bttjvol4no1

Bechtel TelecommunicationsTechnical Journal

v Foreword

vii Editorial

1 Telecommunications Research and Development in the United States: A State of Crisis?

13 IP Multimedia Subsystems (IMS): A Standardized Approach to All-IP Converged Networks

37 A Survey of MEMS-Enabled Optical Devices – Applications and Drivers for Deployment

45 PHY/MAC Cross-Layer Issues in Mobile WiMAX(Invited Paper)

57 ANSI/TIA Standard 222 – Structural Standard forAntenna Supporting Structures and Antennas: A Comparison of Revisions F and G

65 Reducing the Amount of Fiber in Fiber-to-the-Home Networks

73 The Impacts of Antenna Azimuth and Tilt Installation Accuracyon UMTS Network Performance

81 2.4 GHz Wi-FiTM Phased Array Antenna Evaluation

Timothy D. Statton and Jake MacLeod

S. Rasoul Safavian, PhD

Brian Coombe

Jungnam Yun, PhD(POSDATA America R&D Center) and

Prof. Mohsen Kavehrad, PhD(The Pennsylvania State University [CITCTR])

Peter Moskal and Krishnamurthy Raghu

Brian Perkins

Esmael Dinan, PhD, and Aleksey A. Kurochkin

Glenn A. Torshizi

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An Independent Analysis of Current Operational Issues JJ aa nn uu aa rr yy 22 00 00 66

C o n t e n t s

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Bechtel Telecommunications

Technical JournalJanuary 2006

Bechtel Telecommunications Technical Journal

Volume 4, Number 1

ADVISORY BOARDJake MacLeod, Principal Vice President and

Chief Technology OfficerS. Rasoul Safavian, PhD, Vice President of Technology,

Americas Regional Business UnitAleksey A. Kurochkin, Deputy Manager,

Site Development and EngineeringBrian Coombe, Systems Engineer

EDITORIAL BOARD

S. Rasoul Safavian, PhD, Editor-in-ChiefRichard Peters, Senior Technical EditorTeresa Baines, Senior Technical EditorBarbara Oldroyd, Technical EditorRuthanne Evans, Technical Editor

GRAPHICS/DESIGN

Keith Schools, Art DirectorDaniel Kemp, Senior Graphic DesignerJohn Cangemi, Graphic DesignerDavid Williams, Graphic DesignerSusan Gann, Graphic DesignerDiane Cole, Desktop Publishing

The BTTJ is also available on the Web atwww.bechteltelecoms.com/jsp/labs/pubs.jsp

TRADEMARK ACKNOWLEDGMENTS

All product or service names mentioned in this BechtelTelecommunications Technical Journal are trademarks oftheir respective companies. Specifically:

3GPP is a trademark of the European TelecommunicationsStandards Institute (ETSI) in France and otherjurisdictions.

AirMagnet is a trademark of AirMagnet, Inc.

Aironet is a trademark of Aironet Wireless Communications,Inc.

Antenex is a registered trademark of Antenex, Inc.

cdma2000 is a registered trademark of the Telecommunications Industry Association (TIA-USA).

PLS-POLE is a registered trademark of the Power LineSystems, Inc.

Telcordia is a registered trademark of TelcordiaTechnologies, Inc., in the United States, other countries, or both.

TOWER is a registered trademark of the Power Line Systems, Inc.

UNIX is a registered trademark of The Open Group.

Wi-Fi is a trademark of the Wireless Ethernet CompatibilityAlliance, Inc.

Contents

Foreword v

Editorial vii

Telecommunications Research and Development 1in the United States: A State of Crisis?Timothy D. Statton and Jake MacLeod

IP Multimedia Subsystems (IMS): A Standardized Approach 13 to All-IP Converged NetworksS. Rasoul Safavian, PhD

A Survey of MEMS-Enabled Optical Devices – 37Applications and Drivers for DeploymentBrian Coombe

PHY/MAC Cross-Layer Issues in Mobile WiMAX 45 (Invited Paper)Jungnam Yun, PhD (POSDATA America R&D Center) andProf. Mohsen Kavehrad, PhD (The Pennsylvania State University [CITCTR])

ANSI/TIA Standard 222 – Structural Standard for 57 Antenna Supporting Structures and Antennas: A Comparison of Revisions F and GPeter Moskal and Krishnamurthy Raghu

Reducing the Amount of Fiber in Fiber-to-the-Home Networks 65Brian Perkins

The Impacts of Antenna Azimuth and Tilt Installation Accuracy 73on UMTS Network PerformanceEsmael Dinan, PhD, and Aleksey A. Kurochkin

2.4 GHz Wi-FiTM Phased Array Antenna Evaluation 81Glenn A. Torshizi

© 2006 Bechtel Corporation. All rights reserved.

Bechtel Telecommunications is a business unit of the Bechtel group of companies, including Bechtel Corporation inthe United States.

Bechtel welcomes inquiries concerning the BTTJ. For further information or for permission to reproduce any paperincluded in this publication in whole or in part, please contact Bechtel Telecommunications, 5275 Westview Drive,Frederick, MD 21703, telephone 301-228-7500 or toll-free 800-946-3232, and ask for the Chief Technology Officer.

Although reasonable efforts have been made to check the papers included in the BTTJ, this publication should notbe interpreted as a representation or warranty by Bechtel Corporation of the accuracy of the information containedin any paper, and readers should not rely on any paper for any particular application of any technology withoutprofessional consultation as to the circumstances of that application. Similarly, the authors and Bechtel Corporationdisclaim any intent to endorse or disparage any particular vendors of any technology.

January 2006 • Volume 4, Number 1 iii

TELECOMMUNICATIONS

It is with great pleasure that we bring you this latest issue of the Bechtel TelecommunicationsTechnical Journal (BTTJ). The BTTJ is a compilation of expert commentaries on global operationalissues important to wireless and wireline telecommunications systems operators.

This issue leads off with an interesting historical analysis of the impact in the United States of the BellLaboratories divestiture in the mid-1980s. Entitled “Telecommunications Research and Developmentin the United States: A State of Crisis?,” the paper is not so much operationally focused as it is adiscussion of the importance of maintaining substantial emphasis on long-term telecommunicationsresearch for the benefit of the global society. You’ll also find a paper by Peter Moskal andKrishnamurthy Raghu entitled “ANSI/TIA Standard 222 – Structural Standard for AntennaSupporting Structures and Antennas: A Comparison of Revisions F and G.” This paper is a significanttechnical discussion of the importance of structural and mechanical standards for antenna support structures.

This is the second issue in which we have invited guest authors to contribute. Dr. Jungnam Yun of the POSDATA America R&D Center in Santa Clara, California, and Professor Mohsen Kavehrad,PhD, of The Pennsylvania State University Center for Information and Communications TechnologyResearch in University Park, Pennsylvania, have collaborated on a very insightful technicaldiscussion of “PHY/MAC Cross-Layer Issues in Mobile WiMAX.” The paper provides an overviewof mobile WiMAX, especially on OFDMA/TDD systems, and addresses issues that need to beresolved to increase throughput, cell coverage, and spectral efficiency.

My congratulations and sincere appreciation are extended to this issue’s authors and contributors. A special level of gratitude goes to Dr. Rasoul Safavian, vice president of Technology, AmericasRegional Business Unit, for accepting the responsibility of editor-in-chief. Dr. Safavian bringsextensive technical knowledge and industry expertise to the BTTJ.

Thank you for your interest and your continued support. We encourage and welcome your comments regarding the topics chosen. You can submit comments and suggestions for improving the BTTJ by visiting the Bechtel Telecommunications Web site and clicking on the BTTJ “Contact Us” section. Past issues can be downloaded from our technology Web site atwww.bechteltelecoms.com/jsp/labs/pubs.jsp.

May your new year be safe, productive, and prosperous.

Sincerely,

Jake MacLeodPrincipal Vice President and Chief Technology OfficerBechtel Telecommunications

January 2006 • Volume 4, Number 1 v

Foreword

Editorial

January 2006 • Volume 4, Number 1 vii

Welcome to the latest issue of the BTTJ! We focused our journal’s spotlight on the trulyexciting times in which we are living. Technical advances are occurring at a phenomenalpace. The dot.com collapse and the telecommunications slowdown of the early 2000s have

been replaced by a burgeoning array of new offerings to put the latest news, sports, music, and gamesright in the consumers’ hands.

In the arena of wireless telecommunications, 3G network deployments and R&D on even morepowerful technologies are both well underway. With the December 2005 ratification of the WiMAXmobile standard (802.16e or 2005), vendors are energetically engaged in developing and testing newWiMAX mobile infrastructures. The IMS promises to further revolutionize the mobile landscape bymerging the power of the Internet with the convenience of wireless services. And FMC is also pickingup significant momentum.

Significant wireline developments are also occurring. The MEMS-based optical components nowbeing deployed increase speed while reducing costs, power consumption, and size. The prospect ofreduced capital costs is spurring a closer look at ways to decrease FTTH waste by reconfiguringserving areas and hub locations. Even antennas and their supporting structures are in the spotlight asthey are about to benefit from state-of-the-art practices incorporated into the latest building codes and standards.

At the same time, I would like the readers to pay particular attention to this issue’s insightful leadpaper by Tim Statton and Jake MacLeod, which provides a thorough analysis of the state ofTelecommunications R&D in the U.S. The article puts U.S. efforts in perspective with global R&D activities, and highlights the essential long-term R&D areas required for the benefit of the global community.

I also encourage you to read the timely and perceptive paper contributed by our invited guest authors,Dr. Yun and Professor Kavehrad, who put the spotlight on an overview of mobile WiMAX, especiallyOFDMA/TDD systems and some of the PHY/MAC cross-layer issues, and possible resolutionsthrough radio resource management.

I hope you find this new issue of the BTTJ informative and useful. As always, we look forward to yourcomments and contributions.

Happy reading!

Dr. S. Rasoul SafavianEditor-in-Chief

© 2006 Bechtel Corporation. All rights reserved. 1

BACKGROUND

In the 1907 American Telephone and TelegraphCorporation (AT&T) annual report, AT&T

president Theodore Vail promoted a positionthat, given the direction of the industry and thetechnology, the business of telephony would bemuch more efficiently and uniformly deployed toUS citizens if telecommunications was treated asa legally sanctioned monopoly. Vail suggestedthat regulation, “provided it is independent,intelligent, considerate, thorough and just,”would be the most effective approach versus abusiness driven by market competition [1]. In1913, the US government accepted theproposition and formed an agreement, entitledthe Kingsbury Commitment, wherein the BellSystem was allowed to evolve as a naturalmonopoly whose focus was to deploy a uniformtelecommunications system to provide universalservice to US citizens. Strict adherence to thetechnical uniformity of the network designspecifications provided a foundation for reliablecommunications that was unprecedented in theglobal theater. The resulting telecommunicationsnetwork contributed significantly to the economicand social successes of the US through periods of peacetime and periods of war and crisis. The telecommunications network provided aplatform that was an essential element fordeveloping an efficient and effective commercialand governmental environment. It formed the basis of what is known today as theinformation highway.

Universal ServiceOne objective of the development of thetelephone network in the early days was“universal service.” Essentially, the Bell Systemwas charged with providing affordable, highquality, reliable telephone service to business andresidential customers in cities and rural areas.Universal service was partially defined in the USCommunications Act of 1934: “For the purpose ofregulating interstate and foreign commerce incommunications by wire and radio so as to makeavailable, so far as possible, to all of the people ofthe United States, without discrimination on thebasis of race, color, religion, national origin or sex,a rapid, efficient, nation-wide and world-widewire and radio communication service withadequate facilities at reasonable charges for thepurpose of the national defense, for the purposeof promoting safety of life and property throughthe use of wire and radio communication….” [2]

The natural tendency of the corporate focus is toserve the more profitable market sectors andavoid the less profitable. Obviously, the morelucrative telephone customers are located incentral business districts. However, to encourageextension of telecommunications into rural andresidential areas, the US government allowed theBell System to subsidize rural and residentialsectors with the profits realized from thebusiness sector. The Bell System was alsoprovided a guaranteed rate of return oninvestment on all capital expenditures involved

TELECOMMUNICATIONS RESEARCH AND DEVELOPMENT IN THE UNITED STATES: A STATE OF CRISIS?

Abstract—The United States of America has held a leadership position in telecommunications R&D for thepast several decades and has significantly contributed to the global community’s deployment of high qualitycommunications systems. As a result of the divestiture of the Bell System in the 1980s, Bell Laboratories—theprimary telecommunications R&D facility in the US—was divided and refocused to concentrate on producingnear-term profits for the Regional Bell Operating Companies. This paper examines the impact of the lack of acentrally funded mechanism for telecommunications R&D in the US and the declining position of the US as aglobal contributor to the telecommunications industry. Suggestions for resolving this matter are offered forconsideration to the telecommunications industry and to the federal government.

Issue Date: January 2006

Timothy D. Statton [email protected]

Jake [email protected]

Bechtel Telecommunications Technical Journal 2

ABBREVIATIONS, ACRONYMS, AND TERMS

2.5G “second-and-a-half” generation, a marketing designation for an intermediate level of digital mobile phone service responding to demands for greater bandwidth

3G third generation enhanced digital mobile phone service at broadband speeds enabling both voice and nonvoice data transfer

3GPP™ Third Generation Partnership Project

4G fourth generation, enhanced digital mobile phone service boosting data transfer rates to 20–40 Mbps

AMPS advanced mobile phone service

ARIB Association of Radio Industries and Businesses

AT&T American Telephone and Telegraph Corporation

ATIS Alliance for Telecommunications Industry Solutions

CCSA China Communications Standards Association

CD compact disk

CEPT Conference of European Posts and Telegraphs

CTO chief technology officer

CTR communications technology research

DARPA Defense Advanced Research Projects Agency

DoD Department of Defense

DSP digital signal processor

EDGE enhanced data rates for GSM evolution

ETSI European Telecommunications Standards Institute

EU European Union

FDD frequency division duplex

FP Framework Programme

GPRS general packet radio service

GSM global system for mobile communications

HSDPA high speed downlink packet access

HSUPA high speed uplink packet access

IEEE Institute of Electrical and Electronics Engineers

IETF Internet Engineering Task Force

IMT-2000 International Mobile Telecommunications-2000

IPv4 Internet Protocol Version 4

IPv6 Internet Protocol Version 6

ITR information technology research

ITU International TelecommunicationUnion

laser light amplification by stimulated emission of radiation

LSN large scale networking program

MIMO multiple input, multiple output

NIAP National Information Assurance Partnership

NIST National Institute of Standards and Technology

NSF National Science Foundation

NTIS National Technical Information Service

OECD Organisation for Economic Co-operation and Development

OTP Office of Technology Policy

QoS quality of service

R&D research and development

radar radio detection and ranging

RBOC Regional Bell Operating Company

RTD research and technological development

SHRCWC Shanghai Research Centre for Wireless Communications

sonar sound navigation and ranging

SME subject matter expert

TA Technology Administration (US Department of Commerce)

TDD time division duplex

TIA Telecommunications Industry Association

TTA Telecommunications TechnologyAssociation

TTC Telecommunications TechnologyCommittee

U.S. United States Alliance for A-TEAM Technology and Engineering for

Automotive Manufacturing

UMTS universal mobile telephone service

USCAR United States Council for Automotive Research

UTRA universal terrestrial radio access

WCDMA wideband code division multipleaccess

Bell Labs was abrain trust, a

national treasure,wherein the nation’s

best and brightesttelecommunicationsresearch scientists

explored newavenues of

communicationstechnology.

January 2006 • Volume 4, Number 1 3

in deploying telephony networks. The morecapital equipment the Bell System purchased andcommissioned into service, the more profit theBell System would realize because the capitalexpenditures were added to the rate base,thereby increasing the volume of the return. Theformula worked quite well, and the telephonenetwork expanded rapidly and uniformly acrossthe nation, in cities and rural areassimultaneously. The US led the world intelecommunications technology as well as inpercentage of population penetration.

It was recognized in 1925 that the development oftelecommunications network technology andequipment relied heavily on scientific researchand development [3]. Therefore, AT&Testablished Bell Laboratories for the sole purposeof advancing telecommunications industrytechnologies. Bell Labs was funded with a fewcents from every telephone bill. This centrallyfunded research and development (R&D)organization was stable with regard to the sourceof funding and secure in the sense that thescientists could depend on being employed byBell Labs for life. Bell Labs became a magnet forthe world’s leading scientists and engineers.

The network’s continued success andmodernization were ensured by thetelecommunications innovations that emanatedfrom Bell Labs. Bell Labs was a brain trust, anational treasure, wherein the nation’s best andbrightest telecommunications research scientistsexplored new avenues of communicationstechnology. Bell Labs was intentionally isolatedfrom the pressure for economic performance, tostimulate the creativity of the scientists. As aresult, Bell Labs produced an impressive list ofinnovative products and discoveries:

• Transistor—1947—Revolutionized not onlytelephony, but the world

• Laser (light amplification by stimulatedemission of radiation)—1958—Enabled newmultibillion-dollar industries within thefields of medicine, communications, andconsumer electronics

• Optical communications—Advances inphotonics extending from enhancement ofoptical lasers to significant improvements inthe quality and capacity of optical fiber

• Data networking—1925: first fax; late 1940s:first remote operation of a computer—Continued significant developments in datatransport technology

• Digital transmission and switching—1962—First digitally multiplexedtransmission of voice signals

• Cellular telephone technology—1945:cellular telephone concept developed;January 1979: Bell System Technical Journalissued commercial design guidelines foradvanced mobile phone service (AMPS)

• Communications satellites—1962—Firstorbiting communications satellite, Telestar I,built and successfully launched by Bell Labs

• Digital signal processors (DSPs)—1979—First single-chip digital signal processor builtby Bell Labs, launching numerousmultibillion-dollar businesses. DSPs are usedin almost every aspect of our current dailylives, from compact disk (CD) players to carsand trucks.

• Touch tone telephone—1963—Replacementfor the rotary dial telephone

• UNIX® Operating System and CLanguage—1969 and 1972, respectively—Made large-scale networking of diversecomputing systems and the Internet practical

In addition, the following technologies wereeither developed or significantly enhanced at Bell Labs:

• Radar (radio detection and ranging)—Invented in Europe but further developedand miniaturized for use in aircraft during WWII

• Sonar (sound navigation and ranging)—Enhanced by Bell Labs with datacompression and noise analysis techniques

• Hearing aid—Made possible in its modernversion by use of the junction transistorinvented at Bell Labs

• Artificial larynx—Invented by Bell Labs in 1928

• Talking movies

• Two-way television

Bell Labs has produced some of the world’sleading scientists in several disciplines andwhose global recognition is evidenced by:

• 6 Nobel Prizes in physics shared by11 scientists

• 9 US Medals of Science

• 7 US Medals of Technology

• 1 Draper Prize

• 6 Marconi International Fellowship Awards


Centralized federalfunding of focused

telecommunicationsresearch has not

been supported bythe federal budget

in past years.

• 7 Computer & Communication Prizes sharedby 12 scientists and engineers

• 28 IEEE Medal of Honor winners

• 28,000 patents since 1925

DivestitureOn January 1, 1884, it was deemed necessary bythe US government, via a decision by FederalJudge Harold Greene, that the Bell System shouldbe split apart and its monopolistic hold on thetelecommunications industry be released. Theresulting seven Regional Bell OperatingCompanies (RBOCs) had specific marketmandates for service, competition, andprofitability, but the role of the centralized BellLabs was uncertain. Bell Labs was notaccustomed to the market pressures focused onquarterly returns and profitability rather thanproviding a platform for visionaries developingproducts for the future use of society. The RBOCsattempted to maintain the brain trust at Bell Labsby establishing a governing board that wouldjointly direct the efforts of the scientists, buteventually each RBOC had different businessobjectives for the common pool of scientists. Aftermuch effort and wrangling, each RBOCwithdrew its proportional funding of Bell Labsand the scientists were integrated into the variousRBOCs to establish decentralized, RBOC-focusedlaboratory facilities. This disaggregation of thebrain trust marked the end of Bell Labs as weknew it. The core teams were required to morphinto several smaller profit-generating machines.

THE SITUATION TODAY

Today the US depends on corporate budgetallocations as its primary source of funding

for telecommunications R&D. The problem isthat the US financial market demands near-termperformance to the detriment of long-term

research; therefore, the corporate funds that arereluctantly allocated to the R&D equation areheavily weighted to the development (zero to 18 months) versus the research (24 to 60 months)portion of the equation. Although this formulamay be effective to produce short-termperformance, when the momentum of theprevious research efforts reaches a zero state, theresults will unfortunately be very detrimental tothe US economy.

We are at a point in history when the US is nolonger driving the technology and setting thedirection of telecommunications. If the trendcontinues and no immediate action is taken, the US will simply remain a passenger on the bus of technology, dependent on others (notably,Europe and the Asia Pacific countries) to take usto destinations that may not be in the nation’sbest interest. The most significant recent telecom-munications advancements have not originatedfrom US R&D laboratories, but rather fromexcellent, state-of-the-art laboratories abroad. Nolonger are US laboratories the primary magnetfor exceptional scientific talent; in their place, thesecure, coordinated R&D focus of technicalcommunities abroad is attracting the top talent.

Federal Funding in the USCentralized federal funding of focusedtelecommunications research has not beensupported by the federal budget in past years. Ascan be seen in Table 1, the US governmentbudgeted approximately $2 billion for net-working and information technology R&D forfiscal years 2004 and 2005 [4]. The funding wasspread across several agencies, so the focus of thefunded research was immediately diluted. The $2 billion represents less than 1.6 percent of the$132 billion actually requested for R&D by allagencies for 2005. Further, only $330 million ofthe $2 billion was requested for the large-scale

Table 1. Agency Detail of Selected Interagency R&D Efforts ($ million) [6]

Networking and InformationTechnology R&D

2003 Actual 2004 Estimate 2005 ProposedDollar Change:2004 to 2005

Percent Change:2004 to 2005

National Science Foundation 743 754 761 7 1

Health and Human Services 376 368 371 3 1

Energy 308 344 354 10 3

National Aeronautics and SpaceAdministration

213 275 259 –16 –6

Defense 296 252 226 –26 –10

Commerce 26 26 33 7 27

Environmental Protection Agency 2 4 4 0 0

Total 1,964 2,023 2,008 -15 -1


It is clearly evidentthat the EU

recognizes thecriticality of

communications todeveloping and

sustaining societyand economic

stability.

networking (LSN) program, which includes both computing and noncomputing (telecom-munications) research. Less than half of the LSNallocation was dedicated to telecommunicationsR&D. Hence, less than one-tenth of total federalspending on networking and informationtechnology R&D was allocated for telecom-munications research [5].

Although the US has verbally recognized forseveral years the need (e.g., September 11, 2001;Hurricane Katrina) for reliable, secure,interoperable, feature-rich communications, littleor no focused strategic funding mechanism hasmaterialized. Other countries have madestrategically focused, concerted efforts to enhancetelecommunications R&D by providing centralrepositories to fund long-term initiatives.

The European Centrally Funded FrameworkModelThe European Union (EU) has established acoordinated multi-annual, multinational fundingmechanism, the Framework Programme, thathelps to organize and financially supportcooperation among universities, research centers,and industries—including small and medium-sized enterprises. Oversight of this program is theresponsibility of the EU’s Research DirectorateGeneral.

The Research Directorate General’s mission issummarized as follows:

• “to develop the European Union’s policy in the field of research and technologicaldevelopment and thereby contribute to the international competitiveness ofEuropean industry;

• to coordinate European research activitieswith those carried out at the level of theMember States;

• to support the Union’s policies in other fieldssuch as environment, health, energy, regionaldevelopment etc;

• to promote a better understanding of the roleof science in modern societies and stimulate apublic debate about research-related issues atEuropean level.” [7]

As shown in Table 2, the First FrameworkProgramme for research and technologicaldevelopment (RTD) was established in 1984 witha budget of €3.750 billion, while the current SixthFramework Programme covering the period2002–2006 has a budget of €17.5 billion.

For the upcoming Seventh Framework Program(FP7), entitled “ManuFUTURE Vision for 2020,”the period of funding has been extended from thenormal 4 years to 7. FP7’s concentrated focus overthis period will be to develop innovations thatwill integrate digital technologies into themanufacturing processes. Competitive advantagein the global market place is the objective. Of the€72,726 billion that will be allocated to FP7,€12,670 billion (17.4 percent) is focused onInformation Society research (see Table 3).

It is clearly evident that the EU recognizes thecriticality of communications to developing andsustaining society and economic stability. Theapparent lapse, on the other hand, in the US’srecognition that all manner of commerce andsociety rides on the back of communicationstechnology, has caused us to lose ourcommanding position in the telecommunicationsresearch community and will continue todiminish our global status if we do not establish astrategic and focused mechanism to encourageprimary research.

A partial listing of significant telecom-munications developments that have had theirgenesis in Europe or that have been heavilyinfluenced by the excellent technologists in theEU includes the following:

• GSM—In 1982, the Conference of EuropeanPosts and Telegraphs (CEPT) formed a studygroup called the Groupe Spécial Mobile(GSM) to study and develop a pan-Europeanpublic land mobile system that had to meetcertain criteria. In 1989, the responsibility fordeveloping what is now known as the globalsystem for mobile communications (whichretains the GSM acronym) was transferred to the European Telecommunications

Table 2. Budgets for the EU Framework Programmes [8]

Framework Programme

Time Frame

Funding(€ billion)

First 1984 to 1987 3.75

Second 1987 to 1991 5.36

Third 1990 to 1994 6.60

Fourth 1994 to 1998 13.12

Fifth 1998 to 2002 14.96

Sixth 2002 to 2006 17.50

Seventh 2007 to 2013 72,726

Standards Institute (ETSI). Phase I of theGSM specifications was published in 1990,and commercial service was started in 1991.Today, GSM accounts for more than 75 percent of the world’s mobile telephonemarket (1.2 billion subscribers) and is used inmore than 210 countries and territories. [10]

• GPRS—General packet radio service is aGSM-based mobile data service widely usedthroughout the world. It is commonlyreferred to as 2.5G technology and provides a transmission data speed of 171 kbps (54 kbps effective).

• 3GPP™—Third Generation PartnershipProject is a collaboration agreement that wasestablished in December 1998. The agreementbrings together a number of globally locatedtelecommunications standards bodies. These“Organizational Partners” are theAssociation of Radio Industries andBusinesses (ARIB), China CommunicationsStandards Association (CCSA), ETSI,Alliance for Telecommunications IndustrySolutions (ATIS), TelecommunicationsTechnology Association (TTA), and Telecom-munications Technology Committee (TTC).

The original scope of 3GPP was to produceglobally applicable technical specificationsand technical reports for a 3G mobile systembased on evolved GSM core networks andthe radio access technologies that theysupport (i.e., universal terrestrial radio access[UTRA], both frequency division duplex[FDD] and time division duplex [TDD]modes). The scope was subsequentlyamended to include the maintenance anddevelopment of the GSM technicalspecifications and technical reports,including evolved radio access technologies(e.g., GPRS and enhanced data rates for GSM evolution [EDGE]).

• UMTS—Universal mobile telephone service,commonly referred to as 3G mobiletechnology, is an advanced mobilecommunications technology that providesenhanced features and functions at a reducednetwork cost. UMTS was derived out of theInternational Mobile Telecommunications-2000 (IMT-2000) and 3GPP effort at theInternational Telecommunication Union(ITU). At the end of 2004, there were in excessof 16 million 3G UMTS subscribers in


Table 3. Budget Breakdown for the Seventh Framework Programme 2007–2013 [9]

Category Element Amount (€ million)

% Category

% Total

Cooperation

Health 8,317 18.72

Food, Agriculture, and Biotechnology 2,455 5.53

Information and Communications Technologies 12,670 28.52

Nanosciences, Nanotechnologies, Materials andnew Production Technologies 4,832 10.88

Energy 2,931 6.60

Environment (including Climate Change) 2,535 5.71

Transport (including Aeronautics) 5,940 13.37

Socio-economic Sciences and the Humanities 792 1.78

Security and Space 3,960 8.91

Total Cooperation 44,432 61.10

Ideas European Research Council 11,862 100 16.31

People Marie Curie Actions 7,129 100 9.80

Capacities

Research Infrastructures 3,961 53

Research for the Benefit of SMEs 1,901 25

Regions of Knowledge 158 2

Research Potential 554 7

Science in Society 554 7

Activities of International Cooperation 358 5

Total Capacities 7,486 100 10.29

Nonnuclear Actions of the Joint Research Center 1,817 2.50

Total Budget 72,726


But in 2004, the top 10 list,

which included fourUS companies, five Japanese

companies, and one South Korean

company, was marked by the absence of

a domestictelecommunications

company.

60 networks in 25 countries. The growth rate of UMTS is exceeding the GSM growth curves at the comparable stage ofmarket introduction.

• HSDPA—High speed downlink packetaccess is a packet-based data service in thewideband code division multiple access(WCDMA) downlink with data transmissionspeeds up to 14 Mbps (20 Mbps with multipleinput, multiple output [MIMO]) over 5 MHzchannel bandwidth. HSDPA enhances thedata transmission capabilities of UMTS andis expected to be widely deployed in 2006.

• HSUPA—High speed uplink packet access,like HSDPA, is an enhancement to the UMTScommunications systems and will providedata rates that will enable full-rate videoconferencing from mobile telephones. It is apart of the 3GPP Release 6 standards.

• IPv6—Internet Protocol Version 6 is the next-generation IP designed by the InternetEngineering Task Force (IETF) to replaceIPv4. IPv6 will enhance IP features andfunctionality, will provide a tremendousincrease in the number of availableaddresses, and will allow network auto-configuration and quality-of-service(QoS) levels.

PatentsA statistical indication highlighting the erosion ofthought leadership in the US is the number ofpatents filed in the US Patent and TrademarkOffice by US companies versus foreigncompanies. In the past, the statistics were heavilyweighted toward US companies, includingtelecommunications companies like AT&T andMotorola (see Table 4). But in 2004, the top 10 list,which included four US companies, five Japanesecompanies, and one South Korean company, was marked by the absence of a domestictelecommunications company.

China’s Long-Term, Strategic PlanningChina is emerging as a significant world power inthe realm of telecommunications R&D. Like theEU, China has a coordinated, long-term, strategicplan with specific, focused objectives that are centrally funded. According to recentOrganisation for Economic Co-operation andDevelopment (OECD) reports, China’s total R&Dinvestments lag only those of the US and Japan inabsolute terms.

The Proposal of the Central Committee of theChinese Communist Party for Formulating the10th Five-Year Plan (2001–2005) for National

Economic and Social Development, examinedand approved by the Fifth Plenary Session of the15th Central Committee of the Communist Partyof China, states, “We shall support, encourageand guide the healthy development of privateenterprises, especially small and medium-sizedtechnology-intensive companies.” The proposalalso says, “The means of planning, finance andbanking must be used comprehensively. Theleverage functions of price, tax, interest rates andexchange rates must be allowed to play their part,so as to guide and promote economicrestructuring and safeguard the stable growth ofthe economy.” Additionally, the proposal notedthat promoting the application of informationtechnology is a trend in current world economicand social development and the key link toupgrading China’s industrial structure andrealizing industrialization and modernization.

China’s Ministry of Information Industry hasstarted compiling the 11th Five-Year Plan for2006–2010. According to the ministry, thefollowing projects are to be included in the plan:“information technology development in the nextfive to 10 years, domestic and overseas market,third generation mobile communications, digitaltelevision, new display devices, software and

Table 4. Top Patenting Corporations [11, 12]

Company Patents

1977–1996

General Electric Corp. 16,206

International Business Machines Corp. 15,205

Hitachi Ltd. 14,500

Canon Kabushiki Kaisha 13,797

Toshiba Corp. 13,413

Mitsubishi Denki Kabushiki Kaisha 10,192

U.S. Philips Corp. 9,943

Eastman Kodak Co. 9,729

AT&T Corp. 9,380

Motorola Inc. 9,143

2004

International Business Machines Corp. 3,248

Matsushita Electric Industrial Co. Ltd. 1,934

Canon Kabushiki Kaisha 1,805

Hewlett-Packard Development Company, L.P. 1,775

Micron Technology, Inc. 1,760

Samsung Electronics Co. Ltd. 1,604

Intel Corporation 1,601

Hitachi Ltd. 1,514

Toshiba Corporation 1,310

Sony Corporation 1,305


China is solidlyfocused on the

long-term benefitsof investing in

telecommunicationsR&D.

integrated circuits, automotive electronics,communications laws and regulations, study oftelecom competitiveness, telecom networkinteroperation, value-added service and otherissues concerning the telecom trade, network and information security system, governmentonline, e-commerce application and regionalinformation industry development.” If sustained,this intense focus on telecommunications and information technologies will definitelypropel China into a leadership position in theglobal telecommunications marketplace in thenear term.

Evidence of the strength of China’s strategy is seenin Ericsson’s recent press announcement by CEOCarl-Henric Svanberg announcing that, in astrategic move in the field of R&D cooperation,Ericsson has signed the first research agreementwith Shanghai Research Centre for WirelessCommunications (SHRCWC). Under theagreement, Ericsson will collaborate withSHRCWC in undertaking research projects onfuture telecommunications technology such as“Super 3G” and 4G. The agreement furtherdemonstrates Ericsson’s commitment to drive thedevelopment of the telecoms industry in China.During the Summit, Mats H. Olsson, president ofEricsson Greater China, confirmed Ericsson’slong-term commitment to the country,announcing a US$1 billion investment for the next5 years [13].

China is solidly focused on the long-term benefitsof investing in telecommunications R&D. China isnot hindered by the unbalanced competitivemarket pressures that unreasonably emphasizenear-term performance to the detriment of long-term investment. In forward-focused societies,enterprises are severely penalized if they do nothave a long-term perspective to balance short-termperformance objectives.

SUGGESTED PATH FORWARD

The Telecommunications Industry Association(TIA), in coordination with its Chief

Technology Officer (CTO) Council, has carefullyconsidered several potential options to resolve theUS’s R&D crisis and to re-establish the leadershipposition of the US telecommunications R&Dcommunity. The TIA has distilled the numerousoptions down to the following four mechanisms toaddress the funding problem and then hassuggested five critical areas that require technicalfocus [14].

Potential Funding Mechanisms1. Establish a National Technology Council,

whose charter would be to define and guidestrategic areas in communications thatrequire further research critical to the futuregrowth of the US economy. Such a Councilshould include representation from differentsectors, such as government, academia, andindustry.

a. To use scarce financial resources effectively,representatives from government, aca-demia, and industry should be sought toestablish long-term priorities. Additionalresearch would help identify thetechnologies likely to be most relevant toUS economic growth and competitiveness.

b. This Council should be modeled after theEU’s Sixth Framework Programmeinitiative, wherein the Council receivesproposals from industry consortiaregarding specific areas of focused researchand development and has availablesubstantial funding from the government tohelp fund those proposals.

c. This Council should also borrow from theUnited States Alliance for Technology andEngineering for Automotive Manufacturing(U.S. A-TEAM), a partnership createdbetween the US Department of Commerce’sTechnology Administration (TA) (consisting of the Office of TechnologyPolicy [OTP], the National Institute ofStandards and Technology [NIST], and theNational Technical Information Service[NTIS]) and the United States Council forAutomotive Research (USCAR). The U.S. A-TEAM brings together engineers fromgovernment and industry bodies that areparties to the agreement to facilitatetechnological research and technologypolicy analysis focused on improving themanufacturing competitiveness of the USautomotive industry.

d. The Council, in cooperation with industry,would determine the priority of the specificresearch initiatives of national concern.

2. Prioritize communications research fundingwithin Department of Defense (DoD) 6.1Basic Research Programs.

a. In the 1990s, the DoD and the DefenseAdvanced Research Projects Agency(DARPA) began to rely heavily on dual useand industry research funding. Thus, DoD funding became unavailable for


commercially available technologies. As aresult, DoD restricted its research fundingto military-unique needs, which wasacceptable at the time because private-sector-led research was driving high endresearch.

b. With the communications downturn, how-ever, the commercial sector has ceased to bethe major driver of high-end, long-termresearch. As a result, DoD and DARPAneed to increase their focus on andinvestment in dual-use technologies.

3. Prioritize communications research fundingwithin the National Institute of Standardsand Technology.

a. Miniaturization of electronic components incommunications devices continues,resulting in faster, more powerful, morereliable products. Yet, the continuedshrinking of component parts at thenanoscale is hindered by metrology andmanufacturing challenges. NIST programsaddress some of these key issues andshould be adequately funded.

b. Additionally, the TIA supports thecontinuation of the National InformationAssurance Partnership (NIAP), a collab-oration between NIST and the NationalSecurity Agency. The long-term goal of NIAP is to help increase the level of trustconsumers have in their informationsystems and networks through the use ofcost-effective security testing, evaluation,and validation programs.

4. Prioritize communications research fundingwithin National Science Foundation (NSF)research programs.

a. Federal funding for physical sciencesresearch, the foundation of our nation’seconomic competitiveness, hasdramatically decreased. Technologicaladvances driving the economy require thereversal of this trend.

b. The National Science Foundation Autho-rization Act of 2002 called for doubling theNSF budget over 6 years; fulfillment of thatgoal is lagging.

c. In conjunction with increasing NSF’sbudget, the TIA advocates for the creationof an NSF communications technologyresearch (CTR) program similar to theinformation technology research (ITR)program that recently concluded. Such aprogram would greatly benefit the

communications sector by creatingopportunities at the frontiers ofcommunications research and education.

Recommended Areas of Research FocusThe following TIA-recommended areas of focusedresearch closely parallel the technology researchbudget recommendations by the Office of thePresident of the United States [15]:

1. Universal Broadband—Affordable broad-band access and connectivity, using allavailable media (copper, coax, fiber,spectrum, etc.), carrying all services (voice,data, video) to all customers everywhere(urban, suburban, rural, mobile) to enable agreatly upgraded “superhighway.”

a. Broadband Internet access is critical tosupport technology convergence andadvanced communications. A forward-looking US government should supportuniversal access for broadband Internet, aswell as policies that promote widespreadconnectivity. Infrastructure upgrades createincreasing returns to our economy andencourage the development of busi-nesses, entertainment, education, and e-government solutions and capabilities.

b. Additional federally funded research in thisfield is needed, particularly because specialtechnologies will be needed for rural access,and corporate and venture capital financingfor research has dropped significantly overthe last several years. Extremely significantcost reductions are necessary to meet thetechnology needs of rural areas.Additionally, the provision of broadbandaccess in rural areas is costly due tochallenges associated with terrain, lowpopulation density, etc.

2. Security—New authentication, encryption,and monitoring capabilities for all publicbroadband networks to protect communi-cations assets from attack.

a. The US is a post-industrial informationsociety and, as such, has a cyber-infrastructure that is vulnerable to attack.

b. Continued research is needed to preventsystemic attacks to infrastructure and mayprovide an opportunity for university-based “centers of excellence.”

3. Interoperable Mobility—The ability to accesscommercial mobile services and emergencyservices over any mobile network from anymobile instrument.


It is time for the USto stop eating theseed corn and tobegin to plant for

future harvest.

a. Interoperable mobility enables public safetyand law enforcement officials to use thevarious public safety and cellular mobilenetworks while avoiding the necessity ofcarrying multiple mobile devices. It alsopromotes coordinated communicationsamong various public service agencies andallows higher priority use of scarcespectrum resources for emergency use.

b. Federally funded research is necessarybecause the emergency services market iscritical for the common good. Also, bringingcommercial technologies and emergencyservices technologies closer together willresult in lower costs and more advancedfeatures for critical emergency services.

4. Communications Research for HomelandSecurity, including interoperability, security,survivability, and encryption.

a. Homeland Security is a superset of severalother visions. Security technologies canhelp protect public networks and otherpublic infrastructure from maliciousattacks. A large amount of economicactivity today depends on the continuedavailability of public broadband networksand infrastructure. Successful attacks canslow national economic activitysignificantly and can have other disastrousconsequences (e.g., in case of identity theft).

b. Research is needed in all areas(interoperability, security, survivability,and encryption) because the needs of firstresponders and critical infrastructureprotection far exceed the needs of “typical”commercial applications. Further researchis also needed because new worms andviruses are constantly being invented, andnew techniques are needed to preventattacks before there is significant resultingdamage.

c. The country needs a broad program toaddress our vulnerabilities and ensure theintegrity of first responder systems. Thegovernment should support these “extremecase” applications, since they are unlikely tobe sufficiently developed in normalcommercial systems.

5. Nanotechnology

a. Many communications advances have beendriven by fundamental scientificdiscoveries of nanoscale materials.

b. Examples of important research areasinclude sensors, displays, power systems,radio frequency, and nanomicrophones.

c. Advances will reduce cost; increase mobility; decrease power consumption; andimprove healthcare, homeland security,and public safety.

CONCLUSIONS

The US has recently experienced the migrationof manufacturing services to regions of the

world where total costs of manufacturing aremuch reduced. Now the US is witnessing a similar migration of the communications research initiatives to Europe and Asia Pacific. Pre-competitive communications research ismigrating to laboratories abroad for three basic reasons:

• The total cost of conducting communicationsresearch is lower in emerging countries.

• The financial pressure on public companies to generate near-term profit severely limits the allocation of budget to pre-competitive research.

• There is a lack of stable, centrally funded andfocused communications research programsin the US.

The US began to lose its research leadershipposition in communications with the dissolutionof Bell Laboratories in the mid-1980s when marketforces and the pressure on delivery of short-termearnings replaced “cost-of-service” rate-basedmonopolies. The erosion of the research leadershipposition has continued and will continue at anaccelerated rate unless immediate and decisiveaction is taken. In the near to medium term, the USwill likely remain the dominant consumer ofexternally developed and manufactured goods,but the voice of the US will dwindle to a whisperregarding its influence on the productdevelopment and evolution. The US must chooseits areas of emphasis and importance. Consumer-focused product development and manufacture isan area unlikely to hold promise for revitalizationin the US. The US should, however, considerfunding pre-competitive research on commu-nications products and technologies that involvenational security (including cyber security),personal health, and safety and disaster recovery.The suggestions offered by the TIA CTO Counciladdress exactly those issues.

The aforementioned suggestions are offered forserious consideration and discussion . . . andaction. It is time for the US to stop eating the seedcorn and to begin to plant for future harvest. �


TRADEMARKS

3GPP is a trademark of the EuropeanTelecommunications Standards Institute (ETSI) inFrance and other jurisdictions.

UNIX is a registered trademark of The OpenGroup.

REFERENCES

[1] A Brief History: The Bell System(http://www.att.com/history/history3. html).

[2] Communications Act of 1934, Title 1, Section 1(http://www.fcc.gov/reports/1934new.pdf).

[3] Capsule History of the Bell System(http://www.bellsystemmemorial.com/capsule_bell_system.html).

[4] Office of Science and Technology Policy,Executive Office of the President, 2005 BudgetSummary (http://www.ostp.gov/html/budget/2005/ap05.pdf).

[5] National Coordination Office for Networking and Information Technology Research andDevelopment (http://www.itrd.gov/pubs/2006supplement/2006supplement.pdf).

[6] Office of Science and Technology Policy,Executive Office of the President, 2005 BudgetSummary, page 62 (http://www.ostp.gov/html/budget/2005/ap05.pdf).

[7] European Commission, Research Directorate-General (http://europa.eu.int/comm/dgs/research/index_en.html).

[8] European Commission Research Presentation,Towards the Sixth Framework Programme, page 9 of 62 (http://www.lnl.infn.it/pages/Conferenze/Ferrini.pdf).

[9] Budget Breakdown for Seventh FrameworkProgramme (http://www.cordis.lu/fp7/breakdown.htm).

[10] GSM World; GSM Facts and Figures(http://www.gsmworld.com/news/statistics/index.shtml).

[11] Top Patenting Corporations(http://www.nsf.gov/sbe/srs/seind04/c6/tt0603.htm).

[12] US Patent and Trademark Office Calendar Year2004, Preliminary List of Top PatentingOrganizations (http://www.uspto.gov/web/offices/ac/ido/oeip/taf/top04cos.htm).

[13] Ericsson Press Release, September 7, 2005(http://www.3g.co.uk/PR/Sept2005/1867.htm).

[14] Telecommunications Industry Association WhitePaper, Investing in Communications for Tomorrow’sInnovations: The Case for Increased CommunicationsResearch Funding, September 2005.

[15] Executive Office of the President – Office ofScience and Technology Policy, Memorandum forthe Heads of Executive Departments and Agencies, re:FY 2007 Administration Research and DevelopmentBudget Priorities, July 8, 2005.

ADDITIONAL READING

Additional information sources used todevelop this paper include:

• The Task Force on The Future of AmericanInnovation, The Knowledge Economy: Is the UnitedStates Losing Its Competitive Edge?, February 16, 2005.

• N. Gehani, Bell Labs, Life in the Crown Jewel, Silicon Press, 2003, ISBN 0 929306-27-9.

BIOGRAPHIESTim Statton is president ofBechtel Telecommunicationsand an executive vice presidentand member of the Board ofDirectors of Bechtel Group, Inc.Before his present appointment,Tim was president of BechtelEnterprises(BEn),the company’sproject development, financing,and ownership subsidiary.

Before joining BEn, Tim was a member of theChairman’s Leadership Council and president ofBechtel Energy, leading all energy-related engineeringand construction activities within the company. Beforethis, he was managing director for energy and waterdevelopment at BEn. He also served as manager of AsiaPacific operations, representing Bechtel’s engineeringand construction interests in the region. During hisextensive career in the power sector of the company,Tim held numerous field and home office positions, wasa project manager and business development manager,and was eventually elevated to president of Bechtel’spower and industrial company.

Tim has been a member of the Board of the UnitedStates Energy Association (USEA), the leading energytrade organization in the US, and has volunteered fornumerous charitable and community activities. He alsoserved as chairman of the Board of Nexant, an energyconsulting business, and was a member of the Board of Control of InterGen, an independent powerdevelopment company. He currently sits on the UnitedStates Telecommunications Training Institute (USTTI)Board of Directors.

Tim received his BS in Mechanical Engineering fromSan Francisco State University, California, and his BS in Business/Economics from Juniata College,Pennsylvania. He is a member of the AmericanCogeneration Society and the U.S. Energy Association.

Jake MacLeod is the chieftechnology officer, Engineeringand Technology, for BechtelTelecommunications and aBechtel principal vice president.

Jake joined Bechtel in May 2000 and is responsible for expanding the scope ofBechtel’s telecommunicationsengineering services to include

all aspects of technical design, from network planningto commercial system optimization. Jake initiated anddeveloped Bechtel’s RF and Network Planning team, which has grown to over 150 world-class


engineers. He also designed and established Bechtel’stwo world-class telecommunications laboratories toprovide clients with applied research and developmentservices ranging from interoperability testing toproduct characterization.

Jake was the first Bechtel Telecommunications person toenter Baghdad in 2003, immediately after the conflictpaused. He and his teams assessed the Iraqitelecommunications network, then designed andreplaced 12 wire centers (equivalent to 240,000 POTSlines) in a period of 4 months, an unprecedentedachievement in telephony. Jake and his teams alsoanalyzed and replaced the air traffic control system atBaghdad International Airport. Under Jake’s purview,Bechtel’s technology teams are currently in the finalstages of developing the Virtual Survey Tool, anautomated network planning tool that has the potentialto radically change the conventional methods ofnetwork design. Jake’s laboratories are currentlyworking with two global wireless equipmentmanufacturers to analyze and characterize UMTS,HSDPA, Node B hotels, WiMAX, and intuitivenetworks. Under Jake’s direction, the laboratoriesannually produce two issues of the BechtelTelecommunications Technical Journal, an authoritativetechnical publication focused on operational matters.The laboratories also host semi-annual globaltechnology debates focused on the pros and cons of the most advanced telecommunications technologies.Jake, himself, provides an average of six to eightkeynote and technology-based presentations per year atindustry conferences.

Jake started his career in the telecommunicationsindustry in 1978, beginning in transmission engineeringwith Southwestern Bell Telephone Company (SWBTC)in San Antonio, Texas. His responsibilities at SWBTCincluded design and implementation of radio systemsin Texas west of Ft. Worth. He participated in theoriginal cellular telephone system designs for SWBTCin San Antonio, Dallas, and Austin. After SWBTC, Jakebecame the second employee for PageNet/CellNet andvice president of Engineering for their cellular division.He designed over 135 cellular network systems,including San Francisco’s, and filed them with the FCC.In addition to his responsibilities at PageNet/CellNet,Jake was asked to chair the FCC’s OperationalRelationships Committee.

Jake has held executive management positions withNovAtel (Calgary), NovAtel (Atlanta), WesternCommunications, and West Central Cellular. Morerecently, Jake spent 9 years with Hughes NetworkSystems (HNS), where he was instrumental inestablishing their cellular division. He designed andestablished cellular and WLL systems in areas ranging from central Russia to Indonesia, as well as in 57 US markets.

Jake holds a BS degree from the University of Texas in Austin.


INTRODUCTION

In recent years, both the Internet and wirelesscommunications have experienced a surge of

activities and successes. The success of theInternet stems from two main facts: It providesuseful applications such as e-mail, World WideWeb (www), and instant messaging (IM), and ituses readily available open protocols thatpromote and facilitate the development ofvarious services and applications. The success ofmobile communications is self-evident. Thistechnology has experienced explosive growthand provides wide coverage, touching almostevery aspect of peoples’ lives. Currently, there aremore than 1 billion mobile customers, and thisnumber is growing!

Wireless systems dubbed as beyond third-generation (3G+) seek to merge these twosuccessful communications modes to provide thepower of the Internet—and all of its services andapplications—with the convenience of ubiquitouswireless access. The new architecture expected toaccomplish this, the Internet Protocol (IP)multimedia subsystem (IMS), is defined in astandard created by the Third GenerationPartnership Project (3GPP™), one of the mainstandards organizations for 3G networks.

Different aspects of the IMS (call control,charging, roaming, etc.) have been formalizedand published in various 3GPP technicalspecifications [1–15]. The first version of the IMSwas published by the 3GPP in Release 5 of the

universal mobile telecommunications system(UMTS). Release 6 of the UMTS, introduced inMarch 2005, provided some enhancements to the first release and introduced new concepts,such as support for access independence, wirelesslocal area network (WLAN) integration, and IM and presence services. Release 7, underdevelopment with an expected release in mid-2007, will focus primarily on fixed and mobileconvergence issues.

The Third Generation Partnership Project 2(3GPP2), the main standardization body for 3Gnetworks based on cdma2000®, has establishedthe multimedia domain (MMD), its own versionof the IMS specification. The IMS and the MMDare very similar, with the main differencescentering on two issues:

IP MULTIMEDIA SUBSYSTEMS (IMS):A STANDARDIZED APPROACH TO ALL-IP CONVERGED NETWORKS

Abstract—The IMS is a standardized approach to offering Internet services anywhere at any time usingcellular technology. The first release of the IMS is tailored for GPRS/UMTS, whereas later releases will allowaccess independence, including WLANs and even fixed networks such as xDSL and cable modem.

The IMS is based on open interfaces and common elements that make it possible to provide integrated, secure,IP-based, multimedia, multisession applications to mobile and fixed users, with guaranteed end-to-end QoS androaming capabilities, while providing the same services as the home network, and across different accesstechnologies. The IMS also allows a single sign-on authentication and provides a flexible architecture forcharging and billing. Deployment of the first version of IMS products is expected in early 2006.


Rasoul Safavian, PhD [email protected]

The 3GPP, formed in December 1998, is acollaboration of telecommunications industryorganizations to produce globally applicabletechnical specifications for 3G mobilesystems and to maintain and develop GSMstandards. Participants include major regionaltelecommunications standards bodies suchas ARIB of Japan, CSCA in China, CommitteeT1 of the US, and TTA of Korea, as well asmarket representatives such as the UMTSforum, 3G America, GSM associations, andthe IPv6 forum.



2G second generation, the original digital mobile phone service

2.5G “second-and-a-half” generation, a marketing designation for an intermediate level of digital mobile phone service responding to demands for greater bandwidth

3G third generation, enhanced digital mobile phone service at broadband speeds enabling both voice and nonvoice data transfer

3G+ beyond 3G, envisioned as all-digital, entirely packet-switched radio networks involving hybrid networking and access technologiesthat globally integrate services and technology while providing the multiple QoS of an ATM network and the flexibility of an IP network

AAA authentication, authorization, and accounting

ACA accounting answerACR accounting requestAoR address of recordAPI application program interfaceAS application serverATM asynchronous transfer modeAUC authentication centerAUTN authentication tokenAV authentication vectorBGCF breakout gateway control functionBICC bearer independent call controlCAMEL customized application for mobile

network enhanced logicCAP CAMEL application partcapex capital expenseCCA credit control answerCCC credit control clientCCF charging collection functionCCR credit control requestCDF charging data functioncdma2000® A family of standards, developed

through comprehensive proposals from Qualcomm, describing the use of code division multiple access technology to meet 3G requirements for wireless communication systems

CDR charging data recordCGF charging gateway functionCk cipher key

COPS common open policy serviceCS circuit switchedCSCF call SCFCSEQ context sequence (number)DEC decisionDiameter A protocol that provides a

framework for any services requiring AAA/policy support across many networks and that primarily supports mobile IP, accounting, network access, and strong security

DiffServ differentiated servicesDNS domain name systemDSCP differentiated services code pointECF event charging functionFTP file transfer protocolGGSN gateway GPRS support nodeGPRS general packet radio serviceGSM/gsm global system for mobile

communicationGTP GPRS tunneling protocolHLR home location registerHSS home subscriber serverHTTP hypertext transport protocolI-CSCF interrogating CSCFIk integrity keyIKE Internet exchange keyIM instant messagingIMS IP multimedia subsystemIM-SSF IP multimedia service switching

functionIntServ integrated servicesIP Internet ProtocolIP-CAN IP connectivity access networkIPSec IP securityIPv4 Internet Protocol version 4IPv6 Internet Protocol version 6ISC IMS service control (interface)ISDN integrated services digital networkISIM IMS subscriber identity moduleISUP ISDN user partIWF interworking functionMAA multimedia authentication answerMAP mobile application partMAR multimedia authentication requestMegaco media gateway control


MGCF media gateway control functionMGW media gatewayMIME multipurpose Internet mail extensionMMD multimedia domainMRF multimedia resource functionMRFC MRF controlMRFP MRF processorMSC mobile switching centerMTP message transfer partNAI network access identifierOCF online charging functionopex operating expenseOSA open service accessOTA over the airPCM pulse code modulationPCS personal communication systemP-CSCF proxy CSCFPDA personal digital assistantPDF policy decision function (same as

policy decision point [PDP])PDG packet data gatewayPDP packet data protocol; also: policy

decision point (same as PDF)PEP policy enforcement pointPHB per-hop behaviorPLMN public land mobile networkPS packet switchedPSTN public switched telephone networkPTT push to talkQoS quality of serviceRADIUS remote authentication dial-in user

serviceRAND random numberREQ requestRES responseRFC Request for CommentsRSVP resource reservation protocolRTCP real-time transport control protocolRTP real-time transport protocolRTSP real-time streaming protocolSA security associationSAA server assignment answerSAR server assignment requestSBLP service-based local policySCF session control functionSCP service control point

SCS service capability serverS-CSCF serving CSCFSCTP streaming control transmission

protocolSDP session description protocolSEG security gatewaySGSN serving GPRS support nodeSGW signaling gatewaySIM subscriber identity moduleSIP session initiation protocolSLF subscriber location functionSMS short message serviceSMTP simple mail transfer protocolSS7 Signaling System Number 7, a

common channel signaling system defined by the ITU and used to provide a suite of protocols that enables circuit and noncircuit-related information to be routed about and between telecommunications networks

SSP subscriber service profileTCP transmission control protocolTHIG topology hiding internetwork

gatewayTLS transport layer securityTUP telephone user partUA user agentUAA user authorization answerUAC UA clientUAR user authorization requestUAS UA serverUDP user datagram protocolUICC UMTS integrated circuit cardUMTS universal mobile

telecommunications systemURI uniform resource identifier URL uniform resource locatorUSIM UMTS subscriber identity moduleVoIP voice over IPWiMAX worldwide interoperability for

microwave accessWLAN wireless local area networkwww World Wide WebxDSL term used for all forms of

technology using a digital subscriber line

XRES expected response


The need to deploya new domain

may naturally bequestioned,

especially at a timewhen networkoperators are

struggling with costsof deploying 3G

networks and arealso facing reduced

voice revenues.

• Mobility. The 3GPP2’s MMD is built on top ofmobile IP, whereas the 3GPP’s IMS managesmobility through general packet radioservice (GPRS), which provides a Layer 2tunneling mechanism.

• IP versions supported. The MMD supportsboth IPv4 and IPv6, whereas the IMS initiallysupported only IPv6. However, due topressure from telecommunications operators,newer versions of the IMS also support bothIPv4 and IPv6.

This paper focuses on the 3GPP’s IMS. Thediscussion opens with an overview of IMS andthe advantages it holds for network operators.Since an IMS network is basically an advancedmobile session initiation protocol (SIP) network,the paper then explores SIP networks and theirmain features and functionalities. Next, IMS basicarchitecture, components, interfaces, etc., areexamined. The discussion then turns to IMSfunctionalities and operations issues, such as IMSprerequisites, registration, session setup,roaming, security, quality of service (QoS), and charging. Finally, key benefits offered by the IMS approach are summarized in theconcluding remarks.

OVERVIEW OF THE IMS AND WHY IT IS NEEDED

As defined by the 3GPP, the IMS is a newsubsystem that enables convergence of data,

speech, and mobile network technology over anIP-based infrastructure. It provides an integratedservice control platform that allows the creationof multimedia and multisession applicationsusing wireless (and recently wireline) transportcapabilities. It is a combination of new networkelements and interfaces, i.e., a new core networkdomain, which creates a new service deliveryenvironment.

The need to deploy a new domain may naturallybe questioned, especially at a time when networkoperators are struggling with costs of deploying3G networks and are also facing reduced voicerevenues. After all, second-generation (2G)terminals can act as modems to transmit IPpackets over a circuit, and 2G and 3G terminalscan use native packet-switched (PS) technology toperform data communications. To evaluate theneed to deploy the IMS, the following discussionexamines conventional network domains, theservices they can offer, and how they handlevarious new service offerings. The end users’experiences are also considered.

Shortcomings of Conventional Network DomainsThe circuit switched (CS) domain is used bytraditional CS networks, which offer simple user-to-user voice services or short-message services(SMSs). Even though voice services were, andperhaps to some degree will continue to be, amajor source of revenue for network operators,voice revenues have been dropping in the lastfew years, mainly due to increased competitionfor existing subscribers.

Operators have been looking for ways todifferentiate themselves by offering new andcreative data services. The PS domain of PSnetworks or basically 2.5 and 3G networks hashelped PS network operators to introduce user-to-server data services, where a user directlyaddresses a specific server to execute the servicein question; furthermore, these services takeadvantage of IP transport and provide “alwayson” connectivity. Unfortunately, PS networkshave not been very successful, perhaps due toinsufficient bandwidth, lack of enticingapplications, confusing charging schemes, longdelays in service offerings, etc. Furthermore,increasing the bandwidth alone may not provesufficient to enable the plethora of new anddesirable services that customers may demand.

ORGANIZATIONS MENTIONED IN THIS PAPER

3GPP™ Third Generation Partnership Project, formed in December 1998 as a collaboration agreement bringing together a number of telecommunication standards bodies to produce globally applicable technical specifications for 3G mobile systems and to maintain and develop GSM standards

3GPP2 A sister project to 3GPP that is a collaboration agreement dealing with North American and Asian interests regarding 3G mobile networks based on cdma2000

ARIB Association of Radio Industries and Businesses (Japan)

CSCA Consortium on Standards and Conformity Assessment (China)

IETF Internet Engineering Task Force

ITU International Telecommunications Union

TTA Telecommunications Technology Association (Korea)


Thus, a newplatform or domain

is needed withunified features;

common elements;and open,

standardizedinterfaces that can

be used by allexisting and futureapplications and

services. The IMS—the domain of

services—meets this need.

Also, network operators, in a rush to offer data services, deployed specialized isolated island solutions that often did not integrate wellwith the other services. These dedicated solutionsare typically proprietary and use dedicatedcomponents and interfaces that cannot be usedfor other applications. This is particularly truewhen the applications are provided by differentvendors. The island solutions also typically donot support roaming. While the island approach,also known as the vertical application platform,may be acceptable for deploying small-scale dataapplications or services, it neither scales well indeploying many diverse services or applications,

nor provides synergy among the applications.Furthermore, use of this traditional approachincreases not only capital expense (capex), butalso operating expense (opex), since eachapplication may have different operating andmaintenance requirements [16–18].

Thus, a new platform or domain is needed withunified features; common elements; and open,standardized interfaces that can be used by allexisting and future applications and services. TheIMS—the domain of services—meets this need.Figure 1 shows the position of the three domains(CS, PS, and IMS) in a mobile network [17].

Figure 1. CS, PS, and IMS Domains in a Mobile Network (After [16])


Desirable Characteristics of the IMSThe desirable features, requirements, andarchitecture for this new domain are delineated inthe 3GPP’s technical specification for IMS servicerequirements [1, 2].

The IMS domain supports the following keyrequirements:

• IP multimedia sessions, i.e., delivery ofmultimedia sessions over PS networks

• A mechanism to negotiate and enforce QoS

• Integration with Internet and CS networkssuch as public switched telephone networks(PSTNs) and existing cellular networks

• Full roaming capabilities

• Single sign-on and authentication

• Single converged billing

• Strong operator controls with respect toservices delivered to end users

• Rapid service creation without requiringstandardization

• Access independence, i.e., allows accesstechnologies other than GPRS and UMTS(e.g., WLANs and x-type digital subscriberline technology [xDSL])

The IMS also provides improved end-userexperience over that offered by the other twodomains. CS domain users can access only onetype of service per bearer or session. And whilemultiple, parallel IP sessions may be available toPS domain users, bandwidth, QoS, and chargingissues may hinder effectiveness. Implementationof the IMS infrastructure will enable three generaltypes of services to be offered:

• User-to-user

• Multiuser

• Server-to-user

The user-to-user services enabled by IMS extendbeyond today’s simple voice call or SMS textmessaging to include services such as voice overIP (VoIP), video telephony, chat sessions, andpush-to-talk (PTT). The IMS multiuser servicesinclude one-to-many and many-to-one servicessuch as multimedia conferencing, group chatsessions, and multiuser PTT services. Theseservices are enabled in the IMS via a dedicatedmedia server, the multimedia resource function(MRF), which is discussed in the section of thispaper on IMS architecture. Finally, deployingIMS enables operators to introduce manyinnovative server-to-user services by virtue of (1) its ability to locate users within the network,

i.e., its mobility management; (2) its signalingabilities allowing servers to act as user agents(UAs), i.e., initiating and receiving SIP messages;and (3) its ability to integrate SIP-based enablingservices such as IM and presence. Server-to-userservices include click-to-dial, dynamic pushservices, etc., and could be based on presence,status, geographic location, device type andcapabilities, media preference, etc. To fullyappreciate these capabilities, a more thoroughunderstanding of SIP and SIP networks is needed.Therefore, a discussion of SIP networks has beenprovided in the next section of this paper.

Using IMS, end users also have much morecontrol over the services than they do under theCS and PS domains. For instance, an end user caninitiate multiple services within a single session,also known as dynamic media control; add ormodify some components of a session (e.g., byadding a video component to an ongoing voicecall); or add or drop a user during an ongoingconference call or chat session. In summary, IMS,while not designed to create new services, offersall the services, current and future, that theInternet provides.

Protocols Used in the IMSIn any communications network, protocols usedfall into two basic categories: signaling andcontrol plane, and media or user plane. The IMSis built based on IP protocols, and signaling andcontrol protocols for session initiation and controlare based on the SIP (Request for Comments,[RFC] 3261 [19]) and session description protocol(SDP) (RFC 2327 [20]). To transport IMS signalingprotocols, the reliable streaming control trans-mission protocol (SCTP) or transmission controlprotocol (TCP) is used. Media plane protocolsused for media delivery are based on the real-time transport protocol/ real-time controlprotocol (RTP/RTCP) (RFC 3550 [21]) fortransporting real-time media such as audio orvideo. Near-real-time streaming media aretransported using the real-time streamingprotocol (RTSP). Both RTP/RTCP and RTSPtypically use the user datagram protocol (UDP) as the transport protocol to avoid TCP’s setup, teardown, and retransmission delays.Non-real-time media are delivered using hyper-text transport protocol (HTTP) (RFC 2616), simple mail transport protocol (SMTP), or file transfer protocol (FTP), with TCP as thetransport protocol.

IP protocols are developed mainly by the IETFand the ITU and are published by their respectiveorganizations as RFCs and ITU-T documents.

Other protocols used in the IMS include:

• CS call control

— Telephony user part (TUP) (ITU-T Q.721)

— Integrated services digital network(ISDN) signaling user part (ISUP) (ITU-TQ.761)

— Bearer independent call control (BICC)(ITU-T Q.1901). An evolution of ISUP,BICC completely separates the signalingplane from the media plane and can runover asynchronous transfer mode (ATM),Signaling System Number 7 (SS7), or IP.

• Multimedia session establishment andcontrol

— Packet-based multimedia communica-tions systems ITU-T H.323. Unlike BICC,this protocol is designed from scratch tosupport IP technologies in establishingmultimedia sessions.

— Media gateway control (Megaco) (ITU-TH.248). This protocol is used to controlthe IMS media gateway (MGW).

• Authentication, authorization, andaccounting (AAA)

— Diameter (RFC 3588 [22]). An evolutionof remote authentication dial-in userservice (RADIUS) (RFC 2866 [23]),Diameter:

– Is used by the network and the userto authenticate and authorize eachother

– Has a base protocol, complementedby so-called Diameter applicationsthat are customized extensions to thebase Diameter to suit a particularapplication in a given environment

– Interacts with SIP during sessionsetup in one application, performscredit control accounting in anotherapplication, etc.

• Policy and QoS control

— Common open policy service (COPS)(RFC 2748 [24]). This is a request/response protocol used between thepolicy server (the policy decisionfunction [PDF], also known as the policydecision point [PDP]) and the policyclient (the policy enforcement point[PEP]). COPS supports two modes:outsourcing and configurable orprovisioning. In the outsourcing mode,the PEP contacts the PDF each time a

policy decision is needed. The PDFmakes the decision and communicatesthis information to the PEP forenforcement. In the configurable orprovisioning mode, the PDF configuresthe PEP with the enforcement policy,which the PEP stores and uses forcurrent and future decisionmaking.

Since the main protocols used for sessioninitiation, description, control, modification, andtermination in IP networks are SIP and SDP, theIMS is considered basically an advanced mobileSIP network. The essentials of SIP/SDP and SIPnetworks are examined next.

SIP NETWORKS

Purpose of SIPSIP is a general-purpose application-layerprotocol designed to establish, modify, andterminate multimedia sessions in IP networks [25,26]. It also allows other participants to be invitedto ongoing sessions. The main goal of SIP is todeliver a session description to a user at the user’scurrent location. Once the user has been locatedand the initial session description delivered, SIPcan deliver new session descriptions to modifythe characteristics of the ongoing session or toterminate the session. In short, SIP supports thebasic aspects of the multimedia session: userlocation, user availability, user capabilities,session negotiation, and session management.

Session DescriptionsA session description contains enoughinformation for a remote user to be able toestablish, join, modify, or terminate a session. Asession description could include informationsuch as the IP addresses and port numbers towhich the media services need to be sent and thecoder-decoders (codecs) used to encode the voice,image, and video elements. SIP uses SDP, themost common format to describe a multimediasession. SDP has a text-based format and consistsof two basic parts: session-level information andmedia-level information.

Even though SIP uses SDP to transport thesession description, SIP is completelyindependent of the format of the objects ittransports. Objects that SIP transports could besession descriptions written in formats other thanSDP, or any other piece of information.

SIP is a two-way session description exchangeprotocol, also known as the offer/answer orrequest/response transaction model. Since SIP is


In short, SIPsupports the basic

aspects of themultimedia session:

user location, user availability,user capabilities,

session negotiation,and session

management.


SIP uses a SIP URI(RFC 3261) to

identify users. A SIPURI is similar to ane-mail address and

consists of the user’s name and the domain

name of the homenetwork operator.

based on HTTP, it is text-based, which makes SIPeasier to debug and to extend, but less efficient in terms of size and required time/bandwidth to transmit.

SIP TransactionsA SIP transaction consists of a request from aclient or user agent (UA), usually referred to as aUA client (UAC); zero or more provisionalresponses; and a final response from a server,usually referred to as a UA server (UAS). SIPmessages begin with a start line, a header field, and a message body [26].

Start LineThe start line is also called a request line in therequest message and a response line in theresponse message. It consists of a method name, therequest-uniform resource identifier (URI), and theprotocol version, currently SIP/2.0. The methodname indicates the purpose of the request, andthe request-URI contains the destination addressof the request. The response line consists of theprotocol version (e.g., SIP/2.0) and the status of thetransaction, including both a number and itsequivalent readable phrase (e.g., 100 TRYING). Asample SIP transaction is shown in Figure 2. Inthis transaction, (1) a request is made by a UAC(Bob) to establish a session with a UAS (Alice) viaan INVITE request (method), and (2) the replyfrom the UAS (Alice) is 100 TRYING, confirmingthat Bob’s request has been received.

Some of the major SIP methods (request types)are listed in Table 1.

Header FieldFollowing the start line is the header field, whichis composed of a mandatory part and an optionalpart. The mandatory part contains fields such asTo, which carries the destination URI; From,which carries the originator’s URI; and ContentSequence (CSEQ), which includes the sequencenumber and method name used to match repliesto requests.

Message BodyThe message body carries multipurpose Internetmail extension (MIME) encoded messages.Message bodies starting with Content-Dispositionare session descriptions. Message bodies are sentin their entirety, i.e., they are not parsed at proxyservers in between and may be encrypted by the UAC.

User IdentificationSIP uses a SIP URI (RFC 3261) to identify users. A SIP URI is similar to an e-mail mail address andconsists of the user’s name and the domain nameof the home network operator. It may also includeoptional descriptions placed after a semicolon.An example of a SIP URI is:

[email protected]

This address, the public user identity, is assignedby the home network operator. Public useridentities are used to route SIP signaling.

Since the PSTN can only interpret digits, andsince the PSTN is going to be in use for theforeseeable future, the IMS also accommodatespublic user identity telephone uniform resourcelocators (URLs) (RFC 2806), which carry atelephone number. An example of a telephoneURL is:

+1-123-456-7890

IMS operators typically assign at least one SIPURI and one telephone URL to each subscriber.Operators may assign more than one public useridentity to a user, i.e., one or more for personaluse and one or more for business use.Figure 2. SIP Transaction: (1) INVITE Request Message and

(2) 100 TRYING Reply Message

Table 1. Sample SIP Methods

Method Description

INVITE Establishes a session

ACK Acknowledges the establishment of the session

CANCEL Cancels a pending request

REGISTER Maps a public URI with the user’s current location

UPDATE Modifies characteristics of a session

MESSAGE Carries an instant message

SUBSCRIBE Subscribes to an event

NOTIFY Notifies an IMS terminal about a certain event

BYE Terminates the session


Along with a public user identity, subscribersalso receive a private user identity—basically auser name and a password—that takes the formof a network access identifier (NAI) (RFC 2486).The private user identity is not used to route SIPsignals, but only to authenticate subscribers.Private user identity is usually stored in the IMSterminal’s UMTS integrated circuit card (UICC).The UICC typically contains a subscriber identitymodule (SIM) card needed for a global system formobile communication (GSM) call, a UMTSsubscriber identity module (USIM) card neededfor UMTS calls, and an IMS subscriber identitymodule (ISIM) card needed for IMS calls. Withouta UICC, only emergency calls can be placed usingthe terminal.

The ISIM card not only stores the user’s privateuser identity, but also public user identity, homenetwork domain URI, and long-term secrets. Thelong-term secret is used for authentication and forcalculating the integrity key (Ik) and cipher key(Ck) used between the terminal and the network.The IMS terminal uses the Ik to protect SIPsignaling integrity between the IMS terminal andthe network. IMS security is discussed in detail inthe IMS Operations section of this paper.

SIP Network Extensions and EntitiesIn the IMS, extensions are (and continue to be)made to SIP. New methods and header fields canbe defined and easily integrated into the coreprotocol. For instance, SIP has an extension todeliver instant messages and an extension tohandle subscriptions to events. SIP uses eitherTCP or UDP as the transport protocol. UDP isusually preferred, because it does not have theoverhead associated with TCP setup, teardown,and retransmission.

The main entities in a SIP network are:

• UA. The UA—that is, the SIP endpoint—isthe entity that initiates and receives SIPrequests and generates the provisional andfinal responses.

• Registrar server. This entity keeps track of userlocations. The UA sends a registrationmessage to the registrar. This information issaved in a location server for future use; thisserver may or may not be co-located with theregistrar.

• Proxy server. This entity is simply a SIP routerthat receives, processes, and forwards SIPrequests and responses. It receives SIPmessages from a UA or from another proxyand routes them to its destination UA oranother proxy.

• Redirect server. This entity receives SIPrequests and returns an alternative locationwhere the user may be available.

SIP RegistrationRegistration is the process by which a useridentifies himself/herself and his/her currentlocation to the network. To illustrate how thisworks, the basic registration process performedin a SIP (not necessarily mobile or IMS) networkis outlined for a hypothetical user, Alice Smith[26]. Alice has a public user identity—also known as her address of record (AoR)—[email protected], where domain is thedomain of Alice’s home network operator. Thisaddress is used by Alice no matter where she logson. When Alice logs on at her computer at work,her workplace SIP URI is [email protected];when she logs on at her computer at school, sheuses a SIP URI of [email protected]. Alice’sAoR is the address known by the public, i.e., the address that anyone trying to contact Alicewould use. Alice must always register her current location with the registrar so that theregistrar knows where to forward incomingrequests to Alice.

All SIP requests arrive at the proxy server, whichmay or may not be co-located with the registrar.If the two are co-located, SIP messages arriving atthe proxy server are directly forwarded to Alice’snew location. If the proxy server and theregistrar—more specifically, the location server ofthe registrar—are separate, then the proxy serverneeds to first contact the location server to learnthe current location of Alice, and then forwardthe message. Figure 3 illustrates the latter process.

Figure 3. Proxy Server and Registrar (including Registrar’s Location Server)


The IMS networkcan basically

be divided intothree layers:

• Application orservice layer

• Control orsignaling layer

• Access orconnectivitylayer

Conducting a SIP Session SetupHaving covered the basic SIP transactions, thediscussion turns to setup of a multimedia sessionusing SIP. Returning to the hypothetical userexample, Bob and Alice are assumed to havealready registered their current locations with thenetwork; now, Bob wants to establish a sessionwith Alice. Alice is using her personal digitalassistant (PDA), and the proxy server, registrar,and location server are all co-located. Thefollowing process takes place (see Figure 4):

1. Bob sends an INVITE request to Alice’s AoRat the proxy server.

2. The proxy server sends a receipt via a 100 TRYING message.

3. The proxy server looks up Alice’s currentlocation and forwards the request to Alice’s PDA.

4. Alice’s PDA sends a confirmation reply via a100 TRYING message.

5. Alice’s PDA sends a 180 RINGING messageindicating that it has been alerted to therequest to establish a session.

6. Alice’s PDA sends a 200 OK reply messageback to the proxy server. This reply includes a Contact header field in the message (so that Bob can use this URI to contact Alicedirectly for all future transactions), along with an SDP message describing Alice’s PDA session components such as audio andvideo; the codec used for each sessioncomponent; and the transport informationsuch as port numbers, IP addresses, andtransport protocol.

7. The proxy server forwards this 200 OK replymessage along with the SDP to Bob.

8. Bob sends an ACK message directly to Alice.At this point, Bob and Alice can initiate asession, say, an audio session.

9. Bob decides to terminate the session andsends a BYE message directly to Alice.

10. Alice sends an OK reply directly back to Boband the session terminates.

Note that if, in the middle of the session, eitherBob or Alice wants to modify the session (forinstance, by adding a video component), he or she can send either an UPDATE request oranother INVITE request with an updated session description.

Building on the foregoing discussion of SIPnetworks, the following sections examine the IMS network’s architecture; the entities involved in the IMS; and the performance ofvarious operations such as registration, sessionsetups, roaming, security, QoS and policysupport, and charging.

IMS ARCHITECTURE

As depicted in Figure 5, the IMS network canbasically be divided into three layers:

• Application or service layer

• Control or signaling layer

• Access or connectivity layer

The application or service layer containsapplication servers (ASs) such as the SIP AS,

Figure 4. Conducting a SIP Session


third-party open service access (OSA) AS, andlegacy service control point (SCP) AS. The IMScontrols service via the subscriber’s homenetwork and those signaling network elementsdistributed in the application layer and thecontrol layer. This arrangement enablessubscribers to receive the same types of serviceswhile they are roaming.

The control layer contains signaling networkelements or control servers for session setup,modification, and termination or for managingcalls. The heart of the control layer consists of thecall session control function (CSCF) servers, alsoknown as SIP servers. This layer also includes thehome subscriber server (HSS) database,subscriber location function (SLF) database, PDF,and breakout gateway control function (BGCF).

Figure 5. IMS Network Architecture (After [26])

The heart of thecontrol layer

consists of the CSCF servers, also known as

SIP servers.


The connectivity or access layer is used totransport signaling traffic and media streams.This layer contains switches, router, and media-processing entities (MGWs, signaling gateways[SGWs], MRF controls [MRFCs], and MRFprocessors [MRFPs]). Since IMS is designed to beaccess independent, it can connect to differenttypes of existing and emerging access networksas long as they have IP connectivity. Accessnetworks that can connect with the IMS includeGPRS/UMTS, 2G networks such as GSM viagateways, PSTNs via gateways, enterprise fixednetworks via IP Centrex, residential fixednetworks via xDSL or cable modem, WLANs, and worldwide interoperability for microwaveaccess (WiMAX).

Next, elements of each layer are examined alongwith their respective roles or functions inestablishing, modifying, and terminating asession; supporting roaming; providing desiredQoS; providing charging information; etc. Thediscussion begins with the control layer elements,since the IMS calls basically start from this point:

• CSCF server is also known as the SIP server,since it processes SIP signaling. There arethree types of CSCFs: proxy (P-CSCF),interrogating (I-CSCF), and serving (S-CSCF).

— P-CSCF is the first point of contact, in the signaling plane, between the IMSterminal and the IMS network. From the SIP point of view, the P-CSCF acts as a SIP proxy server, i.e., all requestsinitiated by the IMS terminal or destinedfor the IMS terminal traverse the P-CSCF,which provides data integrity andconfidentiality by using IP security(IPSec) to maintain a security associationbetween itself and each IMS terminal.The P-CSCF handles the chargingrecords for billing purposes by creatingand maintaining a charging data record(CDR) that can be consolidated at acharging gateway function (CGF). The P-CSCF also provides QoS authorizationand control by providing the necessaryinformation to the PDF for resourceauthorization and QoS control. The IMSalso supports roaming services via the P-CSCF. (Roaming, QoS, charging, etc.,are discussed in more detail in the IMSOperations section of this paper). Also,once the P-CSCF authenticates the user,it asserts the identity of the user to therest of the nodes (or IMS elements) in the network, so those nodes do not need to authenticate the user again. IMS networks usually have several P-CSCFs for the sake of scalability andredundancy, and each P-CSCF serves acertain number of IMS terminals, basedon its capacity.

— I-CSCF is a proxy server that is located at the edge of an administrative domainand that interfaces with SLF and HSSdatabases. These interfaces are based on the Diameter protocol. The I-CSCFretrieves user location information and routes the SIP requests to theirappropriate destinations, typically an S-CSCF. The I-CSCF also assigns an S-CSCF if there is more than one. The

COMMONLY USED IMS INTERFACEDESIGNATORS

Cx between HSS and I-CSCF/S-CSCF

Dx between SLF and I-CSCF/S-CSCF

Gm between I-CSCF/S-CSCF and user equipment

Go between PDF and PEP

Gq between PDF and P-CSCF

Mb between IM-MGW and entities such as user terminal, MRFP, AS

Mi between BGCF and I-CSCF/S-CSCF

Mj between BGCF and MGCF

Mk within a given BGCF (intra-BGCF)

Mn between MGCF and MGW

Mp between MRFC and MRFP

Mw between CSCFs within the same network

Ro between OCF and S-CSCF, MRFC, SIP-AS, et al.

Sh between HSS and AS (SIP-AS or OSA-SCS)

Si between HSS and IM-SSF

Za between security domains (inter-domain)

Zb within a given security domain (intra-domain)

Note: X/(Y) notation on the interfacesindicates that the interface is X and theprotocol used on that interface is Y.


I-CSCF selects the S-CSCF based on theinformation queried from the HSSthrough the Diameter-based Cx interface.If there are multiple HSSs, the I-CSCFmust first contact the SLF (explainedbelow) through the Diameter-based Dxinterface to obtain the HSS addresses.The I-CSCF may also act as a topologyhiding internetwork gateway (THIG) byhiding sensitive information about thedomain, such as the number of servers,their domain name system (DNS) names,and their capacities. An IMS networkmay have several I-CSCFs for the sake of scalability and redundancy.

— S-CSCF is essentially a SIP proxy thatrelays SIP messages, a SIP UA thatinitiates and terminates SIP transactions,and a SiP registrar that authenticatesusers during registration. Mostimportantly, the S-CSCF controls thesession. The S-CSCF interfaces with the HSS via Diameter protocol anddownloads the user’s profile andauthentication vector (AV) to be used in user authentication. The user profileincludes the subscriber service profile(SSP), which has trigger points and user-specific filter criteria. The S-CSCFuses this information to control a user’saccess to different ASs. The S-CSCF also collects data for charging purposes.An IMS network may have several S-CSCFs for the sake of scalability andredundancy. Both the S-CSCF and the P-CSCF maintain session timers, i.e., they are stateful proxies.

• HSS is a stateless Diameter server and adatabase that holds all of the subscriber’sinformation. It is basically an advanced homelocation register (HLR) that holds userinformation, including location, security data(AV), user profile SSP, trigger points andfilter criteria, and the user’s allocated S-CSCF. The HSS may also support HLR/authentication center (AUC) functionalityand mobile-application-part (MAP)-basedinterfaces for legacy 2G and 2.5G networks.Subscriber data stored in the HSS is the key enabler for service mobility acrossdifferent types of access networks and foruser roaming between different networkoperators. A network may require more thanone HSS due to the number of subscribersand the capacity of the HSS. Because of itsimportance, the HSS is always implementedin redundant configuration.

• SLF is a Diameter-based redirect agent orserver that maps the user’s address to aspecific HSS. A network with a single HSSdoes not require an SLF.

• PDF may be part of the P-CSCF or astandalone entity. It interacts with the P-CSCF via the Diameter-based Gq interfaceand with the PEP at the packet data gateway(PDG) via the COPS-based Go interface. ThePDG for the GPRS/UMTS network is thegateway GPRS support node (GGSN).

• AS is a SIP entity that hosts and executesservices. New IMS-specific services areexpected to be developed in SIP ASs. An ASmay host several different applications.

• IP multimedia service switching function (IM-SSF) is a specialized AS that allows reuseof a GSM network’s customized applicationfor mobile network enhanced logic (CAMEL)services in the IMS. The IM-SSF allows thegsmSCF to control an IMS session.

• MRF provides the source for media in thehome network. It enables the home networkto play announcements, mix media streams,transcode between different codecs, performmedia analyses, and provide statistics. TheMRF is divided into two parts: MRFC andMRFP. MRFC acts as a SIP UA and containsan interface with the S-CSCF. It also controlsresources in the MRFP via a Megacointerface. The MRFP implements all media-related functions, such as playing media.

• BGCF is the SIP server with routingfunctionality based on telephone numbers. It is used in sessions that are initiated by IMS terminals and addressed to users in a CS network such as PSTN or other cellular network. The BGCF’s mainfunctionality is to select an appropriatePSTN/CS gateway.

• Media gateway control function (MGCF) is the main node of the PSTN/CS gateway. It has two primary functions: (1) to performcontrol signal protocol conversion from SIPto BICC or ISUP with the SGW, and (2) tocontrol the resources in the MGW. The MGWconverts the media formats between RTP onthe SIP side and pulse code modulation(PCM) on the PSTN side.

IMS OPERATIONS

Some of the more important issues in IMSnetwork operations, such as prerequisites,

registration, session setup, security, QoS and

An applicationserver is a

SIP entity that hosts and executes

services. New IMS-specificservices are

expected to bedeveloped in

SIP ASs. An AS may host several

differentapplications.


policy support, and charging, are examined inthis section.

IMS Operation PrerequisitesBefore a user can access IMS service, the IMS terminal must perform three major tasks:Access the IMS home or visited network via theIP connectivity access network (IP-CAN),discover the P-CSCF address, and perform IMS level registration.

The IP-CAN could be GPRS/UMTS, WLAN, orxDSL, although Release 5 of the 3GPP (the firstrelease of the IMS) supports only GPRS/UMTS.Thus, the IMS terminal must perform a GPRSAttach with the serving GPRS support node(SGSN) and perform bearer level authenticationwith the HLR. To discover the IP address of the P-CSCF, which is stored at the GGSN, theterminal must perform a packet data protocol(PDP) context activation with the GGSN. Finally,the terminal must perform IMS level registrationwith the CSCF and the HSS. At this point, the IMSterminal can access various applications on ASsvia the CSCF.

IMS User Authentication and SecurityAssociationsBefore a user can complete IMS level registration,the user must perform user authentication and establish security associations. Userauthentication in the IMS is based on achallenge/response algorithm and the secretsstored in the ISIM part of the UICC and in theHSS. The S-CSCF is the entity that performs theauthentication, and the P-CSCF is the entity that

performs the integrity and confidentialityprotection of the messages.

The authentication process begins after theprimary PDP context activation, when the IMSterminal sends a SIP register request to the CSCF. The CSCF contacts the HSS and obtains theuser’s AV. The AV is composed of a randomnumber (RAND) challenge, an authenticationtoken (AUTN), the expected response (XRES), theCk, and the Ik. The XRES is calculated based on aknown algorithm RAND and a stored secret. The CSCF replies with a SIP 401 message andincludes both the RAND and the AUTN. Uponreceiving this message, the IMS terminalcomputes its own response (RES) based on thereceived RAND, the same known algorithm usedby the CSCF, and the secret keys retrieved from its ISIM card. The IMS terminal sendsanother SIP register request message thatincludes its own RES. Upon receiving thismessage, the CSCF compares the XRES with theRES, and, if they agree, the CSCF replies with theSIP 200 OK message. This process is summarizedin Figure 6.

To provide integrity and confidentiality, the P-CSCF and the IMS terminal establish two IPSecsecurity associations (SAs) between themselves.One SA is established from the terminal’s client-protected port to the P-CSCF’s server-protectedport, and the other SA is established from the P-CSCF’s client-protected port to the terminal’sserver-protected port. Both SAs support traffic inboth directions. The P-CSCF and the IMS terminalneed to agree on a set of parameters to establishthe two IPSec SAs. The P-CSCF obtains the Ck

and the Ik in the SIP 401 unauthorized replymessage from the S-CSCF. The P-CSCFremoves both keys before relaying themessage to the IMS terminal. To negotiatethe rest of the IPSec parameters, the P-CSCFand the IMS terminal use the same two SIPregister messages that are used forauthentication. After these steps arecompleted, IPSec is permanently set up forthe duration of the session hop-by-hopbetween the P-CSCF and I-CSCF, I-CSCF andS-CSCF, S-CSCF and AS, AS and HSS, P-CSCF and S-CSCF, and S-CSCF and otherentities such as BGCF and MGCF. Alongwith the foregoing actions, encryption mayalso take place between the IMS terminal andthe GGSN on the radio bearer end. Thisprocess is shown in Figure 7.

Figure 6. IMS User Authentication

To provide integrity and

confidentiality, the P-CSCF and the IMS terminal

establish two IPSec SAs

betweenthemselves.


IMS Level RegistrationIMS level registration involves the followingbasic actions:

• The P-CSCF and the S-CSCF are assigned tothe IMS terminal, and a path is establishedbetween them.

• The IMS terminal is authenticated.

• Integrity and confidentiality securityassociations are set up.

• The S-CSCF downloads the user profilefrom the HSS.

• The S-CSCF stores the address informationin the HSS.

Specific steps in the IMS level registration processare as follows:

Step 1: The IMS terminal sends a SIPregister request message to the P-CSCF and compresses thismessage to save transmissionbandwidth and minimize signalingdelays. The receiving P-CSCFdecompresses the request message.

Step 2: If there are multiple S-CSCFs, the P-CSCF forwards the request to the I-CSCF, so that the I-CSCF can select an S-CSCF to serve theuser’s session.

Step 3: The I-CSCF sends a Diameter-baseduser authorization request (UAR)message to the HSS, requesting theaddresses of all available S-CSCFs.

Step 4: The HSS replies with a user authori-zation answer (UAA) message thatincludes a list of all available S-CSCFs.

Step 5: The I-CSCF selects one S-CSCF andforwards the register request to theselected S-CSCF.

Steps 6–7: The S-CSCF retrieves the user AVfrom the HSS via a Diameter-basedmultimedia authentication request(MAR) message and a Diameter-based multimedia authenticationanswer (MAA) message.

Steps 8–10: From the AV, the S-CSCF computesthe user-specific challenge data andsends this information via a 401UNAUTHORIZED message to theIMS terminal through the I-CSCF,P-CSCF, and IMS terminal.

Steps 11–15: The IMS terminal computes itsauthentication response and sendsit to the S-CSCF via another registerrequest message.

Steps 16–17: The S-CSCF verifies the responseand, if correct, downloads thesubscriber profile from the HSS viaDiameter-based server assignmentrequest (SAR) and server assign-ment answer (SAA) messages. TheS-CSCF may contact an AS (or ASs)for service control as specified inthe SSP.

Figure 7. IMS Security Associations


Steps 18–20: Finally, the S-CSCF sends a 200 OKmessage back to the IMS terminalvia the I-CSCF and the P-CSCF.

A graphic representation of the above steps isshown in Figure 8.

IMS Session SetupSession setup is the process of discoveringnetwork nodes and signaling paths.

Building on the SIP session setup functionalitydetailed earlier in this paper, the IMS offers additional functionality, including SIP signaling compression, SIP messageintegrity/confidentiality protection, QoS-relatedcapabilities and features, routing capabilities tothe PSTN/CS networks, tracking and recordingof charging information, etc.

IMS SecurityIMS security can be divided into two areas [9, 10, 26]: access security and network security.Access security involves authentication of usersand networks and protection of traffic betweenIMS terminals and networks. Network securityinvolves protection of traffic between securitydomains. (A security domain is a networkmanaged by a single administrative authority.)

For access security, as indicated in the earlierdiscussion of IMS level registration, an IMSterminal request message travels through the P-CSCF and the S-CSCF during the registrationprocess. The S-CSCF performs the authenti-cation, and the P-CSCF establishes two IPSecassociations to protect the traffic to and from the terminal.

To provide network security, all traffic enteringor leaving a security domain passes through asecurity gateway (SEG), as shown in Figure 9.Security associations between SEGs areestablished and maintained using the Internetkey exchange (IKE) protocol (RFC 2409). Thetraffic between SEGs is protected using IPSecencapsulation security payload (ESP) (RFC 2406)running a tunnel mode.

Within a security domain, network nodes useIPSec to exchange traffic with each other and withthe SEG. The inter-domain interfaces, Za, requiremandatory authentication, integrity protection,and encryption. The intra-domain interfaces, Zb,carry only intra-domain signaling traffic andrequire mandatory integrity protection.Encryption on these interfaces is optional. Figure 10 illustrates this process.

Figure 8. IMS Level Registration

Within a securitydomain, network

nodes use IPSec toexchange traffic

with each other andwith the SEG.


In addition to the mandatory network layersecurity (IPSec), the IMS also provides optionaltransport layer security (TLS) and applicationlayer security (e.g., HTTP digest authenticationfor SIP).

The IMS also contains three interfaces, Cx, Dx, andSh, that use authentication functions. Interface Dxis the interface between the SLF and the I-CSCFand P-CSCF. Interface Cx is the interface betweenthe HSS and the I-CSCF and P-CSCF. Andinterface Sh is the interface between the HSS andthe ASs. In all three interfaces, Diameter is theauthentication protocol. Diameter runs overreliable transport protocols such as TCP andstreaming control transmission protocol (SCTP).

IMS-CS InterworkingSince CS networks and the PSTN will remain inuse for the foreseeable future, the IMS alsoprovides interworking capabilities with BICC-and ISUP-based legacy networks (e.g., PSTN,ISDN, and CS public land mobile networks[PLMNs]). The main IMS functional entitiesinvolved are the BGCF, MGCF, IM-MGW, andSGW. IMS-CS interworking architecture is shownin Figure 11.

The BGCF’s main functionality is to select anappropriate network in which interworking withthe CS domain will take place, or to select an

appropriate CS/PSTN gateway, if interworkingwill take place in the same network in which theBGCF resides. The MGCF is the component thatcontrols the IM-MGW and that performs SIP-to-BICC or SIP-to-ISUP call-related signaling

Figure 9. Network Security Between Domains

Figure 10. Intra-Domain and Inter-Domain Network Security

Figure 11. IMS-CS Interworking Architecture (Source [11])

Since CS networks and thePSTN will remain

in use for theforeseeable future,

the IMS also providesinterworking

capabilities withBICC- and

ISUP-based legacy networks.


interworking. The IM-MGW converts mediaformats provided in one type of network to theformat required in another type of network; forinstance, it provides the interface between the PSdomain and the CS domain. The IM-MGW mayalso be connected via the Mb interface to variousnetwork entities such as a user terminal (via aGPRS tunneling protocol [GTP] tunnel to aGGSN), an MRFP, or an AS. The SGW performsthe call-related signaling conversion to or fromthe BICC/ISUP-based MTP transport networks tothe BICC/ISUP-based SCTP/IP transportnetworks and forwards the converted signalingto or from the MGCF. The SGW may beimplemented as a standalone entity or located inanother entity in the CS network or the IM-MGW.

IMS QoS Support MechanismThe QoS support mechanism ensures that thecritical elements of IP transmission such astransmission rate, gateway delays, and error ratescan be measured and guaranteed in advance. Thisfunction is performed mainly via the PDF, whichinteracts with and controls the underlying packetnetwork (i.e., the access network resources) viathe Go interface with an element in the PDGcalled the PEP. The PDG for the GPRS/UMTS isthe GGSN, which hosts the PEP. Policy-relatedinformation is transmitted between the PDF andthe PEP using COPS (RFC 2478).

Currently, two basic methods support QoS on the Internet: integrated services (IntServ)

(RFC 2215 [27]) and differentiated services(DiffServ) (RFC 3260 [28]).

IntServ is designed to provide end-to-end QoSwith two classes of services: controlled load andguaranteed. IntServ uses resource reservationprotocol (RSVP) to reserve resources with thedesired QoS. RSVP also ensures that the routersreceiving resource reservation requests are therouters that will actually route the packets. Thisfunction is performed via a two-way handshake,in which one endpoint (endpoint A) sends aPATH message to the other endpoint (endpointB), recording all the visited intermediate nodes.Then, in the reverse direction starting at endpointB, a RES message is sent through all the nodesrecorded in the PATH message, this time actuallyreserving the resources.

Note that a router in the path can reject a resourcereservation request either because it does nothave the required resources or because therequester does not have the permissions toreserve those resources. Thus, RSVP can beconsidered as not only a resource reservationprotocol, but also an admission control protocol.

The main drawback of IntServ is that it does notscale well. This is primarily because (1) thenetwork needs to store a large amount ofinformation, and (2) routers need to look up large tables before they can route the packets. To address these issues, DiffServ architecture was proposed.

The main drawbackof IntServ is that it does not scale

well. This isprimarily because

(1) the networkneeds to store alarge amount ofinformation, and

(2) routers need tolook up large tables

before they canroute the packets.

Figure 12. QoS Authorization


In DiffServ, routers identify packet treatmentwithout the need for table lookup. Packettreatments, known as per-hop behaviors (PHBs),are identified by 8-bit codes called differentiatedservices code points (DSCPs). Packets are markedat the edge of the network with a certain DSCP, sothat routers in the path apply the correct PHB tothem. DSCPs are encoded in the Types of Servicefield of IPv4 and the Traffic Classes field of IPv6.Two examples of PHBs are expedited forwardingand assured forwarding. In expedited forwardingPHB, packets never experience congestion in the network. In assured forwarding PHB, packet-drop precedence is determined, allowinglow priority packets to be discarded before high priority packets; some packets may in fact be discarded.

In the IMS, end-to-end QoS involves both QoSover the access network and QoS in the corenetwork. This implies that QoS-requiredresources have to be provisioned and enforced onboth sides. This can be done by using a link-layerRSVP on the access network side and the DiffServmethod (or RSVP) on the network side. On theGPRS/UMTS access network, the link-layerresource reservation is performed via PDPcontext activation, and the GGSN maps link layerresource reservation flows to DiffServ code pointsin the network [7, 8, 26].

Specific steps in the QoS provisioning process aredescribed below.

For inbound sessions:

Step 1: An INVITE request message arrivesat the P-CSCF/PDF.

Step 2: The P-CSCF adds a mediaauthorization token to the messageand forwards this message to theIMS terminal.

Step 3: The IMS terminal creates a PDPcontext activation request messageand sends it to the SGSN.

Steps 4–6: The SGSN receives this messageand checks the user’s subscriptioninformation stored in the HSS usingmobile application part (MAP)protocol. If the IMS terminalrequests more resources than it isallowed to use, the SGSN adjuststhe requested resources to theappropriate level and sends a PDPcontext request message to theGGSN, along with the authorizationtoken.

Step 7: The GGSN extracts the token andthe packet flow identifier and sendsthis information to the PDF usingthe COPS REQ (request) message.The packet flow identifier containsthe source address, the destinationaddress, the source port number,the destination port number, andthe transport protocol used.

Step 8: The PDF responds to the GGSNwith a COPS DEC (decision)message that contains the QoScharacteristics of the IMS terminal’sauthorized session. This is knownas service-based local policy (SBLP)information. The GGSN uses thisinformation to install packet filtersthat allow only authorized packetflows to be transmitted over a givenPDP context.

Step 9: The GGSN (actually the PEPresiding in the GGSN) sends an RTP message to the PDF indicatingthat it will comply with the PDF’s policy.

Step 10: The GGSN sends a PDP contextresponse message back to theSGSN, authorizing the SGSN for therequested PDP context.

Step 11: The SGSN forwards this response tothe IMS terminal.

Figures 12 and 13 summarize the above process.

For outbound sessions of the QoS provisioningprocess, only the initial steps differ from those forinbound sessions. Replacing Steps 1 and 2 ofinbound sessions, the corresponding stepsinvolved in outbound sessions are:

Step 1a: The IMS terminal sends an INVITE request message to the P-CSCF/PDF.

Step 1b: The P-CSCF/PDF forwards thismessage to the callee.

Step 2a: The P-CSCF sends a sessionprogress message to the callee.

Step 2b: The P-CSCF also adds theauthorization token to this messageand forwards it to the IMS terminal.

Steps 3 through 11 of outbound sessions areidentical to those of inbound sessions of the QoSprovisioning process.

On the GPRS/UMTSaccess network,

the link-layerresource reservation

is performed viaPDP context

activation, and theGGSN maps link

layer resourcereservation flows to DiffServ code

points in thenetwork.


As was mentioned earlier, the GGSN/PEP mustalso play the role of a DiffServ edge router andmap the required QoS to the appropriate DSCP toprovide the desired QoS on the core network side,assuming DiffServ is used there.

IMS Service DeliveryOne of the main features of the IMS is its servicedelivery capabilities. The primary IMS networkelements involved in service delivery are the S-CSCF, which acts as the central session controlpoint; the ASs, where the actual services resideand service-specific SIP processing is performed;the HSS, which provides information on theservices and qualities accessible to the IMS user;and the MRF (along with MRFC and MRFP),which controls the media resources.

The IMS employs three types of service deliveryplatforms: SIP-AS, OSA-AS, and legacy SCP. Thesubscriber’s S-CSCF interacts with the serviceplatforms through a SIP-based, intra-operatorinterface known as the IMS service control (ISC).ASs may access user data in the HSS forapplication-specific information via the Shinterface; as mentioned earlier, the S-CSCFdetermines whether to invoke a particular ASbased on specific filter criteria from the user’s SSPstored in the HSS. Multiple applications canreside on a single AS.

Service control in the IMS is home network based,which means that the user receives the sameservices whether operating in the home networkor roaming into a visited network.

IMS RoamingAs indicated earlier, roaming is handled via theP-CSCF. The P-CSCF is the only node or elementin the IMS that must be located in either the homenetwork or the visited network. Also, when IPconnectivity access is via GPRS or UMTS, thelocation of the P-CSCF is subordinate to thelocation of the GGSN. In roaming scenarios,GPRS and UMTS allow the GGSN to be locatedeither in the home network or the visitednetwork, while the SGSN is always located in thehome network. Furthermore, in the IMS, both theGGSN and the P-CSCF share the same network,and the P-CSCF controls the GGSN via the Go interface.

Currently, the IMS allows two configurations forroaming. In the first configuration, shown inFigure 14a, the P-CSCF (and the GGSN) arelocated in the home network. This arrangementwould probably be used in the early stages of IMSdeployment. In this configuration, the visitednetwork is not required to have an IMS-compliant(i.e., 3GPP Release 5 or newer) GGSN. The visitednetwork only provides the radio bearers and theSGSN services for the roaming terminal. The only

Figure 13. QoS Authorization Flow (Inbound Session)

The IMS employsthree types of

service deliveryplatforms: SIP-AS,

OSA-AS, and legacy SCP.


negative aspect of this configuration is that mediaare first routed to the home network and then totheir destination, introducing unnecessary andundesirable delays. This configuration, however,allow immediate deployment of the IMS and IMS roaming [26].

In the second configuration, shown in Figure 14b,the P-CSCF and the GGSN are located in thevisited network, i.e., the visited network has anIMS-compliant GGSN. There may be a move tothis configuration after initial IMS deployment.

IMS ChargingThe IMS supports both offline (or post-paid) andonline (or real-time pre-paid) charging services.The IMS charging architecture is shown in Figure 15 [12–14, 29].

The online charging services are handled by the online charging function (OCF) AS; the offline charging services are handled by the charging collection function (CCF) AS. In Release 6, the CCF is upgraded to the chargingdata function (CDF).

The CCF/CDF is a stateless Diameter-based ASthat does not maintain session states, but keepstrack of transaction states. It creates, updates, andcloses charging data records (CDRs) based onaccounting request (ACR) messages it receivesfrom IMS elements. There are three types of ACRmessages: ACR (Start) to generate the CDR andstart accounting, ACR (Update) to update theCDR, and ACR (Stop) to update and close theCDR. Figure 16 depicts the signaling flow for asession-based offline charging scenario in whichan ACR (Start) is triggered upon receipt of anINVITE request message to start a session; anACR (Update) is started when the interim periodelapses, and an ACR (Stop) is triggered uponreceipt of a session termination BYE requestmessage.

The OCF is a stateful Diameter-based AS thatmaintains both session states and transactionstates for online charging. IMS elementsinteracting via the Ro interface with the OCFinclude the S-CSCF, MRFC, and SIP ASs. The S-CSCF interacts with the SCF of the OCF forsession-based pre-paid service control, while the

Figure 14a. P-CSCF and GGSN Located in a Home Network (After [26])

Figure 14b. P-CSCF and GGSN Located in a Visited Network (After [26])

The IMS supportsboth offline

(or post-paid) and online (or

real-time pre-paid)charging services.


SIP AS and the MRFC interact with the eventcharging function (ECF) of the OCF for content-based pre-paid control. The SIP-Diameterinterworking function (IWF) provides SIPto/from Diameter format conversion and alsoacts as a credit control client (CCC).

Depending on the received SIP messages and theservice usage condition, the CCC sends aDiameter-based credit control request (CCR)message to the OCF/SCF, and the OCF/SCFsends a credit control answer (CCA) replymessage. There are basically three types of CCRs(and CCAs): CCR (Start), CCR (Update), and CCR (Stop).

CCR (Start) corresponds to the initial CCRmessage to start a session and is triggered whenthe CCC receives an INVITE request to start thesession. Via this CCR message, the CCC checkswith the OCF/SCF to verify that the IMS terminalhas enough credit left for session initiation andusage and then reports back in the CCA replymessage. At this point, the IWF/CCC startstiming or counting session usage. When thegranted units or account balances near depletion,the CCC sends a CCR (Update) message to theOCF/SCF to request more credit. The OCF/SCFreplies to this request via a CCA and includes theamount of additional credit, if any. Finally, if atermination request is received via a BYE requestmessage, the CCC sends a CCR (Stop) message to

Figure 15. IMS Charging Architecture

Figure 16. Session-Based Offline Charging


the OCF/SCF and reports the amount of creditused and the amount left over. Of course, if thecredit is depleted before a SIP termination requestarrives, and additional credit cannot be obtained,the CCC can force the session to be terminated.

CONCLUSIONS

The IMS allows an evolutionary move to all-IP converged networks. But in terms of

capabilities and features offered, the IMS is truly a revolutionary approach to multimedia,multisession service deployment. It providesflexible session control with desirable featuressuch as guaranteed end-to-end QoS, roamingcapabilities, security, and easy and convenientcharging. It also allows horizontal servicedeployment by offering a common platform withreusable components and open interfaces. Forinstance, the IMS will be able to offer PTT with almost immediate user access. DeployingPTT service will be as easy as uploading the AS software in an AS and uploading the client application software over the air (OTA) tothe user’s terminal. With the IMS in place,services and applications can be deployed morequickly, easily, and economically than everbefore. Thanks to the IMS, the vision of a personalcommunication system (PCS) can finally becomea reality! �

TRADEMARKS

3GPP is a trademark of the EuropeanTelecommunications Standards Institute (ETSI) in France and other jurisdictions.

cdma2000 is a registered trademark of the Telecommunications Industry Association (TIA-USA).

REFERENCES

[1] 3GPP TS 23.228, “Service Requirements for theInternet Protocol (IP) Multimedia Core NetworkSubsystem (IMS), Stage 1.”

[2] 3GPP TS 23.228, “IP Multimedia Subsystem(IMS); Stage 2.”

[3] 3GPP TS 24.229, “IP Multimedia Call ControlProtocol Based on SIP and SDP.”

[4] 3GPP TS 23.002, “Network Architecture.”[5] 3GPP TS 32.299, “Diameter Charging

Applications.”[6] 3GPP TS 24.228, “Signaling Flows for the IP

Multimedia Call Control Based on SIP and SDP.”[7] 3GPP TS 23.207, “End-to-End Quality of Service

(QoS) Concept and Architecture.”

[8] 3GPP TS 29.208, “End-to-End Quality of Service(QoS) Signaling Flows.”

[9] 3GPP TS 33.203, “3G Security; Access Security for IP-Based Services.“

[10] 3GPP TS 33.210, “Security Requirements.”[11] 3GPP TS 29.163, “Interworking Between the IP

Multimedia (IM) Core Network (CN) Subsystemand Circuit Switched (CS) Networks.”

[12] 3GPP TS 32.225, “TelecommunicationManagement; Charging Management; ChargingData Description for the IP MultimediaSubsystem (IMS).”

[13] 3GPP TS 32.260, “TelecommunicationManagement; Charging Management; IPMultimedia Subsystem (IMS) Charging.”

[14] 3GPP TS 32.295, “TelecommunicationManagement; Charging Management; ChargingData Record (CDR) Transfer.”

[15] 3GPP TS 32.260, “IP Multimedia Subsystem (IMS) Charging.”

[16] “Siemens IP Multimedia Subsystems (IMS),”Siemens White Paper.

[17] “IP Multimedia Subsystem IMS Overview andApplications,” 3G America White Paper.

[18] “IMS–IP Multimedia Subsystem,” Ericsson White Paper, October 2004.

[19] RFC 3261, “SIP: Session Initiation Protocol,” June 2002.

[20] RFC 2327, “SDP: Session Description Protocol,”April 1998.

[21] RFC 3550, “RTP/RTCP: Real Time Protocol/Real Time Control Protocol,” July 2003.

[22] RFC 3588, “Diameter,” September 2003.[23] RFC 2866, “RADIUS: Remote Authentication

Dial-In User Services,” June 2000.[24] RFC 2478, “COPS: Common Open Policy

Services,” January 2000.[25] G. Camarillo, SIP Demystified, McGraw-Hill,

2001.[26] G. Camarillo and M.–A. Garcia-Martin, The 3G IP

Multimedia Subsystems (IMS): Merging the Internetand the Cellular World, John Wiley & Sons, August 2004.

[27] RFC 2215, “IntServ: Integrated Service,”September 1997.

[28] RFC 3260, “DiffServ: Differentiated Services,”April 2002.

[29] “IMS Signaling Architecture,” Ulticom WhitePaper, 2005.

BIOGRAPHY

Rasoul Safavian brings morethan 15 years of experience in the wired and wirelesscommunications industry to his position as BechtelTelecommunications’ new vicepresident of Technology,Americas Regional BusinessUnit. He is charged withestablishing the overall

technical vision for Bechtel’s American markets andproviding guidance and direction to its specific

The IMS allows an evolutionarymove to all-IP

converged networks.But in terms of

capabilities andfeatures offered,

the IMS is truly a

revolutionaryapproach tomultimedia,

multisession servicedeployment.


technological activities. In fulfilling this responsibility,he will be well served by his background in cellular/PCS, fixed microwave, satellitecommunications, wireless local loops, and fixednetworks; his working experience with major 2G, 2.5G,3G, and 4G technologies; his exposure to the leadingfacets of technology development as well as itsfinancial, business, and risk factors; and his extensiveacademic, teaching, and research experience.Before joining Bechtel in June 2005, Dr. Safavianoversaw advanced technology research anddevelopment activities, first as vice president of theAdvanced Technology Group at Wireless Facilities, Inc.,then as chief technical officer and vice president ofengineering at GCB Services. Earlier, over an 8-yearperiod at LCC International, Inc., he progressedthrough several positions. Initially, as principalengineer at LCC’s Wireless Institute, he was in charge ofCDMA-related programs and activities. Next, as leadsystems engineer/senior principal engineer, heprovided nationwide technical guidance for LCC’s XMsatellite radio project. Then, as senior technicalmanager/senior consultant, he assisted key clients withthe design, deployment, optimization, and operation of3G wireless networks.Dr. Safavian is quite familiar with the ElectricalEngineering departments of four universities: TheGeorge Washington University, where he has been anadjunct professor for several years; The PennsylvaniaState University, where he is an affiliated facultymember; Purdue University, where he received his PhD in Electrical Engineering, was a graduate researchassistant, and was later a member of the visiting faculty;and the University of Kansas, where he received bothhis BS and MS degrees in Electrical Engineering andwas a teaching and a research assistant. He is a seniormember of the IEEE and a past official reviewer ofvarious transactions and journals.


BACKGROUND

The beginning of this decade was marked bytales of the coming unlimited capacity and

endless bandwidth to be provided by ultra-fast,ultra-cheap optical equipment. Multipleequipment manufacturers launched newphotonic, or all-optical, switches, promising theability to switch hundreds of wavelengths at atime—in fractions of the space and power, and ata fraction of the cost—of existing equipment. Thecomponents that made up the core of these newswitches would not be semiconductors based onsilicon, as in existing technologies, but novelmaterials that enabled the devices to redirect thepath of light. This would eliminate optical-electrical-optical (OEO) conversions, whichaccount for the bulk of the cost, as well as thespace and power consumption, in a modernoptical network [1].

Several novel techniques for controlling anddirecting an optical signal were proposed byvarious manufacturers. These methods includedusing polarized thin films, ceramic or polymermaterials, liquid crystals, or even bubbles.However, the most popular method for opticalswitching, micro-electrical mechanical systems(MEMS), used tiny arrays of tilting mirrors.Controlled electrical signals were used to adjustthe arrays of mirrors to the proper angle,allowing for the desired output signal to appearon the correct port. Figure 1 illustrates the MEMSmirror used in one manufacturer’s optical switch.MEMS-based systems allowed significantlyhigher port-count switches than competingtechnologies. These systems also offered the

benefit of being a mature technology, as MEMS-based technology is widely deployed in otherindustries, including the automotive, aerospace,and medical fields.

Unfortunately for the proponents of all-opticalnetworks and photonic switches, MEMS-basedand otherwise, the next few years were markedby the largest downturn in the history of thetelecommunications industry. Overcapacities onroutes, abundances of dark fiber, and excessequipment in the network were noted time andagain, and the once emerging desires for hugeoptical switches disappeared. Equipmentmanufacturers scrapped photonic switch designs,placed upgrades on hold, or even disappearedentirely, as carriers suddenly realized theirgrowth forecasts were upside-down.

A SURVEY OF MEMS-ENABLED OPTICAL DEVICES – APPLICATIONS AND DRIVERS FOR DEPLOYMENT

Abstract—Telecommunications equipment manufacturers are deploying MEMS-based optical components innext-generation equipment. These components, whether active or passive, enable gains in speed and reductionsin cost, power consumption, and size. They also can enable carriers to offer new services and reduce the cost ofdeploying existing services. Components include VOAs, ROADMs, and optical switches. Carriers will face newchallenges, however, when deploying these devices.


Brian [email protected]

Figure 1. Optical MEMS Mirror Used in an Optical Switch [2]

As market demands catch up to the glut ofnetwork capacity, carriers are beginning to buildout newer infrastructure. Not only must thisinfrastructure support new bandwidth, it mustalso support new services, with reducedprovisioning intervals and lower capital andoperational costs. Together, these capabilitiesmake up what are known as “intelligent opticalnetworks.” New systems and subsystems aremaking use of the properties of MEMS to enablecarriers to build these networks.

This paper outlines several technologies based onMEMS components, including optical switches,attenuation and equalization devices, and tunablecomponents. The applications and drivers fordeployment of these devices and systems aredetailed. Included in the discussion are thechallenges, issues, and risks that networkoperators face in deploying these technologies.

OPTICAL NETWORK MARKET DRIVERS

With bankruptcy, consolidation, andovercapacity woes in carriers’ pasts, the

former glut of optical capacity has disappeared.Shifts in carriers’ business drivers and operatingmodels have changed their requirements foroptical equipment; no longer is there an outcry forlarger and larger systems, handling endlessamounts of capacity. Instead, carriers are adopting

leaner operating models, trying to do more withless. This is driving network operators to considera new set of requirements when evaluating opticalequipment. Features of interest now includeautomatic provisioning, remote configurability,and reduced power and space needs. These drivethe all-important reductions in operating expensesthat are key to positive cash flows [3].

The need for reduced operating expenses does notenable equipment providers to sell “gold-plated”equipment to carriers for their networks. Thecompetitive landscape has also driven carriers’capital budgets to lower levels, especially in corenetworking and optical transports. Equipmentproviders must find ways to reduce theircustomers’ operational expenses, with lessexpensive equipment.

With the overbuilt core, carriers are paying moreattention to access—literally the “on-ramps” to thecore network. Nearly every major local carrier hasannounced a fiber-to-the-node, fiber-to-the-curb,or fiber-to-the-home/premise (collectively knownas FTTX) strategy, with some carriers beginningconstruction, and others even offering servicesover these networks. The sheer amount of datathat can be delivered over these networks willdrive capacity increases in the core transportmechanisms as well. The new fiber infrastructuresput in place by these efforts will also require newmethods of supervision and surveillance, ascarriers have never managed this scale of opticalconnectivity.

MEMS-BASED OPTICAL COMPONENTS

Optical networking systems manufacturers arenow using MEMS in a variety of components,

beyond the large, scalable optical switches thatwere seen in previous generations of equipment.MEMS components can be used to perform avariety of functions, including redirecting,reflecting, and attenuating light. The systemsmade up by these components can be divided intoseveral subsets: core optical switches, automatedfiber management platforms, variable opticalattenuators (VOAs), and reconfigurable opticaladd-drop multiplexers (ROADMs).

Core Optical SwitchesThe initial application of MEMS components, core optical switches, is still relevant; however, the opportunities for deployment are reducedfrom the initial projections. Optical switches can enable extremely high bandwidth connections and services, supporting high-performanceapplications such as high-speed simulation,



2D MEMS two-dimensional (axes) MEMS

3D MEMS three-dimensional (axes) MEMS

DWDM dense WDM

FTTX fiber-based access network topologies that include fiber-to-the-home/premise, fiber-to-the-curb, and fiber-to-the-node

GMPLS generalized MPLS

MEMS micro-electrical mechanical systems

MPLS multiprotocol label switching

ODF optical distribution frame

OEO optical-electrical-optical

ROADM reconfigurable optical add-drop multiplexer

VOA variable optical attenuator

WDM wavelength division multiplexing

Equipmentproviders must find

ways to reduce their customers’

operationalexpenses, with less expensive

equipment.

visualization, and grid computing (super-computing technology using the resources ofmany disparate networked computers).

Photonic switches use MEMS-based corematerials to provide all-optical light switching.Tiny reflective components, resembling mirrors,are adjusted to steer an optical signal. MEMSswitch cores come in two designs: 2D MEMS,where the mirrors are arrayed on a single level(and therefore can be adjusted only in twodimensions), and 3D MEMS, where the mirrorsare on multiple planes. More flexible and scalablethan the 2D systems, 3D MEMS allow for morelightpaths through the switch. However, 3D MEMS are more complex and costly than thegenerally smaller and easier-to-manufacture 2D design [3]. These devices are usually referredto as A x A in size, where A is the number of inputand output ports. Thus, a 32 x 32 switch can directany of 32 input signals to any of 32 output signals.Due to their complexity, 3D MEMS devicestypically support much larger switch core sizes.

Figure 2 illustrates the operation of 2D and 3DMEMS switches.

Photonic switches can provide significant costsavings over traditional OEO optical switches, asa certain percentage of traffic at any node is“express” traffic, i.e., it does not terminate locally.In a traditionally designed OEO switch, thistraffic would be regenerated electrically beforebeing transmitted to its next destination. Photonicswitches can direct this traffic to the properinterface without regeneration. The trafficdestined for local equipment can be connected toa smaller OEO switch, sized just to handle thelocal traffic, as shown in Figure 3. This hybridimplementation can offer lower capital expensesand a lower cost of ownership by reducing spaceand power consumption.

Another driver for photonic and hybridOEO/photonic switch applications is the“agnostic” feature of photonic switches. Aphotonic switch will direct a stream of light,regardless of the number of wavelengths or “color” of that stream, the underlying protocol, the bitrate, etc. As such, photonicswitches are considered more future-proof, asdifferent OEO devices are required for differentwavelengths, different protocols, differentbitrates, etc. When new protocols or higherbandwidth services are deployed, existing OEOinfrastructure can become worthless, requiringnew equipment to be purchased. Photonicswitches can ensure that a carrier’s investmentwill be viable for a longer term.

Photonic switches also enable the framework forgeneralized multiprotocol label switching(GMPLS). GMPLS is an extension of the signalingprotocols of MPLS to lower-layer entities in thenetwork, including optical and physical layerdevices. GMPLS-enabled photonic switches allowautomated provisioning and bandwidth-on-demand services, as well as new services likeoptical virtual private networks.

Photonic switchescan ensure that a

carrier’s investmentwill be viable for a

longer term.


Figure 2a. 2D MEMS Operation [4]

Figure 2b. 3D MEMS Operation [4]


Although the applications and drivers forphotonic switches are strong, several pitfalls havehindered their deployment. Carriers mustconsider the loss across the fabric of the switch.Each component does have a finite loss, and thisloss must be carefully calculated into the linkbudget of the optical path. This often has theunintended consequence of requiring additionalamplification, or worse, OEO regeneration—aproblem for designers, who had sought to avoidthe need for OEO regeneration in the first place!

The complexity of MEMS-based devices is also anissue—each mirror needs very complexelectronics to drive its movement and stabilize itsposition. These devices add cost to the system aswell, counteracting some of the potential costsavings of the technology. Another complexityconsideration is the finite switching timeinvolved in repositioning the reflectivecomponents. This time must be carefullyconsidered when examining applications, as theswitching for these devices is not “wire speed”(data processed or switched at its native rate).

Complexity is also cited as an issue in terms ofdurability. While the photonic core is future-proof, concern that the equipment controlling the core configuration may need to be

replaced quickly has hindered deployment.Finally, carriers have cited concerns about dust,dirt, and even resiliency in the face of vibration,such as in an earthquake zone. Several manu-facturers of optical components are counteringthis claim by certifying their equipment withstandards institutes or organizations, such asTelcordia® Technologies, Inc.

Table 1 summarizes the similarities anddifferences between optical and photonicswitches.

GMPLS-enabledphotonic switchesallow automatedprovisioning andbandwidth-on-

demand services, aswell as new services

like optical virtualprivate networks.

Figure 3. Photonic Switch Integration Used to Terminate Local Traffic

Table 1. Optical Versus Photonic Switches

CharacteristicOptical Switch

Photonic Switch

Cost per port Higher Lower

CapacityLimited by size ofswitch core

Virtually unlimited

Granularity Circuit Wavelength

Switching/RoutingWire-speed,processes signals

Out-of-band

MaturityDeployed in largenumbers

Limited deployments

FlexibilityLimited by granularityof core

Future-proof


Systemsmanufacturers will

have to justify alarge gap in capitalsavings with a much

lower cost ofownership, or

reduce that gap with maturing,

mass-producedtechnology.

Automated Fiber Management PlatformsPhotonic switches can be used for more than justcore optical nodes; a new application is emergingfor MEMS-based switching systems. As carriersdeploy FTTX technologies and continue to extend the reach of their optical infrastructure,they face the challenge of managing literallyhundreds of fiber interfaces in a single location.Managing these interfaces is typically donemanually via an optical distribution frame (ODF)or patch panel. Manual patch cords are installedby a technician, placing the proper signal on theproper interface. Any moves, additions, orchanges must be done manually, andconfiguration records are often developed byhand as well. Testing of fiber cables terminatedon the ODF panel is also done manually, and acostly error can occur if the technicianaccidentally tests an in-service fiber pair!

This manual work is costly and labor-intensiveand also introduces many opportunities forerrors. In the case of an unmanned site, repairs, testing, and provisioning cannot occuruntil a technician arrives on site. This delay canincrease the mean time to repair or time to turn-up a new circuit.

Automated fiber management systems make useof MEMS components to mechanize this process.Traffic can be switched or rerouted electronicallyand remotely. Since signals do not need to beswitched dynamically or in real time, lower speed(and lower cost) components are used.

Automated fiber management platforms alsoenable advanced features. They allow for a circuitlayout inventory to be developed electronically,so carriers can monitor exactly what signal orcircuit is riding on what fiber, and when and

where it takes place. This can virtually eliminatethe outages that result when the “wrong” fiberpair is disconnected. In addition, remote,automated testing as well as test equipmentconsolidation is made possible. A single piece oftest equipment can be used in one site, and theautomated fiber management platform can beused on the tested fibers to direct the signals fromthat site to the correct strands [5].

While automated fiber management is a potentialbeneficial application for carriers, severalobstacles may hinder its deployment. ODF panelsare entirely passive components—the only activecomponent is the technician who is cablingthem—while these platforms are active. Thismeans that the system will require power andtelemetry and that, typically, some experience inusing its management platform will be necessary.Carrier personnel will require training in usingthis equipment as opposed to the ODF panel,which requires a very low skill level.

Figure 4 highlights the differences betweenautomated and manual fiber management.

A final hurdle is deployment cost. While carriersare eager to reduce operational expenses, whichclearly is possible with this platform, they areunlikely to pay a premium for equipmentfunctions to do so. Systems manufacturers willhave to justify a large gap in capital savings witha much lower cost of ownership, or reduce thatgap with maturing, mass-produced technology.

VOAs

VOAs are components that allow theattenuation of selected optical signals or

wavelengths. These components are integral in

Figure 4. Automated Versus Manual Fiber Management


dense wavelength division multiplexing(DWDM) systems, as they are used todynamically compensate for skewed gains innetwork wavelength amplification. Theamplifiers used in typical DWDM systems have varying degrees of “flatness”—signals ofcertain wavelengths are amplified more or less than signals of other wavelengths. Afterseveral chains of amplification, the wavelengthsreceiving the most amplification may be powerful enough to saturate the receiveelectronics, while the wavelengths receiving theleast amplification are just strong enough toregister at the receiver. VOA components canthen attenuate only the most powerfulwavelengths, bringing the entire signal intoconformance with DWDM receiver specifications.

VOA components are also crucial in protectionswitching functions, as a cable cut can reduce thenumber of aggregate wavelengths on a fiber.Optical amplifiers have a fixed amount ofamplification, which is typically linked toamplifier pump laser current; the amplification is then spread across all of the relevantwavelengths. If a broken or disconnected fiberresults in the amount of wavelengths being cut inhalf, the remaining wavelengths will be amplifiedtwice as much. This effect is often cascadedthrough multiple amplifiers; the end receivercomponent may then receive a signal that issignificantly higher than the component’soperating specification for optical power,rendering the received signal unintelligible. VOAcomponents can prevent failure during aprotection switching event by attenuating theoffending wavelengths until they are in theoperating range for receiver input power.

MEMS-based devices use tuned components thatcan be adjusted, typically by raising or loweringthem, to accomplish attenuation by partially orcompletely blocking a stream of light. Thesecomponents can be thought of as being verysimilar to a mechanized camera shutter, albeit ona much smaller scale. Multiple actuators can beused to provide a very highly resolved range ofattenuation; this arrangement is potentiallysuperior to alternative optical attenuationtechnologies. Figure 5 illustrates a MEMS-basedVOA. The cylindrical items are two optical fibers;attenuation is achieved by raising a metallicshutter in the gap [6].

As they are significantly smaller and less power-hungry than competing technologies, MEMS-based devices do not require temperaturecompensation, which affords several advantages.Temperature control requires additional

electronics, which adds cost, complexity, powerconsumption, and real estate requirements to thecomponent. Temperature control requirementsalso typically prevent non-MEMS-based VOAcomponents from being deployed outside the central office or other controlled,telecommunications-oriented environment. Onthe other hand, MEMS-based VOA componentscan be deployed in a wider variety ofenvironments, including outside plant and accessnetworks. Potential applications for MEMS-basedcomponents in distribution environments includeamplifiers for cable television systems, and gaincontrol devices for active FTTX deployments.

For these reasons, MEMS VOA components areexperiencing widespread market acceptance;several design wins have been announced inrecent months. Systems designers must considerother issues in addition to the cost andcomplexity of this particular subsystem. Theelectronics required to control and drive thesedevices must also be assessed and taken intoaccount as part of an entire design. Whenevaluating a particular design or component, thesystem architect must also examine items such asscalability. Unlike wideband optical componentssuch as optical switches, these devices may haveto attenuate individual wavelengths or smallbands of wavelengths. Systems designers mustask themselves: Will one device be sufficient aslarger numbers of wavelengths are deployed?

ROADMs

ROADMs have received significant attentionfrom carriers in recent months. These

components allow for remote provisioning andmanagement of DWDM systems. Selectedwavelengths can be transparently transported orserviced—added to and dropped from clientequipment—at particular nodes. Changes can

Potentialapplications for

MEMS-basedcomponents in

distributionenvironments

include amplifiersfor cable televisionsystems, and gaincontrol devices for

active FTTXdeployments.

Figure 5. MEMS-Based VOA [6]


ROADM designsoffer further

benefits, such asremote wavelength

testing, dynamicpower balancing(also known as

equalization), andremote signalsurveillance.

be made remotely and in real time via anintegrated control plane such as GMPLS. Thesecomponents hold great implications forimproving service velocity and reducingoperational expenses. Figure 6 illustrates thefunctionality of a ROADM [7].

In addition to these advantages, several ROADMdesigns offer further benefits, such as remotewavelength testing, dynamic power balancing(also known as equalization), and remote signalsurveillance. The dynamic power balancingfeature of a ROADM may seem very similar tothe features of a VOA; this is not entirelycoincidental. VOAs are one of the componentsthat make up a typical ROADM.

Two types of component subsystems are used in ROADMs—wavelength blockers andwavelength-selective switches. Wavelengthblockers are devices offering the capacity toadd/drop wavelengths and that have sometunability—the ability to add/drop selectedwavelengths. Wavelength-selective switches aremore widely tunable and flexible. Mostwavelength-selective switches can be configuredto add/drop or pass through any number orcombination of wavelengths at a node, allowingfor more modularity and deployment ease whilereducing spacing requirements. It should benoted that this flexibility typically comes at aprice premium [8].

ROADM technologies were first deployed inmetropolitan networks, where channel counts arelow and wavelength spacing is accordingly high.Integrated silicon components, using no movingparts, perform well for these applications. As

metropolitan networks grow, systems designerstypically increase wavelength counts byinterleaving channels in between existingchannels, allowing for retention of amplifiercomponents. This approach presents a problemfor silicon-based ROADMs, as flatnessrequirements for these interleaved systems aregenerally higher. Additionally, ROADMapplications are being proposed for long-haulnetworks. Long-haul networks typically featurehigher capacity, higher channel countwavelength division multiplexing (WDM)systems, which present the same challenges as theinterleaved metropolitan systems [9].

MEMS-based wavelength-selective switches areemerging that provide solutions to this flatnessproblem. Subsystems are based on eitherdiffractive MEMS components or MEMS mirrors.Diffractive MEMS components use arrays ofpiezoelectric materials suspended over asubstrate; in this condition, the device “looks”like a mirror to light. Applying a voltage to thematerials causes them to move in the direction ofthe substrate and thus become a diffractiongrating, attenuating a specific frequency of light.Arrays of MEMS materials can be used toselectively reflect or attenuate specific channels.

Diffractive MEMS do not require contact toactuate (instead relying on applied voltage) andare relatively simple to manufacture. They arealso more reliable than mirror-based MEMS, dueto the lack of moving parts, and can be packagedin a smaller subsystem. The only limitation isscalability—arrays of tuned components becomemore complex by orders of magnitude whenchannel counts increase.

Figure 6. ROADM Subsystem Functionality [7]


Mirror-based MEMS subsystems, long used foroptical switches, can be built for much higherchannel-count applications. Designers have tocontend with the same challenges found inoptical switches—cost, complexity, reliability,and size. Systems designers are now developinghybrid techniques in an attempt to takeadvantage of the positives of both technologies.Several manufacturers now offer hybriddiffractive MEMS/mirror MEMS wavelengthselective switches.

While optical systems using ROADMtechnologies are an enabler for scalable,dynamically reconfigurable networks, systemsdesigners must pay additional attention toimplementation details. Granular powermeasurement and control are required for eachwavelength. Fault isolation at the wavelengthand component level is a necessity, since a singlemisdiagnosed fault can cause networkwideinstability. Additional hardware and softwarerequired to perform these functions can entailadditional cost, real estate, and complexity. Assuch, ROADM technologies may hold a “first-cost” penalty for smaller, lower channel-countnetworks when compared to more maturetechnologies. Carriers must consider the growthforecast and timeframe when evaluating choicesfor new optical networks.

CONCLUSIONS

After much early hype, MEMS-based opticaldevices are experiencing continued

deployment in next-generation optical networks.These components enable a number of advancednetworking features, such as dynamicreconfigurability, while potentially reducingpower consumption, network real estate, andcapital and operational expenses. The benefitsoffered by systems using these components oftencome at a cost, however. Also, not everyapplication or deployment is suitable for MEMS-based systems. To achieve an optimum networkdesign, network engineers must continue to usestrong planning methodologies, combined withdetailed knowledge of ideal applications forvarious technologies and the associated cost-versus-benefit tradeoffs. �

TRADEMARKS

Telcordia is a registered trademark of TelcordiaTechnologies, Inc., in the United States, othercountries, or both.

REFERENCES

[1] T. Freeman, “MEMS Devices Put Their Stamp onOptical Networking,” Fibre Systems Europe,September 2004.

[2] J. Ford, J. Walker, and K. Goosen, “OpticalMEMS: Overview and MARS Modulator,”Presentation made by Lucent Technologies.

[3] Goldman Sachs and McKinsey & Company, “USCommunications Infrastructure at a Crossroads:Opportunities Amid the Gloom,” August 2001.

[4] M. Fernandez and E. Kruglic, “MEMS TechnologyUshers in a New Age in Optical Switching,”Lightwave, August 2000.

[5] C. Matsumoto, “Calient Patches Its Strategy,”Light Reading, February 14, 2005(http://www.lightreading.com/document.asp?site= lightreading&doc_id=68031).

[6] D. Horsley, “Image Gallery”(http://mae.ucdavis.edu/faculty/horsley/photoalbum.html).

[7] “ROADM Architectures and Implementations,”Metconnex(http://www.metconnex.com/Products-page/roadm.htm).

[8] C. Matsumoto, “ROADMs: Almost Famous,”Light Reading, May 20, 2005(http://www.lightreading.com/document.asp?doc_id=74210).

[9] C. Matsumoto, “ROADMs Roll On,” Light Reading, June 7, 2005(http://www.lightreading.com/document.asp?doc_id=75148&site=supercomm).

BIOGRAPHYBrian Coombe joined BechtelTelecommunications in 2003.Currently, he is a systemsengineer for the federaltelecommunications groupwithin Bechtel National, Inc.,Bechtel’s government sub-sidiary. He designs andanalyzes carrier, large-scaleenterprise, and government and

military communications systems.

Previously, as a lead Bechtel Telecommunicationsengineer for the Verizon fiber-to-the-premises program,Brian managed the planning and network design of twowire centers. His Bechtel experience also includes workas an RF engineer providing design solutions for theAT&T Wireless GSM overlay program.

Before joining Bechtel, Brian was a systems engineerand applications specialist at Tellabs®, an opticalnetworking equipment manufacturer. He was amember of the team that launched Tellabs’ DWDMplatform and supported Tellabs’ first DWDM customer.

Brian received a BS with honors in ElectricalEngineering from The Pennsylvania State University.He is a member of the Institute of Electrical andElectronics Engineers and also Eta Kappa Nu, thenational electrical engineering honor society. Brianauthored “Reliable Electric Power Transmission andDistribution under National Restructuring,” whichappeared in the IEEE/WISE Journal of Engineering andPublic Policy in August 1999.

To achieve anoptimum networkdesign, networkengineers mustcontinue to usestrong planningmethodologies,combined with

detailed knowledgeof ideal applications

for varioustechnologies and

the associated cost-versus-benefit

tradeoffs.


INTRODUCTION

IEEE 802.16e, the standard for Mobile WiMAX,is expected to be published by the end of 2005,

and Mobile WiMAX service will launch in SouthKorea and possibly in North America in late 2006or early 2007. Originally, the IEEE 802.16standard was developed for fixed wireless in thesearch for a new tool to allow homes andbusinesses to link with the worldwide corenetworks. It was envisioned that the IEEE 802.16standard would offer a better solution for last-mile connections, compared to fiber, cable, ordigital subscriber line (DSL) links, becausewireless systems are less costly to deploy overwide geographic areas. The publication in June 2004 of the IEEE 802.16d (IEEE 802.16-2004) standard provided assurance thatthe WiMAX market and its competitiveness fornon-line-of-sight (NLOS) wireless broadbandaccess are maturing [1–4].

Since the IEEE 802.16e standard (the mobileversion of the IEEE 802.16-2004 standard) will bepublished soon, the focus of WiMAX is expectedto shift from fixed subscribers to mobilesubscribers with various form factors: personaldigital assistant (PDA), phone, or laptop. TheIEEE 802.16e standardization group promises tosupport mobility at speeds of up toapproximately 40 to 50 mph; some vendors arealready claiming success in testing prototypesystems with mobility at speeds over 50 mph for

data rates of 5 Mbps or better. Hence, MobileWiMAX is not only expected to compete withother last-mile connections such as fiber, cable,and DSL; it also threatens Wi-Fi™ and codedivision multiple access (CDMA) voicecommunications with voice-over-Internet-Protocol (VoIP) services.

Unlike wired networks, wireless networks arehighly dependent on communications channels;radio channels are dynamic, correlated,unreliable, and very expensive. This is whyperformance will be highly dependent on howwell radio resource management supportsquality-of-service (QoS) requirements, even ifQoS might be a luxury in the early stages of theMobile WiMAX market. Therefore, several cross-layer issues between the medium access control(MAC) layer and the physical (PHY) layer need tobe optimally resolved on the radio resourcemanagement side of Mobile WiMAX systems.

In multiuser environments, especially on wirelessfading channels, multiuser diversity is a key radioresource management element for maximizingthroughput. Multiuser diversity is a form ofselection diversity. Since different usersexperience independent time-varying fadingchannels in wireless networks, resources areallocated to the user with the best channel qualityto maximize system throughput. Multiuserdiversity has drawn attention since tracking userchannel fluctuations is becoming more accurate

INVITED PAPER

PHY/MAC CROSS-LAYER ISSUES IN MOBILE WiMAX

Abstract—After the IEEE 802.16-2004 standard was published, much attention was drawn to providingbroadband access in rural and developing areas over fixed wireless channels. Now, the IEEE 802.16e standardfor Mobile WiMAX is about to be published. It is known that Mobile WiMAX will incorporate error-correctioncapability and will be an enhanced version of the IEEE 802.16 standard with mobility support. Therefore, it isexpected that Mobile WiMAX will not only compete with the broadband wireless market share in urban areaswith DSL, cable, and optical fibers, but also threaten the hot-spot-based Wi-Fi™ and even the voice-orientedcellular wireless market. This paper first provides an overview of Mobile WiMAX, especially on OFDMA/TDDsystems. Then, the paper addresses some PHY/MAC cross-layer issues that need to be resolved through radioresource management to increase throughput, cell coverage, and spectral efficiency.


Jungnam Yun, PhDPOSDATA America R&D Center

[email protected]

Professor Mohsen Kavehrad, PhDThe Pennsylvania State University(CITCTR)

[email protected]



AAS adaptive antenna system

AMC adaptive modulation and coding

ARQ automatic repeat request

ASCA adjacent subcarrier allocation

ATM asynchronous transfer mode

BE best effort

BS base station

CC convolutional coding

CDMA code division multiple access

CP cyclic prefix

CQI channel quality indicator

CRC cyclic redundancy code

CS convergence sublayer

CTC convolutional turbo coding

DC direct current

DCD downlink coding descriptor

DHCP dynamic host configurationprotocol

DL downlink

DLFP DL frame prefix

DSCA distributed subcarrier allocation

DSL digital subscriber line

D-TDD dynamic TDD

ertPS extended real-time polling service

FEC forward error correction

FBSS fast BS switching

FCH frame control header

FDD frequency division duplex

FRF frequency reuse factor

FUSC full usage of subchannels

HARQ hybrid ARQ

H-FDD half-duplex FDD

HO handover

HT header type

IR incremental redundancy

LDPC low density parity check

LSB least significant bit

MAC medium access control

MAN metropolitan area network

MAP mobile application part

MIMO multiple input, multiple output

MISO multiple input, single output

MPEG Moving Picture Experts Group

MS mobile station

MSB most significant bit

NLOS non-line of sight

nrtPS non-real-time polling service

OFDM orthogonal frequency divisionmultiplexing

OFDMA orthogonal frequency division multiple access

OFUSC optional full usage of subchannels

OPUSC optional partial usage of subchannels

PDA personal digital assistant

PDU protocol data unit

PHY physical

PMP point to multipoint

PN pseudorandom noise

PUSC partial usage of subchannels

QoS quality of service

QAM quadrature amplitude modulation

QPSK quadrature phase shift keying

RTG receive/transmit transition gap

rtPS real-time polling service

SDU service data unit

SINR signal-to-interference+noise ratio

SISO single input, single output

SNR signal-to-noise ratio

STC space-time coding

TDD time division duplex

TSA time slot allocation

TTG transmit/receive transition gap

TUSC tile usage of subchannels

UBC uplink coding descriptor

UGS unsolicited grant service

UL uplink

VoIP voice over Internet Protocol

Wi-FiTM wireless fidelity

WiMAX worldwide interoperability for microwave access

and faster. Hence, diversity gain increases whenthe dynamic range of the fluctuation increases,but the gain is limited in environments with slowfading [5]. In slow fading, multiuser diversityhardly satisfies all QoS parameters at the sametime, especially fairness among users. Ultimately,radio resource management needs to implementa combined form of multiuser diversity andfairness scheduling [6].

As broadband wireless networks encompassvarious services such as World-Wide Web(www), voice, video, and data, network trafficbecomes very dynamic and unbalanced betweenthe uplink (UL) and downlink (DL) streamvolumes. To provide the highest transportefficiency in broadband networks, time divisionduplex (TDD) is preferred over frequencydivision duplex (FDD) because it offers moreflexibility in changing the UL and DL band-width ratio according to the dynamic trafficpattern [7]. It has been assumed that theswitching points for the UL and DL of TDDschemes are determined by network operatorsand are not changeable. Moreover, switchingpoints in adjacent cells have been synchronizedto avoid severe inter-cell interference. Having thesame switching points in adjacent cells, acentralized controller could set switching pointsfor all cells by observing the traffic characteristics[8]. For the duplex scheme, FDD, TDD, and half-duplex FDD (H-FDD) are all available options inMobile WiMAX. This paper addresses only theTDD mode for the orthogonal frequency divisionmultiple access (OFDMA) PHY layer. Eventhough it seems too early to adopt dynamicchanges in the DL/UL ratio for Mobile WiMAXsystems, several investigations show thatappropriate time slot allocation (TSA) and beamforming can bring higher spectral efficiency indynamic TDD (D-TDD) systems compared toconventional TDD systems [9, 10, 11].

This paper first describes the general PHY and MAC layers and then provides views onseveral cross-layer issues related to MobileWiMAX, specifically focusing on the point-to-multipoint (PMP) mode of OFDMA/TDDsystems, multiuser diversity, zone adaptation forinterference cancellation, hybrid automatic repeat request (HARQ), and variable DL/ULratio or D-TDD.

PHY LAYER IN OFDMA/ TDD MOBILE WiMAX

The three different PHY layers in MobileWiMAX are single carrier, orthogonal

frequency division multiplexing (OFDM), and

OFDMA. The WiMAX Forum’s Mobile TaskGroup is developing a Mobile WiMAX profilebased on the OFDMA PHY layer only [1–4]. Atthe same time, the WiMAX Forum’s EvolutionTask Group is developing technical specificationsfor the evolution of OFDM-based networks fromfixed to nomadic connections. Hence, theOFDMA PHY layer will be the baseline forMobile WiMAX.

OFDMAThe wireless metropolitan area network (MAN)-OFDMA PHY layer based on OFDM modulationis designed for NLOS operation in the frequencybands below 11 GHz. OFDMA inherits OFDM’simmunity to intersymbol interference andfrequency selective fading.

An inverse Fourier transform may be used tosynthesize the OFDMA waveform during asymbol time. A small end-portion of the symboltime, called a cyclic prefix (CP), is copied at thebeginning of the symbol time duration to collectmultipath while maintaining orthogonalityamong the subcarriers.

Within the OFDMA symbol time frame, theactive subcarriers are split into subsets ofsubcarriers; each subset is termed a subchannel.On the DL, a subchannel may be intended for different (groups of) receivers. On the UL, a transmitter may be assigned one or more subchannels and several transmitters may transmit simultaneously. The subcarriersforming one subchannel may, but need not, be neighbors. A slot—the minimum possible OFDMA data allocation unit—consists of one or more symbols in the timedomain by one subchannel in the frequencydomain. Hence, OFDMA can fully use multipleuser channel variations via two-dimensionalresource allocation.

TDDThe three different duplex modes for OFDMAMobile WiMAX systems are TDD, FDD, and H-FDD (see Figure 1). TDD systems use the samefrequency band for DL and UL, and the frame isdivided into DL subframes and UL subframes inthe time domain. FDD systems use differentfrequency bands, and DL and UL subframes areoverlapped in the time domain. H-FDD systemshave two different frequency bands for DL andUL, and DL and UL subframes do not overlap inthe time domain.

Mobile WiMAX has an optional channel-sounding feature for TDD systems. Channel


OFDMA can fully use multiple

user channelvariations via

two-dimensionalresource

allocation.


There are two main types

of subcarrierallocation

techniques:distributed and

adjacent.

sounding is a signaling mechanism that enables the base station (BS) to estimate BS-to-mobile station (MS) channel response based onthe UL signals transmitted by the MS. Channelsounding works only under the assumption ofTDD reciprocity.

Due to channel reciprocity and DL/UL ratioadaptability, TDD is the most favored duplexmode in Mobile WiMAX [7]. It is the only modeaddressed in this paper.

OFDMA/TDD Frame Structure Figure 2 shows an example of the OFDMA framestructure for the TDD mode. Each frame isdivided into DL and UL subframes bytransmit/receive transition gaps (TTGs) andreceive/transmit transition gaps (RTGs). Each DLsubframe has a preamble in the first OFDMAsymbol and then starts with the frame controlheader (FCH) in the second symbol. The FCHspecifies the subchannel groups used for thesegment, the burst profile, and the length of theDL-mobile application part (MAP) message,which directly follows the FCH. The UL-MAPmessage is carried by the first burst allocated inthe DL-MAP. Each UL subframe may have one ormore ranging slots, which are used for thenetwork entry procedure. UL subframes mayhave fast feedback channels for fast channelquality indicator (CQI) reports or other fastoperational requests or responses. (Fast feedbackchannels are not shown in Figure 2.)

Subcarrier AllocationThere are three types of subcarriers: data, pilot,and null. Data subcarriers are used for datatransmissions, pilot subcarriers are used forchannel estimation and various synchronizationpurposes, and null subcarriers are used for thedirect current (DC) carrier and guard bandstransmitting no signals. Multiple data sub-carriers are grouped into a subchannel, and asubchannel forms a slot with one or moreOFDMA symbols. A slot is a channel and MAPallocation unit; it contains 48 data subcarriers.

There are two main types of subcarrier allocationtechniques: distributed and adjacent. In general,the distributed allocations perform very well inmobile applications, while adjacent subcarrierpermutations can be properly used for fixed,portable, or low mobility environments. Theseoptions enable the system designers to trademobility for throughput.

Distributed Subcarrier Allocation In a distributed subcarrier allocation (DSCA),multiple data subcarriers are grouped into asubchannel. Although subcarriers in asubchannel are not usually adjacent to each other,they may be in some cases. DSCAs for the DL areDL-partial usage of subchannels (PUSC), fullusage of subchannels (FUSC), optional FUSC(OFUSC), and tile usage of subchannels (TUSC) 1 and 2. DSCAs for the UL are UL-PUSC and UL-optional PUSC (OPUSC).

• DL-PUSC: The default DL subcarrierallocation method. All DL subframes start inthe DL-PUSC zone. Subchannels are groupedinto six major groups and assigned to threesegments (three sectors) of a cell. Assigningtwo major groups to each segment, the cellcan be viewed as frequency reused by afactor of three. By switching to a DL-PUSCzone that assigns all subchannel groups toeach segment, the cell can realize a frequencyreuse factor of one. DL-PUSC is designed tominimize the probability of using the samesubcarrier in adjacent sectors or cells.

• FUSC: Uses all subchannels and minimizesthe performance degradation of fadingchannels by frequency diversity. FUSC is alsodesigned to minimize the probability ofusing the same subcarrier in adjacent sectorsor cells. FUSC pilots are in both variable andfixed positions.

• OFUSC: Also designed to fully usefrequency diversity. One difference fromFUSC is that OFUSC uses a bin structure like band adaptive modulating and coding(AMC).

Figure 1. Duplex Modes TDD, FDD, and H-FDD


• TUSC: For use in the adaptive antennasystem (AAS) zone; similar in structure toUL-PUSC.

• UL-PUSC: The default UL subcarrierallocation method. It is not necessary to startthe UL subframe in the UL-PUSC zone. UL-PUSC has a tile structure, with each tile comprising four subcarriers by threesymbols. The four corner subcarriers are usedas pilots, and the remaining eight subcarriersare used as data subcarriers.

• OPUSC: Also has a tile structure, with eachtile comprising three subcarriers by threesymbols. The center subcarrier is used as apilot, and the remaining eight subcarriers areused as data subcarriers.

On the DL side, DL-PUSC with all subchannelgroups performs similarly to FUSC and to DL-PUSC with partial subchannel groups, which canavoid co-channel interference by deploying afrequency reuse factor of three. On the UL side,UL-PUSC, with its four pilot subcarriers, has abetter channel estimation performance thanOPUSC. However, OPUSC has more data slotsthan UL-PUSC because it has a smaller tile sizewith the same number of data subcarriers.

Adjacent Subcarrier AllocationBand AMC and PUSC-adjacent subcarrier allo-cation (ASCA) are ASCA techniques. WhileDSCAs can gain frequency diversity in frequency

selective slow fading channels, ASCAs can gainmultiuser diversity in frequency non-selectivefading channels. In the adjacent subcarrierpermutation, symbol data within a subchannel isassigned to adjacent subcarriers, and the pilot anddata subcarriers are assigned fixed positions inthe frequency domain within an OFDMA symbol.This permutation is the same for both the UL and DL.

• Band AMC: In defining a band AMCallocation, a bin—the set of nine contiguoussubcarriers within an OFDMA symbol—isthe basic allocation unit on both the DL andUL. A group of four rows of bins is called aphysical band. An AMC slot consists of sixcontiguous bins in the same band, and fourtypes of AMC slots are defined in the IEEE802.16-2004 standard. But in Mobile WiMAX,only one type of slot, defined as two bins bythree symbols, is used.

• PUSC-ASCA: PUSC-ASCA uses distributedclusters for the PUSC mode. The symbolstructure uses the same parameters as thoseof the regular PUSC, and the same clusterstructure is maintained; only the subcarrierallocation per cluster is different from that ofthe regular PUSC.

Adjacent subcarrier allocations are preferred inthe AAS zone.

While DSCAs can gain

frequency diversityin frequency

selective slowfading channels,ASCAs can gain

multiuser diversityin frequency

non-selective fading channels.

Figure 2. Example of the OFDMA Frame Structure for the TDD Mode


MIMO and AAS are seriously

being considered for implementation

in the early stage to boost spectralefficiency and topromote success in the broadbandwireless market.

Ranging For the purposes of network entry, connectionmaintenance, bandwidth request, and efficienthandover (HO), Mobile WiMAX provides ranging channels with CDMA-like signaling. Amaximum of 256 sets of 144-bit pseudo-noiseranging codes are generated and divided intofour groups: initial, periodic, bandwidth request,and HO ranging.

One or more groups of six subchannels (forPUSC) or eight subchannels (for OPUSC)constitute a ranging channel. For initial and HOranging, two OFDMA symbols are used, and thesame ranging code is transmitted on the rangingchannel during each symbol, with no phasediscontinuity between the two symbols. Hence,initial and HO ranging have a wide range for thepurpose of timing adjustments. Meanwhile,periodic and bandwidth request ranging aretransmitted over one OFDMA symbol becauseactive MSs are aligned mostly via frame time andthe timing deviations are very small.

HARQ HARQ greatly increases the data rate when thesignal-to-noise ratio (SNR) is very low; hence, itincreases the coverage of Mobile WiMAXsystems. The major difference betweenconventional ARQ and HARQ is that aconventional ARQ discards erroneous packetswhen retransmitting lost and/or subsequentpackets, whereas an HARQ does not discard theerroneous packets, but combines them withretransmitted packets to gain time diversity. Forevery MS, each packet transmission over a radiochannel faces different channel characteristics.Therefore, HARQ can also be used to maximizethe throughput from the time diversity gain.

There are two types of HARQ: chase andincremental redundancy (IR). For chase HARQ,each retransmission is identical to the originaltransmission; hence, implementation complexityis lower than for IR HARQ. Meanwhile, an IRHARQ transmits a different redundancy versionfor different subpackets. An IR HARQ is flexiblein adapting the subpacket transmission rateaccording to the most recent channel qualityfeedback; this obviously has the potential ofachieving better performance than that of a chaseHARQ. However, chase HARQ using CTC is thepreferred HARQ option because it is less complexthan IR HARQ using CTC.

Channel Coding Mobile WiMAX has four channel coding steps:randomization, forward error correction (FEC),

interleaving, and modulation. A pseudorandomnoise (PN) sequence generator is used torandomize each FEC data block. Multiple FECtypes are available for encoding randomizeddata: tail-biting convolutional coding (CC), zero-tailed CC, convolutional turbo coding (CTC), andlow density parity check (LDPC). Among thesechannel coding schemes, tail-biting CC and CTCare mandatory; the others are optional. CC isused for the FCH DL frame prefix (DLFP) and ismandatory. Except for the FCH, it is highly likelythat CTC will be used for all control informationand data bursts. FEC encoded data is interleavedin two steps: the scattering step and the least-significant-bit (LSB)/most-significant-bit (MSB)switching step. For modulation in OFDMAsystems, quadrature phase shift keying (QPSK)and 16 and 64 quadrature amplitude modulation(QAM) are available.

Multiple Input, Multiple Output/Adaptive AntennaSystem Since single input, single output (SISO) systemscannot achieve high spectral efficiency, multipleinput, multiple output (MIMO) systems drawmuch attention, and they are included in theWiMAX profile as optional features. MIMOsystems have various advantages over SISO andmultiple input, single output (MISO) systems;these include multiplexing gain, diversity gain,interference suppression, and array gain. In ahighly scattering channel, transmitting inde-pendent data from different antennas increasescapacity linearly. Also, there are receiverdiversity gains with multiple receiver antennasand space-time coding (STC) gains with multipletransmitter antennas. As is true of smart antennasystems, beam forming is also an advantage inMIMO systems when channel information isavailable.

Even though MIMO and AAS are optional MobileWiMAX features, they are seriously beingconsidered for implementation in the early stageto boost spectral efficiency and to promotesuccess in the broadband wireless market.

MAC LAYER IN OFDMA/ TDD MOBILE WiMAX

The MAC layer of Mobile WiMAX provides amedium-independent interface to the PHY

layer and is designed to support the wireless PHYlayer by focusing on efficient radio resourcemanagement. The MAC layer supports both PMPand mesh network modes; this paper focuses onlyon the PMP mode.


There are five service classes in Mobile WiMAX:UGS, rtPS, ertPS,

nrtPS, and BE.

The MAC layer schedules data transmissionbased on connections. Each MS creates one ormore connections having various service classes:unsolicited grant service (UGS), real-time pollingservice (rtPS), extended real-time PS (ertPS), non-real-time PS (nrtPS), and best effort (BE) service.The MAC layer is intended to manage radioresources efficiently to support QoS for eachconnection; to maintain link performance usingAMC, ARQ, and other methods; and to maximizethroughput. The MAC layer handles networkentry for the MS and creates the MAC protocoldata unit (PDU). Finally, the MAC layer providestwo convergence sublayer (CS) specifications:asynchronous transfer mode (ATM) CS andpacket CS.

Service Classes When created, each connection is assigned to acertain service class based on the type of QoS guarantees required by the application. TheIEEE 802.16e standard provides the followingservice classes:

• UGS: Designed to support real-time serviceflows that periodically generate fixed-sizedata packets, such as T1/E1 and VoIPwithout silence suppression.

• rtPS: Designed to support real-time serviceflows that periodically generate variable-sizedata packets, such as Moving Picture ExpertsGroup (MPEG) video.

• ertPS: A scheduling mechanism that buildson the efficiency of both UGS and rtPS. TheBS provides unsolicited unicast grants as inUGS, thus saving the latency of a band-width request. However, UGS allocations are fixed in size, whereas ertPS allocationsare dynamic.

• nrtPS: Offers regular unicast polls, whichensures that the service flow receives requestopportunities even during networkcongestion.

• BE: Intended to provide efficient service forBE traffic. Typical BE service is Web surfing.

Network Entry When an MS wants to enter the network, it follows the network entry process: (1) down-link channel synchronization, (2) initial ranging, (3) capabilities negotiation, (4) authen-tication message exchange, (5) registration,(6) IP connectivity, and (7) periodic rangingafterwards. The following paragraph amplifiesthis process.

(1) The MS first scans for a channel in the definedcarrier frequency list and detects its framesynchronization, using the preamble at the PHYlayer. (2) Once the PHY level is synchronized, theMS can obtain the DL-MAP, downlink codingdescriptor (DCD), and uplink coding descriptor(UCD) for the DL and UL parameters. When theMS has all parameters and information regardingthe UL ranging allocation, it starts sending aCDMA ranging code, followed by several MACmessages, and then adjusts timing and poweraccording to the BS command. (3) After initialranging is completed, the MS negotiates with theBS regarding its modulation level, codingscheme, MAP support, and other capabilities, sothat the BS knows exactly what the MS is capableof and can allocate resources efficiently. (4) Oncecapabilities are negotiated, the BS authenticatesthe MS and sends important key material for dataciphering. (5) After the MS is authenticated, itfinally registers onto the networks and starts thedynamic host configuration protocol (DHCP) toobtain the IP address and other parametersneeded to establish IP connectivity. (6) After that,transport connections are made. The BS initiatesthe establishment of pre-provisioned connectionswhile the MS initiates the establishment of non-pre-provisioned connections. (7) Finally, the MSconducts periodic ranging as needed.

MAC PDU Construction and Transmission The MAC layer of Mobile WiMAX supports bothfragmentation and packing of MAC service dataunits (SDUs) for ARQ-enabled and non-ARQ-enabled connections. Also, multiple MAC PDUscan be concatenated into a single transmission ofeither DL or UL connections. For ARQ-enabledconnections, fragments are formed for eachtransmission by concatenating sets of ARQ blockswith consecutive sequence numbers. Eventhough ARQ implementation is mandatory, ARQmay be enabled on a per-connection basis.Furthermore, a connection cannot have both ARQand non-ARQ traffic. A service flow may requirethat a cyclic redundancy code (CRC) be added toeach MAC PDU carrying data for that serviceflow. In this case, for each MAC PDU with headertype (HT) = 0, a CRC32 is appended to thepayload of the MAC PDU; i.e., request MACPDUs are unprotected. The CRC covers thegeneric MAC header and the payload of the MACPDU. The CRC32 calculation methods for theOFDM mode and the OFDMA mode aredifferent, which makes fixed wireless OFDMsystems using the IEEE 802.16d standard andMobile WiMAX (mobile OFDMA systems) usingthe IEEE 802.16e standard incompatible.


The addition of an HO schememakes MobileWiMAX muchdifferent from

fixed broadbandwireless systems.

Packet Scheduling and Radio ResourceManagementThe main goal of packet scheduling and radioresource management is to maximize throughputwhile satisfying QoS requirements.

The BS packet scheduler works closely with theradio resource management entity to guaranteeQoS while maximizing throughput by efficientlyusing the opportunistic characteristics of wirelesschannels. Meanwhile, the MS packet scheduleronly schedules packets from the connectionqueues into the transmission buffer so that theMS can transmit packets when the BS allocatesbandwidth in certain frames.

Radio resource management is the main task ofthe scheduler, whose functions are to allocate DL and UL bandwidth, construct MAPs in the DL and UL subframes, and decide on the bestburst profile for each connection. Bandwidthallocation and MAP construction should be donejointly, and the burst profile should bedetermined per connection beforehand, based onthe signal-to-interference+noise ratio (SINR)report from each MS. For MAP construction, DLand UL subframes can be divided by multiplezones, i.e., a normal zone with multiple choices ofsubcarrier allocation, STC, AAS, and MIMO,based on how the optional features may beimplemented and used.

Handover The addition of an HO scheme makes MobileWiMAX much different from fixed broadbandwireless systems. Seamless HO is a must whenthe connection is for real-time service such asVoIP. There are three types of HO: hard HO, fastBS switching (FBSS), and macro-diversity HO.Also, either the MS or the BS can initiate HO. Thefirst type of HO, hard HO, disconnects the MSfrom the previous serving BS before the MSconnects with the target BS. The second type ofHO, FBSS, uses a fast switching mechanism toimprove HO quality. The MS can onlytransmit/receive data to/from one active BS atany given frame, and all active BSs should be ready for downlink data for the specific MS atany frame. For FBSS, all BSs should besynchronized based on a common time sourceand use the same frequency channel. The BSs arealso required to share or transfer MAC contextthrough networks. The third type of HO, macro-diversity HO, allows one or more BSs to transmitthe same MAC/PHY PDUs to the MS so that theMS can perform diversity combining. This macro-diversity HO is also called a soft HO. Due to thecomplexity of the macro-diversity HO, it is not

likely to be implemented in the early stage ofMobile WiMAX service.

To prepare and expedite a potential HO in thenear future, the MS should be able to scan nearbyBS signals and associate with possible target BSsto acquire and record ranging parameters andservice availability information.

PHY/MAC CROSS-LAYER ISSUES IN MOBILEWiMAX

The challenges inherent in implementingMobile WiMAX arise from its deployment

of a frequency reuse of one and its adoption ofmany state-of-the-art technologies such asHARQ, MIMO, and AAS. This section describessome cross-layer issues that need to be addressedin the radio resource allocation management ofMobile WiMAX.

Zone Switch and Frequency Reuse Since the OFDMA PHY layer has many choices of subcarrier allocation methods, multiple zonescan use different subcarrier allocation methods to divide each subframe. One benefit of usingzone switching is that different frequency reusefactors (FRFs) can be deployed in a cell (or sector), dynamically.

Figure 3 shows an example of deploying differentFRFs in one frame. For the first half of each frame,the entire frequency band is divided by three andallocated in each sector. For the second half ofeach frame, the whole same frequency band isused in each sector. The benefits of deployingdifferent FRFs in one frame are: (1) the FCH andDL-MAP are highly protected from severe co-channel interference; (2) edge users, who arereceiving co-channel interference from othersectors in other cells, also have suppressed co-channel interference; and (3) users around the cell center have the full frequency bandbecause they are relatively less subject to co-channel interference.

Multiuser Diversity In the wireless multiuser environment, it is wellknown that multiuser diversity is a veryimportant leveraging factor of resource allocationmanagement [5, 6, 12]. Each MS faces a differentfading channel; hence, radio resourcemanagement can use multiuser diversity tomaximize system throughput. The difficulty liesin the fact that radio resource allocation alsoshould satisfy fairness among subscribers.Moreover, in slow fading, multiuser diversity


hardly satisfies all QoS parameters at the sametime, especially fairness among users. Ultimately,radio resource management should follow acombined form of multiuser diversity andfairness scheduling.

Figure 4 illustrates the deployment of multiuserdiversity with a band AMC zone. Similarly,multiuser diversity can also be deployed with aDSCA zone such as PUSC, FUSC, optional FUSC,and TUSC. The only thing different from a bandAMC zone is that the multiuser diversity gain canbe obtained only from time domain allocations.

Dynamic TDD Usage Each MS and BS experiences not only differentchannel characteristics, but also various datatraffic. In other words, UL and DL streamvolumes that have been considered symmetricalfor conventional voice transmissions areunbalanced, and the ratio is time varying. Toprovide the highest transport efficiency inbroadband networks, TDD is preferred to FDDbecause it enables real-time adaptation of UL andDL bandwidth according to the dynamic trafficpattern. Even though FDD can also be used forasymmetric traffic, DL and UL channel bands

should be matched to the ratio of DL and ULtraffic. Moreover, FDD channel bands cannot beadjusted dynamically in response to the varyingratio of DL and UL traffic, due to hardwarelimitations. The ratio of UL to DL streams is fixedfor FDD.

It has been assumed that network operatorsdetermine the switching points for TDD UL andDL schemes and that once such systems aredeployed, the DL/UL ratio is not changeable.Moreover, switching points in adjacent cells must be synchronized to avoid severe inter-cell interference.

Figure 5 shows various co-channel interferencecases. In conventional TDD systems, only DL/DLand UL/UL cases can occur. However, if theDL/UL ratio is changed dynamically frame byframe and independently cell by cell, co-channelinterference can exist in all four cases.

A cross-layer D-TDD scheme considering trafficand channel condition together may be adopted.Since each cell can have different offered loads forUL and DL, cell switching points are setindependently. Although this may cause severeco-channel interference at time slots around the

Ultimately, radio resourcemanagement

should follow acombined form

of multiuserdiversity and

fairness scheduling.

Figure 3. Zone Switch for FRF Change


DL/UL UL/DL

DL/DL UL/UL

Figure 5. Various Co-channel Interference Cases

Figure 4. Usage of a Band AMC Zone Based on Multiuser Diversity

Although D-TDD is complex, it has the

potential of offering higher bits-per-hertz

efficiency whenneeded.


switching points, it still produces morethroughput than the conventional TDD schemewith a fixed DL/UL ratio.

Switching points can be updated daily or byframe. In the daily approach, the networkoperator monitors for a certain amount of time, then updates the switching point to allocateresources based on switching point information.In the frame approach (used in this paper), theswitching point is changed dynamically for eachframe based on traffic characteristics and channelstatus. Based on the resource allocationalgorithm, each user is given a number of timeslots, with corresponding indices and amodulation format. Modulation is determined bythe estimated channel state, and the time slotindices are determined by the TSA algorithm.

Figure 6 illustrates the deployment of differentDL/UL ratios for different cells. In this example,for the early symbols of the UL subframe in Cell 2, the BS experiences severe co-channelinterference coming from Cells 1 and 3 becausethey are all in the DL period and signals fromtheir BSs interfere with the UL signals of users inCell 2. To suppress interference in this case, agood TSA algorithm with beam forming isnecessary. Several investigations of TSAalgorithms show that D-TDD systems out-perform conventional TDD systems when thedynamic traffic has unbalanced DL/ULcharacteristics [9, 11]. Although the complexity of D-TDD makes its adoption in the early stages of Mobile WiMAX unlikely, it has thepotential of offering higher bits-per-hertzefficiency when needed.

CONCLUSIONS

This paper has provided an overview of theIEEE 802.16e standard and Mobile WiMAX

and has provided some simple suggestions toaddress certain PHY/MAC cross-layer issues.

Mobile WiMAX is expected to bring fast, broad,seamless data communications, not only for fixedhome and small business subscribers, but also formobile subscribers. Mobile WiMAX will begincompeting in fixed broadband markets to linkhomes and businesses with worldwide corenetworks, before ultimately penetrating mobilecommunication market shares. To strengthen themarket power of Mobile WiMAX, radio resourcemanagement that deals with PHY/MAC cross-layer issues needs to be developed accurately andclose to optimally. �

TRADEMARKS

Wi-Fi is a registered trademark of the WirelessEthernet Compatibility Alliance, Inc.

REFERENCES

[1] Wireless MAN Working Group(http://www.wirelessman.org/).

[2] IEEE Std 802.16-2004, “IEEE Standard for Localand Metropolitan Area Networks – Part 16: Air Interface for Fixed Broadband WirelessAccess Systems,” October 2004.

[3] IEEE P802.16-Cor1/D5, “Draft Corrigendum to IEEE Standard for Local and Metropolitan Area Networks – Part 16: Air Interface for Fixed Broadband Wireless Access Systems,”September 2005.

[4] IEEE P802.16e/D11, “Draft Amendment to IEEE Standard for Local and Metropolitan AreaNetworks – Part 16: Air Interface for Fixed andMobile Broadband Wireless Access Systems —Amendment for Physical and Medium AccessControl Layers for Combined Fixed and MobileOperation in Licensed Bands,” September 2005.

[5] P. Viswanath, D.N.C. Tse, and R. Laroia,“Opportunistic Beamforming Using DumbAntennas,” IEEE Transactions on InformationTheory, Vol. 48, No. 6, pp. 1277–1294, June 2002.

[6] H. Fattah and C. Leung, “An Overview ofScheduling Algorithms in Wireless MultimediaNetworks,” IEEE Wireless Communications, pp. 76-83, October 2002.

[7] TDD Coalition white paper, “The Advantagesand Benefits of TDD Broadband Wireless AccessSystems,” September 2001.

[8] D.G. Jeong and W.S. Jeon, “Time Slot Allocationin CDMA/TDD Systems for Mobile MultimediaServices,” IEEE Communications Letters, Vol. 4, No. 2, February 2000.

[9] J. Li, S. Farahvash, M. Kavehrad, and R. Valenzuela, “Dynamic TDD and Fixed Cellular Networks,” IEEE Communications Letters, Vol. 4, pp. 218–220, July 2000.

[10] W.C. Jeong and M. Kavehrad, “Co-channelInterference Reduction in Dynamic-TDD FixedWireless Applications, Using Time Slot Allocation Algorithms,” IEEE Transactions onCommunications, Vol. 50, No. 10, pp. 1627–1636,October 2002.

Figure 6. Various DL/UL Ratios for Different CellsMobile WiMAX is expected to

bring fast, broad,seamless data

communications,not only for fixedhome and small

businesssubscribers, butalso for mobile

subscribers.


[11] J. Yun and M. Kavehrad, “Adaptive ResourceAllocations for D-TDD Systems in WirelessCellular Networks,” Proceedings of MILCOM, Vol. 2, pp. 1047–1053, November 2004.

[12] M. Ergen, S. Coleri, and P. Varaiya, “QoS AwareAdaptive Resource Allocation Techniques for Fair Scheduling in OFDMA Based BroadbandWireless Access Systems,” IEEE Transactions onBroadcasting, Vol. 49, No. 4, pp. 362–370,December 2003.

BIOGRAPHIESJungnam Yun is a member ofthe Technical Staff at thePOSDATA America R&DCenter, Santa Clara, California,where he works as a systemengineer for the OFDMA PHYalgorithm and MAC scheduler.His research interests are signalprocessing, communicationtheories, and cross-layer

optimization for radio resource allocations inbroadband wireless communication systems.

Dr. Yun received his BS and MS in ElectricalEngineering from Korea Advanced Institute of Scienceand Technology (KAIST), Taejun, South Korea, and hisPhD in Electrical Engineering from The PennsylvaniaState University, State College, Pennsylvania.

Mohsen Kavehrad was with the Space CommunicationsDivision, Fairchild Industries,and Satellite Corporation andLaboratories, GTE, from 1978 to1981. In December 1981, hejoined Bell Laboratories, and in March 1989, he joined the Department of ElectricalEngineering, University of

Ottawa, Ottawa, Ontario, Canada, as a full professor. Atthe same time, he also was director of the BroadbandCommunications Research Laboratory, director ofPhotonic Networks and Systems Thrust, project leaderin Communications and Information TechnologyOntario (CITO), and director of the Ottawa-CarletonCommunications Center for Research (OCCCR). Hewas an academic visitor (senior consultant) at NTTLaboratories, Yokosuka, Japan, in the summer of 1991.He spent a 6-month sabbatical term as an academicvisitor (senior consultant) with Nortel, Ottawa, Ontario, Canada, in 1996. In January 1997, he joined the Department of Electrical Engineering, ThePennsylvania State University, University Park, as the AMERITECH (W.L. Weiss) professor of ElectricalEngineering and director of CommunicationsEngineering. Later, in August 1997, he was appointedfounding director of the Center for Information andCommunications Technology Research (CICTR). From1997 to 1998, he also was chief technology officer andvice president with Tele-Beam Inc., State College,Pennsylvania. He visited, as an academic visitor (seniorconsultant), Lucent Technologies (Bell Laboratories),Holmdel, New Jersey, in the summer of 1999. He spenta 6-month sabbatical term as an academic visitor (seniortechnical consultant) at the AT&T Shannon ResearchLaboratories, Florham Park, New Jersey, in 2004.

Dr. Kavehrad has been a consultant to a score of majorcorporations and government agencies. He haspublished over 300 refereed journal and conferencepapers, several book chapters, and books. His researchinterests are in the areas of technologies, systems, and network architectures that enable the vision of the information age, e.g., broadband wirelesscommunications systems and networks and opticalfiber communications systems and networks. He holdsseveral key issued patents in these areas.

Dr. Kavehrad received three Exceptional TechnicalContributions Awards for his work on wirelesscommunications systems while he was with BellLaboratories; the 1990 TRIO Feedback Award for hispatent on a “Passive Optical Interconnect”; the 2001IEEE Vehicular Technology Society Neal Shepherd BestPropagation Paper Award; three IEEE Lasers andElectro-Optics Society Best Paper Awards; and aCanada National Science and Engineering ResearchCouncil (NSERC) PhD dissertation Gold Medal award,jointly with his former graduate students, for work onwireless and optical systems. He is a fellow of the IEEEand has lectured worldwide as an IEEE distinguishedlecturer and as a plenary and keynote speaker atleading conferences.

Dr. Kavehrad received his PhD in Electrical Engineeringfrom Polytechnic University (Brooklyn Polytechnic),Brooklyn, New York.


INTRODUCTION

This paper describes the provisions of theANSI/TIA/EIA-222-F-1996 standard [1] for

the design, purchase, fabrication, and installationof telecommunication tower superstructures andfoundations and compares these provisions withthose of the new ANSI/TIA-222-G-2005 standard[2]. The new Revision G standard was issued inAugust 2005 with an effective date of January 1,2006. This standard is the governing documentfor telecommunication towers in the UnitedStates. The contents of the document are beingapplied extensively as new telecommunicationtower sites are built and existing sites areupgraded to accommodate the growth in thewireless communication industry. This paperprovides insight into new Revision G of thestandard and how it will affect projects involvingwireless telecommunication tower sites.

The Telecommunications Industry Association(TIA) subcommittee TR-14.7, which wasresponsible for preparing the standard, states:

The objective of this Standard is to providerecognized literature for antenna supportingstructures and antennas pertaining to: (a)minimum load requirements as derived fromASCE 7-02, “Minimum Design Loads forBuildings and Other Structures,” [3] and (b)design criteria as derived from AISC-LRFD-99,“Load and Resistance Factor DesignSpecification for Structural Steel Buildings,”[4] and ACI 318-05, “Building CodeRequirements for Structural Concrete” [5]. Theinformation contained in this Standard was

obtained from available sources andrepresents, in the judgement of thesubcommittee, the accepted industryminimum structural standards for the designof antenna supporting structures andantennas. While it is believed to be accurate,this information should not be relied upon fora specific application without competentprofessional examination and verification ofits accuracy, suitability and applicability by alicensed professional engineer. This standardutilizes loading criteria based on an annualprobability and is not intended to cover allenvironmental conditions which could exist ata particular location.

The standard provides the requirements for thestructural design of new and the modification ofexisting structural antennas and antenna-supporting structures, i.e., towers, mounts,structural components, guy assemblies,insulators, and foundations.

This paper compares the provisions of RevisionsF and G of ANSI/TIA Standard 222 as they relateto five key aspects of tower superstructure andfoundation design:

• Design method evolution from a traditionalworking strength design approach to acontemporary load resistance factor designapproach for the tower superstructure

• Wind load definition

• Seismic requirements

• Ice loading definition

• Design of existing structures

Abstract—A new revision of ANSI/TIA Standard 222 will take effect on January 1, 2006. The new standard—Revision G—is the most comprehensive revision of Standard 222 since its first publication in 1949. A revisionon this order can cause anxiety in the industry as to its impact on tower design and can raise questions. This paper explains the differences in the basic design philosophies of the standard (Revision F) and the newRevision G. It also discusses the impetus behind this major revision triggered by the latest understanding andstate-of-the-art practices of the current codes and standards in the building industry.


Peter Moskal [email protected]

[email protected]

ANSI/TIA STANDARD 222 – STRUCTURAL STANDARD FOR ANTENNA SUPPORTINGSTRUCTURES AND ANTENNAS: A COMPARISON OF REVISIONS F AND G

In each case, the two standards are contrasted bybriefly describing the provisions of Revision Fand then those of Revision G, followed by asummary of the significance of the changes.

This paper also discusses related areas of interest.Figure 1 shows a typical guyed latticetelecommunication tower structure governed bythe ANSI/TIA-222 standard.

DESIGN METHOD

Asignificant change in the standard reflectsthe migration from the typical working

stress design to the contemporary limit-statesdesign approach for structural steel. This bringsthe standard into compliance with current codes.

Revision FThis standard uses the working stress designmethod. The American Institute of SteelConstruction (AISC) refers to this approach as theallowable stress design (ASD) method. Stresses ina structure are determined by application ofdefined service loads using the principles ofstatics and dynamics. The loads are applied to asuitable arrangement of structural elementsforming a stable assembly. The assembly hassufficient strength and is designed usingmembers and connections of defined geometricshape and known material properties inaccordance with applicable code provisions. Theresulting actual member stresses are compared to allowable member stresses, based on the code, which are less than the member materialyield stresses. The allowable stresses provide an inherent factor of safety to account foruncertainties related to typical simplifyingassumptions and the use of nominal or averagecalculated stresses as the basis for manualmethods of analysis. The margin between theallowable stresses and the material yield stressesprovides the margin of safety. In applying thisapproach, engineering judgment must beexercised. The working stress approach hasserved as the principal design philosophy for a century.

Allowable stress design may be formulated asfollows:

(1)

In this elastic design approach, all loads Qi areassumed to have the same variability, and theterm φ/γ may be thought of as the safety factorapplied to the material resistance. For materials indirect tension, the allowable stress is limited to0.60 times the material yield strength. This resultsin a safety factor of 1/0.60 = 1.67. The AISCspecification defines the allowable stresses for tension members, columns and othercompression members, beams and other flexuralmembers, plate girders, bolts, and welds.

Per Revision F, design for structural members,unless otherwise noted, must be in accordancewith the appropriate AISC or American Iron andSteel Institute (AISI) specification. AmericanSociety of Civil Engineers (ASCE) Standard 10 isused to adjust the AISC allowable compressionstresses for the effects of eccentric axial loadingand partial end restraint for structural steelsingle-angle compression members.



ACI American Concrete InstituteAISC American Institute of Steel

ConstructionAISI American Iron and Steel

InstituteANSI American National Standards

InstituteASCE American Society of

Civil EngineersASD allowable stress designCOA Certificate of AuthorizationERC engineer in responsible

chargeIBC International Building CodeNWS National Weather ServiceTIA Telecommunications Industry

Association

A significant changein the standard

reflects themigration from the

typical workingstress design to

the contemporary limit-states

design approach for structural steel.

γ = ∑QiφRn

Figure 1. Guyed Lattice Tower Structure Governed byANSI/TIA-222

Revision GThe new standard is based on limit-states design.Structural design has been moving toward thismore rational probability-based approach over thepast 25 years. The generic term “limit states” usedin the new standard is synonymous with the AISC’s use of load and resistance factor design terminology.

Structures (tower superstructure and foundation),members, and connections must have adequatestrength to function safely over their service life.Reserve strength must exist in a structure toaccount for possible overload and under-strengthconditions. Overloads may occur because thestructure’s use may be changed or the design loadsmay be judged to be less than those actuallyexperienced. Under strength may result fromvariations in member sizes due to manufacturingor construction practices, or from installedmaterial strengths that are less than specified.

A structure must contain an adequate safetymargin. Safety has been studied usingprobabilistic methods to assess the chance offailure or the limit state. Limit states are reachedwhen a structure no longer performs its intendedfunction. Limit states are divided into twocategories: strength and serviceability. Strengthrefers to the ability of elements to sustain intendedloads and maintain stability in conditions such assliding or overturning. Serviceability involvesconsideration of requirements such as deflectionand permanent deformation.

The loads acting on a structure and thestructure’s resistance to the loads must balancewith an appropriate allowance for a margin ofsafety. The variables that affect loads andstrength have been studied probabilistically toassess variability. The limit-state approachassumes that the load Q and the resistance R arerandom variables. The goal is to have theresistance R exceed the load Q by a reasonablemargin of safety. Unless the margin of safety isvery large, some probability exists that R may beless than Q. When assessing the relationship forthe natural logarithm of resistance divided byload, i.e., ln(R/Q), a probability distributionfunction is obtained. Values for the number ofstandard deviations that are appropriate betweenthe mean value of ln(R/Q) and the lowerspecification limit are established and designatedas the reliability index [6] (see Figure 2).

This leads to the following limit-state requirement:

(2)

The left side of this equation represents resistanceor strength, i.e., strength of concrete, steel, or othermaterial. The right side represents the loads thatthe structure is designed to carry. On the left side ofthe equation, the nominal resistances Rn aremultiplied by a strength reduction factor φ toobtain a design strength limit. On the right side, theapplied loads Qi (e.g., wind, ice, and seismic) aremultiplied by a load factor γi to obtain the factoreddesign loads for which the structure is designed.The strength reduction factors are less than one toestablish material strength limits with anappropriate conservatism applied based onstatistical assessments of materials. The load factorsare greater than one to allow margin in the designloads that are also based on statistical assessments.

Additional discussion and comparison of theworking stress design and limit-state designapproaches can be found in Chapter 1 of [7].

The new standard specifies the requisite loads,load combinations, strength reduction factors,and load factors for tower design andmodification use.

Design Method Summary The limit-state design approach is thecontemporary method for structural analysis anddesign. It is now the method of design fortelecommunication towers as well as concreteand structural steel. This is a rational approachthat will facilitate incorporation of additionalinformation that becomes available on loads andresistances and their variation. It provides aframework for handling unusual loads that maynot be covered by specifications. Safer structuresmay result because the method should lead toimproved awareness of structural behavior. Inmost cases, it should also improve the economyof structures.

The limit-statedesign approach isthe contemporary

method forstructural analysis

and design.


Figure 2. Reliability Index ββ= ∑ γiQiφRn


WIND LOADS

Another significant change relates to themigration from fastest-mile wind speed to a

3-second-gust wind speed definition, whichbrings the standard into compliance with othercurrent standards, codes, and guides.

Revision F Section 2.6.1 of the standard defines the basicwind speed as the fastest-mile wind speed at 33 feet above the ground corresponding to anannual probability of 0.02 (50-year recurrenceinterval) for Exposure C. This may be thought ofas the average speed obtained during the passageof 1 mile of wind. For a 70 mph wind, this wouldmean that 1 mile of wind passes in 51.4 seconds.The wind speed is measured by a weather devicecalled an anemometer at a height of 33 feet abovethe ground. The annual probability is based onthe probability distribution function.

As shown in Figure 3, the value of 0.02 indicatesa 2 percent chance that the 70 mph speed will beexceeded in a year or a 98 percent chance that itwill not be exceeded. Wind determinationrequires a probabilistic formulation, and thenecessary probability measures must beestimated on the basis of experimentalformulations. The average time between twoconsecutive annual occurrences of the windexceeding 70 mph is the reciprocal of theprobability of the event within one time unit, i.e.,1/0.02 = 50 years. Therefore, the probability thatthe wind will exceed 70 mph is a 2 percent chancein a given year, and the chance that this speedwill be exceeded in consecutive years occurs onceevery 50 years. The National Oceanic andAtmospheric Administration’s National WeatherService (NWS) branch collects and publisheswind speed data throughout the US.

Revision GIn the new standard, wind speed definition isbased on the 3-second gust. Wind speeds aredeveloped for 3-second gusts at 33 feet aboveground in open-country exposure Category C fora 50-year mean recurrence interval. The NWS hasphased out the measurement of fastest-mile windspeeds, and the basic wind speed has beenredefined as the peak gust, which is recorded andarchived for most NWS stations. The wind speedsare not representative of speeds at which ultimatelimit states are expected to occur. Load factors orallowable stresses used in design equations leadto structural resistances substantially higher thanthe design wind speeds. Figure 4 illustrates awind speed time history record and provides aninterpretation of the two speed definitions.Figures 5 and 6 show lattice towers withsignificant coaxial cable wind load areas.

Figure 4. Wind Speed Record

Figure 3. Largest Annual Fastest-Mile Velocity (V)


The new standard defines

four methods of structuralanalysis for

seismic loads.

Wind Loads SummaryThe fastest-mile wind speed of 70 mph mentionedpreviously is equivalent to an 85 mph 3-second-gust wind speed. The new standard provides atable of wind speed equivalencies.

This approach to wind loads makes the standardconsistent with ASCE Standard 7 and codes suchas the International Building Code (IBC). It is thecontemporary wind load approach in the US.This standard enables accurate site-specific windmodeling using exposure categories andtopographic effects.

The new standard will use an averaging period of 3 seconds for the US. Other countries usedifferent averaging periods (e.g., 1 hour or 10 minutes). Weather data in the US is collected at465 stations throughout the country.

A description of how to convert the fastest-milewind speed to the 3-second-gust definition can befound in Section 4.0 of Reference [8].

SEISMIC LOADS

Another significant change in the standard isthe inclusion of seismic provisions. This

brings the standard into compliance with othercurrent standards, codes, and guides.

Revision FThe standard does not contain seismic provisions.When seismic loads were considered to apply to aspecific tower design, an equivalent static load,response spectrum modal analysis, or time

history analysis would be applied based on theprovisions contained in applicable buildingcodes.

Revision GThe new standard defines four methods ofstructural analysis for seismic loads. A table isprovided to identify which method is to be usedfor which tower type in accordance with mass andstiffness irregularities. The four approaches are:

• Method 1 – Equivalent Lateral Force

The total weight of the structure, includingappurtenances, is determined; the totalseismic shear is determined and distributedas specified in the standard. The structure isthen analyzed statically using the seismicforces as external loads.

• Method 2 – Equivalent Modal Analysis

The fundamental frequency of the structureis determined along with the seismic forcesfor each level of the structure. The structure isthen analyzed statically using the seismicforces as external loads.

• Method 3 – Modal Analysis

A mathematical model of the structure iscreated that represents the spatialdistribution of the mass and stiffness. Themode shapes—the period, shape factor, andparticipation factor—are determined. Thedesign response spectrum is established inaccordance with the standard. The baseshears and forces at each level of the

Figure 5. Coaxial cable is a major contributor of wind load on a tower. Wind computations are handled

in detail in Revision G.

Figure 6. Towers can be loaded with a very complex array of coaxial cable. Revision G addresses

several load scenarios.


structure for each mode are determined andthen combined by the square root of the sumof the squares of the modal values.

• Method 4 – Time History Analysis

A mathematical model of the structure iscreated that represents the spatial distributionof the mass and stiffness consideringstructural damping to be 5 percent of criticaldamping. Two orthogonal ground motiontime histories are selected from recordedevents with seismicity compatible to the site,or synthetic time histories are developed in accordance with accepted practice. A 5 percent horizontal response spectrum isconstructed, and the foregoing time historiesare combined according to rules specified inthe standard to develop two scaled, site-specific time histories. These two timehistories are used to perform a structuralanalysis. Load effects for design aredetermined by selecting the maximum valuesfrom the time history analysis.

Seismic Loads SummaryUnder the new standard, wirelesstelecommunications structures will typically beanalyzed using either Method 1 or 2.

The standard contains provisions for groundmotions, torsional moments, mathematical modelrequirements, and earthquake loads forstructures supported on buildings or othersupporting structures. A seismic approachconsidering the uniqueness of towers will now beincluded and considered as part of the design.

ICE LOADS

The inclusion of ice loading provisionsrepresents a significant change that brings the

standard into compliance with the currentstandards, codes, and guides.

Revision F Though it recognizes a consideration of radial solidice uniform thickness with a density of 56 lb/ft3, the standard does not specifically state an icerequirement. The standard does recognize that ice may be a significant load for structures to belocated in areas of significant ice accumulation andprovides information for consideration in an annex.

Revision G The design ice thickness specified in the newstandard is a uniform radial thickness of glaze ice at 33 feet above the ground in exposure

Category C for a 50-year mean recurrenceinterval. Escalation of ice thickness and wind onice over the height of the structure is required. Ice is assumed to be glaze ice with a density of 56 lb/ft3. Accumulation of ice is considered on thestructure, guys, and appurtenances. All elementsare assumed to be covered with a uniformthickness of ice that results in a wind drag.Design ice thickness is also escalated with heightand is based on regional climatic data. Forengineering design, all members are traditionallyassumed to be covered with a uniform thicknessof ice, which together with the ice density may beused to calculate the ice weight as well as thewind drag.

Ice Loads SummaryThe new standard provides a rational approachfor considering ice thickness in the tower andfoundation design.

EXISTING STRUCTURES

The standard includes provisions for thedesign of existing tower superstructures and

foundations.

Revision F The standard indicates that towers andsupporting structures should be analyzed whenchanges in the original design or operatingconditions take place. Recommended criteriaappear in an annex to the standard.

Revision G A section of the new standard addresses thestructural analysis of existing structures. Anothersection discusses the evaluation of structuresregardless of the standard used in the originaldesign. Existing structures are to be evaluated inthe following circumstances:

• A change in type, size, or number of appurtenances such as antennas,transmission lines, platforms, and ladders

• Any structural modification (except formaintenance) made to the structure

• A change in serviceability requirements

• A change in structure classification inaccordance with the categories identified in the standard

Existing Structures SummaryThe new standard indicates that existingstructures need not be re-evaluated for each

The standardincludes provisions

for the design ofexisting tower

superstructures and foundations.


The primaryresponsibility of a

registered licensedProfessional

Engineer is theprotection of public,life, safety, health,

welfare, andproperty.

revision of the standard except in thecircumstances listed above. Analyses andmodifications will definitely include modeling,analysis, design, and modification using theprovisions of the standard in computer modelsand calculations.

SOFTWARE

The commercially available software productstypically used for tower analysis, design,

and modification are Power Line Systems, Inc.’s TOWER® and PLS-POLE®, and RISATechnologies’ RISATower. Both companies’software packages are now standard computerapplications in accordance with engineeringdepartment procedures. Both companies areincluding the provisions for the new standard intheir software; this capability will be available inJanuary 2006.

TRAINING

Training in the following subjects is availablefor engineers who work in this area.

Customers may also wish to take advantage ofthis training.

• Telecommunication Tower Overview

The overview training is an orientation for allengineering personnel, regardless ofdiscipline, involved in wireless site projecttower activities. Practical background isprovided to enhance understanding of thedifferent types of towers and foundations,the codes and standards that apply, and thework associated with tower activities.

• ANSI/TIA-222-G-2005 Structural Standardfor Antenna Supporting Structures andAntennas

This offering focuses on the meaning and useof the Revision G standard.

• Telecommunication Tower StructuralAnalysis and Design

This course provides instruction forcivil/structural engineers in the use ofRevisions F and G of the ANSI/TIA-222standard. Training is provided in the use ofthe TOWER, PLS-POLE, and RISATowersoftware packages. Participants model anddesign a lattice steel guyed tower and a self-supporting, tapered tubular steel pole tower.The course provides the backgroundnecessary for an engineer to independentlyanalyze and design telecommunicationtowers. Upon successful completion of the

course and after suitable experience,participants will possess the skill set necessaryto assess, evaluate, analyze, and modifytowers using the ANSI/TIA-222 standard.

LICENSED PROFESSIONAL ENGINEER SEAL

The primary responsibility of a registeredlicensed Professional Engineer is the

protection of public, life, safety, health, welfare,and property. All tower work is to be carried outunder the direction of a licensed ProfessionalEngineer in the Civil and/or Structuresdisciplines in responsible charge as defined instate licensing laws, rules, and regulations. In amajority of states, all companies performingengineering must possess a Certificate ofAuthorization (COA) with an engineer inresponsible charge (ERC) designated on thecertificate. The COA is the state corporate licenseto practice engineering. In states that do notrequire a COA, all companies performingengineering must have an ERC with the appropriate Professional Engineeringregistration.

The drawings, calculations, and reports preparedby civil/structural engineers for owners aremanually wet-stamp sealed or manuallyembossed with a seal and manually signed anddated by a licensed Professional Engineerregistered in the state where the tower structureis located. States are also allowing electronicseals, signatures, and dates on engineeringdeliverables.

CONCLUSIONS

The comprehensive Revision G of Standard 222governing telecommunication tower

activities in the US will continue to guide thecreation of tower structures that successfullyperform their intended function. The standardaddresses both the tower superstructure and the foundation, as both are necessary for thesuccessful performance of a tower in service. The term “tower“ encompasses both thesuperstructure above the ground and thefoundation below the ground functioningtogether to support loads under all designconditions.

The standard has been prepared by individualsand companies who work and practice in thisarea, i.e., fabricators, erectors, consultants,contractors, and architect-engineers. Designs areexpected to be in line with those that result fromapplication of Revision F of the standard.


In summary, ANSI/TIA Standard 222 Revision Greflects contemporary engineering practice andincorporates the latest appropriate technicalmethods and academic information. It is awelcomed evolution that brings the science andart of tower design and analysis into conformancewith current provisions of the AISC, ACI, ASCE,IBC, and other national codes, standards, andpractices. �

TRADEMARKS

PLS-POLE and TOWER are registeredtrademarks of Power Line Systems, Inc.

REFERENCES

[1] American National Standards Institute(ANSI)/Telecommunications IndustryAssociation (TIA)/EIA-222-F, Structural Standardsfor Steel Antenna Towers and Antenna SupportingStructures, June 1996.

[2] ANSI/TIA-222-G, Structural Standard for AntennaSupporting Structures and Antennas, August 2005.

[3] Structural Engineering Institute (SEI)/AmericanSociety of Civil Engineers (ASCE) 7-02, MinimumDesign Loads for Buildings and Other Structures,2003.

[4] American Institute of Steel Construction (AISC),Manual of Steel Construction, Load and ResistanceFactor Design.

[5] American Concrete Institute (ACI) 318, BuildingCode Requirements for Structural Concrete.

[6] A.H.-S. Ang and W.H. Tang, Probability Conceptsin Engineering Planning and Design, Volume 1 –Basic Principles, John Wiley & Sons, Inc., 1975.

[7] C.G. Salmon and J.E. Johnson, Steel StructuresDesign and Behavior, Fourth Edition, HarperCollins, 1996.

[8] M.K.S. Madugula, “Dynamic Response of Lattice Towers and Guyed Masts,” ASCE, Reston, Virginia, 2002.

BIOGRAPHYPeter Moskal is chief engineer,civil, structures, and architecture,for Bechtel Telecommunications.He is responsible for the wire-less and wire line infrastructureengineering deliverables, thepeople who prepare thosedeliverables, the processesemployed, and the tools used toexecute the work on behalf of

Bechtel Telecommunications customers. He overseesthe work of civil engineers, structural engineers,architects, and CAD designers and drafters within theTelecommunications organization.

Peter implements defined standard work processes inaccordance with engineering department procedures

and selects and provides the software, design guides,specifications, training, and tools needed to perform the work. He also regularly supports businessdevelopment activities.

Peter is a voting member of ANSI/TIA-222-G-2005standard committee TR-14.7. He holds ProfessionalEngineer licenses in multiple states and serves as thedesignated engineer in responsible charge on a numberof corporate licenses to practice engineering.

Peter received a BS in Civil Engineering and an MS in Civil Engineering (Structures and Soil Mechanics)from the University of Pittsburgh and a BusinessManagement Certificate from Golden Gate University.

Krishnamurthy Raghu is anengineering supervisor inBechtel Telecommunicationscurrently responsible forsupporting Telecommunicationsstaff engineering and self-performance engineering inFrederick, Maryland. Acivil/structural engineer, hehas also worked in Bechtel’s

Power and Petroleum & Chemical business lines. He isexperienced in wireless tower engineering, sitedevelopment, and zoning as well as in the transmissionand distribution industry. He is a technical specialist intower and pole structures and telecommunicationsstandards.

Krishnamurthy is a member of ANSI/TIA-222-G-2005standard committee TR-14.7.

Krishnamurthy received a BE in Civil Engineering fromBangalore University, India, and an MS in CivilEngineering from Virginia Polytechnic Institute andState University (Virginia Tech).


INTRODUCTION

An excessive amount of fiber is beingdeployed in today’s fiber-to-the-home

(FTTH) networks. In many cases, more than halfthe fiber deployed may be unnecessary. Althoughthe excess fiber is increasing the cost of networks,this phenomenon is occurring largely unnoticedfor several reasons:

• Fiber is generally regarded as “cheap.”Little attention is paid to conserving fiber,since its cost is generally consideredinsignificant. While it is inexpensive, fiber isnot free.1 The cost of fiber in a network canreach hundreds of dollars per householdpassed. Significant savings can be realized ifexcess fiber is eliminated.

• There is a common belief that deployingextra fiber is “good.” It is considered less expensive to have extra fiber in place to meet future demands than to installadditional fiber later on. Indeed, there is some economy in deploying extra fiber for maintenance and foreseeable growth opportunities. A threshold exists, though,between extra fiber and excessive fiberdeployment. Installing fiber in excess of the foreseeable demand is an inefficient useof capital.

• It is difficult to determine when a networkdesign contains excessive fiber. Each designis a unique reflection of an engineer’sjudgment applied to the peculiarities of differing neighborhoods. No twoneighborhoods will yield the same design,nor will two engineers produce the samedesign for the same neighborhood.Identifying excessive fiber is a challengingtask and the impetus for this paper. Twometrics are developed in this paper that canbe simply applied to any network design toindicate the presence of excessive fiber.

BACKGROUND

Fiber is the principal component of any FTTHnetwork, as the name implies. Fiber from

homes is routed to an aggregation point in theneighborhood. At this aggregation point or hub,traffic from multiple fibers is combined ontofewer fibers for transport back to the centraloffice. The hub can combine traffic passively viaoptical splitters in a passive optical network(PON) or actively via an Ethernet switch in anactive Ethernet network. Within the central office,traffic is routed to the appropriate destination.Traffic flows in both directions: home to centraloffice (upstream) and central office to home(downstream). The hub not only aggregatesupstream traffic but also distributes downstream

REDUCING THE AMOUNT OF FIBER IN FIBER-TO-THE-HOME NETWORKS

Abstract—Reducing the amount of unnecessary fiber deployed in fiber-to-the-home networks can significantlyreduce capital costs. Two principal causes of fiber waste—excessive serving area size and poor hub location—are examined. Methods to detect and quantify excessive fiber usage attributable to these causes are developed.


Brian [email protected]

____________________________

1 While the price of fiber-optic cable can vary based onthe number of fibers, cable construction, and purchasecommitments, $0.01 per fiber-foot is a useful rule-of-thumb approximation in today’s market.

traffic from the central office across the multiplefibers to the homes (see Figure 1).

The design of FTTH networks typically occurs intwo phases: planning and engineering. Duringthe planning phase, homes are grouped intoserving areas. The serving area boundaries areestablished to contain an “ideal” number of

homes suited to the capacities of the availablehubs. Once the serving areas are defined, a hubsite is selected within each serving area. Hubs arebest located near the center of the serving areas,but safety, accessibility, and aesthetic concernsoften cause hubs to be located near the servingarea edges (see Figure 2).

The completed plan, with serving areaboundaries and hub locations established, is usedas a template for the engineering phase. Thenetwork design is completed by routing andsizing fiber cables that connect every householdto the hub location within each serving area. Onlyminor modifications of the plan, to compensatefor constructability issues, are considered duringthe engineering phase.

CAUSES OF EXCESSIVE FIBER DEPLOYMENT

Reviews of several FTTH network designsidentified two principal causes of excessive

fiber deployment:

• Overly large serving area

• Poor hub location

The following sections discuss serving area sizeand hub centricity (HC) and their effect on theamount of fiber deployed. Models and metrics are presented to detect and assess the impact ofthese causes.



FTTH fiber to the homeHC hub centricityPON passive optical networkPSTN public switched telephone

network

Two principalcauses of excessivefiber deployment:

•Overly large serving area

•Poor hub location

Figure 2. Hub Locations Within Serving Areas

Figure 1. FTTH Network Architecture

Serving Area SizeThe serving area size for each network design ischosen during the planning phase. Armed withguidelines and an approved list of hub sizes, theplanner uses “good judgment” to group anacceptable number of households into servingareas. Previous work has suggested that expectedsubscription rate (or take rate) should also beconsidered when establishing serving area size tomaximize resource utilization [1]. In general,resource utilization improves as serving area size

increases, lowering the cost per subscriber.However, there is a penalty to be paid forincreasing serving area size. A larger serving arearequires additional fiber per household to beinstalled. This principle is easily demonstratedand quantified with two simplistic examples.

Linear Serving AreaConsider a serving area composed of eighthouseholds arranged as shown in Figure 3.

Each household lot has frontage of x feet. Withthe hub placed in the center, four fibers wouldexit east and west and traverse one lot,connecting to drops to each of the fourhouseholds on either side of the hub. The totalfiber required is 8x fiber-feet or x fiber-feet per household.

Consider now a serving area of 24 households, asshown in Figure 4.

The dimensions are the same as before; each lothas a frontage of x feet. The hub is centered with12 households to either side. In this case, 12 fibersexit east and west from the hub and traverse 5 lots to connect all the households.2 The total fiber required for this serving area is 120x fiber-feet or 5x fiber-feet per household.

The previous two examples demonstrate thattripling the serving area size results in a fivefoldincrease in the fiber required per household. For

linear serving areas of any size n, the total fiberTFl required is:

(1)

The total fiber per household TFl /n is simply:

(2)

For large linear serving areas, total fiber perhousehold is directly proportional to the servingarea size—splitting the serving area in half halvesthe fiber cost per household.

The linear serving area model may be too severea characterization of the real world. Therefore, itmay be useful to investigate the other extreme—an overly generous characterization.

Ideal Mesh Serving AreaConsider a serving area composed of 16 households arranged in a square, as shown in Figure 5.

Each household lot is x by x feet. The hub islocated at the center of the serving area with eighthouseholds north and south of the hub. A maindistribution cable of eight fibers exits north andsouth of the hub and traverses one lot.Subtending distribution cables of four fibers

A larger serving arearequires additionalfiber per household

to be installed.


Figure 4. Linear Serving Area with 24 Households

Figure 3. Linear Serving Area with Eight Households

____________________________

2 The fiber count could be reduced by four at each droplocation. However, it is more expensive to splice smallercables at each taper point than to continue the original 12 fibers the entire length.

=n4– 1 x⎞⎟⎠

⎞⎟⎠TFl

n

≈ x4

n >>4TFln n

ONT ONT ONT ONT

ONT ONT ONT ONT

X

ONT ONT ONT ONT

ONT ONT ONT ONT

ONT

ONT ONT

ONT

12 Fiber Cable

ONT ONT ONT ONTONT ONT


12 Fiber Cable

X

=n2

4– n x⎞⎟⎠

⎞⎟⎠TFl


traverse one lot east and west of both the northand south main cable to connect all thehouseholds. The total fiber used is 32x fiber-feet(16x main cable + 16x subtending cable) or 2x fiber-feet per household.

Now consider a similar serving area of 144 households, as shown in Figure 6.

The lot dimensions are the same: x by x feet. Thehub is located at the center. The main distributioncable is now 72 fibers and traverses 5 lots. There are 12 subtending cables of 12 fibers, each traversing 5 lots. The total fiber is 1,440x fiber-feet or 10x fiber-feet per household.

In general, the total fiber TFim required for anideal mesh serving area is:

(3)

Figure 5. Ideal Mesh Serving Area with 16 Households

X

XONT ONT ONT ONT

ONT ONT ONT ONT

ONT ONT ONT ONT

ONT ONT ONT ONT

Figure 6. Ideal Mesh Serving Area with 144 Households

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

HUB

72 Fi

ber C

able

72 Fi

ber C

able

X

X

























= nTFim n – 2 x⎞⎟⎠⎞⎟⎠


The total fiber per household TFim/n is:

(4)

For large ideal mesh serving areas, total fiber perhousehold is proportional to the square root ofthe serving area size. Splitting a serving area in half reduces total fiber per household by 30 percent.

Optimum Serving Area SizeBoth models indicate a direct relationshipbetween the size of the serving area and the totalfiber per household. The expectation is for realworld behavior to be bounded by the predictionsof the two models. However, the models revealanother important relationship. The total fiber perhousehold and the geographic density—the xfactor—are directly related. While the x factor isvery specific to geographic locations, some usefulgeneralizations can be made. In more ruralenvironments with households separated bygreater distances, the total fiber per household isgreater than in urban environments. Figure 7plots the relative total fiber per household versusserving area size for rural and urbanenvironments.

To determine the optimum serving area size for aspecific region, the fiber cost per subscriberF$ /sub of a specific design must be calculated:

(5)

Where:

TF$ = total fiber cost

n = number of households in serving area

τ = expected take rate

Using the models, the fiber cost per subscribercan now be extrapolated for any serving area size,as shown in Figure 8.

An equipment cost per subscriber plotted againstserving area size can be derived from equipmentcost and calculated utilization (see Figure 9).

≈ n >>4TFimn nx

=TFimn n – 2 x⎞⎟⎠

⎞⎟⎠

Figure 7. Total Fiber per Household Versus Serving Area Size (Rural and Urban)

Figure 8. Fiber Cost per Subscriber Versus Serving Area Size

Figure 9. Equipment Cost per Subscriber Versus Serving Area Size

32 232 432 632 832 1032

Series 2 Rural Linear Series 3 Rural MeshSeries 4 Urban Linear Series 5 Urban Mesh

32 232 432 632 832 1032Serving Area Size

Fiber

Cos

t per

Sub

scrib

er

Linear Mesh

53243233223213232Serving Area Size

Equi

pmen

t Cos

t per

Sub

scrib

er

=F$

subTF$

n1τ•


The optimal serving area size is obtained byfinding the minimum total cost—fiber cost persubscriber + equipment cost per subscriber (seeFigure 10).

Prior to calculating optimum serving area size,excess fiber associated with HC must beidentified and removed.

Hub CentricityFTTH network planning, as described previously,is a two-step process: establish the service areaboundaries, then locate the hub. There is ageneral perception that hub location has littleimpact on cost. As such, little effort is expendedon selecting the least cost location. Locations atthe edge of the serving area—considered more“convenient”—are often chosen. However,serving areas with hubs located away from thecenter require additional fiber strands to beengineered to gain full connectivity. Tounderstand the impact, the earlier models arerevisited.

Linear Serving AreaFrom Figure 4, with the hub centered in the linearserving area, the total fiber required was 120x fiber-feet or 5x fiber-feet per household. Ifthe hub is moved to the far right, as shown inFigure 11, additional fiber is required.

In this case, 20 fibers must now exit to the west ofthe hub and traverse 10 lots. The total fiberrequired is 200x fiber-feet or 8.3x fiber-feet perhousehold—a 66 percent increase.

Ideal Mesh Serving AreaFrom Figure 6, with the hub centered in the idealmesh serving area, the total fiber required was1,440x fiber-feet or 10x fiber-feet per household. Ifthe hub moved to the extreme south end of theserving area, as shown in Figure 12, additionalfiber is required.

In this case, 120 fibers exit the hub to the north asthe main distribution cable and traverse 10 lots.The subtending distribution cables remain thesame. The total fiber required is 1,920x fiber-feetor 13.33x fiber-feet per household—a 33 percentincrease.

Hub Centricity MetricTo quantify the degree to which the hub iscentered, an HC metric is created. HC is definedas the ratio of the total number of fibers exitingthe hub less the greatest number of fibers exitingthe hub in one direction to the greatest number offibers exiting in one direction.

In the Figure 4 example, HC = (24 – 12)/12 = 1.0.



In the Figure 12 example, HC = (144 – 120)/120 =0.17.

An HC metric of 1 (or higher) indicates that thehub is well-centered within the serving area. AnHC metric below 1 indicates that the hub is notFigure 10. Total Cost per Subscriber Versus Serving Area Size

Common Equipment + Fiber Cost per Subscriber vs. Serving Area Size

Serving Area Size

Com

mon

Equi

pmen

t + Fi

ber C

ost p

er S

ubsc

riber

Equipment + Fiber Linear ModelEquipment + Fiber Mesh Model

32 132 232 332 432 532

Optimal Serving Area Size

ONT ONT ONT ONT

ONT ONT ONT ONT

ONT

ONT ONT

ONT



X

Figure 11. Linear Serving Area with 24 Households with Hub Off-Center


A very low HC metricindicates a

significant amountof excess fiber—over50 percent in some

instances.

centered; the lower the metric, the more the hubis offset from center.

While the two models predict an increase in totalfiber per household, it is difficult to predict howmuch excess fiber is present in a design with thegiven HC metric. To understand the behavior ofthe metric, 50 actual designs were reviewed. Themetric was calculated for each design. Where thehub was not centered, a better, more central, hublocation was sought. The fiber required wasrecalculated for each design with improved hublocation. The difference in fiber requirementsbetween the two designs was classified as excessfiber and is plotted against the HC metric of theoriginal design (see Figure 13).

The plot suggests a good correlation between theHC metric and the amount of excess fiber. A verylow HC metric indicates a significant amount of

excess fiber—over 50 percent in some instances.The decision to find a more suitable hub locationor alter serving area boundaries to center the hub

Figure 12. Ideal Mesh Serving Area with 144 Households with Hub Off-Center

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

12 Fiber Cable

HUB

120

Fibe

r Cab

le12

0 Fi

ber C

able

XX

























Figure 13. Excess Fiber Versus Hub Centricity

Centricity

Was

te

1.61.41.21.00.80.60.40.20.0

0.6

0.5

0.4

0.3

0.2

0.1

0.0


should be made after evaluating the potentialsavings to be realized. Reworking a networkdesign in an urban area with an HC of 0.6 and afiber cost of $30 per household may not bewarranted to realize a potential savings of $0 to $3per household. In contrast, a design with an HCof 0.1 and a fiber cost of $200 per household withthe potential to recover $100 per household mostcertainly warrants rework.

CONCLUSIONS

The two primary causes of excessive fiberdeployment—creation of too large a serving

area and poor hub centering—originate in theplanning phase.

Fiber costs of today’s FTTH networks can behundreds of dollars per household. Identifyingand removing excessive fiber can save tens tohundreds of dollars per household.

The amount of fiber per household is directlyproportional to the size of the serving area.Reducing a serving area by half reduces fiber perhousehold by 30 to 50 percent. Equipmentutilization is also affected by serving area size.Fiber cost and equipment cost should be jointlyconsidered to arrive at an optimum serving areasize.

The network requires the least amount of fiberwhen the hub is centered in the serving area.When the hub is not centered, excess fiber isrequired. An HC metric has been developed toidentify how well the hub is centered within theserving area and to estimate the fiber impact. Apoorly centered hub may double the fiber perhousehold requirements. With an understandingof the cost impact, an informed decision can bemade on possible adjustment of hub location orserving area boundaries. �

REFERENCES

[1] B. Perkins, “Optimal Splitter Placement in PONs,”Bechtel Telecommunications Technical Journal, Vol. 2,No. 2, September 2004, pp. 49–52.

BIOGRAPHYBrian Perkins joined BechtelTelecommunications in 2004with more than 15 years of experience in the tele-communications industry. Assenior director of technology—optical networking, heevaluates new products andtechnologies and establishesbest practices for designing and

implementing fiber-optic networks in support ofBechtel’s global projects.

Before joining Bechtel, Brian worked for QuantumBridge Communications, where he was heavilyinvolved in the deployment of PONs. Before QuantumBridge, he led a networks integration department at Lucent Technologies. Brian began histelecommunications career with AT&T BellLaboratories. He has an MS and a BS in ElectricalEngineering from Worcester Polytechnic Institute.

The two primarycauses of excessivefiber deployment—

creation of too largea serving area and poor hub

centering—originate in theplanning phase.


INTRODUCTION

Antenna azimuth and downtilt are twoimportant optimization parameters in

universal mobile telecommunications system(UMTS) networks. Optimization of these twoparameters can significantly improve systemperformance. However, new networks sometimesuse inefficient optimization techniques andimplement default values. Furthermore, incon-sistencies in setting these parameters duringinstallation vary the network coverage andcapacity. This paper presents the results of aquantitative study that investigated the effect ofthese parameters on UMTS network performance.

Many techniques are used to measure antennaazimuth and tilt during installation. The accuracyin setting up the azimuth and tilt depends on the antenna installation processes and humanand instrumentation errors. Inefficient imple-mentation and rigging processes may also causeazimuth or tilt errors. The overall accuracy iswithin ±10 degrees using most traditionaltechniques. Usually, antenna azimuth errors areindependent for antennas belonging to differentsectors. New processes and instruments mayreduce these errors by several degrees, reducerandomness in antenna orientations, and bringerrors consistently within the set tolerance.

This paper investigates the effects of azimuth andtilt inaccuracies on network coverage andperformance and considers the three main UMTSnetwork system quality parameters: service

coverage, the ratio of chip energy to interference(Ec/Io), and soft handoff areas. Two exercises are defined. A variety of errors are introduced for all antennas, and a simulation is performedfor each case. At the end, the results are compared and analyzed. Consistent use of thenew antenna installation processes is promoted tolimit the impact of inconsistencies. Suggestionsare also provided on acceptable installation error limits for use as a baseline to developimplementation processes.

ANTENNA AZIMUTH AND TILT SETTINGS ANDINCONSISTENCIES

Antenna azimuth and tilt errors (Figure 1) arerandomly distributed among the sites and

sectors. For the purpose of this paper, azimutherror is measured as the absolute differencebetween the actual azimuth installed in the fieldand the designed azimuth, as illustrated in Figure 1a. In this definition, all azimuth errors are positive. Tilt errors can be positive ornegative—uptilt errors are considered negative,while downtilt errors are considered positive, asshown in Figure 1b.

An antenna installation technician sets up theazimuth using a compass and alignment tool. Onthe top of the tower, the technician can useseveral mechanisms to install the antenna.However, the technician’s capabilities arerestricted by uncomfortable climbing status,limited time, limited available tools, and

THE IMPACTS OF ANTENNA AZIMUTH AND TILT INSTALLATION ACCURACY ON UMTS NETWORK PERFORMANCE

Abstract—Inconsistencies in setting up antenna azimuth and tilt during installation may reduce overallnetwork performance. However, the degree of quality degradation depends on the amount of the discrepancybetween the designed and installed parameters. The paper investigates the effect of these errors on UMTS RF KPIs, including coverage, signal quality (Ec /Io), and soft-handoff areas. Two examples are studied that include real measurement data. The studies show the effect of azimuth and tilt installation inaccuracies onUMTS network quality.


Esmael Dinan, PhD [email protected]

Aleksey A. [email protected]

environmental factors. An example of aninstallation mechanism using landmarks and anoptical alignment tool is shown in Figure 2. Thisfigure shows two pre-specified landmarks for thetechnician to use from the top of the tower. In this example, the respective angles between the antenna aim point and Landmarks A and Bare set to 40 degrees (counterclockwise) and –25 degrees (clockwise) from aim point to target.Once the alignment is set, antenna tilt is adjustedusing a mechanical tilt bracket. Antenna tilt errorsare caused by imperfect vertical adjustment of theantenna support structure.



Ec/Io ratio of chip energy to interference

GPS global positioning systemKPI key performance indicatorQoS quality of serviceRF radio frequencyRSCP received signal code powerUMTS universal mobile

telecommunications system

Designed Tilt

NegativeErrorPositive

Error

Field Azimuth

Positive Error

Designed Azimuth

Figure 1. Antenna Azimuth and Tilt Errors(a) Azimuth Error; (b) Tilt Error

True North

Optical Alignment Tool

Antenna Support Structure

Target A

Antenna Aim Point

50° Actual Bearing

40° Offset Angle

-25° Offset Angle

90° Specified AntennaAzimuth

115° Actual Bearing

Target B

Figure 2. Example of an Antenna Azimuth Setup and Installation

(a) (b)

The accuracy insetting up the

azimuth and tiltdepends on the antennainstallation

processes andhuman and

instrumentationerrors.

Using the Six Sigma process improvementmethodology, Bechtel initiated a task force tomeasure antenna installation accuracies [1]. Theimplementation team analyzed the data relatedto repeatability and reproducibility of differentantenna azimuth adjustment mechanisms. Theresults demonstrated up to 10 degrees of error insimple global positioning system (GPS)-basedadjustment methods. More advanced mecha-nisms can provide accuracies within 5 degreeswith 95 percent probability of confidence.

Figure 3 illustrates another element used in thestudy that is the subject of this paper: thecorrelation of errors between sectors of the samesite. Scenario A illustrates the traditionaltechnique of pointing antennas individually,leading to independent error in each sector. This paper proposes using a technique that offers a consistent error or the same error forantennas belonging to the same site. In thistechnique, shown in Scenario B, the azimuths ofthe second and third antennas are adjustedrelative to the azimuth of the first-installed

antenna. This paper shows that this scenario,offered by recent installation techniques,provides better network performance than thetraditional method.

SIMULATION MODEL AND ASSUMPTIONS

This paper examines two example networkclusters—one with 20 sites and one with

42 sites—that were simulated using planning andoptimization tools. These clusters are shown inFigure 4. The simulation results help to analyzethe effect of azimuth and tilt settings on someaspects of network performance. The followingtasks were included in the study:

• Select cluster areas, antenna types, defaultsite configuration, and system parameters

• Develop simulation scenarios, objectives,and plans

• Develop project setup in the planning andoptimization tools and configure all theparameters


Error = ∝Error = γ

Error = β

Field Azimuth

Designed Azimuth

Error = ∝Error = ∝

Error = ∝

Field Azimuth

Designed Azimuth

Figure 3. Correlation of Errors Between Sectors of the Same SiteScenario A – Traditional Azimuth Setting; Scenario B – Proposed Azimuth Setting

Figure 4. Cluster Area Elevation Map(a) 20 UMTS Sites – Traffic and Coverage Relevant Area: 17.17 km2

(b) 42 UMTS Sites – Traffic and Coverage Relevant Area: 26.14 km2

Scenario A Scenario B

(a) (b)


• Optimize all antenna azimuths and tilts usingrecursive optimization algorithms (Thisdesign will be considered to be the baselinedesign.)

• Execute the simulation and record thestatistics for the above scenarios and errorparameters

• Analyze the data and compile the finalgraphs

A standard default site configuration wasconsidered. Cell sites included in the test clusterhad the following configuration parameters:

• Antenna radiation center heights in therange of 20 to 25 meters

• Node B transmission power = 20 watts

• Pilot power = 2 watts

• Traffic load = 50 percent, uniform distribution

• Total antenna feeder loss = 3 dB

• Frequency = 2,150 MHz (downlink)

Two example projects were created in theplanning and optimization tools using the aboveconfiguration parameters. Other UMTS systemparameters were set to default values. In thebaseline design, antenna azimuth and tiltconfigurations were optimized for maximumoverall performance of the test cluster. Therefore,changes in these parameters would result inreduced network performance. Antenna azimuthand tilt were optimized using an automatedrecursive optimization tool (Radioplan GmbH’sWireless Network System [WiNeS]). The toolprediction parameters and path loss matrix weretuned using drive test data. For the baselinedesign, a simulation was performed, includingcoverage, interference, and soft handoff analysis.

In the next step, a series of simulations wereperformed to investigate the effect of azimuthand tilt errors on network performance. For bothScenarios A and B, a variety of errors wereintroduced for all the antennas. These errors wererandomly distributed among the cells. For eacherror set, the simulation was executed repeatedlyuntil a steady, consistent result was achieved.Then the performance statistics, includingcoverage, interference, and soft handoff area were calculated and compared. Performancestatistics were recorded and then analyzed toproduce the final graphs.

The exercises described above were performedmultiple times, each using a different antennatype. The results help provide an understandingof the effect of antenna types on the performancegraphs and conclusions. Overall behavior isconsistent with antennas having the samehorizontal and vertical beamwidth. UMTSnetwork performance sensitivity to azimuth andtilt error increases as beamwidth is reduced. Therelationship between error type and beamwidthis as follows:

• Horizontal beamwidth ↔ Azimuth error

• Vertical beamwidth ↔ Tilt error

Simulation results presented in this paper wereperformed with antennas that have 65-degreehorizontal beamwidth and 7-degree verticalbeamwidth, which is considered to be a typicalantenna type in most UMTS networks.

SIMULATION RESULTS

Simulation results are presented in Figures 5, 6,and 7. Figure 5 considers a simple single site

UMTS networkperformancesensitivity to

azimuth and tilterror increases as beamwidth

is reduced.

0 5 10 15 20 25 3096

96.5

97

97.5

98

98.5

99

99.5

100Single Site Coverage Versus Antenna Azimuth Error

Average Antenna Azimuth Error

Norm

alize

d Co

vera

ge A

rea

RSCP < – 86 dBm

– 3 – 2 – 1 0 1 2 370

75

80

85

90

95

100

105Single Site Coverage Versus Antenna Tilt Error

Average Antenna Tilt Error

Norm

alize

d Co

vera

ge A

rea

RSCP < – 86 dBm

Figure 5. Network Performance Versus Antenna Azimuth and Tilt Installation Error in a Single-Site Configuration(a) Azimuth Error; (b) Tilt Error

(a) (b)


0 5 10 15 20 25 300

1

2

3

4

5

6

7Coverage Gap Versus Antenna Azimuth Error


Incr

ease

in C

over

age G

ap (P

erce

ntag

e)RSCP < –86 dBm, ARSCP < –86 dBm, BRSCP < –92 dBm, ARSCP < –92 dBm, B

–3 –2 –1 0 1 2 3–2

0

2

4

6

8

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12

14

16Coverage Gap Versus Antenna Tilt Error


Incr

ease

in C

over

age G

ap (P

erce

ntag

e)

RSCP < –86 dBmRSCP < –92 dBm

0 5 10 15 20 25 300

0.5

1

1.5

2

2.5

3

3.5

4

4.5QoS Gap Versus Antenna Azimuth Error


Incr

ease

in S

ervic

e Qua

lity G

ap (P

erce

ntag

e)

Ec/Io < –12 dB, AEc/Io < –12 dB, BEc/Io < –13 dB, AEc/Io < –13 dB, B

–3 –2 –1 0 1 2 3–0.5

0

0.5

1

1.5

2

2.5QoS Gap Versus Antenna Tilt Error


Incr

ease

in S

ervic

e Qua

lity G

ap (P

erce

ntag

e)Ec/Io < –12 dBEc/Io < –13 dB

0 5 10 15 20 25 300

1

2

3

4

5

6

7

8

9

10Soft Handoff Area Versus Antenna Azimuth Error


Incr

ease

in S

oft H

ando

ff Ar

ea (P

erce

ntag

e)

SHO Margin = 5 dB, ASHO Margin = 5 dB, BSHO Margin = 3 dB, ASHO Margin = 3 dB, B

–3 –2 –1 0 1 2 3–6

–5

–4

–3

–2

–1

0

1Soft Handoff Area Versus Antenna Tilt Error


Incr

ease

in S

oft H

ando

ff Ar

ea (P

erce

ntag

e)

SHO Margin = 3 dBSHO Margin = 5 dB

(a) Area with RSCP < –86 dBm = 12.32%, Area with RSCP < –92 dBm = 4.80%

(b) Area with Ec /Io < –12 dB = 4.0%, Area with Ec /Io < –13 dB = 1.01%

(c) Soft Handoff Area = 28.05% (Soft Handoff Margin = 5 dB), Soft Handoff Area = 17.58% (Soft Handoff Margin = 3 dB)

Figure 6. Performance Graphs for 42-Site Cluster


(a) Area with RSCP < –86 dBm = 5.76%, Area with RSCP < –92 dBm = 2.0%

(b) Area with Ec /Io < –12 dB = 4.42%, Area with Ec /Io < –13 dB = 0.94%

(c) Soft Handoff Area = 36.34% (Soft Handoff Margin = 5 dB), Soft Handoff Area = 23.0% (Soft Handoff Margin = 3 dB)

SHO Margin = 3 dBSHO Margin = 5 dB

–3 –2 –1 0 1 2 3–6

–5

–4

–3

–2

–1

0

1Soft Handoff Area Versus Antenna Tilt Error


Incr

ease

in S

oft H

ando

ff Ar

ea (P

erce

ntag

e)

0 5 10 15 20 25 300

0.5

1

1.5

2

2.5

3

3.5

4

4.5QoS Gap Versus Antenna Azimuth Error


Incr

ease

in S

ervic

e Qua

lity G

ap (P

erce

ntag

e)

Ec/Io < –12 dB, AEc/Io < –12 dB, BEc/Io < –13 dB, AEc/Io < –13 dB, B

–3 –2 –1 0 1 2 30

0.5

1

1.5

2

2.5

3

3.5

4QoS Gap Versus Antenna Tilt Error


Incr

ease

in S

ervic

e Qua

lity G

ap (P

erce

ntag

e) Ec/Io < –12 dBEc/Io < –13 dB

0 5 10 15 20 25 30

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2Coverage Gap Versus Antenna Azimuth Error


Incr

ease

in C

over

age G

ap (P

erce

ntag

e)

RSCP < –86 dBm, ARSCP < –86 dBm, BRSCP < –92 dBm, ARSCP < –92 dBm, B

–3 –2 –1 0 1 2 3–1

0

1

2

3

4

5

6

7Coverage Gap Versus Antenna Tilt Error


Incr

ease

in C

over

age G

ap (P

erce

ntag

e)

RSCP < –86 dBmRSCP < –92 dBm

–

0 5 10 15 20 25 300

1

2

3

4

5

6

7

8Soft Handoff Area Versus Antenna Azimuth Error


Incr

ease

in S

oft H

ando

ff Ar

ea (P

erce

ntag

e)

SHO Margin = 5 dB, ASHO Margin = 5 dB, BSHO Margin = 3 dB, ASHO Margin = 3 dB, B

Figure 7. Performance Graphs for 20-Site Cluster


Azimuth error in the range of 6 to 8 degrees

is tolerable,depending on

the installationscenario and initial

coverage area.

configuration to provide an initial reference resultfor comparison purposes. In this example nointerference or inter-cell soft handoff areas exist;only coverage plots are shown. Only Scenario Awas considered because in Scenario B all the site’santennas were rotated with the same azimutherror; therefore, overall coverage performancedid not change.

As illustrated in Figure 5a, coverage shrinkswhen azimuth error increases. Coverage isreduced by 4 percent when there is a 30-degreeerror in azimuth setting. The coverage area has analmost inverse linear relationship with azimutherror. Figure 5b shows that coverage is also verysensitive to downtilt errors. Coverage changes upto 29 percent when downtilt error varies in therange of –3 to +3 degrees. This example shows asystem with no interference and inter-cell softhandoff coverage. To study and capture realnetwork performance behavior, multiple sites are needed.

Figure 6 shows the results for a 20-site cluster,and Figure 7 shows the results for a 42-sitecluster. These provide realistic examples inperformance graphs.

Azimuth errors in the range of 0 to 30 degreeswere considered for both Scenarios A and B. Tilterrors varied between –3 and +3 degrees. Theareas are represented as the percentage of thecluster area. The performance graphs arecategorized by coverage area, coverage quality,and soft handoff area.

Coverage AreaCoverage area is measured in reference toreceived signal code power (RSCP). Twodefinitions were considered for coverage gap: thearea with less than –86 dBm RSCP and the areawith less than –92 dBm RSCP. Figures 6a and 7ashow the variations in coverage gaps when thereare inconsistencies in antenna azimuth and tiltsettings. A higher coverage percentage and fewer coverage gaps is desirable whenimplementing a UMTS network.

Coverage QualityQuality of service (QoS) or coverage quality ismeasured by Ec/Io. Two definitions wereconsidered for QoS gap: the area with Ec/Io lessthan –12 dB and the area with Ec/Io less than –13 dB. Figures 6b and 7b show the variations inareas with QoS gaps when there are incon-sistencies in antenna azimuth and tilt settings. Ahigher QoS and fewer QoS gaps is desirable whenimplementing a UMTS network.

Soft Handoff AreaSoft handoff area is defined as the area coveredby more than one sector belonging to differentNode Bs. Two different settings were consideredfor soft handoff threshold. Performance graphsare shown for soft handoff areas when the softhandoff margin is 3 dB and 5 dB. Figures 6c and7c show the variations in soft handoff areas whenthere are inconsistencies in antenna azimuth andtilt settings. It is desirable to achieve the targetsoft handoff area recommended by the serviceoperator when implementing a UMTS network.A smaller soft handoff area results in increasedcall drop rate, and a higher soft handoff arearesults in inefficient use of radio resources andexcessive interference.

Careful investigation of the results of the graphsin Figures 6 and 7 leads to the followingconclusions:

• Antenna Azimuth: Network performancevariations depend on antenna azimuth errorvariations and the installation process.Overall degradation in Scenario B is 40 to 60percent less than in Scenario A. Therefore,the same error in all sectors is preferable.Azimuth error in the range of 6 to 8 degreesis tolerable, depending on the installationscenario and initial coverage area.Performance degrades noticeably if the erroris greater than 10 degrees. Soft handoff areas are the least sensitive to azimuth error. The coverage gap is 30 percent greater with 30 degrees of error in antennaazimuth. A comparison of the coveragegraphs in Figures 6 and 7 shows that when the coverage/quality gap is smaller, itssensitivity to error is higher.

• Antenna Tilt: Both coverage and qualityperformances are very sensitive to antennatilt variations. There is up to a 100 percentincrease in coverage and quality gaps with ±3 degrees of tilt error. Soft handoff areas arethe least sensitive to tilt error. The graphs inFigures 6c and 7c show less than a 10 percentvariation in soft handoff area with ±3 degreesof tilt error.

SUMMARY AND CONCLUSIONS

Both the 20- and 42-site examples produceconsistent network performance behavior

and lead to the same conclusions. If equal errorsare introduced to cell site sectors, there is lessnetwork performance degradation (Scenario A),compared with random errors (Scenario B). For


practical purposes, azimuth error in the range of 6 to 8 degrees is tolerable for networkperformance. Performance degradation isnoticeable if the azimuth error is greater than 10 degrees. Network performance is almost tentimes more sensitive to antenna tilt variations,compared with azimuth variations. Bothcoverage and quality gaps increase by up to 100 percent with ±3 degrees of tilt error.

If possible, only one antenna should be orientedand the other antenna azimuths set in reference to that one (Scenario B). However, rooftop sizeand configuration may interfere with thisrecommendation. If Scenario B installationtechniques can be applied to the site, simplermethods (instead of the more expensive methods)have the same effect on network performance.

Considering these conclusions, the followingUMTS network implementation standard can bepractically recommended for antenna azimuthand tilt tolerances:

1. For the Scenario A technique: Azimuthsetting tolerance of ±6 degrees

2. For the Scenario B technique: Azimuthsetting tolerance of ±8 degrees

3. For both scenarios: Tilt setting tolerance of±0.5 degrees

The cluster with more sites experiences lessnetwork quality degradation due to azimuth andtilt errors. However, this could be a subject forfurther studies. �

ACKNOWLEDGMENTS

The authors would like to thank Lacy Kiserfrom the Bechtel Six Sigma Team and Jeff

Bryson from the Bechtel Construction Team forthe valuable data and information they provided.Special thanks go to Radioplan GmbH forproviding WiNeS software for this study.

REFERENCES

[1] Six Sigma PIP TI-81, Report and Data Analysis,Bechtel Telecommunications, 2005.

[2] E. Dinan, “UMTS RF Network OptimizationProcess,” Document Number 3DP-T04G-50009,Bechtel Telecommunications Network PlanningDepartment, 2005.

BIOGRAPHIES

Esmael Dinan, a senior RF technologist with BechtelTelecommunications, hasbeen instrumental in manyaspects of the business unit’sresearch activities and the Cingular RF engineeringproject. He has designedand engineered an RF

engineering data management system, developedCingular project RF engineering processes andprocedures, designed UMTS networks, andverified and tested Dupont cryogenic TMAperformance.

Before joining Bechtel in 2002, Dr. Dinan was product manager for the GMPLS controlplane of the RAYStar DWDM optical switch at Movaz Networks, and lead network architect at MCI. He has conducted research and development on access methods andperformance modeling of 3G wireless commu-nications and high-speed optical networks.

Dr. Dinan received his PhD in ElectricalEngineering from George Mason University,Fairfax, Virginia, and is a registered ProfessionalEngineer in Maryland. He has authored morethan 25 conference papers and journal articlesand has filed a patent on a novel signalingmechanism developed for 3G cellular networks.He is a member of the Institute of Electrical andElectronics Engineers.

Aleksey Kurochkin iscurrently senior director,Site Development andEngineering, in the BechtelT e l e c o m m u n i c a t i o n sTechnology group, a groupthat he originated. He isexperienced in internationalt e l e c o m m u n i c a t i o n s

business management and network imple-mentation. Before joining Bechtel, he worked at Hughes Network Systems, where he built an efficient multi-product team focused on RF planning and system engineering. Hisengineering and marketing background hasgiven him both theoretical and hands-onknowledge of most wireless technologies.

Aleksey has an MSEE/CS degree in AutomaticTelecommunications from Moscow TechnicalUniversity of Communications and Informatics,Russia.

Both coverage and quality gaps

increase by up to 100 percent

with ±3 degrees of tilt error. Tilt setting

tolerance of ±0.5 degrees

is recommended.


INTRODUCTION

Emerging advances in wireless communicationtechnology and its application and the ever-

increasing demand for the availability of wirelesshigh-bit-rate devices have resulted in numeroushigh-tech innovations. Recently, phased arrayantennas have been proposed to reduce the time and cost of providing high-bit-rate wireless fidelity (Wi-Fi™) to indoor and outdoorwireless users.

This paper examines the application, coverage,and security of 2.4 GHz IEEE 802.11b (Wi-Fi)phased array antennas. Two of these antennas areinstalled in real-world scenarios on the roof ofBechtel Park Building 5 (BP5) in Frederick,Maryland. One antenna covers Bechtel ParkBuilding 2 (BP2) (outdoor to indoor) and thebusiness area behind it (outdoor to outdoor), andthe other antenna covers a large outdoorresidential area (outdoor to outdoor).Throughput is measured using a variety oftypical application methods.

This paper describes the theory, test architecture,and setup of these 2.4 GHz Wi-Fi phased array antennas; provides extensive experimentalresults and a technical interpretation of these results; summarizes the advantages anddisadvantages of using this technology; andprovides practical suggestions for its use.

THEORY

Information capacity I, which limits the amountof information that can be sent between a single

transmitter and a single receiver, is provided byClaude Shannon in the following formula [1]:

In the above formula, S is the received signalpower and N is the noise power, and theinformation is measured in bits per second perhertz of transmission bandwidth available. Dueto the maximum power limitation set by theFederal Communications Commission (FCC) onan already crowded unlicensed spectrum,Shannon’s formula does not seem to be useful inincreasing the information capacity. However,multi-antenna arrays or smart antennas havebeen introduced to increase the informationcapacity.

The simplest multi-antenna array—the “steeredbeam” or “phased” array—consists of manyindividual antennas that each transmit the samesignal using different phase shifts, which arearranged so that different signals interfereconstructively in one direction and destructivelyin every other direction. Transmitting andreceiving antenna phase shifts can be steeredelectronically.

Beam steering increases the signal directedtoward an intended receiver and reduces thereception of stray signals intended for othertargets. The receiver perceives these stray signalsas noise. Overall, the relative power gainobtained using an array with m antennas isroughly a factor of m [2].

Glenn A. Torshizi [email protected]

2.4 GHz Wi-FiTM PHASED ARRAYANTENNA EVALUATION

Abstract—This paper examines the application, coverage, and security of a 2.4 GHz IEEE 802.11b (Wi-FiTM)phased array antenna system. Two antennas are installed in real-world scenarios. One antenna providesoutdoor-to-indoor and outdoor-to-outdoor coverage, and the other antenna provides outdoor-to-outdoor coverageonly. Throughput is measured using a variety of typical application methods. The results show both theadvantages and disadvantages of using a phased array antenna system.


I = log (1 + S/N) bits/s/Hz

TEST SETUP AND PROCEDURES

The phased array antenna used in thisevaluation is a three-channel unlicensed (FCC

Part 15) wireless device. It allows point-to-pointpacket communication to client devices throughan integrated high gain, electronically steeredtransmitting and receiving antenna. The antennais configured in a 100-degree pattern horizontallyand a 12-degree pattern vertically. The antennapattern is divided into 13 focused areas. Channelassignments and other settings for each area canbe changed to optimize overall Wi-Fi operationwithin the full antenna pattern. The antenna isapproximately 43 by 36 by 7 inches and weighs80 pounds. Figure 1 shows the placement of two

antennas on the roof of BP5. BP2, a typical three-story office building approximately 70 by 210 feetand 45 feet high, is positioned directly in front ofand 600 feet from the first antenna, which isinstalled on top of BP5 at a height ofapproximately 55 feet above ground level (AGL)with no downtilt. See Figure 2 for a photographdepicting a view of BP2 from Antenna 1 on BP5.

The tools used to measure the radio frequency(RF) environment and signal quality includedAirMagnet™ for iPaq, AirMagnet laptop, CiscoAironet™ 350 personal computer (PC) card, andAntenex® YE240015 antenna.

The antenna uses a Wi-Fi switch that supports aninfrastructure basic service set (BSS) mode. Thenetwork consists of one access point connected tothe wired network infrastructure and a set ofwireless end stations. The independent basicservice set (IBSS), in which wireless stationsintercommunicate directly without using anaccess point or connection to a wired network, isnot supported.

Wireless wall software [3] was used to providesystem security, since wired equivalent privacy(WEP) is susceptible to wireless interception and fraud [4, 5]. This software manages theauthentication process using an IEEE 802.11framework and software-specific extensions toprevent session hijacking or denial of serviceattacks at any point. The software adheres to Advanced Encryption Standard (AES) [6] to protect sessions and networks. AES is a Federal Information Processing Standard (FIPSPublication 197) that specifies a cryptographicalgorithm to be used by US governmentorganizations to protect sensitive information.The wireless wall software operates at Layer 2 ofthe networking stack, providing the highest levelof protection against radio-based attacks.



AES Advanced Encryption Standard

AGL above ground level

BSS basic service set

EIRP effective isotropic radiated power

FCC Federal CommunicationsCommission

FIPS Federal Information ProcessingStandard

IBSS independent BSS

LOS line of sight

PC personal computer

RF radio frequency

WEP wired equivalent privacy

Wi-FiTM wireless fidelity

WLAN wireless local area network

Figure 1. Placement of Antennas on BP5 Roof on Bechtel Campus in Frederick, Maryland

Figure 2. View of BP2 from Antenna 1 on BP5

Beam steeringincreases the signaldirected toward anintended receiverand reduces the

reception of straysignals intended for

other targets.


The test setup goal was to map antenna range andcoverage in open outdoor environments andinside BP2. The effective isotropic radiated power(EIRP) level was 41 dBm.

EXPERIMENTAL RESULTS AND ANALYSIS

Signal, noise, speed, and throughput weremeasured at various locations inside, just

outside of, and on the roof of BP2. Similarmeasurements were also taken at locations off of the Bechtel campus.

Outdoor-to-Indoor MeasurementsData was collected from 11 locations on the thirdand second floors of BP2 and from 10 locations onthe first floor of BP2. Figure 3 shows the variationof the signal level as a function of the relativeposition of the measurement points with respectto the antenna. For each building floor, themeasurement points were divided into threegroups:

• Group A: Points in the areas directly facing the antenna

• Group B: Points in the middle of the building

• Group C: Points deeper inside the building

The measurements for each group were averagedover the nodes in that group and then comparedto the averages of the other groups for each floor.The noise measurements show an approximatelyflat noise level that averages –95 dBm. Speed andthroughput measurements are summarized inFigures 4 and 5, respectively.

The results show that good indoor coverage existsat locations near the windows facing Antenna 1on the first, second, and third floors; however, thesignal degrades significantly away from thewindows toward the inside of the building.

Outdoor-to-Outdoor MeasurementsFigure 6 shows the outdoor measurements forAntenna 1 around BP2. Signal strength wasmeasured at several points in the front and backand on the roof of BP2. The measurements showthat there is a good signal just outside of BP2 onthe ground facing Antenna 1 and on the roof ofBP2. However, the signal did not completelypenetrate through the building, resulting in littleor no coverage immediately behind BP2 at groundlevel. At roof level, if the signal was blocked bythe metal skirt around the air conditioningsystem, the signal degraded noticeably.Otherwise, the signal level was the same or better.

Figure 3. Signal Level Measurements

Figure 4. Speed Measurements

Figure 5. Throughput Measurements

Figure 6. Outdoor Measurements for Antenna 1 Around BP2

The noisemeasurements

show anapproximately flat noise level that averages

–95 dBm.

Outdoor measurements for Antennas 1 and 2were taken at several locations off of the Bechtelcampus. Figure 7 shows the outdoor-to-outdoormeasurement points from Antenna 1, and Figure 8 shows the outdoor-to-outdoor points forAntenna 2.

The test locations were chosen so that line of sight (LOS) or near LOS could be maintained. To ensure that LOS existed, point-to-pointprofiles were evaluated and examined for allmeasurement points. Figure 9 is an example ofone of these profiles.

Figure 10 shows the measurement results from points B, C, and G. The following are worth noting:

• At other locations away from Antennas 1 and 2, the signal was very weak. At severallocations, the signal level was adequate, but connectivity to the test laptop could not be established using just the wirelesslocal area network (WLAN) adapter. Whenan Antenex antenna was used, connectivitywas established.

• Many IEEE 802.11 applications are alreadybeing used around the test locations.

• The Antenex antenna greatly enhanced theperformance in low signal strength areas.

• In marginal areas, the AES algorithmdegraded performance beyond use.


Figure 8. Test Locations G, H, I, J, K, L, and M from Antenna 2 at Location A

Figure 7. Test Locations B, C, D, E, and F from Antenna 1 at Location A


CONCLUSIONS

This paper examined the use of phased arrayantennas for outdoor-to-indoor and outdoor-

to-outdoor applications using extensivemeasurements at several indoor and outdoorpoints. As the experimental results reveal, using the system has both advantages anddisadvantages; these are mainly applicationspecific. The system is conveniently easy and fastto install, and it appears to be the only choice forapplications such as a marina. For the outdoor-to-indoor scenarios, it is a good choice for narrowmultistory buildings. The signal degrades towardthe inside of wide buildings (such as BP2). Use of repeaters is recommended to enhance thesignal in deep building areas that are not covered.Phased array antennas are also a good choice foroutdoor-to-outdoor scenarios where LOS can bemaintained; however, since the frequency isunlicensed, interference is an issue over anextended range. When security is enhanced usingthe AES algorithm, there is also a tradeoffbetween the level of security in the system and throughput. �

TRADEMARKS

AirMagnet is a trademark of AirMagnet, Inc.

Aironet is a trademark of Aironet WirelessCommunications, Inc.

Antenex is a registered trademark of Antenex, Inc.

Wi-Fi is a trademark of the Wireless EthernetCompatibility Alliance, Inc.

REFERENCES

[1] C.E. Shannon, “A Mathematical Theory ofCommunication,” Bell System Technical Journal,Vol. 27, pp. 379–423 and pp. 623–656, 1948.

[2] S.H. Simon, A.L. Moustakas, M. Stoytchev, andH. Safar, “Communication in a DisorderedWorld,” Physics Today online, 2001.

[3] Cranite Systems, Inc., “Best Practices: WirelessLAN Design, Implementation and Management,”White Paper, September 2003.

[4] W.A. Arbaugh, N. Shankar, Y.C.J. Wan, and Z. Kan, “Your 802.11 Wireless Network Has No Clothes,” IEEE Transactions on WirelessCommunications, December 2002, Vol. 9, No. 6, pp. 44–51.

[5] T. Karygiannis and L. Owens, “Wireless NetworkSecurity: 802.11, Bluetooth and HandheldDevices,” NIST Special Publication 800-48,November 2002.

[6] D.J. Welch and S.D. Lathrop, “A Survey of802.11a Wireless Security Threats and SecurityMechanisms,” Army G6 Technical Report ITOC-TR-2003-101, 2003.

Phased arrayantennas are a good choice for

outdoor-to-outdoorscenarios where

LOS can bemaintained.

Figure 10. Measurement Results from Points B, C, and G

Figure 9. Point-to-Point Profile from Antenna 2 at Location A to Test Location J


BIOGRAPHY

Glenn Torshizi joined Bechtelin 2001 and is currently a staffscientist/engineer at BechtelTelecommunications’ Training,Demonstration, and Research(TDR) Laboratory in Frederick,Maryland. Before relocating toFrederick, he spent more than3 years working as an RF designengineer on the Bechtel AWS

GSM, GPRS, and UMTS Program in Philadelphia,Pennsylvania, and as the market RF lead in Harrisburg,Pennsylvania, and Hackensack, New Jersey.

Before joining Bechtel, Glenn was involved in planning,optimizing, and integrating the Triton PCS TDMAsystem in Norfolk, Virginia, and the CricketCommunications CDMA system in Pittsburgh,Pennsylvania. As a technical expert witness onnumerous planning and zoning boards, he was verysuccessful in obtaining final site approvals.

Glenn has a BS in Physics from SouthwesternOklahoma State University and an MS in Physics fromthe University of Tennessee, Knoxville. He has doneresearch in relativistic heavy ion physics at Oak RidgeNational Laboratory and Brookhaven National Laboratory.

Bechtel Telecommunications, a unit of BechtelCorporation, provides turnkey deployment services

that include network planning, RF design, engineering,and project and construction management for thedeployment of wireless, wireline, and othertelecommunication facilities worldwide. We areheadquartered near Washington, DC, and have majoroffices in London and Sydney, and numerous projectoffices worldwide.

Bechtel has performed telecommunications work forover 35 years. We have successfully completed morethan 140 major telecommunications projects worldwide,including more than 85,000 wireless sites; 23,000kilometers of wireline fiber; and communication centerssuch as POPs, NOCs, and data centers.

Bechtel Telecommunications is a global leader inapplying proven project and construction managementtools to geographically and technically complexnetwork builds. We deliver the essential elements ofproject success while setting records for speed,reliability, quality, and cost control.

BECHTEL TELECOMMUNICATIONS

Telecommunications Leadership

TIMOTHY D. STATTONExecutive Vice President, Bechtel Corporation and President, Bechtel Telecommunications

ROBERT CASAMENTOPrincipal Vice President and General Manager

LARRY ALBEE Principal Vice President and Manager of Functional Operations

JAMES A. IVANYPrincipal Vice President and Chief Financial Officer

J. S. (JAKE) MACLEODPrincipal Vice President and Chief Technology Officer

LEE LUSHBAUGH Principal Vice President and General Manager, Americas

RICK ASTLEFORDPrincipal Vice President andGeneral Manager, Europe, Africa, Middle East, Asia, and Asia-Pacific


Bechtel Corporation is a privately heldcompany headquartered in San Francisco,

California, with approximately 40,000 employees,more than 40 offices worldwide, and 2004revenues of approximately $17.4 billion. Foundedin 1898, Bechtel has been under the leadership ofits founding family for four generations. Riley P.Bechtel, great-grandson of the founder, is thecurrent chairman and chief executive officer.Bechtel is one of the world's premier engineering,construction, and project management companies.

• The Financial Times recently listed Bechtelas the “World’s Most Respected Company”in the property and construction industries.

• Global Finance magazine named Bechtel the “World’s Best Company” in theconstruction sector for 5 years in a row.

• Engineering News-Record ranked Bechtel asNumber 1 on the list of the “Top 400Contractors” for 7 consecutive years.

• Engineering News-Record also rankedBechtel Number 1 TelecommunicationsContractor from 2002 to 2004.

Because of our unparalleled industry recognitionand financial stability, we are the partner of choicefor network deployment.

Bechtel Corporation

TECHNOLOGY EVALUATION

Key to Bechtel's deployment success is ourtechnology expertise. To provide our

clients with complete turnkey deploymentservices, we offer expert technology researchand testing services. We have assembled afull-service Technology Group of leadingprofessionals that provides our customerswith neutral technology services andoversees our in-house telecommunicationslaboratories: the Bechtel Wireless Test Bed(BWTB) and the Training, Demonstration and Research (TDR) Laboratory.

The BWTB, an over-the-air wireless test bed,enables us to analyze network products andtechnologies for our clients in a real-worldenvironment. It operates 24/7 and can bedynamically configured to meet specifictesting requirements without sacrificing theclient’s network.

The TDR Laboratory offers our clients the ability to integrate equipment frommultiple vendors to identify interoperabilityissues, monitor product performance, andensure compatibility with legacy systems,thereby reducing the risk of imple-menting new technologies and paving the way for investment in network upgrades or expansion.

88

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Bechtel TelecommunicationsTechnical Journal

v Foreword

vii Editorial

1 Telecommunications Research and Development in the United States: A State of Crisis?

13 IP Multimedia Subsystems (IMS): A Standardized Approach to All-IP Converged Networks

37 A Survey of MEMS-Enabled Optical Devices – Applications and Drivers for Deployment

45 PHY/MAC Cross-Layer Issues in Mobile WiMAX(Invited Paper)

57 ANSI/TIA Standard 222 – Structural Standard forAntenna Supporting Structures and Antennas: A Comparison of Revisions F and G

65 Reducing the Amount of Fiber in Fiber-to-the-Home Networks

73 The Impacts of Antenna Azimuth and Tilt Installation Accuracyon UMTS Network Performance

81 2.4 GHz Wi-FiTM Phased Array Antenna Evaluation

Timothy D. Statton and Jake MacLeod

S. Rasoul Safavian, PhD

Brian Coombe

Jungnam Yun, PhD(POSDATA America R&D Center) and

Prof. Mohsen Kavehrad, PhD(The Pennsylvania State University [CITCTR])

Peter Moskal and Krishnamurthy Raghu

Brian Perkins

Esmael Dinan, PhD, and Aleksey A. Kurochkin

Glenn A. Torshizi

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An Independent Analysis of Current Operational Issues JJ aa nn uu aa rr yy 22 00 00 66

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