GSM

From Wikipedia, the free encyclopedia
For other uses, see GSM (disambiguation).

The GSM logo is used to identify compatible handsets and equipment. The dots symbolize three clients in the home network and one roaming client.[1]

GSM (Global System for Mobile Communications, originally Groupe Spécial Mobile), is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe protocols for second-generation (2G) digital cellular networks used by mobile phones, first deployed in Finland in July 1992.[2] As of 2014 it has become the default global standard for mobile communications – with over 90% market share, operating in over 219 countries and territories.[3]

2G networks developed as a replacement for first generation (1G) analog cellular networks, and the GSM standard originally described a digital, circuit-switched network optimized for full duplex voice telephony. This expanded over time to include data communications, first by circuit-switched transport, then by packet data transport via GPRS (General Packet Radio Services) and EDGE (Enhanced Data rates for GSM Evolution or EGPRS).

Subsequently, the 3GPP developed third-generation (3G) UMTS standards followed by fourth-generation (4G) LTE Advanced standards, which do not form part of the ETSI GSM standard.

“GSM” is a trademark owned by the GSM Association. It may also refer to the (initially) most common voice codec used, Full Rate.

History

In 1982, work began to develop a European standard for digital cellular voice telephony when the European Conference of Postal and Telecommunications Administrations (CEPT) created the Groupe Spécial Mobile committee and later provided a permanent technical support group based in Paris. Five years later, in 1987, 15 representatives from 13 European countries signed a memorandum of understanding in Copenhagen to develop and deploy a common cellular telephone system across Europe, and EU rules were passed to make GSM a mandatory standard.[4] The decision to develop a continental standard eventually resulted in a unified, open, standard-based network which was larger than that in the United States.[5][6][7][8]

In 1987 Europe produced the very first agreed GSM Technical Specification in February. Ministers from the four big EU countries cemented their political support for GSM with the Bonn Declaration on Global Information Networks in May and the GSM MoU was tabled for signature in September. The MoU drew-in mobile operators from across Europe to pledge to invest in new GSM networks to an ambitious common date. It got GSM up and running fast.

In this short 37-week period the whole of Europe (countries and industries) had been brought behind GSM in a rare unity and speed guided by four public officials Armin Silberhorn (Germany), Stephen Temple (UK), Philippe Dupuis (France), and Renzo Failli (Italy).[9] In 1989, the Groupe Spécial Mobile committee was transferred from CEPT to the European Telecommunications Standards Institute (ETSI).[6][7][7][8]

In parallel, France and Germany signed a joint development agreement in 1984 and were joined by Italy and the UK in 1986. In 1986 the European Commission proposed reserving the 900 MHz spectrum band for GSM. The world’s first GSM call was made by the former Finnish prime minister Harri Holkeri to Kaarina Suonio (mayor in city of Tampere) on July 1, 1991, on a network built by Telenokia and Siemens and operated by Radiolinja.[10] The following year in 1992, the first short messaging service (SMS or “text message”) message was sent and Vodafone UK and Telecom Finland signed the first international roaming agreement.

Work began in 1991 to expand the GSM standard to the 1800 MHz frequency band and the first 1800 MHz network became operational in the UK by 1993. Also that year, Telecom Australia became the first network operator to deploy a GSM network outside Europe and the first practical hand-held GSM mobile phone became available.

In 1995, fax, data and SMS messaging services were launched commercially, the first 1900 MHz GSM network became operational in the United States and GSM subscribers worldwide exceeded 10 million. Also this year, the GSM Association was formed. Pre-paid GSM SIM cards were launched in 1996 and worldwide GSM subscribers passed 100 million in 1998.[7]

In 2000, the first commercial GPRS services were launched and the first GPRS compatible handsets became available for sale. In 2001 the first UMTS (W-CDMA) network was launched, a 3G technology that is not part of GSM. Worldwide GSM subscribers exceeded 500 million. In 2002 the first Multimedia Messaging Service (MMS) were introduced and the first GSM network in the 800 MHz frequency band became operational. EDGE services first became operational in a network in 2003 and the number of worldwide GSM subscribers exceeded 1 billion in 2004.[7]

By 2005, GSM networks accounted for more than 75% of the worldwide cellular network market, serving 1.5 billion subscribers. In 2005 the first HSDPA capable network also became operational. The first HSUPA network was launched in 2007. High-Speed Packet Access (HSPA) and its uplink and downlink versions are 3G technologies, not part of GSM. Worldwide GSM subscribers exceeded three billion in 2008.[7]

The GSM Association estimated in 2010 that technologies defined in the GSM standard serve 80% of the global mobile market, encompassing more than 5 billion people across more than 212 countries and territories, making GSM the most ubiquitous of the many standards for cellular networks.[11]

It is important to note that GSM is a second-generation (2G) standard employing Time-Division Multiple-Access (TDMA) spectrum-sharing, issued by the European Telecommunications Standards Institute (ETSI). The GSM standard does not include the 3G UMTSCDMA-based technology nor the 4G LTE OFDMA-based technology standards issued by the 3GPP.[12]

Macau planned to phase out its 2G GSM networks as of June 4, 2015, making it the first region to decommission a GSM network.[13] Singapore will also be phasing out 2G services by April 2017.

Technical details

The structure of a GSM network
Main article: GSM services

Network structure

The network is structured into a number of discrete sections:

Base station subsystem

GSM cell site antennas in the Deutsches Museum, Munich, Germany

GSM is a cellular network, which means that cell phones connect to it by searching for cells in the immediate vicinity. There are five different cell sizes in a GSM network—macro, micro, pico, femto, and umbrella cells. The coverage area of each cell varies according to the implementation environment. Macro cells can be regarded as cells where the base stationantenna is installed on a mast or a building above average rooftop level. Micro cells are cells whose antenna height is under average rooftop level; they are typically used in urban areas. Picocells are small cells whose coverage diameter is a few dozen metres; they are mainly used indoors. Femtocells are cells designed for use in residential or small business environments and connect to the service provider’s network via a broadband internet connection. Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in coverage between those cells.

Cell horizontal radius varies depending on antenna height, antenna gain, and propagation conditions from a couple of hundred meters to several tens of kilometres. The longest distance the GSM specification supports in practical use is 35 kilometres (22 mi). There are also several implementations of the concept of an extended cell,[14] where the cell radius could be double or even more, depending on the antenna system, the type of terrain, and the timing advance.

Indoor coverage is also supported by GSM and may be achieved by using an indoor picocell base station, or an indoor repeater with distributed indoor antennas fed through power splitters, to deliver the radio signals from an antenna outdoors to the separate indoor distributed antenna system. These are typically deployed when significant call capacity is needed indoors, like in shopping centers or airports. However, this is not a prerequisite, since indoor coverage is also provided by in-building penetration of the radio signals from any nearby cell.

GSM carrier frequencies

Main article: GSM frequency bands

GSM networks operate in a number of different carrier frequency ranges (separated into GSM frequency ranges for 2G and UMTS frequency bands for 3G), with most 2G GSM networks operating in the 900 MHz or 1800 MHz bands. Where these bands were already allocated, the 850 MHz and 1900 MHz bands were used instead (for example in Canada and the United States). In rare cases the 400 and 450 MHz frequency bands are assigned in some countries because they were previously used for first-generation systems.

Most 3G networks in Europe operate in the 2100 MHz frequency band. For more information on worldwide GSM frequency usage, see GSM frequency bands.

Regardless of the frequency selected by an operator, it is divided into timeslots for individual phones. This allows eight full-rate or sixteen half-rate speech channels per radio frequency. These eight radio timeslots (or burst periods) are grouped into a TDMA frame. Half-rate channels use alternate frames in the same timeslot. The channel data rate for all 8 channels is 270.833 kbit/s, and the frame duration is 4.615 ms.

The transmission power in the handset is limited to a maximum of 2 watts in GSM 850/900 and 1 watt in GSM 1800/1900.

Voice codecs

GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between 6.5 and 13 kbit/s. Originally, two codecs, named after the types of data channel they were allocated, were used, called Half Rate (6.5 kbit/s) and Full Rate (13 kbit/s). These used a system based on linear predictive coding (LPC). In addition to being efficient with bitrates, these codecs also made it easier to identify more important parts of the audio, allowing the air interface layer to prioritize and better protect these parts of the signal.

As GSM was further enhanced in 1997[15] with the Enhanced Full Rate (EFR) codec, a 12.2 kbit/s codec that uses a full-rate channel. Finally, with the development of UMTS, EFR was refactored into a variable-rate codec called AMR-Narrowband, which is high quality and robust against interference when used on full-rate channels, or less robust but still relatively high quality when used in good radio conditions on half-rate channel.

Subscriber Identity Module (SIM)

One of the key features of GSM is the Subscriber Identity Module, commonly known as a SIM card. The SIM is a detachable smart card containing the user’s subscription information and phone book. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking.

Phone locking

Main article: SIM lock

Sometimes mobile network operators restrict handsets that they sell for use with their own network. This is called locking and is implemented by a software feature of the phone. A subscriber may usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or use software and websites to unlock the handset themselves.

In some countries (e.g., Bangladesh, Belgium, Brazil, Chile, Germany, Hong Kong, India, Iran, Lebanon, Malaysia, Nepal, Pakistan, Singapore, South Africa) all phones are sold unlocked.[16]

GSM Security

GSM was intended to be a secure wireless system. It has considered the user authentication using a pre-shared key and challenge-response, and over-the-air encryption. However, GSM is vulnerable to different class of attacks, each of them aiming a different part of the network.[17]

The development of UMTS introduces an optional Universal Subscriber Identity Module (USIM), that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user, whereas GSM only authenticates the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation.

GSM uses several cryptographic algorithms for security. The A5/1, A5/2, and A5/3stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was developed first and is a stronger algorithm used within Europe and the United States; A5/2 is weaker and used in other countries. Serious weaknesses have been found in both algorithms: it is possible to break A5/2 in real-time with a ciphertext-only attack, and in January 2007, The Hacker’s Choice started the A5/1 cracking project with plans to use FPGAs that allow A5/1 to be broken with a rainbow table attack.[18] The system supports multiple algorithms so operators may replace that cipher with a stronger one.

Since 2000, different efforts have been done in order to crack the A5 encryption algorithms. Both A5/1 and A5/2 algorithms are broken, and their cryptanalysis has been considered in the literature. As an example, Karsten Nohl developed a number of rainbow tables (static values which reduce the time needed to carry out an attack) and have found new sources for known plaintext attacks.[19] He said that it is possible to build “a full GSM interceptor…from open-source components” but that they had not done so because of legal concerns.[20] Nohl claimed that he was able to intercept voice and text conversations by impersonating another user to listen to voicemail, make calls, or send text messages using a seven-year-old Motorola cellphone and decryption software available for free online.[21]

New attacks have been observed that take advantage of poor security implementations, architecture, and development for smartphone applications. Some wiretapping and eavesdropping techniques hijack the audio input and output providing an opportunity for a third party to listen in to the conversation.[22]

GSM uses General Packet Radio Service (GPRS) for data transmissions like browsing the web. The most commonly deployed GPRS ciphers were publicly broken in 2011.[23]

The researchers revealed flaws in the commonly used GEA/1 and GEA/2 ciphers and published the open-source “gprsdecode” software for sniffing GPRS networks. They also noted that some carriers do not encrypt the data (i.e., using GEA/0) in order to detect the use of traffic or protocols they do not like (e.g., Skype), leaving customers unprotected. GEA/3 seems to remain relatively hard to break and is said to be in use on some more modern networks. If used with USIM to prevent connections to fake base stations and downgrade attacks, users will be protected in the medium term, though migration to 128-bit GEA/4 is still recommended.

Standards information

The GSM systems and services are described in a set of standards governed by ETSI, where a full list is maintained.[24]

GSM open-source software

Several open-source software projects exist that provide certain GSM features:

Creator of Cells

eland.[2] He was married and had three children.

Radio telephony and the cell phone

DynaTAC 8000X;1983, $3,995. 13″ tall, weighed 30 oz.[9][14] First commercial portable cell phone. Dubbed the “Boot,” later, a slimmer version was called the “Brick.” DynaTAC=Dynamic Adaptive Total Area Coverage. -First Patent Shown Here:[15] -Mitchell & DynaTAC, 04/03/73 [16]

Mitchell became Motorola’s chief engineer for its mobile communication products in 1960. Prior to the development of the cell phone, Mitchell and his team of engineers produced and marketed the first transistorized pager[3][17] and obtained a patent for the concept of portable cell telephony, including small antennae used to help free mobile phone units from car trunks where they were typically installed.[18]

Mitchell,[4][10][11] Motorola’s chief of portable communication products and Martin Cooper’s[10][11] boss in 1973, played a key role in advancing the development of handheld mobile telephone equipment. Mitchell successfully pushed Motorola to develop wireless communication products that would be small enough to use anywhere and participated in the design of the cellular phone.[19][20] (See Wiki History of Mobile Phones)

Other initiatives

In 1983, Mitchell was appointed U.S. President Ronald Reagan‘s National Security Telecommunications Advisory Committee.[2] Mitchell was a senior member of the IEEE, Institute of Electrical and Electronics Engineers;[3] also served a chairman of the Electronic Industries Alliance; and a recognized expert on world trade,[21] serving as director of the National Association of Manufacturers;[4] and an expert on federal fiscal policy.[22] Mitchell was a director of Bell & Howell Company; trustee Engineering Advisory Council, Marquette University;[5] and active participant in the Easter Seals(US) Campaign;[6] and member of the Presidents Council of the American Lung Association.[7][21] Mitchell was trustee at Illinois Institute of Technology (IIT); the Dublin City University, Ireland; and the University of Limerick Foundation. Mitchell was one of the architects, which also included Chuck Feeney,[8] of the huge expansion of the University of Limerick during the 1990s and 2000s through his work[23] on the University of Limerick foundation.[24][25][26] As a philanthropist and member of IIT’s Philip Danforth Armour Society, [9] Mitchell established endowed scholarship funds for the Camras Program,[10] the Leadership Academy, and electrical engineering students. To date, the John F. Mitchell Scholarship Funds have supported more than 70 students at the university.[18][21]

Awards

Mitchell was awarded Honorary Degrees from the Illinois Institute of Technology (IIT) in Humane Letters & Science, 1995;[27] an honorary doctorate of business administration from Iowa Wesleyan College, on May 18, 1985.;[28][29] and from Dublin City University, Ireland (Ollscoil Chathair Bhaile Átha Cliath), Ireland, on October 25, 1996.[30] Recipient of the 2003 Chicago Innovation Award.[31] IIT Alumni Association award for Professional Achievement,1985 [11]. Alumni Medal 1994, IIT Alumni Association. IIT Alumni Award for Lifetime Achievement Award (posthumous) 2010.[8][32]

Publications

U.S. Patent 2,833,994, July 2, 1954 for High Frequency Long-Line Variably End-Loaded with Clarence P. Pipes.

U.S.Patent 2,912,573, November 10, 1959 for Receiver having frequency and amplitude modulation detecting.

U.S. Patent 3,087,117, April 23, 1963 for Portable Transmitter Apparatus with Selective.

U.S. Patent 2,975,274, March 14, 1961 for Frequency Modulation Radio Receiver.

U.S. Patent 3,126,514,October 13, 1961 for Noise Reducing system with Jack Germain. Germain retired as Director of Quality Assurance.[33]

U.S. Patent 3,906,166, September 16, 1975 for a Radio Telephone System. the cell phone.[34] Martin Cooper, Richard W. Dronsurth, Albert J. Leitich, Charles N. Lynk,[35] James J. Mikulski,[36][37][38] John F. Mitchell, Roy A. Richardson, and John H. Sangster. NOTA BENE: Two names were botched in the original filing; Albert Leitich’s surname was erroneously omitted, and included herein, and the first name of Mikulski was omitted, but included herein. The original document was refiled by Motorola’s legal staff, but has not yet been identified. The seeds of the idea for a portable cell phone can be traced to James J. Mikulski, which were rejected by Mitchell for lack of sufficient business justifications. It is rumored that when John Mitchell suddenly recognized during an attempted phone call that his 400MHz phone had inherent limitations, he immediately reversed his previous decision and championed the portable cell phone concept.[38][39]

U.S. Patent 5,650,776, July 22, 1997 for a Communications Receiver with Thomas F. Holmes

References

  1. University of Limerick Foundation Memorial for John Francis Mitchell – Trustee (a): (b): “During his 45-year career, Mitchell shaped the creation of nearly all of the wireless communications industries in the latter half of the 20th century.” (c) “Blackhawks Water Polo Team,” (d): “lifeguarding,” (e): “shaped the creation of nearly all of the wireless communications industries” (f): his wife of 68 years
  2. John Francis Mitchell Obituary: (a): “During his 45-year career, Mitchell shaped the creation of nearly all of the wireless communications industries in the latter half of the 20th century.” (b): (e): the birth of the cell phone industry – “the cell phone industry was born” (c): Appointed by Reagan to National Security Telecommunications Advisory Committee by Jennifer Erikson, Motorola Spokesperson, date=June 20–21, 2009, Chicago Tribune
  3. John Francis Mitchell Retirement-(a):”the son of Irish Immigrants,” (b):”retiring in 1998 as Vice Chairman,” (c): “-John F. Mitchell charted the development of the revolutionary first generation of cellphones, popular for their forward-looking technology but unaffectionately nicknamed “boot” for their size and heft, and later with the trimmed down version – referred to as “the brick”,” (d):”-son of an Irish born Chicago Patrolman.” (e):”John Francis Mitchell chief engineer“, (f): “One early hit was the first transistorized pager,” (g):”Mitchell pushed hard.” (h): “John Francis Mitchell delayed his retirement to raise another $1.59 billion for Iridium.” (i): “Iridium was a “triumph of technology over business,” said Howard Anderson, Professor MIT” (j): “Mitchell was pleased to watch Iridium…..go on to perform as advertised in high stress situations…war zones….natural disasters.” by Stephen Miller, June 20–21, 2009, The Wall Street Journal
  4. John F. Mitchell biography
  5. The John Mitchell Quality Tester. Chicago Tribune. June 14, 2009.
  6. John F. Mitchell, Longtime Motorola Leader: (a): (h):”Do it the engineering way, the proper way” (b): “Mitchell was very intelligent..creative..original..just a very good guy.” (c): “John F. Mitchell had a reputation of being frugal,” (d): “It was not about him or his perks, but rather the Team, ” (e): “Colleagues stood in awe of his brilliance and his stand up management style.” (f): Summary of Motorola Career. (g): John F. Mitchell, Longtime Motorola Leader – “You couldn’t put one over on John,” (i): John F. Mitchell, Longtime Motorola Leader, “kept everyone on their toes.” by Sandra Guy, Chicago Sun-Times, July 2, 2009
  7. Tennant, Geoff (2001). SIX SIGMA: SPC and TQM in Manufacturing and Services – Quality Returns to America. Gower Publishing, Ltd. p. 6. ISBN 0566083744. Quality Returns to America
  8. Commencement Awards & Notes Illinois Institute of Technology IIT, dated 1995, – (a): “led the evolution of the first generation of cell phones, a field that Motorola would dominate for years.” (b): Awards for John F. Mitchell. (c): Posthumous Award 2010.(d): “led the evolution of the first generation of cell phones, a field that Motorola would dominate for years” (e): the first cell phone was affectionately known as the Brick
  9. Time Magazine Memorial John F. Mitchell by Frances Romero, dated July 6, 2009, Time Magazine, (a):-60% of the world’s population would use mobile phones just a quarter-century after his (Mitchell’s) brainchild in 1983. (b): (c): (d): (i): “comments on early life in last paragraph – rough and tumble, erector set”. (e): “technical specs of DynaTAC”. (f): vision, “virtually anyone”. (g): “60% of the world” – unprecedented success of the cell phone, $20 billion in sales. (h): “Motorola at the forefront of…revolution”.
  10. The top 15 Giants in Telephony
  11. Who Invented the Cell Phone?
  12. Wikipedia: History of Cell Phone
  13. William & Bridie Mitchell, parents of John F. Mitchell
  14. “Details on the original DynaTAC”
  15. Patent for the First Cell Phone System (Radio Telephone System) Announced April 3, 1973.
  16. From Brick to Slick; John F. Mitchell in NYC on April 3, 1973 at Patent Office on announcement of the DynaTAC cell phone , by Howard Wolinsky, Chicago Sun-Times, April 3, 2003
  17. Cell Phones, From Brick to Slick: (a): “invented…marketed the first transistorized pager,” (b): large 45 pound affairs…hard wired…lots of trunk space, (c): (d): (e): . by Howard Wolinksky, date=April 3, 2003, Chicago Sun-Times-pages 57-61
  18. “J.F.Mitchell Biography”. IIT Illinois Institute of Technology, numerous references herein; e: Mitchell granted an honorary doctorate of Humane letters & Science . 2010.
  19. “Motorola Executive Helped spur Cellphone Revolution, Oversaw Ill-fated Iridium Project”. The Wall Street Journal, 20–21 June 2009, p. A10.
  20. “John F. Mitchell, 1928–2009: Was president of Motorola from 1980 to ’95, Chicago Tribune, June 17, 2009, retrieved June 17, 2009”. Chicagotribune.com. Retrieved 29 July 2009.
  21. Iowa Wesleyan College – Bio of John F. Mitchell
  22. “Iowa Weslyan College Commencement Address”. May 18, 1985 – Mitchell known as an expert on ‘fiscal policy; Purple & White, Iowa Wesleyan College. Check date values in: |date= (help)
  23. “Jimmy Deenihan MP, Kerry”. Chicago Tribune a:Comments on Mitchell’s Philanthropic Work with University of Limerick Foundation, b:helping the Irish people. June 16, 2009.
  24. http://www.ulfoundation.com/news/john-mitchell-1928-2009/
  25. http://www.brophy.net/PivotX/images/univoflimericksearchjfm.jpg Search ULFoundation John Mitchell
  26. “John Francis Mitchell Obituary”. Chicago Tribune-The expansion of the Universality of Limerick. June 19, 2009.
  27. “IIT Commencement Award notes”. IIT Mitchell granted an honorary doctorate of Humane letters & Science. 1995.
  28. “IWC Commencement Award”. May 18, 1985, – Mitchell granted an honorary doctorate of business administration. Purple & White, Iowa Wesleyan College. Check date values in: |date= (help)
  29. John Francis Mitchell, Doctor of Business Administration, IWC, May 18, 1983
  30. “John Francis Mitchell Honorary Graduate Dublin City University (Ollscoil Chathair Bhaile Átha Cliath)”.
  31. John Francis Mitchell, Ton Kobrinetz, Marty Cooper, Chicago Sun-Times-Innovation Awards
  32. insert link on IIT site of vips and notables; create new category for business leaders in wiki; also link back to Bob Galvin for Six Sigma}
  33. Jack Germain & Art Sundry Key Movers in Creation of Motorola’s Six Sigma CultureSix Sigma at Motorola by Robert Knight, senior rewrite editor, City News Bureau, Chicago, January 1995, IPO Issue 29 (Illinois Periodicals Online)
  34. {{citation correction: need to come back to correct wiki Radiotelephone to include Mitchell}} (the cell phone){correct in wiki Mobile phone to include Mitchell}
  35. Letter to Middle Schooler, granddaughter of Chuck Lynk, co-inventor of cell phone, by James J. Mikulski, co-inventor of first cell phone April 3, 1973
  36. Comments by Albert (Jim) Mikulski, co-inventor of first cell phone, June 6, 2009, Chicago Tribune (a):“Mitchell known as a hands on manager” (b): (c): (e): (f): (g): “willing to give credit to those who worked in the trenches.” (c): (d): “I remember his delegating his task as…GM to work in the Applied Research Lab and in give and take with the engineers as the Federal Trade Commission (FTC) docket 18262 that would shape Motorola’s future…in the 1970s.” (h): Mitchell team member, (i) patent holder
  37. Co-inventor, First Cell Phone, J.J.Mikulski
  38. Discontinuance of Product Line, Business Case Study Cell Phone, Macher, Jeffrey and Richman, Barak D., Organizational Responses to Discontinuous Innovation: A Case Study Approach. International Journal of Innovation Management, Vol. VII, No. 1, March 2004. Available at SSRN: http://ssrn.com/abstract=485282
  39. need a companion reference here
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