switching to allow multiple channels with the data rate on each channel dynamically set
on demand.
Local Area Networks
As with WANs, a LAN is a communications network that interconnects a variety of
devices and provides a means for information exchange among those devices. There
are several key distinctions between LANs and WANs:
1. The scope of the LAN is small, typically a single building or a cluster of buildings. This difference in geographic scope leads to different technical solutions,
as we shall see.
2. It is usually the case that the LAN is owned by the same organization that owns
the attached devices. For WANs, this is less often the case, or at least a significant
fraction of the network assets is not owned. This has two implications. First, care
must be taken in the choice of LAN, because there may be a substantial capital
investment (compared to dial-up or leased charges for WANs) for both purchase
and maintenance. Second, the network management responsibility for a LAN
falls solely on the user.
3. The internal data rates of LANs are typically much greater than those of
WANs.
LANs come in a number of different configurations. The most common are
switched LANs and wireless LANs. The most common switched LAN is a switched
Ethernet LAN, which may consist of a single switch with a number of attached
devices, or a number of interconnected switches. Two other prominent examples are
ATM LANs, which simply use an ATM network in a local area, and Fibre Channel.
Wireless LANs use a variety of wireless transmission technologies and organizations. LANs are examined in depth in Part Four.
Wireless Networks
As was just mentioned, wireless LANs are common are widely used in business
environments. Wireless technology is also common for both wide area voice and
data networks. Wireless networks provide advantages in the areas of mobility and
ease of installation and configuration. Chapters 14 and 17 deal with wireless WANs
and LANs, respectively.
1.5 / THE INTERNET
25
1.5 THE INTERNET
Origins of the Internet
The Internet evolved from the ARPANET, which was developed in 1969 by the
Advanced Research Projects Agency (ARPA) of the U.S. Department of Defense.
It was the first operational packet-switching network. ARPANET began operations
in four locations. Today the number of hosts is in the hundreds of millions, the number of users in the billions, and the number of countries participating nearing 200.
The number of connections to the Internet continues to grow exponentially.
The network was so successful that ARPA applied the same packet-switching
technology to tactical radio communication (packet radio) and to satellite communication (SATNET). Because the three networks operated in very different
communication environments, the appropriate values for certain parameters, such
as maximum packet size, were different in each case. Faced with the dilemma of
integrating these networks, Vint Cerf and Bob Kahn of ARPA started to develop
methods and protocols for internetworking; that is, communicating across arbitrary, multiple, packet-switched networks. They published a very influential paper
in May of 1974 [CERF74] outlining their approach to a Transmission Control Protocol. The proposal was refined and details filled in by the ARPANET community,
with major contributions from participants from European networks, such as
Cyclades (France), and EIN, eventually leading to the TCP (Transmission Control
Protocol) and IP (Internet Protocol) protocols, which, in turn, formed the basis for
what eventually became the TCP/IP protocol suite. This provided the foundation
for the Internet.
Key Elements
Figure 1.4 illustrates the key elements that comprise the Internet. The purpose of
the Internet, of course, is to interconnect end systems, called hosts; these include
PCs, workstations, servers, mainframes, and so on. Most hosts that use the Internet
are connected to a network, such as a local area network (LAN) or a wide area network (WAN). These networks are in turn connected by routers. Each router
attaches to two or more networks. Some hosts, such as mainframes or servers, connect directly to a router rather than through a network.
In essence, the Internet operates as follows. A host may send data to another
host anywhere on the Internet. The source host breaks the data to be sent into a
sequence of packets, called IP datagrams or IP packets. Each packet includes a
unique numeric address of the destination host. This address is referred to as an IP
address, because the address is carried in an IP packet. Based on this destination
address, each packet travels through a series of routers and networks from source to
destination. Each router, as it receives a packet, makes a routing decision and forwards the packet along its way to the destination.
Internet Architecture
The Internet today is made up of thousands of overlapping hierarchical networks.
Because of this, it is not practical to attempt a detailed description of the exact
26
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
Router
Standalone
mainframe
Local area
network
Wide area network
(e.g., ATM)
Router
Router
Wide area network
(e.g., ATM)
Ethernet
switch
Local area
network
Ethernet
switch
Router
Information
server
LAN PCs
and workstations
Figure 1.4 Key Elements of the Internet
architecture or topology of the Internet. However, an overview of the common, general characteristics can be made. Figure 1.5 illustrates the discussion and Table 1.2
summarizes the terminology.
A key element of the Internet is the set of hosts attached to it. Simply put, a
host is a computer. Today, computers come in many forms, including mobile phones
and even cars. All of these forms can be hosts on the Internet. Hosts are sometimes
grouped together in a LAN. This is the typical configuration in a corporate environment. Individual hosts and LANs are connected to an Internet service provider
(ISP) through a point of presence (POP). The connection is made in a series of steps
starting with the customer premises equipment (CPE). The CPE is the communications equipment located onsite with the host.
For many home users, the CPE is a 56-kbps modem. This is perfectly adequate
for e-mail and related services but marginal for graphics-intensive Web surfing.
Newer CPE offerings provide greater capacity and guaranteed service in some
cases. A sample of these new access technologies includes DSL, cable modem, and
satellite. Users who connect to the Internet through their work often use workstations or PCs connected to their employer-owned LANs, which in turn connect
through shared organizational trunks to an ISP. In these cases the shared circuit is
often a T-1 connection (1.544 Mbps), while for very large organizations T-3 connections (44.736 Mbps) are sometimes found. Alternatively, an organization’s LAN
1.5 / THE INTERNET
27
Corporate
LAN
Residential
subscribers
Regional
ISP
Backbone
ISP
Backbone
ISP
te
riva
g
rin
pee
P
Server
Regional
ISP
LAN
switch
Regional
ISP
ISP Web
farm
Server
Server
Corporate
LAN
Open circle NAP
Filled circle POP
Figure 1.5 Simplified View of Portion of Internet
may be hooked to a wide area network (WAN), such as a frame relay network,
which in turn connects to an ISP.
The CPE is physically attached to the “local loop” or “last mile.”This is the infrastructure between a provider’s installation and the site where the host is located. For
example, a home user with a 56K modem attaches the modem to the telephone line.
The telephone line is typically a pair of copper wires that runs from the house to a
central office (CO) owned and operated by the telephone company. In this instance
the local loop is the pair of copper wires running between the home and the CO. If the
home user has a cable modem, the local loop is the coaxial cable that runs from
the home to the cable company facilities. The preceding examples are a bit of an oversimplification, but they suffice for this discussion. In many cases the wires that leave a
home are aggregated with wires from other homes and then converted to a different
media such as fiber. In these cases the term local loop still refers to the path from the
home to the CO or cable facility. The local loop provider is not necessarily the ISP. In
many cases the local loop provider is the telephone company and the ISP is a large,
national service organization. Often, however, the local loop provider is also the ISP.
The ISP provides access to its larger network through a POP. A POP is simply
a facility where customers can connect to the ISP network. The facility is sometimes
owned by the ISP, but often the ISP leases space from the local loop carrier. A POP
can be as simple as a bank of modems and an access server installed in a rack at the
CO. The POPs are usually spread out over the geographic area where the provider
28
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
Table 1.2 Internet Terminology
Central Office (CO)
The place where telephone companies terminate customer lines and locate switching equipment to interconnect
those lines with other networks.
Customer Premises Equipment (CPE)
Telecommunications equipment that is located on the customer’s premises (physical location) rather than on
the provider’s premises or in between. Telephone handsets, modems, cable TV set-top boxes, and digital
subscriber line routers are examples. Historically, this term referred to equipment placed at the customer’s end
of the telephone line and usually owned by the telephone company. Today, almost any end-user equipment can
be called customer premises equipment and it can be owned by the customer or by the provider.
Internet Service Provider (ISP)
A company that provides other companies or individuals with access to, or presence on, the Internet. An ISP
has the equipment and the telecommunication line access required to have a POP on the Internet for the
geographic area served. The larger ISPs have their own high-speed leased lines so that they are less dependent
on the telecommunication providers and can provide better service to their customers.
Network Access Point (NAP)
In the United States, a network access point (NAP) is one of several major Internet interconnection points that
serve to tie all the ISPs together. Originally, four NAPs—in New York, Washington, D.C., Chicago, and San
Francisco—were created and supported by the National Science Foundation as part of the transition from the
original U.S. government—financed Internet to a commercially operated Internet. Since that time, several new
NAPs have arrived, including WorldCom’s “MAE West” site in San Jose, California and ICS Network Systems’
“Big East.”
The NAPs provide major switching facilities that serve the public in general. Companies apply to use the
NAP facilities. Much Internet traffic is handled without involving NAPs, using peering arrangements and
interconnections within geographic regions.
Network Service Provider (NSP)
A company that provides backbone services to an Internet service provider (ISP). Typically, an ISP connects at
a point called an Internet exchange (IX) to a regional ISP that in turn connects to an NSP backbone.
Point of Presence (POP)
A site that has a collection of telecommunications equipment, usually refers to ISP or telephone company
sites. An ISP POP is the edge of the ISP’s network; connections from users are accepted and authenticated
here. An Internet access provider may operate several POPs distributed throughout its area of operation to
increase the chance that their subscribers will be able to reach one with a local telephone call. The largest
national ISPs have POPs all over the country.
offers service. The ISP acts as a gateway to the Internet, providing many important
services. For most home users, the ISP provides the unique numeric IP address
needed to communicate with other Internet hosts. Most ISPs also provide name resolution and other essential network services. The most important service an ISP provides, though, is access to other ISP networks. Access is facilitated by formal peering
agreements between providers. Physical access can be implemented by connecting
POPs from different ISPs. This can be done directly with a local connection if the
POPs are collocated or with leased lines when the POPs are not collocated. A more
commonly used mechanism is the network access point (NAP).
A NAP is a physical facility that provides the infrastructure to move data
between connected networks. In the United States, the National Science Foundation
(NSF) privatization plan called for the creation of four NAPs. The NAPs were built
and are operated by the private sector. The number of NAPs has grown significantly
1.6 / AN EXAMPLE CONFIGURATION
29
over the years, and the technology employed has shifted from Fiber Distributed
Data Interface (FDDI) and Ethernet to ATM and Gigabit Ethernet. Most NAPs
today have an ATM core. The networks connected at a NAP are owned and operated by network service providers (NSPs). A NSP can also be an ISP but this is not
always the case. Peering agreements are between NSPs and do not include the NAP
operator. The NSPs install routers at the NAP and connect them to the NAP infrastructure. The NSP equipment is responsible for routing, and the NAP infrastructure
provides the physical access paths between routers.
A small hypothetical example can help make the picture clearer. In this example there are two companies, one named A, Inc. and the other B, Inc. and they are
both NSPs. A, Inc. and B, Inc. have a peering agreement and they both install routers
in two NAPs, one located on the east coast of the United States and the other on the
west coast. There are also two other companies known as Y, Inc. and Z, Inc. and they
are both ISPs. Finally, there is a home user named Bob and a small company named
Small, Inc.
Small, Inc. has four hosts connected together into a LAN. Each of the four
hosts can communicate and share resources with the other three. Small, Inc. would
like access to a broader set of services so they contract with ISP Y, Inc. for a connection. Small, Inc. installs a CPE to drive a leased T-1 line into a Y, Inc. POP. Once the
CPE is connected, software automatically assigns a numeric address to each Small,
Inc. host. The Small, Inc. hosts can now communicate and share resources with any
other host connected to the larger ISP network. On the other side of the country,
Bob decides to contract with ISP Z, Inc. He installs a modem on his phone line to
dial into a Z, Inc. POP. Once the modem connects, a numeric address is automatically assigned to his home computer. His computer can now communicate and share
resources with any other computer connected to the larger ISP network.
Bob’s home machine and the hosts owned by Small, Inc. cannot yet communicate. This becomes possible when their respective ISPs contract with NSPs that have
a peering agreement. In this example, the ISP Y, Inc. decides to expand its service
coverage to the opposite coast and contracts with the NSP A, Inc. A, Inc. sells bandwidth on its high-speed coast-to-coast network. The ISP Z, Inc. also wishes to expand
its service coverage and contracts with the NSP B, Inc. Like A, Inc., B, Inc. also sells
bandwidth on a high-speed coast-to-coast network. Because A, Inc. and B, Inc. have a
peering agreement and have implemented the agreement at two NAPs, Bob’s home
machine and the hosts of Small, Inc. can now communicate and share resources.
Although this example is contrived, in principle this is what the Internet is. The differences are that the Internet has millions of hosts and many thousands of networks
using dozens of access technologies, including satellite, radio, leased T-1, and DSL.
1.6 AN EXAMPLE CONFIGURATION
To give some feel for the scope of concerns of Parts Two through Four, Figure 1.6
illustrates some of the typical communications and network elements in use today.
In the upper-left-hand portion of the figure, we see an individual residential user
connected to an Internet service provider (ISP) through some sort of subscriber
connection. Common examples of such a connection are the public telephone
30
CHAPTER 1 / DATA COMMUNICATIONS, DATA NETWORKS, AND THE INTERNET
Subscriber
connection
Residential
user
High-speed link
(e.g., SONET)
Internet service
provider (ISP)
Router
Internet
ATM
switch
Firewall
host
High-speed
link
ATM network
Ethernet
switch
Router
Private
WAN
Information
server
LAN PCs
and workstations
Figure 1.6 A Networking Configuration
network, for which the user requires a dial-up modem (e.g. a 56-kbps modem); a digital subscriber line (DSL), which provides a high-speed link over telephone lines
and requires a special DSL modem; and a cab...
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