Cisco Press - Design and Implementation of DSL

7/30/2019 Cisco Press - Design and Implementation of DSL

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Design and Implementation

of DSL-Based AccessSolutions

Design and Implementation of DSL-Based Access Solutions addresses variousarchitectures for DSL-based networks. It focuses on how to design and implement an

end-to-end solution for service providers, considering various business models suchas retail, wholesale, VPN, etc.

This book depicts the different architectures, and helps you understand the keydesign principles in deploying them. It covers both access encapsulations such asbridging, PPPoA, PPPoE, and routing, as well as core architectures such as IP, L2TP,

MPLS/VPN, and ATM. Because it focuses on end-to-end solutions, Design andImplementation of DSL-Based Access Solutions talks about how to do massprovisioning of subscribers and how to manage networks in the most efficient way. Italso includes discussions of real-life deployments, their design-related issues, and

their implementation.


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About the Authors

Sanjeev Mervana, CCIE #4006, has over 10 years of experience in networkingand has been with Cisco Systems since 1998. A CCIE since 1998, Sanjeev was theTechnical Leader in the Customer Support Organization at Cisco for resolving

complex networking issues before moving to the Technical Marketing Group. Sincejoining the Technical Marketing Group, Sanjeev has established a lead in definingbroadband architectures for customers and has published several service architecture

papers internally to Cisco as well as for customers. His primary job involves him inarchitectural discussions with leading service providers to offer various value-addservices. Sanjeev has been a key contributor and instrumental in defining some of

the requirements for next-generation products at the edge of the network.

Chris Le, CCIE #5235, is a Technical Marketing Engineer for the Service ProviderLine of Business for Cisco Systems. Chris has provided DSL internetworking design,

implementation, and performance engineering to several service providers. He hasworked extensively on design and implementation in the IP aggregation space and

has provided training to engineers on leased-line and broadband aggregation.


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About the Technical Reviewers

Kumar Reddy is Manager of Technical Marketing at Cisco Systems. Kumar hasauthored a number of technical papers and presentations for both internal andcustomer audiences in the area of broadband aggregation and is a regular technical

presenter and trainer at Cisco events. He works extensively with Cisco customersand pre-sales teams on service architectures and deployments for IP accessnetworks. Prior to joining Cisco, Kumar worked in Paris, teaching and developing

network protocols and software.

Jay Thontakudi is a Technical Marketing Engineer with Cisco, working in the fieldon architectural design, network migration, and issues related to DSL. He has been

with Cisco since 1999. Jay's prior experience includes nine years in the oil andnatural gas industry with job functions in project engineering, process controlnetworks, and enterprise networking. Jay holds a masters degrees in computer

science and mechanical engineering.

Brian Melzer, CCIE #3981, is an Internetwork Solutions Engineer for ThruPoint,Inc., out of its Raleigh office. He has worked as a consultant for ThruPoint since

September 2000. Thrupoint is a global networking services firm and one of the fewcompanies selected as a Cisco System Strategic Partner. Before working forThrupoint he spent five years working for AT&T Solutions on the design and

management of outsourcing deals involving Fortune 500 clients. As a member of theWolfpack, Brian received his undergraduate degree in electrical engineering and hismasters degree in management at North Carolina State University.


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Acknowledgments

We would like to thank Stephane Lamarre first for encouraging us with writing DSLarchitecture white papers that were the basis and inspiration for this book. Thanks toCharles Ford with his expertise in DSL and to Jay Thontakudi for his expertise in IP

DSL switching. Special thanks to Kumar Reddy and Jay for their dedication inreviewing the book, for their constructive suggestions and criticisms, and for keepingus honest.

Our thanks also go to John Kane and Christopher Cleveland from Cisco Press whoassisted us with producing this book and believed in our ideas.

Last but not least, we'd like to thank our Technical Marketing Engineer colleagueswhom we learned so much from and made our work much more enjoyable.


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Introduction

This book is aimed at network engineers who need to have an understanding of DSLtechnology and how it is deployed in real networks today. Some backgroundinformation is provided so that readers can appreciate the problems that DSL can

solve and understand some of the challenges still ahead. Readers need to know basicIP operation in order to easily grasp the concept of different ATM encapsulationsoften used in this book. Although not required, knowledge of dial-up networks may

help readers breeze through the encapsulation portion of this book because the sametechnology used in dial-up networks is also used in DSL aggregation.

Not only network engineers can benefit from this book. Network designers can also

enjoy reading the case studies and the pros and cons of each of the encapsulationmethods. Case studies are provided with in-depth discussion of severalimplementation options, from a small DSL network to a large network supporting

millions of subscribers.

Motivation for the Book

There are many books available in the market today that explain how DSL works atLayer 1, its modulation techniques, and so forth. There are also certain books thatcover some of the access encapsulations briefly. What we couldn't find when westarted working on this book was clear understanding of various architectures, when

to deploy a certain architecture, and what the implications were if we deployed thosearchitectures. There are several service providers out there today with DSL networksranging from very small ones to ones that support millions of customers. The lessons

learned from those deployments in addition to various proof-of-concept labs andperformance tests done in-house will help readers understand and implement thosearchitectures.

Goals of the Book

The purpose of this book is to get the readers more familiar with various DSL accessand core architectures and how they are implemented. By presenting readers with

real-life deployment scenarios and case studies, the book helps readers understandthe pitfalls and benefits of DSL architecture and apply those principles whendesigning and deploying their own DSL networks. The in-depth sections on DSL

architectures will also help you to understand different encapsulation methods usedin networks today.

How This Book Is Organized

Chapter 1 takes you back to the time when 300-baud modem was the onlyaccess technology available to consumers. It covers the progress in access

networks for the last 15 to 20 years and gets you to appreciate how far alongwe've come in bringing high-speed access home to consumers.


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This chapter also discusses the basic building block for telephone networks,which is a DS-0, and the frequency it operates on. Other follow-on access

technologies such as ISDN still did not solve the problem of overloading thePublic Switched Telephone Network (PSTN), leading to the adoption of DSL asthe access technology of choice for phone companies.

Chapter 2 introduces the different flavors of DSL along with the advantagesand disadvantages of each flavor. This chapter looks at Layer 1 designconsiderations, such as copper loop issues, noise margin, and reach, and

what can be done in the DSLAMs and in the Central Office (CO) to resolvethose problems. This chapter also addresses the protocols running over DSLATM and Frame Relayshowing why ATM dominates most, if not all, ADSL

deployments today. Chapter 3 covers DSL functional segments and responsibilities and where the

demarcations end. Companies that own the copper loop are traditionally the

sole provider for both voice and data running over that same copper loop, butbecause of government deregulation, other companies can move in and leasethose lines from the phone companies and offer data to their customers. This

is followed by the services offered by wholesale and retail ISPs. Chapter 4 focuses on the encapsulation methods of subscriber ATM VCs.

PPPoA, PPPoE, RFC 1483 bridge, RFC 1483 routed, and RBE are some of the

encapsulations discussed in detail. The chapter addresses the advantages,disadvantages, and design considerations for customer premises equipment(CPE), aggregation devices, and ATM switches for each encapsulation method.

Coverage also includes the different methods of carrying subscribers data tothe ISP and corporate home gateways, such as L2TP, Service SelectionGateway (SSG), and MPLS.

Chapter 5 is an exciting chapter that presents you with real-life DSLdeployment scenarios. This chapter includes four case studies, ranging from afew hundred thousand subscribers to millions of subscribers with downloadspeeds ranging from 384 kbps to 4 Mbps. All elements of DSL are considered

and calculated including the number of subscribers per DSLAMs, the numberof DSLAMs per CO, the number of ATM switches, and the number ofaggregation devices. When scaling the network, these and other elements

need to be closely examined. In addition to the wealth of practical informationin these case studies, you will also have a chance to look at the migrationpath and scaling up of networks to support a much larger number of

subscribers. Chapter 6 provides an overview of the provisioning model today, which

involves a lot of manual steps in order to roll out a new DSL line. To achieve

the goal of installing one million DSL lines that some service providers haveset, there has to be a better way to automatically provision these linesquickly.

This chapter provides an overview of the flowthrough provisioning conceptand Cisco Element Manager Framework, and how each Element Manager tiesinto the framework that integrates into the existing service provisioning

applications. With the right tools, a line can be tested and provisionedautomatically, and the CPE will have its images and configurationsdownloaded, and other elements in the network will also have the appropriate

configurations downloaded to it automatically.


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Icons Used in This Book

Command Syntax Conventions

The conventions used to present command syntax in this book are the sameconventions used in the IOS Command Reference. The Command Reference

describes these conventions as follows:

Vertical bars (|) separate alternative, mutually exclusive elements. Square brackets [ ] indicate optional elements. Braces { } indicate a required choice. Braces within brackets [{ }] indicate a required choice within an optional

element. Boldface indicates commands and keywords that are entered literally as

shown. In actual configuration examples and output (not general commandsyntax), boldface indicates commands that are manually input by the user

(such as a show command). Italics indicate arguments for which you supply actual values


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Chapter 1. History of Remote AccessTechnology

This chapter covers the following key topics:

History of Remote Access Technology A brief history of the remote accesstechnology and its evolution from analog to digital.

The World of Analog Earlier applications that used analog modems weremainly text-based, but with the explosion of the Internet and its graphicalapplications, analog modem has put a tremendous burden on the PublicSwitched Telephone Network (PSTN).

ISDN ISDN was supposed to be the next big thing, replacing analogmodems; however, because telcos failed to push the technology and investmore upfront for a long-term, rather than a short-term financial gain, the

technology failed to take off. ADSL Comes of Age A brief history of ADSL and its adoption by different

phone companies. Each phone company chose a different ADSL vendor, whilea few ganged up to form a Joint Procurement Contract and together chose

one vendor to supply them with ADSL equipment. ADSL Benefits ADSL offered some of the benefits that analog modems and

ISDN were never able to offer. High-speed access, always-on, offload data

from the voice network, and so much more, are the main reasons everyphone company today is pushing the technology.

Applications That Drive High-Speed Access High-speed access applicationssuch as Voice over IP (VoIP) and video on demand (VoD) enable phonecompanies the possiblity for additional revenues over their existing copperwire infrastructure.

Remember back when a 300 baud modem was the fastest access technology thatyou could get your hands on. And when 2400 baud came out, you thought modemspeed could not get any faster? As encoding algorithms continue to improve and

telephone lines have become cleaner of electrical interference, analog modem speedhas improved greatly with the latest V.90 standard capable of 56 kbps of datatransfer.

In today's digital age, inexpensive cameras that are available in a neighborhoodelectronic store can store images in a digital format, music we listen to is stored

digitally on CDs, and movies are encoded into digital format and stored on DVDs.

The recent MP3 format for music and MPEG-2 for movies have ignited a series oflawsuits from recording and movie studios. Music can be encoded in a digital format

small enough to be shared among friends. It doesn't matter how many times thesong has been copied, the quality of the song doesn't decrease. What worries therecording studios and music labels even more is that these songs are easily

accessible by anyone that has a connection to the Internet.

When it comes to remote access, we are still using decade old technology to transmitthese digital files over analog phone lines. We spent so much time and energy


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building faster computers and digitizing everything from music, voice, and movies,that there has not been much improvement when it comes to access technology.

There have been attempts to bring faster access speed to consumers in the past,each with various degree of success. The most popular broadband accesstechnologies today are cable modems and digital subscriber lines (DSL), with

wireless gaining more popularity. Although each technology has its advantages anddrawbacks, DSL seems promising because it is backed by virtually all phonecompanies, plus the technology has room to grow.

This chapter will cover the history of remote access including analog modem andIntegrated Services Digital Network (ISDN), and the problems that led to the

adoption of DSL by major telephone companies as the next method of remoteaccess.

The World of Analog

Earlier applications that used analog modem involved text-based applications. Theseapplications were mainly used by corporations who had a need for employees orvendors to access and use a corporate database. Digital bits and bytes from the PC

applications are converted into analog waves by analog modems. Those analogwaves then are transmitted through a Public Switched Telephone Network (PSTN).These waves are received by another modem, which in turn converts these signals

into digital bits and bytes that the far-end (receiving) computer can understand.Figure 1-1 illustrates how the signals are converted from one PC to another fromdigital to analog and then back again to digital.

Fi g u r e 1 - 1 . Si g n a l Co n v e r s i o n B e t w e e n T w o Com p u t e r s U s i n g

A n a l o g M o d em s

Because early applications were mainly text-based and early computers were

relatively slow, low-speed modems were not a big issue. A person from their homewould dial in to a computer, and once a session was established, users would send a

couple of keystrokes to the mainframe asking it to do a series of computations. Aftersome time, the mainframe would then display the result of the operation back to theuser, one screen of text at a time. As you can see, there's not much need for a high-speed connection because the majority of the time is spent waiting for themainframe to finish its jobs. Remote access was limited to a very small group of

scientists and engineers involved with number crunching and other mathematicalapplications, and only the final result of the computations were of any interest to theend user.


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When multimedia intensive applications, such as the World Wide Web, began to takeoff, there was an obvious need for remote access speed to improve. Over time,

modem speed has become faster with better modulation/demodulation algorithmsand cleaner copper wires from the Central Office (CO) to your residence. Modemspeed has been able to improve from 300 bps to 1200 bps to 9600 bps. With clever

engineering, modem speed has achieved 56 kbps. But as with most technologies,

there is an upper limit of how fast modem speed can go. With the 56-kbps modemtechnology, we are hitting the upper limit of analog technology, and 56 kbps is the

highest analog speed that we will be able to achieve over the existing copper wiresused to carry voice. Due to the modem limitation, alternative technologies, such asbroadband access technologies, are needed that allow subscribers to access the

Internet at a much higher speed.

Telephone companies had been converting their analog switches to digital fordecades. That along with digital phone switches comes Integrated Services Digital

Network (ISDN), which is supposed to solve the digital to analog back to digitalconversion problem. ISDN enables much faster access speeds. With ISDN, voice anddata can be offered simultaneously over the same pair of copper wiring. A channel is

available for voice and another channel for data. When the voice channel is not inuse, ISDN equipment can use both channels to achieve speeds up to 128 kbps. ISDNwas offered around the time when modem speed was at 9600 bps, making it

attractive to users who demanded high-speed access. Figure 1-2 illustrates theconcept of 2 B channels used in both voice and data.

Fi g u r e 1 - 2 . V o ic e a n d D a t a O v e r t h e Sam e I S D N Li n e

ISDNISDN brings some relief to the speed limitations of analog modems by offering speedup to 1.5 Mbps for Primary Rate Interface (PRI), or a more commonly used rate that

is offered by most phone companies, the Basic Rate Interface (BRI). BRI offers speedup to 128 kbps, significantly faster than traditional analog speed.


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So what makes ISDN able to achieve such high speeds that analog modems can't?To fully understand the difference between analog modems and ISDN, we need to

take a look at how voice is encoded into a digital signal level 0 (DS-0) channel.

To convert an analog voice into digital signal, the required sampling rate is 8000samples per second; therefore, 8 bits are needed to represent each sample.

Therefore, the bandwidth needed is 8000 * 8 = 64,000 bits per second, which iscalled a DS-0 channel. A traditional analog line runs over a DS-0 channel and has 64kbps of total bandwidth. Out of this total bandwidth, 8 kbps is taken out for

telephone signaling, leaving 56 kbps for voice calls. The services that are run overthe 8-kbps channel are necessary for your phone to ring. That's where caller IDinformation is sent, plus other maintenance services so that the telephone switch

knows when you have hung up your call and to stop billing you. This method is calledin-band signaling, or robbed bit signaling, because an 8 kb chunk is taken out, orrobbed out of the original 64 kbps for signaling. If digital signals are converted into

analog and back to digital again, only 56 kbps worth of bandwidth is available to doit in, so this is where the 56-kbps limitation in analog modems comes in. Figure 1-3illustrates the breakdown in robbed bit signaling.

Fi g u r e 1 - 3 . Ro b b e d B i t S ig n a l i n g

ISDN, on the other hand, carries three logical channels over the same pair of copper,

two B, or Bearer, channels and one D, or Delta, channel. Each B channel has abandwidth of 64 kbps and is capable of carrying voice or data. Instead of the 8 kbpsin-band signaling channel from the POTS (plain old telephone system) case, there isnow a separate signaling channel called a D channel that has 16 kbps and can

support signaling for both B channels. The two B channels can then be combinedyielding up to 128 kbps of speed. The attractive feature of ISDN is integrated voiceand data running over the same copper wire. A computer can have both B channelsconnected to the Internet, but when a person needs to make a phone call, he can

simply pick up the phone using one of the B channels. The ISDN equipment willautomatically detect that there is a phone call and will drop one B channel from thecomputer automatically.

You can choose from many variations of ISDN configurations. You can choose one Bchannel to carry voice alone and one D channel for signaling, or you can choose only

one B channel for voice, one B channel for data, and one D channel for signaling. Themost popular method is called 2B+D BRI. Figure 1-4 shows a BRI line has threelogicalchannels: two B channels and one D channel.

Fi g u r e 1 - 4 . L o g i ca l Ch a n n e l s O v e r a BRI I n t e r f a c e


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Besides the voice capability, ISDN also offers other attractive features such as quickcall setup and teardown. In the analog world when a modem needs to call the

Internet, your modem has to train with the modem on the other end, meaning themodems need to negotiate the connecting speed that both computers can send andreceive data reliably. Because each B channel always operates at 64 kbps and there's

a separate dedicated D channel for signaling, the setup time for ISDN is in seconds,negligible to the end user. Because ISDN is entirely digital, this effectively makescommunication between two ISDN devices all digital without converting signals to

analog and back to digital again. Figure 1-5 shows ISDN communication betweentwo PCs.

Fi g u r e 1 - 5 . I SD N Com m u n i ca t io n B e t w e e n T w o Com p u t e r s

This setup seems to be the answer to low-speed analog modem problem but therewere several issues that prevented ISDN from taking off to become the de factoremote access method. The following list examines some of the issues associated

with ISDN that have prevented it from becoming the top remote access method:

The first issue with ISDN is that it's not available to anyone who wants theservice. Before offering ISDN, phone companies must upgrade their phone

switches to make sure they are ISDN-ready. ISDN support for phone switchesrequires expensive software upgrades, costing millions of dollars for eachswitch. Because of this, not all telephone companies embraced ISDN

technology, making ISDN availability spotty in the U.S. Europe and Japan,however, have enjoyed a much higher success rate of ISDN deployment. Withlow availability of ISDN services and phone companies unable to commit to

this technology in the U.S., ISDN service remains expensive, preventing itfrom spreading to lots of consumers and consequently lowering the cost ofISDN.

To make matters worse, common ISDN deployment takes up to three voiceports from the expensive phone switch. Typically, every phone in yourneighborhood will run back into a CO. Then each copper pair will terminate ona phone switch, the most popular of which are the Lucent 5ESS and Nortel

DMS-100. These phone switches are responsible for, among other things,providing a dial tone so that when you pick up the phone you can be sure that


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it's ready for you to dial. Phone ports on these switches are expensive andoften take years of service to recover the initial investment cost. With three

voice ports taken just for ISDN, it becomes too expensive to offer to themasses and, therefore, each phone company has a different view on offeringISDN. Some charge hundreds of dollars for installing ISDN, not including

monthly service and usage charges.

To the phone switch, an ISDN call looks the same as a voice call, tying up thesame 56-kbps channel on the switch. Telephone companies design theirphone network for voice calls that typically last on the average of five to sixminutes per call. Internet users who use analog or ISDN modems typicallystay connected for a much longer period of time, usually from 30 to 35

minutes, tying up phone lines for other people to use. This results in thephone switch getting congested and unable to serve other customers. Thisproblem is similar to the Mother's Day problem where the phone switch runsout of capacity resulting in a busy signal. With everybody rushing to connect

to their Internet at 6:00 p.m. after dinner, this Mother's Day problem nowhappens almost every night, prompting the phone companies to find otherways to offer remote access methods to their customers.

Besides spotty availability and expensive monthly charges, ISDN doesn'tattract a lot of customers because the most affordable and commonly usedform of ISDN (BRI) has a limitation of 128 kbps, making it unattractive for

subscribers who use bandwidth-hungry applications.

As you can see, ISDN offered faster speed access to customers but presented

another set of problems to phone companies. Some phone companies areuncommitted to the technology because of the cost of equipment upgrade and longerreturn on their investment, although ISDN has enjoyed a much higher deployment inEurope and Japan. The best method for phone companies to solve remote access is

to get data off the voice network without running another pair of copper wiring toevery house. The next few sections examine how DSL can help relieve the voicenetwork and at the same time see why DSL technology does not run into the same

56-kbps limitation like the analog modem does.

ADSL Comes of Age

Asymmetric digital subscriber line (ADSL) is a technology invented by BellCore in themid-1980s as a method to offer video and voice over the same copper loop. Theintent was to offer VoD to their customers when they wanted it. When thetechnology fizzled out because VoD failed to take off and typical deployment of ADSL

was too slow to run any real-time video over it; however, the technology seems tohave been forgotten for almost a decade.

ADSL had a renewed interest from phone companies again after their voice networkbecame overloaded with data. ADSL was being looked at again, but this time themain application that was driving it was datanot video like it was originally

designed for. In the early 1990s, phone companies wanted to offer DSL as a methodto prevent data from overtaking their voice network but there was a problem.There's no DSL standard, and phone companies don't make the equipment

themselves. Southwestern Bell, Bell Atlantic, and BellSouth got together to form theJoint Procurement Contract (JPC) to search for an ADSL equipment maker. Theypicked Alcatel as the main supplier of their Digital Subscriber Line AccessMultiplexers (DSLAMs) and customer premises equipment (CPE). US West chose not


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to participate in the consortium and instead picked a startup company calledNetSpeed (later acquired by Cisco). Bell Atlantic went with Westell as the main

supplier for their DSL equipment. Other competitive local exchange carriers (CLECs)such as Northpoint Kovad, Rhythms, and Cincinnati Bell all offered DSL services lateron each choosing ADSL equipment from different vendors.

Unlike ISDN, the announcements by major U.S. telcos are a big boost to ADSLtechnology, signaling that ADSL is at the forefront of new remote access technology.Most importantly with ADSL, voice and data are separated into two different

networks preventing data from overloading the voice infrastructure. Figure 1-6shows data offloaded from the PSTN resulting in two networks: data network andvoice network.

Fi g u r e 1 - 6 . O ff l o a d in g D a t a f r o m P ST N N e t w o r k

Though offering tremendous benefits, ADSL, like most new technologies, has someproblems inherited from the current cable wiring for phone services and massdeployments. In the next chapter, we'll look at these benefits and problems that

earlier ADSL deployments have run into.

ADSL Benefits

In May 2001, 2,914,003 ADSL lines were recorded in service (Telechoice DSLDeployment Summary, www.xdsl.com/content/resources/deployment_info.aspwww.xdsl.com/content/resources/deployment_info.asp). Thistremendous growth in DSL technology was the result of major telcos pushing this

technology. So far, DSL has been the most promising technology that enables telcosto get incremental revenues by offering voice, video, and data over the same copperwire. Telephone companies embraced the technology because of the followingbenefits:

High-speed access For an affordable price of around $40 a month,customers now can get speeds of up to 1 Mbps, which is necessary to

download web pages that are loaded with graphics. Sharing pictures on the
http://www.xdsl.com/content/resources/deployment_info.asphttp://www.xdsl.com/content/resources/deployment_info.asphttp://www.xdsl.com/content/resources/deployment_info.asp


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web is now more practical and downloading large MP3 music files from theweb is more attractive largely due to advances in DSL technology.

Always-on If you have ever used a modem before, you know how long ittakes to boot up your computer, turn on the modem, dial a number, andlisten to a barrage of noise from the modem training. If you get lucky, you

will connect on the first try and then you have to make sure everybody in the

house does not pick up the phone while you are online. With DSL, it is aservice that's always on, meaning you are always connected to the Internet.

You no longer have to dial your service provider or fear that someone will pickup the phone while you are on the Internet. The fact that DSL is always onmakes people come up with great ideas of utilizing the service from putting

up their own web server in the garage to monitoring their homes remotelythrough a web camera set up inside their house.

Low maintenance For phone companies deploying hundreds of thousandsof DSL lines, low maintenance is a big requirement. The phone lines between

the CO and your house are usually underground and protected, barringconstruction nearby that results in a cable cut. Once DSL is installed, yourphone line is left aloneno maintenance needed.

Security Because every house has its own copper pair running back to theCO, the voice or data traffic traversing the line does not have to mix with anyother traffic until it gets to the CO, meaning no one else can listen to your

phone message or sniff your data from their computer. Cable technologyrelies on a shared network, similar to a LAN where everybody on the LAN canlisten to all the traffic on that LAN. With some clever hacking or spoofing, a

computer might be hacked into. The implication here is that hacking is harderon a dedicated line like DSL. The nature of a dedicated line to the CO makesthings a little harder for hackers to compromise your home PC.

ADSL provides a tremendous opportunity for telephone companies to offer newservices such as data and voice over their existing copper infrastructure. Newer DSLstandards, such as very-high-data-rate DSL (VDSL), provides an extremely fast pipe

to the consumer, enabling services such as video over DSL, VoD, video conferencing,and VoIP, all on top of the regular POTS line. The next few sections examine howDSL will enable new service offerings and how high-speed access plays an important

role in the future.

Applications That Drive High-Speed Access

Having a big data pipe to the house opens up a new world of applications that werenot possible before with analog modems or even ISDN. Applications such as theWorld Wide Web and e-mail can be run on top of IP. Plus now with plenty ofbandwidth that DSL provides, other bandwidth hungry applications can also run on

top of IP as well. With a theoretical speed of 8-Mbps download for ADSL and 52 Mbpsfor VDSL, data, voice, and video can be run concurrently over the same copper wire.The following sections discuss applications that are bandwidth intensive, including

Video over DSL VoIP Residential Gateway High-speed Internet Access and Virtual Private Network (VPN)


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Video over DSL

With over 827 million copper lines installed worldwide, phone companies have amassive customer base and they also want to offer new services to their customers.Right now, phone companies would love to have recurrent revenue of about $40from each resident. But running cable to every neighborhood proves to be expensive.

Due to their existing infrastructure, phone companies find it very attractive to offernew services utilizing the existing copper pair. With ADSL, they have been able tooffer voice and affordable high-speed data over the same wire and without clogging

up their expensive voice network.

As mentioned previously, ADSL was invented by BellCore to offer VoD over existing

copper pair. Using MPEG-2 compression, each video stream takes about 3 Mbps toachieve 30 frames per second, a standard rate at which broadcast televisionoperates.

Although ADSL was initially designed to operate at 7 Mbps of download speed,distance limitation has cut down the typical ADSL deployment to about 1.5 Mbps

peak download.

As DSLAMs are pushed closer to the neighborhood, copper length is becomingshorter, allowing ADSL to operate at a much higher speed. It also allows other highspeed technologies, such as VDSL, to operate at a maximum theoretical 52 Mbps of

download speed. This availability of high bandwidth allows phone companies to offermultiple real-time feeds of TV channels to their subscribers over VDSL or a singlefeed of real-time or VoD channels over ADSL.

Customers now can use multiple VoIP phone lines, surf the Internet, and watch TV atthe same timeall over the same copper wire. Because there isn't a need to run

additional wiring to your home, the cost savings as a customer is significant. This

news is great for the telephone companies because the more bandwidth they canoffer to their customers, resulting in the capability to offer more robust services (agreater amount of digital TV channels, for example). Figure 1-7 shows a Video over

DSL setup.

Fi g u r e 1 - 7 . Vi d e o o v e r D S L A p p l i c a t i o n


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In the Figure 1-7, a multicast server gets its programming either from a videotapedclassroom or from a satellite feed. A content manager such as an IPTV content

server will broadcast its programming information to any multicast clients thatrequest for one. The ISP point of presence (POP) will pull down a multicast stream

only if there's a client that requests that programming. The beauty of multicast isthat even if there's a hundred clients wanting the same programming, the ISP POPwill have to pull down only one stream and replicate that stream a hundred times onan aggregation device, pushing those streams down the DSL lines to the clients. Inthe case of an IP DSL switch, the replication will be done on the switch itself, pushing

the content closer to the subscriber.

Voice over IP

Corporations have long known that running two networks, one for voice and one fordata, is very expensive both to deploy and to maintain. Separate pieces ofequipment have to be bought and maintained, while renting leased lines from telcos

incurs on-going monthly expenses. As the phone and data networks begin toconverge, corporations are increasingly running voice over their fast data networks.Figure 1-8 shows a typical VoIP application, sometimes called toll bypass.

Fi g u r e 1 - 8 . V o I P A p p l i c a t i o n

Let's say a large corporation has four regional offices across the U.S. with high-speedInternet connections. Voice calls outside the company go out of the PSTN, while datacalls go out of the public Internet connection. Because all four regional offices havehigh-speed Internet connections, voice calls placed to the other regional offices use

VoIP (over the public Internet). Special Premium Quality of Service (QoS) is appliedto these calls to ensure there's minimal delay. This is sometimes referred to as a tollbypass: a remote office or a regional office can dial the headquarters using the four-

or five-digit extension, essentially bypassing the CO.


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There's no reason why this technology will not work for telecommuters using DSL asa data connection to the corporation. Instead of running one phone line for voice and

one phone line for data, both voice and data can be run over the same high-speedDSL connection.

Residential Gateway

Phone companies are already invested heavily in the voice infrastructure, includingphone switches and copper runs to the neighborhood. Besides offering DSL over the

same set of copper, phone companies also would like to offer voice for incrementalrevenue. Running an additional pair of copper lines to the home for a second phoneline is expensive, and voice over the same pair enables the phone companies to

provision a second line without the expensive cable run.

CLECs, unlike phone companies, have no voice infrastructure but would like to offervoice service to their data customers. Imagine when you finally give in and agree to

let your teenage daughter add another phone line to your house. Instead of waitingfor the phone companies to run another pair of copper to your house, these CLECs

can simply enter a couple of keystrokes to activate another phone line for yourhouse.

All that is needed here is a residential gateway. This is a gateway that will sit in yourhome, terminating the DSL line from your provider and, in turn, provide your house

with different services, such as high-speed Internet access, multiple phone lines, andvideo feeds. Figure 1-9 shows how a residential gateway might look.

Fi g u r e 1 - 9 . T y p i c a l Re s i d e n t i a l Ga t e w a y

The residential gateway has several important features built into the box:


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On the PC side, a Dynamic Host Configuration Protocol (DHCP) server,network address translation (NAT), and an Ethernet switch allow multiple PCs

to obtain IP addresses from the residential gateway by way of a traditionalEthernet cable. A wireless base station might be built into the gatewayinstead of an Ethernet hub for wireless 802.11 users. The ideal gateway

should also include VPN and encryption software that enable telecommuters

to connect to their office securely. Several voice ports are also built into this box that enable regular phones to

plug into these ports. As mentioned earlier, when a new phone line is needed,the phone provider can configure their switch and the gateway for a newphone number. There is no need to run an additional pair of copper from the

CO to the house. Last but not least, a coax port should be available for future VoD over DSL

programming. VoD can be achieved easily, even today, when the entire movieis downloaded into the high capacity internal hard drive in the gateway,

enabling later playback at any convenient time.

Some manufacturers have come out with residential gateways, although not all of

them have a lot of the rich features. Cisco, 2Wire, and 3COM have all come out withtheir own version of residential gateway, but you can bet that the functionality ofthese boxes will be a lot richer once they become more popular.

High-Speed Internet Access and VPN

The main application of DSL today is still web and e-mail traffic, which does not needguaranteed bandwidth and delay. One or two seconds delay will not affect yoursending or receiving e-mail. As we move to more time-sensitive traffic such as voice,a different level of service must be implemented by service providers to distinguish

time-sensitive traffic over the rest of the nontime-sensitive applications.

Corporations can take advantage of high-speed Internet access by allowingemployees to access their corporation from home by various techniques. Figure 1-10

shows a typical DSL deployment where a subscriber is connected to the Internet viaan ISP POP. Later chapters will discuss different techniques of building a VPN.

Fi g u r e 1 - 1 0 . H ig h - S p e e d I n t e r n e t A c c es s o v e r D SL


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Looking Ahead

In this chapter, we've looked at how the access technology has evolved from the

traditional analog modems to ISDN and finally DSL. We've also discussed theexciting new applications that can be offered at a low cost to DSL subscribers such as

high-speed Internet access, voice service, VPN, and video over DSL.

We'll take a closer look at different xDSL flavors in the next chapter. Differentencoding algorithms such as Carrierless Amplitude and Phase Modulation (CAP) anddiscrete multitone (DMT) will be discussed, as well as a more in-depth discussion of

the frequencies in which voice and data operates.

References

xDSL.com, which provides an analysis of DSL technologies, can be found at:www.xdsl.com/content/resources/deployment_info.aspwww.xdsl.com/content/resources/deployment_info.asp
http://www.xdsl.com/content/resources/deployment_info.asphttp://www.xdsl.com/content/resources/deployment_info.asphttp://www.xdsl.com/content/resources/deployment_info.asp


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T1/E1, which we are familiar with. DSL technology in its simplest form is nothing buta modem technology. Data from the subscriber gets modulated by the subscriber-

end DSL modem before being put on the physical copper loop. The CO equipment atthe other end comprised of banks of modemsdemodulates the signals and makesnecessary switching decisions based on the transport layer used. For example, in the

case of ADSL, the most widely and commonly used data link layer is ATM. Other

vendors, however, also use Frame Relay as the data link layer.

To understand any DSL connectivity, you must understand the components of the

entire DSL network, as shown in Figure 2-1. This diagram is also a graphicalrepresentation of the ADSL Forum reference model.

Fi g u r e 2 - 1 . ADS L Fo r u m R e f e r e n c e M o d e l

As Figure 2-1 illustrates for any DSL connectivity, you have a DSL modem at the

subscriber end at a minimum, noted in the diagram as an ATU-R (ADSL terminationunit-remote) and more commonly known as customer premises equipment (CPE). Atthe CO, a corresponding DSL modem demodulates the signals modulated by thesubscriber modem. The CO is equipped with a digital subscriber line access

multiplexer (DSLAM), which consists of banks of ATU-Cs (ADSL termination unit-central). Depending on the region that CO is serving, the DSLAM should have acorresponding ATU-C for each ATU-R at the subscriber end. Because we are depicting

ADSL in the diagram, we made reference to ADSL termination units. If the flavor ofDSL is SDSL, the subscriber-end modem then would be called STU-R and so forth.

The splitters shown in Figure 2-1 reflect a device that differentiates DSL data from

the regular analog voice. It is important to note here that any DSL flavor usually

makes use of the frequency spectrum, which is higher than that used by regularanalog voice (typically 4 kHz). In simplistic terms, the job of the splitter is to identify

whether the signal is below 4 kHz or higher. This is achieved using a simple low-passfilter technology. By making use of this splitter technology and by DSL using theupper frequency spectrum, utilizing the same pair of copper for both regular analog

voice as well as DSL data is possible. If a single pair of copper is used for both theanalog voice and DSL data, a splitter will be used at both the subscriber end as wellas one in the CO.


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In the CO, when a signal is received from the subscriber end, the POTS splitter sendsthe voice spectrum to a regular phone switch in the CO. Additionally, it sends the

data spectrum to the ATU-C in the DSLAM. The ATU-C in turn demodulates thesignal. Depending on which transport layer the CPE and DSLAM agreed on using(whether it is ATM or Frame Relay), the DSLAM makes the necessary switching

decision to forward the subscriber traffic to its final destination. We will discuss about

the data link layer in the later section, "Considerations at the Data Link Layer (Layer2)."

One other important aspect worth noting on the reference model in Figure 2-1 isupstream and downstream:

Upstream is always referred to as the direction from the subscriber towardsthe CO.

Downstream is referred to as the direction towards the subscriber.Table 2-1 depicts the most commonly used xDSL flavors with their correspondingdistance limitations and bandwidth.

T a b l e 2 - 1 . DSL Fl a v o r Ch a r a c t e r i s t i c s

Upstream Bit Rate Downstream Bit Rate Reach

ADSL Up to 800 kbps Up to 8 Mbps Up to 17,000 feet

G.Lite Up to 176 kbps Up to 1.5 Mbps Up to 18,000 feet

SDSL Up to 768 kbps Up to 768 Kbps Up to 10,000 feet

VDSL Up to 20 Mbps Up to 52 Mbps Up to 3000 feet

G.shdsl Up to 2.3 Mbps Up to 2.3 Mbps Up to 22,000 feet

The distance or reach specified in Table 2-1 reflects the distance from the CPE to theCO. Depending on which modulation technology you use or which flavor of xDSL youselect, you can achieve different bandwidth in both upstream and downstream

direction. The key point that needs to be noted here is that a tradeoff between reachand bandwidth always occurs. The further down the CPE is from the CO, the lesserthe bandwidth it gets. A number of factors can play a big role in determining the

exact bandwidth at a certain distance, and they will be discussed briefly in thesection, "Design Considerations at the Physical Layer (Layer 1)."

ADSL

ADSL, as explained in the first chapter, is the most widely deployed flavor of xDSLtoday. The reason for this is simple: ADSL provides the right suite of bandwidth bothin upstream and downstream directions, required by the most consumers today.

ADSL has gained popularity in today's consumer space, not only as the always-ontechnology but also as a cheaper and more suitable alternative to the common dialmodem technology. In addition, ADSL is being offered and gaining popularity as a


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cheaper alternative to the traditional T1/Frame Relay circuits for small offices, homeoffices, and business customer space.

ADSL has become a relatively mature technology having already been throughseveral years of development and physical deployment by many service providers.ADSL can offer rates of 8 Mbps in the downstream direction and approximately 1

Mbps in the upstream direction.

ADSL uses CAP or DMT to encode the data traffic on copper loop. Both CAP and DMT

are quite different in how they encode. CAP is not standardized; however, DMT wasinitially standardized as ANSI T1.413 and was then forwarded to ITU as G.992.1.Since then, the DMT standard has gone through various versions (or issues). Issue

One provides the basic framework, while Issue Two provides better interoperabilityand includes references to ATM and rate adaptation.

Although both CAP and DMT are frequency domain techniques, CAP relies more

heavily on the time domain than does DMT. CAP sends high bandwidth symbolsacross a wider spectrum for a shorter period of time. On the other hand, DMT relies

on smaller bandwidth channels sending longer duration symbols at a narrowerfrequency.

As shown in Figure 2-2, CAP relies on single downstream and upstream bandoccupying a larger proportion of the available bandwidth. As shown in the diagram,the spectrum is divided into two single carriersthe upstream starts at f1 and thedownstream starts at f2. CAP modems can accept ATM, packet, and bit synchronoustraffic. As seen with most ADSL deployments, however, ATM predominates across

the loop. CAP defines a number of downstream and upstream baud rates (number ofsymbols per second), which then derive the actual bandwidth.

Fi g u r e 2 - 2 . CAP Sp e c t r u m f o r ADS L

CAP was used as the initial choice of encoding method for most initial ADSLdeployments, but with the standardization of DMT and the availability of Issue Two,most vendors started adopting the DMT encoding method to achieve the

interoperability with other vendors. Another reason for vendors to standardize on


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DMT is the fact that DMT is more susceptible to noise and therefore offers a betterrate adapting technique.

DMT encodes the data into a number of narrow subcarriers, or tones, transmitted atlonger time intervals than CAP. As shown in Figure 2-3, DMT consists of 256 4 kHztones.

Fi g u r e 2 - 3 . DMT Sp e c t r um f o r ADS L

The modem can modulate each of these tones at a different bit density (a maximum

of 60 kbps/4 kHz tone) depending on the line noise. For example, the modem canachieve a higher rate at lower frequencies. By contrast, the modem achieves a lowerrate at higher frequencies where there is higher attenuation of the signal. In an

event where the interference or the line noise is high, some of these tones zero out,

meaning those tones are not used and, hence, the aggregate rate that can beachieved is rate adapted. This use of tones and its maintenance is one of the reasonsDMT is considered to be a bit more complex than CAP; however, this use has lately

been overcome by development and advances in a newer generation of digital signalprocessors (DSPs).

DMT defines two data paths: fast and interleaved. Fast offers low latency, while thepurpose of interleaving is to avoid consecutive errors delivered to the Reed-Solomon(RS) forward error correction (FEC) algorithm at the receiving end of the circuit. RS

is much more effective on single errors or errors that are not consecutive.

Most ADSL systems are set up to use frequency-division multiplexing (FDM) toestablish upstream and downstream channels. In FDM mode, the upstream and

downstream frequency bands are separated. Using FDM, the upstream channelallocation ranges from 26 kHz to 138 kHz and downstream ranges from 138 kHz to1.1 MHz. See Part A ofFigure 2-4 for a representation of FDM-based ADSL.

Fi g u r e 2 - 4 . ADSL Ec h o Can c e l la t i o n


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An alternative to FDM is to use echo cancellation, which enables upstream anddownstream signals to use the same spectrum. Echo cancellation also adds more

available spectrum to the downstream channel. The objective of echo cancellation isto enable the downstream data to use lower frequencies than are available in FDMmode. Using lower frequencies results in less attenuation, which theoretically allowsfaster downstream data rates on longer loops (telephone lines). Part B ofFigure 2-4

illustrates this concept.

Echo cancellation mode has the potential to increase downstream bandwidth, but

with a compromise in upstream bandwidth. This mode can also increase crosstalk,resulting in higher noise levels and degraded signal-to-noise ratios (SNRs).

Echo cancellation is used to separate the far-end transmitted signal from the near-

end echo in the overlapped band. Echo is the reflection of a transmitted signal back

into a receiver at the same end of the circuit. The source of echo can be a reflectionfrom the far-end modem or from the two-wire interface of the near-end modem.

When a transmitter and receiver share the same two-wire interface, a four-wire totwo-wire conversion is done. The two-wire interface is split into a receive circuit anda transmit circuit. While line card circuits have been used in ADSL modems to form

four-wire to two-wire hybrids and bandpass filters, there is an increasing use ofdigital signal processors to perform the separation of transmit and receive. Any


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phase errors in an analog circuit can cause a received signal to be coupled into thetransmit path, or vice versa.

A lower data rate version ADSL has also been proposed as an extension to ANSIstandard T1.413 by the Universal ADSL Working Group (UAWG) led by Microsoft,Intel, and Compaq. This is known as G.Lite in the ITU standards committee. The

standard calls for reducing the maximum transmit power, thus reducing thedownload speed of up to 1.5 Mbps and upload speed of up to 512 kbps. G.Liteoperates at a lower frequency than ADSL, allowing for a longer distance between a

CO and its subscribers and, therefore, reaching more subscribers than ADSL. G.Liteuses the same DMT modulation scheme as ADSL but eliminates the POTS splitter atthe customer premises. As a result, the ADSL signal is carried over all the house

wiring, resulting in lower available bandwidth due to greater noise impairments.

VDSL

VDSL is an emerging technology that plans to deliver data rates as high as 52 Mbpsin the downstream direction to the subscriber at a shorter loop. VDSL is not yetstandardized, however both American (ANSI) and European (ETSI) standards bodiesare working actively in standardizing this technology.

With downstream speeds of up to a blazing 52 Mbps, VDSL is the next step up thespeed ladder beyond ADSL. The price paid for VDSL's increased speed, however, is a

shorter distance range. This means that VDSL can be a promising technology forapplications requiring a very high bandwidth in the downstream direction towards tosubscriber, especially in the Multi Dwelling Unit/Multi Tenant Unit (MDU/MTU) space.This also applies for hospitality suites like hotels and so forth that would like to offer

voice, video, and data on a common pair of copper. A typical VDSL-basedarchitecture is shown in Figure 2-5.

Fi g u r e 2 - 5 . MDU / M TU A r c h i t e c t u r e

VDSL comes in two variants: a symmetrical version and an asymmetrical version,

which is one of the key differences from ADSL. Over short ranges (1000 feet), VDSL


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can offer up to 52 Mbps downstream capacity compared to the ADSL capacity of upto 8 Mbps; upstream, the asymmetrical version of VDSL offers a slower data rate of

approximately 1.5 Mbps to 2.3 Mbps. Over short distances, VDSL offers a speedincrease of approximately three to five times over ADSL. Over longer distances,VDSL will offer a speed increase of two to three times but VDSL's distance range will

always be less than that for ADSL.

To provide high bandwidth to the subscriber, the copper loop to the subscribershould be as short as possible, which means that the DSLAMS consisting of VDSL

cards might mainly be located in basements of the buildings for MDU/MTUapplications as well as the hospitality suites. Plus, they are likely to be fed by a highcapacity fiber optic link from a central site but occasionally directly from a CO or

switch.

The asymmetrical version of VDSL works well in the consumer space and would beused mainly to offer various applications to the consumers. Some of the ideal

applications for VDSL include providing video over the copper loop, provisioningmultiple TV channels, video conferencing, streaming video, and so on. On the other

hand, the symmetrical version of VDSL can be a suitable option for business classservices. The bandwidth offered by VDSL today is far more than most people need.This would change, however, with the growing demand of video applications onxDSL.

IDSL

IDSL makes use of the ISDN 2B1Q encoding method but for permanent (or always-on) connectivity, unlike ISDN that enables dynamically initiating/terminating the

connection. IDSL allows for the use of the D channel along with the 2 B channels ofISDN and, hence, can achieve the bandwidth of 144 kbps.

The maximum achievable bandwidth for IDSL is 144 kbps in both directions, and it issymmetric. IDSL is repeatable (which implies that you can have a repeater in theloop to amplify the signal), so it can be offered at longer distances where ADSL

usually cannot be offered. Because of this distance advantage over ADSL, IDSL hasbecome an attractive option for regions where ADSL can't be offered and thesubscribers want to take advantage of high-speed access instead of the slower

analog modem connection. One good question everyone asks at this juncture is,"Why not use plain ISDN, which also provides 128 kbps using two B channels?" Themain difference between ISDN and IDSL is that, with ISDN, the D channel is usuallynot used to carry dataalthough some real applications utilize the D channel to carry

data, like X.25 over D Channel, fewer companies in Europe adopted it consequentlysacrificing 16 kbps. In the case of IDSL, the D channel is used along with two B

channels for carrying data. The other major difference between IDSL and ISDN isthat ISDN is typically not an always-on technology and can initiate the call as andwhen required. However, IDSL is always onyou cannot tear down any of the B or Dchannels if they are idle.

Because a majority of the telephone systems in Europe makes use of ISDNtechnology, ADSL over ISDN is gaining popularity in Europe. ADSL over ISDN is

accomplished by overlaying the DMT encapsulation over the 2B1Q encoding,consequently separating the frequency spectrum of DMT and 2B1Q so that they both


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can coexist and DMT can be carried over 2B1Q. Because in countries, such asGermany, where there is a widespread deployment of ISDN, the coexistence of ADSL

and ISDN on the same copper loop is a requirement. Therefore, the ADSL spectrumin this case can only start at 140 kHz because ISDN occupies a much largerspectrum in the baseband. You need the splitters the same way as discussed in the

ADSL section, but in this case, the splitter is used to separate ISDN from DMT.

SDSL

SDSL is symmetric in nature, which in simple language means you get the same

bandwidth in both upstream and downstream direction. With SDSL, the maximumbandwidth that can be achieved is 2.3 Mbps. The typical maximum reach for SDSL isapproximately 4 km, or 12,000 feet.

SDSL, like IDSL, makes use of 2B1Q encoding. SDSL, however, is not repeatable,meaning that you cannot connect repeaters to boost the signal. The inability to userepeaters with SDSL results in the same distance limitation approach as that of

ADSL. SDSL, unlike ADSL, is not rate adaptive, implying that once the two modemsare trained up they will not rate adapt to a lower rate in case of noise interference orother factors.

SDSL because of its symmetric nature becomes an attractive option for small offices,home offices, or business customer spaces that are looking for cheaper alternative

for traditional time-division multiplexing (TDM) lines. SDSL has been a means ofentry for CLECs to take the leased line business and has been quite successful in thatspace. However, SDSL implementation is not based on any standard, and it isproprietary with each vendor based on the chipsets they use. For this reason, vendor

compatibility with SDSL is a big issue and, therefore, has not become the ultimatechoice of technology for leased line replacement. Most of the CLECs and incumbentlocal exchange carriers (ILECs) are awaiting the arrival of standards-based G.shdsl,

which is presumed will ultimately dominate the Leased Line Replacements and will bethe preferred choice of most DSL providers for business class users.

G.shdsl

Recently, International Telecommunications Union (ITU) determined the highlyanticipated G.shdsl standard. This new standard was developed to replace or

enhance many older or existing DSL technologies and other transport options suchas HDSL, DSL, HDSL2, ISDN, T1, E1, and IDSL. Until now, telecommunicationequipment vendors were required to develop several different line cards toaccommodate each of the services that were offered by the technologies listed

above. The new G.shdsl Draft Standard will enable equipment manufacturers todevelop CO loop access equipment and CPE around a single standard, enabling themto address the interoperability issues faced by other xDSL implementations as

discussed earlier.

G.shdsl is a technology that encompasses all functions which are currently provided

by the European SDSL standard and HDSL2 including Overlapped Phase Trellis-codedInterlocking Spectrum (OPTIS) spectral shaping. G.shdsl is multirate, multiservice,extended-reach, and repeatable.


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It is multirate because it supports data rates from 192 kbps to 2.3 Mbps. G.shdsluses G.hs (handshake) to negotiate the framing protocol. Protocols supported

include ATM, T1, E1, ISDN, and IP. The flexibility of G.shdsl enables the transport ofvirtually any type of service. It makes use of Trellis Coded Pulse AmplitudeModulation (TC-PAM) line coding that enables interoperation due to the low

complexity level of the transceivers. G.shdsl is suppose to deliver approximately 30

percent greater reach than currently deployed transport technologies. G.shdsl isexpected to rapidly replace the proprietary SDSL implementations of today and is

mainly used for business-class users.

Comparing xDSL Flavors

Table 2-2 summarizes the various features of each xDSL flavor we discussed and

which segment of market they fit in.

T a b le 2 - 2 .

ADSL IDSL SDSL G.shdsl

Standard Yes, ANSI T1.413, ITU G.992.1 No No Yes (ITU)

Rate Adaptive Yes No No Yes

Repeatable No Yes No Yes

ATM Framing Yes No No Yes

T1 Framing No No No Yes

E1 Framing No No No Yes

Design Considerations at the Physical Layer(Layer 1)

The purpose of this section is to make you aware of some of the challenges faced atLayer 1 while deploying DSL. Additionally, because this book focus mainly on ADSL,we will discuss some important issues related with DMT and its various options.

Copper Loop Issues

One of the major challenges to next-generation data networking over PSTN is the

copper pair itself. The nature of copper windings and the traditional cabling practicesused in local loops creates huge barriers to technologies, such as ADSL. Thisinfluenced a requirement for rate adaptive and resilient signaling techniques to

compensate for the issues that are inherent in a twisted-pair local loop.

The following issues are all potential hindrances to a voice or DSL network:

Since the early days of telephony, load coils have been applied to long loopsto boost and flatten the frequency response of the line at the upper edge of


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the voice band. These load coils increase attenuation greatly at frequenciesabove the voice band; therefore, load coils must be removed for DSL to

function on a local loop. In multicarrier systems, inter-carrier interference can occur. This happens as

a result of frequency dependent attenuation and dispersion, which can vary

between carriers. With the infringement of a variety of carriers into the CO,

this becomes a serious concern for DSL deployments. In the process of connecting and disconnecting portions of a loop cabling,

portions of open-circuit wiring can sometimes be left connected to a workingtwisted pair. The presence of these bridge taps can affect the frequencyresponse of the loop and cause distortion and interference.

Reflections and distortions can also be caused by loops made up of wires ofdiffering diameter. This is a frequent problem in older wiring that has beenmodified over time.

Impulse noise (a brief, high amplitude jolt of noise introduced into the cablingfrom outside sources) is a major problem in copper wiring. Lightning andswitching equipment transients are common causes of impulse noise. This iscommon in older COs, which are not properly insulated and older network

equipment that has not been properly maintained.

Radio frequency (RF) interference is very common in twisted pairs, especiallywith the cabling techniques common in the U.S. Various radio sources can

impair with a copper transmission medium, the foremost of which is theamateur radio transmitter. Ham radio operators can disrupt telephonecommunication in a large area around their transmitters if the local loops are

extremely long or vertical. RF interference becomes a bigger problem asfrequency increases, making DSL applications more susceptible.

The impact of these points can vary widely depending on particular network

topologies and the specifics of the local loop cabling plant. Both voice and DSLswitching systems have designed solutions for dealing with each of these issues tosome degree.

Reach Versus Bandwidth

One of the challenges faced by the DSL providers today is that they would like tooffer DSL services to as many subscribers as possible, but that is not easilyachievable because of the distance limitations seen in various DSL technology. Alsothe further the subscriber is from the CO the less bandwidth the subscriber can get.

The next couple of sections discuss some of the factors that affect the rate that canbe offered to subscribers.

Noise Margin

It is important to note that higher DSL rates result in lower SNRs and lower DSLrates result in higher SNRs. Therefore, the noise margin becomes lower at longer

cable lengths and at higher DSL rates. When a bit error rate (BER) of 107 can nolonger be maintained, an automatic reduction in DSL rate normally occurs in DSLflavors that are rate adaptive, such as ADSL and G.shdsl.


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RADSL (rate adaptive DSL) is a technique that automatically adjusts the DSLtransmission speed to a rate where the appropriate noise margin can be maintained

and enables a 107 BER to be maintained.

When DSL service is provisioned in a DSLAM, the minimum acceptable noise marginis usually specified. CAP DSL service is typically provisioned with a downstream

margin of 3 dB and an upstream margin of 6 dB. Research has shown that theoptimum margins for DMT service are 6 dB downstream and 6 dB upstream.

Avoiding configuring a DSL service with more noise margin than appropriate isimportant because the system will train to an unnecessarily low DSL rate to providethe specified margin. It is also important to avoid specifying an exceptionally low

margin, such as 1 dB downstream and 1 dB upstream because a small increase innoise level on the transmission line would probably result in excessive errors and asubsequent retraining to a lower DSL rate.

If a DSL line is at maximum reach (maximum cable length) and the modems will nottrain, the margins may be set to zero for troubleshooting purposes only. For

example, if a DSL line trains with the margins set to 0 but will not train when themargins are set to 6 dB, the line length is probably at maximum reach (typically 18kft of 24-gauge wire). If providing service at any cost is necessary, a compromisemargin between 0 and 6 dB can be selected. In such cases, determining whether the

source of the problem is in the downstream spectrum or in the upstream spectrum isimportant, and make adjustments accordingly. For example, a margin setting of 3 dBin the downstream spectrum can be necessary to provide a preferred DSL rate at 18

feet of cable.

Increasing the transmit power levels will also improve the noise margin but at thecost of interfering with other services in the same cable.

Most DSLAMs and CPE report both the provisioned and actual noise margins for eachDSL line. If the actual margin is higher than the provisioned margin, the line shouldprovide an acceptable error rate at the present DSL line rate. As the actual margin

drops below the provisioned margin, there is a high probability of an excessive errorrate and subsequent retrain to a lower DSL rate.

The general indicator of acceptable DSL performance is when the actual margin isbetter than an appropriately set minimum margin, and the DSL line is running at thedesired data rate.

Forward Error Correct ion

FEC refers to the process of correcting errors mathematically at the receiving end ofa transmission path, rather than calling for a retransmit of the erred data.

Retransmitting data to correct errors uses the available bandwidth to repeatedlysend the same information, and the user perceives very slow throughput. FEC resultsin greater effective throughput of user data because valuable bandwidth is not being

used to retransmit erred data.

When errors can be corrected without retransmitting the data, the errors arereported as corrected errors. When errors can't be corrected by the algorithm, they


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are reported as uncorrected errors. The ratio of corrected to uncorrected errorsshows the relative effectiveness of the error correction algorithm, or the relative

intensity of the errors.

FEC Bytes

Also called check bytes or redundancy bytes, FEC bytes are added to the user datastream to produce a means of calculating the presence of erred data and generatinga corrected frame. The appropriate number of FEC bytes provisioned generallydepends on the type of errors being detected and corrected. It is important to note

that the more FEC bytes that are added to the data stream the more bandwidth theusers' data shares with the FEC bytes.

The tradeoff is being able to correct errors without retransmission versusdisplacement of user data. It is generally observed that much better throughput,however, is achieved by increased efficiency in FEC rather than by retransmittingerred data.

In many systems, the selectable number of FEC bytes is 0 (none), 2, 4, 8, 12, or 16.As a very basic and general rule, the more FEC bytes used the more effective is the

error correction. But in error-free transmission paths, an unnecessary number of FECbytes serves only to displace user data in the available bandwidth. For example, 16FEC bytes per frame displaces much more user data at a transmission rate of 256

kbps that the same number of FEC bytes displaces at a transmission rate of 8 Mbps.That is the presence and number of FEC bytes at 256 kbps is more apparent to theuser as reduced throughput than the same number of FEC bytes at 8 Mbps.

Note that some chipsets optimize the number of FEC bytes versus the requested ATMtransmission rate. For example if a downstream rate of 8.032 Mbps and 16 FECbytes is specified in the line configuration but the error rate is within the 107 BER

limit, the chipset may set the number of FEC bytes to a lower number to create therequested user bandwidth. When DMT performance statistics are reviewed, thechipset reports a fewer number of FEC bytes than what was specified in the

configuration.

Before specifying the most efficient number of FEC bytes and the most efficientinterleave delay, we must determine the pattern of errors occurring on the

transmission path. Errors occur in two general patterns: bursty or dribbling.

Interleaving

Interleaving is the process of scrambling user data in a very precise sequence. The

purpose of interleaving is to avoid having consecutive errors delivered to the RS FECalgorithm at the receiving end of the circuit. RS is much more effective on singleerrors or errors that are more spaced in time (not consecutive).

If a noise burst occurs on the copper transmission line, several consecutive data bitscan be affected resulting in consecutive bit errors. Because the data was interleaved

at the transmitter, de-interleaving at the receiver produces not only the original bitsequence, but separates the erred bits over time. (The erred bits appear in separate


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bytes.) Therefore, the erred bits are no longer consecutive and the RS FEC process ismuch more effective.

Latency

In many systems, interleaving can be set to 0 (off, none), 1, 2, 4, 8, or 16milliseconds of interleave delay or interleave depth. The more milliseconds of time

allocated to interleaving, the more data can be interleaved. But increasinginterleaving delay will cause additional latency or delay in the time the data istransmitted and the time it is available to the receiving user.

As a general rule, the more interleaving delay that is used the better the RSalgorithm can correct consecutive errors. The increased latency normally causes no

problems for general data transmission but digitized voice over a high-latency pathresults in extremely unpleasant echo. For this reason, a minimum interleave delay(or no interleaving) is always used on data channels carrying voice traffic. As delay isadded to voice transmissions, the problem of echo increases radically and requires

additional treatment. Two-way video conferencing can also experience some

undesirable effects from excessive latency in the data stream.

A relatively high error rate can usually be tolerated during voice conversations, and ahuman ear might not even detect the errors. Additional consideration of minimizedlatency versus error correction may be required, however, when analog data or fax is

also running on the voice channel. Conversely, greater latency (delay) is notparticularly detrimental to data transmission. Increasing latency does not usuallyreduce the transmission speed (throughput). Effective FEC, partially resulting from

increased interleaving, can contribute significantly to achieving maximum throughputin a noisy environment.

Bursty Errors

Bursty errors are multiple errors that occur in very short timeframes. For example, ina one-minute timeframe, there might have been a total of 100 errors. The 100errors, however, might have occurred in bursts of 10 errors at a time, spaced several

seconds apart.

To determine if errors are bursty, inspect the total DSL trained-up time and erred-seconds counters. If a unit has been trained for one hour and has reported 100

errors and one erred second, the errors are bursty. By contrast, if the unit has beentrained for one hour and has reported 1000 errors and 1000 erred seconds, theerrors are dribbling. (Dribbling errors are covered in the following section.)

Noting the ratio of corrected errors to uncorrected errors is important. If all reportederrors are corrected (no uncorrected errors), no further action is required.

For treating uncorrected bursty errors, increase the interleave delay (interleavedepth). In most systems, interleave delay can be set as low as 1000 microseconds(one millisecond) and as high as 16,000 microseconds (16 milliseconds). If very few

erred seconds are detected over a period of several hours, additional correctivemeasures might not be necessary.


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Dribbl ing Errors

Dribbling errors occur usually one at a time (spaced by milliseconds to seconds) and

continue to occur over any given timeframe.

To treat a steady stream of uncorrected dribbling errors, increase the number of FECbytes. Increasing the number of FEC bytes adds baggage to the datastream. This

results in a tradeoff between correcting errors and reducing the bandwidth availableto the user. The effect of increasing FEC bytes depends on the data rate: Slower datarates demonstrate more bandwidth loss. As a general guideline, do not add more

FEC bytes than what is required to correct the errors.

Coding Gain

When using FEC, typically RS encoding, errors might be corrected at the receiverwithout having to use TCP for block retransmits. When errors are corrected in thisway, it has the same effect of using higher noise margins without the relatedreduction in DSL trained rates. This effect is called coding gain and is expressed in

dB (the equivalent dB of margin).

Without coding gain, we need a specific SNR to achieve and maintain an ATM data

error rate of 107 or better. With coding gain, we can achieve and maintain a 107error rate with a lower SNR. Errors might still be occurring on the transmission linebut are being corrected by the RS algorithm. The resulting error-free (corrected)

data rate is the same as if a higher noise margin was used.

DSL Modem Train-Up

The initialization sequence conducted between two modems is referred to as training,

or train-up.

During train-up, DSL modems start with no data interleaving and no line coding.(Interleaving and line coding must be negotiated between the two modems.) The

signal-to-noise level is calculated across the DSL spectrum, and a DSL line rate isestablished based on three essential factors:

Provisioning The DSL modems cannot establish a faster DSL rate thanwhat is specified (provisioned) in the DSLAM. The DSL trained rate is typicallyspecified in terms of available user bandwidth rather than actual DSL

spectrum used. DMT options An unnecessarily high noise margin setting results in an

unnecessarily low DSL trained rate. The recommended noise margin settings

for DMT are 6 dB downstream and 6 dB upstream. (The recommendation forCAP systems is traditionally 3 dB downstream and 6 dB upstream.)

Line conditions This includes several factors, but primarily two:attenuation and noise levels. It is important to note that more than one noise

source might be contributing to the total noise spectrum on the line, thusproducing multiple noise frequencies with related noise (power) levels. Somenoise sources are more detrimental to DSL performance than others, again

depending on the spectrum and power level of the noise induced into the DSLcircuit.


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Train-Up M ode Options

In DSLAMs options are available sometimes for specifying the training procedure. For

example, in Cisco DSLAMs, you might encounter the options standard train and quicktrain, or standard train and fast train. Standard train relates to a training procedurespecified in ANSI standards document T1.413, which is considered the standards

reference for DMT ADSL. (Any manufacturer offering the option standard train shouldensure the option is T1.413 compliant.) Quick train, fast train, and so forth can bemore proprietary and perform best when used with the same manufacturer's

modems (and usually the same DSL chipset) on both ends of the telephone line.Furthermore, quick train might not always result in a faster train-up by the modems.

Train-Up Problem s

If a DSL modem cannot train to the ATU-C in the DSLAM, any of the following is apossible problem:

Excessive line length, or exceeding the maximum reach An ILECknows how long any given cable pair is, but testing cable length dynamicallyrequires expensive test equipment. It's also important to note the differencebetween 24- and 26-gauge wire. Without getting into pages of calculations,

we can say that 26-gauge wire is significantly more attenuative at allfrequencies than 24-gauge wire. This becomes especially apparent at DSLfrequencies, where 18,000 feet of 24-gauge cable is roughly equivalent to

15,000 feet of 26-gauge wire. Bridge taps and half taps A bridge tap and half tap are the same thing. A

bridge tap occurs when a section of cable is connected to a telephone line at

some midpoint between the CO and the CPE, usually done to prepare for acable reroute. Bridge taps can significantly alter the impedance of a telephoneline, especially at DSL frequencies. Impedance mismatches cause a wide

variety of symptoms, typically reflection of data bits from the tap-point backto the point of transmission, causing bit errors. A tapped-in stub can evenbecome a tuned filter at DSL frequencies, causing excessive attenuation of

DSL signals. Figure 2-6 illustrates the concept of a bridge tap.

Fi g u r e 2 - 6 . B r i d g e T a p


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Interfering signals This can become extremely complicated and difficultto diagnose without spectrum analysis. If a single DSL line is activated in a

25-pair binder (a group of pairs), we can measure a specific noise margin. IfDSL is activated on 10 more pairs in the 25-pair binder, the margin degradesnoticeably.

AM radio stations have been known to cause a degradation of DSL service butshould not cause a no-train condition unless the modems are already atmaximum reach. The DSL spectrum is roughly from 25 kHz to 1.1 MHz, and

the AM radio band (in North America) is roughly from 550 kHz to 1.7 MHz.The overlap in the two spectrums enables powerful radio stations to interferewith highly attenuated DSL signals on longer cable lengths.

DMT interferes with CAP more than other CAP signals. SDSL interferes withDMT. ISDN interferes with CAP. Basically, everything interferes witheverything else. The important thing is that an acceptable noise margin be

maintained, and this is controlled by attenuation (cable length and wiregauge), bridge taps on the line, and the spectrum and power level of

interfering signals.

Finally, note that a small amount of noise is also produced by the digitalsignal processors and DMT modulation subsystems. The term channel noise is

used to identify noise on the transmission path rather than internal noise in amodem.

Load coils Load coils are typically 88 millihenry (mH) chokes placed in thetip and ring lead of the cable pair at specific intervals to add more inductancethan can be produced by twisting the wire pairs. The added inductancecounteracts the effect of capacitance between thousands of feet of a wire pair

by creating a very effective bandpass filter that is optimized for a flatresponse between 300 and 3000 Hz.

Without load coils on cables of 15,000 feet or longer, excessive capacitance

will begin to radically attenuate the higher frequencies of the voice band.Analog modem and fax performance begins to degrade as well as voicequality.

Load coils must be removed from cable pairs when DSL service is provided.What makes the job somewhat difficult and expensive is that, when load coils

are placed in a cable, they are installed 3 kft from each end and at 6 kftintervals between each end. Fortunately for DSL service, many telephonecompanies do not load cables shorter than 10 kft, so fewer cable pairs haveto be unloaded for DSL service.

Other communication devices connected to the phone line Atelephone or fax machine connected to the phone line without a microfilter or

DSL splitter will cause excessive loading on the line when off the hook. Onlonger cable lengths, a DSL modem cannot train when a phone is off the hookunless a microfilter is used at the telephone or a splitter is placed between

the phone and DSL modem.


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On shorter cable lengths, taking a phone off the hook without a filter orsplitter causes the DSL modem to untrain and then retrain at a lower DSL

rate. But things can get tricky. If a cordless phone base unit is connected tothe phone line without a microfilter, the RF pi-filter in the base unit is muchmore reactive (lower resistance) across the wire pair at lower DSL frequencies

than at higher frequencies. The DSL downstream rate appears normal, but

the upstream rate is lower than normal. Fortunately, installing a microfilter inthe phone line to the base unit corrects the problem.

Radio interference (RFI) filters on the phone line RFI filters areinstalled in many areas where AM radio stations can be heard on telephonesduring conversations. The most basic RFI filters are simply capacitors placed

across the phone line. More sophisticated RF filters also include inductorsplaced in the tip and ring. When capacitors are placed across the phone line(parallel) and inductors are inserted into the phone line (series), DSL

frequencies can be removed completely. RFI filters have no affect at voicefrequencies but appear as a short circuit at radio (and DSL) frequencies. RFIfilters can cause degradation of DSL performance on shorter cable lengths

and can prevent DSL modems from training on longer cable lengths.

Power Levels In DSL Services

DSL power levels are much higher than used in voice, fax, or analog data services.

This is simply because of the much greater attenuation of signals at D

Documents

Cisco Press - Design and Implementation of DSL