MARKETING REPORT MR-001 DSL Anywhere – Issue 2paginas.fe.up.pt/~mleitao/STEL/Tecnico/DSLAnywhere_Issue2.pdf · MARKETING REPORT MR-001 DSL Anywhere – Issue 2 September 2004 Produced

MARKETING REPORT MR-001

DSL Anywhere – Issue 2 September 2004

Produced by the DSL Anywhere Task Force of the Marketing and

Technical Working Groups Editors in Chief: Peter J. Silverman - Pedestal Networks

Tim Waters - Celite Systems

DSL Forum Marketing Chair: Jay Fausch, Alcatel DSL Forum Technical Chair: Gavin Young, Bulldog

Communications Ltd.

Abstract: Over the last several years, DSL service has experienced ever-increasing demand from the subscriber community. However, a portion of the world-wide user community continues to be unable to get DSL service because they are either located at too great a distance from the service provider’s central office (CO) or they are not served by a copper loop directly from the CO. In addition, the need to support higher-bandwidth solutions such as on-demand video services has driven the need for continuous innovation in DSL technologies. This paper offers architectural solutions and techniques for both extending DSL coverage to help service providers deploy ‘DSL Anywhere’, as well as new technologies to support ever-increasing bandwidth requirements.

Notice: The DSL Forum is a non-profit corporation organized to create guidelines for DSL network system development and deployment. This Marketing Report has been reviewed by members of the Forum and approved for publication by the Board of Directors of the DSL Forum to provide useful information to the industry and the public regarding the deployment and use of DSL networks and technology. This document is not binding on the DSL Forum, any of its members, or any developer or service provider involved in DSL. Nothing in this document constitutes endorsement by the DSL Forum of any product, or service. The document is subject to change, but only with approval of the Board of Directors of the DSL Forum. ©2004 Digital Subscriber Line Forum. All Rights Reserved. DSL Forum market reports may be copied, downloaded, stored on a server or otherwise re-distributed in their entirety only. Notwithstanding anything to the contrary, the DSL Forum makes no representation or warranty, expressed or implied, concerning this publication, its contents or the completeness, accuracy, or applicability of any information contained in this publication. No liability of any kind shall be assumed by the DSL Forum as a result of reliance upon any information contained in this publication. The DSL Forum does not assume any responsibility to update or correct any information in this publication. The receipt or any use of this document or its contents does not in any way create by implication or otherwise, any express or implied license or right to any patent, copyright, trademark, or trade secret rights which are or may be associated with the ideas, techniques, concepts, or expressions contained herein.

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About the DSL Forum

The Digital Subscriber Line Forum is a consortium of nearly 200 leading industry players including telecommunications, equipment, computing, networking, and service provider companies. Established in 1994, the Forum continues its drive for a global mass market for DSL broadband, to deliver the benefits of this technology to end users around the world over existing copper telephone wire infrastructures. In ten years, the DSL Forum has moved through defining the core DSL technology to delivering maximum effectiveness in its deployment and use. Best practices for auto-configuration, flow-through provisioning, network architecture, equipment interoperability, and other key facilitators of scalable, global, mass-market deployment of DSL broadband are fast-tracked by the DSL Forum. Outcomes of that work are published as Technical Reports for use throughout the global industry and are available from our website www.dslforum.org/aboutdsl/tr_table.html. Industry-wide support for, and contribution to, DSL Forum's prioritized action plan to support the global market has been unparalleled with a consistently high membership of nearly 200 global DSL companies. Each member company contributes to the work of the Forum through the development of the technology and its effective delivery. They participate in technical and marketing working groups, sharing their knowledge, experience, and expertise to create common, agreed protocols, processes, and best practice recommendations for use by the industry and for standards and other related industry bodies. This work takes place at quarterly, weeklong meetings and through the continuous activity of both the technical and marketing working groups. These Forum meetings foster a sharing of knowledge and best practices between members to make DSL the world's primary choice for broadband services. Through its marketing activities - an extensive, continuous, global public and industry education campaign - the DSL Forum also ensures a growing international understanding of the benefits of DSL broadband. Further information on the DSL Forum, its work, members, and meeting schedule can be found at www.dslforum.org. DSL Forum's website dedicated to providing information to end-users can be found at www.dsllife.com.

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http://www.dslforum.org/aboutdsl/tr_table.html

http://www.dslforum.org/

http://www.dsllife.com/

Table of Contents

Table of Contents........................................................................................................................... iv Acknowledgements........................................................................................................................ vi Acronym Guide............................................................................................................................ viii Preface............................................................................................................................................. 1 1 Introduction................................................................................................................................ 2

1.1 SCOPE..........................................................................................................................................................2 1.2 FIXED LINE AND DSL DEPLOYMENT – MARCH 2004..................................................................................3 1.3 DSL AVAILABILITY WORLDWIDE...............................................................................................................5 1.4 MARKET REQUIREMENTS FOR EXTENDING DSL SERVICE ..........................................................................7 1.5 DSL ANYWHERE ANALYSIS........................................................................................................................9

2 Loop Qualification.................................................................................................................. 10

2.1 NETWORK CONDITIONS AFFECTING DSL PERFORMANCE..............................................................................11 2.1.2 Loading Coils ..........................................................................................................................................11 2.1.3 Bridged Taps ...........................................................................................................................................12 2.1.4 Faults ......................................................................................................................................................12 2.1.5 Pair-gain devices ....................................................................................................................................12 2.1.6 Crosstalk and Noise ................................................................................................................................12

2.2 LOOP QUALIFICATION TECHNIQUES ..............................................................................................................12 2.2.1 Loop Qualification Processes and Systems.............................................................................................13

2.3 SUMMARY .....................................................................................................................................................24 3 Overlay Access Solutions ..................................................................................................... 25

3.1 REMOTE ACCESS MULTIPLEXER....................................................................................................................28 3.1.1 Description of Architecture.....................................................................................................................30

3.2 SAI-BASED DSLAM .....................................................................................................................................30 3.2.1 Description of Architecture/Technique ...................................................................................................31

3.3 FIBER-OPTIC DSL EXTENDERS.......................................................................................................................35 3.3.1 Architecture.............................................................................................................................................35

3.4 PON-FED DSL REMOTE EXTENSIONS ...........................................................................................................37 3.4.1 Advantages ..............................................................................................................................................39 3.4.2 Implementation and Deployment Issues..................................................................................................39

4 Integrated POTS+DSL Access Solutions ............................................................................. 41

4.1 DLC LINECARD ........................................................................................................................................42 4.1.1 Introduction .........................................................................................................................................42 4.1.2 Architecture .........................................................................................................................................44 4.1.3 Advantages of integrated POTS+DSL linecards .................................................................................45 4.1.4 Conclusion ...........................................................................................................................................45

4.2 NEXT-GENERATION DIGITAL LOOP CARRIERS..........................................................................................46 4.2.1 Introduction .........................................................................................................................................46 4.2.2 Architecture .........................................................................................................................................46 4.2.2 Advantages of a Broadband NGDLC ..................................................................................................47 4.2.3 Operational and Deployment Issues .........................................................................................................48

4.3 BROADBAND LOOP CARRIER.....................................................................................................................49 4.3.1 Introduction .........................................................................................................................................49

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4.3.2 Architecture .........................................................................................................................................49 4.3.3 Advantages of a BLC ...........................................................................................................................50 4.3.4 Conclusion ...........................................................................................................................................51

5 Loop Extenders and Repeaters.............................................................................................. 52

5.1 LOOP EXTENSION ...........................................................................................................................................52 5.1.1 Description of Architecture/Technique ..................................................................................................52 5.1.2 Advantages ..............................................................................................................................................53 5.1.3 Implementation/Deployment Issues.........................................................................................................54 5.1.4 Operational Issues ...............................................................................................................................54

5.2 MID-SPAN REPEATER................................................................................................................................54 5.2.1 Description of Architecture/Technique................................................................................................54 5.2.2 Advantages...........................................................................................................................................55 5.2.3 Implementation/Deployment Issues .....................................................................................................56 5.2.4 Operational Issues ...............................................................................................................................57 5.2.5 Network Management Issues ...............................................................................................................57

6 Standardized DSL Technology Options ................................................................................. 58

6.1 DSL STANDARDS ..........................................................................................................................................58 6.1.1 ADSL - G.992.1 & G.992.2 .....................................................................................................................61 6.1.2 ADSL2 - G.992.3 & G.992.4 ...................................................................................................................61 6.1.3 ADSL2plus - G.992.5 ..............................................................................................................................62 6.1.4 ADSL Annexes.........................................................................................................................................63 6.1.5 Reach Extended ADSL2 – G.992.3 Annex L............................................................................................63 6.1.6 SHDSL.....................................................................................................................................................64 6.1.7 VDSL .......................................................................................................................................................65

6.2 BONDED DSL ................................................................................................................................................66 6.2.1 Ethernet-Based Bonding .........................................................................................................................67 6.2.2 ATM-Based Bonding ...............................................................................................................................68 6.2.3 TDIM-Based Bonding .............................................................................................................................70

6.3 FUTURE ENHANCEMENTS ..............................................................................................................................72 7 Additional Solutions ............................................................................................................. 73

7.1 LOW FREQUENCY DSL – IMPROVED REACH TECHNOLOGY......................................................................73 7.1.1 Description ..........................................................................................................................................73 7.1.2 Advantages...........................................................................................................................................74 7.1.3 Implementation / Deployment Issues ...................................................................................................75 7.1.4 Summary ..............................................................................................................................................75

7.2 VDSL2 – NEXT-GENERATION VDSL .......................................................................................................76

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Acknowledgements Creating the second issue of the DSL Anywhere Marketing Report required the work of many people over many months. Although this document is based on the original issue of the DSL Anywhere white paper, issued in 2001, advances in the technology and increases in the expectations of carriers, equipment providers, and end-users regarding the capabilities of DSL services have necessitated a complete rewriting of the original paper. The work of the contributors to this document and the support provided by the companies they represent in allowing this paper to be produced is very much the basis for the strengths of this paper. Special thanks must be given to the ‘Section Editors’ who stewarded each individual chapter and are in large part responsible for the high quality of this document. The following people served as editors and coordinators in producing this paper: Editors in Chief: Peter J. Silverman Pedestal Networks Tim Waters Celite Systems Copy Editing and Layout: Chris Frederick Critical Telecom Administration and Coordination Laurie Gonzales DSL Forum Section Editors Section 1: Tim Waters Celite Systems Section 2 Frank Bauer Teradyne Section 3 Peter Silverman Pedestal Networks Section 4 Gary Bolton CIENA Section 5 Barry Dropping Symmetricom Section 6: Sarah LaLiberte Aware Section 7: Ken Ko Paradyne The following people contributed to the text of the document: Frank Bauer Teradyne Gary Bolton CIENA George Dobrowski Conexant Barry Dropping Symmetricom Jay Fausch Alcatel Chris Frederick Critical Telecom Maryanna Gundal Pedestal Networks Leslie Hansen Nettonet Tim Johnson Point Topic Ken Ko Paradyne Kristi Kosloske DSL Forum Peter LeBlanc Aware

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Sarah LaLiberte Aware Sascha Lindecke Infineon Malcolm Loro CIENA Don McCullough Entrisphere Lane Moss Alcatel Ellis Reid Pedestal Networks Peter Silverman Pedestal Networks Tim Waters Celite Systems Arlynn Wilson Adtran Pete Youngberg Sprint Additionally, Ishai Ilani and Matt Squire provided insight on progress toward standardization of bonding for DSL.

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Acronym Guide Acronym Expansion ADL All Digital Loop ADSL Asymmetric DSL AMI Alternate Mark Inversion ANSI American National Standards Institute ATIS Alliance for Telecommunications Industry SolutionsATM Asynchronous Transfer Mode AWG American Wire Gauge BLC Broadband Loop Carrier B-NGDLC Broadband NGDLC CAPEX Capital Expenditure CBR Constant Bit Rate CEV Controlled Environmental Vault CLEC Competitive Local Exchange Carrier CO Central Office CORBA Common Object Request Broker Architecture CPE Customer Premises Equipment CSA Customer Serving/Service Area DDS Digital Data Storage DLC Digital Loop Carrier DMT Discrete Multitone DOCSIS Data Over Cable Service Interface Specification DSL Digital Subscriber Line DSLAM DSL Access Multiplexer DSP Digital Signal Processing DSx Digital Signal EFM Ethernet in the First Mile EMS Element Management System(s) ETSI European Telecommunications Standards Institute FCC Federal Communications Commission FEC Forward Error Correction FSAN Full Service Access Network FTTP Fiber To The Premises HDSL High-speed DSL HDSL2 The two wire mode of the ANSI T1.418-2002

Standard HDSL4 The four wire mode of the ANSI T1.418-2002

Standard IDSL ISDN DSL IEEE Institute of Electrical and Electronics Engineers ILEC Incumbent Local Exchange Carrier

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IMA Inverse Multiplexing over ATM IP Internet Protocol ISDN Integrated Services Digital Network ITU International Telecommunication Union JWI Junction Wiring Interface LAN Local Area Network LST Line and Station Transfer MAC Medium Access Control MDF Main Distribution Frame MGCP Media Gateway Control Protocol MIB Management Information Base MLPPP Multilink Point-to-Point Protocol MSAP Multi-Service Access Platform NEBS Network Equipment Building System NGDLC Next-Generation DLC NMS Network Management System(s) OAM Operation and Maintenance OC-x Optical Carrier OLT Optical Line Terminal ONT Optical Network Terminal ONU Optical Network Unit OPEX Operational Expenditure OSS Operational Support System(s) PMT-TC Packet Mode Transmission Transconvergence PON Passive Optical Network POTS Plain Old Telephone Service PSD Power Spectral Density PSTN Public Switched Telephone Network QAM Quadrature Amplitude Modulation QOS Quality of Service RAM Remote Access Multiplexer RE ADSL Reach-Extended ADSL RT Remote Terminal SAI Serving Area Interface SDSL Symmetric DSL SHDSL Single-Pair High-Speed DSL SID Sequence ID SLC Subscriber Loop Carrier SNMP Simple Network Management Protocol SNR Signal-to-Noise Ratio SONET Synchronous Optical Network TC PAM Trellis Coded Pulse Amplitude Modulation

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TDIM Time Division Inverse Multiplexing TDM Time Division Multiplexing TDMA Time Division Multiple Access UBR Unspecified Bit Rate UL Underwriters Laboratories VBR Variable Bit Rate VC Virtual Circuit VCI Virtual Circuit Identifier VDC Volts Direct Current VDSL Very-high-speed DSL VoIP Voice Over IP VP Virtual Path VPN Virtual Private Network WAN Wide Area Network Wi-Fi Wireless Fidelity

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Preface Welcome to DSL Forum’s DSL Anywhere White Paper. The original DSL Anywhere White Paper was published in 2001. This is our second release, updated in Summer 2004. The intent of this paper is to help service providers identify a number of different DSL technologies and solutions that enable cost-effective and efficient deployment of DSL to any customer, regardless of where they live. When selecting the appropriate DSL method or solution, service providers should carefully consider the application, the required data rates, the anticipated DSL penetration, and the evolution plans of their network. To assist the reader, the DSL Anywhere White Paper is presented in seven sections: Section 1 provides an overview and description of the current state of DSL deployments around the world. Section 1 also includes DSL availability data, penetration information, and an outline of the key business challenges faced by carriers deploying DSL today. Section 2 provides an update on loop qualification issues, as well as methods and procedures that facilitate the rapid and satisfactory qualification of DSL copper loops. Sections 3 and 4 present DSL solutions and technologies for overlay access and POTS+DSL respectively. Solutions and technologies included in Sections 3 and 4 use the copper loop for access, in conjunction with a standard DSL technology or a DSL technology that is at or beyond the ‘straw ballot’ stage of approval in a recognized standards body, such as the DSL Forum, ITU, ETSI, or ANSI T1E1. Section 5 reviews various DSL loop extender and repeater solutions. The use of loop extenders that use long reach technologies to carry limited reach services over long distances is discussed, as well as the use of repeaters to extend service coverage to practically 100%. Section 6 is an overview of DSL standards. Included in this section are existing, approved technologies, as well as DSL technologies that are at least at the straw ballot stage of approval by a recognized standards body. Lastly, Section 7 documents solutions that are not standards based, nor at this point up for straw ballot in a recognized standards organization. To be included in Section 7, solutions and technologies must be copper loop (DSL) access solutions, enable a wide deployment of DSL coverage to address ‘DSL Anywhere’ issues, and have either a carrier trial of a size and scope that is consistent with industry-accepted field trials, an actual network service deployment, or have carrier standardization support in the form of a carrier-sponsored ITU, T1E1, or similar contribution. We hope you enjoy our updated DSL Anywhere White Paper and find it a helpful tool in your assessment and deployment of DSL around the globe.

DSL Forum MR-01: DSL Anywhere issue 2 Preface Page 1

1 Introduction Over the last several years, DSL service has experienced ever-increasing demand from the subscriber community. The demand for high-speed Internet access, complemented by the continuous emergence of new, higher-bandwidth applications, continues to drive demand for DSL. Around the globe, DSL has seen increasing deployment and is now the most chosen broadband option in the world. Although most users world wide can receive DSL service from the service provider’s central office (CO), there are certain users that are more difficult to serve because they are either located at too great a distance from the CO or they are not served by a copper loop directly from the CO. In addition, the need to support higher-bandwidth solutions, such as on-demand video services, has driven continuous innovation in DSL technologies. This paper offers architectural solutions and techniques for extending DSL coverage to help service providers deploy ‘DSL Anywhere’, as well as new technologies to support ever-increasing bandwidth requirements. 1.1 Scope The initial release of this DSL Anywhere paper in 2001 was targeted primarily toward service providers in the United States. Since then, DSL has experienced explosive growth around the world. This edition is intended to capture a more global view of deployment in terms of numbers, techniques, opportunities, and issues. The term ‘DSL Anywhere’ refers to the DSL Forum’s initiative to encourage service providers to seek methods and solutions that greatly expand DSL availability to as many subscribers as possible. This initiative should also encourage industry vendors to continue to innovate and provide efficient, cost-effective solutions that enable viable business cases and thereby allow service providers to extend DSL availability. The methods and solutions described in this paper are some of the most recent and attractive options that are available to service providers. In the late 1990s, getting ADSL rolled out to the most people possible was the overall goal. Today, availability of DSL is just one metric that service providers need to consider. Service providers also need to consider the target applications and the required data rates for each application. As a result, DSL deployment models that significantly shorten the loop distance to the subscriber should be considered. For example, it may be viable to serve subscribers up to 18 kft (5.5 km) with 384-kbps service for Internet access. However, some service providers plan to provide services that need guaranteed data rates of 1.5 Mbps or more. Historically, very short subscriber loops were required for the delivery of video and other high-bandwidth services. As will be discussed in this paper, technology advances over the last four years, as well as those planned for the future, allow DSL service to reach much further with higher bandwidths. In summary, this DSL Anywhere paper attempts to provide insights into some of the different methods and solutions currently available. While the term ‘DSL Anywhere’ is the goal, we do not pretend that the options identified within will achieve 100% market coverage. We do hope that these options will significantly close the gap between the projected availability of DSL and the large number of subscribers that wish to subscribe to DSL service. DSL Forum MR-01: DSL Anywhere issue 2 Section 1: Introduction Page 2

1.2 Fixed Line and DSL Deployment – March 2004 At the end of 2003, there were approximately 1.1 billion fixed access lines installed in North America, Asia Pacific, Europe, the Middle East, Africa, and Latin America. Figure 1-1 presents a region-by-region view of fixed access line deployment.

Fixed Access Lines by Region (Millions of Lines)

22.36

329.46

87.25

206.19

12.66

433.65AfricaEuropeLatin AmericaNorth AmericaOceanaAsia

Source: ITU 2003

Figure 1-1: Worldwide deployment of fixed access lines – by region As calculated by the market research firm Point Topic in March 2004, the number of DSL lines has grown significantly around the world and has now reached 63.8 million subscribers. Asia Pacific, including China (11 million), Japan (10.3 million), and the U.S. (9.1 million) lead in total DSL line deployments. Western Europe, including France, Germany, the United Kingdom, and Italy has also experienced significant growth in the number of DSL subscribers over the last year. Figure 1-2 summarizes DSL deployment by region, while Figure 1-3 illustrates the top 10 countries per 100 population (penetration), and Figure 1-4 shows the top 10 countries per available phone lines.1

DSL Forum MR-01: DSL Anywhere issue 2 Section 1: Introduction Page 3

1 The DSL Forum periodically updates information about world-wide DSL deployment on its website: www.dslforum.org

11.3

18.7

32.2

1.6

63.8

0

10

20

30

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50

60

70

North America EMEA Asia Pacific Latin America

Mill

ions

of L

ines

DSL Deployment - March 2004

Source: Point Topic, March 2004

Figure 1-2: Worldwide deployment of DSL with associated penetration – by region

0

2

4

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DSL

line

s pe

r 100

pop

ulat

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South

Korea

Taiwan

Hong K

ong

Denmar

kJa

pan

Belgium

Canad

a

Sweden

Finland

Singapore

2000200120022003

Source: Point Topic

Figure 1-3: Top 10 DSL countries by 100 population


0

5

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f DSL

line

s pe

r 100

phon

e lin

es

South K

orea

Taiwan

Hong K

ong

Belgium

Japan

Denmark

Singapo

reIsr

ael

Finland

Canad

a

Source: Point Topic Q4 2003; countries with at least 250,000 lines

Figure 1-4: Top 10 DSL countries per 100 phone lines Underlying this growth and increased penetration has been the service providers’ ability to reach more homes with DSL. This in turn is a result of both increased investment and improved technologies. 1.3 DSL Availability Worldwide Also significant is the expansion in network availability. Point Topic has updated DSL availability data, as of March 2004, for the major regions around the globe, depicted in the following graphs:

Availability of DSL: USA

0%

10%

20%

30%

40%

50%

60%

70%

80%

Acc

ess

lines

with

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ava

ilabl

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BellSouth SBC Qwest Verizon Overall

Source: Point Topic, Q4 2003 DSL Forum MR-01: DSL Anywhere issue 2 Section 1: Introduction Page 5

Availability of DSL: Europe

0%10%20%30%40%50%60%70%80%90%

100%

Austri

a

Belgium

Denmar

k

Finland

France

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yIta

ly

Netherl

ands

Norway

Spain

Sweden UK

Overal

l

Source: Point Topic, Q4 2003 Availability of DSL: Asia-Pacific

0%10%20%30%40%50%60%70%80%90%

100%

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with

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e

Austra

lia

Hong K

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Japan

Korea

New Zea

land

Singapo

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Overal

l

Source: Point Topic, Q4 2003

Figure 1-5: DSL Availability by region


As noted, although DSL is widely available around the globe, there is not yet 100% network coverage. According to various global carriers, two serious hurdles need to be overcome before DSL services can be made available to 100% of the user community: • Distance Limitations - Unlike lower-speed modems that can be connected to long local

loops, DSL modems are not 100% effective on the longer local loops. The achievable reach depends on the transmission rate and there are maximum loop lengths beyond which any DSL ceases to function.

• Higher-Bandwidth Applications – Though basic, high-speed Internet access remains the

primary driver behind the deployment of DSL, there is increasing demand for DSL to accommodate higher-bandwidth applications such as video. For competitive reasons, carriers continue to look for methods to profitably deploy a ‘triple play’ service – voice, data, and video. As such, for an increasing number of carriers, delivering higher-bandwidth DSL to as many of their customers as possible is a key business objective.

1.4 Market Requirements for Extending DSL Service For the past 100 years, POTS (Plain Old Telephone Service) has been the foundation (volume service) of the public telephony access network. As indicated by the 64 million subscribers connected, DSL is now exhibiting the promise to become a volume service as well, but service providers must first be able to make DSL as ubiquitous and affordable to subscribers as POTS service is today. As noted above, for service providers to successfully deploy ‘DSL Anywhere’, they will require network solutions that can address the following issues:

• Reach • Ability to deliver services/applications with different bandwidth requirements (low to high)

It is important to note that reach and service models are not mutually exclusive and as such, different DSL technologies and solutions will accomplish different carrier objectives. In addition, issues such as ubiquitous service availability, rapid service deployment, scalable service provisioning, and migration to next-generation packet networks are critical carrier criteria for continued DSL deployment. The following table summarizes the applicability of the various DSL deployment options to different environments. An ‘X’ indicates that the solution is applicable for enabling support of DSL in that particular situation. The three major situations discussed are:

• Customer premises served directly from a CO • Customer premises served from a Remote Teminal (RT) • Special situations or services


Table 1-1: DSL Anywhere solutions for different deployment applications

Customer Premise Served Directly from Central Office

Customer Premise Served from a Remote Terminal

Special Situations

Class Solution DSL COs

<17kft (<5.3km)

DSL COs

<12kft (<4km)

DSL COs

>17kft (>5.3km)

New DSL COs

SAI DLC upgrade

DLC replacement

NGDLC upgrade

Sym. Business Services

New Growth

Softswitch Convergence

DSLAM X X X X

X

Remote DSLAM

X X X X X

SAI-based DSL

X X X X X X X X

Fiber extender

X X X X X X X X

Overlay

RAM X X X X X

PON-fed DSL

X X X X X X

Bonding X X X X X X

DLC linecard

X X X

NGDLC x X X X X

Integrated

BLC X X X X

Repeater Repeater/ Loop ext.

X

SHDSL X X X X X New Tech

Improved ADSL

X X X X X

Alt. Solutions

Low Freq. DSL

X X

Table 1-1 illustrates that service providers can significantly increase DSL service coverage, improve data-rate offerings, and reduce costs by selecting an appropriate DSL solution. For example, a service provider might choose to use CO DSLAMs to serve CO-fed subscribers that are within 12 kft (4 km), upgrade their existing installed base of legacy DLCs and NGDLCs with integrated POTS+DSL line cards, and deploy BLCs for new growth. This deployment strategy would dramatically improve DSL availability, except for subscribers beyond 17 kft (5 km); those subscribers could be served by reach extended DSL (READSL), SHDSL, repeaters, or Low Frequency DSL, or by installing remote DSL capability in the outside plant, using fiber-optic extenders, SAI-based DSLAMs, DLC (including NGDLC and BB-DLC), Fiber-to-the-Curb (FTTC), or Fiber-to-the-Premises (FTTP).


1.5 DSL Anywhere Analysis The demand for high-speed access and the adoption of DSL has been unprecedented in the telecom industry. The deployment of DSL has grown exponentially from an installed base of fewer than 500,000 lines to well over 64 million lines in just a few years. Driving the bandwidth demand is the increasing consumer appetite for rich Internet content, including multimedia and peer-to-peer file-sharing programs, as well as the telecommuting trend and the requisite need for access to company VPNs and e-mail servers. As a result, service providers must implement cost-effective and scalable DSL product solutions that optimize operations and speed DSL service deployment. In 2001, while most service providers had deployed DSLAMs to the majority of their COs, nearly 100 million subscribers who may have wished to have high-speed access were still not eligible due to a number of hurdles, the most significant being DSL distance limitations. Over the last three years, DSL technology advancements have been able to close this gap. Future developments will further close the gap, making DSL truly ubiquitous. Carriers now have the option to explore a number of avenues to address these formerly ineligible customers, including deployment of broadband-capable remote terminals (RTs), emerging technologies such as loop extenders and repeaters, and scalable methods for automatic and remote provisioning. The following sections go into greater detail about how carriers can meet their business and network objectives. To that end, each section addresses the issues of rate and reach, and how specific technologies and solutions will help providers accomplish certain goals.


2 Loop Qualification Loop qualification systems and methods are used to determine a loop’s suitability for a specific DSL service prior to service provisioning. As these systems have become more accurate, service providers can now use them to: • Identify fringe-area customers who can be reliably served • Identify customers who can be offered high-speed service • Identify the maximum capabilities of the loop to enable service ‘up-selling’ • Avoid service commitments on loops that cannot support service This section discusses new loop-qualification techniques that greatly improve loop assessment accuracy and enable DSL to be provided to customers who would have previously not qualified. These techniques reduce the number of failed turn-ups and, as a result, DSL services are available to more customers, while the cost to provide service is reduced. With on-demand qualification, the service representative initiates an analysis of the loop connected when a customer calls the service provider to request service. This may involve an engineer analyzing the loop characteristics stored in a database, technicians performing field measurements with test-equipment, or a service representative having direct access to a test system that can immediately test any loop. With pre-qualification, every loop connected to a CO is analyzed in advance of customer orders. This is a critical first step toward achieving mass market DSL deployment in a target market. Once loop pre-qualification of a target market area is complete, the service provider is positioned to begin effective mass deployment of DSL services, which includes the following: • Ensuring DSL services can be deployed to the target market area • Ensuring the subscriber loops support the bandwidth required for the target service

applications • Eliminating truck rolls • Minimizing human intervention • Enabling subscribers to self-install CPE • Enabling flow-through provisioning Although the objectives above are universal, deployed loop qualification systems differ due to varying network architectures and operations.

DSL Forum MR-01: DSL Anywhere issue 2 Section 2: Loop Qualification Page 10

2.1 Network Conditions Affecting DSL performance Qualification systems are designed to account for physical network variations. These network conditions can include: • Loop design with respect to segment lengths and gauge mix • Presence of load coils • Presence, length, and gauge of bridged taps • Presence, type, and severity of faults • Presence of pair-gain devices • Interfering noise (crosstalk) caused by neighbor pairs in the cable bundle Not all the conditions above exist in all networks. Many Asian and European networks, for instance, contain few load coils or bridged taps due to their naturally shorter loop lengths. North and South American voice networks were often designed for a more geographically distributed population, and therefore tend to have the added complexity of load coils, bridged taps, and pair-gain devices. 2.1.1 Loop Length and Gauge Mix The length and gauge mix of the loop are factors because wire itself has loss. Longer loops and/or thinner gauges cause more signal loss, resulting in lower transmission rates. The achievable DSL transmission rate is inversely proportional to the loop loss according to a relationship described later in this section. As an example to highlight the interaction of these two factors, shorter loops of smaller gauge (e.g. 12 kft of 26AWG or 4 km of PE04) could have the same or worse DSL performance than longer lengths of larger gauge (e.g.15 kft (4.6 km) / 22AWG / PE06). 2.1.2 Loading Coils Loading coils are in-line inductors used as low-pass filters to balance loop response for voice frequency transmission. These devices effectively block xDSL signals. Loading coils are common on long loops in North America but are not used in most other regions. As a general rule, they exist on any loop longer than 18 kft (5.5 km) (or more accurately, those with more than 8 dB of loss at 1004 Hz). One or more coils can exist on a loop, depending on its length and loss design. However, these DSL-killers are not limited to long loops - over time, some of these long loops have been shortened as intermediate loop carrier and pair-gain systems were introduced. In many cases, the loading coils were not removed from the remaining loop sections as the network was rebuilt. Recently, a new type of loading coil has become available that does not cause severe attenuation in the DSL path. These new loading coils can replace traditional coils, providing an option for both DSL and POTS over long loops where removing the loading coils entirely might degrade POTS beyond an acceptable level.


2.1.3 Bridged Taps Bridged taps are lengths of unterminated wire that are connected in parallel with the loop being evaluated. Bridged taps are typically formed when changes are made to the loop that leaves unneeded cable attached to the loop. Bridged taps can exist between the CO and the customer premises, or can extend beyond the customer. The negative effect of bridged taps on DSL service is directly related to the location, length, and gauge of the wire, the type of DSL service being deployed, and the frequencies used by the service. Although bridged taps can cause some service degradation, especially for higher-speed services, lower speeds can typically be supported without their removal. 2.1.4 Faults A variety of faults can affect DSL service, depending on a number of factors such as their type, severity, and location. Faults can be caused by physical degradation of the wires or human error during network repair and provisioning. Besides obvious service-affecting faults such as hard shorts and opens, the most significant fault types affecting DSL performance are those causing line imbalances. Some of these fault types are: • Series resistance, typically caused by corrosion at a joint and affecting one side of the pair

more than the other. Higher currents in the loop during a voice connection can temporarily clear this fault, only to have it reappear intermittently during data service connections.

• Capacitive imbalances, typically caused by a bridged tap on one side of the pair • Contact faults to ground, battery, or another pair • Split pair, where a wiring error has one leg of the pair crossed over with another pair for a

section of cabling 2.1.5 Pair-gain devices These devices (e.g. DAML, DACS) are installed in the loop and convert one pair to two or more voice-service pairs at the premises. These pair-gain systems effectively block DSL transmission. 2.1.6 Crosstalk and Noise Long stretches of cable running beside other cabling are susceptible to crosstalk. The extent to which crosstalk is a problem depends on many factors, such as the number, strength, and type of the crosstalk sources, the susceptibility of the crosstalk receiver loop, the distance that separates the sources from the receiver, the extent to which the source frequencies and their harmonics overlap the receiver transmission frequencies, and the electrical balance of the loop (see Faults above). When these crosstalk sources, otherwise known as ‘interferers’ or ‘disturbers’, combine with other noise sources, the effective noise floor can be raised to the point where loop transmission is slowed or even halted. 2.2 Loop Qualification Techniques Loop pre-qualification has become more accurate, with performance improvements aimed at minimizing false positives and false negatives. A false positive is defined as an incorrect pre-


qualification result that states a particular loop is qualified for DSL when it cannot support the service. A false negative is defined as an incorrect pre-qualification result that states a loop does not qualify for DSL when it could support the service. Loop pre-qualification systems are generally based on: • Geographic information systems • Design information derived from network records • Loop testing • Some combination of records and testing Geographic information systems are inexpensive to deploy and operate. They are widely available and provide information independent of the service provider. Where records are complete and accurate, DSL service performance is usually predicted by deriving loss or ‘gauge-equivalent’ information for the loop. These data can be directly related to speed for the DSL service desired. Where record systems are inaccurate, incomplete, or cannot keep up with network changes, pre-qualification test systems have been deployed. Many systems are biased toward minimizing false positives, to reduce the number of false commitments made for DSL services. This bias can lead to a corresponding vulnerability to false negatives, reducing the available market for the service. With increasing performance, the systems listed below simultaneously minimize both false positives and false negatives. 2.2.1 Loop Qualification Processes and Systems Geographic-Based Qualification Systems

Geographic-based qualification systems can be categorized into two types of systems. One uses a proxy (postal code) for the location of a subscriber; the second uses the physical distance as measured on a map. Because of the ease of implementation, these systems were among the first developed in the early deployment of DSL and served to provide a qualification statement regarding the ‘sync/no sync’ performance of the line. Unfortunately, these systems cannot provide any additional information with regard to actual speed (or minimum guaranteed speed). Postal Code. This qualification process is based on resolution of the postal code system. A subscriber’s postal code is used as a proxy to determine proximity to the CO. Since there is little differentiation that can be made between subscribers within that postal code, a single ‘length’ is applied to the entire area. While this qualification process can be tuned for a ‘sync/no sync’ boundary, it cannot be used reliably for tiered-speed offerings or grading of services. Also, this qualification process cannot identify other impairments on the telephone line that may block DSL service (e.g. load coils, bridged taps, etc.).

Geographic Distance. The geographic distance qualification system is based on measuring a certain radius from the serving CO. Because most cables follow the road layout, a radius is chosen such that it can accommodate a telephone line that follows a right-angle path between CO and subscriber home. For example, if a telephone company chose to offer service to customers within 2 miles (3.2 km) of the CO, they would draw a circle with a radius of 2 miles. This would DSL Forum MR-01: DSL Anywhere issue 2 Section 2: Loop Qualification Page 13

be equivalent to qualifying lines with a worst-case maximum length of 15.4 kft (4.7 km). This is based on the fact that cables typically follow the road and assumes that the cable follows a right-angle path between the CO and the subscriber home. Again, this process cannot account for other impairments on the telephone line that may block DSL service. Length-Based Qualification Systems Length-based qualification systems can be divided into two general categories: records-based and measured-length based. While these systems are generally more accurate than the geographic-based systems, they can suffer from significant shortfalls with regards to accuracy, ‘sync/no sync’ boundaries, and speed binning. These qualification systems are built with speed breakpoints for the different services offered. Unfortunately, length is not always an accurate predictor of the speed that the telephone line will support, especially when gauge mix and bridged taps are involved. Individual Line Record. Many service providers commonly use line-record qualification systems. These systems provide length indications and possibly a gauge mix/line composition on which a qualification statement can be made. While the line-record qualification system normally provides good information about the lines that there are accurate records for, the system suffers from two major problems.

The first problem is missing records. If the system does not have information about the line, it is impossible to make a qualification statement.

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• The second problem is incomplete records or inaccurate records. Changes in the network due to reassignments, repair, pair swaps, and line conditioning can cause 25+% inconsistency per year in records-based qualification systems.

This system can be tuned to provide very good results for false positives (less than 2%), but it can leave as many as 30% false negatives for ‘sync/no sync’ assessments. The speed prediction error on this qualification system can be as high as 35%. Individual Line Record applied to distribution point. The ‘individual line record applied to distribution-point’ qualification system is a hybrid of the individual line-record qualification system and the geographic qualification system. This system uses line records to determine line length to a distribution point. The longest line length from the distribution point is assigned to all subscribers served from that point. For example, if the length to the distribution point is 12.9 kft (4 km) and the longest length from that point is 4 kft (1.2 km), then all subscribers served from that point have an associated length of 16.9 kft (5.2 km). This qualification system suffers from the same issue as the individual-line-record qualification system. It also cannot differentiate between subscribers close to the distribution point and those further away. In the example above, if the service qualification boundary were 15 kft (4.6 km), those subscribers would all be disqualified even though many might be less than 15 kft from the CO. This qualification system can be tuned to provide very good results for false positives (less than 2%), but can generate up to 35% false negatives for ‘sync/no sync’ assessments. The speed prediction error for this qualification system can be as high as 27%. Individual Line Record Gauge Equivalent Length. The ‘individual line record gauge equivalent’ qualification system uses line record information for gauge mix and calculates an DSL Forum MR-01: DSL Anywhere issue 2 Section 2: Loop Qualification Page 14

equivalent gauge length for the telephone line. By using formulae, 22AWG (PE06), 24AWG (PE05), and 26AWG (PE04) lengths (including taps) are converted to an equivalent gauge length that is used by the qualification process. Once the equivalent gauge length is known, it is compared to a maximum length for that service to determine whether the line qualifies for that particular service. Again, this type of qualification system is prone to the same shortcomings as the individual line-record-based system. In addition, this qualification method is particularly susceptible to inaccurate records, especially the accuracy of the gauge lengths in the line record. The advantage is that it extends the reach of the length rule by taking into account the various gauge mixes. This qualification system can be tuned to provide very good results for false positives (less than 2%) but can generate up to 20% false negatives. The speed prediction error on this qualification system can be as high as 20%. Measured Length. This system uses either a measurement head that is connected to the switch test bus to measure the length of the telephone line, or a hand-held tester to measure the length of the telephone line. The advantage of this system is that it actually measures the line that the telephone number is associated with, thus reducing the churn error (due to reassignments, repair, pair swaps, and line conditioning) associated with telephone lines. Because this method actually tests the pair that the customer is connected to, it can provide more accurate information than the records-based qualification system. This process is susceptible to inaccuracies or inconsistencies vis-à-vis the length returned from the measurement device. It has been demonstrated that some of the central and hand-held test instruments provide inaccurate or inconsistent length results. This is primarily because the length measurement returned by some of these devices is affected by terminations on the line (such as multiple phones, answering machines, modems, etc.) that can generate inaccurate readings that misrepresent the length by up to 3 kft (.9 km). When hand-held and installed test systems provide somewhat inaccurate length measurements, the measurements are de-rated for line qualification. Even with this inaccuracy, these systems can be tuned to provide very good results for false positives (less than 2%) but can generate up to 18% false negatives. The speed prediction error on this qualification system can be as high as 20%. Measured Loss

ADSL modems measure the insertion loss of the line over their bandwidth and compute SNR ratios at these frequencies. Based on the loop attenuation and noise values, the modem assigns data bits to individual carrier signals (DMT DSL), which determine the overall ability of a DSL line to support a certain data rate. Therefore, effective ADSL service qualification techniques include an accurate measurement for loop attenuation and the noise characteristics of the network. Figure 2-1 depicts loop attenuation and the corresponding bits-per-tone distribution used by an ADSL modem to transmit the provisioned rate for a 9-kft 26-AWG (2.7 km PE04) loop. It clearly illustrates the dependence of the modem’s bit assignment scheme on the loss characteristics of the line.


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Figure 2-1: (a) Attenuation characteristics (b) Bits-per-tone distribution for a 9-kft 26-AWG loop Insertion loss is one of the most significant factors in determining the data carrying capability of a loop. The line loss in turn depends upon the construction of the wire pair under qualification. Cable construction includes gauge mix, cable segment lengths, and the presence, gauge, length, and location of bridged taps. Differently constructed lines of the same length can show large insertion loss (and hence modem speed) variations. This is why it is nearly impossible to accurately predict speed based on length of loop alone. The insertion loss of a line at 300 kHz has been suggested as a good indicator of a line’s ability to support ADSL services (see Figure 2-1). Bearing in mind that the downstream data are of primary concern in an ADSL link, note that most of the downstream signal PSD is concentrated around 300 kHz. This is why 300 kHz is considered a reasonable reference frequency for ADSL service qualification. However, there are limitations to using the insertion loss at 300 kHz as a metric for determining modem speeds. This is due to the effect that loop makeup, specifically bridged taps, has on the attenuation characteristics of the line. The insertion loss plot in Figure 2-1 is typical for loops without any bridged taps where loss is a monotonically increasing function of the frequency. In DSL Forum MR-01: DSL Anywhere issue 2 Section 2: Loop Qualification Page 16

fact, it is this monotonic property of the 300-kHz loss that makes it an accurate predictor of the data rates that can be supported by a line; there is a direct relationship between insertion loss and modem speed.

The plots in Figure 2-2 were obtained from the 26-AWG 9-kft loop shown in Figure 2-1 with a 400-ft bridged tap added to it. Observe that the insertion loss for the two loops is approximately the same; however, there is more than 1-Mbps difference in the data rates corresponding to the two loops. Thus, the loss at 300 kHz is not as accurate a measurement for characterizing the performance of the lines in this case, due to the presence of the bridged tap. The impact of bridged taps on loop qualification performance becomes more pronounced when multiple taps are present.

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with a 400-kft bridged tap The 300-kHz insertion loss is an efficient reference in networks with gauge mix and/or minimal or no bridged taps. Most European and Asian telephone networks fit this description. However, in the North American network, more than 70% of loops contain one or more bridged taps. It is therefore necessary to use an alternative loss-measurement technique when addressing the ADSL loop qualification problem in this market. Loop Qualification in the Presence of Bridged Taps. Consider again the plots in Figures 2-1 and 2-2, this time comparing the ‘average loss’ presented by the loop with and without the bridged tap. Average Loop Loss (ALL) characterizes the performance of a loop for ADSL service (a similar scheme is used by ADSL modems to quantify line attenuation). Note that ALL DSL Forum MR-01: DSL Anywhere issue 2 Section 2: Loop Qualification Page 17

(43 dB and 46 dB) is representative of the downstream modem speeds (7008 kbps and 5888 kbps respectively) supported by the loops in this example. The ALL performs equally well for loops with gauge mix and long lengths, as well as loops with or without taps. The performance improvement exhibited by ALL is primarily because it takes into account the non-monotonic variation in the loss characteristics (Figure 2-2) displayed by loops with bridged taps. A single-frequency loss or length-based measurement cannot compensate for the drop in data rate due to resonance dips in the insertion loss vs. frequency characteristics of a line when a bridged tap is present. The data-carrying spectrum for loops with long lengths is restricted due to high insertion loss of the line. As a consequence, the influence of taps can be severe enough to prohibit ADSL service altogether. A more accurate measurement for loop qualification is therefore required to ensure the system performance. These plots are obtained from data acquired in a laboratory environment for a set of standard-based loops representing the North American network. The test loops were selected from standards (e.g. ANSI, CSA, etc.) and are constructed of varying gauge mix and segment lengths. Additional loops with bridged taps that are challenging to DSL are included. Side-by-side comparison of the plots shows that ALL provides the most accurate prediction of modem speeds, as compared to employing loop length and insertion loss at 300 kHz techniques for ADSL loop qualification. This is demonstrated by the improvement in the correlation coefficient for ALL (ρALL = -0.95), loop length (ρLen = -0.82), and loss at 300 kHz (ρIL300 = -0.91).


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measures (a) Length, ρ Len = -0.82 (b) Insertion loss at 300kHz, ρ IL300 = -0.91 and (c) ALL, ρ ALL= -0.95


Insertion Loss Prediction. Insertion loss is typically a double-ended measurement, requiring access to both ends of the line (CO and customer premises). In order to accomplish a network-scale qualification for DSL service, it is essential to assess the line loss through a single-ended measurement. This task becomes increasingly challenging when the switch test bus restricts the test bandwidth. Some test systems have achieved mass qualification of the network through single-ended prediction of insertion loss that is comparable to a double-ended measurement. These systems operate through the same test access used for POTS testing, which is automated but band-limited. In North America, this automated access is provided as the NTT (No Test Trunk) and in other parts of the world as the DSAT (Digital Switch Access Technique). Insertion loss can also be measured by some ‘in-band’ test systems. These require access to the loop through a matrix, requiring the loop to be rewired to the access point prior to testing. This method works well once the DSL service has been (at least partially) provisioned, but does not support pre-qualification. Noise Mask: Future Proofing the ADSL Network. Telephone network binder groups carry multiple broadband services. These services can introduce noise into the adjacent wire pairs within the binder. A qualification system must consider the impact of this crosstalk interference on the data rates supported by the line. Crosstalk noise is dynamic in nature and is expected to grow with time as additional services are launched via the same binder group. Therefore, it is not useful to incorporate present noise measurements into the qualification process since there is no assurance that the level of crosstalk will stay constant in the future. Furthermore, in order to make an accurate measurement of the noise environment on the line, samples need to be taken and analyzed over a long period, which may be prohibitive for an on-demand qualification system. One approach to future proofing a service or guaranteeing a level of service in the presence of noise is to assume a ‘worst-case’ scenario. This worst-case interference is referred to as the ‘noise mask’. The noise mask is the difference between the maximum insertion loss acceptable for support of a service with no noise present and the maximum insertion loss acceptable for support of a service in the presence of worst-case noise. The noise mask is adaptive to the broadband service deployment practices of the service provider. It is determined by the network-specific cable configurations, disturbers, and performance criteria designed for currently deployed DSL services.


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In order to generate the noise mask for a particular network, the services and number of each type in individual binder groups must be determined. Using this information, the noise mask is developed by modeling the total crosstalk influence and adjusting the loss vs. speed relationship accordingly. Figure 2-5 depicts a simplified example of a noise mask implementation. Assuming that the worst-case crosstalk noise model for a particular network is a reduction in model speed due to an 8-dB increase in insertion loss, this 8-dB guard band is applied to the noise-free loss characteristics of the network. For instance, if it is desired to determine the insertion loss threshold corresponding to 2-Mbps service and a particular line supports this data rate at a loss of 51 dB, this line is qualified to operate at 2 Mbps in worst-case noise if its insertion loss is less than 43 dB.


Table 1-1 summarizes the above-described qualification systems and their typical performance.

Table 1-1 System Reason Implemented Performance Postal code Inexpensive

Quickly implemented Independent of service provider Cannot see other impairments (i.e. load coils, bridged taps, faults)

Sync/no sync False positive >20% False negative >30% Speed prediction not available

GIS/map Same as postal code Applied to individual residences instead of an area

Similar to postal code qualification system

Individual line record

Existing information (inexpensive) Not easily obtained from legacy databases

Sync/no sync False positive <2% False negative 20-30% Speed prediction Error >35%

Individual line record with gauge mix

Same as individual line record Improves qualification accuracy for long lines Improves qualification accuracy for lines with significant thin-gauge lengths

Sync/no sync False positive <2% False negative 15-20% Speed prediction error >20%

Line record sample applied to distribution area

Same as individual line record Applied when individual line records are incomplete

Sync/no sync False positive <2% False negative 25-35% Speed prediction error >28%

Length measured per line

Corrects the line records errors Obtains current line information Not invalidated by record aging, re-assignment, repair, or conditioning

Sync/no sync False positive <2% False negative >18% Speed Prediction Error <25%

Loss measured per line

Same as length measured per line This method produces the highest performance, fewest false positives and negatives

Sync/no sync False positive <2% False negative <2% Speed prediction error <15%


2.3 Summary Loop pre-qualification testing is normally performed prior to the initialization or turn up of a DSL service or circuit, using a combination of records and test systems. Pre-qualification test results can fill in missing records and with arbitrating logic, can augment information when records and test outputs conflict. Pre-qualification should be the first of a number of possible test actions taken to ensure end-to-end connectivity and throughput is possible. Although true performance testing cannot be done until customer and provider equipment has been provisioned and brought on line, much can be done to ensure the likelihood of a positive turn-up experience by confirming the condition of the loop. Loop pre-qualification testing is most cost-effectively and practically done if it is performed from centralized locations without the need to reconfigure existing services and dispatch technicians; automation further enhances efficiency. A regular cycle of repeated testing is also necessary to keep records up-to-date and account for changes in the network. A comprehensive testing solution should have the ability to quickly identify DSL circuit problems and locate their source. The system should be capable of providing suggestions as to the nature of the problem(s) and steps necessary to further isolate or resolve the problem(s), as well as support storage, benchmarking, and comparison functions that are applicable to this information. This is especially true as DSL providers ramp up their deployments to scale. To qualify large numbers of copper pairs, automated bulk testing under the control of an OSS is highly desirable. Bulk test results can be stored in inventory systems that normally contain other facilities information. In addition to automated bulk-qualification testing, a system supporting on-demand loop qualification is also very helpful. On-demand loop qualification allows a user to quickly perform a test for a circuit that may not yet have been qualified or to reapply the test to confirm the state of a circuit.


3 Overlay Access Solutions This section describes equipment that overlays existing POTS elements with DSL equipment, without requiring the replacement of the existing POTS-only elements. These remote overlay solutions include: remote-based DSLAMs, RAMs, SAI-based DSLAMs, fiber-optic extension of DSL, and PON-fed DSL. The remote DSLAM is essentially just an ordinary CO DSLAM, except it must be industrially hardened for placement in a remote cabinet in the outside plant. This means that it must operate in conditions from -40°C to +65°C (-40°F to 149°F) and meet a stringent set of requirements for operation in the remote environment. Additionally, requirements for front panel access and remote configuration are important. The Remote Access Multiplexer (RAM) products are super-low-profile products that are designed to fit into nearly any DLC. The RAMs are generally between 1U (1.75"/4.45cm high) and 3U in vertical size, and can often fit into DLC cabinets that are already ‘full’ of POTS equipment. This is highly desirable when the service provider wishes to provide DSL service, has little room in the cabinet for a DSLAM, and does not wish to purchase new POTS equipment or adjunct cabinetry. Both remote DSLAMs and RAMs are placed at existing DLC sites. These sites typically have power provided by the electric company and serve loops up to 12 kft (4 km) in length from the DLC cabinetry to the customer premises.

Figure 3-1: Deployment of remote DSLAMs and RAMs The SAI-based DSLAM is similar to a remote DSLAM but is placed deep in the outside plant at the SAI. This site may not have cabinets to protect the equipment from the elements, so the SAI-based DSLAM is industrially hardened to meet a stringent set of requirements for operation in this unprotected remote environment. Additionally, these sites typically do not have an external source of power, therefore the SAI-based DSLAM must be line powered. However, since SAI-based DSLAMs meet these hardening requirements and are line-powered they are flexible; there

DSL Forum MR-01: DSL Anywhere issue 2 Section 3: Overlay Access Solutions Page 25

is no need to place the unit in a DLC cabinet or draw local power from the DLC cabinet. However, the DLC cabinet could be the source of the line power for the SAI based DSLAM. Fiber-optic extenders transparently extend an existing CO DSLAM signal to a cross-connect cabinet using an optical multiplex. This effectively brings the DSLAM line interface out to the customer serving area (the DSLAM is physically resident in the central office but logically located in the outside plant). DSL data streams from a CO-based DSLAM are converted into a multiplexed optical signal that is sent from the CO to a cross-connect cabinet (using Gigabit Ethernet transport components, for example). At the outside-plant cabinet, the electrical signal is reconstructed and demultiplexed, restoring the original DSL signals for distribution to subscribers over existing copper connections. Fiber-optic extenders share the flexibility of SAI-based DSLAMs in that they too must be line-powered and hardened for deployment in unprotected remote environments. PON-fed DSL uses an FSAN (G.983 or G.984) PON fiber architecture to serve a remote site. The G.983 optical multiplex is converted to DSL at an ONT placed at a remote site (such as the SAI) in the carrier’s outside plant and is carried as a DSL physical layer over the carrier’s copper plant from the remote site to the customer’s site. Unlike a pure optical fiber-to-the-home deployment, this solution requires that the optical overlay only reach the remote site in the carrier’s plant. SAI-based DSLAMs, fiber-optic extenders, and PON-fed DSL all place the DSL remote quite deep into the carrier’s outside plant - typically, in North America, at the SAI cabinet, which is usually 4 kft (1.2 km) or less from the customer. In many metropolitan PSTN service areas in North America, loops extending from SAIs may serve customers up to 6 kft distant, although most industry estimates are that at least 80% of customers are within 4 kft. Globally, loop lengths from the SAI depend on the population density of the area served. In sparsely populated locations, the loop lengths would be expected to increase, while in very densely populated regions they would likely be shorter. While the terminology used for components is also very country specific, copper cross-connect cabinets and other splice points occur in all carriers’ networks. To understand the architecture of these remotes, it is important to examine the typical local POTS loop. As shown in Figure 3-2, the North American POTS network typically comprises four segments: feeder, distribution, drop, and inside wiring. The feeder pair segment extends from the main distribution frame (MDF) in a central office or from a remote DLC site, and typically consists of large 500-pair-count cables, each typically containing 600 to 3,600 cable pairs that feed several neighborhoods or subdivisions. The feeder cable (also called F1 loops) is terminated on an SAI, which is also referred to as a cross-box, SAC box, or JWI. The SAI provides a junction point between the feeder and distribution cables and provides easy test access for loop maintenance and installation. From the SAI, the network is divided into multiple, smaller distribution cables that serve portions of a neighborhood or small subdivisions. Distribution cables can range from 25 pairs to 600 pairs, but 50 pairs is typical. This distribution network is where the deployment of SAI-based DSLAMs, fiber-optic extenders, and PON-fed DSL is focused. RAMs and remote DSLAMs are designed to be deployed along with the RT, on the F1 loop, as shown in Figure 3-1 above. By


deploying at the RT, RAMs can take advantage of the availability of power and use the RT cabinet to provide additional environmental protection.

Figure 3-2: The outside plant architecture

The SAI typically does not have an external power source and is subject to extreme environmental conditions (it cannot be assumed that there is any environmental protection, even an unconditioned cabinet, at that point in the network). Therefore, all four of the solutions that can be placed deep in the outside plant may require line powering from the CO, and a level of environmental hardening that is more rigorous than that required for the remote DSLAM or RAM. However, the SAI infrastructure provides a web of convenient locations for telcos attempting to deliver broadband access, as the relatively short loops between the SAI and customers’ premises can support higher-bandwidth DSL services. ADSL can run as fast as 8 Mbps downstream over 6.5 kft (2 km) of copper wire. ADSL2+ can support between 15 and 20 Mbps downstream over 5 kft (1.5 km), depending on cross talk and operational conditions.


Table 3-1 summarizes the five remote overlay solutions.

Table 3-1: Characteristics of remote overlays Solution Location Expected

maximum loop length

Backhaul technology

Electrical power

Remote DSLAM

At remote site of existing DLC

12,000 ft (4000 m)

Optical or copper

External power available

RAM (section 3.1)

At remote site of existing DLC

12,000 ft (4000 m)

Optical or copper

External power available

SAI-based DSLAM (section 3.2)

At SAI 4000 ft (1200 m)

Optical or copper

Line powered

Fiber-optic DSL extension (section 3.3)

At SAI 4000 ft (1200 m)

Optical Line powered

PON-based DSL (section 3.4)

At SAI 4000 ft (1200 m)

Optical Line powered

3.1 Remote Access Multiplexer Because of their small size and relatively low cost, RAMs are quick and easy to deploy and often serve as excellent competitive tools against cable modem deployments, since DSL service providers can use them to target neighborhoods where cable modem services are being deployed. Today, many thousands of RAMs are deployed in many networks around the world. RAMs are DSL devices that can be deployed in the remote terminal/DLC environment. The units are industrially hardened and typically support ambient temperatures of -40°C to +65°C (-40°F to 149°F). In North America, the NEBS level III, UL, and FCC requirements would apply. Today’s RAMs typically offer from 8 to 48 subscribers per unit in sizes ranging from 1U to 3U or more. Figure 3-3 shows a typical DLC deployment scenario using a RAM. The picture shows how the RAM unit is literally ’squeezed‘ into any available space in the DLC cabinet.


Figure 3-3: DSL deployment using a RAM RAM products typically allow a mixture of DSL (ADSL and SHDSL) services on a single platform. Some units also support VDSL. The backhaul is supported using from one to eight T1s or E1s bonded with IMA, or sometimes with DS3 or higher optical interfaces. For very remote locations, long-reach technologies like HDSL2/4 and SHDSL can be used for backhauling. In the case of an ATM-based network, the device acts as an ATM bridge, bridging traffic from the customer to the ATM WAN network. It can be deployed either directly off the ATM switch, off ATM aggregation devices, or off DSLAMs. In the case of an Ethernet-based architecture, the device could act as an Ethernet switch. Many RAM products incorporate the CO-side POTS splitter inside its housing, allowing very simple tip-and-ring connectivity in the DLC environment. Some use special cabling for quick tip-and-ring connections to the protector panel. Most RAMs contain a fully functioning ATM switch that supports multiple QoS types (UBR, CBR, VBR, etc.). Ethernet-based architectures can also be supported from a RAM. In some products, the multiple T1s or E1s can be used simultaneously for backhaul and subtending devices.


The RAM is capable of providing DSL service at the RT without the need to deploy an adjunct cabinet or swap out existing DLC equipment for NGDLC equipment. This limits service providers’ capital costs for providing service and allows them to bring up DSL lines quickly and efficiently. With subtending supported on many RAMs, a service provider could bring up 48

DSL lines, POTS splitters included, in three rack-units of space inside a remote terminal just utilizing ‘dead’ space within the cabinet. 3.1.1 Description of Architecture RAMs are designed to be small devices that provide easy access to DSL within the remote environment. RAM devices are hardened devices, which makes them suitable for deployment in the harshest environments. Most devices range in size from 1U to 3U. They usually range in density from 8 to 48 DSL lines and include splitters. RAMs typically use DS1/E1 facilities for connection to the core network. Advantages

The overall benefit of the RAM is that it allows quick, easy DSL access in virtually any environment. These small, industrially hardened devices can be deployed in the CO, building basement, equipment closet, or RT. Quick-connect cables for power, DS1/E1, and tip-and-ring pairs usually allow installation of RAM devices in one hour or less. Implementation/Deployment Issues

RAMs are simply mini-DSLAMs and hence have the same deployment issues as any other DSLAM. Operational Issues

RAMs are designed to be easy to install and manage. Once the unit is inserted in the rack, power, alarms, DS1s/E1s, and tip-and-ring pairs are connected. Once the RAM is powered up, a self-test is typically initiated. After the self-test, the RAM will be pre-provisioned for basic ADSL service, and if the ADSL device at the customer’s site is already connected, service will commence upon training completion. RAMs use an integrated network management system common to most DSLAM vendors, so users can easily be provisioned and de-provisioned, and equipment can be managed. The units usually have very few serviceable parts, since they are self-contained devices. Typically, only fan unit replacement is necessary, and this is a modular device that is easily replaced. Spectrum-management issues with a RAM are typically the same as with a DSLAM. Since RAMs deployed in North America often contain both T1 and ADSL lines, the T1s ideally should be in separate binders to minimize noise in the DSL binder. This is typically not a problem, since the binder back to the CO is almost always separated from the downstream customer pairs. Network Management Issues

The Management Information Bases (MIBs) used by RAMs are the same used by any other DSLAM. Typically, this includes the AToM MIB, ADSL MIB, DS1 MIBs, and a few enterprise MIBs for managing the actual device. In most cases, every RAM manufacturer has a management system that is integrated with their own DSLAM, and some provide an uplink interface so that the management systems can be integrated into carrier network management systems. 3.2 SAI-based DSLAM SAI-based DSLAMs are DSL devices that extend DSL services to subscribers who heretofore could not receive service because they were too far from the CO or were served by DLCs that are DSL Forum MR-01: DSL Anywhere issue 2 Section 3: Overlay Access Solutions Page 30

not upgradeable to support DSL. Like a RAM, the SAI-based DSLAM is quick and easy to deploy and has a small, compact size. The difference, however, is that the SAI-based DSLAM is line-powered and therefore has fewer restrictions on where it can be deployed. The overall benefit of the SAI-based DSLAM is that it allows quick, easy DSL access anywhere without additional outside plant or right-of-way issues. The SAI-based DSLAM is a small device that can be deployed in harsh environments and provides easy access to DSL within the remote environment. The device is hardened (industrial temperature range, NEBS Level III, etc.) and because it is directly exposed to the elements, units must be able to withstand the elements (e.g. GR-487 or equivalent environmental requirements outside North America). In addition, some devices are installed in underground and flood-prone locations such as manholes and thus are able to operate fully immersed under water. 3.2.1 Description of Architecture/Technique SAI-based DSLAMs are designed to be deployed either at the SAI or on the F2 loops (see Figure 3-2). This allows deployment at locations much closer to the subscribers, but also requires that the DSLAM be line powered and meet stringent environmental hardening requirements. By not being limited to the RT location, SAI-based DSLAMs can be effectively used to extend DSL service to customers currently served by legacy DLC systems. They can also be used to deliver full-rate ADSL services to customers on long loops as far as 50 kft (15 km) from the CO. Figure 3-4 shows a typical pole-mount deployment scenario using a line-powered SAI-based DSLAM. The picture shows a 24-port ADSL DSLAM connected to an aerial splice containing 50 POTS circuits.


Figure 3-4: DSL deployment using a SAI-based DSLAM The SAI-based DSLAM typically allows a mixture of standard DSL services on a single platform. The backhaul can be provisioned using bonded DSL or an optical medium. The device acts as an ATM bridge, bridging traffic from the customer to the ATM WAN network. It can be deployed either directly off the ATM switch, off ATM aggregation devices, or off many DSLAMs. Architectures based on an Ethernet network could also be supported, in which case the SAI-based DSLAM would act as an Ethernet Switch. Since it is directly spliced into the POTS feeder pairs, the SAI-based DSLAM should incorporate both the CO-side and subscriber-side interfaces, including integrated POTS splitters, allowing very simple tip-and-ring connectivity to the feeder pairs. Ideally, the units are manufactured with the POTS ‘in’ and ‘out’ pairs already pre-terminated with connectors used by the carrier in their outside plant. The use of a physical-layer cross connect allows a concentration stage between the loops and the xTU-C, and pre-connection of the DSLAM to all the customer loops at the SAI. The SAI-based DSLAM is capable of providing DSL service either at the SAI or anywhere along the distribution cable, without the need to install a cabinet, pour a concrete pad, or deploy -48 VDC power and battery backup systems. This minimizes the operational complexity seen by the service providers when providing service and allows them to bring up DSL lines quickly. Advantages

SAI-based DSLAMs allow carriers to provide service to subscribers ‘on the fringes’ of the network in a manner similar to the service delivery provided to subscribers close to the CO. Key advantages of this architecture are:


• Customers on long loops can receive the same ADSL services available to other customers using the same CPE.

• The architecture supports high DSL connect speeds because it uses short ADSL loop lengths (from the SAI to the customer premises) and fewer bridge taps.

• There are fewer loop qualification issues because load coils are typically not found on shorter loops; in North America, CSA rules do not allow load coils on F2 loops.

• There are fewer right-of-way issues due to the small footprint and deployment flexibility. • SAI-based DSLAMs are standards-based and do not require modifications to the carrier’s

core network. • They are simple to install by existing carrier personnel, with minimal modifications to the

outside plant and existing DSL operations/processes. Implementation/Deployment Issues

The SAI-based DSLAM has many of the same characteristics and deployment issues as any other DSLAM. However, one of the advantages to a SAI-based DSLAM is that existing load coils on long POTS lines do not need to be removed. As long as the DSLAM is placed beyond the last load point, there are no issues with the ADSL service. Typically, the SAI-based DSLAM is fully compatible with a loaded POTS network. Note however, that the HDSL2/4 pair(s) used for backhaul of the system must be unloaded. One of the key highlights of a SAI-based DSLAM is its ability to be line-powered and independent from any local power source. The line-powering feature in the DSLAM removes constraints on where the service provider can place the device. Close proximity to a -48 VDC power source is no longer a requirement, and the service provider now has the freedom to locate the DSLAM wherever there are spare copper pairs available. This allows him to fully optimize both the upstream and downstream data rate capabilities of the system, by placing it within 12 kft (4 km) of the subscriber homes. Considerations for line powering of a SAI-based DSLAM include:

• SAI-based DSLAMs should be designed to minimize power consumption in order to attain the maximum reach using the smallest possible number of copper pairs for power and transport.

• The SAI-based DSLAM may also include the ability to support dedicated power loops (i.e. the ability to deliver power over pairs dedicated solely to power delivery) to ensure that adequate power can be delivered to the unit. This is especially useful in applications where the SAI-based DSLAM is a long distance from the line power source or has an optical backhaul.

• Although Telcordia GR 1089 does recommend the use of negative voltages for line powering, the rate of corrosion depends upon many factors and there is a fundamental tradeoff between pair savings and corrosion concerns.

Operational Issues

SAI-based DSLAMs are designed to easily fit into a carrier’s current operating practices. Their use of line powering, environmental hardening, small size, and compatibility with existing loop management practices allows a rapid and uncomplicated installation at the remote site. DSL Forum MR-01: DSL Anywhere issue 2 Section 3: Overlay Access Solutions Page 33

In determining their operational preferences, service providers can make a choice between two physical variants of the SAI-based DSLAM – sealed units and serviceable units. Both variants are small line-powered remote DSLAMs, as previously described. However, they provide different approaches to the total lifecycle of the product. Sealed SAI-based DSLAMs are fixed-line-size units that are easy to install and manage. Once the unit is spliced into the outside plant network, power, alarms, DS1s, and tip-and-ring pairs are connected. Once the HDSL2/4 and power are applied, the unit will power up, self test, and be ready for use. Once installed, sealed units are intended to be left alone. Additional units can be installed to address increased capacity requirements and enable the DSL facility to scale in sealed-unit increments (typically 24-ports per unit). Serviceable SAI-based DSLAMs are field-serviceable and upgradeable, plug-in based platforms. The serviceable unit can be configured as a multi-service platform and can readily accommodate changes in the service mix through card upgrades. This enables the service provider to field provision changes to individual line-side service interfaces, or a system migration through changes to the network side backhaul interfaces. The serviceable unit is able to scale in small increments and allows for diagnosis and servicing of failure points versus complete unit replacement. Spectrum management issues with the SAI-based DSLAM are typically the same as with any other DSLAM. Consideration should be given to the signals running though the cable binder groups including those from other CO-based services. Systems that use HDSL2, and especially HDSL4 or SHDSL, may have some advantages for backhauling since these physical layers are more compatible with ADSL than other backhaul technologies (especially the North American HDSL physical layers). One operational issue that must be addressed is the ability to provide remote management. SAI-based DSLAMs are deployed in remote locations that may be physically difficult to access. Therefore, SAI-based DSLAMs should support remote management through an in-band management channel. While a local craft port might be used for initial basic configuration (e.g. programming IP address, etc.), the remaining configuration, provisioning, and maintenance activities should be possible using the in-band management channel from a remote network operations center. The Management Information Bases (MIBs) used by SAI-based DSLAMs are the same MIBs used by any other DSLAM. Typically, this includes the AToM MIB, the ADSL MIB, DS1 MIBs, IMA MIBs, and a few enterprise MIBs for managing the actual device. In most cases, device manufacturers have a management system that is integrated with their DSLAM and some provide a northbound interface so that the management systems can be integrated into a carrier's network management systems and OSS. Summary

SAI-based DSLAMs provide a standards-based solution to the problem of serving DSL subscribers who are otherwise unable to receive DSL service. By providing an environmentally hardened, line powered unit that can be deployed anywhere in the last mile of the local loop,


SAI-based DSLAMs not only meet the challenges of today but provide the carrier with a platform to support the high-bandwidth requirements of tomorrow’s services. 3.3 Fiber-optic DSL extenders Fiber-optic extenders transparently extend an existing CO DSLAM signal to a cross-connect cabinet. This effectively brings the DSLAM line interface out to the customer serving area (the DSLAM is physically resident in the CO but logically located in the outside plant). Fiber-optic DSL extenders leverage the SAI infrastructure and the distribution (F2) copper plant by fitting in the existing cross-connect cabinets that dot the landscape (there are approximately 700,000 in the U.S.).2 Fiber-optic extenders can also be effectively used to extend DSL service to customers currently served by legacy DLC systems. Fiber-optic extenders are environmentally hardened and have the ability to withstand the snow, rain, dust, and temperature fluctuations that existing outside-plant POTS equipment can. Typically, these outside-plant components use minimal electronics, waterproofed surfaces, and passive cooling so there are no moving parts and frequent maintenance can be avoided. In addition, they can be line powered from the CO over existing, unused copper pairs. They generally share the same deployment flexibility as SAI-based DSLAMs.

Figure 3-5: Fiber-optic DSL extenders deployed in a cross-connect cabinet Using fiber-optic extenders to enhance the reach of full-rate DSL can provide broadband connectivity to unserved areas 82 kft (25 km) or more from a CO. 3.3.1 Architecture DSL data streams from a CO-based DSLAM are converted into a multiplexed optical signal that is sent from the CO to a cross-connect cabinet (using Gigabit Ethernet transport components, for


2 Similar architectures are found throughout the world. As with elsewhere in this document, the North American terminology (SAI, F1, and F2) is used for convenience.

example). At the outside-plant cabinet, the electrical signal is reconstructed and demultiplexed, restoring the original DSL signals for distribution to subscribers over existing copper connections.

Figure 3-6: Fiber-optic DSL extension architecture This approach eliminates the need to deploy complex remote electronics and facilitates delivery of full-rate DSL to all subscribers simultaneously. Moreover, this solution is highly scalable and can support scores of simultaneous full-rate DSL subscribers on a single fiber pair. Fiber-optic DSL extenders do not interfere with the existing PSTN infrastructure, they simply extend the range of DSL using fiber transport. Therefore, rather an implementing the data/POTS cross-connection at the CO MDF, a splitter in the cross-connect cabinet combines data traffic from the fiber with existing POTS traffic from a voice switch. Although the vast majority of cross-connect cabinets are still copper fed, there are usually available fiber feeder cables nearby (often destined for DLC systems). Thus, where fiber is not already available, it can often be pulled aerially or via installed cabinet-to-CO ducting. While FTTP is a long-term goal for many telcos, fiber-enabling SAI cabinets is a logical starting point for this endeavor. Moreover, with short loops and ADSL2+ bandwidths, it is possible to deliver bandwidths similar to FTTP over existing copper. Advantages

Fiber-optic DSL extenders both maximize CO DSLAM utilization and leverage existing outside-plant equipment, without adding new network elements. They also require little planning and time for installation, since they are non-configurable. Transparency. Fiber-optic DSL extenders do not comprise ‘network elements’ and therefore require no OSS integration or management. Furthermore, there are no issues with CO/outside plant interoperability; fiber-optic extenders are DSLAM-agnostic, simply extending the range of the standard DSL output already deployed via the CO DSLAMs. DSL Forum MR-01: DSL Anywhere issue 2 Section 3: Overlay Access Solutions Page 36

Full-rate bandwidth delivery. Because ‘intelligence’ and processing power is centralized in the CO, fiber-optic extenders offer a non-blocking architecture that enables full-rate DSL to all subscribers simultaneously. With no reliance on statistical multiplexing, they eliminate ‘last-mile bottlenecks’ that can happen in remote DSLAM deployments. Simple deployment and redeployment. Deployment of fiber DSL extenders does not require a ‘forklift’ upgrade. The relative simplicity of a fiber extender solution also means that installation time is minimized. When deploying unhardened edge access equipment to extend DSL service, most of the effort is in non-equipment preparations such as cabinet construction and placement, extending power to the remote location (for non-line-powered options), securing municipal permits, and civil works. Fiber-optic extenders reduce that because outside-plant equipment can be deployed in existing cross-connect cabinets and can be powered remotely from the CO using spare copper pairs. This eliminates the need for planning and permits for electrical and civil works, concrete pad, new environmentally hardened cabinet, power trenching, power termination, and battery backup at the remote site. It is worth noting that the aforementioned deployment simplicity also lends fiber extenders a degree of redeployment flexibility and minimizes potential for stranded investment. Fiber extender equipment is easily retrievable for reassignment and this eases administration, pre- and post-deployment, providing some breathing room for network planners (i.e. miscalculated take rates are not as costly). Scalable. Incremental scalability allows a match of equipment to take rates in any given serving area and allows demand for DSL service to be validated without full capital commitment. Operational optimizations. Post-installation, there can be substantial operational optimizations with fiber-optic extenders. Because of their non-configurable nature, they require little training and fewer high-skill technicians to install and maintain than solutions that place complicated electronics in the outside plant. Moreover, by isolating complex electronics in the CO, truck rolls are minimized. Manageability also is enhanced, because DSLAM upgrades and subscriber provisioning are done from the CO rather than multiple remote sites. 3.4 PON-fed DSL Remote Extensions PON-fed DSL uses an FSAN (G.983) PON fiber architecture to serve a remote site. The G.983 optical multiplex is converted to DSL at an Optical Network Terminal (ONT) placed at a remote site (such as the SAI) in the carrier’s outside plant and is carried as a DSL physical layer over copper from the remote site to the customer’s site. Unlike a pure optical fiber to the home deployment, this solution requires that the optical overlay only reach the remote site in the carrier’s plant. The DSL system at the remote site may be a DSLAM with only DSL capabilities, a DLC with narrowband and DSL capabilities, or a multi-service access platform (MSAP) with narrowband, DSL, DS1, and Ethernet service drops. Figure 3-7 shows the generic architecture.


PON-Fed DSL Remote

PSTNOLT

Fiber <20 km ATM, IP,

Ethernet DSL on Copper

OSP ATM/IP

Splitter up to 1:32

Coupler

V-OLT EDFA Video

Figure 3-7: FSAN G.983-compliant PON-fed DSL remote with optional analog video and

narrowband services In this application, the Optical Line Terminal (OLT) in the CO is a standard FSAN G.983 OLT. The fiber output from the OLT is passively split to feed up to 32 standard FSAN ONTs that function as the remote DSL platforms at the other end of the PON. The FSAN standard requires that the OLT and ONT be interoperable with different vendors. Each ONT interworks with a number of DSL interfaces to reach multiple customer premises over copper pairs. FSAN defines a variety of bandwidth options for feeding ONTs:

• 622 Mbps downstream and 155 Mbps upstream • 622 Mbps symmetrical • GPON with 1.2 Gbps downstream and 622 Mbps upstream

The bandwidth is shared by all of the PON-fed remotes (ONTs) and served from one OLT. It is up to the carrier to determine which bandwidth alternative to use and how to split the bandwidth among the ONTs. The optical reach of FSAN PONs is 20 km (over 60 kft). The PON-fed DSL remote mechanical specifications are defined by the FSAN ONT requirements. These requirements include environmental hardening (-40ºC to +65ºC/-40°F to 149°F) and, for units deployed in remote sites, conformance to NEBS Level III and FCC requirements (or other appropriate national requirements) just like any remote access device.


3.4.1 Advantages ADSL2+ and other DSL advances promise to increase upstream and downstream bandwidth per pair on shorter loops. Supporting these increased bandwidth capabilities poses several challenges. The increased bandwidth may exhaust current uplink solutions from the remote DSL site to the CO. The need to shorten loops may require longer uplink reaches from the remote DSL site to the CO. Of course, economics must also be kept in mind. Feeding high-bandwidth DSL (ADSL2+, VDSL, etc.) remote sites with FSAN G.983 PONs can solve these challenges. FSAN-compliant PONs offer three advantages as feeders for high-bandwidth DSL varieties:

• FSAN PON bandwidth of 622 Mbps downstream and 155 Mbps upstream exceeds traditional uplink bandwidth and fits well with the asymmetric nature of ADSL bandwidth

• The FSAN PON reach of 20 km extends uplink distances • The fiber splitting used with FSAN PONs improves the economics of fiber uplinks

The FSAN standard allows for transmission of narrowband services as well as DSL services. The PON-fed remote unit can also be designed to accept narrowband service cards. In this mode, it functions as a broadband DLC. In addition, FSAN PON has the added advantage of carrying analog video on a separate wavelength in the fiber. The analog video can then be split out and launched from the PON-fed remote unit via coax drop cable separate from the DSL drops. By carrying analog video as well as digital content (data and digital video) on the DSL, carriers can offer a wider range of services. 3.4.2 Implementation and Deployment Issues There are two main deployment issues: the fiber build and powering. A PON-fed access system makes most sense when a carrier decides to build a wide-scale PON or FTTP access network. PON-fed access systems may then be deployed as part of the carrier-wide FTTP deployment, in cases where building all the way to the premises does not make sense. With short loops and ADSL2+ bandwidths, it is possible to deliver bandwidths similar to FTTP over existing copper from PON-fed DSL remote extensions. In this way, the PON-fed access systems work as part of the overall FTTP build out. Powering is the other issue for PON-fed remotes. Some sort of line powering or remotely located powering (in a cabinet, for example) must be employed. Operational Issues

The number of DSL lines served from an ONT depends on both the number of homes that are accessible from the remote site and the applications being offered. Since the goal is to shorten the loops from the traditional CSA distances (12 kft/4 km) to a few thousand feet in order to get higher bandwidths, the number of homes served will be fewer than DSL Forum MR-01: DSL Anywhere issue 2 Section 3: Overlay Access Solutions Page 39

for traditional remote access solutions (i.e. fewer than traditional DLC or remote DSLAMs). At the same time, economics will dictate serving more homes per ONT. While the optimal modularity is being determined empirically, it may be worthwhile to deploy chassis-based systems with plug-in DSL cards to allow a range of DSL lines to be installed. Later, as the optimal size is determined, there may be a shift from chassis-based ONTs to PON-fed ‘bricks’, with predetermined numbers of DSL lines. Whatever ONT modularity is chosen, the total number of lines served per PON can be governed by the split ratio, which is the number of ONTs served by a single OLT. If the goal is to deliver more bandwidth per DSL line with less over-subscription, then the number of splits will be low or even 1:1. If the goal is to serve more subscribers, the split could be up to the FSAN maximum of 1:32. Management Issues

PON-fed remote DSL extensions are managed via the FTTP management system. The OLT will have northbound interfaces (including TL1, SNMP, and CORBA) to receive commands from the OSS. Management issues point to the benefits of deploying PON-fed access within a larger FTTP/PON deployment. Within the context of an FTTP deployment, PON-fed access systems will be managed as a standard ONT. Summary

PON-fed access systems solve the bandwidth and reach challenges posed by ADSL2+ and other high-bandwidth/short-loop DSL technologies. Compliance with FSAN G.983 simplifies operational issues within the context of an FTTP architecture.


4 Integrated POTS+DSL Access Solutions This section describes integrated POTS+DSL access solutions for remote terminals, also referred to as IVD (Integrated Voice and Data) systems. Integrated solutions often consume less power and provide higher densities than overlay solutions, and they can speed the installation, provisioning, and deployment of DSL services. DLCs were first introduced in North America in the early 1980s. It was very common for the local loop – the copper line between the subscriber and the CO – to be several kilometers or more in length. DLCs enabled multiple subscriber lines to be terminated at a point close to a neighborhood and then multiplexed on a single copper span for transport back to the CO. In recent years, global DLC service deployment has begun to migrate from voice to data. In particular, DLC-based service has evolved from primarily POTS to DSL. This is particularly true in North America and Western Europe where the percentage of DSL delivered from RT platforms continues to increase, while the POTS infrastructure is considered mature. In these countries, growth in the use of POTS+DSL combo cards is outpacing that of POTS-only cards and DLCs are increasingly considered broadband platforms. In Eastern Europe and developing countries such as China and India, there is little copper infrastructure and DLCs primarily provide basic voice services. However, according to industry analysts RHK, DLCs in these regions will shift from being primarily POTS platforms to broadband platforms by 2008. CO-based DSL deployment models have the benefit of space, and they typically serve high-density areas of 10,000 to 20,000 subscribers, which helps amortize carriers’ capital and operational investments. RT-based DSL deployment models, on the other hand, face significant space restraints and they typically serve lower-density areas. In the U.S. for example, most RTs - 84% - serve fewer than 400 subscribers3. While the challenge of providing DSL from RTs seems daunting, the global subscriber base served from RTs is large and it is growing rapidly. Integrated access solutions enable service providers to meet the demands of this prime DSL subscriber base with quick, economically viable deployment models. This section explores the architecture and advantages of three integrated access solutions that can help service providers increase the deployment of DSL. Digital Loop Carrier (DLC) Line Card. In the early 1980s, the first ‘integrated’ DLCs were introduced, which allowed the RT to be connected directly to the CO switch. The Telcordia TR-08 specification described the requirements necessary for a local digital switch to connect to an RT across a digital interface at the T1 rate of 1.5 Mb/s. Other DLC models also adopted this interface specification and there are still over 20 million lines served from these first-generation DLCs in North America. Some of these vintage DLCs can be upgraded to provide broadband access with simple, integrated POTS+DSL line cards, without any loss of POTS function, capacity, or quality.

DSL Forum MR-01: DSL Anywhere issue 2 Section 4: Integrated POTS+DSL Access Solutions Page 41

3 Millennium-Skyline Project: An Excess of Access: U.S. Access Equipment Market Brief, Issue 2 2003

Broadband Next-Generation DLC (B-NGDLC). DLC capabilities advanced further, beginning in the late 1980s, with the introduction of optical SONET/SDH multiplexing and the Telcordia GR-303 interface specification. While GR-303 is a North American specification, there is a comparable European specification in V5, which was approved by ETSI in 1997. DLCs that incorporated optical transport options and GR-303/V5 interfaces became known as Next-Generation DLCs (NGDLCs). NGDLCs provide greater density and more efficient use of transport facilities via voice concentration. NGDLCs can be equipped with a suite of interchangeable line cards that can provide a number of different services, such as HDSL, IDSL, and ISDN, as well as DS-1 options and applications. An NGDLC can be upgraded to a Broadband NGDLC (B-NGDLC) to support ADSL with the addition of integrated POTS+DSL line cards. The B-NGDLC RT can be connected directly to the residential broadband network or it can use an ATM backhaul facility to carry all data to and from the CO terminal. Some B-NGDLCs use circuit emulation to carry voice traffic along with the data traffic, thus eliminating the need for a separate TDM facility. Broadband Loop Carrier (BLC). The BLC is a new class of access vehicle that provides integrated voice, data, and video services, flexible transport capability, and supports evolution to Softswitch-based VoIP networks. The BLC is a packet-based platform (supporting ATM or IP switching and routing) that is optimized for delivery of high-volume, high-churn service offerings. BLCs can support POTS and DSL on every subscriber port, enabling remote provisioning of any line for voice and/or DSL and thereby eliminating the need for a technician to reconfigure the system as the service mix changes. The BLC also incorporates integrated voice gateway capabilities, including support for both TDM network protocols (e.g. GR303, V5.2) and packet call control protocols (e.g. MGCP, H.248) to operate within today’s TDM voice network infrastructure and enable line-by-line migration to packet-based voice networks. 4.1 DLC Linecard 4.1.1 Introduction While many COs are now equipped to address DSL demands, DLC serving areas have been largely unaddressed. However, as both existing and new subscribers are being migrated from COs to RTs, subscriber loops are becoming shorter, significantly increasing their DSL bandwidth capabilities. The most common architecture in COs consists of a POTS switch, mechanical POTS splitters, and a DSLAM, as shown in Figure 4-1.


Figure 4-1: Initial DSL deployment model This deployment model is difficult to extend to DLCs in the remote plant for the following reasons: Space constraints

While COs usually have some flexibility in locating DSLAM equipment, there is rarely space available to place overlay DSL equipment in DLCs, because they are located at the edge of neighborhoods in small, outside-plant cabinets, CEVs (Controlled Environmental Vaults), or huts.

Right-of-way issues, esthetics, and the expense of new boxes deter service providers from building cabinet farms at the edge of neighborhoods to house overlay DSLAMs and POTS splitters. As a further deterrent, the SAI is not always located with the DLC equipment, which means that service providers must implement non-standard wiring to gain access to subscriber loops. Lack of justification for high initial investment

While DSLAMs in COs serve thousands to tens of thousands of subscribers, DLC serving areas are very small by comparison. Service providers have difficulties justifying the high initial investment required for new cabinets, pouring pads, and increased commercial power to create the DSL overlay infrastructure to compete for such a limited number of subscribers. Inability to respond quickly to service requests

Some overlay DSL remote-site solutions require pouring pads, remote cabinets for POTS splitters and DSLAMs, and complex setups. These installations can be time-consuming and


resource intensive, making it difficult for service providers to respond quickly to DSL service requests. 4.1.2 Architecture To significantly increase DSL service coverage, especially for the growing base of subscribers served from RTs, service providers need a DSL deployment model that is simple, quick, and without the high initial investment required by the current deployment model. Integrated POTS+DSL linecards represent such a solution. The integrated POTS+DSL linecard fits into the existing DLC linecard slot. DSL gains access to the POTS loop, thus eliminating any complex and time-consuming wiring to the protection block, SAIs, or POTS splitters. In addition, the integrated POTS+DSL linecard eliminates the need for additional or larger cabinets, new pouring new pads, and related equipment. With integrated POTS+DSL linecards, the POTS service remains intact, and the voice traffic continues to be backhauled to the CO over the existing POTS transport infrastructure. There are no changes to the existing voice operations, maintenance, or procedures. The DSL traffic is directed to a new, common ATM network interface card, placed in an available slot with backplane access to each linecard. The DSL traffic is aggregated on the ATM card and interfaces to the service provider’s transport system via E1/T1s, E3/DS-3s, or STM1/OC-3s. The DSL traffic is backhauled to an ATM switch at the CO, where it is unbundled and made available to the service providers. Figure 4-2 illustrates the simplified architecture made possible by integrated POTS+DSL linecards.

PSTN

Voice Switch

Central OfficeCustomer Premise

Data Network

Voice Traffic

Data Traffic

Broadband Equipped Remote Terminal

ATM Aggregator

POTS+DSL

Figure 4-2: DSL deployment model for digital linecards


4.1.3 Advantages of integrated POTS+DSL linecards The integrated POTS+DSL linecard architecture makes it possible for the millions of residential subscribers served from DLCs to receive broadband DSL services. Integrated POTS+DSL linecards offer service providers the following key advantages: • DSL coverage • Reliability • Low initial investment • Scalability • Rapid deployment • Amortized backhaul DSL coverage

DLC serving areas account for a significant portion of the target DSL subscriber base. Integrated POTS+DSL linecards enable service providers to offer DSL service to this widely dispersed, suburban, and rural subscriber base. Reliability

Integrated POTS+DSL linecards eliminate the need for pouring pads, overlay cabinets, complex wiring, and resource-intensive installations, simplifying the access network and reducing the number of failure points. Low initial investment

DLCs can be equipped for DSL service on a linecard-by-linecard basis. This level of granularity enables service providers to match investments to returns. Scalability

The continued advancements in DSL silicon technology enable service providers to upgrade legacy DLCs on a linecard-by-linecard basis to address required and projected DSL penetration levels, without any reduction to POTS functions, capacity, or quality. Rapid deployment

POTS+DSL linecards can be installed quickly, since only a simple linecard swap is required. New DLCs can be deployed pre-equipped with integrated POTS+DSL linecards. Remote service activation further speeds up the deployment process. Amortized backhaul

All DSL traffic is backhauled to the ATM switch for service unbundling. The DSL backhaul facilities are amortized over the entire DSL subscriber area. This is an efficient architecture for delivering DSL services to the subscriber base. 4.1.4 Conclusion The integrated POTS+DSL linecard architecture is a simple and efficient approach to providing DSL service to many of the millions of subscribers served from DLCs, without the need for high initial investments that may be difficult to justify.


While POTS+DSL linecards are ideally suited to enable DSL from the most widely deployed first-generation DLCs, the integration of POTS and DSL on linecards will also enable a new generation of broadband-capable RTs, as explored in the following sections. 4.2 Next-Generation Digital Loop Carriers 4.2.1 Introduction Next-Generation Digital Loop Carriers (NGDLCs) have been deployed since the 1980s as an access platform for voice and data services for residences and businesses. NGDLCs offer service providers multiple features, primarily for the deployment of narrowband services based on DS0s: • With NGDLCs, each CO can serve end users far beyond the local CSA of 12 to 18 kft (3.5 to

5.5 km). Fiber or E1/T1 span lines can be deployed deeper into the access network to multiple CSAs, to serve multiple remote residences and businesses.

• Service providers can rely on open standards (SONET/SDH, GR-303, V5.2, T1/E1 span lines) between the NGDLC and CO to select vendors based on price and performance, assured of integration into the network.

• Maximizing the use of Class 5 switches means equipping them with high-capacity digital links instead of analog voice lines only. Additionally, Class 5 switches have evolved so that one local switch can serve an extended geographic area of up to 100,000+ subscribers.

• Fiber transmission is superior compared with a large number of copper loops deployed all the way to the CO. Fiber feeder plants are more economical in the long run and pave the way for broadband deployment.

The versatility of NGDLCs, manifested in the support of different drop-side services over several transport facilities, makes them flexible enough to fit into residential areas and small or medium businesses, in different service-provider arrangements. Service providers have a number of options for equipping NGDLCs with DSL capability, but most fall into one of three categories: • Deploying an overlay remote DSLAM or mini-RAM at the NGDLC location • Upgrading the NGDLC to support DSL • Replacing the NGDLC to support voice and DSL This section provides insight about the second option—upgrading the NGDLC. A B-NGDLC is defined as an NGDLC upgraded to support broadband DSL services. What follows are guidelines for assuring that a B-NGDLC has the flexibility of a CO-based DSLAM, while maintaining the functions, capacity, and quality of its narrowband services. 4.2.2 Architecture


NGDLCs were originally designed to provide narrowband services. The need for integrated DSL in NGDLCs has brought along with it the requirement of a cell-based fabric. To rate as an effective, integrated solution, a B-NGDLC must meet several important requirements:

Efficient transport

Provisioning DSL service at the NGDLC requires a cell-based transport to the CO. As the DSL is integrated with narrowband services, the transport needs to accommodate TDM and cell-based traffic. In certain applications, a service provider may choose to physically separate the DSL backhaul traffic from the TDM traffic at the remote. In such configurations, the TDM traffic and ATM traffic are separated into two different physical backhaul facilities; each can be based on copper (HDSL, T1/E1) or fiber. The CO-based equipment is similarly separated into TDM and ATM functions. In other applications and deployment models, service providers may benefit from using the same backhaul facility to carry both data and voice traffic. This eliminates the need to deploy new fiber or copper between the RT and the CO to carry the DSL traffic. Sufficient DSL density

The B-NGDLC must provide sufficient density of DSL services to address projected DSL take rates, and POTS density should not be compromised as a result of DSL support. The capability to integrate splitters on the B-NGDLC line card is also important, because it eliminates the space required for a separate splitter shelf. In addition, advancements in DSP technologies allow relaxation of the stringent requirements for splitters, enabling lower cost implementations of combined POTS and ADSL linecards. Right market segments

NGDLCs support both residential and business services. B-NGDLCs must also meet the requirements of both these segments. Loop management

Copper loop management, which includes wiring, qualification, and access to metallic test access bus, is essential for the management of the DSL assets in NGDLCs. Network architecture

B-NGDLCs must support the same network topologies and universal/integrated operation modes as NGDLCs. Management and operations

Management and operations must accommodate the deployment of DSL and voice services from the same B-NGDLC box. The B-NGDLC EMS and NMS must be able to manage both services simultaneously. 4.2.2 Advantages of a Broadband NGDLC Upgrading a NGDLC to a B-NGDLC offers service providers the following key advantages: • Low initial investment • Simplicity • Versatility • Scalability • Efficient ATM network interface • Consolidated management DSL Forum MR-01: DSL Anywhere issue 2 Section 4: Integrated POTS+DSL Access Solutions Page 47

Low initial investment

As a first step, the service provider may only need to replace the line cards that require DSL. At this stage, new transport facilities may not be necessary. Simplicity

The solution is simple and does not require changes to equipment, network architecture, or operation. Versatility

The solution supports all existing narrowband services while allowing multiple DSL services to address different market segments. A single remote terminal can provide any mix of narrowband and DSL-based services. Scalability

The solution scales to significant DSL take rates that can serve long-term DSL deployment needs. As bandwidth requirements continue to increase, an upgrade to broader backhaul facilities is also an option. Efficient ATM network interface

Flexible topology options enable the service provider to aggregate multiple subscribers’ data onto a single ATM network interface, or multiple interfaces, for unbundling purposes. Consolidated management

A single element and network management platform can be used for both narrowband and broadband services. 4.2.3 Operational and Deployment Issues Linecard upgrades of both first-generation DLCs (section 4.1) and NGDLCs (section 4.2) present several deployment issues. System Density

RT linecard density and overall shelf density are usually important features as they impact real estate sizing. Compared with indoor applications, heat dissipation requirements are more stringent in the outside plant, which may lead to DSL densities that are lower in DLCs and B-NGDLCs than CO-based DSLAMs. When sizing a remote, consideration needs to be given to overall shelf capacity for DSL, voice, and integrated services. Other equipment requirements in the RT also need to be planned for in advance of deployment. Power system, battery backup, and surge protectors are common components in the RT, and must be engineered for coexistence with DSL. Spectrum of Services

DLCs and NGDLCs provide service to residences and small-business customers. While ADSL is the dominant service for residences, symmetrical DSL (SHDSL) is favored in the business segment. The B-NGDLC is required to support these DSL versions. Usually, implementing different DSL linecards enables the service provider to support both. Copper Loop Management

Copper loop management involves the use of tools to analyze copper pairs for DSL service, designate wiring and physical connections, and provide life-cycle management. Usually, a DSL Forum MR-01: DSL Anywhere issue 2 Section 4: Integrated POTS+DSL Access Solutions Page 48

specialized test head (equipped to test the DSL spectrum) will gain ‘tip and ring’ access to each served copper loop and perform the maintenance. In a CO-based deployment, the cost of a loop qualification system is justifiable across all the DSLAM assets in the CO. In a remote deployment, where DSL port count is significantly lower, a more cost-effective solution is desirable. The need for loop testing in B-NGDLC remotes is increased by the fact that these locations are numerous and less accessible than the CO. Most NGDLCs support access via the metallic test bus, which is an extension that provides metallic test access to the DSL linecards. The location of the POTS splitter may be a challenge to loop qualification in some instances. 4.3 Broadband Loop Carrier 4.3.1 Introduction A new generation of access vehicle, the Broadband Loop Carrier (BLC), is designed to improve the availability of DSL service. The BLC is optimized to address full DSL demand and offers service providers a solid business case for addressing the DSL requirements of the growing subscriber base served from RTs. BLCs also enable a seamless migration from today’s TDM network to a converged, packet-based network. A fundamental characteristic of the BLC is the integration of POTS and DSL on every line. POTS has long been the model for the delivery of widespread and affordable volume service. The full integration of POTS and DSL on a 100% broadband platform brings POTS+DSL operating efficiencies close to those of a POTS-only solution. 4.3.2 Architecture The BLC architecture supports POTS+DSL on all subscriber ports, thereby enabling remote provisioning of voice and/or DSL on any line. This eliminates the need for a technician to reconfigure the system as the service mix changes. DSL on every line

Every subscriber line supports lifeline telephone service and is DSL-ready upon installation. This enables service providers to keep pace with growing DSL demands, without additional equipment or resources. There is no requirement to trade off voice ports for DSL ports, or to change or add cards when the service mix changes. All operations and provisioning can be performed remotely. Simplified access network

Because DSL is available on every line, re-wiring, and wire tromboning4 to POTS splitters and DSLAMs is eliminated. POTS and/or DSL service can be provisioned, tested, monitored, and maintained remotely. Line and station transfers can be virtually eliminated.


4 Tromboning is a term that describes the incremental and complex MDF (Main Distribution Frame) wiring and routing from the POTS switch to the MDF to the DSLAM POTS splitters, back to the MDF, and then on to the subscriber.

Remote terminal architecture

Integrated POTS+DSL linecards eliminate the complex and time-consuming wiring to protection blocks, SAIs, and POTS splitters. These linecards also eliminate the need for incremental cabinets and equipment, new concrete pads, and other aspects required to physically co-locate some overlay solutions. Figure 4-3 illustrates the BLC RT architecture.

Converged Packet Network

Softswitch

DataPacket VoicePOTS + DSL

BroadbandLoop Carrier

Central Office or Remote

Figure 4-3: DSL deployment model for broadband loop carriers The POTS (TDM) traffic can be backhauled to the CO via GR303 or V5.2, or packetized and carried with the data traffic to an ATM switch. The DSL traffic is backhauled to the ATM switch at the CO. The DSL traffic is unbundled at the ATM switch and available to the data affiliate and competitive service providers via virtual circuits (VCs). Alternatively, the BLC can provide an Ethernet uplink to an IP-based network. Packet-ready architecture

The BLC enables a seamless transition from today’s TDM network to a converged, packet-based network by supporting optional voice packetization and packet call protocols. BLCs support both TDM network protocols (e.g. GR303, V5.2) and packet call control protocols (e.g. MGCP, H.248) to operate within today’s TDM voice network infrastructure and enable line-by-line migration to packet-based voice networks. Service providers can packetize voice traffic, on a per-line basis, and then carry both voice and data traffic to the converged packet network. The need for TDM voice grooming and GR303/V5.2 switch-based interfaces is eliminated. In addition, there is no forced tradeoff of packet voice ports for DSL ports, and no requirement to change or add cards as the service mix changes. 4.3.3 Advantages of a BLC The BLC offers service providers the following key advantages: • Network simplification • Network reliability • Operational efficiency; software-activated services • Seamless transition to a converged packet-based network


Network simplification

DSL is available on every line, with no impact to POTS densities. The need to provision and maintain a separate overlay network is eliminated. A single network significantly reduces complexity and points of failure, resulting in enhanced reliability. Network reliability

The BLC’s integration of POTS+DSL eliminates the need for separate overlay access networks. A single network significantly reduces complexity and points of failure, resulting in greater network reliability. Operational efficiency; software-activated services

All DSL service requests are processed using remote provisioning software at the network operations center. No truck rolls to the RT are required, and no manual turn up or turn down of services is required, allowing service providers to rapidly and efficiently deploy DSL services. Many of the traditional outside plant operational activities, such as DSL card insertions and line and station transfers (LSTs) can be eliminated. Seamless transition to the converged, packet-based network

The BLC provides line-by-line migrations to the emerging packet/softswitch network. Whereas most other platforms must cut over in wholesale fashion from TDM to packet, the BLC provides the option to offer differentiated tariffs for VoP services on a discrete line-by-line basis. As the BLC accommodates both MGCP and Megaco/H.248-based call control, only software activation is required to begin operation with a voice-over-packet infrastructure. The transition of voice to packet is completely transparent to the subscriber. 4.3.4 Conclusion The integration of POTS and DSL on linecards has resulted in a new class of BLC whose broadband architecture makes DSL as available as POTS. This architecture helps service providers continue to drive ‘DSL Anywhere’ by simplifying the access network and maximizing operational efficiencies. Equally important, the BLC provides service providers with the means to implement a graceful migration to the converged packet-based network using the same equipment infrastructure.


5 Loop Extenders and Repeaters The major limitation in the delivery of DSL service is the deployment range of the selected DSL technology. This limitation is the result of two major constraints: the noise environment of the copper facility and the reach of the DSL technology used to deliver the DSL service. The noise environment of the copper facility is affected by crosstalk from services co-located in the same binder group, as well as by noise generated by external sources such as power lines and radio transmissions. The reach limitation of a DSL technology is also directly related to the line code and multiplexing scheme used by the DSL technology. This section discusses two techniques available to resolve deployment limitations of the copper facility: loop extension and repeaters. Loop Extension. Loop extension technologies improve the ability to deploy a given service. Depending on the given noise environment, the performance of differing DSL technologies will vary greatly. In these cases, using a different loop technology to transport the DSL service can be the difference between reaching a customer and denying service to a customer. Repeaters. The term repeater refers to both regenerators (devices that recover and regenerate a signal) and amplifiers (devices that amplify the signal level). Repeaters are deployed in the outside plant to extend the range of DSL technologies. A single repeater can often double the range of a DSL technology. Loop extension and repeaters can also be combined to achieve an even longer reach. 5.1 Loop Extension Loop extension is one of the most popular applications of DSL technologies, with HDSL, HDSL2 (ANSI T1.418), and SHDSL (ITU G.991.2) being the leading varieties deployed today. For years, T1/E1 service was deployed using an alternate mark inversion (AMI)-type signal requiring significant loop engineering to remove bridged taps and accommodate repeaters. With the advent of HDSL (and more recently HDSL2 and SHDSL), T1/E1 services can be deployed on copper pairs with bridged taps and without repeaters when deployed over CSA ranges (i.e. 12-kft/3.6-km spans). Loop extension technologies are available today for the delivery of T1, E1, ISDN, DDS, and DSL services. These technologies give service providers the ability to deliver services more cost efficiently than with standard delivery techniques. 5.1.1 Description of Architecture/Technique Loop extension technologies are not designed to replace services deployed in the network, but rather to improve service deployment in the network. Consider the use of HDSL2 in the delivery of T1 services at 1.544 Mbps.

DSL Forum MR-01: DSL Anywhere issue 2 Section 5: Loop Extenders and Repeaters Page 52

The old delivery method for T1 or E1 service used an AMI signal over two pairs. One pair was used for the transmit path; the other for the receive path. Due to crosstalk problems, the two pairs had to be deployed in separate binder groups and each pair had to be engineered to remove all load coils and bridged taps. On a typical CSA loop (12 kft/3.6 km of 24-AWG cable) T1 had to use two repeaters to complete the circuit. Figure 5-1 shows the typical deployment of a T1 service using the original AMI-type signals.

OFFICEREPEATER

NETWORKINTERFACE

UNIT

LINEREPEATER

LINEREPEATER

T1 T1 T1 T1T1

6 kft24 awg

3 kft24 awg

3 kft24 awg

Figure 5-1: T1 service with AMI deployment

The deployment of T1 and E1 services was costly and time-consuming using the original AMI technology. In contrast, HDSL2 and SHDSL have applied advancements in technology to the deployment of T1 and E1 circuits to improve service providers’ ability to deliver services. HDSL2 and SHDSL use Trellis Coded Pulse Amplitude Modulation (TC PAM) with spectral shaping to provide service on a single copper pair. HDSL2 and SHDSL can be deployed at CSA ranges without using repeaters and can tolerate up to 2.5 kft (.8 km) of bridged taps. Figure 5-2 shows the typical deployment of T1 services using HDSL2.

HDSL2CO

UNIT

HDSL2REMOTE

UNIT

1224

kftawg

T1 T1HDSL2/SHDSL

Figure 5-2: T1 service with HDSL2/SHDSL deployment

While HDSL2 has been specifically designed for T1 transport, SHDSL can do the same for E1 but due to its versatility, can also carry many other services. 5.1.2 Advantages Each loop extension technology provides its own set of advantages depending on the application. However, the general purpose of loop extension is to provide the following advantages: Improved Performance

Loop extension technologies typically incorporate advanced algorithms that allow increased performance and overcome different types of interference and impairments on the circuit. Increased Range

Loop extension typically either eliminates or decreases the need for line repeaters. This reduces the deployment and maintenance problems involved in deploying repeatered services to customers.


Ease of Use

The overall goal is to make the service easier to deploy. By improving performance, reducing engineering requirements, and reducing the need for repeaters, loop extension technologies make it easier to deploy services. 5.1.3 Implementation/Deployment Issues Loop extension technologies are widely deployed in all regions of the world. The delivery of ISDN, DDS, T1, and E1 is typically achieved with some means of loop extension. Unfortunately, many loop extension technologies are proprietary to a specific vendor. Proprietary technologies require that service providers coordinate both CO and remote units to ensure proper operation of the technologies. Only with the advent of standards-based loop extension technologies, such as HDSL2 and SHDSL, will ubiquitous deployment of loop extension technologies take place. 5.1.4 Operational Issues Loop extension technologies pose significant maintenance and troubleshooting problems. As previously mentioned, many loop extension technologies are proprietary. Because of this, there usually are not test sets available to provide physical layer testing of the loop extension technology. If such equipment is available, it too can be proprietary and costly. In addition to maintenance and troubleshooting issues, provisioning issues arise with new technologies. When loop extension technologies are first provisioned, it is typically a manual process for the service provider. The deployment of loop extension technologies typically starts out as the exception, not the rule. In this case, it is simple to keep up with the manual engineering of these services. As the technology matures and becomes more cost-effective, it becomes the rule as opposed to the exception. In this case, the service provider’s operational systems must be adjusted to provide automated, flow-through provisioning of the loop extension technology. While this is not a complicated process, it does require planning and commitment. 5.2 Mid-Span Repeater Repeaters extend the range of a DSL service. Typically, a repeater can double the range of a DSL technology. Repeaters are active elements installed in the outside plant, and are either amplifiers or regenerators. Amplifiers amplify and equalize the signal, while regenerators recover and regenerate the signal. 5.2.1 Description of Architecture/Technique Repeaters have been deployed with all major digital local loop technologies to date. DDS, ISDN, T1, E1, SHDSL, and HDSL repeaters operate as regenerators. ADSL repeaters operate as amplifiers. Deployment specifics vary slightly depending on the technology being deployed. In general, repeaters are span-powered over the copper pair from the CO, and the deployment guidelines between any two network elements (CO to remote, CO to repeater, repeater to remote, and repeater to repeater) are the same. Using SHDSL with line powered repeaters, some providers have been able to deliver ADSL service over distances greater than 10 miles (16 km),


which enables a practically unlimited service coverage given the deployment considerations outlined in section 5.2.3 below. Figure 5-3 is a simplified block diagram of a typical repeater deployment.

Repeater Deployment

DSLAM orCross-Connect Modem

or NIURepeater

MD

F

ORB

Span PowerModule

ORB = Office Repeater BayMDF = Main Distribution Frame

CO SIDE OUTSIDE PLANT CPE SIDE

Figure 5-3: Typical DSL repeater deployment

5.2.2 Advantages The most obvious advantage of providing regenerators for DSL technologies is to provide extended range, as this directly affects the coverage area of a service provider. Use of repeaters can extend the reach of a DSL technology by 100%. However, this extended range also increases the radius of the coverage area, so the actual area of coverage might increase by 300%. This technique is of greatest advantage for delivering service to low-density clusters of customers on a per-line implementation basis and is complementary to other high-concentration strategies described in this document. Figure 5-4 depicts the gain in coverage resulting from the use of repeaters. DSL Forum MR-01: DSL Anywhere issue 2 Section 5: Loop Extenders and Repeaters Page 55

ServiceProvider

Regenerator

Business

Regenerator

Regenerator

Government& Education

Consumer

Deploymentw/ Repeaters

Deploymentw/o Repeaters

Figure 5-4: Gain in coverage using repeaters

5.2.3 Implementation/Deployment Issues Deployment methods and procedures are consistent for most repeater installations and require the insertion of an active element in the local loop plant. The only deployment criterion that varies significantly is the distance between elements in a repeatered circuit. However, loop plant deployment presents challenges ranging from physical deployment to spectral compatibility. Repeater Housings

Repeaters must be deployed in environmentally hardened housings to prevent the elements from affecting operation. These housings are typically either metallic, domed enclosures or composite chamber-type housings located in manholes or mounted on telephone poles. Deployment of the housings is costly and time-consuming. Temperature

Even though repeaters are deployed in environmentally hardened housings, they still must operate over a wide temperature range. Regenerators can experience temperatures that range DSL Forum MR-01: DSL Anywhere issue 2 Section 5: Loop Extenders and Repeaters Page 56

from -400F up to 1220F (-400C to 500C). With the addition of solar loading, operational temperatures for regenerators can easily exceed 1500F (650C). Care must be taken in repeater design to ensure performance and reliability over a wide temperature range. Span Power

Power must be provided to repeaters via the copper facility that they are attached to. Care must therefore be taken to safeguard service provider technicians from exposure to high voltages. In some applications, the differential voltages applied to the cable pairs can exceed 200 VDC. A secondary concern with span powering is that the application of positive DC voltages can cause an electrolysis effect that can potentially degrade the performance of the copper. Spectral Compatibility

When repeatered technologies are deployed in the same binder group as non-repeatered technologies, deployment guidelines must be determined to ensure spectral compatibility. The increased power level of the repeatered signal can contribute undesired crosstalk to other services in the binder group. Specific guidelines for analysis of repeaters and regenerators are a subject of ongoing study. The use of repeaters is governed by the spectral compatibility regulations in force in a particular regulatory jurisdiction. These regional rules may limit or even forbid the use of repeater technologies in a particular deployment. 5.2.4 Operational Issues There are several issues of concern associated with the installation and maintenance of repeaters. Installation

As repeaters are installed in the outside plant, the added cost and logistical complexity of a truck roll is required. Troubleshooting

Fault isolation and troubleshooting can be a significant challenge with repeatered systems and can contribute to significant delays in resolving customer troubles. Many modern repeatered systems have introduced diagnostic modes to aid in troubleshooting and isolating faults in the outside plant environment. Repair

A costly truck roll is required for repair/replacement of repeaters. Furthermore, locating the correct repeater housing can be a challenge if adequate record keeping has not been maintained. 5.2.5 Network Management Issues While it is true that repeaters pose operational issues, there have been significant efforts to address these. One such effort is the advent of ‘intelligent repeaters’. Intelligent repeaters allow fault isolation in a repeatered span by allowing each repeater to be addressed individually. Each element in the network then becomes capable of performing diagnostics and performance monitoring. This allows easy fault isolation. Today, T1, E1, DDS, ISDN, HDSL,and SHDSL all have intelligent regenerators available.


6 Standardized DSL Technology Options In order to expand the reach and data rate of the first generation of DSL standards, the industry has spent considerable efforts to push out the limits of DSL through the use of signal processing technologies. In July 2002, the International Telecommunication Union (ITU) completed the ADSL2 standards; in January 2003, the completion of ADSL2plus brought data rates of 24 Mbps to the ADSL2 standards. In October 2003, Reach Extended ADSL2 joined the ADSL2 family, delivering bandwidth on phone lines as long as 22 kft (6.7 km). Collectively, the ADSL2 family of standards delivers significant improvements in performance, as well as valuable new features that will undoubtedly accelerate the growth of the worldwide DSL subscribership. In December 2003, the second version of the ITU SHDSL standard was released, doubling the date rate of the first generation to 5.7 Mbps. Section 6.1 describes the implementation and reach advantages of these new standards, and outlines how they can help service providers cost-effectively improve deployment of ‘DSL Anywhere’. Section 6.2 provides an overview of the emerging standards for bonding multiple DSL loops together to carry traffic at bandwidths higher then could be supported over a single DSL physical layer. 6.1 DSL Standards The international DSL standards were developed at the ITU-T (International Telecommunications Union - Telecommunications Sector) within ‘Study Group 15 Question 4’ (ITU-T SG 15/4), the group responsible for DSL physical layer standards. While the ITU standards are usually versatile, global documents, region or service-specific issues are addressed in Annexes. Further, the major regions have their own standardization bodies, such as ATIS in North America and ETSI in Europe. However, in the case of DSL, these regional standards are aligned as much as possible with region-specific Annexes of the ITU-T standards. The ITU-T categorizes projects under various designations, such as G.dmt, G.lite, G.shdsl, G.voice, G.vdsl, and G.bond. Figure 6-1 summarizes the existing ITU-T DSL standards.

DSL Forum MR-01: DSL Anywhere issue 2 Section 6: Standardized DSL Technology Options Page 58

Figure 6-1: DSL standards

Figures 6-2, 6-3, and 6-4 provide a comparison of downstream data rates versus loop reach for the DSL standards listed in Figure 6-1.5


5 Figures 6-2, 6-3, and 6-4 provide loop reach in kft; 1 kft = 0.3 km. These three figures illustrate the rate and reach performance that the family of DSL technologies can deliver. The plots are based on a middle-of-the-road noise model simulation and represent the target performance that standard xDSL implementations can achieve with the given noise condition. Figures 6-2 and 6-3 provide a comparison of downstream data rates vs. loop reach for the asymmetric DSL standards listed in Figure 6-1. Figure 6-2 provides information for loops up to 10 kft, while Figure 6-3 provides this information for loops between 12 and 20 kft in length. Both these figures are based on a simulation calculated using 12 self NEXT/FEXT disturbers over 26 AWG loops and a noise floor of -140 dBm/Hz with a margin of 6dB. Figure 6-4 illustrates G.991.2 rate vs. reach performance. This figure is based on a simulation using 12 self NEXT/FEXT disturbers over 26 AWG loops and a noise floor of -120 dBm/Hz with a margin of 6dB.

Asymmetric DSL Technology Performance (12 Self Crosstalk)

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Figure 6-2: Asymmetric DSL technology rate/reach comparison (12 Self)

Asymmetric DSL Performance on Long Loops (12 Self)

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Figure 6-3: Asymmetric DSL Performance on Long Loops (12 Self)


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s)

Figure 6-4: G.shdsl G.991.2 - 2003 Performance (-120 dBm/Hz + 12 Self )

6.1.1 ADSL - G.992.1 & G.992.2 Work was completed in 1999 on the G.dmt and G.lite recommendations to publish G.992.1 for full-rate ADSL and G.992.2 for the splitterless version. These standards include Annexes that define the use of ADSL in various environments throughout the world. Since these two standards were ratified in 1999, the ITU-T SG 15/4 has added enhancements to these recommendations that provide increased bandwidth and loop reach. ANSI T1.413, ITU-T G.992.1, and G.992.2 describe an asymmetric transmission method for data transport in access networks. Compliant modems are typically capable of supporting downstream user data rates in the range of 32 kbps to 8000 kbps and upstream user data rates in the range of 32 kbps to 800 kbps in increments of 32 kbps, using a DMT line code. ADSL has been in use since it was approved by the ITU, and is successfully deployed worldwide. As of December 2003, 64 million ADSL lines were in service. With the first set of ADSL standards, providers were faced with several challenges, such as limited rate and reach, inability to perform advanced diagnostics, and support of additional emerging technologies. Solutions were proposed to offer improvements in these areas, and the standards bodies recognized the need to incorporate these improvements into ADSL2. 6.1.2 ADSL2 - G.992.3 & G.992.4 The ITU ADSL2 standards (ITU G.992.3 and G.992.4) add new features and functionality targeted at improving performance and interoperability, including support for new applications,


services, and deployment scenarios. Among the changes are improvements in data rate and reach performance, bonding, rate adaptation, advanced diagnostics, and stand-by mode. Rate and Reach Improvements

ADSL2 was specifically designed to improve the rate and reach of ADSL, largely by achieving better performance on long lines in the presence of narrowband interference. ADSL2 accomplishes this by improving modulation efficiency, reducing framing overhead, achieving higher coding gain, improving the initialization state machine, and providing enhanced signal-processing algorithms. As a result, ADSL2 mandates higher performance for all standard-compliant devices. Other ADSL2 Improvements

• Better performance on long lines. One-bit constellations and an improved framing structure increase data rates over longer loops.

• Better performance with bridged taps and interference. A receiver-allocated pilot tone and the enabling of interference cancellation techniques provide better performance.

• Diagnostic tools. ADSL2 transceivers have extensive diagnostic capabilities, including a double-ended line-testing mode for trouble resolution that provides precise measurements of line noise, attenuation, and noise at both ends of the line.

• Fast start-up. ADSL2 reduces initialization time from more than 10 seconds (as is required for ADSL) to less than 3 seconds.

• Channelization capability. ADSL2 transceivers are able to channelize bandwidth, allocating different link characteristics to different applications. For example, a voice application might have low latency but a higher error rate, while a data application might have high latency but a lower error rate.

• Power management. ADSL2 includes low-power modes that provide statistical power savings and stand-by/sleep modes that reduce the overall transceiver power consumption. This is particularly important for remote DSL equipment, where heat is a challenging engineering problem.

• All-digital mode. ADSL2 enables an optional mode that allows for transmission of data in the voice bandwidth, adding 256 kbps of upstream data rate.

• Support of Packet-Based Services. ADSL2 includes a packet mode transmission trans-convergence layer (PMT-TC) that enables packet-based service (such as Ethernet) to be transported directly over ADSL2.

• Bonding. The ADSL2 standards support the ATM Forum’s inverse multiplexing for ATM (IMA) standard developed for traditional ATM architectures. Through IMA, ADSL2 chipsets can bind two or more copper pairs for higher downstream data rates.

6.1.3 ADSL2plus - G.992.5 The ability to deliver higher data rates is appealing to providers. The ability to deliver 24 Mbps enables providers to offer new content and services, including bandwidth-hungry applications such as movies and video, thereby increasing revenue and the customer base. Again, the standards bodies recognized this need and delivered ADSL2plus. ADSL2plus reached consent at the ITU in January 2003, joining the ADSL2 standards family as G.992.5. The ADSL2plus recommendation doubles the downstream bandwidth, effectively doubling the maximum downstream data rates, and achieving rates of 24 Mbps on phone lines as


long as 5 kft (1.5 km). ADSL2plus solutions will most commonly be multimodal, with chipsets supporting ADSL and ADSL2, as well as ADSL2plus. ADSL2plus enables service providers to evolve their networks to support advanced services such as video in a flexible way, with a single solution for both short-loop and long-loop applications. It includes all the feature and performance benefits of ADSL2, while also interoperating with legacy equipment. As such, carriers are able to overlay new, advanced technologies without having to ‘forklift-upgrade’ existing equipment, allowing a gradual transition to advanced services. While the first two members of the ADSL2 standards family specify a downstream frequency band up to 1.1 MHz and 552 kHz respectively, ADSL2plus specifies a downstream frequency from 1.1 MHz to 2.2 MHz by masking the downstream frequencies below 1.1 MHz. This can be particularly useful for reducing crosstalk when ADSL services from both the CO and an RT are present in the same binder as they approach customers’ homes. The increase in frequency and elimination of crosstalk results in significant data rate increases on shorter phone lines. However, if the loop is longer than 5 kft (1.5 km), the data rate drops to ADSL2 data rates and the significant speed improvement is reduced. 6.1.4 ADSL Annexes ADSL standards include Annexes that specify ADSL operation for particular applications and regions around the world. Generally, the Annexes specify subcarriers (or tones) and their associated transmission power levels used for upstream and downstream transmission. The naming convention is such that Annexes to the DMT-based ITU-T DSL standards use the same identifying letter (e.g. A, B, C, etc.) across each of the recommendations to designate the same area of concern and similar use of the DMT subcarriers.

Figure 6-5: ADSL Standard Annexes

6.1.5 Reach Extended ADSL2 – G.992.3 Annex L One of the main challenges addressed in this white paper is that although there is an increase in demand for high-speed Internet access, a significant number of potential subscribers live outside


the range of current ADSL technology, particularly in North America but also in Europe and Asia. In these cases, carriers are forced to deny service to customers, so the ability to extend the range of ADSL and expand the addressable market is very valuable. In response to this demand, the Reach-Extended (RE-ADSL2) mode of operation in the ADSL2 standard was officially ratified at the annual ITU meeting in Geneva, held in October 2003, as ‘Annex L: Specific Requirements for a Range Extended ADSL System Operating in the Frequency Band above POTS’. RE-ADSL2 contains one mandatory downstream power spectral density (PSD) mask, one optional downstream PSD mask, and two mandatory upstream PSD masks. A tradeoff between bandwidth and PSD levels is an important aspect of the RE-ADSL2 system that allows reach-extended systems to be deployed in the same environment as legacy ADSL systems without causing them harm. The reach-extended PSD masks were carefully chosen to be spectrally compatible according to ANSI Standard T1.417, the standard that specifies spectrum compatibility requirements in North America. Reach-Extended ADSL2 systems provide increased performance on long lines under various crosstalk conditions. The increase in transmit PSD levels result in increased data rates, which are achieved even though the overall transmit bandwidth is less than Annex A ADSL systems. This is primarily because the higher frequency spectrum is not usable on long lines due to the high channel attenuation. As a result, RE-ADSL2 systems provide an increase in data rate on shorter lines (where the high frequency spectrum is good) as compared to Annex A systems, even though they use less overall bandwidth. RE-ADSL2 introduces a new era of standard-compliant products to address the rate/reach challenge. It represents one of many areas where the ITU has responded to carrier requirements in order to enable DSL product vendors to deliver valuable solutions to the marketplace. Reach-Extended ADSL2 promises be yet another example of DSL standards driving interoperability, performance, and healthy competition.

Figure 6-6: Reach Extended ADSL2 Reach Improvements 6.1.6 SHDSL


The ITU-T SG 15/4 completed work on G.SHDSL (G.991.2-2001) in 2001. An enhanced version, commonly referred to as G.shdsl.bis (G.991.2-2003), was released in 2003. The ITU

standard for Single-pair High-speed Digital Subscriber Line (SHDSL) Transceivers Recommendation G.991.2 describes a symmetric transmission method for data transport in access networks. G.991.2 transceivers are capable of supporting selected symmetric user data rates in the range of 192 kbps to 5,696 kbps, using a Trellis Coded Pulse Amplitude Modulation (TC-PAM) line code. G.991.2 modems can be configured to operate at longer ranges than most of the existing DSL technologies, while maintaining spectral compatibility with all other DSL technologies when regional spectral deployment guidelines are followed. G.991.2 was developed as an encompassing technology that addresses the key features and benefits of other symmetric DSL technologies, whether proprietary or standard, to achieve interoperability throughout the DSL world. G.991.2 is significant in that it addresses rate/range adaptability, spectral compatibility, impairment tolerance, and high-speed symmetric deployment for business-based applications such as multiple voice line delivery, Internet access, and remote LAN access. G.991.2 represents the convergence of many traditional DSL technologies into a single, internationally recognized industry standard. TC PAM was chosen as the basis for G.991.2 due to the simplicity of the algorithms and the low latency required for voice traffic. The use of Trellis Coding provides an additional ‘coding gain’ that improves the performance of the digital signal in the presence of interference. The resulting higher level of performance allows the deployment distance to be increased without sacrificing any of the safety ‘margin’ required for practical, real-world implementation. TC PAM is spectrally friendly, ensuring compatibility with other DSL-based services such as ADSL. The TC PAM characteristics that make G.991.2 attractive for spectral compatibility, the use of narrower frequencies for transmission, and the coding gain from the Trellis Coding, all make G.991.2 an attractive option. Though SHDSL already provides an impressive long reach capability, there are two additional options to enable an almost unlimited coverage. The first option is the use of signal regenerators (“repeaters”). As the standard allows for up to eight repeaters the loop reach can be increased by a factor of nine. Another option that can be used, even in addition to repeaters, is the so-called 4-wire mode. In this mode data rate is split over two wire pairs thus bringing down the data rate per line and thereby increasing the loop reach. 6.1.7 VDSL Very High Speed Digital Subscriber Line, or VDSL (G.993.1), is both symmetric and asymmetric and provides up to 52 Mbps of bandwidth over voice on a single twisted-pair copper loop. VDSL, approved by the ITU in 2004, is based on the T1E1 DMT standard with a QAM normative annex. VDSL is twice as fast as ADSL2+ and nearly ten times faster than ADSL. The tradeoff for increased speed is loop length: VDSL has a shorter reach in the loop. Like other DSL technologies, VDSL uses the higher-frequency spectrum available over standard copper above the frequencies used for lifeline POTS and ISDN services. This is commonly referred to as data- and video-over-voice technology. This technology enables telcos to utilize existing copper infrastructure for the delivery of broadband services over the same physical plant. The VDSL spectrum is specified to range from 200 kHz to 30 MHz. Actual spectral allocation varies based on line rates and whether or not asymmetric or symmetric rates are being


used. The baseband for lifeline POTS and ISDN service is preserved by using passive filters commonly known as splitters. 6.2 Bonded DSL Loop bonding solutions provide the industry with a method for combining multiple copper DSL connections with different bit rates together into a single, aggregate connection. This technology is extremely valuable for telcos and service providers that are providing high-speed services such as IP video over ADSL, VoIP, and tiered Internet access connections. In the case of remote customers, bonding can allow delivery of high-bandwidth services to customers even when the bandwidth of individual DSL connections is relatively low. All DSL technologies are limited in the amount of bandwidth they can deliver. This bandwidth also depends on the length of the copper line over which it is delivered, so the ability to use multiple pairs to gain additional bandwidth can literally be the difference between providing the customer the service they desire and not being able to serve the customer at all. There are a number of well-defined existing solutions that allow for support of bonding using connections between applications. These solutions include ATM-based IMA (inverse multiplexing over ATM) and Multilink Point-to-Point Protocol (MLPPP). Although these are stable and commonly implemented solutions to the problem of using multiple connections between hosts in the data communications industry, they have transport capacity inefficiencies and operational complexities in the DSL environment, especially when used to support a mass deployment environment. The Symmetric DSL standards (HDSL2/4 and G.SHDSL) support multiple pair modes, which provide a bonding that is specific to the particular physical layer. A “4-wire mode”, is defined in the SHDSL ITU-T G. 991.2 Recommendation (and also in the ANSI HDSL2/4 T1.418 Standard). In the case of SHDSL, the 4-wire mode can be utilized either to increase the bit rate offered or to extend the service reach. In December 2003, an update to the G.991.2 Recommendation for SHDSL was published by the ITU-T, which increases the maximum SHDSL data rate to 5,696 kbps on a single pair. Use of 4-wire SHDSL mode now enables 10-Mbps, full-duplex service. This mode is included in the IEEE 802.3ah Ethernet in The First Mile (EFM) standard. There are no multi-wire modes defined for the asymmetric DSLs (ADSL, ADSL2/2+, or VDSL). There are three architectures currently being developed into DSL bonding standards by ATIS T1E1.4 and the ITU-T: • Ethernet-based DSL bonding • ATM-based DSL bonding • Time Division Inverse Multiplexing (TDIM) Each of these is designed to be efficient in a DSL environment and independent of the particular physical layer being used. A primary goal of each method is to provide the capability to bond lines that are running at different data rates in order to maximize the bit rate available over the bonded connection. The methods differ in how the aggregation is performed but each takes advantage of the traffic type at the bonding layer.


Four basic functions are needed to bond pairs running at different data rates: • Segmentation - Blocks of data are partitioned into fragments • Framing - Headers and/or footers are used to delineate the fragments • Sequencing - Allows the receiver to reassemble the fragments in proper order • Delineation - Determination of fragments by the receiver Additionally, the following operational issues are dealt with in the proposed DSL bonding standards: • How does a bonded system initialize? How does the bonded system identify which lines

belong to the bonded group? • How can lines be added or deleted from a bonded group? What happens if some of the

individual lines fail? All three of the proposed standards address these issues. 6.2.1 Ethernet-Based Bonding In January 2001, IEEE 802.3 initiated a project, 802.3ah Ethernet in the First Mile (EFM), which included among its objectives the ability to transport Ethernet over multiple pairs of voice-grade copper. Ethernet bonding was the first multi-pair aggregation strategy optimized for packet transport and it solved many of the inefficiencies of previous bonding strategies. With the ability to run over loops of disparate speeds, and the capability for hot insertion and deletion of pairs, multiple obstacles characteristic of previous multi-pair technologies were overcome. Currently, work is in progress in both ATIS T1E1.4 and the ITU-T Study Group 15 to adapt this work to the specific needs of carriers supporting DSL services.

Ethernet bonding utilizes a flexible segmentation and reassembly process that allows vendor-specific algorithmic implementations to interoperate in a robust fashion. As shown in Figure 6-7, the aggregation layer operates over multiple physical connections and below the Ethernet MAC, with a segmentation algorithm on transmit and a reassembly algorithm on receive.

Ethernet MAC

Segmentation Reassembly

Physical pairs

Figure 6-7: Ethernet-based bonding

Ethernet bonding is designed to operate over pairs of disparate speed and qualities, while providing high-efficiency low-latency transport of any packet stream. Ethernet bonding fits into the Ethernet architecture, and is just like any other Ethernet port on a device. The bonding


algorithm operates on up to 32 pairs in an aggregate and supports rate differentials where the fastest link can be up to four times the speed of the slowest link.

Ethernet bonding follows the traditional data philosophy of having a very flexible transmit and receive process. The segmentation algorithm receives a packet and can partition that packet into multiple fragments to be transmitted over independent physical links. The exact algorithm is not specified nor does it need to be – the receive process is flexible enough to allow almost any implementation, subject to a few simple rules. On transmit, each fragment is given a fragmentation header that consists of a 12-bit sequence number and two flag bits – one indicating that the fragment marks the beginning of a packet, and the other indicating that the fragment marks the end of a packet. The segmentation algorithm may take the speed of the links into account when partitioning the packet into fragments, sending more bytes down faster links, as illustrated in Figure 6-8.

Ethernet packet

Frag1 Frag2 Frag3 Frag Hdr

FragHdr

FragHdr

Segmentation

LinkSpeed = 5 Mbps LinkSpeed = 2.5 Mbps LinkSpeed = 2.5 Mbps Figure 6-8: Ethernet packet segmentation

On the other side, the receiver simply re-sequences the received fragments based on the sequence number in the header. When a full packet is received (as indicated by the flags in the header), the packet is handed to the Ethernet MAC for processing by the higher layers. The receiver handles various error conditions (lost fragment, fragment received in error, etc.) with a simple set of rules that allow low-latency high-throughput processing.

Summary

The simplicity and flexibility of Ethernet bonding makes it ideal for packet transport for any application. As it fits into the Ethernet architecture, it can be easily integrated into any product that has an Ethernet MAC to provide efficient, flexible use of Ethernet over the existing copper plant. 6.2.2 ATM-Based Bonding The vast majority of DSL equipment in the network today employs ATM. In fact, the ADSL interoperability specification written by the DSL Forum presupposes the use of ATM in the end-user modem. Traditionally, bonding at the ATM layer is done via IMA (Inverse Multiplexing over ATM). In the ATM Forum IMA specification, all bonded links must operate at the same nominal rate. The


original cells are not modified and control (ICP) cells are inserted for OAM communication between the two ends. Cell sequence information is not transmitted to each receiving end. Instead, cells are distributed to the constituent links in strict round-robin order. ATIS committee T1E1.4 and ITU-T Study Group 15 are currently working on a new method for ATM-layer bonding that will allow bonding of pairs that have different data rates. Of the four basic functions needed to do this, three of those already exist when ATM is used. The only function yet to be defined is the sequencing function. Even though ATM provides the ability to use up to 65,536 VCs per VP between DSLAM and remote, only a few VP/VC values are actually used. The ATM-based bonding standard in T1E1.4 uses some of these VCI bits in the ATM cell header to carry cell sequence information between DSLAM and remote. The System View

The operational principles of the new ATM-based bonding standard are illustrated in Figure 6-9.

Receivingentity

Transmittingentity

Aggregateconnection

Aggregateconnection

Aggregateconnection

Aggregateconnection

Transmittingentity

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Bonded links (downstream direction)

Bonded links (upstream direction)

‘CO’ side ‘CPE’ side

CPE-side status

messagesCO-side status

messages

Figure 6-9: Functional diagram of ATM-based bonding The aggregate connection represents the total bandwidth available to the end user, and the bonded links are the individual pairs in the bonded group. The transmitting entity in each direction is responsible for distributing cells from the aggregate connection to the individual pairs, including insertion of the Sequence ID (SID) in each cell header. The Receiving entity receives cells from each link in the bonded group and reassembles the aggregate stream, based on the SID. There is also a control channel that carries CO-side and CPE-side status messages. These messages communicate information, such as link quality, to the far-end and pairs that do not have sufficient quality will not be used to carry bonded traffic. Advantages of the new ATM-based bonding approach

Uses existing ATM infrastructure. ATM bonding maximizes the reuse of existing ATM-based infrastructure, thereby minimizing the need for new CAPEX investment. Works with existing DSL transceivers. Many off-the-shelf DSL transceiver chips have an ATM interface. While IMA requires modifications to this interface, namely that idle cells shall


neither be generated nor terminated and errored cells shall not be discarded, the ATM bonding standard does not require any changes to the existing ATM interface. Facilitates centralized implementation. Due to the use of a sequence number, the SID, each pair in the bonded group does not have to share a common clock. This facilitates centralized bonding implementations that allow bonding of arbitrary pairs in an access node. 6.2.3 TDIM-Based Bonding In contrast to Ethernet and ATM bonding, TDIM is a synchronous bonding method; the TDIM receiver has to decode the incoming data in the exact order in which the transmitter sent it. This is done by partitioning the incoming data into bonding-sub-blocks, each bonding-sub-block being 125 µsec long. Each transmitted bit has a specified place on one of the copper pair transmitters in the aggregation group of bonded pairs and the bonding receiver collects the data from the individual receivers in the exact order it was transmitted, thus generating the original data that was transmitted. The TDIM bonding method can transport ATM and Ethernet data, which do not have a constant bit rate, as well synchronous traffic that does. It also is able to deal with differential delay between the lines that are bonded to carry the traffic. Figure 6-10 provides an overview of the protocol.

Single-pairmodems / PMDs

FECInterleaver(optional)

Aggregation

Modem

Service Encapsulation

ATM cells TDM PCMstream Ethernet packets

Modem

Modem

Figure 6-10: TDIM data flow model

Differential Delay

The issues of differential delay between lines and detection of line failures are dealt with in the aggregation layer by adding a multi-pair synchronization frame called a Super-Frame every 12 msec. The Super-Frame comprises 12 bytes, with 1 byte sent every millisecond to achieve better error protection and to allow fast detection of a line failure. This Super-Frame allows for bonding control messages to be sent, as well as synchronization between links running at different line rates. DSL Forum MR-01: DSL Anywhere issue 2 Section 6: Standardized DSL Technology Options Page 70

Data CRC

12 0 01234567

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12 0 01234567

Data

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In6

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SF

Header1 Byte

Figure 6-11: Multi-pair synchronization – frame format

Asynchronous Traffic

ATM, Ethernet, or any type of traffic that is not constant bit rate is dealt with in the service encapsulation layer. Each type of traffic is encapsulated by adding a certain header associated with that traffic type. In addition, idle ATM cells or idle Ethernet packets are added as necessary to ensure the constant bit rate of the bonding layer. Whenever a mix of TDM traffic and asynchronous traffic are transmitted concurrently, the TDM traffic is given specified bit locations in the 125-µsec bonding-sub-blocks, while other traffic types use the remaining positions (i.e. at the bonding layer, TDM traffic always has higher priority than asynchronous traffic. If one wants to assign a higher priority to an ATM or Ethernet stream, this is done at the service encapsulation level or at a higher level). High-Quality Services

To ensuring that the quality of delivered service over a bonded system is acceptable, a FEC may be added between the Service Encapsulation layer and the aggregation layer. The TDIM bonding proposed standard defines an optional flexible RS (Reed-Solomon) code and Interleaver. This flexibility allows for various error scenarios and delivers the required quality of transmission with a minimal number of redundancy bytes. Applications

TDIM bonding delivers high-bandwidth TDM traffic with minimal latency and overhead to the incoming traffic. The optional FEC allows it to operate reliably even in harsh environments, which are otherwise characterized by frequent errors on the individual lines due to impulse noise events.


It may be used to deliver high bandwidths to remote DSLAMs or Wi-Fi spots that are not fiber fed, to connect remote sites of an enterprise with a LAN-to-LAN connection, or for any application requiring high and reliable bandwidth, but suffering from a lack of fiber. 6.3 Future Enhancements As discussed, there is global standardization of ADSL, ADSL2plus, SHDSL, and VDSL and more are following, some of which are currently out for ballot at the standards bodies. These will help provide a richer portfolio of DSL technologies designed to deliver ubiquitous broadband services for a wide range of situations and applications.


7 Additional Solutions The solutions described in this section are all technology-based approaches that, at the time of publication, are neither formally standardized nor in the final stages of a standardization process. Each of the solutions meets one or both of the following criteria for inclusion in this paper: • The solution has been deployed in one or more network service deployments, or in one or

more carrier trials consistent in size and scope with industry-accepted field trials • The solution has carrier standardization support in the form of carrier-sponsored ITU, T1E1,

ETSI, or similar work, expected to lead to standardization of the technology The solutions outlined below vary widely in their approaches to providing DSL Anywhere. The first solution, Low Frequency DSL, is optimized to provide coverage on loops that are too long or otherwise too challenged to support reliable service. The second solution, VDSL2, will provide very high data rates from deployments based deep in the outside plant. 7.1 Low Frequency DSL – Improved Reach Technology Low Frequency DSL complements ADSL-based deployments on loops that are too long for, or present other challenges to, ADSL systems. Low Frequency DSL was originally introduced in 1998 and has since been deployed in the Americas, Asia, and Europe to provide increased coverage on unloaded loops. The current generation of the technology has been optimized to enable multimode implementations that support both ADSL and Low Frequency DSL on the same products. 7.1.1 Description Low Frequency DSL combines frequency usage, duplexing method, and adaptation techniques optimized for operation over long loops and at low received signal levels in the presence of time-varying noise and channel conditions. The technology can be implemented in a variety of edge network architectures, including overlay and integrated solutions intended for deployment from the CO or from equipment placed in the outside plant. Low Frequency DSL has been deployed in three versions since its original introduction. Maximum rates have increased with each generation, from 768 kbps to 960 kbps in the second generation, and to 2.2 Mbps in the third and current generation. The current generation was also designed for commonality with critical ADSL parameters, such as sample rate, allowing implementations that include both technologies in the same chipsets. With combination implementations that include Low Frequency DSL and all flavors of ADSL, including RE-ADSL2, service providers can deploy a full range of technologies in one product and enable the technology that works best for each customer, on a loop-by-loop basis, without requiring a truck roll or replacement of customer endpoints.

DSL Forum MR-01: DSL Anywhere issue 2 Section 7: Additional Solutions Page 73

7.1.2 Advantages Low Frequency DSL was designed from the beginning to work on long and challenged loops. The main characteristics that allow it to do so include: Low frequency operation

Low Frequency DSL operates both upstream and downstream in frequencies starting at about 20 kHz. These low frequencies are better able to withstand the attenuation of long loops and suffer less from crosstalk coupling and non-crosstalk noise ingress than the higher frequencies used in the ADSL (and non-overlap RE-ADSL2) downstream passband. Figure 7-1 compares the downstream PSD template frequently used by Low Frequency DSL on the longest loops to the PSD templates for non-overlap ADSL and RE-ADSL2. The templates are shown both at the transmitter and after attenuation through an 18 kft (5.5 km), 26 AWG loop. For equivalent noise levels, the difference in receive levels translates directly to improved SNR at the lower frequencies. Since crosstalk coupling is weaker at lower frequencies, lower noise levels frequently add to the improvement in SNR.

PSD templates

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(a) At transmitter (b) At receiver, after 18 kft (5.5 km) 26 AWG loop loss

Figure 7-1: PSD templates

Adaptive Time Domain Duplexing

This duplexing technique allows overlap of upstream and downstream frequencies, permitting usage of the most desirable frequency range in both directions without requiring echo cancellation. Traffic-based adaptive time allocation is used to allocate transmission time in each direction based on momentary demand, thus providing throughput that approaches the physical-layer line rate in applications such as high-speed Internet access, e-mail, streaming multimedia, etc., which are largely unidirectional, with intermittent traffic in the reverse direction. Highly adaptive operation

Low Frequency DSL is optimized to adapt quickly and reliably in response to changing loop and noise conditions. On long loops where received signal levels are very low, noise ingress from sources outside the loop becomes significant, and these types of ingress can dominate on loops, even without significant crosstalk. The range and variation of noise ingress that can be encountered is generally not well modeled, but it includes the following types of sources:


• AM radio transmission: Sidebands vary in both power and shape based on the material being transmitted. The total power of an AM transmission may vary by over 20 dB between daytime and nighttime transmission, and it may cease transmitting entirely at night.

• Customer premises ingress: Impulses and harmonics from motors and small appliances come and go as the motors switch on and off or change speed. Harmonics from triac-based lighting dimmers are observable throughout the DSL frequency range and come and go with the position of the dimmer. Wiring within the customer premises, which is frequently poorly balanced and even untwisted, compounds these issues by increasing the overall amount of noise ingress.

• POTS signaling: POTS ringing signals are at very high voltage levels and have significant harmonics. These signals can interfere, via crosstalk, with DSL systems on nearby loops as well as those on the same loop. The ringing signals come and go with both the ring cadence and the initiation of a POTS call. In many COs, ringing signal is generated synchronously on multiple loops, resulting in synchronous fluctuating crosstalk from multiple sources. The magnitude of the ring signal may also cause impedance changes due to nonlinear devices on the line.

• Temperature changes, moisture changes, and splices: The long loop channel itself is more subject to disturbances and changes than shorter loops. Temperature variation over the longer length causes more variation in attenuation and there are more opportunities for moisture ingress through cracks and mechanical stress on splices. Changes in balance can be significant during times of weather change.

Low Frequency DSL’s adaptive behavior is designed to respond to any level of variation in the environment. It uses receiver-based equalization to adapt without messaging. Its seamless autorate protocol is specifically designed to be robust, even in response to very large power changes in impairment conditions. 7.1.3 Implementation / Deployment Issues Low Frequency DSL can be deployed in the same manner as ADSL. It is commonly deployed from the same DSLAMs as ADSL, and has been implemented in ADSL/Low Frequency DSL chipsets that allow multimode deployments. It also makes use of the same MIBs to support common configuration and reporting mechanisms. Given that the maximum rate of Low Frequency DSL is 2.2 Mbps, in any given deployment there will be lines over which ADSL or RE-ADSL2 can achieve higher rates, as well as lines over which Low Frequency DSL can achieve higher rates and/or more stable connectivity under fluctuating conditions. If rates beyond 2.2 Mbps are not required for a particular offering, then a service provider can consider deploying Low Frequency DSL on all loops. If higher rates are desired when achievable, then a multimode deployment may be preferred, using products that support both Low Frequency DSL and ADSL, in which the best technology for any individual loop can be negotiated. 7.1.4 Summary Low Frequency DSL provides a complementary alternative to ADSL and its variations on long and challenged loops. The technology has been implemented in multimode chipsets that enable



deployment of both standards-based ADSL and Low Frequency DSL together, allowing performance to be optimized on a loop-by-loop basis using the technology best suited for local conditions. Use of Low Frequency DSL, either alone or in a complementary multimode deployment as described, can provide maximum DSL coverage from the CO or from outside plant-based DSLAMs. 7.2 VDSL2 – Next-Generation VDSL ITU and ANSI committee T1E1.4 have begun work on a VDSL2 standard, based on ADSL2 & T1.424. The goal is to facilitate multimode ADSL2/VDSL2 implementations. VDSL2 is intended to deliver data rates up to 100 Mbps over short loops, in order to enable services such as HDTV. Whereas VDSL uses both DMT and QAM as alternate modulation schemes, VDSL2 will only specify DMT. Although the ITU has just begun work on the VDSL2 standard, the committee has already agreed that VDSL2 will include several features contained in ADSL2, such as loop diagnostics modes, low power modes, and a common management interface with ADSL2. Additional VDSL2 features include Trellis coding, optional fast startup, digital duplexing, windowing, etc.

MARKETING REPORT MR-001 DSL Anywhere – Issue 2paginas.fe.up.pt/~mleitao/STEL/Tecnico/DSLAnywhere_Issue2.pdf · MARKETING REPORT MR-001 DSL Anywhere – Issue 2 September 2004 Produced

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