ByJack Unger

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IndexDeploying License-Free W ireless Wide-Area Networks

ByJackUnger

Publisher: Cisco Press

Pub Date: February 26, 2003

ISBN: 1-58705-069-2

Pages: 352

Best practices for planning and deployment of broadband WWANs.

Learn insider tips from an experienced wireless industry leader

Understand the principles that underlie the operation of all wireless systems

Learn how to provide profitable and reliable wireless Internet access

Select the most effective equipment and antenna systems for your area

Avoid common pitfalls encountered by new wireless network operators

Minimize the effects of noise and interference on your network

Enjoy the satisfaction of providing wireless Internet access to your community

Practice the business principles used by successful wireless ISPs (WISPs)

Use 802.11a, 802.11b, and 802.11g equipment more successfully in your own home, office,

or outdoor environment

Choose the right network architecture for your wireless network

Conduct physical site surveys and radio-frequency (RF) site surveys

License-free broadband wireless wide-area networks (WWANs) provide fast deployment of low-

cost, high-speed "last-mile" wireless Internet access. License-free wireless technology delivers

these benefits without requiring the use of products or services provided by local telephone or

cable companies. WWANs enable Internet service providers (ISPs) and corporate IT managers to

deploy their own cost-efficient broadband networks that deliver high-speed access for buildings

and areas where traditional wired connectivity is either completely unavailable or is cost-

prohibitive.Deploying License-Free Wireless Wide-Area Networks is the first book that providescomplete, real-world "start-to-finish" design, installation, operation, and support information for

wireless ISPs and other organizations deploying outdoor wireless WANs-including coverage of

802.11a, 802.11b, 802.11g, and proprietary-protocol networks. This vendor-neutral book covers


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all brands of broadband wireless equipment and explains the principles upon which all wireless

equipment is based. Inside, you'll find step-by-step instructions and crystal-clear explanations

that walk you through initial planning stages and onto full wireless network operation. End-of-

chapter review questions reinforce important concepts.

Whether you're an IT director, ISP engineer, network architect, or field technician, Deploying

License-Free Wireless W ide-Area Networks is your essential reference. With practical, in-depth

coverage of the real-world challenges of outdoor, license-free wireless WAN deployment, this

book provides a comprehensive, vendor-neutral guide to successful wireless network design andoperation.


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Table of Contents


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Copyright

About the Author

About the Technical Reviewers

Acknowledgments

Sample of Physical Site Survey Data Form

Introduction

Who This Book Is For

How to Use This Book

Chapter1. An Introduction to Broadband License-Free Wireless Wide-Area Networking

Differences Between Wired Networks and Wireless Networks

Wireless Terminology and Evolution Advantages of License-Free Wireless Networks

Challenges of License-Free Wireless Networks

Review Questions

Chapter2. Understanding Wireless Fundamentals

Wireless Propagation

Wireless Frequency

Wireless Wavelength

Attenuation

Free-Space Waves

Reflected Waves

Diffraction

Weather and Other Atmospheric Effects

Working with Wireless Power

Antenna Characteristics

Antenna Beamwidth

Obtaining Wireless Line-of-Sight Paths

Wireless Link Budget

Tips for Planning Long Wireless Links

Review Questions

Chapter3. Choosing Your Network Architecture


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Point-to-Point Architecture

Cellular Architecture

Mesh Architecture

Selecting the Preliminary Network Architecture

Review Questions

Chapter4. Performing Site Surveys

Physical Site Surveys

Radio Frequency (RF) Site Surveys Negotiating a Site Lease

Review Questions

Chapter5. Selecting Antenna Systems

Using Antennas to Focus Power and Reduce Interference

Surveying Common Antenna Types

Combining Antenna Systems

Isolating Antenna Systems

Sector Antenna Systems

Other Antenna System Components

Feed Systems

Review Questions Chapter6. Evaluating and Selecting Wireless Equipment

Overview of the Equipment Selection Process

OSI and TCP/IP Reference Models

Physical Layer Wired-Interface Features

Physical Layer Wireless-Interface Features

Data Link Layer Features

Network Layer Features

Application Layer Features

Major Network Feature Decisions

Review Questions

Chapter7. Installing Outdoor Wireless Systems

Preparing for Outdoor Wireless Installations

Verifying the Installation Documentation

Installing Tower-Mounted Antenna Systems

Installing Rooftop-Mounted Antenna Systems

Testing Wireless Systems

Review Questions

Chapter8. Solving Noise and Interference Problems

Understanding SNR

Maximizing the Signal Level

Minimizing the Noise and Interference Level Using Direction-Finding Techniques to Locate Noise Sources

Review Questions

Chapter9. Providing Broadband Wireless Internet Access

Planning for WISP Success

Designing Reliable Networks

Deploying Stable Networks

Managing Network Throughput

Enhancing Network Security

Coexisting with Other Wireless Networks

Living with the FCC


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Anticipating Emerging Trends

Avoiding B lack Holes

Review Questions

AppendixA. Wireless Standards Summary

AppendixB. Wireless Hardware, Software, and Service Provider Organizations

Wireless Hardware

Low-Cost 802.11b Access Point Hardware

Wireless Hotspot Hardware Free Space Optical Hardware

Security Hardware and Software

Antenna Systems

Coaxial Cable

Towers and Antenna Mounting Hardware

Lightning-Protection Equipment

Grounding Equipment

Test Equipment

Peripheral Equipment Distributors

Wireless Software

Services Suggested Books for Further Reading

Online Magazines

WISP Industry Organizations

E-Mail Lists

FCC Rules and Regulations

Wireless Standards

Locating a WISP

Community Networks and Wireless User Groups

AppendixC. Answers to Chapter Review Questions

Answers to Chapter 1 Review Questions









Index


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Copyright

Copyright 2003 Cisco Systems, Inc.

Publishedby:

CiscoPress201West103rdStreetIndianapolis,IN46290USA

All rights reserved. No part of this book may be reproduced or transmitted in any form or by any

means, electronic or mechanical, including photocopying, recording, or by any information

storage and retrieval system, without written permission from the publisher, except for the

inclusion of brief quotations in a review.

Printed in the United States of America 2 3 4 5 6 7 8 9 0

Second Printing June 2003

Library of Congress Cataloging-in-Publication Number: 2001098196

Warning and Disclaimer

This book is designed to provide information about license-free wireless wide-area networks.

Every effort has been made to make this book as complete and as accurate as possible, but no

warranty or fitness is implied.

The information is provided on an "as is" basis. The author, Cisco Press, and Cisco Systems, Inc.

shall have neither liability nor responsibility to any person or entity with respect to any loss or

damages arising from the information contained in this book or from the use of the discs or

programs that may accompany it.

The opinions expressed in this book belong to the author and are not necessarily those of Cisco

Systems, Inc.

Trademark Acknowledgments

All terms mentioned in this book that are known to be trademarks or service marks have been

appropriately capitalized. Cisco Press or Cisco Systems, Inc. cannot attest to the accuracy of this

information. Use of a term in this book should not be regarded as affecting the validity of any

trademark or service mark.

Feedback Information

At Cisco Press, our goal is to create in-depth technical books of the highest quality and value.

Each book is crafted with care and precision, undergoing rigorous development that involves the

unique expertise of members from the professional technical community.

Readers' feedback is a natural continuation of this process. If you have any comments regardinghow we could improve the quality of this book, or otherwise alter it to better suit your needs,

you can contact us through e-mail at [email protected]. Please make sure to include the


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book title and ISBN in your message.

Credits

We greatly appreciate your assistance.

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CiscoSystemsInternationalBV

HaarlerbergparkHaarlerbergweg13-191101CHAmsterdamTheNetherlandswww-europe.cisco.comTel:310203571000

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CiscoPowered Network mark, the Cisco Systems Verified logo, Cisco Unity, Follow Me Browsing,FormShare, iQ Net Readiness Scorecard, Networking Academy, and ScriptShare are trademarks

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owners. The use of the word partner does not imply a partnership relationship between Cisco


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and any other company. (0303R)

Printed in the USA

Dedications

I dedicate this book to my mother, Virginia Blossom Kaufman-Unger-London and to my father,Milton Unger. Thank you both for loving me unconditionally.


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About the Author

Jack Unger, the founder and president of Wireless InfoNet, Inc., is a pioneer in the broadband

fixed-wireless industry. Since founding Wireless InfoNet in 1993, he has personally designed and

installed hundreds of license-free outdoor broadband wireless network sites and consulted on

countless others. In 1995, he designed and deployed one of the world's first public outdoorwireless Internet access points-of-presence. This wireless POP is still in service today in Palo

Alto, California. In 2001, based on his wireless ISP experiences, Jack created the world's first

vendor-neutral wireless ISP training seminar. To date, in this seminar, he has personally trained

more than 800 ISP personnel across the United States.

Prior to founding Wireless InfoNet, Jack worked for 14 years in the Silicon Valley

telecommunications industry for ROLM, IBM, Siemens, and NEC. Prior to that, Jack worked for 7

years selling wireless communications equipment, including television and radio station

broadcast equipment. He has a total of 45 years of wireless experience since his "initiation" into

the wireless world as an amateur radio operator at the age of 11. Jack has received A.A., A.S.,

and B.A. degrees and has completed extensive work in the U.C. Berkeley Extension

Telecommunications Engineering Program. He holds FCC Amateur Extra and GeneralRadiotelephone licenses as well as an FAA Private Pilot license. He also served as a volunteer

firefighter for 11 years in the mountains of Santa Cruz County, California.

Jack is an experienced technical writer who has written numerous telecommunications hardware

and software manuals and close to 100 Cisco IOS Release Notes for the Cisco 2600, 3600, 3810,

4000, and IAD2420 router product lines. Deploying License-Free Wireless Wide-Area Networks isthe industry's first book written specifically to help wireless ISPs and corporate IT department

personnel successfully deploy outdoor, license-free, wireless WANs and broadband wireless

Internet access. Jack welcomes your comments, suggestions, corrections, and questions via e-mail. You are invited to contact him at [email protected].


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About the Technical Reviewers

Jim Geier is the founder and principal consultant of Wireless-Nets, Ltd., a consulting firm that

assists companies with the development and deployment of wireless LAN products and systems.

His 20 years of experience includes the analysis, design, software development, installation, and

support of numerous client/server and wireless network-based systems for retail, manufacturing,warehousing, healthcare, and airline industries throughout the world. Jim is the author of

several books: 802.11 Unleashed,Wireless LANs,Wireless Networking Handbook, and NetworkReengineering, as well as numerous articles. He is a voting member within the Wireless Ethernet

Compatibility Alliance (WECA), responsible for certifying interoperability of 802.11 (Wi-Fi)

wireless LANs. He served as chairman of the Institute of Electrical and Electronic Engineers

(IEEE) Computer Society, Dayton Section, and chairman of the IEEE International Conference on

Wireless LAN Implementation. He has been an active member of the IEEE 802.11 Working

Group, responsible for developing international standards for wireless LANs. Jim's educationincludes B.A. and M.A. degrees in electrical engineering and an M.A. degree in business

administration. You can contact Jim [email protected] or visit his website atwww.wireless-nets.com.

H. Paul Shuch, Ph.D. is the aerospace engineer who is credited with designing the world's first

commercial home satellite TV receiver. Paul has served as executive director of the grassroots,

nonprofit SETI League, Inc. since its inception eight years ago.

The author of more than 300 publications, Paul holds a Ph.D. degree in engineering from the

University of California, Berkeley and has taught on various campuses for 30 years. His teaching

and research have won several international awards. Paul is a Fellow of the Radio Club of

America and a Fellow of the British Interplanetary Society; he has served on the boards of

directors of several nonprofit organizations.

A licensed commercial pilot and instrument flight instructor, Paul serves as an accident

prevention counselor for the FAA, as a military program evaluator for the American Council onEducation, and as a lecturer for the Air Safety Foundation. He is listed in Who's Who in Aviation

and Aerospace,Who's Who in California,Who's Who of American Inventors,Who's Who inScience and Engineering,American Men and Women of Science,Who's Who in Amer icanEducation , and the International Directory of Distinguished Leadership.

Paul lives on a radio-quiet hilltop in central Pennsylvania with his biologist wife, five of their

seven recombinant DNA experiments, three motorcycles, two radio telescopes, and an antique

MG-TD. He can be found on the web at http://drseti.com.

Jack Unger, the author of this book, is Paul's former student, former employee, fellow radio

amateur, and lifelong friend.
http://drseti.com/


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Acknowledgments

Literally thousands of people have made indirect contributions to this book.

First, thanks to all of my public school teachers (from kindergarten through college) who

patiently helped me, encouraged me, and taught me how to learn. Next, thanks to the manymembers of the amateur radio community who showed me how to actually make wireless

equipment work. Further, I extend my thanks to the many members of the wireless ISP

community who shared their experiences and their ideas with me. Next, thanks to my

colleagues, J.V. Rudnick, Bob Fike, and Phil Marcelis, who many times showed me a better way

to erect an antenna, route a cable, or configure a wireless router. Last but certainly not least, I'd

like to thank my Cisco Press editors, Michelle Grandin and Christopher Cleveland, and my

technical reviewers, Dr. Paul Shuch, Jim Geier, and Greg DesBrisay, for graciously contributing

their energy, their experience, and their knowledge to this book.

Finally, allow me to make a gentle request of you, the reader. If you find this book helps you to

learn more about and to experience the magic of wireless, please feel free to generously pass

your new learning along to others who share your interest in wireless.


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Sample of Physical Site Survey Data Form

Surveyor Name Phone/ E-Mail

Site Address Site Owner Site Manager Phone/ E-Mail

Facility Manager Phone/ E-Mail

Existing W ireless Equipment Existing Antenna L ocations

New Equipment Location Power Source

Path Length Fresnel Zone Clearance

Roof Height Roof Access Location

Antenna Location Antenna Height AboveGround

Antenna Mounting Hardw are

Needed Antenna Heading/ Tilt

Nearby Obstructions Distant Obstructions

Cable Type and Length Cable Entry Point

Cable Rou te

Grounding Locations (Mast and

Building Entrance)

Ground Wire Route (Mast and

Building Entrance)

Lightning Protection Description Lightning P rotection Location

Site Evaluation Good Acceptab le Not

Acceptable

Comments and

Recommendations

Follow-Up Is sues

Drawings Attached


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Who This Book Is For

This book is for you, if you

Find yourself intrigued by the magic of wireless

Want to advance your knowledge and want to learn practical wireless networking skills

Want to advance in your career and in the wireless field

Are interested in providing broadband wireless service to others in their community

Want to save money, have fun, and be creative


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How to Use This Book

The chapters in the book are arranged in a logical order. If you have never been exposed to

wireless ("exposed to wireless" "radio-active"you get it?), you can start reading at Chapter1 and read right through to Chapter 9. On the other hand, if you just want to look up oneparticular topic, use the index or the table of contents, locate the topic, and go directly to that

page or chapter. Here are the topics covered by each chapter:

Chapter 1, "An Introduction to Broadband License-Free Wireless Wide-Area Networking,"presents basic broadband wireless terminology and history. It outlines the advantages and

the challenges of working with outdoor networks. Finally, it discusses the safety issues that

you need to address.

Chapter 2, "Understanding Wireless Fundamentals," helps you understand wirelessfundamentals. Even if you have never worked with wireless before, you can be successful if

you take the time to learn these fundamentals.

Chapter 3, "Choosing Your Network Architecture," describes the four basic wireless networkarchitectures, including the advantages and disadvantages of each architecture.

Chapter 4, "Performing Site Surveys," discusses physical site surveys and wireless sitesurveys. Site surveys are an important step in building a reliable wireless network.

Chapter 5, "Selecting Antenna Systems," describes how antennas work and how to usethem. Proper antenna selection and use is vital to successful wireless WAN operation.

Chapter 6, "Evaluating and Selecting Wireless Equipment," helps you to understand therange of features available in wireless equipment. When you understand the features and

how they determine your network capabilities, you can select the best equipment for your

network.

Chapter 7, "Installing Outdoor Wireless Systems," explains the techniques to safely install,ground, and test your outdoor wireless systems.

Chapter 8, "Solving Noise and Interference Problems," discusses the causes of noise andinterference and explains the techniques that you can use to minimize noise and

interference problems. This chapter also provides some suggestions about how you can

cooperate and coordinate with other wireless network operators to reduce interference

between your networks.

Chapter 9, "Providing Broadband Wireless Internet Access," provides extra tips andtechniques for those of you who want to be wireless Internet Service Providers (WISPs) and

provide wireless Internet access service to your communities.

Appendix A, "Wireless Standards Summary," provides a summary table of the 802.11standards. It also includes related standards, such as 802.1x and 802.16.

Appendix B, "Wireless Hardware, Software, and Service Provider Organizations," contains alisting of organizations that provide wireless hardware, software, peripherals, and services.

Appendix C, "Answers to Chapter Review Questions," provides answers to the reviewquestions that appear at the end ofChapters 1 through 9.


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Chapter 1. An Introduction to BroadbandLicense-Free Wireless Wide-AreaNetworking

This chapter introduces broadband wireless networking terms, summarizes license-free wireless

history, and describes both the advantages and the challenges of deploying license-free wireless

wide-area networks (WANs).


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Differences Between Wired Networks and Wireless

Networks

Most of the time, users of a broadband wireless network do not experience a difference between

using a wireless network and using a wired network. Your experiences as you design and build a

wireless network, however, will be quite different compared to your experiences when youdesign and install a wired network. Table 1-1 summarizes the differences that you need to beaware of.

Tab le 1-1. Differences Between Wired and W ireless Networks

Network

Characteristic

Wired Network Wireless Network

Visualdetermination ofnetwork

connectivity

If you can see the network cable

going to a location, that location

can be connected to the network.

Wireless networks sometimes

connect locations that you cannot

visibly see. Additionally, wireless

networksmight not connectlocations that you can see visibly.

Visibility node-to-node on the same

network

All of the nodes on a wired

network can hear all other nodes.

Many nodes on a wireless network

cannot hear all of the other wireless

nodes on the same network.

Visibility network-to-network

Wired networks are invisible to

other wired networks. The

presence of one wired network hasno effect on the performance of

another wired network.

Wireless networks are often visible

to other wireless networks. One

wireless network can affect theperformance of other wirelessnetworks.

Atmosphericproperties

Wired network performance is not

affected by the properties of the

atmosphere.

Wireless network performance can

be affected by the properties of the

atmosphere.

Terrain properties Wired network performance is notaffected by the properties of the

earth's terrain.

Wireless network performance is

strongly affected by the properties ofthe earth's terrain.

User connectivity

and mobility

Connectivity is possible only to or

from those physical locationswhere the network cablingextends.

Connectivity is possible beyond the

bounds of physical network cabling.


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Wireless Terminology and Evolution

Today, you can design and build wireless networks that possess three characteristics that were

not available in the past. These characteristics are broadband capability, wide-area coverage,

and license-free operation. The definition of these terms is somewhat vague; therefore, it is

important to define the terms clearly. The following sections provide these definitions as well as

some wireless history.

Broadband

Broadband is a subjective term that has been used in various ways throughout the

communications industry. Broadband is used when new communications technologies are

developed that provide enough additional bandwidth for the user experience to feel substantially

faster than it felt before.

Most Internet users today have experienced dialing into the Internet at bandwidths ranging from

28,800 bits per second (28.8 kbps) up to 56,000 bits per second (56 kbps). They perceive afaster Internet connection, such as a 1.5 million bit per second (1.5 Mbps) connection, as a

broadband connection.

Some users have access to the Internet using a web browser on a cell phone. Their connection

bandwidth ranges from 9.6 kbps to 14.4 kbps. Comparing the cell phone connection speed to a

dialup 28.8-kbps connection, the cell phone connection feels slow. It certainly doesn't feel like

broadband; in fact, it feels like "narrowband."

For the remainder of this book, any wireless connection that has a bandwidth of 128,000 bits per

second (128 kbps) will be defined as broadband. Given the Internet experiences of users who

have either browsed the Internet using a cell phone or dialed into the Internet, a 128-kbps

connection is perceived as broadband.

Wide-Area Network

There is no absolute line between the definition of a local-area network (LAN) and the definition

of a wide-area network (WAN). Both terms have been used somewhat loosely. Here's how this

book defines them:

LA N A network that connects stations contained within a single building

WAN A network that connects stations located in different buildings or in different partsof a city

NOTE

Some books use the term MAN to indicate a metropolitan-area network or, in other

words, a citywide network. This book refers to a citywide network as a WAN.


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History of License-Free Wireless Networking

The purpose of this book is to help you deploy broadband wireless WANs without needing to

apply to the Federal Communications Commission (FCC) for a license. The following sections

provide a historical overview of some of the events that led to the present privilege of using

some broadband wireless equipment without needing to obtain a license.

History of Wireless Licensing

Wireless technology has passed through several regulatory phases during its history. In the late

1800s and early 1900s, there were many wireless scientists, experimenters, and hobbyists. In

these early days, transmitting distances were limited and population density was low; it was

unnecessary to require a license for transmitting. As more wireless stations came on the air,

interference between stations became a serious problem. In 1921, the United States government

began requiring licenses for all transmitters, including commercial broadcast transmitters,

experimental transmitters, and amateur radio transmitters.

Radio system usage continued to grow rapidly during the remainder of the 20th century. The

U.S. government continued to regulate radio transmissions and to license radio transmitters.

These licensing regulations played a useful role because they allowed many different radio

systems to share the available radio frequencies without interfering with each other.

The downside to governmental regulation was that it took both time and money to obtain a

license to transmit on a specific frequency. This limited the use of broadband wireless equipment

to those companies or individuals who could afford the cost of obtaining the FCC license and

purchasing the rather expensive wireless equipment.

In 1985, the U.S. Federal Communications Commission (FCC) issued regulations that, for the

first time, allowed the use of broadband wireless transmitting equipment without the need to

apply for, pay for, and wait for a license. To operate license free, the wireless equipment had to

do the following:

Operate at low power levels

Use spread spectrum modulation

Transmit within three specified frequency bands

Broadband license-free wireless equipment began to be manufactured and sold at a much lower

cost than the licensed broadband wireless equipment.

Table 1-2 provides an outline of the wireless events that led full-circle from the days whenlicensing was not required, to the days when licensing was required, to today, when broadband

wireless equipment can be deployed license free.

Table 1-2. Brief History of Wire less Development


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Year Event

1600 Dr. Will iam Gilbert detects electromagnetic activity in the human body and describes itas "electricity."

1837 Samuel F.B. Morse invents the Morse telegraph and sends messages over wires by usingMorse code.

1865 Scientists, inventors, and hobbyists begin performing experiments with wireless.

1867 Scottish mathematician and physicist James Clerk Maxwell develops the theory thatpredicts the existence of electro-magnetic waves.

1886 German physicist Heinrich Hertz is the first person to demonstrate the existence ofelectromagnetic waves as predicted by James Clerk Maxwell.

1896 Italian Guglielmo Marconi demonstrates electric-wave telegraphy.

1901 Marconi transmits the letter "s" in Morse code across the Atlantic Ocean from England toCanada.

1906 Professor Reginald Fessenden broadcasts voice and music in Massachusetts.

1910 The U.S. government requires all ships to be equipped with a wireless telegraph.1912 The "unsinkable" Titanic ocean liner sinks after striking an iceberg in the North Atlantic.

The wireless telegraph is used to summon help from other ships in the area.

1912 The regulation of radio broadcasting is started by the U.S. Department of Commerce,Bureau of Navigation. Transmitting stations simply had to supply the government with a

description of their transmitting equipment.

1912 Radio station KQW makes frequent broadcasts from San Jose, California.

1920 Radio station KDKA begins regular commercial broadcasting from Pittsburgh,Pennsylvania.

1921 The U.S. government requires all broadcasting stations to apply for a broadcastinglicense.

1927 The responsibility for regulating radio transmission is transferred to the Federal RadioCommission.

1934 The Communications Act of 1934 establishes the FCC. The responsibility for regulatingradio transmission is transferred to the FCC.

1985 The FCC authorizes license-free spread spectrum transmission in three industrial,scientific, and medical (ISM) bands at 900 megahertz (MHz), 2.4 gigahertz (GHz), and

5.8 GHz. Maximum legal transmitter output power is 1 watt (1W).

1994 The FCC begins using spectrum auctions. Corporations bid hundreds of thousands tomillions of dollars to buy the right to use specific wireless frequency bands.

1997 The FCC authorizes license-free transmission in three 5.1 to 5.8 GHz Unlicensed-National Information Infrastructure (U-NII) subbands. Maximum legal transmitter

output power ranges from 50 milliwatts (50 mW) to 1 W.

History of Spread Spectrum

Modulation is the process of adding information or intelligence to a wireless signal.


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NOTE

In the wireless world, the term intelligence sometimes has no discernable relationship

to the process of using one's brain to make a wise decision.

A modulated wireless signal carries intelligence to the receiver where the intelligence is removed

and used. The intelligence might be voice, music, data, or video. For example, the process of

amplitude modu lation adds voice and music to AM broadcast station signals. The process of

frequency modulation adds voice and music to FM broadcast station signals. The process of

spread spectrum modu lation adds data to broadband license-free wireless signals.

The development of spread spectrum modulation began during World War II. Hedy Lamar (the

actress) is credited for inventing spread spectrum modulation. The story goes that she was

opposed to the actions of the German military during the war and decided to find a way to

transmit and receive messages that would remain undetected by the German military. She

devised the spread-spectrum principle of frequency hopping. Frequency hopping changes the

transmitter frequency rapidly to prevent the transmitted messages from being detected byanyone except the person intended to receive the messages. The spread spectrum receiver

knows the proper sequence of frequency changes and follows them to decode (demodulate) the

transmitted message.

Two different types of spread spectrum modulation are in general use today, both of which

spread the signal out over a broad band of frequencies:

Frequency hopp ing spread spectrum (FHSS) Changes frequency from 8 to 32 times

each second

Direct sequence spread spectrum (DSSS) Stays on one center frequency but spreadsout the signal at low power over a wide frequency band

The resulting transmitted signal simply looks like weak noise. Only a receiver that knows how to

despread the signal can demodulate it and recover the original intelligence.

Development of License-Free Spread Spectrum

Prior to 1985, the FCC permitted low power, short-range devices such as baby monitors and

garage door openers to operate license free. These devices operated indoors or with short-range

transmitters and did not cause interference problems with other wireless systems. In 1985, the

FCC considered the following points and then decided to allow the operation of license-freespread spectrum systems:

Spread spectrum signals spread their wireless energy over the frequency spectrum rather

than concentrating it all on one frequency. By spreading out the energy, the signals are lesslikely to cause interference to other spread spectrum and non-spread spectrum systems.

Spread spectrum signals are less susceptible to being inter fered with than non-spreadspectrum signals.

Low power spread spectrum transmitters operating in the ISM bands will be limited to line-


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of-sight operation. The signals will not carry very far; therefore, many spread spectrum

systems can operate in the same general area without causing significant interference to

each other.

Many spread spectrum systems will be used indoors. The building walls will absorb much of

the wireless energy before it can go very far and cause interference problems.

The first license-free spread spectrum wireless systems were short-range, indoor LANs. These

systems were used for applications such as retail price marking and inventory management.

Next, wireless equipment manufacturers began to offer spread spectrum equipment with

improved antenna systems that could be used for longer-distance outdoor point-to-point links

between buildings. These outdoor links offered a low-cost alternative to the expensive leased-line connections offered by the local telephone companies. Soon, cities, school districts, and

corporations began to use this outdoor equipment in point-to-multipoint network configurations.

By 1995, the first few Internet service providers (ISPs) began to use the technology to provide

license-free broadband wireless Internet access to the public. The era of the license-free

broadband wireless WAN had begun.

NOTE

Although spread spectrum systems are unlicensed, it does not mean that they areunregulated. The FCC specifies the rules and regulations that govern the manufacture

of spread spectrum systems. Equipment manufacturers are responsible for

manufacturing equipment that complies with the FCC regulations. Equipment installers

are responsible for installing the wireless equipment properly so that it does not violate

FCC regulations.

License-Free Wireless Frequencies

In 1985, the FCC authorized the use of license-free spread spectrum wireless equipment in the

following three ISM bands in the United States:

900 to 928 MHz (900 MHz range)

2.4 to 2.483 GHz (2.4 GHz range)

5.725 to 5.850 GHz (5 GHz range)

Further, in 1997, the FCC authorized the use of license-free low-power non-spread-spectrum

wireless equipment in the following three U-NII bands:

5.15 to 5.25 GHz

5.25 to 5.35 GHz

5.725 to 5.825 GHz


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Advantages of License-Free Wireless Networks

Following are the advantages of deploying a license-free broadband wireless WAN:

Cost savings

Fast deployment speed

Network architecture flexi

Documents

ByJack Unger