Optical Networks: A Practical Perspective - Part 8 potx

Optical add/drop multiplexers and crossconnects are now available as commercial products and are beginning to be introduced into telecommunications networks, stimulated by the fact that

40 INTRODUCTION TO OPTICAL NETWORKS

networks are now widely deployed Today it is common to have high-speed optical interfaces on a variety of other devices such as IP routers and ATM switches

As these first-generation networks were being deployed in the late 1980s and early 1990s, people started thinking about innovative network architectures that would use fiber for more than just transmission Most of the early experimental efforts were focused on optical networks for local-area network applications, but the high cost of the technology for these applications has hindered commercial viability of such networks Research activity on optical packet-switched networks and local-area optical networks continues today Meanwhile, wavelength-routing networks became

a major focus area for several researchers in the early 1990s as people realized the benefits of having an optical layer Optical add/drop multiplexers and crossconnects are now available as commercial products and are beginning to be introduced into telecommunications networks, stimulated by the fact that switching and routing high-capacity connections is much more economical at the optical layer than in the electrical layer At the same time, the optical layer is evolving to provide additional functionality, including the ability to set up and take down lightpaths across the network in a dynamic fashion, and the ability to reroute lightpaths rapidly in case of

a failure in the network A combination of these factors is resulting in the introduction

of intelligent optical ring and mesh networks, which provide lightpaths on demand and incorporate built-in restoration capabilities to deal with network failures There was also a major effort to promote the concept of fiber to the home (FTTH) and its many variants, such as fiber to the curb (FTTC), in the late 1980s and early 1990s The problems with this concept were the high infrastructure cost and the questionable return on investment resulting from customers' reluctance to pay for

a bevy of new services such as video to the home However, telecommunications deregulation, coupled with the increasing demand for broadband services such as Internet access and video on demand, is accelerating the deployment of such net- works by the major operators today Both telecommunications carriers and cable operators are deploying fiber deeper into the access network and closer to the end user Large businesses requiring very high capacities are being served by fiber-based SONET/SDH or Ethernet networks, while passive optical networks are emerging as possible candidates to provide high-speed services to homes and small businesses This is the subject of Chapter 11

Summary

We started this chapter by describing the changing face of the telecom industry the large increase in traffic demands, the increase in data traffic relative to voice traffic, the deregulation of the telecom industry, the resulting emergence of a new set of

carriers as well as equipment suppliers to these carriers, the need for new and flexible types of services, and an infrastructure to support all of these

We described two generations of optical networks in this chapter: first-generation networks and second-generation networks First-generation networks use optical fiber as a replacement for copper cable to get higher capacities Second-generation networks provide circuit-switched lightpaths by routing and switching wavelengths inside the network The key elements that enable this are optical line terminals (OLTs), optical add/drop multiplexers (OADMs), and optical crossconnects (OXCs) Optical packet switching may develop over time but faces several technological hurdles

We saw that there were two complementary approaches to increasing transmis- sion capacity: using more wavelengths on the fiber (WDM) and increasing the bit rate (TDM) We also traced the historical evolution of optical fiber transmission and networking What is significant is that we are still far away from hitting the fundamental limits of capacity in optical fiber While there are several roadblocks along the way, we will no doubt see the invention of new techniques that enable progressively higher and higher capacities, and the deployment of optical networks with increasing functionality

Further Reading

The communications revolution is a topic that is receiving a lot of coverage across the board these days from the business press A number of journal and magazine special issues have been focused on optical networks [GLM+00, CSH00, DYJ00, DL00, Alf99, HSS98, CHK+96, FGO+96, HD97, Bar96, NO94, KLHN93, CNW90, Pru89, Bra89]

Several conferences cover optical networks The main ones are the Optical Fiber Communication Conference (OFC), Supercomm, and the National Fiber-Optic En- gineers' Conference Other conferences such as Next-Generation Networks (NGN), Networld-Interop, European Conference on Optical Communication (ECOC), IEEE Infocom, and the IEEE's International Conference on Communication (ICC) also cover optical networks Archival journals such as the IEEE's Journal of Lightwave Technology, Journal of Selected Areas in Communication, Journal of Quantum Electronics, Journal of Selected Topics in Quantum Electronics, Transactions on

of this subject

There are several excellent books devoted to fiber optic transmission and compo- nents, ranging from fairly basic [Hec98, ST91] to more advanced [KK97a, KK97b,

42 INTRODUCTION TO OPTICAL NETWORKS

Agr97, Agr95, MK88, Lin89] The 1993 book by Green [Gre93] provides specific coverage of WDM components, transmission, and networking aspects

The historical evolution of transmission systems described here is also covered in

a few other places in more detail [Hec99] is an easily readable book devoted to the early history of fiber optics [Wil00] is a special issue consisting of papers by many

of the optical pioneers providing overviews and historical perspectives of various aspects of lasers, fiber optics, and other component and transmission technologies [AKW00, Gla00, BKLW00] provide excellent, although Bell Labs-centric, overviews

of the historical evolution of optical fiber technology and systems leading up to the current generation of WDM technology and systems See also [MK88, Lin89] Kao and Hockham [KH66] were the first to propose using low-loss glass fiber for optical communication The processes used to fabricate low-loss fiber today were first reported in [KKM70] and refined in [Mac74] [Sta83, CS83, MT83, Ish83] describe some of the early terrestrial optical fiber transmission systems [RT84] describes one

of the early undersea optical fiber transmission systems See also [KM98] for a more recent overview

Experiments reporting more than 1 Tb/s transmission over a single fiber were first reported at the Optical Fiber Communication Conference in 1996, and the num- bers are being improved upon constantly See, for example, [CT98, Ona96, Gna96, Mor96, Yan96] Recent work on these frontiers has focused on (1) transmitting terabits-per-second aggregate traffic across transoceanic distances with individual channel data rates at 10 or 20 Gb/s [Cai01, Bak01, VPM01], or 40 Gb/s channel rates over shorter distances [Zhu01], or (2) obtaining over 10 Tb/s transmission capacity using 40 Gb/s channel rates over a few hundred kilometers [Fuk01, Big01] Finally, we didn't cover standards in this chaptermbut we will do so in Chapters 6,

9, and 10 The various standards bodies working on optical networking include the International Telecommunications Union (ITU), the American National Standards Institute (ANSI), the Optical Internetworking Forum (OIF), Internet Engineering Task Force (IETF), and Telcordia Technologies Appendix C provides a list of relevant standards documents

References

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Jan.-Mar 1999

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44 INTRODUCTION TO OPTICAL NETWORKS

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46 INTRODUCTION TO OPTICAL NETWORKS

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Optical Fiber

O P T I C A L F I B E R IS A R E M A R K A B L E communication medium compared to other media such as copper or free space An optical fiber provides low-loss trans- mission over an enormous frequency range of at least 25 T H z ~ e v e n higher with special fibers~which is orders of magnitude more than the bandwidth available in copper cables or any other transmission medium For example, this bandwidth is sufficient to transmit hundreds of millions of phone calls simultaneously, or tens

of millions of Web pages per second The low-loss property allows signals to be transmitted over long distances at high speeds before they need to be amplified or regenerated It is due to these two properties of low loss and high bandwidth that optical fiber communication systems are so widely used today

As transmission systems evolved to longer distances and higher bit rates, dis- persion became an important limiting factor Dispersion refers to the phenomenon where different components of the signal travel at different velocities in the fiber In particular, chromatic dispersion refers to the phenomenon where different frequency (or wavelength) components of the signal travel with different velocities in the fiber

In most situations, dispersion leads to broadening of pulses, and hence pulses cor- responding to adjacent bits interfere with each other In a communication system, this leads to the overlap of pulses representing adjacent bits This phenomenon is called Inter-Symbol Interference (ISI) As systems evolved to larger numbers of wave- lengths, and even higher bit rates and distances, nonlinear effects in the fiber began to present serious limitations As we will see, there is a complex interplay of nonlinear effects with chromatic dispersion

We start this chapter by discussing the basics of light propagation in optical fiber, starting from a simple geometrical optics model to the more general wave

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