Broadband Cable Access Networks The HFC Plant
By David Large James Farmer
MORGAN KAUFMANN Copyright © 2009 Elsevier Inc.
All right reserved. ISBN: 978-0-08-092214-0
Linear Broadband Distribution Systems
Cable television systems have moved far beyond simple delivery of television programming to include high-speed data services, voice telephony, networking, transactional delivery of digital video under the interactive control of customers, and targeted advertising delivery, to name a few. To manage this complex business, what was formerly known simply as the "headend" has also evolved into a hierarchy of national, regional, and local signal processing centers. Similarly, the subscriber's premise has evolved to often include local distribution networks that allow communication among devices as well as with the external network. In the near future, communications will be provided by operators to multiple, diverse end terminals, including wireless devices. Our previous book, Modern Cable Television Technology: Voice, Video and Data Communications, 2nd ed. (Morgan Kaufmann, 2004) covered the entire range of technologies involved in a cable system and should be consulted for topics lying outside the scope of this volume.
At some point in this network, the modulated radio frequency (RF) signals that are to be transported to customer homes are delivered to a linear distribution network whose purpose is to deliver those same signals with no further per-signal processing and with as little degradation as economically possible. This book is devoted to that portion of the system, which we have referred to generically as the "hybrid fiber/coax (HFC) plant," although it need not always include both fiber optics and coaxial transport and may occasionally include microwave links.
The HFC plant is designed to be as transparent as possible and is characterized by its bidirectional RF bandwidth, the maximum level of various impairments to the transported signals, the number of homes or customers who share common signals, its reliability and availability, and its ability to scale to provide greater per-subscriber bandwidth as needed.
It is this portion of a cable television system that distinguishes it from, for instance, a direct-to-home satellite system; that is, a satellite provider is limited to 1 a one-way broadband path that, because of the characteristics of satellite antennas, delivers signals in common to millions of homes and, because of the characteristics of those signals, requires signal processing for every in-home television receiver. Because of the limitations of its networks, direct broadcast satellite (DBS) systems are practically limited to video distribution as their primary business.
By contrast, cable operators have the luxury of a two-way path with the ability to deliver a unique spectrum of signals to each small group of homes with a quality that is sufficiently high to support both analog video- and bandwidth-efficient digital modulation schemes. The HFC plant thus supports both broadcast and high-usagerate transactional and two-way services.
New fiber-based telephone networks have many of the same advantages as cable, although with a different network architecture. In particular, some telephone carriers use a combination of linear and digital optical transport on different wavelengths sharing a single fiber. These emerging fiber-deep (including fiber to the home) networks are also covered in this volume.
1.2 ORGANIZATION OF THIS BOOK
We begin with the fundamentals of coaxial technology in Chapter 2, including all of the elements that make up the coaxial portion of the HFC network. In Chapter 3 we move on to coaxial distribution networks, how they are designed and powered, and the nature of signal degradation through such networks. Linear, single-wavelength fiber optics are introduced in Chapter 4, along with the signal degradation mechanisms that are unique to fiber optics. That chapter begins with some fundamental properties of light and ends with the calculated performance of complete transmitter-fiber-receiver links. Chapter 5 expands the optical topic to include multi-wavelength systems and discusses at length the various ways in which the signals modulating various wavelengths in shared fibers interact to create crosstalk. Because physical plant construction is not always economically possible, linear microwave technology is still used in selected locations as an alternate interconnecting technology. Chapter 6 includes full step-by-step instructions for designing and calculating the performance of such links, including availability under predicted rainfall conditions.
In Chapter 7, we treat the entire HFC network as a system and discuss both performance requirements and typical performance of the cascaded elements. The upstream (subscriber-to-headend) direction of communications has unique characteristics, design and alignment issues, which are treated in Chapter 8. In Chapter 9, we move to overall HFC architecture, with a look at service-specific requirements and architectural options that can address those requirements. Then Chapter 10 looks first at architectural elements and concludes with various examples of end-to-end HFC architectures. Chapter 11 complements that with a treatment of emerging fiber-deep systems, including hybrid analog/digital transport. Finally, Chapter 12 is devoted to reliability and availability, including a methodology for predicting these factors for any architecture.
Each chapter stands alone and can be referred to independently without reviewing the material leading up to it (for those who are already familiar with the subjects covered in preceding chapters), and a glossary is included to assist in identifying terms that may have been introduced earlier in the book. For those for whom broadband distribution systems are relatively new, we recommend starting with the fundamentals chapters. Because it is referred to throughout, we have included the full channelization plan for cable (CEA-542-B) as this book's appendix.
1.3 THE SOFTWARE APPLICATIONS
In a major change from previous publications, we have included with this book access to programs that allow the reader to readily duplicate some of the more complex calculations associated with predicting various aspects of the performance of HFC distribution systems. Any or all of these can be accessed through the following website: www.elsevierdirect.com/companions/9780123744012. The included applications and their functions are as follows:
Cascaded Noise-Distortion Calculator.xls—This Excel spreadsheet, referred to primarily in Chapter 3, allows the user to do two types of calculations: the cascaded C/N, C/CTB, and C/CSO of a system when the performance of its constituent elements are known, and the characteristics of an unknown element when the cascaded performance and the performance of other elements are known.
Single-Wavelength Performance Calculator.xls—This Excel spreadsheet, referred to in Chapter 4, predicts the C/N and C/CSO performance of a single-wavelength fiber-optic link, including contributions from transmitter RIN and chirp, shot noise, postamplifier noise, interferometric intensity noise, and phase noise.
Optical Crosstalk-Individual Mechanisms.xls—This Excel spreadsheet, referred to in Chapter 5, calculates and plots the magnitude of various individual crosstalk mechanisms as they cause interactions between two optical signals. Crosstalk mechanisms include those due to cross-polarization modulation, stimulated Raman scattering, crossphase modulation, and optical Kerr effect.
Optical Crosstalk Summary.xls—This Excel spreadsheet, also referred to in Chapter 5, is an extension of the individual mechanisms sheet that calculates the total crosstalk affecting the top, middle, and bottom optical signal when 16 DWDM optical signals share a single fiber. Total crosstalk is both calculated and plotted as a function of RF modulating frequency.
Micro.xls—This Excel spreadsheet, referred to in Chapter 6, includes all calculations required to design and calculate the performance of a linear amplitude-modulated microwave link, including path design, nominal performance, and predicted availability.
ReturnLevelCalculator-Ex2.xls—This Excel spreadsheet accompanies Chapter 8, the return path. It allows you to input parameters of your upstream transmitter, and the signals you wish to carry in the upstream direction. It then calculates the optimum signal level for each signal based on modulation type and bandwidth, and gives you the carrier-to-noise ratio to expect for the optical portion of the upstream. The numbers in the spreadsheet are those used to generate the second example near the end of the chapter. Instructions appear on the first page of the spreadsheet.
Soar Manual.doc and SOAR.xls—These companion applications, which are related to the material in Chapter 12, include the System Outage and Reliability Calculator Excel workbook and its companion instruction manual, a Word document. Between them they document how to analyze the reliability and availability of most HFC architectures, including those that include redundant elements, given a user-input table of component reliabilities, repair times, and how the components are interconnected. Extensive plotting of results is included.
1.4 WHY THIS BOOK
When the first edition of Modern Cable Television Technology, published in 1999, was about four years old, we determined that the industry had evolved sufficiently that a revision was required, leading to the second edition in 2004. Four years later, we have again reviewed the state of the industry and the need for an updated volume. The authors and publisher were concerned that a single-volume revision would simply be too large to be practical (it grew from 873 to 1053 pages between the first and second editions and a third edition would be larger still), and that there is a significant segment of the cable technical community whose job responsibilities do not extend beyond the distribution plant and for whom a comprehensive book is simply too expensive—hence the current work. We also saw an opportunity to provide distribution plant specialists with the calculation tools necessary to understand, design, and maintain linear distribution networks, and thus decided to provide online access to download application programs for the most complex calculations required in that effort.
Owners of the second edition of Modern Cable Television Technology will recognize that this book covers that same range of topics as Parts 4 and 5 of that work (11 of its 25 chapters). That material has been revised and updated as required to reflect changes in the intervening years. Major plant-related trends since the second edition was written include FCC standards for delivery of digital video signals, negotiated standards for digital "cable-ready" receivers, wide availability of 1-GHz equipment, an imminent change from analog over-air broadcasting to digital (and, increasingly, high-definition) video, and major deployments of wavelength multiplexing equipment as operators split existing nodes in fiber-sparse environments. On the competitive front, two of the largest telephone companies have moved aggressively into video transport, while DBS operators are rolling out large numbers of HDTV channels. It is an interesting time to be working with cable systems. We hope you find this book useful and relevant.
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