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Ethernet
The Definitive Guide
Charles E. Spurgeon
Beijing • Cambridge • Farnham • Köln • Paris • Sebastopol • Taipei • Tokyo
Page iv
Ethernet: The Definitive Guide
by Charles E. Spurgeon
Copyright © 2000 O'Reilly & Associates, Inc. All rights reserved.
Printed in the United States of America.
Published by O'Reilly & Associates, Inc., 101 Morris Street, Sebastopol, CA 95472.
Editors: Mark Stone and Chuck Toporek
Production Editor: David Futato
Cover Designer: Hanna Dyer
Printing History:
                  February 2000:                      First Edition.
Nutshell Handbook, the Nutshell Handbook logo, and the O'Reilly logo are registered trademarks
of O'Reilly & Associates, Inc. Many of the designations used by manufacturers and sellers to
distinguish their products are claimed as trademarks. Where those designations appear in this book,
and O'Reilly & Associates, Inc. was aware of a trademark claim, the designations have been printed
in caps or initial caps. The association between the image of an octopus and the topic of Ethernet is
a trademark of O'Reilly & Associates, Inc. SC connector is a trademark of NTT Advanced
Technology Corporation. ST connector is a trademark of American Telegraph & Telephone.
Some portions of this book have been previously published and are reprinted here with permission
of the author. Portions of the information contained herein are reprinted with permission from IEEE
Std 802.3, Copyright © 1995, 1996, 1999, by IEEE. The IEEE disclaims any responsibility or
liability resulting from the placement and use in the described manner.
While every precaution has been taken in the preparation of this book, the publisher assumes no
responsibility for errors or omissions, or for damages resulting from the use of the information
contained herein.
Library of Congress Cataloging-in-Publication Data
Spurgeon, Charles (Charles E.)
    Ethernet: the definitive guide / Charles E. Spurgeon
        p. cm.
    ISBN 1-56592-660-9 (alk. paper)
        1. Ethernet (Local area network system) I. Title.
    TK5105.8.E83 S67 2000
    004.6'8--dc21
                                                                           99-086932
[M]
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TABLE OF CONTENTS
Preface
xi
I. Introduction to Ethernet
1
1. The Evolution of Ethernet
3
History of Ethernet
3
The Latest Ethernet Standard
8
Organization of IEEE Standards
10
Levels of Compliance
13
IEEE Identifiers
15
Reinventing Ethernet
19
Multi-Gigabit Ethernet
22
2. The Ethernet System
23
Four Basic Elements of Ethernet
24
Ethernet Hardware
29
Network Protocols and Ethernet
34
3. The Media Access Control Protocol
39
The Ethernet Frame
40
Media Access Control Rules
47
Essential Media System Timing
50
Collision Detection and Backoff
53
Gigabit Ethernet Half-Duplex Operation
60
Collision Domain
65
Ethernet Channel Capture
67
High-level Protocols and the Ethernet Frame
70
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4. Full-Duplex Ethernet
76
Operation of Full-Duplex
77
Ethernet Flow Control
82
5. Auto-Negotiation
85
Development of Auto-Negotiation
85
Basic Concepts of Auto-Negotiation
86
Auto-Negotiation Signaling
87
Auto-Negotiation Operation
90
Parallel Detection
94
Management Interface
96
1000BASE-X Auto-Negotiation
96
II. Ethernet Media Systems
99
6. Ethernet Media Fundamentals
101
Attachment Unit Interface
102
Medium-Independent Interface
108
Gigabit Medium-Independent Interface
114
Ethernet Signal Encoding
117
Ethernet Network Interface Card
122
7. Twisted-Pair Media System (10BASE-T)
125
10BASE-T Signaling Components
125
10BASE-T Media Components
128
10BASE-T Configuration Guidelines
132
8. Fiber Optic Media System
134
Old and New Fiber Link Segments
134
10BASE-FL Signaling Components
136
10BASE-FL Media Components
137
Connecting a Station to 10BASE-FL Ethernet
139
10BASE-FL Configuration Guidelines
140
9. Fast Ethernet Twisted-Pair Media System (100BASE-TX)
142
100BASE-TX Signaling Components
142
100BASE-TX Media Components
145
Connecting a Station to 100BASE-TX Ethernet
146
100BASE-TX Configuration Guidelines
147
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10. Fast Ethernet Fiber Optic Media System (100BASE-FX)
149
100BASE-FX Signaling Components
149
100BASE-FX Media Components
152
Connecting a Station to 100BASE-FX Ethernet
153
100BASE-FX Configuration Guidelines
154
11. Gigabit Ethernet Twisted-Pair Media System
(1000BASE-T)
156
1000BASE-T Signaling Components
157
1000BASE-T Signal Encoding
158
1000BASE-T Media Components
160
Connecting a Station to 1000BASE-T Ethernet
162
1000BASE-T Configuration Guidelines
163
12. Gigabit Ethernet Fiber Optic Media System
(1000BASE-X)
164
1000BASE-X Signaling Components
165
1000BASE-X Signal Encoding
166
1000BASE-X Media Components
167
1000BASE-SX and 1000BASE-LX Media Components
168
1000BASE-CX Media Components
169
1000BASE-SX and 1000BASE-LX Configuration Guidelines
171
13. Multi-Segment Configuration Guidelines
173
Scope of the Configuration Guidelines
174
Network Documentation
174
Collision Domain
174
Model 1 Configuration Guidelines for 10 Mbps
176
Model 2 Configuration Guidelines for 10 Mbps
177
Model 1 Configuration Guidelines for Fast Ethernet
184
Model 2 Configuration Guidelines for Fast Ethernet
186
Model 1 Configuration Guidelines for Gigabit Ethernet
190
Model 2 Configuration Guidelines for Gigabit Ethernet
191
Sample Network Configurations
193
III. Building Your Ethernet System
203
14. Structured Cabling
205
Structured Cabling Systems
206
TIA/EIA Cabling Standards
207
Twisted-Pair Categories
211
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Ethernet and the Category System
213
Horizontal Cabling
214
New Twisted-Pair Standards
217
Identifying the Cables
219
Documenting the Cable System
221
Building the Cabling System
222
15. Twisted-Pair Cables and Connectors
224
Category 5 Horizontal Cable Segment
230
Eight-Position (RJ-45-Style) Jack
230
Four-Pair Wiring Schemes
230
Modular Patch Panel
234
Work Area Outlet
235
Twisted-Pair Patch Cables
236
Building a Twisted-Pair Patch Cable
239
Ethernet Signal Crossover
244
Twisted-Pair Ethernet and Telephone Signals
248
16. Fiber Optic Cables and Connectors
249
Fiber Optic Cable
249
10BASE-FL Fiber Optic Characteristics
256
100BASE-FX Fiber Optic Characteristics
257
1000BASE-X Fiber Optic Characteristics
258
17. Ethernet Repeater Hubs
264
Collision Domain
265
Basic Repeater Operation
266
Repeater Buying Guide
269
10 Mbps Repeaters
276
100 Mbps Repeaters
281
1000 Mbps Gigabit Ethernet Repeater
285
Repeater Management
286
Repeater Port Statistics
289
18. Ethernet Switching Hubs
298
Brief Tutorial on Ethernet Bridging
299
Advantages of Switching Hubs
306
Switching Hub Performance Issues
311
Advanced Features of Switching Hubs
314
Network Design Issues with Switches
320
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IV. Performance and Troubleshooting
325
19. Ethernet Performance
327
Performance of an Ethernet Channel
328
Measuring Ethernet Performance
334
Network Performance and the User
338
Network Design for Best Performance
342
20. Troubleshooting
346
Reliable Network Design
347
Network Documentation
348
The Troubleshooting Model
350
Fault Detection
352
Fault Isolation
354
Troubleshooting Twisted-Pair Systems
357
Troubleshooting Fiber Optic Systems
361
Data Link Troubleshooting
364
Network Layer Troubleshooting
368
V. Appendixes
371
A. Resources
373
B. Thick and Thin Coaxial Media Systems
383
C. AUI Equipment: Installation and Configuration
430
Glossary
441
Index
459
Page xi
PREFACE
This is a book about Ethernet, a local area network (LAN) technology that allows you to connect a
variety of computers together with a low-cost and extremely flexible network system. Virtually every
computer manufacturer today supports Ethernet, and this broad support, coupled with its low cost
and high flexibility, are major reasons for Ethernet's popularity.
This book provides a comprehensive and practical source of information on the entire Ethernet
system in a single volume. The goal of this book is to be definitive: to describe the entire range of
Ethernet technology specified in the IEEE standard for Ethernet. This includes 10 Mbps Ethernet,
100 Mbps Fast Ethernet, 1000 Mbps Gigabit Ethernet, full-duplex Ethernet, descriptions of all
Ethernet media systems, and repeaters and repeater configuration guidelines. Also described in this
book are switching hubs, structured cabling systems, network management, troubleshooting and
more.
This book shows how Ethernet components can be combined to create Ethernet LANs. While
some basic network designs are shown in this book, there are an infinity of network designs that can
be built using Ethernet, ranging from the smallest workgroup on up to very large enterprise networks
that support tens of thousands of computers.
The design of complete network systems that use Ethernet to carry data between computers is a
major subject, and a number of books are needed to describe all of the issues that can be
encountered. Since this book is about how Ethernet technology works, we stay focused on that
topic. As anyone who reads the entire book would agree, this topic alone has more than enough
detail for any single book to cover.
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The Ethernet system has grown over the years, becoming ever larger and more complex. It now
includes a wide variety of media systems, each based on its own particular set of hardware and each
with its own configuration guidelines. This book covers all Ethernet systems that have ever been
widely implemented, from the latest Gigabit Ethernet systems all the way back to the original coaxial
cable systems. With this book you can support the entire range of Ethernet technology you may
encounter.
As the Ethernet system has grown more complex, a number of misconceptions and
misunderstandings have arisen about how Ethernet functions and how the system should be
configured. To provide the most accurate information possible and to help combat incorrect
"Ethernet folklore," I kept a complete set of official Ethernet standards at my elbow while writing this
book and referred to them frequently. I have been working with Ethernet technology since the early
1980s, and that experience has included many hard-won lessons in network design and operation
that have also made their way into this book.
Ethernet Is Everywhere
There are a number of factors that have helped Ethernet to become so popular. Among these
factors are cost, scalability, reliability, and widely available management tools.
Cost
The rapid evolution of new capabilities in Ethernet has also been accompanied by a rapid decrease
in the cost of Ethernet equipment. The widespread adoption of Ethernet technology created a large
and fiercely competitive Ethernet marketplace, which drives down the cost of networking
components. As a result, the consumer wins out in the process, with the marketplace providing a
wide range of competitively priced Ethernet components to choose from.
Scalability
The first industry-wide Ethernet standard was published in 1980. This standard defined a 10 Mbps
system, which was very fast for the times, and which remained fast enough for most uses until the
mid-1990s. The development of the 100 Mbps Fast Ethernet system in 1995 provided a tenfold
increase in speed. Fast Ethernet has been a major success, and network interfaces that can
automatically support both 10 and 100 Mbps operation are widely available, making the transition
from 10 Mbps to 100 Mbps systems very easy to accomplish.
Page xiii
Applications tend to grow to fill all available bandwidth. To anticipate the rising demand, Gigabit
Ethernet was developed in 1998, providing yet another tenfold increase in performance. All of this
makes it possible for a network manager to provide high-speed backbone systems and connections
to high-performance servers. Desktop machines can be connected to the original 10 Mbps Ethernet,
100 Mbps Fast Ethernet, or Gigabit Ethernet as required.
Reliability
Ethernet uses a simple and robust transmission mechanism that reliably delivers data day in and day
out at sites all over the world. Ethernet based on twisted-pair media was introduced in 1987,
making it possible to provide Ethernet signals over a structured cabling system. Structured cabling
provides a data delivery system for a building that is modeled on high-reliability cabling practices
originally developed for the telephone system. This makes it possible to install a standards-based
cabling system for Ethernet that is very reliable, as well as being simple, stable, and easy to manage.
Widely Available Management Tools
The widespread acceptance of Ethernet brings another advantage, which is the wide availability of
Ethernet management and troubleshooting tools. Management tools based on standards, such as the
Simple Network Management Protocol (SNMP), make it possible for network administrators to
keep track of an entire campus full of Ethernet equipment from a central location. Management
capabilities embedded in Ethernet repeaters, switching hubs, and computer interfaces provide
powerful network monitoring and troubleshooting capabilities.
Design for Reliability
A major goal of this book is to help you design and implement reliable networks. Network reliability
is of paramount importance to any networked organization. Information sharing between networked
computers is an essential feature of today's workplace, and if the network fails, everything comes to
a halt. This book shows you how to design reliable networks, how to monitor them and keep them
working reliably, and how to fix them should something fail.
The wide range of Ethernet components and cabling systems that are available today provides
enormous flexibility, making it possible to build an Ethernet to fit just about any circumstance.
However, all this flexibility does have a price. The many varieties of Ethernet each have their own
components and their own configuration rules, which can make the life of a network designer
complex. Designing and implementing a reliable Ethernet system requires that you understand how
all
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the bits and pieces fit together, and that you follow the official guidelines for the configuration of the
media systems.
This book provides the complete set of official configuration guidelines for every commercially
available media system, as well as the official guidelines for combining media systems. You'll also
find a great deal of information on how to build media systems that meet the standards and that will
function reliably.
Downtime Is Expensive
Avoiding network downtime is important for a number of reasons, not least of which is the amount
of money that downtime can cost. Some quick ''back of the envelope'' calculations can show how
expensive network downtime can be. Let's assume that there are 1,000 users of the network at the
Amalgamated Widget Company, and that their average annual salary including all overhead
(benefits, pension, etc.) is $75,000. That comes to $75 million a year in employee costs.
Let's further assume that everyone in the company depends on the network to get their work done,
and that the network is used 40 hours a week, for about 50 weeks of the year (excluding holidays).
That's 2,000 hours of network operation. Dividing the annual employee cost by the hours of
network operation shows that the network is supporting $37,500 per hour of employee cost during
the year.
When we total up all of the network outages over the period of a year in our hypothetical
corporation, we find that the network was down 2.5 percent of the time. That's an annual total of 50
hours, or one hour a week, or a mere 12 minutes each day. Fifty hours of network downtime at
$37,500/hour is $1.8 million in lost productivity due to network outage. Obviously, our calculations
are very "quick and dirty." We didn't bother to calculate the impact of network outages during times
when no one is around, but during which times the network is still supporting critically important
servers. Also, we're assuming that a network failure brings all operations to a halt, instead of trying
to factor in the varying effects of localized failures that cause outages on only a portion of the
network system. Nor do we try to estimate how much other work people could get done while the
network is down, which would tend to lessen the impact.
However, the main point is clear: even small amounts of network downtime can cost a company
quite a lot in lost productivity. That's why it's worth investing extra time, effort and money to create
the most reliable network system you can afford.
Page xv
Organization of This Book
The purpose of this book is to provide a comprehensive and practical guide to the entire Ethernet
system. The emphasis is on practical issues, with minimal theory and jargon. Chapters are kept as
self-contained as possible, and many examples and illustrations are provided. The book is organized
in five parts to make it easier to find the specific information you need.
These five parts provide:
• An introduction to the Ethernet standard which describes Ethernet operation in detail. This part of
the book covers those portions of Ethernet operation that are common to all Ethernet media
systems.
• A description of each of the Ethernet media systems, including 10-, 100-, and 1000 Mbps
systems operating over twisted-pair and fiber optic cables. The older coaxial media systems are
described in Appendix B, Thick and Thin Coaxial Media Systems.
• A description of structured cabling systems and the components and cables used in building your
Ethernet system. These include twisted-pair and fiber optic cables, and repeater and switching hubs.
• A description of Ethernet performance and Ethernet troubleshooting.
• Appendixes and glossary.
Part I, Introduction to Ethernet
Chapters 1–5 provide a tour of basic Ethernet theory and operation. This section includes the
portions of Ethernet operation that are common to all of the Ethernet media systems, including the
structure of the Ethernet frame and the operation of the media access control (MAC) system.
Chapter 1, The Evolution of Ethernet
Gives a brief guide to the history of Ethernet and the development of the IEEE 802.3 standard
for Ethernet.
Chapter 2, The Ethernet System
Presents an overview of how the Ethernet system operates, introducing the major concepts.
Chapter 3, The Media Access Control Protocol
Provides an in-depth look at how the original half-duplex Ethernet channel operates.
Chapter 4, Full-Duplex Ethernet
Describes the full-duplex mode of Ethernet operation.
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Chapter 5, Auto-Negotiation
Describes the auto-negotiation mechanisms used to automatically configure Ethernet equipment.
Part II, Ethernet Media Systems
Chapter 6, Ethernet Media Fundamentals, provides an introduction to the basic media system
components used in all Ethernet media systems. This chapter is essential reading before going on to
the individual media systems, described in Chapters 7–12. Chapter 13, Multi-Segment
Configuration Guidelines, completes this part of the book with a description of the configuration
guidelines that apply when linking media systems together with repeaters.
Each of the media system chapters are based on an identical format, which helps to organize and
clearly present the information needed to cover all of the Ethernet media varieties. While every effort
was made to avoid needless duplication of text, the identical format leads to some unavoidable
repetition in these chapters. This is especially noticeable if you read several media chapters in a row.
Chapter 6, Ethernet Media Fundamentals
Describes the Ethernet media components and the basic concepts that are common to each of
the media systems.
Chapter 7, Twisted-Pair Media System (10BASE-T)
Chapter 8, Fiber Optic Media System (10BASE-F)
Chapter 9, Fast Ethernet Twisted-Pair Media System (100BASE-TX)
Chapter 10, Fast Ethernet Fiber Optic Media System (100BASE-FX)
Chapter 11, Gigabit Ethernet Twisted-Pair Media System (1000BASE-T)
Chapter 12, Gigabit Ethernet Fiber Optic Media System (1000BASE-X)
Describe the hardware components and official configuration guidelines for each media variety.
Chapter 13, Multi-Segment Configuration Guidelines
Describes the official guidelines for combining media varieties using repeaters.
Part III, Building Your Ethernet System
Chapter 14 describes the structured cabling standards. Chapters 15 and 16 provide information on
the configuration and construction of twisted-pair and fiber optic cable segments; Chapters 17 and
18 describe how to design and build Ethernet systems using repeaters and switches.
Chapter 14, Structured Cabling
Describes structured cabling systems and the structured cabling standards that specify how
these systems are built.
Page xvii
Chapter 15, Twisted-Pair Cables and Connectors
Describes the twisted-pair cables and components used in twisted-pair network segments.
Chapter 16, Fiber Optic Cables and Connectors
Describes the fiber optic cables and components used in fiber optic network segments.
Chapter 17, Ethernet Repeater Hubs
Describes the operation and management of Ethernet repeater hubs and how to design
networks using them.
Chapter 18, Ethernet Switching Hubs
Describes the operation and management of Ethernet switching hubs and how to design
networks using them.
Part IV, Performance and Troubleshooting
Chapters 19 and 20 cover network performance and network troubleshooting.
Chapter 19, Ethernet Performance
Describes Ethernet system performance and how to measure overall network performance.
Chapter 20, Troubleshooting
Describes how to go about troubleshooting problems when they occur.
Part V, Appendixes
Appendix A, Resources
Describes additional sources of information on Ethernet, including books, periodicals, and web
sites.
Appendix B, Thick and Thin Coaxial Media Systems
Describes the thick and thin coaxial media systems and hardware components.
Appendix C, AUI Equipment: Installation and Configuration
Describes equipment and configuration issues based on the original 15-pin Ethernet AUI.
Glossary
Provides concise definitions of the acronyms and technical terms relevant to Ethernet.
Online References
A number of online references are provided in this book, based on the Universal Resource Locators
(URLs) used on the World Wide Web. Web references are live
Page xviii
in the sense that the Web is constantly evolving and changing, which may render a reference
obsolete. Sometimes a replacement link will be left, pointing to the new location for the information.
If that happens, all you have to do is click on the new link to find what you're looking for.
Other times a site may be reorganized in a manner that leaves no forwarding link to the new location.
If an online reference no longer works, you can try several approaches to finding the material.
One method is to access the top-level web page by using the first part of the URL, which specifies
the domain name of the site. For example, if the URL
http://www.bellereti.com/ethernet/ethernet.html should fail to work, you could try just the
domain name portion of the URL, located inside the first set of slashes, http://www.bellereti.com/,
and see what you find there.
Some web sites may also be equipped with a search feature that allows you to type in the name of
the material you are looking for at that site. If all else fails, you can try one of the many web search
sites that will search the entire Web for the subjects you're looking for.
How to Use This Book
The goal of this book is to provide the information needed for you to understand and operate any
Ethernet system. For example, if you are a newcomer to Ethernet and you need to know how
twisted-pair Ethernet systems work, then you can start with the chapters in Part I. After reading
those chapters, you can go to the twistedpair media chapters in Part II, as well as the twisted-pair
cabling information in Part III. Twisted-pair segments can be connected together with both repeater
hubs and switching hubs, and these are also described in Part III. Experts in Ethernet can use the
book as a reference guide and jump directly to those chapters that contain the reference information
they need.
Conventions Used in This Book
• Constant Willison is used for program examples, attribute value literals, start- and
end-tags, and source code example text.
• Constant Willison Oblique is used for "replaceable" text or variables. Replacement
text is text that describes something you're supposed to type, like a filename, in which the word
"filename" acts as a placeholder for the actual filename.
Page xix
• Garamond Italic is used for filenames and URLs.
• URLs (http://www.oreilly.com/) are presented in parenthesis after the name of the resource they
describe in the book.
The owl icon designates a note, which is an important aside to its
nearby text.
The turkey icon designates a warning relating to the nearby text.
How to Contact Us
We have tested and verified the information in this book to the best of our ability, but you may find
that features have changed (or even that we have made mistakes!). Please let us know about any
errors you find, as well as your suggestions for future editions, by writing to:
O'Reilly & Associates, Inc.
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The web site for Ethernet: The Definitive Guide lists errata and plans for future editions. You can
access this page at:
http://www.oreilly.com/catalog/ethernet/
Page xx
For more information about this book and others, see the main O'Reilly web site:
http://www.oreilly.com/
Acknowledgments
This book would not have been possible without the help of many people. First and foremost, I
would like to thank the inventor of Ethernet, Bob Metcalfe, and his fellow researchers at Xerox
PARC. Their work revolutionized the way computers are used, unleashing a powerful new
communications technology based on information sharing applications running on computers linked
with networks. I also thank the many engineers who have voluntarily given their time in countless
IEEE standards meetings to help develop the Ethernet system and to write the Ethernet
specifications.
I would like to thank Mark Stone, executive editor for O'Reilly's Open Source editorial group, for
his interest in this project and for all the work that he and his colleagues at O'Reilly & Associates
have put into making this book possible. Chuck Toporek at O'Reilly has spent many hours applying
his copyediting skills to excellent effect, for which I thank him. Thank you to Hanna Dyer for the
cover design, and David Futato, the production editor for this book. Chris North, Eric Pearce, Jesse
Robbins, and Rich Seifert provided reviews of the manuscript that helped improve the final work.
It's difficult for busy people to find time to provide a detailed review and to compile useful responses
for a large manuscript such as this one, and I am especially grateful to the reviewers who were able
to do so. Of course, I alone am responsible for any remaining errors.
Finally, I wish to thank my wife, Joann Zimmerman, for enduring yet another book project, and for
her patience, her unstinting support, and her editing skills. Without her very able assistance, this
book would not have been possible.
Page 1
I
INTRODUCTION TO ETHERNET
The first part of this book provides a tour of basic Ethernet theory and operation. These chapters
cover those portions of Ethernet operation that are common to all Ethernet media systems. Common
portions include the Ethernet frame, the operation of the media access control system, full-duplex
mode, and the AutoNegotiation protocol.
Part I contains these chapters:
• Chapter 1, The Evolution of Ethernet
• Chapter 2, The Ethernet System
• Chapter 3, The Media Access Control Protocol
• Chapter 4, Full-Duplex Ethernet
• Chapter 5, Auto-Negotiation
Page 3
1
The Evolution of Ethernet
In this chapter:
• History of Ethernet
• The Latest Ethernet Standard
• Organization of IEEE Standards
• Levels of Compliance
• IEEE Identifiers
• Reinventing Ethernet
• Multi-Gigabit Ethernet
Ethernet is by far the most widely used local area networking (LAN) technology in the world today.
Market surveys indicate that hundreds of millions of Ethernet network interface cards (NICs),
repeater ports, and switching hub ports have been sold to date, and the market continues to grow.
In total, Ethernet outsells all other LAN technologies by a very large margin.
Ethernet reached its 25th birthday in 1998, and has seen many changes as computer technology
evolved over the years. Ethernet has been constantly reinvented, evolving new capabilities and in the
process growing to become the most popular network technology in the world.
This chapter describes the invention of Ethernet, and the development and organization of the
Ethernet standard. Along the way we provide a brief tour of the entire set of Ethernet media
systems.
History of Ethernet
On May 22, 1973, Bob Metcalfe (then at the Xerox Palo Alto Research Center, PARC, in
California) wrote a memo describing the Ethernet network system he had invented for
interconnecting advanced computer workstations, making it possible to send data to one another
and to high-speed laser printers. Probably the bestknown invention at Xerox PARC was the first
personal computer workstation with graphical user interfaces and mouse pointing device, called the
Xerox Alto. The PARC inventions also included the first laser printers for personal computers, and,
with the creation of Ethernet, the first high-speed LAN technology to link everything together.
This was a remarkable computing environment for the time, since the early 1970s were an era in
which computing was dominated by large and very expensive
Page 4
mainframe computers. Few places could afford to buy and support mainframes, and few people
knew how to use them. The inventions at Xerox PARC helped bring about a revolutionary change in
the world of computing.
A major part of this revolutionary change in the use of computers has been the use of Ethernet
LANs to enable communication among computers. Combined with an explosive increase in the use
of information sharing applications such as the World Wide Web, this new model of computing has
brought an entire new world of communications technology into existence. These days, sharing
information is most often done over an Ethernet; from the smallest office to the largest corporation,
from the single schoolroom to the largest university campus, Ethernet is clearly the networking
technology of choice.
The Aloba Network
Bob Metcalfe's 1973 Ethernet memo describes a networking system based on an earlier experiment
in networking called the Aloha network. The Aloha network began at the University of Hawaii in the
late 1960s when Norman Abramson and his colleagues developed a radio network for
communication among the Hawaiian Islands. This system was an early experiment in the
development of mechanisms for sharing a common communications channel—in this case, a
common radio channel.
The Aloha protocol was very simple: an Aloha station could send whenever it liked, and then waited
for an acknowledgment. If an acknowledgment wasn't received within a short amount of time, the
station assumed that another station had also transmitted simultaneously, causing a collision in which
the combined transmissions were garbled so that the receiving station did not hear them and did not
return an acknowledgment. Upon detecting a collision, both transmitting stations would choose a
random backoff time and then retransmit their packets with a good probability of success. However,
as traffic increased on the Aloha channel, the collision rate would rapidly increase as well.
Abramson calculated that this system, known as pure Aloha, could achieve a maximum channel
utilization of about 18 percent due to the rapidly increasing rate of collisions under increasing load.
Another system, called slotted Aloha, was developed that assigned transmission slots and used a
master clock to synchronize transmissions, which increased the maximum utilization of the channel to
about 37 percent. In 1995, Abramson received the IEEE's Koji Kobayashi Computers and
Communications Award "for development of the concept of the Aloha System, which led to modern
local area networks."
Page 5
Invention of Ethernet
Metcalfe realized that he could improve on the Aloha system of arbitrating access to a shared
communications channel. He developed a new system that included a mechanism that detected when
a collision occurred (collision detect). The system also included ''listen before talk,'' in which
stations listened for activity (carrier sense) before transmitting, and supported access to a shared
channel by multiple stations (multiple access). Put all these components together, and you can see
why the Ethernet channel access protocol is called Carrier Sense Multiple Access with Collision
Detect (CSMA/CD). Metcalfe also developed a more sophisticated backoff algorithm, which, in
combination with the CSMA/CD protocol, allowed the Ethernet system to function at up to 100
percent load.
In late 1972, Metcalfe and his Xerox PARC colleagues developed the first experimental Ethernet
system to interconnect the Xerox Alto. The experimental Ethernet was used to link Altos to one
another, and to servers and laser printers. The signal clock for the experimental Ethernet interface
was derived from the Alto's system clock, which resulted in a data transmission rate on the
experimental Ethernet of 2.94 Mbps.
Metcalfe's first experimental network was called the Alto Aloha Network. In 1973, Metcalfe
changed the name to "Ethernet," to make it clear that the system could support any computer—not
just Altos—and to point out that his new network mechanisms had evolved well beyond the Aloha
system. He chose to base the name on the word "ether" as a way of describing an essential feature
of the system: the physical medium (i.e., a cable) carries bits to all stations, much the same way that
the old "luminiferous ether" was once thought to propagate electromagnetic waves through space.*
Thus, Ethernet was born.
In 1976, Metcalfe drew the following diagram (Figure 1-1) "…to present Ethernet for the first time.
It was used in his presentation to the National Computer Conference in June of that year. On the
drawing are the original terms for describing Ethernet. Since then, other terms have come into usage
among Ethernet enthusiasts."**
In July 1976, Bob Metcalfe and David Boggs published their landmark paper "Ethernet: Distributed
Packet Switching for Local Computer Networks," in the Communications of the Association for
Computing Machinery (CACM)>. In late 1977, Robert M. Metcalfe, David R. Boggs, Charles P.
Thacker, and Butler W.
* Physicists Michelson and Morley disproved the existence of the ether in 1887, but Metcalfe decided
that it was a good name for his new network system that carried signals to all computers.
** From The Ethernet Sourcebook, ed. Robyn E. Shotwell (New York: North-Holland, 1985), title page.
Diagram reproduced with permission.
Page 6
Figure 1-1.
Drawing of the original Ethernet system
Lampson received U.S. patent number 4,063,220 on Ethernet for a "Multipoint Data
Communication System With Collision Detection." A patent for the Ethernet repeater was issued in
mid-1978. At this point, Xerox wholly owned the Ethernet system. The next stage in the evolution of
the world's most popular computer network was to liberate Ethernet from the confines of a single
corporation and make it a worldwide standard.
Evolution of the Ethernet Standard
The original 10 Mbps Ethernet standard was first published in 1980 by the DECIntel-Xerox vendor
consortium. Using the first initial of each company, this became known as the DIX Ethernet
standard. This standard, entitled The Ethernet, A Local Area Network: Data Link Layer and
Physical Layer Specifications, contained the specifications for the operation of Ethernet as well as
the specs for a single media system based on thick coaxial cable. As is true for most standards, the
DIX standard was revised to add some technical changes, corrections, and minor improvements.
The last revision of this standard was DIX V2.0.
When the DIX standard was published, a new effort led by the Institute of Electrical and Electronics
Engineers (IEEE) to develop open network standards was also getting underway.* Consequently,
the thick coaxial variety of Ethernet ended up being standardized twice—first by the DIX
consortium and a second time by the IEEE. The IEEE standard was created under the direction of
the IEEE Local and Metropolitan Networks (LAN/MAN) Standards Committee, which identifies
all the standards it develops with the number 802. There have been a number of net-
* The IEEE is the world's largest technical professional society, with members in 150 countries. The IEEE
provides technical publishing, holds conferences, and develops a range of technical standards,
including computer and communications standards. The standards developed by the IEEE may also
become national and international standards.
Page 7
working standards published in the 802 branch of the IEEE, including the 802.3* Ethernet and
802.5 Token Ring standards.
The IEEE 802.3 committee took up the network system described in the original DIX standard and
used it as the basis for an IEEE standard. The IEEE standard was first published in 1985 with the
title IEEE 802.3 Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access
Method and Physical Layer Specifications. The IEEE standard does not use "Ethernet" in the title,
even though Xerox relinquished their trademark on the Ethernet name. That's because open
standards committees are quite sensitive about using commercial names that might imply
endorsement of a particular company. As a result, the IEEE calls this technology 802.3 CSMA/CD
or just 802.3. However, most people still use the Ethernet name when referring to the network
system described in the 802.3 standard.
The IEEE 802.3 standard is the official Ethernet standard. From time to time you may hear of other
Ethernet technology "standards" developed by various groups or vendor consortiums. However, if
the technology isn't specified within the IEEE 802.3 standard, it isn't an official Ethernet technology.
Periodically, the latest IEEE 802.3 standards are presented to the American National Standards
Institute (ANSI), which forwards them on, where they are adopted by the International Organization
for Standardization (ISO). This organization is described in more detail later in this chapter.
Adoption by the ISO means that the IEEE 802.3 Ethernet standard is also a worldwide standard,
and that vendors from around the globe can build equipment that will work together on Ethernet
systems.
Ethernet Family Tree
The title of the latest version of the IEEE standard as of this writing is 802.3, 1998 Edition
Information Technology—Telecommunications and information exchange between
systems—Local and metropolitan area networks—Specific requirements—Part 3:
Carrier sense multiple access with collision detection (CSMA/CD) access method and
physical layer specifications.
This edition contains 1,268 pages and "includes all contents of the 8802-3:1996 Edition, plus
IEEE Std 802.3aa-1998, IEEE Std 802.3r-1996, IEEE Std 802.3u-1995, IEEE Std
802.3x&y-1997, and IEEE802.3z-1998." These latter documents were developed as
supplements to the standard. This edition of the standard can be purchased from the IEEE
through their web site at: http://standards.ieee.org/catalog/IEEE802.3.html.
* Pronounced "eight oh two dot three."
Page 8
The Latest Ethernet Standard
After the publication of the original IEEE 802.3 standard for thick Ethernet, the next development in
Ethernet media was the thin coaxial Ethernet variety, inspired by technology first marketed by the
3Com Corporation. When the IEEE 802.3 committee standardized the thin Ethernet technology,
they gave it the shorthand identfier of 10BASE2, which is explained later in this chapter.
Following the development of thin coaxial Ethernet came several new media varieties, including the
twisted-pair and fiber optic varieties for the 10 Mbps system. Next, the 100 Mbps Fast Ethernet
system was developed, which also included several varieties of twisted-pair and fiber optic media
systems. Most recently, the Gigabit Ethernet system was developed using both fiber optic and
twisted-pair cabling. These systems were all developed as supplements to the IEEE Ethernet
standard.
IEEE Supplements
When the Ethernet standard needs to be changed to add a new media system or capability, the
IEEE issues a supplement which contains one or more sections, or "clauses" in IEEE-speak. The
supplement may consist of one or more entirely new clauses, and may also contain changes to
existing clauses in the standard. New supplements to the standard are evaluated by the engineering
experts at various IEEE meetings and the supplements must pass a balloting procedure before being
voted into the full standard.
New supplements are given a letter designation when they are created. Once the supplement has
completed the standardization process, it becomes part of the base standard and is no longer
published as a separate supplementary document. On the other hand, you will sometimes see trade
literature that refers to Ethernet equipment with the letter of the supplement in which the variety was
first developed (e.g., IEEE 802.3u may be used as a reference for Fast Ethernet). Table 1-1 lists
several supplements and what they refer to. The dates indicate when formal acceptance of the
supplement into the standard occurred. Access to the complete set of supplements is provided in
Appendix A, Resources.
Table 1-1. IEEE 802.3 Supplements
Supplement
Description
802.3a-1985
10BASE2 thin Ethernet
802.3c-1985
10 Mbps repeater specifications, clause 9
802.3d-1987
FOIRL fiber link
802.3i-1990
10BASE-T twisted-pair
Page 9
Table 1-1. IEEE 802.3 Supplements (continued)
Supplement
Description
802.3j-1993
10BASE-F fiber optic
802.3u-1995
100BASE-T Fast Ethernet and Auto-Negotiation
802.3x-1997
Full-Duplex standard
802.3z-1998
1000BASE-X Gigabit Ethernet
802.3ab-1999
1000BASE-T Gigabit Ethernet over twisted-pair
802.3ac-1998
Frame size extension to 1522 bytes for VLAN tag
802.3ad-2000
Link aggregation for parallel links
If you've been using Ethernet for a while, you may recall times when a new variety of Ethernet
equipment was being sold before the supplement that described the new variety had been entirely
completed or voted on. This illustrates a common problem: innovation in the computer field, and
especially in computer networking, always outpaces the more deliberate and slow-paced process of
developing and publishing standards. Vendors are eager to develop and market new products, and
it's up to you, the customer, to make sure that the product will work properly in your network
system. One way you can do that is to insist on complete information from the vendor as to what
standard the product complies with.
It may not be a bad thing if the product is built to a draft version of a new supplement. Draft versions
of the supplements can be substantially complete yet still take months to be voted on by the various
IEEE committees. When buying pre-standard equipment built to a draft of the specification, you
need to ensure that the draft in question is sufficiently well along in the standards process that no
major changes will be made. Otherwise, you could be left out in the cold with network equipment
that won't interoperate with newer devices that are built according to the final published standard.
One solution to this is to get a written guarantee from the vendor that the equipment you purchase
will be upgraded to meet the final published form of the standard. Note that the IEEE forbids
vendors to claim or advertise that a product is compliant with an unapproved draft.
Differences in the Standard
When the IEEE adopted the original DIX standard it made a few changes in the specifications. The
major reason for the changes made between the DIX and IEEE standards is that the two groups had
different goals. The specifications for the original DIX Ethernet standard were developed by the
three companies involved and were intended to describe the Ethernet system—and only the Ethernet
system. At the time the multi-vendor DIX consortium was developing the first Ethernet standard,
there was no open LAN market, nor was there any other multi-vendor
Page 10
LAN standard in existence. The efforts aimed at creating a worldwide system of open standards had
only just begun.
On the other hand, the IEEE was developing standards for integration into the world of international
LAN standards. Consequently, the IEEE made several technical changes required for inclusion in
the worldwide standardization effort. The IEEE specifications permit backward compatibility with
early Ethernet systems built according to the original DIX specifications.* The goal is to standardize
network technologies under one umbrella, coordinated with the International Organization for
Standardization.
Organization of IEEE Standards
The IEEE standards are organized according to the Open Systems Interconnection (OSI) Reference
Model. This model was developed in 1978 by the International Organization for Standardization,
whose initials (derived from its French name) are ISO. Headquartered in Geneva, Switzerland, the
ISO is responsible for setting open, vendor-neutral standards and specifications for items of
technical importance. For example, if you're a photographer you've no doubt noticed the ISO
standard speeds for camera film.
The ISO developed the OSI reference model to provide a common organizational scheme for
network standardization efforts (with perhaps an additional goal of keeping us all confused with
reversible acronyms). What follows is a quick, and necessarily incomplete, introduction to the
subject of network models and international standardization efforts.
The Seven Layers of OSI
The OSI reference model is a method of describing how the interlocking sets of networking
hardware and software can be organized to work together in the networking world. In effect, the
OSI model provides a way to arbitrarily divide the task of networking into separate chunks, which
are then subjected to the formal process of standardization.
To do this, the OSI reference model describes seven layers of networking functions, as illustrated in
Figure 1–2. The lower layers cover the standards that describe how a LAN system moves bits
around. The higher layers deal with more abstract notions, such as the reliability of data transmission
and how data is represented to the user. The layers of interest for Ethernet are the lower two layers
of the OSI model.
* All Ethernet equipment built since 1985 is based on the IEEE 802.3 standard.
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Figure 1–2.
The OSI seven layer model
In brief, the OSI reference model includes the following seven layers, starting at the bottom and
working our way to the topmost layer:
Physical layer
Standardizes the electrical, mechanical, and functional control of data circuits that connect to
physical media.
Data link layer
Establishes communication from station to station across a single link. This is the layer that transmits
and receives frames, recognizes link addresses, etc. The part of the standard that describes the
Ethernet frame format and MAC protocol belongs to this layer.
Network layer
Establishes communication from station to station across an internetwork, which is composed of a
number of data links. This layer provides a level of independence from the lower two layers by
establishing higher level functions and procedures for exchanging data between computers across
multiple links. Standards at this layer of the model describe portions of the high-level network
protocols that are carried over an Ethernet in the data field of the Ethernet frame. Protocols at this
layer of the OSI model and above are independent of the Ethernet standard.
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Transport layer
Provides reliable end-to-end error recovery mechanisms and flow control in the higher level
networking software.
Session layer
Provides mechanisms for establishing reliable communications between cooperating applications.
Presentation layer
Provides mechanisms for dealing with data representation in applications.
Application layer
Provides mechanisms to support end-user applications such as mail, file transfer, etc.
IEEE Layers Within the OSI Model
The Ethernet standard concerns itself with elements described in Layer 2 and Layer 1, which include
the data link layer of the OSI model and below. For that reason, you'll sometimes hear Ethernet
referred to as a link layer standard.
The Ethernet standards describe a number of entities that all fit within the data link and physical
layers of the OSI model. To help organize the details, the IEEE defines extra sublayers that fit into
the lower two layers of the OSI model, which simply means that the IEEE standard includes some
more finely grained layering than the OSI model.
While at first glance these extra layers might seem to be outside the OSI reference model, the OSI
model is not meant to rigidly dictate the structure of network standards. Instead, the OSI model is
an organizational and explanatory tool; sublayers can be added to deal with the complexity of a
given standard.
Figure 1–3 depicts the lower two layers of the OSI reference model, and shows how the major
IEEE-specific sublayers are organized. Within these major sublayers there are even further sublayers
defined for additional MAC functions, new physical signaling standards, and so on. At the data link
level, there are the Logical Link Control (LLC) and the MAC sublayers, which are the same for all
varieties and speeds of Ethernet. The LLC layer is an IEEE mechanism for identifying the data
carried in an Ethernet frame. The MAC layer defines the protocol used to arbitrate access to the
Ethernet system. Both of these systems are described in detail in Chapter 3, The Media Access
Control Protocol.
At the physical layer, the IEEE sublayers vary depending on whether 10-, 100-, or 1000 Mbps
Ethernet is being standardized. Each of the sublayers is used to help organize the Ethernet
specifications around specific functions that must be achieved to make the Ethernet system work.
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Figure 1–3.
The major IEEE sublayers
Understanding these sublayers can also help us understand the scope of the various standards
involved. For example, the MAC portion of the IEEE standard is "above" the lower layer physical
specifications. As such, it is functionally independent of the various physical layer media
specifications and does not change, no matter which physical media variety may be in use.
The IEEE LLC standard is independent of the 802.3 Ethernet LAN standard and will not vary—no
matter which LAN system is used. The LLC control fields are intended for use in all LAN systems
and not just Ethernet, which is why the LLC sublayer is not formally part of the IEEE 802.3 system
specifications.
All of the sublayers below the LLC sublayer are specific to the individual LAN technology in
question, which in this case is Ethernet. To help make this clearer, the Ethernet-specific portions of
the standard in Figure 1–3 are all shown in gray.
Below the MAC sublayer, we get into the portions of the standard that are organized in the Physical
Layer of the OSI reference model. The physical layer standards are different depending on the
Ethernet media variety in use and whether or not we're describing the original 10 Mbps Ethernet
system, 100 Mbps Fast Ethernet, or 1000 Mbps Gigabit Ethernet system.
Levels of Compliance
In developing a technical standard, the IEEE is careful to include only those items whose behavior
must be carefully specified to make the system work. Therefore, all Ethernet interfaces that operate
in the original half-duplex mode (described in
Page 14
Chapter 3) must comply fully with the MAC protocol specifications in the standard to perform the
functions identically. Otherwise, the network would not function correctly.
At the same time, the IEEE makes an effort not to constrain the market by standardizing such things
as the appearance of an Ethernet interface, or how many connectors it should have on it. The intent
is to provide just enough engineering specifications to make the system work reliably, without
inhibiting competition and the inventiveness of the marketplace. In general, the IEEE has been quite
successful. Most equipment designed for use in an Ethernet system fully complies with the standard.
Vendor innovation can sometimes lead to the development of devices that are not described in the
IEEE standard, and that are not included in the half-duplex mode timing specs or the media specs in
the standard. Some of these devices may work well for a small network, but might cause problems
with signal timing in a larger network operating in half-duplex mode. Further, a network system using
equipment not described in the standard or included in the official guidelines cannot be evaluated
using the IEEE half-duplex mode configuration guidelines.
The Effect of Standards Compliance
How much you should be concerned about all this is largely up to you and your particular
circumstances. Another way of saying this is: "Optimality differs according to context."* It's up to
you to decide how important these issues are, given your particular circumstances (or context). For
one thing, not all innovations are a bad idea.
After all, the thin coaxial and twisted-pair Ethernet media systems started life as vendor innovations
that later became carefully specified media systems in the IEEE standard. However, if your goal is
maximum predictability and stability for your network given a variety of vendor equipment and traffic
loads, then one way to help achieve that goal is by using only equipment that is described in the
standard.
One way to decide how important these issues are is to look at the scope and type of network
system in question. For an Ethernet that just connects a couple of computers in your house, you may
feel that any equipment you can find that helps make this happen at the least cost is a good deal. If
the equipment isn't described in the official half-duplex configuration guidelines, you may not care all
that much. In this instance, you are building a small network system, and you
* I am indebted to Mike Padlipsky for this useful advice, which was published in his book, The Elements
of Networking Style, M. A. Padlipsky (Englewood Cliffs, New Jersey: Prentice Hall, 1985), p. 229.
Page 15
probably don't intend for the network to grow very large. The limited scope of your network makes
it easier to decide that you are not all that worried about multi-vendor interoperability, or about your
ability to evaluate the network using the IEEE configuration guidelines.
On the other hand, if you are a network manager of a departmental or campus network system, then
the people using your network will be depending on the network to get their work done. The
expanded scope changes your context quite a bit. Departmental and workgroup nets always seem
to be growing, which makes extending networks to accommodate growth a major priority for you.
In addition, network stability under all sorts of traffic loads becomes another important issue. In this
very different context, the issues of multi-vendor interoperability and compliance with the standard
become much more important.
Equipment Included in the Standard
All Ethernet equipment sold is compliant in some way with the standard; otherwise, it wouldn't be
able to interoperate with other Ethernet equipment. Therefore, mere compliance with the standard
doesn't tell you much. Unfortunately, there's no LAN industry organization that will certify and stamp
equipment, ''This device is described in the standard and included in the official IEEE configuration
guidelines.'' That's why you need to be wary about believing everything you read in equipment
catalogs.
Sometimes vendors may not tell you whether the component they are selling is included in the IEEE
system configuration guidelines, and whether it is a piece of standard and interoperable equipment
that is widely available from other vendors. Some components that are not included in the official
standard or media system configuration guidelines include the 10 Mbps AUI port concentrator,
media converters, and special media segments. These components are described in later chapters
and Appendix C, AUI Equipment: Installation and Configuration.
IEEE Identifiers
The IEEE has assigned shorthand identifiers to the various Ethernet media systems as they have
been developed. The three-part identifiers include the speed, the type of signaling used, and
information about the physical medium.
In the early media systems, the physical medium part of the identifier was based on the cable
distance in meters, rounded to the nearest 100 meters. In the more recent media systems, the IEEE
engineers dropped the distance convention and the third part of the identifier simply identifies the
media type used (twisted-pair
Page 16
or fiber optic). In roughly chronological order, the identifiers include the following set:
10BASE5
This identifies the original Ethernet system, based on thick coaxial cable. The identifier means 10
megabits per second transmission speed, baseband transmission, and the 5 refers to the 500 meter
maximum segment length. The word baseband simply means that the transmission medium, thick
coaxial cable in this instance, is dedicated to carrying one service: Ethernet signals. The 500 meter
limit refers to the maximum length a given cable segment may be. Longer networks are built by
connecting multiple segments with repeaters or switching hubs.
10BASE2
Also known as the thin Ethernet system, this media variety operates at 10 Mbps, in baseband
mode, with cable segment lengths that can be a maximum of 185 meters in length. If the segments
can be at most 185 meters long, then why does the identifier say "2," thus implying a maximum of
200 meters? The answer is that the identifier is merely a bit of shorthand and not intended to be an
official specification. The IEEE committee found it convenient to round things up to 2, to keep the
identifier short and easier to pronounce. This less expensive version of coax Ethernet was
nicknamed "Cheapernet."
FOIRL
This stands for Fiber Optic Inter-Repeater Link. The original DIX Ethernet standard mentioned a
point-to-point link segment that could be used between repeaters, but did not provide any media
specifications. Later, the IEEE committee developed the FOIRL standard, and published it in 1989.
FOIRL segments were originally designed to link remote Ethernet segments together. Fiber optic
media's immunity to lightning strikes and electrical interference, as well as its ability to carry signals
for long distances, makes it an ideal system for transmitting signals between buildings.
The specifications in the original FOIRL segment only provide for linking two repeaters together,
one at each end of the link. While waiting for a larger set of fiber optic specifications to appear,
vendors extended the set of devices that are connected via fiber, allowing an FOIRL segment to be
attached to a station as well. These changes were taken up and added to the newer fiber optic link
specifications found in the 10BASE-F standard (described later in this section).
10BROAD36
This system was designed to send 10 Mbps signals over a broadband cable system. Broadband
cable systems support multiple services on a single cable by dividing the bandwidth of the cable into
separate frequencies, each
Page 17
assigned to a given service. Cable television is an example of a broadband cable system, designed to
deliver multiple television channels over a single cable. 10BROAD36 systems are intended to cover
a large area; the 36 refers to the 3,600 meter distance allowed between any two stations on the
system. These days, the vast majority of sites use fiber optic media for covering large distances, and
broadband Ethernet equipment is not widely available.
1BASE5
This standard describes a 1 Mbps system based on twisted-pair wiring, which did not prove to be a
very popular system. 1BASE5 was superseded in the marketplace by 10BASE-T, which provided
all the advantages of twisted-pair wiring as well as the higher 10 Mbps speed.
10BASE-T
The "T" stands for "twisted," as in twisted-pair wires. This variety of the Ethernet system operates at
10 Mbps, in baseband mode, over two pairs of Category 3 (or better) twisted-pair wires. The
category system for classifying cable quality is described in Chapter 14, Structured Cabling. The
hyphen was added to the ''10BASE-T" identifier to help ensure the correct pronunciation of "ten
base tee." It was felt that without the hyphen people might mistakenly call it "10 basset,'' which is too
close to the dog, "basset hound." Use of the hyphen is found in this and all newer media identifiers.
10BASE-F
The "F" stands for fiber, as in fiber optic media. This is the most recent 10 Mbps fiber optic
Ethernet standard, adopted as an official part of the IEEE 802.3 standard in November 1993. The
10BASE-F standard defines three sets of specifications:
10BASE-FB
This is for active fiber hubs based on synchronous repeaters for extending a backbone system.
10BASE-FP
This is for passive hub equipment intended to link workstations with a fiber optic hub.
10BASE-FL
This includes a set of fiber optic link segment specifications that updates and extends the older
FOIRL standard.
Two of these specifications have not been widely deployed. Equipment based on 10BASE-FB is
scarce, and equipment based on 10BASE-FP is non-existent. The vast majority of Ethernet vendors
sell 10BASE-FL fiber link equipment.
Page 18
100 Mbps Media Systems
100BASE-T
This is the IEEE shorthand identifier for the entire 100 Mbps system, including all twisted-pair and
fiber optic Fast Ethernet media systems.
100BASE-X
This is the IEEE shorthand identifier for the 100BASE-TX and 100BASE-FX media systems.
These two systems are both based on the same 4B/5B block encoding system, adapted from a 100
Mbps networking standard called Fiber Distributed Data Interface (FDDI). FDDI was originally
developed and standardized by ANSI.
100BASE-TX
This variety of the Fast Ethernet system operates at 100 Mbps, in baseband mode, over two pairs
of high-quality, Category 5 twisted-pair cable. The TX identifier indicates that this is the twisted-pair
version of the 100BASE-X media systems. This is the most widely used variety of Fast Ethernet.
100BASE-FX
This variety of the Fast Ethernet system operates at 100 Mbps, in baseband mode, over multi-mode
fiber optic cable.
100BASE-T4
This variety of the Fast Ethernet system operates at 100 Mbps, in baseband mode, over four pairs
of Category 3 or better twisted-pair cable. This variety has not been widely deployed, and
100BASE-T4 equipment is scarce.
100BASE-T2
This variety of the Fast Ethernet system operates at 100 Mbps, in baseband mode, on two pairs of
Category 3 or better twisted-pair cable. This variety was never developed by any vendor, and
equipment based on the T2 standard is non-existent.
1000 Mbps Media Systems
1000BASE-X
This is the IEEE shorthand identifier for the Gigabit Ethernet media systems based on the 8B/10B
block encoding scheme adapted from the Fibre Channel networking standard. Fibre Channel is a
high speed networking system developed and standardized by ANSI.
The 1000BASE-X media systems include 1000BASE-SX, 1000BASE-LX, and 1000BASE-CX.
Page 19
1000BASE-SX
The "S" stands for "short," as in short wavelength. The "X" indicates that this media segment is one
of three based on the same block encoding scheme. This is the short wavelength fiber optic media
segment for Gigabit Ethernet.
1000BASE-LX
This is the long wavelength fiber optic media segment for Gigabit Ethernet.
1000BASE-CX
This is a short copper cable media segment for Gigabit Ethernet, based on the original Fibre Channel
standard.
1000BASE-T
This is the IEEE shorthand identifier for 1000 Mbps Gigabit Ethernet over Category 5 or better
twisted-pair cable. This system is based on a different signal encoding scheme required to transmit
gigabit signals over twisted-pair cabling.
Reinventing Ethernet
No matter how well designed a LAN system is, it won't help you much if you can only use it with a
single vendor's equipment. A LAN has to be able to work with the widest variety of equipment
possible to provide you with the greatest flexibility. For maximum utility, your LAN must be
vendor-neutral: that is, capable of interworking with all types of computers without being
vendor-specific. This was not the way things worked in the 1970s when computers were expensive
and networking technology was exotic and proprietary.
Metcalfe understood that a revolution in computer communications required a networking
technology that everyone could use. In 1979 he set out to make Ethernet an open standard, and
convinced Xerox to join a multi-vendor consortium for the purposes of standardizing an Ethernet
system that any company could use. The era of open computer communications based on Ethernet
technology formally began in 1980 when the Digital Equipment Corporation (DEC), Intel, and
Xerox consortium announced the DIX standard for 10 Mbps Ethernet.
This DIX standard made the technology available to anyone who wanted to use it, producing an
open system. As part of this effort, Xerox agreed to license its patented technology for a low fee to
anyone who wanted it. In 1982 Xerox also gave up its trademark on the Ethernet name. As a result,
the Ethernet standard became the world's first open, multi-vendor LAN standard. The idea of
sharing proprietary computer technology in order to arrive at a common standard to benefit
everyone was a radical notion for the computer industry in the late 1970s. It's a tribute to
Page 20
Bob Metcalfe's vision that he realized the importance of making Ethernet an open standard. As
Metcalfe put it:
The invention of Ethernet as an open, non-proprietary, industry-standard local network was perhaps
even more significant than the invention of Ethernet technology itself.*
In 1979 Metcalfe started a company to help commercialize Ethernet. He believed that computers
from multiple vendors ought to be able to communicate compatibly over a common networking
technology, making them more useful and, in turn, opening up a vast new set of capabilities for the
users. Computer communication compatibility was the goal, which led Metcalfe to name his new
company 3Com.
Reinventing Ethernet for Twisted-Pair Media
Ethernet prospered during the 1980s, but as the number of computers being networked continued to
grow, the problems inherent in the original coaxial cable media system became more acute. Installing
a thick coax cable in a building was a difficult task, and connecting the computers to the cable was
also a challenge. A thin coax cable system was introduced in the mid-1980s that made it easier to
build a media system and connect computers to it, but it was still difficult to manage Ethernet
systems based on coaxial cable. Coaxial Ethernet systems are typically based on a bus topology, in
which every computer sends Ethernet signals over a single bus cable; a failure of the cable brings the
entire network system to a halt, and troubleshooting a cable problem can take a long time.
The invention of twisted-pair Ethernet in the late 1980s by a company called SynOptics
Communications made it possible to build Ethernet systems based on the much more reliable
star-wired cabling topology, in which the computers are networked to a central hub. These systems
are much easier to install and manage, and troubleshooting is much easier and quicker as well. The
use of twisted-pair cabling was a major change, or reinvention, of Ethernet. Twisted-pair Ethernet
led to a vast expansion in the use of Ethernet; the Ethernet market took off and has never looked
back.
In the early 1990s, a structured cabling system standard for twisted-pair cabling systems was
developed that made it possible to provide building-wide twisted-pair systems based on
high-reliability cabling practices adopted from the telephone industry. Ethernet based on twisted-pair
media installed according to the structured cabling standard has become the most widely used
network technology.
* Shotwell, The Ethernet Sourcebook, p. xi
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These Ethernet systems are reliable, easy to install and manage, and support rapid troubleshooting
for problem resolution.
Reinventing Ethernet for 100 Mbps
The original Ethernet standard of 1980 described a system that operated at 10 Mbps. This was
quite fast for the time, and Ethernet interfaces in the early 1980s were expensive due to the buffer
memory and high-speed components required to support such rapid speeds. Throughout the 1980s,
Ethernet was considerably faster than the computers connected to it, which provided a good match
between the network and the computers it supported. However, as computer technology continued
to evolve, ordinary computers were fast enough to provide a major traffic load to a 10 Mbps
Ethernet channel by the early 1990s.
Much to the surprise of those who thought Ethernet was limited to 10 Mbps, Ethernet was
reinvented to increase its speed by a factor of ten. The new standard created the 100 Mbps Fast
Ethernet system, which was formally adopted in 1995. Fast Ethernet is based on twisted-pair and
fiber optic media systems, and provides high-speed network channels for use in backbone systems,
as well as connections to fast server computers and to desktop computers.
With the invention of Fast Ethernet, multi-speed twisted-pair Ethernet interfaces can be built which
operate at either 10 or 100 Mbps. These interfaces are able, through an Auto-Negotiation protocol,
to automatically set their speed in interaction with Ethernet repeater hubs and switching hubs. This
makes the migration from 10 Mbps to 100 Mbps Ethernet systems easy to accomplish.
Reinventing Ethernet for 1000 Mbps
In 1998, Ethernet was reinvented yet again, this time to increase its speed by another factor of ten.
The new Gigabit Ethernet standard describes a system that operates at the speed of 1 billion bits per
second over fiber optic and twisted-pair media. The invention of Gigabit Ethernet makes it possible
to provide very fast backbone networks as well as connections to high-performance servers.
The twisted-pair standard for Gigabit Ethernet makes it possible to provide very high-speed
connections to the desktop when needed. Multi-speed twisted-pair Ethernet interfaces can now be
built which operate at 10-, 100-, or 1000 Mbps, using the Auto-Negotiation protocol (see Chapter
5, Auto-Negotiation) to automatically configure their speed.
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Reinventing Ethernet for New Capabilities
Ethernet innovations include new speeds and new media systems. They also include new Ethernet
capabilities. For example, the full-duplex Ethernet standard makes it possible for two devices
connected to a full-duplex Ethernet media system to simultaneously send and receive data. A port
on a Fast Ethernet switching hub can simultaneously send and receive data at 100 Mbps with a
server when using full-duplex mode, resulting in a total link bandwidth of 200 Mbps. The
Auto-Negotiation standard provides the ability for switching hub ports and computers linked to
those ports to discover whether or not they both support full-duplex mode, and if they do, to
automatically select that mode of operation.
As you can see, the Ethernet system has been reinvented again and again to provide more flexible
and reliable cabling, to accommodate the rapid increase in network traffic with higher speeds, and to
provide more capabilities for today's more complex network systems. The remarkable success of
Ethernet in the marketplace has been based on the equally remarkable ability of the system to adapt
and change to meet the rapidly evolving needs of the computer industry.
Multi-Gigabit Ethernet
In March 1999, the IEEE 802.3 standards group held a "Call for Interest" meeting on the topic of
Ethernet speeds beyond the current 1 Gbps standard. A number of presentations were made on the
general topic, after which the group voted to create a High Speed Study Group. At the time of this
writing, the High Speed Study Group is meeting on a regular basis to review presentations on a
variety of technical issues. It is expected that this work will lead to the development of a new higher
speed Ethernet standard, probably operating at 10 Gbps, within the next few years.
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2
The Ethernet System
In this chapter:
• Four Basic Elements of Ethernet
• Ethernet Hardware
• Network Protocols and Ethernet
An Ethernet Local Area Network (LAN) is made up of hardware and software working together to
deliver digital data between computers. To accomplish this task, four basic elements are combined
to make an Ethernet system. This chapter provides a tutorial describing these elements, since a
familiarity with these basic elements provides a good background for working with Ethernet. We will
also take a look at some network media and simple topologies. Finally, we will see how the Ethernet
system is used by high-level network protocols to send data between computers.
This chapter describes the original half-duplex mode of operation. Half-duplex simply means that
only one computer can send data over the Ethernet channel at any given time. In half-duplex mode,
multiple computers share a single Ethernet channel by using the Carrier Sense Multiple Access with
Collision Detection (CSMA/CD) media access control (MAC) protocol. Until the introduction of
switching hubs, the half-duplex system was the typical mode of operation for the vast majority of
Ethernet LANs—tens of millions of Ethernet connections have been installed based on this system.
However, these days many computers are connected directly to their own port on an Ethernet
switching hub and do not share the Ethernet channel with other systems. This type of connection is
described in Chapter 18, Ethernet Switching Hubs. Many computers and switching hub
connections now use full-duplex mode, in which the CSMA/CD protocol is shut off and the two
devices on the link can send data whenever they like. The full-duplex mode of operation is
described in Chapter 4, Full-Duplex Ethernet.
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Four Basic Elements of Ethernet
The Ethernet system includes four building blocks that, when combined, make a working Ethernet:
• The frame, which is a standardized set of bits used to carry data over the system.
• The media access control protocol, which consists of a set of rules embedded in each Ethernet
interface that allow multiple computers to access the shared Ethernet channel in a fair manner.
• The signaling components, which consists of standardized electronic devices that send and
receive signals over an Ethernet channel.
• The physical medium, which consists of the cables and other hardware used to carry the digital
Ethernet signals between computers attached to the network.
The Ethernet Frame
The heart of the Ethernet system is the frame. The network hardware—which is comprised of the
Ethernet interfaces, media cables, etc.—exists simply to move Ethernet frames between computers,
or stations.* The bits in the Ethernet frame are formed up in specified fields. Figure 2-1 shows the
basic frame fields. These fields are described in more detail in Chapter 3, The Media Access
Control Protocol.
Figure 2-1.
An Ethernet frame
Figure 2-1 shows the basic Ethernet frame, which begins with a set of 64 bits called the preamble.
The preamble gives all of the hardware and electronics in a 10 Mbps Ethernet system some signal
start-up time to recognize that a frame is being transmitted, alerting it to start receiving the data. This
is what a 10 Mbps network uses to clear its throat, so to speak. Newer Ethernet systems running at
100 and 1000 Mbps use constant signaling, which avoids the need for a preamble.
* A computer connected to the network may be a standard desktop workstation, a printer, or anything
else with an Ethernet interface in it. For that reason, the Ethernet standard uses the more general term
"station" to describe the networked device, and so will we.
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However, the preamble is still transmitted in these systems to avoid making any changes in the
structure of the frame.
Following the preamble are the destination and source addresses. Assignment of addresses is
controlled by the IEEE Standards Association (IEEE-SA), which administers a portion of the
address field. When assigning blocks of addresses for use by network vendors, the IEEE-SA
provides a 24-bit Organizationally Unique Identifier (OUI).* The OUI is a unique 24-bit identifier
assigned to each organization that builds network interfaces. This allows a vendor of Ethernet
equipment to provide a unique address for each interface they build. Providing unique addresses
during manufacturing avoids the problem of two or more Ethernet interfaces in a network having the
same address. This also eliminates any need to locally administer and manage Ethernet addresses.
A manufacturer of Ethernet interfaces creates a unique 48-bit Ethernet address for each interface by
using their assigned OUI for the first 24 bits of the address. The vendor then assigns the next 24 bits,
being careful to ensure that each address is unique. The resulting 48-bit address is often called the
hardware, or physical, address to make the point that the address has been assigned to the Ethernet
interface. It is also called a Media Access Control (MAC) address, since the Ethernet media access
control system includes the frame and its addressing.
Following the addresses in an Ethernet frame is a 16-bit type or length field. Most often this field is
used to identify what type of high-level network protocol is being carried in the data field, e.g.,
TCP/IP. This field may also be used to carry length information, as described in Chapter 3.
After the type field can come anywhere from 46 bytes to 1500 bytes of data. The data field must be
at least 46 bytes long. This length assures that the frame signals stay on the network long enough for
every Ethernet station on the network system to hear the frame within the correct time limits. Every
station must hear the frame within the maximum round-trip signal propagation time of an Ethernet
system, as described later in this chapter. If the high-level protocol data carried in the data field is
shorter than 46 bytes, then padding data is used to fill out the data field.
Finally, at the end of the frame there's a 32-bit Frame Check Sequence (FCS) field. The FCS
contains a Cyclic Redundancy Checksum (CRC) which provides a check of the integrity of the data
in the entire frame. The CRC is a unique number that is generated by applying a polynomial to the
pattern of bits that make up the frame. The same polynomial is used to generate another checksum
at the receiving station. The receiving station checksum is then compared to the checksum generated
* The IEEE-SA web page for OUI assignment is listed in Appendix A, Resources.
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at the sending station. This allows the receiving Ethernet interface to verify that the bits in the frame
survived their trip over the network system intact.
That's basically all there is to an Ethernet frame. Now that you know what an Ethernet frame looks
like, you need to know how the frames are transmitted. This is where the set of rules used to govern
when a station gets to transmit a frame comes into play. We'll take a look at those rules next.
The Media Access Control Protocol
The half-duplex mode of operation described in the original Ethernet standard uses the MAC
protocol, which is a set of rules used to arbitrate access to the shared channel among a set of
stations connected to that channel. The way the access protocol works is fairly simple: each
Ethernet-equipped computer operates independently of all other stations on the network; there is no
central controller. All stations attached to an Ethernet operating in half-duplex mode are connected
to a shared signaling channel, also known as a signal bus.
Ethernet uses a broadcast delivery mechanism, in which each frame that is transmitted is heard by
every station. While this may seem inefficient, the advantage is that putting the address-matching
intelligence in the interface of each station allows the physical medium to be kept as simple as
possible. On an Ethernet LAN, all that the physical signaling and media system has to do is see that
the bits are accurately transmitted to every station; the Ethernet interface in the station does the rest
of the work.
Ethernet signals are transmitted from the interface and sent over the shared signal channel to every
attached station. To send data, a station first listens to the channel, and if the channel is idle the
station transmits its data in the form of an Ethernet frame or packet.*
As each Ethernet frame is sent over the shared signal channel, or medium, all Ethernet interfaces
connected to the channel read in the bits of the signal and look at the second field of the frame
shown in Figure 2-1, which contains the destination address. The interfaces compare the destination
address of the frame with their own 48-bit unicast address or a multicast address they have been
enabled to recognize. The Ethernet interface whose address matches the destination address in the
frame will continue to read the entire frame and deliver it to the networking software running on that
computer. All other network interfaces will stop reading the frame when they discover that the
destination address does not match their own unicast address or an enabled multicast address.
* The precise term as defined in the Ethernet standard is "frame," but the term "packet" is sometimes
used as well.
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Multicast and Broadcast Addresses
A multicast address allows a single Ethernet frame to be received by a group of stations. An
application providing streaming audio and video services, for example, can set a station's
Ethernet interface to listen for specific multicast addresses. This makes it possible for a set of
stations to be assigned to a multicast group, which has been given a specific multicast
address. A single stream of audio packets sent to the multicast address assigned to that group
will be received by all stations in that group.
There is also the special case of the multicast address known as the broadcast address,
which is the 48-bit address of all ones. All Ethernet interfaces that see a frame with this
destination address will read the frame in and deliver it to the networking software on the
computer.
After each frame transmission, all stations on the network with traffic to send must contend equally
for the next frame transmission opportunity. This ensures that access to the network channel is fair
and that no single station can lock out the others. Fair access of the shared channel is made possible
through the use of the MAC system embedded in the Ethernet interface located in each station. The
media access control mechanism for Ethernet uses the CSMA/CD protocol.
The CSMA/CD protocol
The CSMA/CD protocol functions somewhat like a dinner party in a dark room where the
participants can only hear one another. Everyone around the table must listen for a period of quiet
before speaking (Carrier Sense). Once a space occurs everyone has an equal chance to say
something (Multiple Access). If two people start talking at the same instant they detect that fact, and
quit speaking (Collision Detection).
To translate this into Ethernet terms, the Carrier Sense portion of the protocol means that before
transmitting, each interface must wait until there is no signal on the channel. If there is no signal, it can
begin transmitting. If another interface is transmitting, there will be a signal on the channel; this
condition is called carrier.* All other interfaces must wait until carrier ceases and the signal channel
is idle before trying to transmit; this process is called deferral.
* Historically, a carrier signal is defined as a continuous constant-frequency signal, such as the one
used to carry the modulated signal in an AM or FM radio system. There is no such continuous carrier
signal in Ethernet; instead, ''carrier'' in Ethernet simply means the presence of traffic on the network.
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With Multiple Access, all Ethernet interfaces have the same priority when it comes to sending frames
on the network, and all interfaces can attempt to access the channel at any time.
The next portion of the access protocol is called Collision Detect. Given that every Ethernet
interface has equal opportunity to access the Ethernet, it's possible for multiple interfaces to sense
that the network is idle and start transmitting their frames simultaneously. When this happens, the
Ethernet signaling devices connected to the shared channel sense the collision of signals, which tells
the Ethernet interfaces to stop transmitting. Each of the interfaces will then choose a random
retransmission time and resend their frames in a process called backoff.
The CSMA/CD protocol is designed to provide fair access to the shared channel so that all stations
get a chance to use the network and no station gets locked out due to some other station hogging
the channel. After every packet transmission, all stations use the CSMA/CD protocol to determine
which station gets to use the Ethernet channel next.
Collisions
If more than one station happens to transmit on the Ethernet channel at the same moment, then the
signals are said to collide. The stations are notified of this event and reschedule their transmission
using a random time interval chosen by a specially designed backoff algorithm. Choosing random
times to retransmit helps the stations to avoid colliding again on the next transmission.
It's unfortunate that the original Ethernet design used the word collision for this aspect of the
Ethernet media access control mechanism. If it had been called something else, such as Distributed
Bus Arbitration (DBA) events, then no one would worry about the occurrence of DBAs on an
Ethernet. To most ears the word "collision" sounds like something bad has happened, leading many
people to incorrectly conclude that collisions are an indication of network failure and that lots of
collisions must mean the network is broken.
Instead, the truth of the matter is that collisions are absolutely normal events on an Ethernet and are
simply an indication that the CSMA/CD protocol is functioning as designed. As more computers are
added to a given Ethernet, there will be more traffic, resulting in more collisions as part of the normal
operation of an Ethernet. Collisions resolve quickly. For example, the design of the CSMA/CD
protocol ensures that the majority of collisions on a 10 Mbps Ethernet will be resolved in
microseconds, or millionths of a second. Nor does a normal collision result in lost data. In the event
of a collision, the Ethernet interface backs off (waits) for some number of microseconds, and then
automatically retransmits the frame.
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Networks with very heavy traffic loads may experience multiple collisions for each frame
transmission attempt. This is also expected behavior. Repeated collisions for a given packet
transmission attempt indicate a very busy network. If repeated collisions occur, the stations involved
will expand the set of potential backoff times in order to retransmit the data. The expanding backoff
process, formally known as truncated binary exponential backoff, is a clever feature of the
Ethernet MAC protocol that provides an automatic method for stations to adjust to changing traffic
conditions on the network. Only after 16 consecutive collisions for a given transmission attempt will
the interface finally discard the Ethernet frame. This can happen only if the Ethernet channel is
overloaded for a fairly long period of time or if it is broken.
So far, we've seen what an Ethernet frame looks like and how the CSMA/CD protocol is used to
ensure fair access for multiple stations sending their frames over the shared Ethernet channel. The
frame and the CSMA/CD protocol are the same for all varieties of Ethernet. Whether the Ethernet
signals are carried over coaxial, twisted-pair, or fiber optic cable, the same frame is used to carry
the data and the same CSMA/CD protocol is used to provide the half-duplex shared channel mode
of operation. In full-duplex mode, the same frame format is used, but the CSMA/CD protocol is
shut off, as described in Chapter 4.
Ethernet Hardware
The next two building blocks of an Ethernet system include the hardware components used in the
system. There are two basic groups of hardware components: the signaling components, used to
send and receive signals over the physical medium; and the media components, used to build the
physical medium that carries the Ethernet signals. Not surprisingly, these hardware components
differ depending on the speed of the Ethernet system and the type of cabling used. To show the
hardware building blocks, we'll look at an example based on the widely used 10 Mbps twisted-pair
Ethernet media system, called 10BASE-T.
Signaling Components
The signaling components for a twisted-pair system include the Ethernet interface located in the
computer, as well as a transceiver and its cable. An Ethernet may consist of a pair of stations linked
with a single twisted-pair segment, or multiple stations connected to twisted-pair segments that are
linked together with an Ethernet repeater. A repeater is a device used to repeat network signals onto
multiple segments. Connecting cable segments with a repeater makes it possible for the segments to
all work together as a single shared Ethernet channel.
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Figure 2-2 shows two computers (stations) connected to a 10BASE-T media system. Both
computers have an Ethernet interface card installed, which makes the Ethernet system operate. The
interface contains the electronics needed to form up and send Ethernet frames, as well as to receive
frames and extract the data from them. The Ethernet interface comes in two basic types. The first is
a board that plugs into a computer's bus slot, and the other relies on chips that allow Ethernet
interfaces to be built into the computer's main logic board. In the second form, all you'll see of the
interface is an Ethernet connector mounted on the back of the computer.
Figure 2-2.
A sample 10BASE-T Ethernet connection
The Ethernet interface connects to the media system using a transceiver, which can be built into the
interface or provided as an external device. Of the two stations shown in Figure 2-2, one is
provided with a built-in transceiver and one uses an external transceiver. The word "transceiver" is a
combination of transmitter and receiver. A transceiver contains the electronics needed to take
signals from the station interface and transmit them to the twisted-pair cable segment, and to receive
signals from the cable segment and send them to the interface.
The Ethernet interface in Station A is connected directly to the twisted-pair cable segment since it is
equipped with an internal 10BASE-T transceiver. The twisted-
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pair cable uses an 8-pin connector that is also known as an RJ-45 plug. The Ethernet interface in
Station B is connected to an outboard transceiver, which is a small box that contains the transceiver
electronics. The outboard transceiver connects to the twisted-pair segment using an 8-pin
connector. The outboard transceiver connects to the Ethernet interface in the station with a
transceiver cable. The transceiver cable in the 10 Mbps Ethernet system uses a 15-pin connector
which is called the Attachment Unit Interface (AUI).
The final signaling component shown in Figure 2-2 is an Ethernet repeater hub which links multiple
twisted-pair segments. This device is called a hub because it sits at the center, or hub, of a set of
cable segments. The repeater connects to the cable segments using the same built-in 10BASE-T
transceivers used by an ordinary station interface. The repeater operates by moving Ethernet signals
from one segment to another one bit at a time; it does not operate at the level of Ethernet frames, but
simply repeats the signals it sees on each segment. Repeaters make it possible to build larger
Ethernet systems composed of multiple cable segments by making the segments function together as
a single channel.
Media Components
The cables and other components used to build the signal-carrying portion of the shared Ethernet
channel are called the physical media. The physical cabling components vary depending on which
kind of media system is in use. For instance, a twisted-pair cabling system uses different components
than a fiber optic cabling system. Just to make things more interesting, a given Ethernet system may
include several different kinds of media systems all connected together with repeaters to make a
single network channel.
By using repeaters, an Ethernet system of multiple segments can grow as a branching tree. This
means that each media segment is an individual branch of the complete signal system. The 10 Mbps
system allows multiple repeaters in the path between stations, making it possible to build repeated
networks with multiple branches. Only one or two repeaters can be used in the path of higher speed
Ethernet systems, limiting the size of the resulting network. A typical network design actually ends up
looking less like a tree and more like a complex set of network segments that may be strung along
hallways or throughout wiring closets in your building. The resulting system of connected segments
may grow in any direction and does not have a specific root segment. However, it is essential not to
connect Ethernet segments in a loop, as each frame would circulate endlessly until the system was
saturated with traffic.
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Round-Trip Timing
In order for the MAC system to work properly, all Ethernet interfaces must be capable of
responding to one another's signals within a strictly controlled amount of time. The signal timing for
Ethernet is based on the amount of time it takes for a signal to get from one end of the complete
media system to the other and back. This is known as the round-trip time. The maximum
round-trip time of signals on an Ethernet system operating in half-duplex mode is strictly limited.
Limiting the round-trip time ensures that every interface can hear all network signals within the
specified amount of time provided for in the Ethernet MAC system.
The longer a given network segment is, the more time it takes for a signal to travel over it. The intent
of the configuration guidelines in the standard is to make sure that the round-trip timing restrictions
are met, no matter what combination of media segments are used in your network. The configuration
guidelines provide specifications for the maximum length of segments and rules for combining various
kinds of cabling segments with repeaters. These specifications and rules ensure that the correct
signal timing is maintained for the entire LAN. The specifications for individual media segment
lengths and the rules for combining segments must be carefully followed. If these specifications and
rules are violated, the computers may not hear one another's signals within the required time limit,
and could end up interfering with one another.
That's why the correct operation of an Ethernet LAN depends upon media segments that are built
according to the rules published for each media type. More complex LANs built with multiple media
types must be designed according to the multi-segment configuration guidelines provided in the
Ethernet standard. These rules include limits on the total length of segments and the total number of
segments and repeaters that may be in a given system. This is to ensure that the correct round-trip
timing is maintained.
Ethernet Hubs
Ethernet was designed to be easily expandable to meet the networking needs of a given site. As
we've just seen, the total set of segments and repeaters in the Ethernet LAN must meet round-trip
timing specifications. To help extend half-duplex Ethernet systems, networking vendors sell repeater
hubs that are equipped with multiple Ethernet ports. Each port of a repeater hub links individual
Ethernet media segments together to create a larger network that operates as a single Ethernet
LAN.
There is another kind of hub called a switching hub. Switching hubs use the 48-bit Ethernet
destination addresses to make a frame forwarding decision from one port of the switch to another.
As shown in Figure 2–3, each port of a switching hub
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provides a connection to an Ethernet media system that functions as an entirely separate Ethernet
LAN.
In a repeater hub the individual ports combine segments together to create a single LAN channel.
However, a switching hub makes it possible to divide a set of Ethernet media systems into multiple
separate LANs. The separate LANs are linked together by way of the switching electronics in the
hub. The round-trip timing rules for each LAN stop at the switching hub port, allowing you to link a
large number of individual Ethernet LANs together.
Figure 2–3.
Switching hub creates separate Ethernet LANs
A given Ethernet LAN may consist of a repeater hub linking several media segments together.
Whole Ethernet LANs can themselves be linked together to form extended network systems using
switching hubs. Larger networks based on repeater hubs can be segmented into smaller LANs with
switching hubs in a process called network segmentation. In this instance, the switching hub is used
to segment a single LAN composed of network segments linked by repeater hubs into multiple
LANs, to improve bandwidth and reliability.
Network segmentation can be extended all the way to connecting individual stations to single ports
on the switching hub, in a process called micro-segmentation. As switching hub costs have
dropped and computer performance has increased, more and more stations are being connected
directly to their own port on the switching hub. That way, the station does not have to share the
Ethernet channel bandwidth with another computer. Instead, it has its own dedicated Ethernet link
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to the switching hub. Switching hub operation and network segmentation are detailed in Chapter 18.
Network Protocols and Ethernet
Now that we've seen how frames are sent over Ethernet systems, let's look at the data being carried
by the frame. Data that is being sent between computers is carried in the data field of the Ethernet
frame and structured as high-level network protocols. The high-level network protocol information
carried inside the data field of each Ethernet frame is what actually establishes communications
between applications running on computers attached to the network. The most widely used system
of high-level network protocols is called the Transmission Control Protocol/ Internet Protocol
(TCP/IP) suite.
The important thing to understand is that the high-level protocols are independent of the Ethernet
system. There are several network protocols in use today, any of which may send data between
computers in the data field of an Ethernet frame. In essence, an Ethernet LAN with its hardware and
Ethernet frame is simply a trucking service for data being sent by applications. The Ethernet LAN
itself doesn't know or care about the high-level protocol data being carried in the data field of the
Ethernet frame.
Since the Ethernet system is unaffected by the contents of the data field in the frame, different sets of
computers running different high-level network protocols can share the same Ethernet. For example,
you can have a single Ethernet that supports four computers, two of which communicate using
TCP/IP, and two that use some other system of high-level protocols. All four computers can send
Ethernet frames over the same Ethernet system without any problem.
The details of how network protocols function are an entirely separate subject from how the
Ethernet system works and are outside the scope of this book. However, Ethernets are installed to
make it possible for applications to communicate between computers using high-level network
protocols to facilitate the communication. Let's take a quick look at one example of high-level
network protocols to see how the Ethernet system and network protocols work together.
Design of Network Protocols
Network protocols are easy to understand since we all use some form of protocol in daily life. For
instance, there's a certain protocol to writing a letter. We can compare the act of composing and
delivering a letter to what a network protocol does to see how each works. The letter has a
well-known form that has been "standardized" through custom. The letter includes a basic message
with a greeting to the recipient and the name of the sender. After you're through writing the
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letter, you stuff it into an envelope, write the name and address of both the recipient and sender on
the envelope, and give it to a delivery system, such as the post office, which handles the details of
getting the message to the recipient's address.
A network protocol acts much like the letter protocol described above. To carry data between
applications, the network software on your computer creates and sends a network protocol packet
with its own private data field that corresponds to the message of the letter. The sender's and
recipient's names (or protocol addresses) are added to complete the packet. After the network
software has created the packet, the entire network protocol packet is stuffed into the data field of
an Ethernet frame. Next, the 48-bit destination and source addresses are provided, and the frame is
handed to the Ethernet interface and the Ethernet signal and cabling system for delivery to the right
computer.
Figure 2–4 shows network protocol data traveling from Station A to Station B. The data is depicted
as a letter that is placed in an envelope (i.e., a high-level protocol packet) that has network protocol
addresses on it. This letter is stuffed into an Ethernet frame, shown here as a mailbag. The analogy is
not exact, in that each Ethernet frame only carries one high-level protocol "letter" at a time and not a