Optical networking a beginners guide

267 Part III Practical Optical Networking ▼8 Building Optical Networks.. We talk about opticalnetworking basics, optical switching and routing, how to design and manage your opti-cal net

Optical Networking:

A Beginner’s Guide

co-author of eBusiness: A Beginner’s Guide.

Toby J Velte,Ph.D., MCSE+I, CCNA, and CCDA, is a respected industry leader in thefield of networking and has co-founded several high tech start-ups Dr Velte is the author

of Cisco®: A Beginner’s Guide, Cisco® Internetworking with Windows® NT and 2000, and

eBusiness: A Beginner’s Guide.

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for my 13 th birthday.

AT A GLANCE

▼ 1 Optical Networking Theory 3

▼ 2 History of Optical Networking 35

▼ 3 Optical Architectures 61

Part II Optical Networking Tools, Applications, and Vendors ▼ 4 Optical Networking Design 101

▼ 5 Optical Switching and Routing 147

▼ 6 Vendors and Their Wares 185

▼ 7 Optical Networking Applications 233

Copyright 2002 by The McGraw-Hill Companies, Inc Click here for Terms of Use

Part III Practical Optical Networking

▼ 8 Building Optical Networks 273

▼ 9 Optical Network Management 319

▼ 10 Optical Maintenance and Tuning 371

▼ 11 Optical Fiber 405

▼ 12 Optical Network Security 437

▼ Index 467

Acknowledgments xix

Introduction xxi

Part I Networking at the Speed of Light ▼1 Optical Networking Theory 3

The Basics 4

The Promise of Optical Networking 6

How Optical Networking Works 8

Fiber 101 12

Let There Be Light 17

Photons Versus Electrons 17

Light Sources 17

Amplification 21

Get a Boost 21

Erbium-Doped Fiber Amplifiers (EDFAs) 22

Semiconductor Optical Amplifiers 25

Raman Amplification 26

xi Copyright 2002 by The McGraw-Hill Companies, Inc Click here for Terms of Use

Optical Networking Obstacles 28

Attenuation 29

Dispersion 30

Nonlinear Effects 30

Limitations 32

Summary 34

▼2 History of Optical Networking 35

Optical Networking Through Time 36

Discoveries and Innovation 36

Spinning Threads of Glass 41

Spanning the Distance 44

Foundations in Science 48

The Birth of Fiber 48

Lasers 51

Optical Network Service 54

Development 54

The Last Mile 57

▼3 Optical Architectures 61

Multiplexing 62

Fundamentals 62

Types of Multiplexing 63

Optical Demultiplexing 65

Add/drop Multiplexing 68

SONET 69

Evolution 70

Overview 71

Rings 71

Structure 73

DWDM 86

Spectral Specifics 86

DWDM Development 87

How DWDM Works 89

Optical Amplification and DWDM 91

SONET Versus DWDM 92

Dynamic Packet Transport 93

DPT Overview 93

Spatial Reuse 94

DPT Versus SONET 97

Part II Optical Networking Tools, Applications, and Vendors ▼4 Optical Networking Design 101

Networking Basics 102

OSI Reference Model 102

Datalink Protocols 103

IP Addressing 106

Optical LAN Considerations 109

Physical Considerations 114

Network Topology 114

Design 119

Wide Area Networks 122

Access 122

The Edge 124

Backbone 124

Long Haul 128

Fabrics 129

Switching 130

Metrics 131

Applications 133

Fabric Size 134

Fabric Overlays 136

MPLS 137

▼5 Optical Switching and Routing 147

Routing and Switching Basics 148

Routers 148

Switches 152

Optical Considerations 159

Optical Switching 161

Basics 162

Switching Technologies 162

Optical Cross-Connects 169

Optical Routing 174

Basics 175

Speed and Scaling 178

Routers Shape the Next-Generation Internet 180

▼6 Vendors and Their Wares 185

Cisco Systems, Inc 186

Overview 186

Transport Systems 187

Switches 191

Routers 194

Tellium, Inc 199

Overview 199

Switches 199

Nortel Networks, Inc 201

Switches 201

Transport 203

Juniper Networks, Inc 206

Overview 207

Routers 208

CIENA Corporation 212

Overview 212

Transport 212

Switching 216

Sycamore Networks, Inc 219

Overview 219

Switches 220

Lucent Technologies, Inc 222

Overview 222

Transport 222

Switches 223

Other Products 224

Foundry Networks, Inc 226

Overview 226

Routers 227

Switches 229

▼7 Optical Networking Applications 233

Submarine Systems 234

Connecting Continents 234

Installation 236

Metropolitan Area Networks 240

The MAN 241

SONET Versus DWDM Round III 241

The Next Generation MAN 244

VoIP 248

Introduction 248

Building VoIP Networks 251

Using Optics 256

Storage Area Networks 258

Storage Needs 258

Fibre Channel 260

Designing and Building a SAN 261

Fiberless Optics 265

Basics 266

No Cables, no Red Tape 267

A Network Without Wires 267

Part III Practical Optical Networking ▼8 Building Optical Networks 273

Design Basics 274

The Three-Layer Hierarchical Design Model 274

Design Methods 280

Optical Networking Gear 283

Designing to Fit Needs 285

Understanding Existing Internetworks 285

Characterizing Networks 286

Optimal Optical Technologies 289

FDDI 289

Gigabit Ethernet 293

10 Gigabit Ethernet 300

Fibre Channel 302

Models 309

Service Provider 309

Backbones 311

LANs 312

MANs 314

▼9 Optical Network Management 319

Overview of Network Management 320

The Roots of Network Management 321

Network Management Tools Today 324

Trends in Enterprise System Management 327

Service Level Agreements 328

Benefits 329

Preparing a SLA 332

Maintenance 335

What to Look for in a SLA 337

SNMP 338

What Is SNMP? 338

SNMP Polling and Managed Objects 339

The MIB 340

Polling Groups and Data Aggregation 346

SNMP Commands 348

Thresholds 349

Events and Traps 350

SNMP Versions 351

TMN 352

Overview 352

Functional Architecture 355

Physical Architecture 357

Information Architecture 358

Logical Layered Architecture 359

Optical Network Management Applications 362

Lucent WaveWrapper 363

Cisco Transport Manager 363

Corvis CorManager 366

Sun Solstice Enterprise Manager 367

▼10 Optical Maintenance and Tuning 371

Logical Maintenance and Tuning 372

Network Design 372

Statistical Multiplexing 376

Errors 378

Physical Maintenance and Tuning 383

Solving Fiber Cuts 383

Solving Bad Fiber Connections 387

Loss Testing 396

Tunable Lasers 400

Tools 401

Digital Lightwave Network Information Computers 402 Acterna CycloneCore IP Optimizer 402

Fluke FiberInspector 403

▼11 Optical Fiber 405

Cables, Construction, and Connectors 406

Types 406

Fiber Construction 411

Fiber Connectors 416

Cabling and Composition 421

Cabling Environments 421

Cable Composition 427

Fiber Protection 430

Care, Caution, and Companies 431

Problems 431

Safety 433

Manufacturers 435

▼12 Optical Network Security 437

Overview of Network Security 438

Traffic-Based Security 438

User-Based Security 440

Types of Attack 443

Protecting Your Network 446

Security Protocols 446

Intrusion Detection Systems 450

Optical Considerations 454

Optical Network Security Features 454

Attack Methods 457

Attack Detection 459

Vulnerabilities 461

Physical Security 463

▼ Index 467

When it comes down to writing a book this size, it is important to

thank all the people who work to make sure all the pieces fittogether Franny Kelly coordinated the whole effort and is always apleasure to work with Alex Corona kept things in order while CarolynWelch crossed the ‘t’s and dotted the ‘i’s We are indebted to our technicaleditor Mac McVicker and Bob Campbell our copy editor They both did atop-notch job under very tight time constraints

A special thanks goes out to Walter Goralski who allowed us to use

figures from his excellent book, Optical Networking and WDM.

xixCopyright 2002 by The McGraw-Hill Companies, Inc Click here for Terms of Use

Try to imagine a time when one lone cable will be able to carry

▼ One million DVD or high-definition television signals

■ Six million high-speed Internet connections

▲ One hundred million dialup Internet connections

If this sounds like the type of data and video networking that only your children will be able to enjoy, don’t be too despondent The truth of the matter isthat fiber optics makes this capacity possible today—on just a single strand of fiber!The capacities of fiber optics are amazing, and every day there seems to be newways to pack even more data on fiber One day, your grandchildren might look

grand-back and ask, “Fiber could only carry a million HDTV channels at the turn of the

century?” Although fiber optic systems were first conceived early in the twentiethcentury (the basics of light and fiber a couple hundred years before that), it wasn’tuntil the mid-1970s that fiber optics were first deployed for communications Sincethen, fiber-based communications have been getting more robust, and it is onlynow that optical networking is coming into its own

xxiCopyright 2002 by The McGraw-Hill Companies, Inc Click here for Terms of Use

It should come as no surprise that networks are getting faster and faster If you askanyone if their network is fast enough, they’ll tell you “no.” This need for speed isspurred by beefier applications and the changing role of data in modern networks It isn’tsufficient for a network to simply transfer a text file from the server to a client Now, it isnecessary to send video files halfway around the globe and conduct online telephoneconversations without any disruption in service.

A way to make networks and computing run faster and with more capacity is to ploy an optical system Optical networks are quite similar to conventional electrical net-works However, optical devices (like routers and switches) are used in place of electricaldevices, and miles and miles of extremely thin glass fiber have supplanted copper wire

em-On the surface, it would seem that spans of glass just a few microns across would be pable of carrying anything Quite the opposite is true Optical networks are able to carrydata much faster and in greater quantities than their electrical counterparts

inca-This book examines optical networking from a variety of angles We talk about opticalnetworking basics, optical switching and routing, how to design and manage your opti-cal network, and how you can keep everything safe and secure

OPTICAL NETWORKING AND YOUR ORGANIZATION

Building an optical network is loaded with the same challenges inherent in electrical works However, the physics and operation of optical networks make the process hard toget a grasp on sometimes

net-No matter how you deploy an optical network, your organization will benefit from it

As networks become more powerful, so do the applications that run on those networks.And as applications demand more bandwidth, the network must grow to match This is anever-ending circle, but you can get a good jump on network speed and capacity if youemploy an optical design

Optical networks are conventionally thought of as something just for phone nies or Microsoft The truth of the matter is that organizations of any size can implement

compa-an optical solution Whether your orgcompa-anization needs to trcompa-ansmit video compa-and data acrossthe sea or if you just need high-capacity links in a local area network, optical networkingcan provide a solution

Of course, there is a financial reality to optical networks They tend to be more sive than their electrical counterparts; however, when you weigh the initial costs of anoptical network with the ongoing costs of an electrical network, the cost savings come in

expen-on the side of the optical network

WHO SHOULD READ THIS BOOK

This book is written for a range of people in your organization or for anyone with aninterest in optical networking First and foremost, it is meant for the chief decision

maker who will be implementing an optical networking solution But the content is

also appropriate for IT staffers, CEOs, and anyone else with an interest or stake in your

organization’s optical goals Anyone reading this book will be able to understand the

principles, theories, and applications behind the optical issues presented within

But this isn’t to suggest that individuals with a higher level of technological expertise

can’t glean valuable information from this book The world of networking is changing

rapidly, and—because of the nature of optical networking—there are different concepts

and ways to use technology that are unique to optical deployments We discuss those

issues here

WHAT THIS BOOK COVERS

Optical Networking: A Beginner’s Guide is organized into three parts Each part presents an

important facet about optical networking in an easy-to-follow manner For example, if

you’re looking for information about optical networking basics, you can refer to the first

three chapters in Part I If you want to read about putting optical networks into practical

deployment, Part III contains chapters germane to that issue The following describes the

sections and the chapters within those sections

Part I: Networking at the Speed of Light In the first section, the first three chapters

exam-ine the basics of optical transmission, how optics were even considered as a transmission

platform, and how networks are built based on optical technology The chapters in this

section are:

▼ Chapter 1: Optical Networking Theory This chapter is really Fiber Optics

101 We cover the basics behind optical networking, including how optical fiber

works, amplification, loss, and how light moves through a span of optical fiber

■ Chapter 2: History of Optical Networking In this chapter we pause for

a moment to consider the history of optical networks We talk about how

pioneers like Charles Vernon Boys made the first uniform threads of glass and

how Swiss physicist Daniel Colladon and French physicist Jacques Babinet

simultaneously discovered that light would follow the flow of water from a

fountain Finally, we wrap up our discussion with a look at former high-speed,

last mile designs and what went wrong Then, we examine passive optical

networks (PONs) and see how they could be the answer to the last mile problem

▲ Chapter 3: Optical Architectures In Chapter 3, we get back to the nitty

gritty of optical networking In this chapter we talk about the types of optical

networks that are responsible for the transmission of optical bits and bytes

Our discussion includes Synchronous Optical Networks (also known as

SONET) and Dense Wavelength Division Multiplexing (DWDM)

Part II: Optical Networking, Tools, Applications, and Vendors In this section, we look at

how you might design an optical network to suit your particular needs, how optical

routing and switching occur, what optical vendors have to sell, and which applicationsare best suited for an optical network The chapters in this section are:

▼ Chapter 4: Optical Networking Design This chapter focuses on the design

of your optical network Whether you need a large-scale deployment orsomething smaller, this chapter explains how optical networks are designed.First, we talk about some networking basics; then we apply that knowledge

to the world of fiber optics

■ Chapter 5: Optical Switching and Routing In this chapter, we talk aboutrouting and switching Both are extremely important in the construction

of networks and internetworks, both electrical and optical In an opticaldeployment, the requirements of a router or a switch are little different,especially when truly all-optical devices are introduced This chapter notonly looks at how routing and switching work, but also how they will workwhen truly all-optical devices are developed

■ Chapter 6: Vendors and their Wares This chapter examines the variousvendors out there who offer optical equipment We cover the most prevalentvendors in the optical world, talk about their place in it, and also discuss theequipment they offer

▲ Chapter 7: Optical Networking Applications It’s all well and good to build

an optical network, but what would you use it for? This chapter discusses theapplications for which an optical network would be best suited Topics includeVoice over IP (VoIP), storage area networks (SANs), and submarine systems

Part III: Practical Optical Networking The final section puts the knowledge of optical

networks into practical application We’ve moved from designing an optical network tobuilding it Once an optical network is built, you need to manage it, tune it, and protect it.Further, we talk about the different types of fiber on the market and why you wouldchoose one type over another Chapters in this section are:

▼ Chapter 8: Building Optical Networks This chapter explains how opticalnetworks can be built Topics include the ever-increasing capacity of Ethernetand Fiber Distributed Data Interface (FDDI) and which technologies would

be best for your optical deployment

■ Chapter 9: Optical Network Management Network management is

important in any network and this is no less true in optical networks Thischapter looks at issues relevant to the management of optical networks andsuggests ways you can manage your own optical networks

■ Chapter 10: Optical Maintenance and Tuning Once a network is built, itdoes not continue to work, error free, for years on end by itself Maintenanceand tuning are constant chores that information technology professionals mustdeal with This chapter discusses issues of optical network management and

tuning and also suggests some third-party resources that can help you tune

your optical network

■ Chapter 11: Optical Fiber When it comes time to build an optical network,

you need to know what types of fiber are out there, how they’re connected,

and how you can keep safe Even though there isn’t electrical current coursing

through optical fiber, lit fiber can still be hazardous, potentially blinding In

this chapter, we discuss safety from a lit fiber standpoint, as well as the

handling and disposal of fiber shards

▲ Chapter 12: Optical Network Security The last chapter focuses on optical

network security We start with a discussion of security that covers both

electrical and optical networks, then move to issues specifically related to

optical networks Not only do we talk about the things you can do to prevent

your network from falling prey to a hacker or being accessed by the wrong

people, we also discuss the physical things you can do to keep your network

from being damaged

HOW TO READ THIS BOOK

This book has been written with beginners in mind It is designed so that you can pick

it up, flip to any chapter, and find the information you need We do assume, however,

that you have some very basic understanding of networking, computers, and the Internet

If you are looking for something specific (like information about optical routing), you can

just flip to that chapter If you want specific information on security, for example, you can

easily turn to that chapter

PART I

Networking at the

Speed of Light

1Copyright 2002 by The McGraw-Hill Companies, Inc Click here for Terms of Use

CHAPTER 1

Optical Networking

Theory

3Copyright 2002 by The McGraw-Hill Companies, Inc Click here for Terms of Use

When primitive man first figured out how to make fire, he was concerned with

keeping warm and cooking food After a while, he learned to use fire to turnnight into day, and then he put it into an engine to make trains and cars run

As we begin the twenty-first century, we’re using a form of fire to communicateacross great distances and share an abundance of information in a way that our hairy,knuckle-dragging ancestors never conceived of (no, I’m not talking about Uncle Cletus).The advent of the laser and its subsequent application to fiber optic cabling is movinguntold gigabits of information every second across continents and under seas

In this chapter, we look at what makes optical networking happen We discuss theconcepts, and we look at how those concepts are put into practical application Eventhough optical networking is discussed in terms of what you can do with it, we also look

at the other side of the coin, specifically, what its limitations are

THE BASICS

In its purest essence, optical networking is some flashing light moving back and forthacross a glass rod This flashing light carries the information traveling between network-ing components, such as optical switches and routers As Figure 1-1 shows, think of thisprocess as two ships signaling each other on a cold dark night in the north Atlantic.But where our two mariner friends are sharing such quick, short messages as “whereare we?” and “is that a hole in your bow?” optical networking can send billions of bits ofinformation in a second An optical network can send pages of text, images, music, andmovies in the blink of an eye

Spotlight

Light flashes

Spotlight

Light flashes

Figure 1-1. Optical networking is much like ships flashing Morse Code messages to each other

In a conventional internetwork, this information would be transmitted across greatdistances using twisted-pair copper wire, across a wide area network (WAN) or even alocal area network (LAN) As useful and utilitarian as twisted-pair cabling and an electri-cal network have been, fiber optics allows information to be transferred at immenselyhigher rates In the past, when computers shared only brief conversations across themiles, electrical networks could handle the load But now, as information is shared as ithas never been shared before, there is a clear need for an upgrade in network capacities.

Let’s go back to our examination of what, exactly, comprises an optical network AsFigure 1-2 illustrates, an optical network is a glass rod connecting networking devices(routers, switches, and so forth) in different locations Clearly, large glass rods spanningthe continents and oceans are not a viable solution So how, then, are light pulses deliv-ered across such a fragile medium? Amazingly thin strands of glass (even thinner thanone of the strands of hair in your Uncle Phil’s comb-over) that can be thousands of mileslong are used to deliver the optical network’s payload Even though these strands of glassare remarkably strong—even stronger than a similar sized piece of steel wire—they areencased in a surrounding cable and buried underground or even submerged on theocean floor

Let’s say you’ve got a short length of a glass rod and you shine a flashlight in one end.Even though you would see the light in the end of the rod, the truth of the matter isthat very little of the originating light would make it all the way to the other end This

is because as clear and clean as glass may look, it is still chock-full of impurities Theseimpurities block the light before it gets to the other end

So how, you might be wondering, does light make it across thousands of miles on astrand of glass thinner than the human hair? Optical networking is enabled because oftwo improvements in the field of shining a flashlight in one end of a glass rod:

▼ The glass used in fiber optics is specially designed so that it is low in

impurities Further, within the fiber itself, the light can bounce around

and propagate to the far end

▲ Very powerful lights called lasers are capable of traveling much farther than

regular light

Glass rod Light

Light rays

Light rays

Light

Figure 1-2. In essence, an optical network is a glass rod with two lights flashing on and off at

each end

Like the Morse Code being sent between the two ships in Figure 1-1, the laser lightflashes in an optical network represent the information that networking devices are shar-ing between one another On the other end of the fiber is the receiver This receiver keeps

up with the flashes of light

The quicker the lasers can flash on and off, the quicker the information can be

trans-mitted This speed is known as the bit rate and is commonly talked about as bits per

sec-ond (bps) The most common bit rate going across conventional optical networks is 10gigabits per second, which breaks down to about 10 billion flashes of light per second Forthe sake of comparison, the preeminent electrical networks top out at 1 Gbps, but mostcommonly operate at 10 or 100 million bits per second (megabits per second)

Though 10 Gbps is impressive, the next step in optical networking speed is 40 Gbps,and some vendors are experimenting with 1.6 terabits per second (1.6 trillion bits per sec-ond) To put this all in perspective, using a 10 Gbps optical network, you could transmitthe text of 1000 books in just one second

The Promise of Optical Networking

It’s no secret that as industry and society increase their dependence on technology, thecapabilities of existing technology will be taxed Conventionally speaking, information in

an internetwork courses through copper wire, be it across LAN connections or across aleased WAN link Unfortunately, copper wire has its limitations

Optical networking, on the other hand, offers enhancements over conventional working because it provides three important network performance improvements: speed,capacity, and distance

net-Speed

Comparing the bit rates in electrical networks to optical networks is like putting WoodyAllen in a prison yard fist fight with Mike Tyson—there’s just no comparison (unlessWoody whined neurotically at Mike long enough to give him an aneurysm, but let’s stay

on track here) The greatest thing that optical networking has going for it is raw speed.Common WAN links that move across electrical networks are T-1 (1.544 Mbps) andT-3 (45 Mbps) On the LAN front, things get a little better Most organizations use 10 or

100 Mbps Ethernet The top-of-the-line Ethernet clocks in at 1 Gbps However, once fiberoptics gets into the race, look out

At their slowest, fiber optic networks speed along much faster than a T-1 or a T-3.Once fiber shifts out of first gear, there ceases to be a comparison When discussing opti-cal networking speeds, you’ll hear the terminology change from T-1 or T-3 to OC OC

stands for optical carrier OC takes over where T leaves off Once the optical carrier gets

involved, speeds not only reach 1 Gbps but even leave 1 Gbps in the rearview mirror.Table 1-1 shows how optical networking line speeds increase

As you can see, the speed rates in optical networks (not to mention their ment) are increasing at an amazing velocity OC-192 is the current top end in optical net-working, although OC-768 is soon to be a reality On the horizon, however, is the

develop-networking equivalent of breaking the sound barrier—1.6 terabits per second Breaking the terabit barrier on a single fiber is due thanks to dense wavelength division multiplexing

(DWDM) and ever- increasing line rates.

NOTE: We talk more about DWDM in Chapter 3.

Capacity

Going hand-in-hand with speed is the notion of capacity Think of capacity as a gardenhose Let’s say that you are trying to water your lawn using a standard, green gardenhose available at any lawn and garden store The hose is good for the job, but what if yourhouse catches on fire? When the fire department shows up, they don’t start unrollinglengths of garden hose Instead, they use hoses that have a much wider diameter Thesehoses can deliver more water than your garden hose The same holds true for network-ing The wider the “pipe,” the more data can get through

To think about capacity, it’s useful to think about your computer and how you use it

If you receive an e-mail message, it could have come through a 640 Kbps Ethernet tion with no problem whatsoever Think of this as the garden hose But what if you want

connec-to watch a large video over a WAN or over the Internet? Your 640 Kbps garden hose justwon’t be able to accommodate your needs It will get clogged up, you’ll drop frames, andthe video will be unwatchable On the other hand, if you use an OC-192 optical network,that’s like backing a pumper truck to your burning house The large volume of video traf-fic will come through clearly because the link has enough capacity

Capacity is also helpful if you are trying to develop an IP Telephony solution, forinstance If you are trying to connect a large company to an IP Telephony solution using10/100 Ethernet, you will run into problems However, using an optical network amelio-rates that problem

The future of motion pictures will owe a lot to optical networking, as well Now thatmore and more motion pictures are being filmed and edited digitally, the next step is toproject the film digitally Rather than mail a disc with the movie to theaters around thecountry, the movie studios want to electronically transmit the movie to the theaters Thisallows the movie to be presented digitally and eliminates the need to ship spools of cellu-loid cross-country to thousands of theaters But it also illustrates the need for a high-capacity internetwork.

Distance

One of the key benefits of fiber optics is its ability to span a few feet or thousands of miles.Depending on the quality of the fiber and the hardware, a stretch of fiber a thousandmiles long needs no repeater or amplification hardware This is rare, however, and moststretches of fiber require periodic boosts in signal strength

NOTE: We talk about the ins and outs of optical networking amplification later in this chapter.

You probably use a fiber optical network day in and day out, but aren’t aware of it.When you make a long distance phone call or access a Web site in Europe, for example,the signal has to find a way to hop the pond But the bulk of the communications aren’tbeing beamed to satellites to make the jump—they cross the oceans using submarine fiberoptic cabling

In order to locate thousands of miles of optical fiber on the ocean floor, the fiber isencased in a very durable material, along with an amplifier as needed, and placed Inorder to put the cable on the seabed as safely as possible, a very detailed map of the oceanfloor is used, then the cable placement is determined, using a route that takes it out of theway of any obstacles, such as oil platforms or fishing lanes The place where submarinelines are most often damaged is not out in the deepest part of the ocean Rather, it is inshallow harbors where boats can snag the line

How Optical Networking Works

When you buy all the components for an optical network, you don’t really need to hire

Mr Wizard to act as a consultant to understand the physics behind the thing You don’t

need to know how refraction and diffusion occur within your system However, a little

primer on some basics will give you a better understanding of the system and what’sgoing on inside those expensive boxes of circuitry

Light Reflection and Refraction

The main job of optical fibers is to guide light waves without losing too much light.Within a run of fiber optic cable, light is transmitted at about two-thirds the speed of light

in a vacuum The transmission of light in optical fiber is most commonly explained using

the principle of total internal reflection (TIR).

This means that 100 percent of light that strikes a surface is reflected For means ofcomparison, a mirror reflects about 90 percent of the light that strikes it, so you can seethat TIR is a high standard to meet.

When light is emitted—be it from a powerful laser or from candlelight—the radiatedlight can bounce, assuming it strikes the right material Light can be manipulated in twobasically different ways:

▼ Reflectionmeans that the light bounces back

▲ Refractionmeans that the light’s angle is altered as it passes through a

different medium (like a glass of water, a prism, or a fiber) The angle is

determined by the angle of incidence The angle of incidence is the angle at

which light strikes the interface between an optically denser material and

an optically thinner one

For TIR to occur, the following conditions must be present:

▼ Beams of light must pass from a dense material to a less dense material

▲ The incident angle must be less than the critical angle The critical angle is

the angle of incidence at which light stops being refracted and is instead

totally reflected

Figure 1-3 illustrates the principle of total internal reflection within a fiber core Asyou see, the core has a higher refractive index than the surrounding cladding, thus allow-ing the beam of light to strike the surface at less than the critical angle The second beamdoes not meet the critical angle requirement and is refracted

As you remember from our earlier discussion about fiber, the core and cladding areconstructed out of optically denser and optically thinner types of highly pure silica glass

These components are mixed with components called dopants (like erbium), which

adjusts their refractive indices The difference between the refractive indices of the twodifferent kinds of glass causes most of the emitted light to bounce off the cladding andstay within the core, traveling to the endpoint

The critical angle requirement is met by controlling the angle at which light is beamedinto the core

Optical Fibers

Earlier in this chapter, we told you that glass (as you know it) is not the best for use inoptical networking Because of its composition and the manufacturing process, it isloaded with impurities, no matter how crystal clear it appears For optical networking tohappen, it must use a fantastically pure kind of glass In this case, silica glass is the blend

of choice Even though it is exceptionally pure, there is still a little loss of light as the lighttravels through The loss, however, is much less pronounced than with ordinary glass.Pure glass isn’t the end of the discussion on how to get information through the fiberwithout obstacles, especially when it comes to sending information through hundreds orthousands of miles The key to sending a flash of light across a continent or beneath theocean is in simple physics Let’s talk about refraction a bit more Think back to seventhgrade science class when the teacher was talking about light and refraction and blah,blah, blah… is this going to be on the final? Anyway, you may remember a demonstrationwhere the teacher held a pencil behind a glass of water As a jaded teenager, you mayhave expected just to see the pencil behind the glass Instead, it was bent at an odd angle.This is how refraction manifests itself Sure, it wasn’t as cool as the baking soda and vine-gar volcano or cooking a hot dog with tin foil, but can you send 10 Gbps with baking sodaand vinegar?

Another junior high science classic is when you shined a light into a block of glass andrather than coming out of the opposite side, it bent and shot out at a funky angle We seethis in Figure 1-4 This happens because the light was refracted when it left the glass Asyou may have noticed when you experimented further, if you beamed the light source into

the glass at an angle greater than the critical angle, it would be bent inside the block until it

left the block at a seemingly random location The departure point may have seemed dom to you; however, if you managed to shine the light at precisely the same location eachand every time, the light would have exited the block at precisely the same spot

ran-Light is refracted within the block (and within a strand of optical fiber) because ofrefractive indices and Snell’s Law Snell’s Law tells us how light—moving from one envi-ronment to another, like between air and water or core and cladding—is bent within amaterial The refractive index of a material tells us how dense a particular material is

Once you have a core of pure silica, an extra layer of glass (called cladding) is wrapped

around the core The cladding has a lower refractive index than the core The difference inrefractive indices guides the light into the core and prevents the light from escapingthrough the sides of the fiber

Earlier, we said that large glass rods weren’t the ideal way to send information across

an optical network But that doesn’t mean that all fiber has to be the smallest diameter

object made by man There can be (and are) varying diameters of fiber core The size of a

fiber’s core will determine how light travels through it A wave of light has a physical size

We may not be able to see it with the human eye, but as Figure 1-5 shows, a wavelength of

light has a physical dimension that must be accommodated by the size of the optical fiber If

the fiber is too narrow, the wavelength won’t be able to fit inside An optical signal can

gen-erate many different light waves, which can travel through the fiber simultaneously

This is the method used in multimode fibers (which provides a medium over which a

number of concurrent transmissions can be sent) Unfortunately, this can also cause

prob-lems as the waves arrive at the end of the fiber and are out of sync Most optical networks

use single-mode fiber, which has a rather small fiber core (about 9 micrometers—a

mi-crometer is a millionth of a meter), thus ensuring that only a single light wave traverses

the fiber, alleviating receiving problems

NOTE: To get an idea of just how tiny a nanometer is, look at the letter “o” anywhere on this page.

That letter is about a millimeter across (using scientific notation, this is expressed as 10–3meters)

Light source

Light beam

Light beam Prism

Light beam

on floor

Figure 1-4. Refraction in a prism

Fiber 101

In a nonoptical networking scenario, the wires connecting networking components aresimple lengths of copper wire The most prevalent is twisted pair, but there are still somenetworks out there using coaxial cable But optical networks, on the other hand, connectcomponents with fiber optic cabling

Fiber optic cables contain extremely thin strands (10 micrometers across) of silicaglass, which are then encased in a thicker, denser layer of silica glass (about 125 microme-ters across) Once a protective wrap is applied, the fiber optic cable is a quarter of a milli-meter in diameter As you remember from our earlier discussion, the glass used withinthe fiber is highly pure, thus ensuring that the photons keep moving Also, these strands

of glass can be hundreds or thousands of miles long

A number of factors go into the design and construction of optical fiber This sectiontakes a closer look at such issues as size, construction, and design

Design

Optical fiber is composed of three parts:

▼ The core, which carries the light

■ Cladding, which traps the light in the core, causing total internal reflection

▲ The Buffer, which is the insulating wrap protecting the fiber

Figure 1-6 shows how these parts fit together to make fiber optic cabling

NOTE: The difference between the optical density of a given material and a vacuum is the material's

refractive index

Wavelength

10 0 -10

1550 nm

Figure 1-5. Though invisible to the human eye, a wavelength of light has a physical size

To get a better understanding of how fiber works, let’s follow a few photons from

their origins, across the fiber to their final destination Figure 1-7 shows our photonic

friends as they make this journey

1 First, the light source (in this case a laser) converts the network’s electrical

signal into pulses of light

2 The light is injected into the core of the fiber

3 The photons bounce off of the border between the core and the cladding

Because the core and the cladding have different refractive indices, the photons

are bounced back into the core

4 The photons continue through the length of the fiber

5 Ultimately, they exit the fiber and are converted back into electrical signals by

the light detector

6 Because the fiber doesn’t exist in a harm-free environment, the core and

cladding are encased in a protective wrap called the buffer The buffer makes

the fiber more durable and easy to handle

011011

000110

Figure 1-7. How light travels through fiber optic cabling