Access Technologies: DSL and Cable

Access Technologies: TE AM FL Y DSL and Cable James Harry Green A McGraw-Hill Original e book Access Technologie...

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Access Technologies:

TE

AM FL Y

DSL and Cable

James Harry Green A McGraw-Hill Original

e

book

Access Technologies: DSL and Cable Executive Briefings in Key Technologies

James Harry Green

McGraw-Hill New York Chicago San Francisco Lisbon London Mexico City Milan New Delhi San Juan Seoul Singapore Sydney Toronto

Madrid

abc

McGraw-Hill

Copyright © 2002 by The McGraw-Hill Companies, Inc. All rights reserved. Manufactured in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. ISBN 0-07-138247-X All trademarks are trademarks of their respective owners. Rather than put a trademark symbol after every occurrence of a trademarked name, we use names in an editorial fashion only, and to the benefit of the trademark owner, with no intention of infringement of the trademark. Where such designations appear in this book, they have been printed with initial caps. McGraw-Hill eBooks are available at special quantity discounts to use as premiums and sales promotions, or for use in corporate training programs. For more information, please contact George Hoare, Special Sales, at [email protected] or (212) 904-4069. TERMS OF USE This is a copyrighted work and The McGraw-Hill Companies, Inc. (“McGraw-Hill”) and its licensors reserve all rights in and to the work. Use of this work is subject to these terms. Except as permitted under the Copyright Act of 1976 and the right to store and retrieve one copy of the work, you may not decompile, disassemble, reverse engineer, reproduce, modify, create derivative works based upon, transmit, distribute, disseminate, sell, publish or sublicense the work or any part of it without McGraw-Hill’s prior consent. You may use the work for your own noncommercial and personal use; any other use of the work is strictly prohibited. Your right to use the work may be terminated if you fail to comply with these terms. THE WORK IS PROVIDED “AS IS.” McGRAW-HILL AND ITS LICENSORS MAKE NO GUARANTEES OR WARRANTIES AS TO THE ACCURACY, ADEQUACY OR COMPLETENESS OF OR RESULTS TO BE OBTAINED FROM USING THE WORK, INCLUDING ANY INFORMATION THAT CAN BE ACCESSED THROUGH THE WORK VIA HYPERLINK OR OTHERWISE, AND EXPRESSLY DISCLAIM ANY WARRANTY, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. McGraw-Hill and its licensors do not warrant or guarantee that the functions contained in the work will meet your requirements or that its operation will be uninterrupted or error free. Neither McGraw-Hill nor its licensors shall be liable to you or anyone else for any inaccuracy, error or omission, regardless of cause, in the work or for any damages resulting therefrom. McGraw-Hill has no responsibility for the content of any information accessed through the work. Under no circumstances shall McGraw-Hill and/or its licensors be liable for any indirect, incidental, special, punitive, consequential or similar damages that result from the use of or inability to use the work, even if any of them has been advised of the possibility of such damages. This limitation of liability shall apply to any claim or cause whatsoever whether such claim or cause arises in contract, tort or otherwise. DOI 10.1036/007138247X

Contents

1 Introduction to Access Technologies 1 2 Data Transmission Fundamentals 15 3 Telephone Local Loop Characteristics 37 4 Digital Subscriber Line Technology 49 5 Cable Access Technology 61 6 Wireless Access Technology 75 7 Fiber-optic Access Technology 91 8 Application Considerations 99 Appendix 1 Glossary 109 iii

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1

Introduction to Access Technologies

T

elecommunications networks have made some remarkable strides in the last few decades. Fiber-optic technology has revolutionized information transport, providing enormous amounts of high-quality bandwidth that the older transcontinental microwave network could not support. Highspeed routers and switches and new protocols have brought the Internet to the masses, making it possible to obtain information and communicate anywhere in the world for a flat monthly fee. Despite these advances, a major bugbear remains. The highspeed backbone network is accessed over a copper cable local loop that was designed for a nineteenth-century network. To be sure, copper wire quality increased significantly during the twentieth century, and local loop technology improved, but local access remains the chokepoint for the vast majority of Internet users. 1 Copyright © 2002 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.

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Access Technologies: DSL and Cable

From the start, data networks were designed around the voice-frequency circuit because nothing else was available. Older telecommunications systems had a high error rate because of noise that came from a variety of sources such as clattering switches and relays, atmospheric conditions, and technician activity. Since noise was an analog phenomenon induced into analog circuits, the longer the path, the higher the noise level, and the greater the error rate. Although a high noise level is annoying to voice sessions, it is fatal to data, so when fiber optics came along to replace the analog microwave network that laced the country, it brought a revolution in transmission quality. No longer was bandwidth constrained by the channelized voice circuits in the wide area. Common carriers and large users served by fiber optics could now obtain an ample supply of digital capacity in whatever bandwidth they chose. The irony of the fiber-optic revolution is that, except for users large enough to justify the expense of bringing fiber directly to their premises, bandwidth is still constrained by the capacity of a twisted pair of copper wires. The reason is that the local architecture was designed long before any potential for broadband data communications existed and the cost of changing it is higher than the revenue potential can support. The wide area network is concentrated into a backbone of shared switches and circuits, fanning out to millions of dedicated connections to every business and household in the developed world. Not only are the multipliers huge, but also these cables must pass through some expensive real estate. Ideally, service providers would bring fiber optics to every business and residence, but the cost of digging up the streets is enormous and the only way it can be justified is by bringing services for which subscribers will pay enough to justify the cost. Telephone service is a given—a lifeline service that few can do without. Beyond that are entertainment services, most of which ride on coaxial cable today. The community antenna television (CATV) providers have already invested the capital to bring a broadband channel to more than 80 percent of the households. Were either telephone or CATV to start from scratch today, the billions that were invested in


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twisted-pair wire and coax would instead be invested in fiber optics, but replacement is an expensive proposition. Only in the past decade has a third service emerged for which subscribers are willing to pay: information. Before the advent of the Internet, it was difficult to conceive that information would become such an important commodity, but today access to the Internet is a third revenue stream that has important implications for the future. The issues are clear to the service providers. Large business users require fiber optics because no other medium can support their bandwidth requirements. Therefore, not only the incumbent local exchange carriers (ILECs, i.e., the traditional telephone companies), but also competitive local exchange carriers (CLECs) and competitive access providers (CAPs), are bringing fiber to those users with enough bandwidth demand to justify it. That leaves the small businesses and residences, which is where the real multipliers are. The fact is that the copper wire and coaxial cable plant is already in place and the providers don’t expect enough revenue to justify replacing it. Therefore, they are deploying methods of extending the life of their existing plant. The ILECs are applying digital subscriber line (DSL) technologies to their twisted-pair wire. CATV providers are converting their one-way systems, which were originally designed to deliver entertainment services, to two-way systems. Wireless providers plan to bypass wired alternatives altogether by using radio frequency devices. They have already made inroads into the CATV market with direct broadcast satellite services. Wireless providers such as Teligent and Wavestar have made broad forays into the voice and data market, but have encountered financial problems that tend to dampen the enthusiasm of future investors. These three classes of service providers are eyeing each other’s markets hungrily. Satellite providers are including local channels to woo CATV customers. Cable companies are preparing to offer telephone service, and local exchange carriers (LECs) are positioning themselves to deliver video on demand (VOD). All three offer Internet access, either by providing their own service or as common carriers for Internet service providers (ISPs).

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For larger businesses, Internet access has become a necessity that falls just behind telephone service in importance. These companies and agencies require full-time connections to the Internet. In addition, most multilocation organizations have a wide area network to tie all sites together for access to corporate databases and e-mail. Residential subscribers and small businesses, on the other hand, cannot justify the cost of broadband access that most corporations enjoy, so they are left with dial-up access. Once users become accustomed to high-speed access at the office, however, the comparatively interminable waiting time of dial-up access becomes so painful that they are willing to pay for a better way. Better alternatives are becoming available, but it is difficult to know what to believe. Service providers’ advertisements often make extravagant claims of access speed based on idealized conditions that most users will not experience. Horror stories of lengthy delays and inept technicians abound, many of which are unfortunately true. The objective of this book is to provide users with information about the various access technologies, what to expect from them, and where they fit. Let’s begin by looking at the default method that most users employ, the public switched telephone network (PSTN).

The Public Switched Telephone Network From its inception in the nineteenth century, the PSTN has had a simple and straightforward architecture. Subscribers are connected by copper wire to a central switching system located in a building known as the central office (CO). The CO is also known as a serving wire center (SWC), so called because all of the cables from the subscribers route to this center. The CO houses one or more switching systems. In the early days of the telephone these were manually operated, but today’s switching systems use computer-driven electronic switches. Older systems are analog, but analog technology is obsolete and is being replaced by digital switches as economics permit. Large metropolitan areas have multiple wire


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centers that are connected by groups of local trunks, as Fig. 1.1 shows. A trunk is any circuit between switches. All users share trunks and the central switching fabric within the CO. These shared pathways are engineered to a low probability of blockage, but calls may be blocked during unusual calling peaks such as disasters. If a call cannot be completed because of blockage, the network returns a fast busy signal. This point is key to understanding access technologies: the bandwidth restrictions in the PSTN are not in the local loop. The switching and trunking networks limit the bandwidth to that needed for a voice session. Except for longer telephone loops, the local loop has far more bandwidth than a voice session requires. The local network connects to the long-haul network over access trunks. The interexchange carriers (IXCs) provide trunks from their switching systems to the LECs and interconnect their switches with intertoll trunks. Today, virtually all of these trunks ride over fiber-optic facilities. For more information on how the optical network functions, refer to the I-book Optical Networking, available at http://shop.mcgrawhill.com/cgi-bin/pbg/indexebooks.html. The optical backbone provides a high-quality facility with low error rates that is used by voice and data alike. Before optical networking became prevalent, those constructing data networks had little choice but to obtain analog circuits from the public telephone network. These circuits were identical to telephone circuits except that they were not switched. Instead, they were connected directly to form dedicated or private line circuits. In prefiber days the circuits were derived over analog microwave or coaxial cable. In either case, for data the error rate was high and the bandwidth was constrained by the voice circuit, which is nominally 0 to 4 kHz—actually about 300 to 3300 Hz. Into this network the Bell System began to deploy digital toll switches, converting the analog circuits to digital circuits through a device known as a channel bank. Gradually, as fiber optics replaced the microwave and as competitive carriers entered the picture, channel banks gave way to direct digital connections. For voice connections, the all-digital network resulted in a dramatic increase in quality. With analog circuits, noise is

Fax

Local Switch

AM FL Y

Local trunks

Access trunks

Telephone

Local Switch Telephone

SONET/SDH Backbone

Toll Switch

Local trunks

TE

6

Local Switch

PBX

Customer Premise Equipment

Figure 1-1

Local Loop

Local Switching

Local Access Trunks

Toll Switch

Transmission Equipment

Architecture of the Public Switched Telephone Network


7

cumulative. As amplifiers along the way boost the signal, they also increase the noise. Digital circuits are periodically regenerated, which keeps noise at a low level. Coupled with the immunity to interference that is inherent in fiber optics, digital circuits ensure voice signal quality. They also lower the error rate for data to the point that errors may occur only once in a trillion bits. Digital circuits also increase the bandwidth by a factor of three or four compared to the maximum data capacity of an analog circuit, but that is not enough for broadband applications such as the Internet. The PSTN is built on a time-division multiplexing (TDM) model. The lowest level in this model, a DS-0, operates at 64 kbps, which is the widest bandwidth the PSTN is designed to switch. If more bandwidth than that is needed, say for a conferencequality videoconference, it is necessary to bond multiple channels together through inverse multiplexing. The problem is further aggravated by the characteristics of the local loop. The cable plant in the telephone network was designed to support analog telephones. Most of the intelligibility in the human voice is contained within the narrow pass band of 0 to 4 kHz. The local telephone networks in the world were designed at a time when either a private entity such as the Bell System and independent telephone companies in North America or the postal telephone and telegraph (PTT) agencies in the rest of the world owned everything including the telephone set. These entities had the objectives of minimizing investment and controlling maintenance cost. Therefore, until recently with the advent of speed dial and analog display telephones, all of the intelligence in the network resided in the CO. Even as the world has converted to digital, additional shortcomings of the PSTN remain. For one thing, the digital circuit hierarchy does not scale well. Voice circuit bandwidth is 64 kbps. The next step up from a single digital channel is T1/E1, which has 24 channels in North America and Japan and 30 channels in most of the rest of the world, with nominal transmission speeds of 1.5 and 2.0 Mbps, respectively. The next step up the hierarchy, T3/E3, has transmission speeds of 45 and 34 Mbps. In the wide area, users can obtain

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fractional T1/E1 or T3/E3, but these are not generally available in the local loop. The problem is further aggravated in North America and Japan by the fact that 64 kbps is not available in the local loop. For reasons we will explain later, a digital local loop in countries that use the T1 multiplexing scheme has only 56 kbps of bandwidth. Until recently, these constraints affected only businesses. Residences needing access to information databases had no choice but dial-up modems. Modems have become increasingly sophisticated and quite inexpensive over the years. A V.90 modem is theoretically capable of up to 56 kbps of fullduplex transmission (meaning data is sent in both directions simultaneously), but various constraints such as noisy loops and ISP configuration limit the actual transmission speed to something like 33.6 kbps in many cases. Dial-up modems do an excellent job of cramming a lot of information into a small amount of bandwidth, but the limits of the technology have been reached. No further advances are likely. Dial-up modems did a reasonably good job when the typical database provider offered primarily textual information with limited graphical content, but the World Wide Web changed that forever. The essence of a Web page is bitmapped graphics, which transfer slowly across a dial-up connection. The telecommunications industry, aware of the deficiencies of the analog network, began to deploy digital circuits in the 1960s, and a few years later began to develop the integrated services digital network (ISDN) to provide end-to-end digital connectivity. Basic rate ISDN (BRI) provides two digital “bearer” channels of 64 kbps each. An external signaling channel of 16 kbps completes the circuit, which is supported on a single copper cable pair. Primary rate ISDN (PRI) provides full T1/E1 bandwidth. ISDN is an improvement over an analog dial channel, but it has several drawbacks. First is the lack of general availability. Many small LECs don’t provide ISDN, and for those that do, the cost is often high. Furthermore, the LECs do not support the service well. The same service representatives who handle plain old telephone service (POTS) often can’t take orders for ISDN, and the customer either has to get assistance in applying ISDN or learn a lot more about the service than most want to know.


9

Perhaps a greater drawback of BRI is the fact that it is just too slow in today’s arena. Even with both channels bonded together to give a full-duplex bandwidth of 128 kbps, it still seems slow to people who are accustomed to high-speed Internet access. From the ISPs’ viewpoint, any dial-up access has numerous drawbacks. Modems are expensive to own and troubleshoot. Furthermore, if the customer remains connected, as many do, ISDN or analog ports are sitting idle, perhaps waiting for an occasional events such as arrival of an e-mail message. Business customers have an even greater problem. The need for Internet access for the small business is much the same as for residences except for the fact that many more users need simultaneous access. Therefore dial-up access is feasible only for the smallest of businesses. Full-time access is essential for nearly any business that requires an Internet connection. In addition, businesses with branch offices almost invariably need data connectivity for exchanging files and e-mail. For years, the only alternative was to use dedicated circuits. Now, broadband connections to the home are becoming essential for many users. The emphasis has shifted from video to Internet access. In addition, many people are interested in working from home at least part of the time. Telecommuting requires access to files in the office computer, and using a remote-access server is a poor substitute for being online. The length of time it takes to download large files over a dial-up modem makes the remote-access server effective only for traveling employees who do not have a broadband alternative. In addition to providing universal access to corporate databases and connection to the Internet, many call centers would like home users to function as call center agents at least part of the time. The motivation for full-time access to the Internet doesn’t rest only with the subscribers. The LECs are also interested in getting Internet users off the dial-up network. The PSTN was originally engineered to support an average connection holding time of about four minutes. Calls into the Internet are far longer than that and have upset the engineering calculations that were used for designing the PSTN. Besides the revenue potential of providing Internet access via DSL, the LECs can

10


benefit from transferring Internet connections from the dial network to dedicated circuits.

The Cable Network The history of cable is only about one-third as long as that of the PSTN. The first application of cable was in Astoria, Oregon, in 1950. From that beginning, cable expanded to the point that it can now reach about 80 percent of the households in the United States, although fewer than that subscribe to the service. Early cable systems received distant channels off the air and repeated them over a coaxial network, or even open wire transmission facilities, to subscribers who were unable to receive television directly. That gradually gave way, however, to the concept of cable as a medium that provides premium entertainment channels. Figure 1.2 shows the architecture of a typical CATV system. The CATV equivalent of the LEC central office is the headend. Here, signals are picked off the air, downlinked from satellites, or originated locally. The channels are applied to modulators that link them to the cable at any compatible frequency. The conventional very-high-frequency (VHF) broadcast band in North America consists of 12 channels, each of which is 6 MHz wide. These cover the frequency range of 54 to 88 MHz and 174 to 216 MHz. The ultra-high-frequency (UHF) channels 14 to 69 cover the range of 470 to 806 MHz. Cable has an advantage over conventional television in that ranges that are assigned to other services can be used to support cable channels. Today’s cable systems cover the range of 54 MHz to as much as 1 GHz, a bandwidth that can support more than 150 channels. Most systems stop at 750 MHz, however, reserving the top 250 MHz for future services. Cable was originally constructed as a one-way broadcast medium. The transmission direction from the headend to the subscriber is referred to as the downstream direction. (The other direction, obviously, is upstream.) As Fig. 1.2 shows, amplifiers are placed at intervals to overcome the losses in

Amplifiers Splitter

Trunk Cable

Headend

Termination Bridger Amplifier

Splitter

11

Feeder Cables

Subscriber Drops

Figure 1-2 Cable Television Architecture

Taps

12


the coax, which are high at frequencies approaching 1 GHz. Amplifiers were one-way in earlier systems, but some systems were constructed with bidirectional amplifiers. In some cases the service providers did this to support enhanced services that required two-way communication. In other cases their franchises required them to provide two-way communication services for the municipalities in exchange for use of the public right-of-way. Modern two-way systems use the frequencies below 40 MHz for upstream communications, providing a downstream bandwidth that is far greater than upstream. Cable provides a broadband pipe that can potentially reach a high percentage of the population in most developed countries, and would appear to be an ideal medium for data communications. As we will discuss in Chaps. 5 and 6, however, the shared-medium nature of cable imposes some restrictions that are costly to overcome.

Wireless Access Wireless has long played a vital role in telecommunications, primarily in the long-haul transmission network and in mobile applications. Satellite services once were—and for some countries still are—a way of bringing telecommunications services to locales that lack heavy enough demand to justify undersea cable. Point-to-point terrestrial microwave was once the primary means of intercontinental telecommunications service, and still is a convenient way of bypassing obstructions that are expensive to cross with cable. In the access market, however, wireless has been impaired by several factors. First is the fact that broadband demand didn’t really develop in the local network until fiber optics was available and the quality of fiber is so much higher that is always preferred if it is economically feasible. The most limiting aspect of wireless has been the lack of available frequencies. Microwave got its start from radar technologies that were first employed in World War II and then converted to commercial telecommunications service


13

after the war. The lowest microwave bands were developed first because the technology was easy to use and the transmission characteristics are superior to those of the higher bands, where rain, fog, and obstructions can limit the usability of the medium. Microwave frequencies are a finite, limited resource, and competition for the available wireless spectrum is intense. In recent years, the low-band microwave spectrum centering around 2.4 GHz has been released for telecommunications services. The common carrier personal communications service (PCS) occupies two sub-bands, with unlicensed spectrum between the two bands. One of the objectives of splitting the band in this manner is to enable transceivers to hop easily from the licensed PCS band to the unlicensed spectrum that is available for private wireless use. A single instrument can serve users in both bands, even enabling users to move from a private system to the public network without interrupting a session. This same spectrum plus others are available for public and private wireless use, including access services, as we will discuss in later chapters. Wireless appears at first glance to be an ideal replacement for the telephone local loop. So far, however, most wireless local loop (WLL) application has been in developing countries. In the developed nations wired services are in place, and there has been little impetus to replace them.


2

Data Transmission Fundamentals

T

he subject of access technologies revolves primarily about data transmission. The traditional networks work fine for voice, for which fixed-bandwidth circuits are perfectly adequate, but most data applications need something more. The purpose of this chapter is to discuss the nature of data transmission for those who may not be familiar with the details. Those who are may choose to move to the next chapter. Bandwidth requirements for data depend on the nature of the application. Some applications are bursty—that is, they need a substantial amount of bandwidth for a short period of time and then need next to nothing. Internet access is a good case in point. When the user downloads a graphic file, it typically puts a heavy instantaneous demand on the network. After the file is received, the user may browse it on a local personal computer (PC) and put no demands on the network until the 15 Copyright © 2002 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.

16


TE

AM FL Y

next download. Other applications—video, for example— have a steady flow. The video coder-decoder (codec) is pumping information in both directions simultaneously. This leads to a second characteristic of data flow: whether the application is symmetrical or asymmetric. An asymmetric flow has more data flowing in one direction than the other. Internet browsing is asymmetric. An upstream information request takes a relatively small amount of bandwidth compared to the response the network delivers. E-mail is largely symmetrical in that information flows in both directions in about the same amount. In between are a variety of other applications. For example, voice applied to a data network is somewhat bursty, and flows in only one direction at a time. The ping-pong nature of voice sessions results because one person speaks while the other listens. Another characteristic of a data session is the amount of delay it can tolerate. Response time in a data network is the elapsed time from pressing the enter key following an information request until the response arrives. Most data applications are tolerant of delay within limits. In practice, the user is most likely to lack tolerance for slow response, but some data protocols time out if the response is delayed excessively. Furthermore, some applications, such as voice and video, are real-time. When delay exceeds certain bounds, the session is unusable because the participants tend to talk over one another and miss the short listening responses that indicate that the message is understood. When voice and video are applied to data networks, quality must be controlled tightly or the session becomes unusable. These characteristics are discussed more fully in the I-book Voice and Video over IP, available at http://shop.mcgraw-hill.com/cgi-bin/pbg/indexebooks.html.

Modulation Methods Data is passed across a network by having its signal encoded on the transmission medium through a process known as modulation. Baseband modulation applies the signal directly to the physical facility. A telephone system, for example, applies the


17

signal directly to copper wires using amplitude modulation. By contrast, a CATV system divides the bandwidth of the medium into frequency segments and modulates the signal onto a carrier frequency. The carrier itself carries no intelligence; it is merely a frequency within the pass band of the transmission medium that can be modulated to carry information. In a CATV system, multiple carriers or channels are used, a process that is known as frequency-division multiplexing (FDM) of a broadband medium. Technically, the International Telecommunications Union (ITU, www.itu.int) defines broadband as any bandwidth greater than primary rate interface (PRI), but the term is widely used in conjunction with DSL bandwidths that are less than T1/E1. Three different parameters of an electrical wave can be modulated: frequency, amplitude, and phase. Early modems used frequency-shift keying (FSK), in which one frequency was assigned to a 0 and the other frequency to a 1. Highcapacity modems use a complex combination of phase and amplitude modulation. Figure 2.1 shows how these modulation schemes can convey information at a higher rate. For example, 2 binary 1 quaternary (2B1Q) coding, shown on the left side of the figure, assigns a 2-bit code to each of four voltage levels. Phase-shift keying (PSK) assigns a 2-bit code to each of four different phases. Since there are four combinations of 2 bits, i.e., 00, 01, 10, and 11, any 8-bit binary number can be encoded by sending a combination of four phase shifts. These changes in state are known in the industry as symbols, or sometimes by the old telegraph term baud. Amplitude shifts may be combined with phase shifts to form a modulation method known as quadrature amplitude modulation (QAM), as shown in the other two diagrams. QAM is a combination of amplitude and phase shift. With QAM, two carriers are phase-shifted 90˚ with respect each other, a condition described as quadrature. Each carrier is amplitude-modulated with half the data. For example, a 16QAM signal could have eight phase and two amplitude points, or four phase and four amplitude points, to detect. The eightlevel QAM signal constellation in Fig. 2.1 shows how bits are transmitted three at a time. Complex QAM modulation

000

111

10 10

00

11

18

01

Carriers 11

001

101

010

01

00 2B1Q

110

100

011

4-Level QAM 8-Level QAM

Figure 2-1 Modulation Constellations


19

schemes require circuits that are relatively free of noise and distortion. In addition, many modems employ forward error correction (FEC). FEC systems transmit redundant information that the receiver uses to determine bits that are in error and to change these bits to what they should have been. In the final analysis, the error-correcting protocol employed by the end devices ensures that the signal is received free of errors.

T1/E1 Carrier The basic digital facility over which virtually all voice and data signals pass is known as T1/E1 multiplex or carrier. Data signals can be connected directly to a digital circuit through a device known as a channel service unit (CSU). Analog signals are digitized through a process known as pulse code modulation (PCM). Nyquist’s law states that if the amplitude of an analog signal is sampled at twice the highest frequency it contains, the samples can be used to reconstruct the original signal with a reasonable degree of fidelity. Since the highest frequency in the audio pass band is 4 kHz, this means that a telephone signal is sampled 8000 times per second. Each sample is scaled into an 8-bit word through a process called quantizing. This signal, which is known as a DS-0 in North America, comprises a 64-kbps signal, which is derived from the product of 8000 samples per second × 8 bits per sample. In North America, 24 DS-0s are time-division multiplexed into a T1 frame. The E1 frame in Europe consists of 32 circuits, of which two are used for signaling and 30 for information. Although the T1/E1 signal is channelized into 24 or 30 channels for voice, it can be obtained unchannelized for wideband data. In ISDN terms, a T1/E1signal is known as a PRI. PCM is not a new process. It was developed in England in 1938, but it wasn’t practical with vacuum tube technology because of the problems of power drain and need for floor space. A decade later, the invention of the transistor brought PCM to the realm of practicality, but not until the 1960s did

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digital carrier begin to replace the older analog multiplex then in use. North American and European digital standards developed somewhat independently and without any requirement for interoperability. Transoceanic circuits were analog, and nothing was on the horizon to suggest that a lack of interoperability would be a problem. Fiber optics changed that. The DS hierarchy in North America was programmed to go as high as DS-4, which is 274 Mbps—the highest rate that coaxial cable was expected to support. When fiber optics came on the scene in the late 1970s, the industry bypassed the DS-4 rate without a backward glance. Numerous manufacturers were producing fiber-optic multiplexing equipment, but each had its own hierarchy. The lack of uniformity and interoperability quickly became a problem and led to the development of the synchronous optical network (SONET) protocol, known as synchronous digital hierarchy (SDH) in Europe. Table 2.1 shows the most common SONET/SDH standards. One benefit of SONET/SDH is that the North American and European standards meet at OC-3. The entire Internet backbone is composed of circuits riding on high-speed fiber-optic connections. The fiber-optic carriers provide bandwidth in OC-x increments to Internet providers and IXCs. This bandwidth is time-division multiplexed and completely synchronous. By this we mean that a signal can be extracted from a higher-bandwidth signal if its Table 2.1 SONET/SDH Hierarchy SONET Signal

Bit Rate (Mbps)

OC-1

51.840

OC-3

155.520

OC-12

622.080

OC-48

2488.320

OC-192

9953.280

SONET Capacity 28 DS-1s or 1 DS-3 84 DS-1s or 3 DS-3s 336 DS-1s or 12 DS-3s 1344 DS-1s or 48 DS-3s 5376 DS-1s or 192 DS-3s

SDH Signal

SDH Capacity

STM-0

21 E1s

STM-1

63 E1s or 1 E4

STM-4

252 E1s or 4 E4s

STM-16

1008 E1s or 16E4s

STM-64

4032 E1s or 64 E4s


21

Figure 2-2 Bipolar and 2B1Q Coding

2B1Q

Level 2 -

Level 1 -

Level 1 +

Level 2 +

Bipolar

Unipolar

+

1

11

1

01

0

00

1

10

0

0

Superimposed Sine Wave

1

0

position in the bit stream is known. This synchronicity was not the case with the older multiplexing methods. T carrier uses a bipolar line-coding scheme in which alternate ones digits are poled in opposite directions. Each symbol on the line represents 1 bit. ISDN doubles the carrying capacity per symbol by using 2B1Q modulation. Four different voltage levels (one quad or Q) are imposed on the cable pair with each level representing 2 bits (2B). Figure 2.2 illustrates the difference between bipolar and 2B1Q line coding. This coding scheme uses the entire bandwidth of the cable pair down to 0 Hz or direct current (DC), which makes it

22


impossible to support voice and data simultaneously. As we will discuss in Chap. 4, some types of DSLs separate voice from data with filters, making it possible to retain the analog voice channel below the data. In the case of ISDN, the signals are being sent simultaneously in both directions over a single cable pair. ISDN modems employ echo canceling to prevent the transmitted signal from being reflected back to the receiver. When a T1 signal is applied to a line, it uses a coding scheme known as alternate mark inversion (AMI). In an AMI signal, every other ones bit is inverted, as shown in Fig. 2.2. This coding scheme accomplishes two results. First, in looking at a sine wave superimposed on Fig. 2.2, it can be seen that the frequency of the sine wave is half the frequency of a corresponding unipolar signal. This allows greater range between regenerators. Second, AMI serves as a means of error detection. If a line hit adds or deletes a ones pulse, it results in two adjacent ones pulses poled in the same direction, a condition known as a bipolar violation. Terminating devices such as the office repeater on a trunk or a CSU on a loop can display bipolar violations as a means of detecting trouble. T1 carrier is designed as a trunk carrier and does not fit well into the distribution network. It uses separate pairs for sending and receiving, and these must be sufficiently separated, either by sending and receiving in separate cables or by the use of D screen cables, which separate the transmit and receive directions with shielding. T carrier is designed to fit the physical structure of loaded trunk cables. These have inductors spaced at 6000-ft (1830-m) intervals with end sections of 3000 ft (915 m). To fit into this spacing, the first regenerator is placed at 3000 ft, with additional regenerators at every 6000 ft and a final 3000-ft end section. If the end section doesn’t fit the spacing, it is built out electrically. A further factor that must be understood about the North American digital hierarchy is the effects of its signaling protocol. When the DS hierarchy was developed, much of the signaling between central offices was carried over the


23

talking channel. This concept is called in-band signaling. Analog circuits use tones and DC to convey signals. Digital circuits use a protocol that robs the least significant bit of every 6th and 12th frame for signaling the on-hook or off-hook status of the trunk. This bit-robbed signaling is of no consequence to voice or analog data, but it obviously cannot be tolerated for digital data applied directly to the line. Therefore, only 7 of the 8 bits can be used for digital data, resulting in bandwidth of 56 kbps. An option known as extended super frame (ESF) is available in most North American and Japanese localities to provide a clear channel with 64 kbps of bandwidth. This is generally available only in T1 or greater bandwidths. T1/E1, and its derivative, 56/64 kbps, are widely used for access as well as point-to-point circuits. The problem with T1/E1 access has been a lack of scalability. The ILECs provide either a single 64/56-kbps circuit or a full T1/E1. Most ILECs do not provide a fractional T1 in the local loop. The reason is that the architecture for fractional T1 is identical with that for a full T1. Since the rate is based, at least to some degree, on the cost of providing the service, the ILECs have no incentive to provide fractional T1. As we will discuss the next chapter, however, DSL changes that situation.

Data Protocols Data devices communicate with one another by using protocols, which are sets of programmed instructions that a processor can execute. Protocols are used to set up sessions between devices, determining such factors as which end controls the session, what participants are authorized, what transmission speed and code set will be used, and myriad other functions. Protocols handle other such functions as addressing, error detection and correction, and recovery from network failures. Before devices can communicate, they

24


must confirm that they are prepared to use the same protocols and resolve how they will apply any optional features. Data protocols are classified as connection-oriented or connectionless. In a connection-oriented protocol the path is established at session setup and remains for the duration of the session. The telephone network is a prime example. A connectionless protocol launches data packets into the network, where they are routed to the destination as an independent unit. Internet Protocol (IP) is a connectionless protocol. As with most computer programs, protocols are built in modules or layers. Each module has a specific function and has clearly identified application programming interfaces (APIs) at the boundary so developers can write to and use the functions of the protocol. Table 2.2 shows the layers and their functions in the International Standards Organization (ISO) Open Systems Interconnect (OSI) model. OSI is not intended to be a complete protocol. Instead, it is a conceptual model with descriptions of the functions that fit within each layer so developers can build on its structure. Table 2.2 shows a brief summary of the functions of each layer in OSI. Each end of the session uses the same layer definitions. The sending end of the session passes information to the receiving end by appending records to the information block. The receiving end receives the instructions in the block, executes them, and strips the extra records until only the inforTable 2.2 The ISO Open Systems Interconnect Model Layer Number

Layer Name

Unit

Function

7 6 5

Application Presentation Session

Packet Packet Packet

4

Transport

Packet

3

Network

Packet

2

Datalink

Frame

1

Physical

Bit

User application support Data syntax and encryption Session setup and termination End-to-end transport and error correction Packet routing across the network Frame transfer and error correction Transfer of bits from point to point


25

mation block remains free of errors and with all information correctly sequenced. Data networks can operate without identifying each of the layers exactly as defined. For example, TCP/IP, which is the protocol that supports the Internet, uses layers 1, 2, and 3 of OSI. IP is the network or layer 3 protocol, but its definition doesn’t conform exactly to OSI’s layer 3, and Transmission Control Protocol (TCP), which is the transport protocol, combines many of the layer 4 and 5 functions. The OSI model is customarily shown with layer 1 on the bottom because it is the base on which the other layers are built. Two devices can communicate using only layer 1 simply by connecting their serial ports together, but this connection has no ability to signal, correct errors, send data to multiple addresses, or perform numerous other functions that higher-level protocols accomplish.

LAN Protocols The operation of local area network (LAN) protocols illustrates in practical terms how the OSI model functions. This will also aid in understanding the operation of 802.11b wireless, discussed in Chap. 7. The IEEE 802 Committee, which developed the LAN protocols, designed the protocols to operate within the first two layers of the OSI model. The most popular LAN protocol, Ethernet, is used almost universally as a LAN protocol to connect to the Internet. Figure 2.3 shows the layered structure of the Ethernet protocol stack. Ethernet drivers are available to connect to any of the popular transmission media. Early implementations used RG-8 and RG-58 coaxial cable, but unshielded twisted-pair wire (UTP) is by far the most common medium. Drivers are also available for fiber optics and wireless systems. Ethernet’s link layer is divided into two portions; media access control (MAC) and logical link control (LLC). The MAC layer is responsible for controlling access to the medium—which, in the case of Ethernet, is based on contention. A station with information to transmit listens to the network to see if it is idle. When it determines that no other station is sending, it launches an Ethernet frame, which is shown in

26

Access Technologies: DSL and Cable Higher Layer Protocols

Logical Link Control LLC Data Link Layer

Media Access Control MAC Physical Link Signaling PLS

Physical Layer

AM FL Y

Attachment Unit Interface Physical Medium Attachment

Medium-Dependent Interface Medium (Wire, coax, fiber, radio, etc.)

TE

Figure 2-3 Ethernet Protocol Stack

Fig. 2.6. If two stations transmit simultaneously, their frames collide and are mutilated. Any station on the network hearing the collision sends a jamming signal. This signals the stations to cease transmitting and back off for a random length of time before attempting to transmit again. The reason for the random time is so that stations don’t immediately attempt to transmit and collide again. As Fig. 2.4 shows, each Ethernet frame has a 4-byte cyclical redundancy check (CRC) block immediately following the data block. The transmitting MAC calculates CRC using a complex algorithm. The receiving MAC uses the same algorithm to calculate CRC and matches it to the received CRC. If the CRCs match, the block is accepted and acknowledged; if they don’t match, the receiving end notifies the sending end to retransmit. The LLC communicates with the higher-order protocols. It funnels data streams to and from the network layer if one

Start of Frame Delimiter

27 Preamble (7)

(1)

Destination Address (2 or 6)

Figure 2-4 Ethernet Frame

Source Address (2 or 6)

Length (2)

Data (0-1500)

Pad (0-46)

CRC (4)

28


exists. A network layer is not required in a LAN. If all communication is carried over a closed network such as a single Ethernet segment, link layer communication is all that is needed. Segments can also be interconnected at the link layer. A bridge is a two-element device that can be used to divide Ethernet networks into multiple segments. The bridge listens to and learns MAC addresses on each segment and keeps traffic confined to its home segment unless it is destined for an address on the other segment. An Ethernet switch is a multiport bridge that connects segments together long enough to pass a frame between them. If each station has its own port on the Ethernet switch, collisions are eliminated.

Routing Since a bridge is a two-port device, it lacks the ability to make routing decisions. Some bridges may deliberately or inadvertently develop more than one path between segments. This condition is prohibited by the Ethernet protocol, and is prevented by using a protocol known as spanning tree. More frequently, a routing protocol such as IP is used and the network is connected through routers. The TCP/IP protocol falls under the blanket of packet switching. The U.S. Department of Defense initially conceived packet switching as a security measure. If a message is broken into small packets and sent through the core network over random patterns, it is difficult to reassemble the message except at the access point. If the access circuit is kept short, it is easy to secure. Furthermore, if the core network has plenty of bandwidth, packets from other sessions can be interleaved to use the excess capacity. The problem with a connectionless network is that packets can arrive with errors or out of sequence, and some method must be used to preserve the integrity. Packet reassembly is one function of data protocols such as TCP. IP operates on top of the Ethernet LLC at layer 3 in the OSI model, and introduces a second layer of addressing. The MAC address is always the means by which a station can be identified. It is permanently burned into the network inter-


29

face card (NIC) as a unique 48-bit address. The IP address, on the other hand, can easily change. If the station moves from one segment to another, for example, the IP address changes, perhaps by as little as one digit. In addition, IP addresses are often assigned only as long as the computer is active. Most networks use a protocol known as Dynamic Host Configuration Protocol (DHCP) to assign IP addresses to station as they boot up. A protocol known as Address Resolution Protocol (ARP) links IP addresses with the corresponding MAC addresses. Once a router is connected to the network, packets can flow anywhere in the world. Routers understand the IP addresses and know what to do with any address that is handed to them, but users rarely communicate by means of IP addresses. More likely, they use a uniform resource locator (URL), which has the form of user_name@domain_name.aaa, where aaa is a suffix such as com, gov, edu, and so forth. The router must have a way of translating the URL into an IP address. It does this by accessing a server running a protocol known as domain name service (DNS). The process of setting up a session is handled by a fast interchange of messages among devices on the network. Although devices could theoretically communicate using nothing more than IP, it wouldn’t be satisfactory to the application. IP is a connectionless datagram protocol. A datagram is a single unacknowledged packet that flows from node to node on the network until it finally arrives at the destination. IP is an unreliable protocol, which means that data delivery is not guaranteed. If a router doesn’t deliver in time or if it encounters congestion and can’t deliver the packet, it simply discards it. Furthermore, IP doesn’t check for errors. In data communications, a packet with a single bit error is worse than no packet at all. This need for purity, incidentally, is true only for data packets and not for other types of packets such as voice. To turn IP into a reliable network, we need another protocol, and this is where TCP comes into the picture. TCP sets up a logical path with its peer at the other end of the connection. The two ends carry on a dialog in which they transfer IP

30


packets across the network complete with error checking, sequencing, and receipt acknowledgment. Instead of acknowledging packets one at a time, TCP acknowledges an entire sequence of packets with a single message. TCP has an interesting method of controlling congestion. With each acknowledgment, the receiving end returns a window number, which indicates the number of packets it is prepared to receive in the next transmission. In the absence of congestion, it opens the window wide. If it is receiving more packets than its buffers can handle, instead of dumping packets (which it is permitted to do), it closes the window, which informs the sending end to throttle back. As mentioned earlier, some applications, such as voice over IP, do not need packet acknowledgment and error checking. Therefore, instead of running under TCP, they run under a much simpler protocol known as User-Datagram Protocol (UDP). UDP greatly reduces the number of overhead (noninformation) bits that are transferred across the network. Other protocols also use UDP. For example, Simple Network Management Protocol (SNMP), which is used to report alarms and accept orders, also runs under UDP. Applications can talk directly to TCP/UDP. For example, Fig. 2.5 shows the TCP/IP protocol stack, including some of the standard application protocols such File Transfer Protocol (FTP) and Simple Mail Transfer Protocol (SMTP).

Asynchronous Transfer Mode Asynchronous transfer mode (ATM) was initially intended as a broadband ISDN protocol to replace circuit switching in the carrier backbone network. In the 1990s it was welcomed with unbounded enthusiasm by developers who planned to run it to the desktop so data could flow seamlessly from device to device without media conversion. ATM delivers bandwidth on demand and provides service quality that the application and the network negotiate. The universal protocol vision has faded as ATM has proved to be complex and expensive, particularly in face of Ethernet, which has proved


31

Application Services File Transfer Protocol (FTP) Simple Mail Transfer Protocol (SMTP) TELNET Hypertext Transport Protocol (HTTP)

Transmission Control Protocol (TCP)

User Defined Protocol (UDP)

Internet Protocol (IP)

Address Resolution Protocol

ARP

RARP

Datalink

Physical

Figure 2-5 TCP/IP Protocol Stack

Reverse Address Resolution Protocol

32


so durable. Gigabit Ethernet delivers the speed that developers intended for ATM at a lower cost. Nevertheless, ATM has developed a role for itself in the local loop. It is the most popular protocol for multiplexing services on DSL lines, and it is also used in wireless and fiber. ATM is a connection-oriented protocol that divides the data stream into short cells. The cell payload is 48 bytes with a 5-byte header that steers the cells over a virtual path to the destination. There are two principal reasons for using short fixed-length cells instead of the long variable-length packets that TCP/IP employs. The first reason is that cells can be switched in hardware, which is faster than routing. The fixed cell length makes it easier to maintain a high degree of utilization. The second reason is that it is easier to control latency and jitter, which are critical quality of service (QoS) elements in real-time applications such as voice and video. If a cell is lost in transmission, it has a minimal impact on quality because the amount of voice or video information contained in one cell is negligible. An ATM header carries a virtual path indicator (VPI) and a virtual channel indicator (VCI). These correspond to ATM’s two types of circuits: virtual paths and virtual channels. Virtual channels are groups of channels between ATM devices, and virtual paths are groups of virtual channels. A connection between end points can be provisioned as a permanent virtual circuit (PVC) or by switching and signaling to establish a switched virtual circuit (SVC). A PVC is analogous to a private line. The carrier sets it up and it remains set up until disconnected by service order. An SVC is set up by the application and is charged like a telephone call. Like all modern protocols, ATM is a layered protocol. User applications communicate with the ATM adaptation layer (AAL). The AAL is divided into two sublayers, the segmentation and reassembly (SAR) and the convergence. The SAR segments outbound traffic and reassembles it inbound. The convergence sublayer protocols are different for the various types of information such as voice, video, and data. The AAL supports five different classes of traffic:


33

1. Class A traffic is constant-bit-rate, connection-oriented

real-time traffic such as voice and video. 2. Class B traffic is variable-bit-rate, connection-oriented

real-time traffic such as packet video. 3. Class C traffic is variable-bit-rate, connection-oriented

traffic such as bursty data. 4. Class D traffic is variable-bit-rate, connectionless traffic

such as datagram services. 5. Class X allows user-defined traffic and timing relation-

ships. The ATM adaptation layer recognizes that different classes of traffic have different bit rates, and provides for four classes: constant bit rate (CBR), variable bit rate (VBR), available bit rate (ABR), and unspecified bit rate (UBR). CBR is the highest class of service, providing the ATM equivalent of a T1/E1 or T3/E3 dedicated line. It is connection-oriented and designed for time-sensitive applications such as voice and video. VBR is also connection-oriented, and is designed for any application, such as LAN interconnection and frame relay, that requires a variable amount of bandwidth. ABR is offered as a discounted service to take advantage of the bandwidth that is left over after CBR and VBR traffic have been accommodated. The industry has not settled the issue of whether ATM or IP should be used as the information transport protocol in the local loop. As we will discuss in Chap. 9, the passive optical network (PON) has advocates for both protocols, and the same is true of DSL. IP has the advantages of being less complex and having a broad range of applications and software drivers to support it. ATM is more complex and expensive, but it has QoS built into the design, and many of the ILECs use it as the backbone of their DSL networks.

Frame Relay Toward the end of the 1980s, a new protocol known as frame relay began getting attention. Unlike fixed circuits, frame relay

34


operates on the principle of access circuits from multiple customers that are statistically multiplexed into the carrier’s backbone. The service provider can oversubscribe the bandwidth because users aren’t all transmitting simultaneously. Oversubscription means that less bandwidth is available in the network than the potential demand. We’ll have more to say about frame relay in Chap. 10, but note that each subscriber node on the network requires an access circuit. The access circuit is typically T1/E1 or 56/64 kbps and extends from the subscriber’s premises to the carrier’s nearest switch node. These access circuits are distance- and bandwidth-sensitive—that is, the faster the circuit and the longer the span, the more it costs. As we mentioned earlier, fractional T1/E1 is generally unavailable in the access network. Moreover, the carriers’ switch nodes are usually in large metropolitan areas. In smaller cities, carriers often have a rating point of presence (POP) and backhaul the traffic at their expense to the switch node. Traffic is generally channelized time-division multiplexing (TDM) and fed through a device such as a digital cross-connect system (DCS) so it can be concentrated into a wideband circuit to the frame relay switch. A DCS is a digital switch in which the connections are established in software and remain connected until rearranged by the carrier’s provisioning process. Carriers have considerable motivation to avoid channelized circuits, which, because of the bursty nature of most data applications, are wasteful of bandwidth. Therefore, frame relay is a major market for improved access service. The architecture of frame relay is very much like that of a predecessor protocol known as X.25 packet switching. The principles of X.25 are similar to those of frame relay with some notable exceptions. First, X.25 users often have analog access circuits. These are dialed or dedicated, but the nature of X.25 is such that its applications are rarely graphical because the bandwidth is insufficient. However, X.25 establishes the principle of packet switching and virtual circuits. A virtual circuit is one that is implemented over shared bandwidth as opposed to being dedicated to the session. Users deliver their media to the edge of the network, where the carrier may packetize it.


35

X.25 is an edge-oriented protocol. The core of the network is up to the carrier. It decides how much bandwidth to provide between nodes and what routing protocols to use within the network. Routing is a matter of determining how packets flow from one node to another. The route can be fixed, i.e., set up at the start of the session and left for the duration of the session. The problem with fixed routes is that a circuit failure or node failure disrupts the session and congestion slows the session down. More effective is the ability to reconfigure the session within the core network. The service provider can use a routing protocol to enable nodes to exchange information about their loads and circuit status. Circuit failures and congestion can be made almost transparent to the user application. X.25 checks for errors at every node. A packet is not transferred to the next node until it has been received error-free. This process was logical for circuits that had a high error rate, but when the error rate is as low as it is with fiber optics in the backbone, node-bynode error checking is unnecessary. Frame relay leaves it up to the end user to check for errors. A frame relay network has three billing elements: the access circuit, a PVC between end points, and the port into the network. The PVC is defined with a specific amount of bandwidth. It represents the minimum bandwidth the carrier agrees to transport. Frames above that bandwidth are carried if the backbone has sufficient capacity. If not, frames can be dropped. Frame relay is a convenient and flexible protocol, but the access circuit is a problem for both carrier and the customer. Only two choices are available: 56/64 kbps or full T1/E1. The carriers pass the access cost along to the user, but they bear the cost of transporting the data to their switch node. Newer access technologies such as DSL are advantageous because they can concentrate the traffic into a broadband circuit much closer to the customers. Since DSL terminates at the wire center, frame relay frames can be routed over a cable pair to the wire center, where they are concentrated onto the LEC’s backbone and routed over a shared network to the IXC. The result is a substantial reduction in the cost of access. Whether this cost is passed along to the customer in the form of lower rates is up to the IXC.

TE

AM FL Y


3

Telephone Local Loop Characteristics

A

broad understanding of how the PSTN, and particularly the local loop, functions is a prerequisite to understanding access technologies. As mentioned in Chap. 1, the PSTN consists of a network of switches that interconnect circuits to form end-to-end connections that are exclusive to the parties for the duration of a session. When one of the users hangs up, the connection drops and the circuits are returned to a pool of inactive circuits until they are seized by the next session. All circuits and apparatus in the PSTN are shared except for the local loop, which is the cable that extends from the central office to the subscriber. The telephone network was initially constructed with the objective of line sharing. Party lines were once the norm, particularly in rural areas where the cost of running a line to each house was prohibitively expensive. When party lines were prevalent, the cable plant was constructed so that the 37 Copyright © 2002 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.

38


same pair might appear on cable legs running down different streets, improving the chances of finding a vacant pair to serve a subscriber. That architecture, which is known as multiple plant, often resulted in bridged tap, a situation in which dead wire is connected across an active cable pair. Furthermore, the subscriber’s service is connected to a distribution cable pair by means of drop wire. Drop wire is bridged across a pair, but the cable extending beyond is normally not trimmed off, also resulting in unterminated bridged tap. The party line has gradually surrendered to the individual line, which relieves subscribers of the contention that once characterized the telephone system. Some of the vestiges of multiple plant remain, however, and have an adverse impact on high-frequency transmission. At the heart of the local telephone network is the wire center, which is the building in which copper cable and fiber extending to the subscriber terminate. Copper cable starts at a cross-connection point called a main disturbing frame (MDF). The MDF includes protectors that prevent injury to personnel and damage to equipment resulting from lightning strikes or crosses with electrical power. The cable is composed of 50-pair groups that are known as binder groups or cable complements. These are formed into large cables of up to 3600 pairs. The wire gauge varies from 26 AWG (0.4 mm) to 19 AWG (0.9 mm). Fine gauge (26 AWG) is used nearest the central office so that more pairs will fit in a 4-in (10.2-cm) conduit. Each cable pair is constructed with precise twists and is twisted around its neighboring pairs. The manufacturing objective is to expose each wire in the cable to approximately the same amount of potential interference from signals both within and outside the cable. When cable pairs are carefully balanced in manufacturing, they are capable of carrying bandwidths well outside the voice frequency range. For short distances, as in the EIA/TIA 568B structured cabling standards, precisely manufactured category 7 twisted-pair wire can carry bandwidths of 600 MHz for distances of up to 100 m. Wire centers in most communities have existed in the same location for decades. Wire centers are located to achieve a rea-


39

sonable balance between cost and transmission quality. The cable connecting the central office to subscribers is known as feeder cable. At distribution points the cable is connected to the distribution cable, which terminates at the subscriber’s premises. A serving area interface is a large cross-connect point where feeder pairs are connected to distribution pairs. Figure 3.1 shows the general structure of cable distribution. Collectively, the cables, conduits, and poles are known as outside plant. Cables are constructed with a layer of shielding material surrounding the copper pairs. An outer sheath—typically of polyethylene—surrounds the shielding, keeping the pairs reasonably free of moisture. Many cables are kept under air pressure or filled with a gel substance as a further defense against moisture. If cable pairs do get wet, much of their protection against interfering signals is lost. While cable manufactured in the last three or four decades is normally impervious to water and other elements, older cable is not. Older cables are built with lead sheath and paper insulation on the pairs. Splices deteriorate in time, and as a result the

Serving Area Interface Distribution Cable Feeder Cables

Central Office

Serving Area Interface Distribution Cable Subscriber Drops

Figure 3-1 Telephone Cable Plant

Subscriber Drops

40


cable is incapable of carrying high-bandwidth signals. Wire is carefully twisted to reduce crosstalk between pairs. Cable pairs are formed into 50-pair complements. Copper cable provides a DC path from the subscriber to the CO. The CO feeds −48 V of DC toward the subscriber. When the telephone is inactive or on hook, no current flows in the line. When the subscriber lifts the handset off the hook, current begins to flow. This change in current signals the central office to return a dial tone, which signals the user to begin dialing. Some large businesses are served over fiber optics, but the default method of connecting subscribers to the serving CO is over a copper cable pair. At the heart of the CO is a switching system. Since most systems today are digital, an analog signal originating at the subscriber’s premises must be digitized. This is done at the line interface in the switch. ISDN loops are an exception. With ISDN, digitizing is done in the telephone set. Many large companies have private branch exchanges (PBXs) that also connect to the CO over digital loops. Unless these loops are fiber optics, the digital signal is connected to the CO over some form of DSL technology. Central offices are connected to one another by means of interoffice trunks. These trunks are digital circuits with 64 kbps of bandwidth. It is important to understand that both switched and dedicated circuits are identical in all respects except that dedicated circuits are permanently connected rather than switched. This process is called provisioning in the telephone industry, and it involves connecting circuits by means of jumper wire or logical connections through a DCS.

Telephone Electrical Concepts To understand why access technologies behave as they do, it is necessary to have some appreciation of electrical terms. Any circuit, such as a twisted-wire cable pair or a coaxial cable, has inherent electrical characteristics such as resistance and impedance. Direct current flows in a predictable manner in response to Ohm’s law, which states that the amount of


41

current flow is directly proportional to the voltage or electrical pressure and inversely proportional to the resistance. In mathematical terms this is denoted as I = E/R, where I is current, E is voltage, and R is resistance. When an alternating current (AC) signal from a human voice or a high-speed modem is applied to the circuit, the phenomenon of impedance arises. If we turn Ohm’s law around to express it as R = E/I, we can see that resistance is the ratio of voltage to current. In alternating current terms we express resistance as impedance, and denote it with the letter Z. AC flows are affected by circuit elements of capacitance and inductance that do not affect DC. Without going into detail, suffice it to say that any transmission line has capacitive and inductive qualities that are distributed along its length. Capacitance and inductance impose opposite effects with respect to frequency. As shown in Fig. 3.2, the higher the frequency, the greater the resistance to AC an inductor imposes. (It’s actually called reactance, but for our purposes we can think of it as resistance). Capacitance, on the other hand, offers less resistance as frequency increases. Since capacitance is distributed along the length of the line between the two wires of a pair, the higher the frequency, the more the capacitance takes on the electrical effects of a short circuit, which is a direct connection between the two sides of a cable pair. If a transmission line is infinitely long, it begins to assume a characteristic impedance, which is the ratio of voltage to current along the line. This characteristic impedance is stated in ohms (Ω), which is the same unit for DC resistance or AC impedance. For example, category 5 unshielded twisted-pair wire used for LANs has a characteristic impedance of 100 Ω. Transmission lines are never infinitely long, but if they are terminated in a circuit that has the same characteristic impedance, the line assumes uniform electrical characteristics. A line with perfect impedance doesn’t exist outside the laboratory. In the real world, many things happen to alter the impedance of lines. In twisted-pair cable, gauge changes, irregular splices, and moisture are examples of things that can cause impedance changes. The effects of some of these may be

42


d

In

Frequency

e nc ita ac ap

C

e

nc

ta uc

Reactance

Figure 3-2 Reactive Effects of Capacitance and Inductance

moderate, but an unterminated cable causes a severe irregularity, the effects of which are difficult to predict. When an electrical wave reaches the end of an unterminated line, some of the energy is reflected back to the source. Although the architecture and transmission media of a CATV network are completely different from those of the PSTN, the same impedance considerations apply to coaxial cable as well. Kinks and dents can cause impedance irregularities in the trunk cable, but the main issue is ensuring that all subscribers are connected to the distribution coax in a nondisruptive manner. The network must be constructed so that nothing subscribers do on their premises affects the integrity of the signal on the backbone. In other words, each household must be electrically isolated from the network. This is achieved by using high-impedance taps that allow the video signal to pass to the subscriber, but that isolate each subscriber from the rest of the network. As a signal is applied to copper wire, it is attenuated over distance. The amount of attenuation depends on the length of the cable, its gauge, and several other factors of lesser importance such as temperature. A cable pair looks electrically like a capacitor across the cable pair. Capacitors pass higher frequencies more readily than lower frequencies As a result, the higher the frequency, the greater the tendency of the cable to


43

short-circuit the signal, so the greater the attenuation. This makes little difference to a voice circuit up to a point, but the loss of high frequencies results in a loss in intelligibility. To compensate, ILECs connect load coils in series with cable pairs when the cable length exceeds more than 18,000 ft (5,500 m) from the central office. A load coil is an inductor that has high attenuation to frequencies above the voice band. High-frequency services such as DSL cannot operate through load coils, so they must be removed if the cable is to support high-speed services. As discussed earlier, bridged tap results when an unterminated length of wire is connected in parallel with the talking path. It also exists when a length of cable extends beyond the termination point. Most types of DSL cannot operate with more than a small amount of bridged tap. Removing it is not a trivial matter because it may involve opening splices to cut off the offending wire. Furthermore, the ILECs’ records are not always reliable enough to show exactly where it exists. Before DSL can be provided, it is often necessary for the ILEC to remove load coils and bridged tap, an operation that we will refer to as conditioning.

Crosstalk Considerations The local loop was designed and constructed to support voice-frequency signals. The use of one pair to serve one subscriber does not begin to use the bandwidth capacity of a cable pair. For short distances, plain copper wire is costeffective, but for longer subscriber loops or for large concentrations of loops into a single subscriber, multiplexing can increase the capacity. Therefore, the ILECs sometimes use multiplex equipment in the local loop. Several factors instigated this. One was the development of digital loop carrier (DLC), which allows the ILECs to extend subscriber lines to the outer reaches of the wire center using a system similar to T1/E1. The DLC is housed in an environmentally controlled enclosure and fed from the CO with fiber optics. This is a cost-effective alternative to using coarser cable gauges

44


and range extension. The latter amounts to electronic amplification of the voice signal and boosting the DC signaling range of the switch. The second factor was demand for direct IXC access. Divestiture took the ILECs out of the long-haul toll business and gave this function to IXCs. When the ILEC switches the call from the subscriber to the IXC, The IXC pays time-sensitive access charges. To avoid access charges, large users bypass the ILEC with dedicated lines. The most economical way of providing toll bypass is with T1/E1. As discussed in Chap. 2, T1/E1 was designed as a shorthaul interoffice trunk carrier. Interoffice trunks in a metropolitan network were originally provisioned on copper cable that was usually loaded. T1/E1 was designed so that regenerators would fit in manholes, which are spaced at 6000-ft (1830-m) intervals with 3000-ft (915-m) end sections. An office repeater feeds power to midspan regenerators. Much of this trunking network has been replaced by fiber optics. When the demand began to develop for T1/E1 services in the local loop, the logical approach was to use T1/E1 carrier or DLC on a cable system that was designed for voice frequencies. Digital signals contain high-frequency components that are subject to crosstalk, which is the reception of any unwanted signal that is induced from another source. Consider the diagram in Fig. 3.3. As a T1 signal is injected into the transmit pair of a circuit, it is at a high level compared to the signal on the receive pair, which has been attenuated by the transmission loss from the previous regenerator. Unless precautions are taken, an excessive amount of the transmit signal radiates from the source and is coupled into the receive path of its own circuit or others in the cable. This path is known as near-end crosstalk (NEXT). Crosstalk is also possible at the far end of the circuit, but, as we have seen, the receive signal level is attenuated compared to the transmit level, so the coupling from the receive pair into the transmit pair is a minor concern. The best way to eliminate NEXT is by separation. If separate transmit and receive cables are available, this is the preferred method. Another method is by using so-called D screen cable, in which transmit and receive complements are

High Signal Level Low Level Signal

Near End Transmitter

Cable Pairs

45

Far End Receiver Far End Transmitter Near End Receiver

Figure 3-3 Crosstalk

Near End Crosstalk

Far End Crosstalk

46


enclosed in separate shielding. Crosstalk may also be reduced by physical separation of the complements carrying T carrier. In a large cable with multiple binder groups, cable pairs used for other services can provide the separation, but generally only one T1/E1 service can exist in each binder group.

Echo Considerations

TE

AM FL Y

Another impairment that must be considered is echo. Echo occurs whenever a four-wire circuit is converted to a two-wire circuit. Trunks are almost always four-wire because they are provisioned on multiplex equipment that inherently has separate transmit and receive paths. Local loops were designed as two-wire to conserve cable. Many older switches were also two-wire, but all digital switches are four-wire, meaning the transmit and receive directions of a session are separated. A device known as a hybrid does the four-wire-to-two-wire conversion. A hybrid works on the principle of balancing the impedances of the two-wire and four-wire loops. As shown in Fig. 3.4, the four-wire transmit and receive sides of the hybrid feed into two of its ports. The two-wire cable pair and a balancing network feed into the other two ports. If the balancing network is a perfect impedance match for the cable pair, then the four-wire legs are completely isolated. The balance is never perfect, however, because cable pairs have widely varying characteristics. Therefore, a balancing network is selected as a compromise that is less than perfect. Some of energy from the receive side of the hybrid feeds into the transmit side shown in Fig. 3.4 as the echo path. Echo is present in every connection, but it is noticeable only if there is a significant time delay before the talker’s voice returns. A short time delay of less than about 45 ms is heard as sidetone, the faint feedback that makes the telephone sound alive when you talk into it. Echo is not a problem in the local exchange network unless an application such as voice over IP is being used. Where echo is a problem, it is controlled by means of echo cancelers. An echo canceler compares the transmitted voice with the signal on the return


4-wire Receive Pair

Echo Path

2-wire Pair

Balancing Network 4-wire Transmit Pair

47

Hybrid

Figure 3-4 Hybrid Circuit

path, and where they are identical the echo signal is canceled out. Echo cancelers are built into the IXC’s network, and are not needed in the local exchange. T carrier is excellent in the trunking network where crosstalk can be controlled by use of dual cables or shielded cables. It is not an ideal medium for the subscriber loop, however, because of crosstalk limitations. The line-coding method of 1 bit per symbol change is not as efficient as newer linecoding methods that are used in ISDN and other services. Therefore, T carrier is generally replaced in the local loop by one of several DSL technologies, which are the subject of the next chapter.


4

Digital Subscriber Line Access Technology

D

SL is the prime prospect the ILECs have for capitalizing on the information age. The millions of miles of cable that are in the ground and strung from poles are functionally obsolete, yet they are an asset that has enabled the ILECs to preserve their monopoly in the face of incipient competition. POTS has about run its course as a dynamic growth business. Nearly every household in the developed world has at least one line now, and cell phones are fulfilling much of the demand for additional residential lines. Growth in the telephone industry follows the trend of business and residential development to a large degree—steady, reasonably predictable, and only marginally profitable. Although the revenues from local service are enormous (estimated to be in excess of $70 billion per year in the United States alone), the service is regulated. While profits are all but guaranteed, regulation prevents the ILECs from 49 Copyright © 2002 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.

50


achieving the profitability of more exciting industries. About all the ILECs can count on to increase their revenue stream is additional telephone services such as their custom signaling services, which include caller ID and other profitable services. While these command high margins, consumers find them easy to drop during downturns in the economy. DSL, however, is a new and profitable service that enables the ILECs to enter new markets. The copper cable to support it is in place, it is inexpensive, and it does the job well. The bandwidth restrictions that limit modem speed are in the central office and trunking network, not in the local loop. From an architectural standpoint, DSL consists of a specialized modem at the subscriber’s premises, feeding into a DSL access multiplexer (DSLAM) in the central office. Some types of DSL ride on top of a POTS line, while others use the cable pair down to a DC level, which precludes the use of a POTS line. Figure 4.1 shows the layout of DSL service. The DSLAM demultiplexes the signal from the subscriber, splitting the service so the telephone service connects to the central office while the data portion feeds into the ILEC’s backbone network, where it is routed to the service provider. DSL is not an ILEC monopoly service. ILECs typically did not lease bare cable pairs until the Telecommunications Act of 1996 required them to make unbundled network elements (UNEs) available at any feasible interconnection point. The interface to the cable pair in the central office is such a point, which makes it feasible for service providers other than the ILECs to provide DSL service. To obtain access, a service provider must register as a CLEC. The CLEC has two alternatives for physical connection to the cable pair. If space is available, it can lease space from the ILEC, install its own DSLAM in the ILEC’s central office, and connect directly to cable pairs. The other alternative is to run a copper cable extension from the CO to a nearby building. Since DSL is distance-limited, this has the effect of reducing the range from the central office to the DSL subscriber. Several companies, such as Covad and Rhythms, provide DSL service, but with the downturn in the economy in 2000–2001, competitive DSL providers have had difficulty achieving profitability.

Voice Switch

Local Loop PC Bay Networks

51 LEC Frame Relay or ATM

ISP

DSLAM= DSL Access Multiplexer

Figure 4-1 DSL Configuration

DSL Router DSLAM LEC Central Office

Telephone

Customer Premises

52


An attractive aspect of DSL is the dual use of cable pairs for voice and data service. When competing carriers first began to emerge, the ILECs refused to permit them to provide DSL over existing telephone lines. That meant that either competitors leased a single-purpose cable pair or the customer purchased an additional telephone line. The DSL carriers appealed to the Federal Communications Commission (FCC), and in its 1999 line sharing order, the FCC required the ILECs to permit competitive DSL carriers to provide service over existing telephone lines.

DSL Standards One problem with DSL has been a glut of standards. The service comes in an alphabet soup of options going by the names of HDSL, SDSL, IDSL, ADSL, RADSL, and VDSL, often collectively described by the term xDSL. Each of these has different characteristics and fits a different niche in the marketplace. To add to the confusion, two different modulation methods—discrete multitone (DMT) and carrierless amplitude phase (CAP)—are used. The muddle isn’t quite as severe as it seems at first. High-speed DSL (HDSL), and to some degree single-pair DSL (SDSL), are used by the carriers themselves to provision point-to-point T1/E1, so the subscriber is insulated from these. The industry itself drops the first letter in its promotions, so users don’t have to know what variety of DSL they are getting. Table 4.1 lists the types of DSL and their approximate transmission ranges. The inability of ILECs to respond to service requests has hampered DSL’s growth. Many users have complained about long delays in receiving services they have ordered and then difficulties in getting the service to work. Technicians must sometimes make multiple trips to get the PC and modem properly configured. To resolve these issues and to bring some order to the chaos of multiple standards, the industry formed the Universal ADSL Working Group in 1998 to develop a single standard. The carriers’ principal objective was to make it easy for subscribers to purchase self-configuring modems and

Table 4.1

DSL Types

53

Type of DSL

Acronym

Upstream Bandwidth

Downstream Bandwidth

Range in ft (m)

Asymmetric High-bit-rate ISDN Single-pair Splitterless Very-high-bit-rate

ADSL HDSL IDSL SDSL G.lite VDSL

16–640 kbps 1.544 or 2.048 Mbps 144 kbps 1.544 or 2.048 Mbps 16–640 kbps 1.5–2.3 Mbps

1.5–9 Mbps 1.544 or 2.048 Mbps 144 kbps 1.544 or 2.048 Mbps 1.5–6 Mbps 13–53 Mbps

18,000 (5,500) 12,000 (4,000) 18,000 (5,500) 12,000 (4,000) 18,000 (5,500) 1,000–4,500 (330–1,500)

54


set up the service themselves. In the industry vernacular, “truck rolls” (i.e., dispatching a technician to the subscriber’s premises) were hampering profitability. The result, a protocol known as G.lite, is acceptable within limits, but it has drawbacks that prevent its becoming a universal solution. Today users can purchase modems that operate on either DSL or cable and, theoretically at least, can set the service up without a technician, but the results have been mixed. Some of the difficulty lies in the nature of the ILECs’ plant. As discussed in Chap. 3, bridged tap is a major problem because the cable records don’t always show whether it exists, or, in some cases, where it is. Loading is not as great a problem because load coils are generally placed on loops that are more than 18,000 ft (5,500 m) long, and these loops are too long for most varieties of DSL. The solution to the loop length limit is, to some degree, resolvable. For years, ILECs have used DLC to serve subscribers that are located outside the range boundaries of the central office. These are often known as GR-303 remotes after the BellCore (now Telcordia) standard for interfacing DLC to central office switches. Where the T1/E1s supporting DLCs are provisioned over fiber from the central office, it is not difficult to equip the copper cable extension for DSL. One alternative is simply to install a DSLAM in the enclosure housing the DLC equipment and separate voice and data at that point. Specialized DLCs are available that are equipped with combination line cards and with the DSLAM function built into the system. They do not, however, work with legacy DLCs. Enclosures in which both POTS and DSL are collocated are often called neighborhood gateways. Part of the motivation for using neighborhood gateways is the ability to use very-high-bit-rate DSL (VDSL), which is capable of handling video and high-speed data signals within a limited range of 1000 to 2000 ft (330 to 660 m). Other architectures are also in use. Some CLECs offer voice over DSL (VoDSL), in which case the customer’s voice and data signals are fed into an integrated access device (IAD) at the subscriber end. At the central office end, the sig-


55

nal may feed into a media gateway, which in effect is a front end that separates the voice and data signals. The alternatives are many and varied, and more are yet to come.

DSL Technology DSL lives in an environment that was originally designed for voice transmissions. As discussed in Chap. 3, the principal limiting factor is crosstalk. Since cable plant is called on to carry high-speed signals that were not intended for copper wire, care must be used to prevent interference from various types of line signals. DSL can be categorized by two primary criteria: the line coding and modulation methods.

Coding Methods HDSL and ISDN DSL (IDSL) use the entire bandwidth of the cable pair by applying a DC signal directly to the cable pair. These types of DSL cannot coexist with a POTS line. Other types of DSL separate voice from data with filters and do permit the use of a POTS line. High-speed modems encode information in symbols instead of a raw bit stream. By using complex coding schemes, a single symbol can be made to represent some number of consecutive bits. As we have discussed, T1/E1 uses a simple coding scheme that encodes only 1 bit per symbol. ISDN improves on that by using 2B1Q line coding. Each pair of bits represents one quaternary signal. Higher modem speeds are achieved with QAM. With QAM, two carriers, each having the same frequency, are phase-shifted 90˚ with respect each other. One signal is called the I signal and the other the Q. Each carrier is amplitude-modulated with half the data. The two signals are combined at the source and transmitted to the receiver, where the signals are separated and demodulated to produce the original data stream. Early QAM modem systems used eight phases and two amplitude levels to transmit 4 bits over each symbol. These

56


are known as 8-QAM. An interesting form of error correction can be obtained by increasing this to 16-QAM. Here, 12 phase changes are used together with two amplitude levels. Amplitude changes occur on only four of the phase changes. The result is that both a phase change and an amplitude change are valid in some symbols but not in others. Trellis coding defines the way in which signal transitions are allowed to occur. Signal transitions that do not follow the expected pattern are detected as errors. By using finer phase and amplitude changes, QAM can be extended to 6 bits per symbol (64QAM) and 8 bits per symbol (256-QAM).

Modulation Methods

TE

AM FL Y

The second issue is the modulation method. DSL uses either DMT or CAP. DMT separates the spectrum above the voice band into 256 narrow channels called bins. Each bin is 4 kHz wide. Data is modulated onto each channel using QAM modulation with a 4-kHz symbol rate, resulting in up to 60 kbps per bin. A simple splitter separates voice from the data signal. The modems detect which channels may be impaired and spread the data to unimpaired channels. The upstream direction can use bins 6 to 38, which is about 25 to 163 kHz. Bins 33 to 255 (142 to 1.1 MHz) are used downstream. This method of modulation is inherently rate-adaptive because it uses all the available channels and ignores those that have poor transmission quality. For example, if an AM broadcast station is interfering with a band of frequencies, these bins are skipped. DMT is the American National Standards Institute (ANSI) standard T1.413. The European Telecommunications Standards Institute (ETSI) and ITU also recognize the standard. CAP is closely related to QAM. The data rate is divided in two and modulated onto two orthogonal carriers phaseshifted 90˚ to one another. CAP systems use a combination of amplitude and phase states to encode the data. The two channels are combined, fed into a digital-to-analog converter, and transmitted. A simple filter separates voice and data. Hybrids and echo cancelers are not needed with CAP systems because the two directions of transmission are separate. The


57

modems use adaptive equalization to overcome, to some degree, the effects of bridged tap, gauge changes, and splices. CAP is a proprietary protocol, but despite its lack of standard status, it has a large following.

Spectral Compatibility The degree to which DSL signals can coexist in the same cable is a matter of concern to the ILECs. The limiting factor is usually near-end crosstalk. NEXT can be controlled by limiting the nonvoice signals that are assigned to particular groups of pairs. The NEXT problem is most severe nearest the central office because the cable density is highest there. DSL systems therefore use the lowest frequencies for the upstream direction to compensate for the fact that the higher the frequency, the greater the coupling from one pair to another. To limit interference, each DSL standard includes a power spectral density (PSD) mask that describes the PSD by frequency in the upstream and downstream directions. The degree of crosstalk that one DSL type imposes on another defines their spectral compatibility. The degree of compatibility depends on such issues as the spectrum the DSL type uses and its placement in the cable.

Types of DSL Each of the DSL types listed in Table 4.1 has a particular function. This is usually not a matter of concern to the subscriber because the service provider determines which type will be used. This section briefly discusses the characteristics and applications of each type. HDSL T1/E1, which is designed as a trunk carrier, doesn’t fit well into exchange cables. Not only is repeater spacing critical, but also, crosstalk considerations limit the number of systems that can be installed in a 50-pair cable complement. HDSL is designed to overcome these limitations. The ILECs have used

58


HDSL for years to provide T1/E1 service over two twistedpair wires up to 12,000 ft (4,000 m) long. This enables them to avoid the regenerators that T1/E1 requires, which are expensive and difficult to install on many loops. Terminating equipment is installed both in the central office and at the subscriber’s premises. Unlike other DSL versions, HDSL does not support regular telephone service on a cable pair. SDSL SDSL (also known as symmetrical DSL) serves the same market as HDSL, with two important functional differences. Instead of dual cable pairs, it provides T1/E1 service on a single cable pair and it also derives a POTS line under the data signal. It transmits and receives in the same band of frequencies using an echo-canceling protocol, making it susceptible to NEXT. SDSL uses 2B1Q line coding that is adopted from basic rate ISDN. IDSL IDSL provides 128 kbps of bandwidth using the same protocol as BRI. The main difference is that IDSL terminates in a router instead of an ISDN switch port. This precludes it from carrying a voice signal. This technique has limited application because its limited bandwidth cannot compete effectively with other DSL technologies. VDSL VDSL was developed as a means of providing video on demand in fiber-to-the-curb implementations. Fiber is used to bring the video signal to an access node with twisted-pair wire supporting the last span. VDSL supports data rates as high as 52 Mbps, which is enough to carry a DS3 over short spans of copper wire. Rates of 52 Mbps can be supported over a 1000ft (330-m) range, but the rate drops to about 13 Mbps beyond 4500 ft (1500 m). The technology is asymmetric, with upstream speeds of 1.5 to 2.3 Mbps. It can be overlaid over POTS lines. ADSL Asymmetric DSL (ADSL) can carry as much as 1.5 Mbps downstream over a range of up to 18,000 ft (5,500 m). Data


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travels much more slowly in the upstream direction—in the 64- to 640-kbps range. For residential Web surfing, ADSL is an excellent alternative because the nature of the application is asymmetric. Many business applications require symmetrical bandwidth, however, so ADSL may not be adaptable in many cases. ADSL is most attractive for casual users, while SDSL is the most effective for businesses and telecommuters. Rate-adaptive DSL (RADSL) is a variation of ADSL. The DMT modulation method is inherently rate-adaptive, but CAP is not. When the industry refers to RADSL, it is usually with respect to a version of CAP in which the modems automatically optimize the line to the highest effective transmission rate. Rate adaptation compensates for changes that occur because of weather changes and aging of components. If the line quality degrades, the modems step down to a lower rate to preserve transmission integrity. Standard ADSL requires a splitter at the customer’s premises. The splitter not only separates the voice and data signals, it also terminates the line with a constant impedance. The telephone wire in households is not a reliable termination, because wire quality varies greatly, unterminated wire runs are common, and telephone sets being unplugged or going off hook cause sharp impedance changes. Also, the telephone sets themselves are of varying quality, and, since they are connected in parallel across the line, the quantity of sets on the line affects the impedance. The splitter isolates the line from these variations. The Universal ADSL Working Group developed G.lite, an ADSL variation, to eliminate the need for a splitter. The objective was to enable subscribers to install the service without the need for a technician visit. G.lite uses the DMT modulation method and is ANSI standard T1.413. G.lite uses 96 upstream bins, which limits the throughput to about 400 kHz. The downstream direction may support up to 1.5 Mbps, depending on the length of the loop to the central office. ATM is used as the transport medium. G.lite is designed to retrain itself rapidly in the face of sudden impedance changes, as when a phone is lifted off-hook.

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DSLAM The DSLAM separates voice-frequency signals from data at the CO end. The DSLAM aggregates data traffic from several loops into the backbone. In the case of ILEC-provided DSL, the backbone is usually ATM. Competitive access providers may use IP. ISPs and other network providers connect to the network with PVCs. A multiservice DSLAM can handle traffic from SDSL, IDSL, and ADSL. It may also support video access in addition to Internet access. Multiservice capability may also include multiple service types such as frame relay, ATM, TDM, and IP running to the subscriber from a single platform. Frame relay is provisioned to the subscriber using TDM or ATM.

VoDSL DSL started out to keep voice and data separated by means of splitters and filters, but several new protocols and products have been developed or are under development to merge voice and data at the circuit level. The products compress the voice signal down to a fraction of the 64-kbps bandwidth of a conventional voice channel. Voice can be compressed to 8 kbps with only a small loss of intelligibility. The quality is equivalent to a solid cell phone connection. The protocols add some overhead, so the number of channels is less than a straight multiple of the compressor output, but the voicecarrying capacity is substantial. Note, however, that faxes and modems require an uncompressed channel. The amount of bandwidth the voice channel occupies depends on the protocol used. ATM is the most common protocol. With 5 bytes of header per cell, the overhead is approximately 10 percent. Equipment vendors claim from 16 to 20 voice channels per DSL line, but of course that depends on the actual data rate the line can sustain and also on the amount of bandwidth needed for data.

5

Cable Access Technology

C

able television facilities pass more than 80 percent of the homes in the United States, and about 65 percent of these subscribe to the service. Cable was originally developed as an entertainment medium, but having a broadband pipe into the majority of residences makes it an obvious candidate as an information medium as well. In fact, in the 1960s the Bell System made a few forays into the cable business for just that reason. These were, however, thwarted by regulatory authorities that opposed adding cable to the Bell companies’ telephone monopoly. Many of these restrictions are disappearing, however, as cable companies prepare to offer telephone service and telephone companies investigate DSL as a means of delivering video-on-demand. Despite the fact that technology may make it possible to cross between the traditional media, there is often a wide gap between what is possible and what is feasible. Differences in telephone and 61

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

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video signals still leave the conventional architectures more suitable for their traditional markets. One of the impediments to expanding cable for other than video service has been the one-way nature of the conventional system. At the time cable systems were constructed, entertainment was inherently one-way. That is still the case to a large degree because television is not interactive, but games and Internet access are. Two-way cable opens the potential for additional revenues for the system operators, leading most providers to upgrade their systems. In some cases, franchising authorities compel cable operators to enter the twoway business. In exchange for permission to use the public right-of-way, some municipalities require cable operators to provide two-way networks for public purposes. Although cable has plenty of bandwidth for voice and data, it has some inherent drawbacks. At the time the early two-way systems went into operation, the industry lacked standards, so systems were constructed using proprietary protocols. Only in the last few years, with the development of the Data over Cable System Interface Specifications (DOCSIS), has a standard existed. Before that the industry relied on proprietary equipment such as the LANcity (now a Nortel company) transceiver that provided access to the physical cable medium. Even now, DOCSIS is not fully accepted as an international standard. The Institute of Electrical and Electronic Engineers (IEEE) organized the 802.14 committee to develop a standard, but DOCSIS was completed first, leading many to question whether 802.14 has missed the boat. Some cable operators still maintain proprietary equipment, which requires matching modems to communicate. DOCSIS, which is discussed in more detail later, is today the best protocol the cable industry has. Cable Labs (www.cablelabs.com), the industry association that developed DOCSIS, tests and certifies modems as compatible with the protocol, thereby enabling users to obtain their modems from sources other than the cable provider. A major issue in many jurisdictions is that of open access to the medium. The municipalities that regulate cable generally prefer that their constituents have free access to any ISP,


63

but at this point, cable providers have been successful in warding off requirements to open their networks. Consequently, most cable users have access only to the cable provider’s captive ISP. The city of Portland, Oregon, attempted to require AT&T Broadband to provide open access to all ISPs, but AT&T defeated the order in court. The open access issue is still boiling on the back burner, and undoubtedly will resurface as the industry matures. One consequence of the lack of open access was the widespread disruption that occurred in December, 2001. AT&T’s provider Excite@home disconnected the service following its bankruptcy necessitating changes to every AT&T subscriber.

Conventional Cable Architecture A cable system is constructed on a completely different architecture than the telephone system. Where the telephone companies extend one or more cable pairs from the central office directly to each subscriber, cable companies connect their subscribers in parallel across a cable that delivers all channels to all subscribers. For those who choose not to subscribe to premium channels, the cable company either filters such channels out or scrambles them so that a device provided by the cable company is needed to descramble them. The architecture of a cable system is shown in Fig. 1.2. The headend consists of off-the-air receiving apparatuses for picking up local channels, satellite receivers for receiving premium channels, modulators to stack the video signals in the appropriate frequency slots, and equipment to receive upstream transmissions and connect them to the appropriate service. The headend feeds trunk cables with various entertainment and information channels. The trunk cable is a high grade of coaxial cable with diameters of 1.9 to 2.5 cm (0.75 to 1 in) and amplifiers placed at intervals of about 500 m. Automatic gain control in the amplifiers compensates for changes in transmission loss with temperature variations. The system feeds power to amplifiers over the coaxial center conductor, with main power feeds located approximately

64


every 1.5 km. To continue essential services during power outages and amplifier failures, the cable operator provides redundant amplifiers and backup battery supplies. Bridger amplifiers separate the feeder or distribution cable from the trunk cable. Multiple feeders are coupled with splitters, which match the impedance of the cables. Feeder cable is smaller and less expensive and has higher loss than trunk cable. Subscriber drops connect to the feeder cable through taps, which are passive devices that isolate the feeder cable from the drop. The tap must have enough isolation so that disturbances at the subscriber’s premise do not affect other users. The FCC assigns 6 MHz of bandwidth to each TV channel (8 MHz is assigned in Europe). Only 4 MHz is used for transmission; the remaining bandwidth is used for guard bands to prevent interference between adjacent channels. Cable is inherently an analog frequency-division medium. The earliest cable systems were intended strictly for off-the-air pickup and retransmission. Channels 2 to 6 of the broadcast spectrum in North America cover the range of 54 to 88 MHz. Channel 7 starts at 174 MHz, with channel 13 assigned 210 to 216 MHz. UHF channels 14 to 69 cover the frequency range of 470 to 806 MHz. Set-top converters and cable-ready television sets can receive as many as 100 channels, which is approximately the capacity of a high-bandwidth cable system. Cable has a major advantage over regular broadcast channels. Households on the fringe of a broadcast coverage area can easily experience interference on adjacent channels despite the guard bands. The FCC, accordingly, normally does not assign adjacent channels in the same metropolitan area. Cable is under no such restrictions because all channels are transmitted at the same power level so adjacent channel interference is not a problem. The headend equipment can pick signals off the air and assign them to any channel. The audio and video signals are stripped from the channel carrier, reapplied to channel modulators, and inserted into the trunk cable. The earliest cable systems were constructed to transmit only the 12 VHF channels. Any UHF channels in the area were applied to a vacant VHF channel on the cable. Gradually, however, cable systems were expanded to the


65

point that the typical system today has a bandwidth of 1 GHz, of which the top 250 MHz is reserved for future applications, leaving room for about 100 channels. An analog cable system could amplify signals to extend the range almost indefinitely, except for the problems of noise and distortion, which limit the serving radius of 80channel CATV to about 8 km from the headend. As with any analog medium, when the signal is amplified any accumulated noise is amplified along with the desired signal. The measure of quality in a cable system is the signal-to-noise ratio (SNR). The SNR should be at least 35 dB to provide a satisfactory signal. The video portion of the signal is amplitude-modulated. The lowest level in the picture portion of the signal is black, so any noise spiking above the black level shows as snow on the screen. Cable is engineered to keep noise within prescribed boundaries, but the more media that are carried on a cable, the more susceptible it is to noise. Each subscriber is a potential noise source. Since hundreds of subscriber drops are bridged in parallel across the cable, these can induce unwanted signals. Amateur radio and any apparatus with a high electrical noise level can introduce noise into the cable. Cable systems must therefore be carefully constructed to prevent interference with other services. Signals leaking into the cable from the outside would distort the picture and signals radiating from the cable could interfere with other services.

Two-Way Systems Two-way cable systems are constructed on the same principles as one-way systems, but with bidirectional amplifiers. Filters split the signal into the high band for downstream transmission and the low band for upstream transmission. The ratio of downstream to upstream bandwidth is heavily weighted in the downstream direction. The 5- to 40-MHz range is used for upstream bandwidth, with a guard band about 15 MHz wide to separate the two directions of transmission. The upstream direction shares frequencies with

66


TE

AM FL Y

high-powered short-wave transmitters operating at frequencies of 5 to 30 MHz, so interference from these sources is a potential problem. Cable and amplifiers must be adequately shielded to prevent interference. Headend equipment is more complex in a two-way than in a one-way CATV system. The cable system is constructed as a shared medium, which means that some method is required to regulate access. The downstream direction is simple to regulate. Communicating devices are assigned addresses and programmed to respond only to their address, much in the same way as Ethernet operates. Upstream is another matter. User devices access the upstream cable by contention, token passing, or being polled from the headend. Some systems use a transponder at the user end to receive and execute orders from the headend. For example, a polling message might instruct the transponder to read utility meters and forward the reading over the upstream channel. One solution to the twoway cable problem is using a telephony return. One of the 6MHz TV channels is devoted to downstream, while the upstream direction uses a conventional modem on the telephone line. Although this method works for some services, it has obvious disadvantages for any service requiring alwayson access.

Hybrid Fiber-Coax Architecture The traditional cable architecture shown in Fig. 1.2 is being displaced as cables are converted to two-way transmission. One problem with traditional cables is that amplifiers are connected in series, leaving the service vulnerable to amplifier failures. Failure of a trunk amplifier can disrupt service to an entire neighborhood. Redundant amplifiers are available, but a more effective method is to bypass the trunk cable with fiber optics, which doesn’t require amplifiers. The typical cable network today uses fiber to a neighborhood node, where the signal is converted from optical to electrical. This architecture is known as hybrid fiber-coax (HFC). Fiber has more than enough bandwidth, and has the additional advan-


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tage of eliminating a string of amplifiers, which increases reliability. Furthermore, dense wave-division multiplexing (DWDM) can expand the capacity of the fiber. In concept, DWDM is much like frequency-division multiplexing, which is used on coax. Signals are applied to different “colors,” with multiple wavelengths applied to the fiber simultaneously. The signals aren’t truly colors, because the wavelengths used in fiber are below the visible spectrum, but the multiple-color concept is a useful way of understanding DWDM. Data signals may be transported upstream and downstream as digital signals in the fiber portion of the network and combined with the video signal in the coaxial portion. The result is a medium that is inherently shared for data with all the subscribers in the neighborhood coaxial section, which typically numbers from 500 to 2000 subscribers.

Cable Access Technology In many ways, cable is an ideal Internet access technology in the downstream direction. With around 750 MHz of bandwidth, the cable operator can easily sacrifice an entertainment channel to enable subscribers to receive data transmissions addressed to them. The addressing scheme is standard IP, and, using QAM modulation, one 6-MHz television channel can carry 27 Mbps of downstream data. The asymmetric nature of Internet access fits well into the cable transmission scheme with its limited upstream and abundant downstream bandwidth. On the surface, it would appear that cable is a superior method to DSL, which rarely exceeds 1.5 Mbps downstream and is usually much less. Cable would be preferable to DSL except for three limiting factors: 1. Cable is inherently a shared medium, which means that the bandwidth available to a subscriber is a direct function of the number and activity of other subscribers on the same coaxial segment. By contrast, DSL is a dedicated connection to the DSLAM, and is shared only in the service provider’s access circuit into the backbone network.

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2. Access to upstream bandwidth must be regulated by some

sharing protocol. The upstream bandwidth must divided among users or users must contend with one another for access. DSL provides a direct channel that is exclusive to the subscriber. 3. Cable is primarily intended as a residential service. It may not be available to some businesses, and even if it is, its asymmetric nature may preclude its use for many business applications. A data network using a shared medium must have a method of regulating access. Token passing is one method. The token is a software mark that circulates past all stations in turn. A station can transmit only when it possesses the token. Then it can transmit a frame, after which it relinquishes the token to the next station in line. Token passing is used successfully in the IEEE 802.4 token bus protocol, but it is too complex for a cable network serving hundreds of independent stations that may come online and drop off at random. If a station drops off just at the time it possesses the token, the other stations must regenerate the token, and stations must at all times know the identity of immediately adjacent stations to regulate token flow. A more practical method is a protocol known as Aloha, developed by the University of Hawaii for data communications between the islands over radio. When a station has data to send, it transmits it. If two stations transmit simultaneously, their transmissions collide, and the data is mutilated. The transmitting station, receiving no acknowledgement of receipt, sends again until the message is acknowledged. The problem with Aloha is similar to the problem in a cable network. Since the stations can’t hear transmissions of all the other stations, they have no way of waiting until the channel is clear before transmitting. The busier the network, therefore, the greater the probability of collision. Ethernet evolved from the Aloha protocol and is the predominant protocol for resolving network access, but it works well because all stations on the network can hear transmission from the other stations. When a station is transmitting


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just at the time other stations have data to send, the stations wait until the network is idle. The Aloha situation is similar to that in a cable network. Stations can hear the transmissions of upstream stations, but they cannot hear downstream stations. If stations simply listen to the channel and wait for an idle moment to transmit, the upstream stations grab a disproportionate share of the bandwidth and squeeze out the downstream stations. This can be resolved by using a protocol known as slotted Aloha. Stations are granted a time slot within which they can transmit. The headend is responsible for ensuring that bandwidth is evenly distributed by assigning transmission timeslots to active stations. Proprietary methods were used for years on cable networks, but the result was that users could obtain only the types of modems that the cable operator certified for its network. Proprietary equipment tends to be more expensive than standard equipment, and also limits the growth of the technology, which led the cable industry to develop a protocol that all could employ.

DOCSIS DOCSIS was developed by CableLabs, Inc., a consortium of equipment manufacturers that collaborated on creating a standard for data transmission over cable. In Europe, DOCSIS is known as a Euro-DOCSIS, and is derived from the U.S. version. The European cable community adopted the standard in early 2000. DOCSIS consists of several components. The cable modem (CM) connects the subscriber’s PC to the cable network. At the headend is the cable modem termination system (CMTS) and a variety of specialized servers. This equipment operates with the subscriber’s modem as either a bridge or router. In between is the HFC plant, which forms the radio frequency (RF) link between the CMTS and the CM. DOCSIS specifies the RF physical layer with respect to modulation methods and symbol rates. It specifies modem initialization procedures, security, and data

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control. At the subscriber’s premises a splitter separates the data and television portions of the signal. Cable companies can use DOCSIS standards to facilitate delivery of Internet and other information services over their CATV networks. The standard relieves users of the need to purchase a proprietary modem to operate over cable. Although DOCSIS is designed for cable, with certain modifications it can also be used in wireless multipoint multichannel distribution service (MMDS) and local multipoint distribution service (LMDS). The headend contains several specialized servers in addition to the CMTS server. These include a DHCP server, which downloads IP addresses when CMs become active. A user authorization server controls access to the system and may provide accounting. A time-of-day server enables the CM to synchronize its time with the rest of the system. The DNS correlates URLs to IP addresses. A Trivial File Transfer Protocol (TFTP) server is provided to facilitate file transfer. In addition, a service provider will undoubtedly provide an email server. Many cable operators cache popular Web pages in servers close to the user to reduce the amount of bandwidth required in the Internet backbone. Security is a potential problem with a shared medium because any station with a protocol analyzer on a shared coaxial leg can see the transmissions of the upstream stations. If that station happens to be closest to the fiber node, it could see all upstream packets and intercept confidential information such as passwords. To prevent this, the DOCSIS protocol includes encryption and key exchange. A cable modem is effectively a frequency-agile RF transceiver that is tuned to upstream and downstream channels. Downstream, the cable operator typically allocates a single 6-MHz channel for data. The cable modems are all tuned to the same channel and use the IP address to pick their packets from the stream. The CM usually connects to a PC with Ethernet. Some universal serial bus (USB) modems are available, but these have the disadvantage of allowing only one PC to connect to the modem, whereas Ethernet can connect multiple PCs through a hub or switch.


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When a CM is first turned on, it begins scanning the cable for a downstream data channel. From this it learns the frequency of the upstream channel. It broadcasts its presence to the CMTS, which validates its right to use the service, obtains an IP address from the DHCP server, and sends the IP address to the CM. With this initial handshake completed, the CM is prepared to communicate over the cable. Cable modems may also use a telephone line for the return path, a method known as a telephony return interface (TRI) system, but it is not suitable for always-on access. First, it ties up a telephone line, adding to the expense, and second, the telephone line is limited to a maximum of 33.3 kbps upstream, which is much slower than the CM. DOCSIS uses a frame format similar to Ethernet in both the upstream and downstream directions. The LLC is standard IEEE 802.2 protocol. The rest of the data link layer has two other sublayers in addition to the LLC: link security and MAC. The link security sublayer has three sets of requirements: baseline privacy interface (BPI), security system interface (SSI), and removable security module interface (RSMI). The BPI encrypts data traffic between the user’s modem and the CMTS. The MAC layer includes collision detection and retransmission, error detection and recovery, and procedures for registering modems. It also performs ranging, which enables the CMTS to evaluate the time delay to each cable modem and allocate the upstream time slots accordingly. Table 5.1 may help to put the architecture in perspective with respect to the OSI model. Since cable is an analog medium, cable modems communicate with the headend over modulated analog channels. The downstream channel, which is standardized as ITU J.83, is either 64-QAM or 256-QAM. The downstream payload of 64-QAM is approximately 27 Mbps. Eight amplitude levels are used to modulate the carriers and 6 bits of data are transmitted at a time. The payload of 256-QAM is 39 Mbps using 16 amplitude levels and transmitting 8 bits of data at a time. The bandwidth of the RF signal is 180 kHz to 6.4 MHz, with the data rate varying from a low of 320 kbps to a high of 20.5 Mbps depending on the modulation method and the RF

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Table 5.1 DOCSIS Compared to OSI OSI

DOCSIS

Higher layers

Applications

Transport

TCP/UDP

Network

IP

Data link

IEEE 802.2 Link security (BPI, SSI, RSMI) Media access control Upstream TDMA 5–40 MHz QPSK/16-QAM

Physical

DOCSIS control messages

Downstream MPEG-2 54–850 MHz 64/256-QAM

bandwidth used. All users on the coaxial portion share the aggregate bandwidth, so the actual throughput any user will experience is much lower and varies with the amount of activity. The upstream direction uses either quadrature phaseshift keying (QPSK) or 16-QAM. Although 16-QAM has twice the data rate of QPSK, the latter is more tolerant of interference. The upstream direction is much lower in speed than the downstream channel, having a typical bandwidth of 300 kbps to 1 Mbps. Downstream data is encapsulated in MPEG-2 frames. MPEG stands for the Motion Picture Experts Group, which developed a standard method of transmitting compressed digital video. MPEG-1 was developed to compress video onto audio CDs using a low-bit-rate process that resulted in a picture with about the same resolution as VHS tape. MPEG-2 provides studio-quality video, including support for high-definition TV (HDTV). It allows multiple channels to be multiplexed into a single data stream. DOCSIS specifies Reed-Solomon FEC as a means of improving error performance. FEC adds redundant bits to the bit stream and sends them along with the information bits. If errors occur, the decoder attempts to correct them before the bit stream is presented to the application, with the final error check done at the receiving apparatus. The Reed-Solomon code used in video transmits 204 bytes per frame, of which


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188 bytes are MPEG header and information and the remainder are parity bits to correct errors. The purpose of forward error correction is to reduce the number of frames that must be retransmitted because errors occurred. Each frame starts with a packet identifier (PID). Standard PIDs are assigned for video, audio, clock, and other data such as the program guide. MPEG-2 can multiplex multiple audio and video programs together on the same bit stream. One feature of DOCSIS is the provision of various classes of service. Cable providers can offer priority to customers who are willing to pay, just as the airlines offer first-class, business class, and tourist seats. Although some compromises are required to use cable as an access medium, it is a reliable system that provides sufficient bandwidth to meet nearly all residential requirements. From the cable operator’s standpoint, it is a profitable service. The main problem is that as demand grows, response time slows. The only solution is to segment the coaxial portion of the network, which is an expensive proposition and not always practical.


6

Wireless Access Technology

M

any companies are relying on wireless to loosen the ILECs’ stranglehold on the local loop. Humpty Dumpty could have been talking about the WLL when he said that words mean exactly what he wanted them to mean. By WLL some people mean cellular, others mean one of the wireless local distribution systems, and some writers even categorize cordless telephones as WLL. We will reject that definition because cordless telephones can’t connect to the wide area voice or data network directly without going through a POTS line. The other technologies qualify, though, and in this chapter we will briefly discuss the following: • Cellular modems: Specialized modems connected to an ordinary cell phone. The speed is limited and the usage is metered. 75

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• Next-generation cellular: Also known as 3G and 2.5G cel-

•

•

•

•

AM FL Y

•

lular. New hand-held devices overcome the speed limitations of conventional cellular data. Cellular digital packet data (CDPD): Makes use of idle cellular channel capacity. TCP/IP connections at up to 19.2 kbps can be supported. IEEE 802.11b wireless: Wireless LAN protocol used for in-building applications and for fixed point-to-point connections at up to 54 Mbps. Multipoint multifrequency distribution system (MMDS): Delivers video bandwidth from a central transmitter to multiple receivers in the 2.5-GHz band. Local multipoint distribution system (LMDS): Uses the 25-GHz and higher spectrum to deliver point-to-point or point-to-multipoint broadband services. Satellite services: Both equatorial and low-orbiting satellite services provide bandwidths ranging from voice circuit to video.

TE

Wireless has much in common with cable in that the medium is shared. As with cable, sharing is no problem in the downstream direction. All that is needed is to program the receiving device to respond to its address and ignore the others. The upstream direction is a problem because multiple contenders are vying for access to the same bandwidth. Upstream sharing takes place by one of three multiple access methods: frequency division, time division, or code division. From a high-level standpoint, FDM is ultimately used in every wireless technology. The available spectrum is divided into frequency segments and allocated to the various services. The radio division of ITU regulates bandwidth assignments through international treaties, but the countries have considerable latitude, particularly in microwave bands where radio waves can be confined within a country’s borders. An exception to this is satellite broadcasting, which cannot be confined to national boundaries. Microwave frequencies used for satellite can be focused narrowly and reused many times, with terrestrial and satellite services often sharing the same frequency spectrum. Within the assigned frequency segments, FDM is


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often used to segregate channels. The earliest mobile and cellular services used FDM with analog modulation. The second method of sharing is time-division multiple access (TDMA), which is used in digital cellular and satellite uplinks. On wired media, Ethernet in LANs and DOCSIS in cable systems use this method. Each device receives a share of the bandwidth. The key is to allocate bandwidth so every station gets an equitable share. When stations can hear one another as they do with Ethernet, they simply wait for an idle moment. When they cannot hear one another, as is the case with satellites, cable, and cellular, access is regulated by some other means such as assigning time slots from a master station. Code-division multiple access (CDMA), also known as direct sequence spread spectrum (DSSS), is somewhat more difficult to grasp. Instead of waiting for an available time slot, stations simply launch their signals across a wide band of frequencies. A code embedded in the transmission enables the receiver to pick the desired signal from the jumble. A useful analogy is to visualize a crowded room filled with people speaking many different languages. Each listener is able to focus on his or her native language and pick out that one conversation from a cacophony of voices. Wireless has a lot to recommend it. In fact, for some applications it is the only alternative. Today, cruise ships and airlines offer their passengers telephone and Internet access. Emergency vehicles and mobile services, even down to the much-maligned cell phone in the restaurant, all rely on radio waves for access to international voice and data networks. The industry provides a variety of wireless alternatives, some of which provide access options that can’t be matched by wired services.

Categories of Wireless Service Wireless service can be characterized as broadband or narrowband. Broadband alternatives generally provide downstream bandwidth on 6-MHz television channels and provide

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a wireless service that is similar to wired service on cable. Some even use a version of DOCSIS protocol. Narrowband services provide bandwidths ranging from voice-grade services to the equivalent of T1/E1. Wireless can further be characterized as licensed or unlicensed. Unlicensed systems can operate in specified frequency bands, but the users must resolve any interference problems because the FCC or other regulatory authorities make no guarantee of an interference-free channel. Licensed systems are more likely to be free of interference, but the permit process requires interference studies and a sometimes lengthy licensing process. A third factor classifying radio is whether the application is fixed or mobile. Fixed radio access substitutes a radio link for wire; otherwise the wired and wireless services produce an equivalent result. The data output from the wireless link feeds a LAN for distribution inside the building. The frequency band further classifies wireless applications. The higher the frequency, the more spectrum that is available and the more bandwidth that an application can occupy. Although more bandwidth is available at higher frequencies, coverage becomes more of an issue. The highest microwave bands in use today are on the order of 40 GHz and may have a range as short as 2 mi or so. The lower bands are heavily used and therefore more congested. The higher frequencies are more readily adaptable to frequency reuse than the lower bands. Cellular, which operates in the 800-MHz band, is based on the principle of reuse. Distance and terrain are used to separate cell sites using the same frequencies. With the use of directional antennas, different sectors can be served by transmitters on the same frequency without mutual interference. At the shorter wavelengths, signals can operate on the same frequency by cross-polarization, i.e., one signal is polled vertically and the other horizontally. All services and the bands we have been discussing share a common characteristic: they operate only within the line of sight. They may penetrate walls, but communication is not reliable. The higher the frequency, the shorter the wavelength and the more metal framework and masonry walls


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attenuate waves. Microwave frequencies above about 10 GHz are subject to rain fading. In areas of heavy rainfall, such as the Gulf Coast of the United States, raindrop size is a significant fraction of a wavelength, so that rainfall may depolarize or absorb microwave signals. Attenuation from rain and fog can be overcome with higher power, which can be achieved by increasing use of transmit output power, receiver sensitivity, or antenna gain. These effects are predictable and a properly designed system should operate reliably over narrow ranges. Radio signals are also attenuated by multipath fading. This phenomenon occurs when a signal is reflected. The reflected path takes a longer route to the receiving antenna and arrives at the antenna slightly out of phase with the direct path, resulting in a reduction of the received signal level. Under normal conditions and with a properly designed system, multipath fading does not occur, but a sudden fog bank can introduce unpredictable reflections. Because of the transient nature of multipath reflections, not much can be done about them except to tolerate the outage. Commercial microwave systems have a variety of diversity methods that improve the path reliability, but these are generally not feasible for access systems. Microwave radio is an excellent broadband downstream information system. Just as a cable system can devote a 6-MHz channel to downstream information, a satellite or terrestrial microwave can do the same because of the oneto-many nature of the channel. Each receiver is tuned to recognize its address and pick its information out of the data stream. Upstream, however, is another matter because of its many-to-one characteristics. No license is needed to receive data, but in many cases the upstream direction must be licensed, which adds delays and some complexity. Using a telephone return may solve the upstream problem, but this has the same disadvantages we discussed in the Chapter 5. With this background in mind, we will discuss some of the methods of providing local access service over wireless. As with the other services, this discussion will emphasize data.

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Cellular Dial-up Within limits, cellular dial-up can be an effective access option. The first limitation is low data throughput, which makes it unsatisfactory for Web surfing. Ordinary commercial modems may not work well with cellular, particularly at high speeds. The modulation methods of these modems are complex, and they do not gracefully handle interference, fades, and signal dropout. Handoff between cells causes a momentary interruption, and may cause the connection to drop. Furthermore, many modems are designed to operate only after they recognize dial tone, which cellular does not provide. Therefore, special cellular modems that can adjust speed to the signal conditions are required. For occasional use such as mobile file transfer, cellular is a satisfactory alternative, but it is expensive for short messages. Most cellular operators levy a one-minute per-call minimum charge, which makes cellular a poor choice for short transactions that may last only a few seconds. Also, the setup time is long compared to other alternatives. Cellular has the advantage of good coverage. In general, its applications are similar to those of the PSTN: it is acceptable for facsimile and file transfers, but poor for short, bursty messages.

Next-Generation Cellular The cellular industry is touting so-called third-generation cellular as the solution for Web surfing from a hand-held mobile device. The first cellular generation was analog frequency-division cellular, some of which is still in operation. As the industry ran out of capacity in the cellular bands, which are between 824 and 894 MHz, second-generation digital cellular was introduced to make more efficient use of the spectrum. Most of the world elected to use the global system for mobile communications (GSM) modulation method. In the United States, the FCC elected to let the market determine the most effective method. As a result, two methods are used, neither of which is compatible with GSM. AT&T uses


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TDMA to apply three digital channels in the same spectrum used by one analog channel. Several other carriers have adopted CDMA. Since these methods use voice compression to make better use of the spectrum, they are not suitable for high-speed data communications. The third generation, which goes by the acronym of 3G cellular, promises data transmission at as much as 2 Mbps, which makes it competitive with the best wired alternatives. The only problem is that 3G cellular will have a difficult time fulfilling its promise. Several technical hurdles remain to be overcome before 3G is practical. One problem is power drain. The circuitry needed to sustain higher data rates gives off heat, which means high battery drain—more than today’s batteries can provide. One solution is to reduce the data rate, perhaps by a factor of four or more. This will limit some of the expected applications while still providing a respectable speed for Web surfing. Furthermore, the small screens and keyboards of hand-held devices put additional limits on the applications. This is causing manufacturers to scale back their plans to something called 2.5G cellular, which has data rates in the order of 384 kbps. Although this is far from the promise of 3G, it is still considerably more than dial-up landline services. A major issue with both 2.5G and 3G wireless is spectrum. The existing PCS spectrum is insufficient to meet the bandwidth requirements, which has led the industry to ask the FCC to allocate bands in the 1750- to 1850-MHz and the 2500- to 2690MHz ranges. This spectrum is currently occupied by fixed wireless and the Department of Defense. Whatever the outcome of this request, it is clear that next-generation wireless has a long way to go before it is a mainstream access choice. Another technology getting a lot of attention is Bluetooth, which is intended to enable devices to communicate with one another over short ranges using unlicensed 2.4-GHz frequencies. Bluetooth is intended to link devices such as personal digital assistants (PDAs) with master devices over a wireless link. Bluetooth is not intended as a public communications medium, so it does not fit into our definition of an access technology. It is worthy of attention by potential customers if

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for no other reason than the fact that it occupies the same frequencies as 802.11b wireless and that coexistence may be a problem.

CDPD CDPD is a good alternative for obtaining wireless coverage in metropolitan areas and along major highways because it offers the same coverage area as cellular if the service provider has elected to equip its cell sites for the service. CDPD is a packetswitched data service that rides on top of cellular and uses idle analog channel time. It can be added to existing cell sites at a moderate cost. Carriers charge by the packet or kilobyte instead of by connect time, and the long call setup time and minimum connect time charges are eliminated. This makes CDPD good for short, bursty messages such as point of sale, dispatch, package tracking, telemetry, and e-mail. CDPD is available in most metropolitan areas. CDPD operates at 19.2 kbps using a TCP/IP type of protocol, which raises the problem of IP addressing because the subnet is mobile. For CDPD to be entirely effective for some applications, a laptop user should be able to disconnect from the LAN, travel to another location while remaining in contact with the network through a wireless connection, reconnect to the LAN at the distant location, and become part of the network again. The process is possible today, but the user needs to understand how to do it. Therefore, it is not yet feasible for the true mobile laptop or PDA application. To implement CDPD, carriers install mobile database stations (MDBSs), which retrieve packets from the wireless network, and a mobile data intermediate system (MDIS), which routes them. Frames are picked up by the MDBS and handed off to the MDIS. Mobile stations use a protocol called Digital Sense Multiple Access with Collision Detection (DSMA/CD) for access to the network. The access method is similar to Ethernet’s Carrier Sense Multiple Access with Collision Detection (CSMA/CD). A station wishing to transmit listens to the outbound channel to determine if a carrier is


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present. If not, it transmits a packet. If so, it waits and attempts a short time later. While TCP/IP is an excellent protocol for wired services, it is not optimized for mobile use, particularly where a perpacket charge applies. Frequent acknowledgment packets are returned by TCP/IP, and, although these are short, unless the charge is byte-oriented instead of packet-oriented, as much as one-third of the cost of a session may be taken up with acknowledgment packets. The hazards of mobile and portable communications make dropped and out-of-sequence packets likely. The main advantage of CDPD is the coverage it can offer. The main population areas of the country are well covered for cellular, and if the carrier elects to overbuild the network with CDPD, data coverage can be equivalent. CDPD is good for short, bursty applications, but for lengthy file transfers, dial-up application over regular cellular may be less costly. The major drawback of CDPD is its narrow bandwidth, which makes it a poor choice for Web surfing.

The 802.11b Wireless Protocol For portable and mobile operations, wireless is the only way to get freedom of movement. The 802.11b protocol, also known as “Wi-Fi” (wireless fidelity) is effectively Ethernet over a radio link. The protocol is a bit different than Ethernet and the throughput is not as high, but the principle is the same. Transceivers can be placed in locations such as offices, hotels, airports, and warehouses, and users can link up with hand-held and laptop devices. Tiny wireless modems fit in a PC card slot and enable users to roam the coverage area. The service can be private or offered by a public service provider, which enables travelers to establish accounts to log on to the Internet. The 802.11b specification is similar to 802.3 with a wireless physical layer that uses DSSS. As with 802.3, the data link layer is subdivided into the MAC and LLC sublayers. The access control protocol is Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). The standard CSMA/CD

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protocol that Ethernet uses is not feasible with wireless because the portable receiver cannot hear the transmissions of all the other units. Collision avoidance requires the stations to broadcast a request-to-send (RTS) packet containing the sending and receiving addresses and the length of the information packet. If the receiving station is in a position to accept a packet, it broadcasts a clear-to-send (CTS) packet. Stations on the network that receive the RTS or CTS packets set internal timers that prevent them from sending during the expected duration, thereby reducing the probability of collisions. Wireless LANs operate on unlicensed industrial, scientific, and medical (ISM) band frequencies in the ranges of 2.4 to 2.4835 GHz and 5.1 to 5.825 GHz. The FCC requires such devices to use spread spectrum modulation with a maximum of 1 watt of power. Most of the wireless LAN products on the market use the 2.4-GHz band, but the 5.1GHz frequencies offer the potential of faster speeds. Since the frequencies are unlicensed, interference is always a possibility, but spread spectrum can distinguish the wanted signal. In the DSSS method used in 802.11b, the band is divided into 14 overlapping 22-MHz channels. The protocol operates over a 2.4-GHz carrier wave that is modulated with different techniques depending on speed. The protocol is designed to sustain the highest throughput possible, which is 11 Mbps under ideal conditions but may step down to as low as 1 Mbps. Since the output power is limited to 1 watt, as the mobile device moves away from the wireless access point, the data rate drops. Binary phase-shift keying is used at 1 Mbps, encoding 1 bit for each phase shift. At 2 Mbps, QPSK is used to encode 2 bits of information per symbol. The top speed of 11 Mbps is achieved with a unique QPSK coding scheme that encodes 8 bits per symbol. The 802.11b protocol promises to be one of the chief methods of obtaining wireless communication. Service providers are beginning to offer Internet access service in public locations such as airports and hotels. ISPs use 802.11b wireless bridges to connect to their subscribers. Inside office buildings, 802.11b substitutes for wired LANs, which is a principal objective of the protocol.


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The main limitation of 802.11b wireless is transmission speed, which is much slower than 100-Mbps Ethernet. Typical speeds do not exceed 1 or 2 Mbps, which is still plenty for most access applications. The IEEE is working on a new protocol, 802.11g, which is intended to double the speed to 22 Mbps. In addition, IEEE 802.11a is being developed to use the 5-GHz band. This protocol operates at 54 Mbps. Both Wi-Fi and Bluetooth operate in the same frequency range, which raises compatibility issues that have yet to be resolved.

LMDS In 1998 the FCC auctioned 1.3 GHz of spectrum in the 28to 31-GHz range for LMDS. In each geographical area, WLL providers bid for the right to use frequencies known as the A block, with 1150 MHz of bandwidth, and the B block, with 150 MHz of bandwidth. LMDS is intended for such services as multichannel video, interactive gaming, streaming video, telephone service, and Internet access. LMDS is a point-tomultipoint service. The service provider locates a hub in the center of a serving area that may serve several thousand homes. Subscribers are equipped with small rooftop antennas and transceivers and feed data into the pipeline using an ATM-like protocol or IP. Multiple hubs are linked with fiber optics. The bandwidths available to customers range from one DS-1 to as much as OC-3. Figure 6.1 shows the architecture of a typical LMDS network. In Canada a similar service is called local multipoint communication systems, and similar services are available in many European countries. LMDS is intended to allow service providers to bypass the copper local loop with an economical service that can be deployed rapidly. There is no need to dig up the streets to place cable, and the scalable nature of the service enables providers to meet customer demand in a few days after the hub is installed. A major objective is to defer the amount of unused investment. With copper wire plant, the ILECs must build initially for what they forecast will be the ultimate service demand because of the high cost of adding more. This

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AM FL Y

PSTN

OC3 to 12

TE

ATM Base Station

Figure 6-1

Local Multipoint Distribution Service Architecture

ATM Core Switch

Internet


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results in unused investment sitting idle until demand develops. With LMDS, the major investment is the hub. The customer premises equipment can be installed only as needed. The base station connects to the wide area network over a fiber-optic link. The structure of the base station is up to the service provider. It could include a telephone switch, highspeed routers for Internet access, backbone pipes to other service providers such as frame relay, and so on. The microwave signal is fed into antennas with narrowly focused beams to transmit the signal in multiple sectors of, perhaps, 30 degrees beam width. Upstream access is typically allocated with TDMA using PSK or QAM modulation. The customer premises configurations will include outdoor microwave equipment connected to an internal distribution network that could be a LAN, T1/E1, or whatever is required by the service the customer subscribes to. A key issue with LMDS is availability, i.e., the percentage of time the service is available for use. Availability figures are quoted as a percentage of total uptime. An availability figure of 99.9 percent would equate to 9 hours of downtime. This is calculated from the following: 365 days × 24 h per day = 8760 h per year × 0.999 = 8751 h of uptime = 9 h of downtime The service provider must take into account factors that cause the signal to fade. With factors such as transmitter output power, receiver sensitivity, and antenna gain held constant, availability will be a function of the distance from the customer’s site to the hub. The longer the signal path, the lower the availability. The maximum distance a subscriber can be located from a cell site while still achieving acceptable service reliability is referred to as the link budget. A link of 8.5 mi (14 km) may be achievable in some climates, while in heavy rainfall areas the link budget may drop to 1.5 mi (2.5 km).

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MMDS MMDS was originally intended as a wireless replacement for cable. For TV signals with 6 MHz of bandwidth, 33 channels can fit into the 200 MHz of spectrum that is allocated for MMDS use between 2.5 and 2.7 GHz. The architecture of MMDS is similar to that of LMDS, consisting of headend equipment similar to that used in a CATV system and receiving equipment at subscriber locations. For video reception, a set-top converter demodulates the incoming signal to the frequency of a conventional television channel. The MMDS signal is transmitted from an omnidirectional antenna. Repeaters may be used to extend the range or to fill dead areas caused by shadows in the coverage area. Although MMDS was initially intended for one-way video, it is now authorized for two-way service, making it applicable to Internet access as well. Typical service offerings provide downstream transmission rates of 1 Mbps or higher, scalable up to 10 Mbps, and upstream speeds up to 512 kbps, which makes MMDS competitive with DSL and cable access. The MMDS spectrum is shared with instructional television fixed service (ITFS), which is intended for distance-learning video. The 6-MHz video channels can be modulated with data signals using the same concepts, and in many cases the same hardware, as cable modems.

Satellite Service One of the major attractions of satellite service has always been the elimination of the local loop. Satellite is ideal for broadcast applications where it is desirable to uplink a signal from one location and downlink it to a vast area. This is the nature of the direct broadcast satellite services that have been competing with cable for the past few years. These services are acceptable for Internet use as well, except that their oneway nature means that a telephone return is required, with its attendant disadvantages.


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Two-way satellite services are readily available, and are feasible for multiple-site operations. This service, known as very-small-aperture terminal (VSAT), uses a small receiver about the size of a PC at each remote site. These terminals are supervised by a central hub having uplinks to a geosynchronous satellite, which orbits the earth at an altitude of 22,238 mi (35,580 km). At this altitude the satellite appears stationary with respect to a point on the earth’s surface. The customer connects to the VSAT hub over a dedicated terrestrial circuit. The hub sends the data signal to the satellite, which converts it to the downlink signal and broadcasts it over the coverage area. The VSAT terminals copy messages with their address and ignore the rest. VSAT operators generally use TDMA to regulate uplink access. VSAT is typically used for applications that have multiple sites and reasonably slow data transmission requirements, such as department stores, service stations, and the like. The principal drawback of satellite services is the time delay. The signal travels from earth to the satellite at the speed of light, but at the distances involved, the round-trip delay of a quarter of a second is still slow for some services. Many data protocols rely on receiving acknowledgements within an expected interval. If the acknowledgement is not received in time, the sender assumes the packet has been lost and retransmits. One solution to this is to use a delay compensator, which returns the acknowledgement to the sending station and communicates with the receiving station over the satellite link using a separate protocol. A better solution, many satellite operators believe, is to use a lower orbit to reduce the delay. Low-earth-orbit satellite (LEOS) is as yet an unproven service. The technology has been proven, but the economic realities have yet to be worked out. The first LEOS system was Motorola’s Iridium satellite network, which failed to fulfill its promise and fell into bankruptcy. A competing network, Teledesic, is not yet in operation. These services are worth following because they may have application in the future.


7

Fiber-optic Access Technology

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f you could see a few decades into the future, what might the access network look like? No one is certain at this point because there are so many conflicting opinions. It seems clear, however, that the twisted-pair wire and coaxial cable of the past must give way to a medium with greater bandwidth. Based on what is now known, fiber optics will be that medium. Today, copper wire, coaxial, and wireless alternatives all rely on fiber trunking to bring information to neighborhood nodes. There, depending on the service provider, the information jumps on a short twisted-pair wire run, coax, or a wireless link of some kind. The fiber stops at the neighborhood node because carrying it all the way to the subscriber is too costly. As long as the infrastructure can be installed before streets and sidewalks are installed, the cost of fiber is supportable, but fiber still has to fit into the existing service provider’s 91 Copyright © 2002 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.

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architecture. It doesn’t pay to design a new fiber architecture that will be installed going forward into new developments and gradually retrofit it into existing developments without having an accepted architecture, and that issue is not settled. The issues are political and economic, not technical. The technology for an all-fiber network exists today. The question is whether the service revenues are high enough to justify the cost. It’s clear that the revenues from the three legs of the stool—voice, video, and Internet—are not of themselves sufficient to support a fiber network to residences. The technology to combine the three on the same fiber can easily be developed, but political considerations intervene. None of the three service provider classes is willing to concede the market to the others. The logical conclusion would be to develop a local information distribution utility that owns the backbone and carries anyone’s information for a fee. In other words, service provision would be separated from ownership of the physical facility. The CLECs have been arguing this point for the past few years. The ILECs own the physical facility now, and also provide telephone and leased-line services. Their motivation to assist the CLECs to share the physical facility is less enthusiastic than support of their own services. Furthermore, the process is unbelievably complex. While separating the ownership of the physical facility from the provision of subscriber services is technically possible, it would require some major changes that the ILECs would undoubtedly oppose. For larger businesses, fiber is readily available in most metropolitan areas today. ILECs and competitive access providers have constructed fiber networks to serve concentrations of population. Fiber is available in most major office buildings, and more is being installed daily.

Overview of Fiber-optic Technology To appreciate the issues involved in applying fiber in the loop, a basic understanding of the technology is needed. This section provides a brief overview. For more detailed information


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on fiber optics, the I-book Optical Networking is available at http://shop.mcgraw-hill.com/cgi-bin/pbg/indexebooks.html. Fiber is a hair-thin waveguide made of ultra-pure glass. The core of the fiber is purer than the outer cladding, and guides light pulses along the way. When pulses strike the cladding, they are refracted so that they follow the path defined by the core. Fiber comes in two varieties: singlemode and multimode. Multimode fiber has a relatively wide core of 50 or 62.5 µm (millionths of a meter). As a light pulse passes down the fiber, it takes multiple paths, which leads to rounding of the pulse, a condition known as dispersion. Because of dispersion, multimode fiber has higher loss than single-mode fiber, which has a core of 9 to 10 µm. The core of single–mode fiber is so small that the light pulse is not dispersed by reflections. Multimode fiber is largely confined to building and campus applications. Metropolitan and wide area networks use single-mode fiber almost exclusively. On the transmitting end, electrical pulses are fed into a tiny laser, which injects light pulses into the fiber. The light pulses travel through the core to the distant end, where an avalanche photodiode converts them back to an electrical signal. The distances fiber can span are a function of numerous factors such as transmitter output power, receiver sensitivity, fiber loss, and miscellaneous losses that result from couplers and splices. Fiber can connect most metropolitan areas without regeneration, although nonregenerative light amplifiers may be needed to boost the signal. Fiber operates below the visible spectrum. Three transmission windows are typically available: 850, 1300, and 1550 nm. A single fiber can carry light pulses simultaneously at these three different wavelengths by using a technology known as wave-division multiplexing (WDM). Long-haul fibers often employ dense wave-division multiplexing (DWDM), in which 40 or more wavelengths operate simultaneously. DWDM can also be used in metropolitan areas, but generally it is less expensive to install multiple fibers than to invest in DWDM equipment. Fiber is deployed in one of three topologies. Point-to-point is a common configuration for fiber originating in a location

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such as a central office and radiating out directly to the users. Fiber installed in this configuration is vulnerable to failure, and, because of its enormous carrying capacity, a failure may disrupt many important services. Service providers may offer self-healing service, switching to a protection fiber with only a few milliseconds of delay. To gain the maximum protection from this and other self-healing services, full route diversity should be reviewed. If any portion of the main fiber path is carried over the same cable route as the protection fiber, the degree of protection is less than complete. Even if the fibers run in separate cable sheaths, a single event such as a fire or dig-up is likely to kill both paths. Greater protection is secured with a self-healing ring configuration. SONET multiplexers are generally capable of both add-drop and self-healing protection. The fact that the carrier has deployed its fiber in a metropolitan ring does not necessarily mean that the ring is self-healing. The carrier may provide point-to-point service without self-healing capability. The same precautions mentioned in the preceding paragraph about single points of failure must be reviewed. A self-healing ring is ineffective protection if both directions of transmission are in the same cable sheath or the same conduit run. Also, be on alert for spurs, which are unprotected fiber segments that branch off the protected ring. The third configuration is a branching tree, which has a topology similar to that of a cable network. This architecture is used in the PON. As a general rule, fiber is the only solution when bandwidths greater than T1/E1 are required. Multiple T1/E1s can be used on copper, but the point is reached where it is more economical to provide service over fiber. The primary drawback of fiber is the expense of demultiplexing it. When a service provider brings fiber into a building, the minimum bandwidth is usually OC-3, which is capable of supporting 84 DS-1s. That is a lot of bandwidth for most users, with the exception of ISPs, but it is only a fraction of what a fiber pair is capable of carrying. The multiplexing equipment to break this down to T1/E1 is expensive and can be economically justified only where the service provider can support multiple customers. Therefore, fiber


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has been supplied primarily to carriers, large end users, and large office complexes. As discussed later, the PON may change that situation. From a user’s point of view, an important issue is whether to obtain fiber bandwidth or dark fiber (fiber without the electronics to illuminate and multiplex it). Many common carriers, as a matter of policy, do not provide dark fiber for variety of reasons. For one thing, the fiber is rarely located where the user wants it without extensive cutting, splicing, and rearranging, the result of which may be small isolated fiber segments that have no commercial use. More important to most carriers is that fiber bandwidth can be multiplexed to carry enormous quantities of information and multiplexing is far more profitable than leasing dark fiber. Some nontraditional providers, however, are willing to provide dark fiber, particularly if they have more than they expect to sell service on in the foreseeable future. Companies that have access to the public right-of-way, such as power companies, gas and water utilities, CATV companies, and even some municipalities, may have idle capacity and be willing to lease dark fiber. From a user’s standpoint, multiplexing dark fiber is inexpensive. Gigabit Ethernet, for example, provides a lot of capacity for a wide variety of applications. Gigabit switches are designed to interface fiber. The bandwidth is sufficient and voice and video can be supported without undue concern for QoS issues. If 1 Gbit is not enough, 10-Gbit Ethernet is on the near horizon. Fiber optics is the default transmission medium for gigabit and 10-Gbit Ethernet. Many industry observers believe that Ethernet will eventually take the place of SONET, largely because it is simpler and less expensive to deploy.

The PON The PON is a logical physical structure for single-pipe information services in the local loop. Conceptually, the service is not much different than cable except for the physical medium, which provides significantly more bandwidth. Figure 7.1 shows the PON architecture. It is typically installed with a

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tree topology, but it can also be a star or ring. The network is passive in that power is not required except at the originating and terminating ends. The fiber-optic trunk network starts at a central location, which may be the same as or completely distinct from the ILEC wire center. Fiber is not limited to the loop length that constrains copper wire, so the center of the PON network could be anywhere. The network is capable of extending up to 12 mi. A single fiber is used for both upstream and downstream transmissions. The data stream is reflected into the various legs of the fiber through passive splitters. Each group of 32 or more subscribers shares the bandwidth of one fiber. A PON can deliver up to 622 Mbps downstream to the users and up to 155 Mbps upstream. The PON can also serve as a trunk between a larger system, such as a CATV system, and a neighborhood, building, or home. Since nothing installed in the middle is specific to the bit rate, the PON can carry any kind of data, analog or digital. The fiber originates in an optical line termination (OLT) at the service provider’s headend. It terminates at the subscriber’s end in an optical network unit (ONU) that performs the optical-to-electrical conversion, likely connecting to an Ethernet LAN. The ONUs listen to the downstream addresses, copying the packets addressed to them and discarding the rest. In the upstream direction, ONUs are assigned time slots and are permitted to transmit in turn. The current architecture has no way of using vacant time slots, but the bandwidth is high enough that it doesn’t matter. PONs can use either Ethernet or ATM as their protocol. To distinguish between the two, they are known respectively as EPONs or APONs. The choice between the two rests on which protocol the service provider furnishes. As with DSL, ATM has an advantage in that the ILEC as service provider normally uses ATM in its backbone. Ethernet, however, is a simpler protocol that supports full-length frames in contrast to the 48-byte payload of an ATM cell. PONs have not been widely developed yet. The ITU has assigned G.983.1 to the protocol. The Full Service Access Network Initiative (www.fsanet.net) is a group of telecom-


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ONU ONU

Optical Line Termination

Feeder Fibers

ONU

ONU

Passive Splitters ONU

ONU Central Office

ONU = Optical Network Unit

ONU ONU

Figure 7-1 Passive Optical Network

munications companies and equipment suppliers that are working on PON standards that have been presented to several standardization bodies. SBC Corporation has announced the use of PONs in its Operation Pronto, which is intended to bring broadband service to about 80 percent of its subscribers.


8

Application Considerations

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he choice of access technology depends to a large degree on what is available. For Internet access, the only universally available choice is dial-up, which is a poor alternative for full-time access. Even ISDN with its 128 kbps of bandwidth is insufficient for many users, and, if it is a measured service, the cost may be prohibitive. Only the smallest businesses will find dial-up access satisfactory, not only because of a lack of bandwidth, but also the need for multiple accounts, the difficulty of distributing e-mail, and numerous lesser problems. For a business with multiple employees who must have access to e-mail, a full-time connection is essential. At the other end of the scale are large businesses that directly engage in electronic commerce. For these, T1/E1 and higher multiples are the only alternative. In between are residences and countless smaller businesses that cannot justify the cost of T1/E1. The choice may boil down to the question of what is available. Cable 99 Copyright © 2002 by The McGraw-Hill Companies, Inc. Click here for Terms of Use.

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access is available only in localities in which the cable operator has upgraded the facility to two-way and provides access service over the cable. This excludes many, if not most, small communities and rural areas, at least in the near term. DSL also has availability problems. First is the question of loop length and bridged tap, which probably precludes DSL for around 40 percent of telephone subscribers. In many cases the lack of availability isn’t technical, but is simply due to the fact that no one offers the service. A certain critical mass of subscribers is needed to make it economical to install the DSLAMs and connecting data networks. Many rural communities have no fiber-optic link to the outside world, which may mean that backbone capacity isn’t available. SBC Corporation, which serves at least a third of subscribers in the United States, has announced its Project Pronto, which is designed to bring DSL to a least 80 percent of its subscribers. This still leaves a substantial percentage of users and exchanges for which DSL is available but that still cannot get the service. Furthermore, many smaller communities simply do not have enough potential subscribers to make it worthwhile for the LECs to provide DSL. Internet access is not defined as a lifeline service and DSL is generally not regulated, so market conditions dictate its availability. This leaves wireless as an alternative. Terrestrial wireless depends on having a line of sight, which precludes it for a significant number of potential subscribers. Furthermore, it is a shared medium and therefore is subject to the same kind of bandwidth limitations that cable experiences. Satellite technology circumvents line-of-sight requirements, but radio uplinks are expensive. Many satellite services use the telephone line for the upstream direction. This is acceptable for Internet service for residences, but it requires a second telephone line, which is expensive. A telephone uplink for businesses is unsatisfactory because of bandwidth limitations for e-mail attachments.

The Oversubscription Issue All telecommunications service providers oversubscribe their bandwidth. By oversubscription, we mean that the total


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potential demand is more than the available capacity. The principle is built into every telecommunications network because the service provider can predict with some degree of confidence that users will not all require service simultaneously. Occasionally, abnormalities such as storms, national emergencies, earthquakes, and critical news events cause traffic to exceed the peaks against which the network was designed. In such a case, every network has some method of protecting itself from collapse while still providing service to crucial subscribers. The Internet, in particular, has numerous chokepoints, and it is impossible for the end user to determine where the bandwidth restriction actually is. A dial-up user knows that about 53 kbps downstream and 33.6 kbps upstream is the maximum a modem can transfer, so the other chokepoints in the network are probably irrelevant. The objective of subscribers with always-on access is to eliminate the access restriction or at least to provide predictable performance. DSL can deliver predictable performance, but cable cannot because of the way it is designed. The only way a service provider can reduce the oversubscription ratio is by reducing the number of households served by a coaxial segment, which is expensive.

Voice and Video Service Most of the applications we have discussed are asymmetric in nature, so the limited upstream bandwidth of cable, wireless, and ADSL present no problems. Once voice and video are added to the equation, however, a symmetrical channel is required. Furthermore, these services, which can be lumped under the label voice over IP (VoIP), have other characteristics that impose entirely different requirements on the network than data. First, VoIP operates in real time. A packet stream leaving the transmitter must not be delayed on its way to the receiver or intelligibility is lost. Anyone who has talked over a satellite circuit knows the delay can be disconcerting unless the parties to the conversation change their normal mode of talking. The nature of the IP protocol is that

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packets do not arrive evenly spaced. This variation in spacing between packets is called jitter. If allowed to remain, jitter distorts the voice and further affects intelligibility. Buffering packets and releasing them in a steady stream can cure jitter, but this adds to delay, so it is effective only within limits. These factors are discussed in more detail in the McGraw-Hill I-book Voice and Video over IP, available at http://shop.mcgraw-hill.com/cgi-bin/pbg/indexebooks.html. Another difference is that error correction is unnecessary with voice. Bit errors that would render a data file unusable are of no consequence in voice and video, and furthermore, real-time services cannot tolerate the time delay involved in retransmission. Therefore, VoIP runs under UDP instead of TCP. Since data cannot tolerate errors, this makes it impractical to combine it with voice at the source. Since the ILECs own the cable plant, they have little interest in deviating from their traditional architecture. The CLECs, however, must lease cable pairs, so they have considerable interest in increasing the carrying capacity through the use of VoDSL. Because of the nature of voice sessions, one of the symmetrical DSL options—generally SDSL—is used. As we have seen, the loop length is limited to about 10,000 ft (3,000 m) from the central office, which means that VoDSL is not a universal alternative. It has an advantage over cable, however, in that the upstream direction is not a shared medium. Moreover, if the line protocol is ATM all the way to the subscriber, the delays inherent in IP are avoided. Voice over cable is feasible and is part of the serving plan of large cable service providers. The latest version of DOCSIS supports voice over cable networks, but the service has not yet been widely used.

Branch Offices and Telecommuters The old model of a fixed office where everyone reports to work in a central building is largely a thing of the past. Today’s office has multiple locations, and many pressures


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weigh in favor of enabling workers to access the corporate network from home or in a neighborhood office. Crowded highways, air pollution standards, and personal preference make it desirable to enable people to work away from the office at least part of the time. Telecommuting solves staffing problems for companies that need call center agents during calling peaks, and is an ideal application for workers who have small children, disabilities, or other reasons for working less than full time. Remote office alternatives have been limited until recently. For corporate branch offices the choices have been expensive and not truly suitable. A 64-kbps fixed or frame relay access circuit is insufficient for remote applications that need wide bandwidth on demand. T1/E1 is plenty of bandwidth, but it is expensive with nothing in between. Now, new access technologies, principally DSL, can provide a satisfactory alternative under the right conditions. The conditions have to do with the suitability of the network between the remote and central locations. QoS standards are not yet fully developed. When they are, it will probably be years before the Internet can provide reliable transport. Other alternatives may be feasible if plenty of bandwidth is available.

T1/E1 Access If a full point-to-point T1/E1 can be justified to a branch office, the same bandwidth is available in both the access circuit and the backbone and the facility has plenty of stability to handle both voice and data. T1/E1 is an ideal medium if the bandwidth requirements justify the cost. This is often the case with branch offices, but it is rarely justified for telecommuting.

DSL Access If the branch or home office is within range, DSL can be an ideal medium. Scalability problems that are typical of T1/E1 service are resolved, and the service provider’s architecture

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may solve the QoS issue. As we have seen, most ILECs use ATM in the access circuit and in their backbone. If a single carrier provides the service end-to-end, the quality should be controllable within tight limits and could be enough to support both voice and data. Many PBX manufacturers provide a proprietary IAD that works with their digital telephone. Security is tight, and the telecommuter or branch office worker should have facilities equivalent to a station collocated with the PBX. Quality is less predictable when more than one service provider is involved. In a metropolitan area with two ILECs, if each one has an ATM network with a network-to-network interface, the service may be equivalent to the ATM network of a single provider. When the network extends between two metropolitan areas, it may be necessary to connect them over an IXC’s IP or ATM backbone. Some IXCs provide service level agreements (SLAs) that may be high enough in quality to support a branch office or telecommuting application. Be aware, however, that many IXCs quote their SLAs as an average over 30 days. To obtain satisfactory quality of service, you must know the worst-case delay and jitter figures to determine whether quality will be satisfactory to support the application.

Cable and Wireless Cable and wireless can also be used for telecommuting access, with certain provisos. First is the question of whether the service provider permits any but its own services to use its facility. If it does not, it may not be possible to control the path sufficiently to obtain satisfactory QoS. In addition, the nature of the medium, with its asymmetric data flow and shared bandwidth, may render it unsatisfactory for voice communication unless the service provider has a method of prioritizing voice packets. Branch offices can often use the PSTN for all voice communication, leaving the private network to handle only data. This is not the case with most home office applications,


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however. Telecommuters usually prefer to have real-time access to both voice and data, receiving telephone calls and e-mail as if they were in the office. DSL is clearly the best access medium for this application.

Dial-up access For some access applications, dial-up is the only feasible alternative. Despite the bandwidth limitations of the PSTN, it is almost universally available, although not necessarily at a low cost. Many ISPs provide dial access to the Internet as well as e-mail. A premium price may be charged, and access is available almost anywhere, but it is almost precluded as a telecommuting medium because of lack of bandwidth. Some PBX manufacturers provide devices to enable the use of a digital telephone over an analog loop, but these are not highly effective for full-time telecommuting. If bandwidth limitations prevent the use of dial-up, then ISDN may be an alternative. Basic rate ISDN suffers from the same problems as DSL, however, in that it has an 18,000ft (5,500 m) limitation. Furthermore, unless the channels are bonded, the bandwidth may not be enough of an improvement over analog to justify the extra cost. BRI is often metered, even in locations where flat-rate analog is available, and an always-on connection to the Internet or headquarters can be prohibitively expensive. Finally, BRI suffers from a general lack of availability in the United States. Many European countries have much better ISDN availability, but many central offices in the United States have not been equipped for BRI. Some multioffice wire centers have ISDN available in one switching system, but a number change may be required to switch from analog to ISDN service. One promising service, always-on dynamic ISDN (AO/DI), uses the D channel for a full-time connection to the Internet. If more bandwidth is needed for Web surfing or to transfer an e-mail file attachment, the service brings a B channel online long enough to provide the necessary bandwidth, after which it disconnects. The theory of AO/DI is

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appealing, but the practice is much less so because few LECs are equipped to offer the service.

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DSL can be an effective network access method for both frame relay and point-to-point data services. The primary consideration is whether the providers have a service offering that takes advantage of DSL as the access method. Nothing technical prevents the data network provider from connecting over the ILEC’s network or that of a competitive DSL provider. Security becomes a concern when the network goes outside the bounds of the service provider’s closed network and connects to the public Internet. A network that can be reached over the Internet is technically known as a virtual private network (VPN). The access circuit for a VPN can be DSL, cable, dedicated, or wireless. Any of these should work well for typical VPN applications, which are e-mail and file transfer. Time-sensitive applications such as voice require tight control over latency. Before using shared media technology or any Internet connectivity for a voice VPN, QoS must be carefully considered. For a straight data VPN, any of the access technologies should be satisfactory. Security considerations, which include authorization, authentication, encryption, and firewalling, must be taken into account. These functions are normally included in VPN access devices.

Summary and Conclusions The principal conclusion that most users will reach after studying the access arena is that the perfect solution does not exist. Every technology discussed in this volume has its drawbacks, and it is important that prospective subscribers understand them. The first factor affecting the choice is like-


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ly a simple question of what is available. A significant percentage of users will go no further than a POTS line with a V.90 modem—not because it is ideal, but just because it is available and affordable. For many users, dial-up is not an acceptable option because of the nature of the application. Data wide area networks will require some form of dedicated access. If DSL is not available or if it has insufficient bandwidth, then the choice turns to T1/E1 or a 56/64-kbps access loop. The latter will normally be of interest only for business applications. Dedicated 56/64-kbps connections lack the necessary bandwidth and T1/E1 is prohibitively expensive. Exceptions to these general comments will occur, but they are rare. Except for telecommuting, most residential data applications are confined to Internet access. If DSL and cable are both available, and if the prices are competitive, the deciding factor is performance. It is impossible to generalize which will provide the best performance. The answer is that it depends on a variety of factors including distance, number of users sharing the medium, time of day, and the ISP’s service quality. The best way to evaluate the service is to ask questions of other users. Unfortunately, what they tell you today may not be valid tomorrow.


Appendix 1: Glossary

AAL: ATM adaptation layer. ABR: available bit rate. AC: alternating current. ADSL: see asymmetric digital subscriber line. alternate mark inversion (AMI): the T carrier line-coding system that inverts the polarity of alternate ones bits. always-on dynamic ISDN (AO/DI): an ISDN service that keeps the D channel actively connected to a service provider. B channels are called in as needed. AMI: see alternate mark inversion. ANSI: American National Standards Institute. AO/DI: see always-on dynamic ISDN. API: application program interface. ARP: Address Resolution Protocol.

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

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asymmetric digital subscriber line (ADSL): a technology for multiplexing a high-speed data or compressed video signal above the voice channel in a subscriber loop. asynchronous transfer mode (ATM): a broadband connectionoriented switching service that carries data, voice, and video information in fixed-length 48-octet cells with a 5-octet header. ATM: see asynchronous transfer mode. AWG: American wire gauge. bandwidth: the range of frequencies a communications channel is capable of carrying without excessive attenuation. baseband: a form of modulation in which data signals are pulsed directly on the transmission medium without frequency division. basic rate interface (BRI): the basic ISDN service consisting of two 64-kbps information or bearer channels and one 16-kbps data or signaling channel. baud: the number of data signal elements or symbols per second a data channel is capable of carrying. B channel: the 64-kbps “bearer” channel that is the basic building block of ISDN. The B channel is used for voice and circuit switched or packet switched data. bin: frequency segments into which a discreet multitone DSL line is divided. binder group: a 50-pair group of cable pairs that is bound together with a colored binder. BRI: see basic rate interface. bridge: circuitry used to interconnect networks at the MAC layer. bridged tap: any section of a cable pair that is not on the direct electrical path between the central office and the user’s premises, but is bridged onto the path. broadband: a term used to describe high-bandwidth transmission of data signals. Technically, any signal greater than primary rate (T1/E1). cable modem termination system (CMTS): a system of interfacing the cable network with a data network. CAP: carrierless amplitude phase; competitive access provider. Carrier Sense Multiple Access with Collision Detection (CSMA/CD): a system used in contention networks where the network interface unit listens for the presence of a carrier before


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attempting to send and detects the presence of a collision by monitoring for a distorted pulse. CATV: community antenna television. CBR: constant bit rate. CDPD: cellular digital packet data. central office (CO): a switching center that terminates and interconnects lines and trunks from users. channel service unit (CSU): an apparatus that terminates a T1 line providing various interfacing, maintenance, and testing functions. CLEC: see competitive local exchange carrier. CM: cable modem. CMTS: see cable modem termination system. CO: see central office. competitive local exchange carrier (CLEC): a company offering local service in competition with an incumbent local exchange carrier. complement: a group of 50 cable pairs (25 pairs in small cable sizes) that are bound together and identified as a unit. CRC: cyclical redundancy checking. cross-connect: a wired connection between two or more elements of a telecommunications circuit. crosstalk: the unwanted coupling of a signal from one transmission path into another. CSMA/CA: Carrier Sense Multiple Access with Collision Avoidance. CSMA/CD: see Carrier Sense Multiple Access with Collision Detection. CSU: see channel service unit. CTS: clear-to-send. Data over Cable Interface Specification (DOCSIS): an industry specification for providing data and voice communications over CATV. DC: direct current. D channel: the ISDN 16-kbps data channel that is used for outof-band signaling functions such as call setup. DCS: digital cross-connect system.

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dedicated circuit: a communications channel assigned for the exclusive use of an organization. dense wave-division multiplexing (DWDM): the process of multiplexing fiber optics with multiple wavelengths—40 or more with today’s technology. DHCP: see Dynamic Host Configuration Protocol. digital loop carrier (DLC): A multichannel digital device that enables several subscribers to share a single facility in the local loop. digital subscriber line access multiplexer (DSLAM): a device in the central office that splits voice and data signals and connects voice to the PSTN and data to a high-speed backbone. dispersion: the rounding and overlapping of a light pulse that occurs at different wavelengths because of reflected rays or the different refractive index of the core material. DLC: see digital loop carrier. DMT: discrete multitone. DNS: see domain name service. DOCSIS: see Data over Cable System Interface Specifications. domain name service (DNS): a service that translates host names to IP addresses. downstream: data flowing from the headend to the subscriber. DSL: digital subscriber line. DSLAM: see digital subscriber line access multiplexer. DSMA/CD: Digital Sense Multiple Access with Collision Detection. DSSS: direct sequence spread spectrum. DWDM: see dense wave-division multiplexing. Dynamic Host Configuration Protocol (DHCP): a protocol that allocates IP addresses to network clients at startup. echo canceler: an electronic device that processes the echo signal and cancels it out to prevent annoyance to the talker. ESF: see extended super frame. ETSI: European Telecommunications Standards Institute. extended super frame (ESF): a T1 carrier framing format that provides 64-kbps clear channel capability, error checking, 16-state signaling, and other data transmission features. FCC: Federal Communications Commission.


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FDM: see frequency-division multiplexing. FDMA: frequency-division multiple access. FEC: forward error correction. firewall: a device that protects the connection between a network and an untrusted connecting network such as Internet. The firewall blocks unwanted traffic from entering the network and allows only authorized traffic to leave. frame relay: a data communication service that transports frames of information across a network to one or more points. Cost is based on three elements: committed information rate, access circuit, and port speed. frequency-division multiplexing (FDM): dividing the bandwidth of a transmission medium by separating frequency segments. FSK: frequency-shift keying. FTP: File Transfer Protocol. Gbps: gigabits per second. G.lite: a modified type of ADSL that enables users to connect to a DSL line without a splitter. HDSL: see high-speed digital subscriber line. HFC: see hybrid fiber-coax. high-speed digital subscriber line (HDSL): a protocol for delivering T1/E1 over two pairs of wire for up to 12,000 ft (4,000 m). hybrid: a multiwinding coil or electronic circuit used in a fourwire terminating set or switching system line circuit to separate the four-wire and two-wire paths. hybrid fiber-coax (HFC): a cable television transmission method that uses a combination of fiber optics to a neighborhood node and coaxial cable to the subscribers. IAD: integrated access device. IDSL: see ISDN digital subscriber line. ILEC: see incumbent local exchange carrier. impedance: the ratio of voltage to current in an alternating current electrical circuit. in-band signaling: telephone signaling in which the signals are carried in the talking path. incumbent local exchange carrier (ILEC): the traditional telephone company that serves a particular franchised area.

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integrated services digital network (ISDN): a set of standards promulgated by ITU-T to prescribe standard interfaces to a switched digital network. interexchange carrier (IXC): a common carrier that provides long-distance service between local access transport areas. IP: Internet Protocol. ISDL: see ISDN digital subscriber line. ISDN: see integrated services digital network. ISDN digital subscriber line (IDSL): a DSL protocol that uses a basic rate ISDN signal. Provides symmetrical speeds of 144 kbps. ISM: industrial, scientific, and medical. ISO: International Standards Organization. ISP: Internet service provider. ITFS: instructional television fixed service. ITU: International Telecommunications Union. IXC: see interexchange carrier. jitter: variation in arrival intervals of a stream of packets. kbps: kilobits per second. kHz: kilohertz. LAN: local area network. LEC: see local exchange carrier. LEOS: see low-earth-orbit satellite. link budget: the amount of loss that can be tolerated between the two ends of a link while still maintaining the required service quality. LLC: logical link control. LMDS: see local multipoint distribution service. local exchange carrier (LEC): any local exchange telephone company that serves a particular area. See also competitive local exchange carrier; incumbent local exchange carrier. local multipoint distribution service (LMDS): a microwavebased service that serves multiple voice and data users from a central base station hub. low-earth-orbit satellite (LEOS): a global personal communications service technology that uses a constellation of satellites orbiting earth at a few hundred miles up for communications with hand-held units.


115

MAC: media access control. main distributing frame (MDF): the cable rack used to terminate all distribution and trunk cables in a central office. Mbps: megabits per second. MCU: multipoint control unit. MDBS: mobile database station. MDF: see main distributing frame. MDIS: mobile data intermediate station. MHz: megahertz. MMDS: multipoint multichannel distribution service. modulation: the process by which some characteristic of a carrier signal, such as frequency, amplitude, or phase, is varied by a lowfrequency information signal. MPEG: Motion Picture Experts Group. multiple plant: cable plant in which cable pairs are connected in parallel. near-end crosstalk (NEXT): the amount of signal received at the near end of a circuit when a transmit signal is applied at the same end of the link. neighborhood gateway: an enclosure that houses telephone equipment such as DSL or digital line carrier in a neighborhood to shorten the length of the wire run. NEXT: see near-end crosstalk. NIC: network interface card. Open Systems Interconnect (OSI): a seven-layer data communications protocol model that specifies standard interfaces that all vendors can adapt to their own designs. PBX: private branch exchange. PCM: see pulse code modulation. PCS: personal communication service. PDA: personal digital assistant. PDU: protocol data unit. permanent virtual circuit (PVC): in a data network a PVC is defined in software. The circuit functions as if a hardware path is in place, but the path is shared with other users. plain old telephone service (POTS): the standard analog dial-up service provided by the PSTN.

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PON: passive optical network. POP: point of presence. POTS: see plain old telephone service. PRI: see primary rate interface. primary rate interface (PRI): in North America, a 1.544-Mbps information-carrying channel that furnishes ISDN services to end users and consists of 23 bearer channels and one signaling channel. In Europe, a 2.048-Mbps channel consisting of 30 bearer and two signaling channels. private line: see dedicated circuit. protocol: the conventions used in a network for establishing communications compatibility between terminals and for maintaining the line discipline while they are connected to the network.

AM FL Y

provisioning: the process of assembling all of the elements that make up a circuit. PSD: power spectral density. PSK: phase-shift keying.

PSTN: see public switched telephone network. PTT: postal telephone and telegraph.

public switched telephone network (PSTN): a generic term for the interconnected networks of operating telephone companies.

TE

pulse code modulation (PCM): a digital modulation method that encodes a voice signal into an 8-bit digital word representing the amplitude of each pulse. PVC: see permanent virtual circuit. QAM: see quadrature amplitude modulation. QoS: quality of service. QPSK: see quadrature phase-shift keying. quadrature amplitude modulation (QAM): a method of modulating digital signals on a carrier using both amplitude and phase modulation. quadrature phase-shift keying (QPSK): a method of modulating digital signals on a carrier encoding two digital bits on each of four phase states. RADSL: rate-adaptive digital subscriber line. ranging: the process by which a cable modem evaluates the time delay in transmitting to the headend.


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RF: radio frequency. RTS: request-to-send. SAR: segmentation and reassembly. SDH: synchronous digital hierarchy. SDSL: see single-pair digital subscriber line. serving wire center: the ILEC wire center that serves a particular subscriber. short: a circuit impairment that exists when two conductors of the same pair are connected at an unintended point. sidetone: the sound of a talker’s voice audible in the handset of the telephone instrument. single-pair digital subscriber line (SDSL): a protocol similar to HDSL except that it uses only one cable pair. It also provides a POTS line under the data. SLA: service level agreement. SNMP: Simple Network Management Protocol. SNR: signal-to-noise ratio. SONET: synchronous optical network. splitter: a device that connects and splits two different media types. For example, a DSL splitter separates the data and telephone signals. Also, a device used to connect branches of a cable system. spread spectrum: a radio modulation method that transmits a signal over a broad range of frequencies (direct sequence method) or rapidly jumps from one frequency to another (frequency hopping). Provides excellent security and resists interference. SVC: switched virtual circuit. SWC: serving wire center. symbol: see baud. tap: a device for connecting a subscriber’s premises to a coaxial CATV line. The tap isolates the subscriber from other services on the coax. TCP/IP: Transmission Control Protocol/Internet Protocol. TDM: see time-division multiplexing. TDMA: time-division multiple access. time-division multiplexing (TDM): a method of combining several communications channels by dividing a channel into time increments and assigning each channel to a time slot. Multiple

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channels are interleaved when each channel is assigned the entire bandwidth of the backbone channel for a short period of time. time slot: in a TDM system, a unit of time sufficient for the transmission of 8 bits that is assigned to each byte. This unit of time recurs at the same instant in a transmission frame. TRI: telephony return interface. trunk: a communications channel between two switching systems equipped with terminating and signaling equipment. UDP: User Datagram Protocol. UNE: unbundled network element. upstream: data flow direction from the subscriber to the headend. URL: uniform resource locater. USB: universal serial bus. UTP: unshielded twisted pair. VBR: variable bit rate. VC: virtual channel. VCI: virtual channel identifier. VDSL: see very-high-bit-rate digital subscriber line. very-high-bit-rate digital subscriber line (VDSL): a DSL type that delivers 13 to 52 Mbps downstream and 1.5 to 2.3 Mbps upstream over a single copper twisted pair over a short range. video on demand (VOD): the delivery of video services to customers in response to their specific request. VOD is contrasted to conventional cable television, where all channels are delivered over the medium. VOD: see video on demand. VoDSL: voice over DSL. VoIP: voice over IP. VPI: virtual path identifier. VPN: virtual private network. VSAT: very-small-aperture terminal. WDM: wave-division multiplexing. WLL: wireless local loop. 2 binary 1 quaternary (2B1Q): a line-coding technique in which 2 bits are mapped to one quaternary symbol.

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