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Metropolitan area network

A metropolitan area network (MAN) is a computer network that interconnects users with computer resources in a geographic region of the size of a metropolitan area. The term MAN is applied to the interconnection of local area networks (LANs) in a city into a single larger network which may then also offer efficient connection to a wide area network. The term is also used to describe the interconnection of several LANs in a metropolitan area through the use of point-to-point connections between them.

History
By 1999, local area networks (LANs) were well established and providing data communication in buildings and offices. For the interconnection of LANs within a city, businesses relied primarily on the public switched telephone network. But while the telephone network was able to support the packet-based exchange of data that the various LAN protocols implemented, the bandwidth of the telephone network was already under heavy demand from circuit-switched voice, and the telephone exchanges were ill-designed to cope with the traffic spikes that LANs tended to produce.

To interconnect local area networks more effectively, it was suggested that office buildings are connected using the single-mode optical fiber lines, which were by that time widely used in long-haul telephone trunks. Such dark fibre links were in some cases already installed on customer premises and telephone companies started to offer their dark fibre within their subscriber packages. Fibre optic metropolitan area networks were operated by telephone companies as private networks for their customers and did not necessarily have full integration with the public wide area network (WAN) through gateways.

Besides the larger companies that connected their offices across metropolitan areas, universities and research institutions also adopted dark fibre as their metropolitan area network backbone. In West Berlin the BERCOM project built up a multifunctional broadband communications system to connect the mainframe computers that publicly funded universities and research institutions in the city housed. The BERCOM MAN project could progress at speed because the Deutsche Bundespost had already installed hundreds of miles of fibre optic cable in West Berlin. Like other metropolitan dark fibre networks at the time, the dark fibre network in West Berlin had a star topology with a hub somewhere in the city centre. The backbone of the dedicated BERCOM MAN for universities and research institutions was an optical fibre double ring that used a high-speed slotted ring protocol developed by the GMD Research Centre for Innovative Computer Systems and Telephony. The BERCOM MAN backbone could thus support two times 280 Mbit/s data transfer.

The productive use of dense wavelength-division multiplexing (DWDM) provided another impetus for the development of metropolitan area networks in the 2000s. Long haul DWDM, with ranges from 0 to 3000+ km, had been developed so that companies that stored large amounts of data on different sites could exchange data or establish mirrors of their file server. With the use of DWDM on the existing fibre optic MANs of carriers, companies no longer needed to connect their LANs with a dedicated fibre optic link. With DWDM companies could build dedicated MANs using the existing dark fibre network of a provider in a city. MANs thus became cheaper to build and maintain. The DWDM platforms provided by dark fibre providers in cities allow for a single fibre pair to be divided into 32 wavelengths. One wavelength could support between 10 Mbit/s and 10 Gbit/s. Thus companies that paid for a MAN to connect different office sites within a city could increase the bandwidths of their MAN backbone as part of their subscription. DWDM platforms also alleviated the need for protocol conversion to connect LANs in a city, because any protocol and any traffic type could be transmitted using DWDM. Effectively it gave companies wishing to establish a MAN choice of protocol.

Metro Ethernet uses a fibre optic ring as a Gigabit Ethernet MAN backbone within a larger city. The ring topology is implemented using Internet Protocol (IP) so that data can be rerouted if a link is congested or fails. In the US the Sprint was an early adopter of fibre optic rings that routed IP packets on the MAN backbone. Between 2002 and 2003 Sprint built three MAN rings to cover San Francisco, Oakland and San Jose, and in turn connected these three metro rings with a further two rings. The Sprint metro rings routed voice and data, were connected to several local telecom exchange points and totalled 189 miles of fibre optic cable. The metro rings also connected many cities that went on to become part of the Silicon Valley tech-hub, such as Fremont, Milpitas, Mountain View, Palo Alto, Redwood City, San Bruno, San Carlos, Santa Clara and Sunnyvale.

The metro Ethernet rings that did not route IP traffic instead used one of the various proprietary spanning tree protocol implementations; each MAN ring had a root bridge. Because layer 2 switching can not operate if there is a loop in the network, the protocols to support L2 MAN rings all need to block redundant links and thus block part of the ring. Capsuling protocols, such as Multiprotocol Label Switching (MPLS), were also deployed to address the drawbacks of operating L2 metro Ethernet rings.

Metro Ethernet was effectively the extension of Ethernet protocols beyond the local area network (LAN) and the ensuing investment in Ethernet led to the deployment of carrier Ethernet, where Ethernet protocols are used in wide area networks (WANs). The efforts of the Metro Ethernet Forum (MEF) in defining best practice and standards for metropolitan area networks thus also defined carrier Ethernet. While the IEEE tried to standardise the emerging Ethernet-based proprietary protocols, industry forums such as the MEF filled the gap and in January 2013 launched a certification for network equipment that can be configured to meet Carrier Ethernet 2.0 specifications.

Metropolitan Internet exchange points

Internet exchange points (IXs) have historically been important for the connection of MANs to the national or global Internet. The Boston Metropolitan Exchange Point (Boston MXP) enabled metro Ethernet providers, such as the HarvardNet to exchange data with national carriers, such as the Sprint Corporation and AT&T. Exchange points also serve as low-latency links between campus area networks, thus the Massachusetts Institute of Technology and the Boston University could exchange data, voice and video using the Boston MXP. Further examples of metropolitan Internet exchanges in the USA that were operational include the Anchorage Metropolitan Access Point (AMAP), the Seattle Internet Exchange (SIX), the Dallas-Fort Worth Metropolitan Access Point (DFMAP) and the Denver Internet Exchange (IX-Denver). Verizon put into operation three regional metropolitan exchanges to interconnect MANs and give them access to the Internet. The MAE-West serves the MANs of San Jose, Los Angeles and California. The MAE-East interconnects the MANs of New York City, Washington, D.C., and Miami. While the MAE-Central interconnects the MANs of Dallas, Texas, and Illinois.

In larger cities several local providers may have built a dark fibre MAN backbone. In London, the metro Ethernet rings of several providers make up the London MAN infrastructure. Like other MANs, the London MAN primarily serves the needs of its urban customers, who typically need a high number of connections with low bandwidth, a fast transit to other MAN providers, as well as high bandwidth access to national and international long-haul providers. Within the MAN of larger cities, metropolitan exchange points now play a vital role. The London Internet Exchange (LINX) had by 2005 built up several exchange points across the Greater London region.

Cities that host one of the international Internet exchanges have become a preferred location for companies and data centres. The Amsterdam Internet Exchange (AMS-IX) is the world's second-largest Internet exchange and has attracted companies to Amsterdam that are dependent on high-speed internet access. The Amsterdam metropolitan area network has benefited too from high-speed Internet access. Similarly Frankfurt has become a magnet for data centres of international companies because it hosts the non-profit DE-CIX, the largest Internet exchange in the world. The business model of the metro DE-CIX is to reduce the transit cost for local carriers by keeping data in the metropolitan area or region, while at the same time allowing long-haul low-latency peering globally with other major MANs.

title: Metropolitan area network


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Borel set

In mathematics, a Borel set is any set in a topological space that can be formed from open sets (or, equivalently, from closed sets) through the operations of countable union, countable intersection, and relative complement. Borel sets are named after Émile Borel.

For a topological space X, the collection of all Borel sets on X forms a σ-algebra, known as the Borel algebra or Borel σ-algebra. The Borel algebra on X is the smallest σ-algebra containing all open sets (or, equivalently, all closed sets).

Borel sets are important in measure theory, since any measure defined on the open sets of a space, or on the closed sets of a space, must also be defined on all Borel sets of that space. Any measure defined on the Borel sets is called a Borel measure. Borel sets and the associated Borel hierarchy also play a fundamental role in descriptive set theory.

In some contexts, Borel sets are defined to be generated by the compact sets of the topological space, rather than the open sets. The two definitions are equivalent for many well-behaved spaces, including all Hausdorff σ-compact spaces, but can be different in more pathological spaces.

Generating the Borel algebra
In the case that X is a metric space, the Borel algebra in the first sense may be described generatively as follows.

For a collection T of subsets of X (that is, for any subset of the power set P(X) of X), let
 * $$T_\sigma $$ be all countable unions of elements of T
 * $$T_\delta $$ be all countable intersections of elements of T
 * $$T_{\delta\sigma} = (T_\delta)_\sigma.$$

Now define by transfinite induction a sequence Gm, where m is an ordinal number, in the following manner:
 * For the base case of the definition, let $$ G^0$$ be the collection of open subsets of X.
 * If i is not a limit ordinal, then i has an immediately preceding ordinal i − 1. Let $$ G^i = [G^{i-1}]_{\delta \sigma}.$$
 * If i is a limit ordinal, set $$ G^i = \bigcup_{j < i} G^j. $$

The claim is that the Borel algebra is Gω1, where ω1 is the first uncountable ordinal number. That is, the Borel algebra can be generated from the class of open sets by iterating the operation $$ G \mapsto G_{\delta \sigma}. $$ to the first uncountable ordinal.

To prove this claim, any open set in a metric space is the union of an increasing sequence of closed sets. In particular, complementation of sets maps Gm into itself for any limit ordinal m; moreover if m is an uncountable limit ordinal, Gm is closed under countable unions.

For each Borel set B, there is some countable ordinal αB such that B can be obtained by iterating the operation over αB. However, as B varies over all Borel sets, αB will vary over all the countable ordinals, and thus the first ordinal at which all the Borel sets are obtained is ω1, the first uncountable ordinal.

The resulting sequence of sets is termed the Borel hierarchy.

Example
An important example, especially in the theory of probability, is the Borel algebra on the set of real numbers. It is the algebra on which the Borel measure is defined. Given a real random variable defined on a probability space, its probability distribution is by definition also a measure on the Borel algebra.

The Borel algebra on the reals is the smallest σ-algebra on R that contains all the intervals.

In the construction by transfinite induction, it can be shown that, in each step, the number of sets is, at most, the cardinality of the continuum. So, the total number of Borel sets is less than or equal to $$\aleph_1 \cdot 2 ^ {\aleph_0}\, = 2^{\aleph_0}.$$

In fact, the cardinality of the collection of Borel sets is equal to that of the continuum (compare to the number of Lebesgue measurable sets that exist, which is strictly larger and equal to $$2^{2^{\aleph_0}}$$).

Standard Borel spaces and Kuratowski theorems
Let X be a topological space. The Borel space associated to X is the pair (X,B), where B is the σ-algebra of Borel sets of X.

George Mackey defined a Borel space somewhat differently, writing that it is "a set together with a distinguished σ-field of subsets called its Borel sets." However, modern usage is to call the distinguished sub-algebra the measurable sets and such spaces measurable spaces. The reason for this distinction is that the Borel sets are the σ-algebra generated by open sets (of a topological space), whereas Mackey's definition refers to a set equipped with an arbitrary σ-algebra. There exist measurable spaces that are not Borel spaces, for any choice of topology on the underlying space.

Measurable spaces form a category in which the morphisms are measurable functions between measurable spaces. A function $$f:X \rightarrow Y$$ is measurable if it pulls back measurable sets, i.e., for all measurable sets B in Y, the set $$f^{-1}(B)$$ is measurable in X.

Theorem. Let X be a Polish space, that is, a topological space such that there is a metric d on X that defines the topology of X and that makes X a complete separable metric space. Then X as a Borel space is isomorphic to one of (This result is reminiscent of Maharam's theorem.)
 * 1) R,
 * 2) Z,
 * 3) a finite space.

Considered as Borel spaces, the real line R, the union of R with a countable set, and Rn are isomorphic.

A standard Borel space is the Borel space associated to a Polish space. A standard Borel space is characterized up to isomorphism by its cardinality, and any uncountable standard Borel space has the cardinality of the continuum.

For subsets of Polish spaces, Borel sets can be characterized as those sets that are the ranges of continuous injective maps defined on Polish spaces. Note however, that the range of a continuous noninjective map may fail to be Borel. See analytic set.

Every probability measure on a standard Borel space turns it into a standard probability space.

Non-Borel sets
An example of a subset of the reals that is non-Borel, due to Lusin, is described below. In contrast, an example of a non-measurable set cannot be exhibited, although the existence of such a set is implied, for example, by the axiom of choice.

Every irrational number has a unique representation by an infinite continued fraction

 * $$x = a_0 + \cfrac{1}{a_1 + \cfrac{1}{a_2 + \cfrac{1}{a_3 + \cfrac{1}{\ddots\,}}}} $$

where $$a_0$$ is some integer and all the other numbers $$a_k$$ are positive integers. Let $$A$$ be the set of all irrational numbers that correspond to sequences $$(a_0,a_1,\dots)$$ with the following property: there exists an infinite subsequence $$(a_{k_0},a_{k_1},\dots)$$ such that each element is a divisor of the next element. This set $$A$$ is not Borel. However, it is analytic (all Borel sets are also analytic), and complete in the class of analytic sets. For more details see descriptive set theory and the book by A. S. Kechris (see References), especially Exercise (27.2) on page 209, Definition (22.9) on page 169, Exercise (3.4)(ii) on page 14, and on page 196.

It's important to note, that while Zermelo–Fraenkel axioms (ZF) are sufficient to formalize the construction of $$A$$, it cannot be proven in ZF alone that $$A$$ is non-Borel. In fact, it is consistent with ZF that $$\mathbb{R}$$ is a countable union of countable sets, so that any subset of $$\mathbb{R}$$ is a Borel set.

Another non-Borel set is an inverse image $$f^{-1}[0]$$ of an infinite parity function $$f\colon \{0, 1\}^{\omega} \to \{0, 1\}$$. However, this is a proof of existence (via the axiom of choice), not an explicit example.

Alternative non-equivalent definitions
According to Paul Halmos, a subset of a locally compact Hausdorff topological space is called a Borel set if it belongs to the smallest σ-ring containing all compact sets.

Norberg and Vervaat redefine the Borel algebra of a topological space $$X$$ as the $$\sigma$$-algebra generated by its open subsets and its compact saturated subsets. This definition is well-suited for applications in the case where $$X$$ is not Hausdorff. It coincides with the usual definition if $$X$$ is second countable or if every compact saturated subset is closed (which is the case in particular if $$X$$ is Hausdorff).

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Borel set


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Talk:Checkers (disambiguation)

- Checkers was the name of a cocker spaniel that saved Richard Nixon's political career in 1952. Nixon, then a Vice President candidate for Eisenhower, was accused of setting up a secret slush fund. Nixon appeared on television, tearfully saying that he only gift he'd received as a politician was Checkers, that his children loved the dog, and that he and he wasn't going to give it back -- even if it was a crime. The audience loved it, and Nixon became VP and later President. - "Checkers" is the name of the Prime Minister of the United Kingdom's secondary country residence.

That one's 'Chequers'. -

Disambiguation
Could someone explain why the disambiguation page is at Checkers (disambiguation), rather than here at Checkers? It makes the whole notion of a disambiguation page rather pointless if no-one can find it. -- Visviva 08:52, 6 August 2005 (UTC)

Move discussion in progress
There is a move discussion in progress on Talk:Draughts which affects this page. Please participate on that page and not in this talk page section. Thank you. —RMCD bot 01:17, 20 December 2021 (UTC)

title: Talk:Checkers (disambiguation)


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Professional Association of Diving Instructors

The Professional Association of Diving Instructors (PADI) is a recreational diving membership and diver training organization founded in 1966 by John Cronin and Ralph Erickson. PADI courses range from entry level to advanced recreational diver certification. Further, they provide several diving skills courses connected with specific equipment or conditions, some diving related informational courses and a range of recreational diving instructor certifications. They also offer various technical diving courses. As of 2020, PADI claims to have issued 28 million scuba certifications. The levels are not specified and may include minor specialisations. Some of the certifications align with WRSTC and ISO standards, and these are recognised worldwide. Some other certification is unique to PADI and has no equivalence anywhere, or may be part of other agencies' standards for certification for more general diving skill levels.

History
In 1966, PADI was founded by John Cronin and Ralph Erickson. Cronin was originally a NAUI instructor who decided to form his own organization with Erickson, and to break diver training down into several modular courses instead of the single universal course then prevalent. Erickson developed continuing education scuba courses during this time and wrote the initial issue of the first trade magazine for scuba instructors, The Undersea Journal. Cronin got the idea for PADI's Positive Identification Card at a trade show. PADI established the Master Scuba Diver certification, the industry's first certification awarded for accumulation of specified certification and experience, and not based on a specific training program, in 1973, later launching the modular scuba program. By 1979, PADI was producing 100,000 certifications a year after previously hitting 25,000 a year. PADI was the first organization to use confined water or pool dives for training new divers and introduced the PADI Rescue Diver course and manual for rescue training during the 1980s.

In 1989, PADI founded Project AWARE to help conserve underwater environments. In 1992, Project AWARE Foundation became a registered nonprofit organization with an environmental mission and purpose. PADI continues to partner with PADI AWARE, supporting the organization with in-kind services, donations through its processes and connection to the PADI network of divers, dive professionals and dive centers. PADI AWARE information has been integrated in most courses and divers are offered the chance to exchange their normal certification card for an AWARE-certification card by making a donation to the program when sending in their application for a new certification.

In 2006, PADI was severely criticized by a Coroner's court in the United Kingdom for providing what experts regarded as short and insufficient training. Although PADI training standards differ from those formerly prevalent in the United Kingdom under the BSAC system, PADI training standards are consistent with World Recreational Scuba Training Council standards.

On 9 August 2012, Lincolnshire Management and Providence Equity Partners jointly acquired PADI from Seidler Equity Partners. In 2015, Providence Equity Partners acquired majority stake of PADI from Lincolnshire Management.

In 2017, Providence Equity Partners LLC sold PADI to Canadian investment firm Altas Partners and French private equity firm Florac for 700 million USD, through an entity called Mandarin fish Holding.

In 2018, PADI launched PADI Travel, an online dive travel resource and booking platform for dive resort and live-aboard packages.

In 2021, PADI reported it had a membership of over 128,000 professional members and 6,600 dive centers, and had awarded more than 28 million diving certifications internationally. PADI operates in 186 countries and territories. From 2015-2020, the average male to female membership split was approximately 63% to 37% respectively. The organization hosts Women's Dive Day events across the globe in an effort to increase awareness for women divers.{{clarify|To what end? does this provide a useful function?

Training system
PADI courses are performance-based diver training programs, and at the introductory level emphasizes practical knowledge, safety and motor skills. The basics of diving physics and physiology are introduced during entry level programs. The details of these concepts are left for later courses when they are necessary for the required competences of the specific training. According to PADI, these practices fall within current modern learning philosophies and receive regular updates via peer review.

The PADI training system is composed of modules with standardized learning objectives divided into theory and practical skills development. Each module is a stand-alone course for which certification is provided to the participant on successful completion of the course. Theory is mainly conveyed by way of self-study using books or digital training using PADI E-Learning. All study options are supplemented with video and, in most cases, live instruction to help the participant visualize what they have read. Confirmation of the student diver's level of competence in standardized knowledge review sessions is carried out by a scuba instructor using both written tests and personal observation during dives to verify the student's knowledge and skills. Practical skills are obtained through confined water training (pools or relatively shallow water) and performance evaluations in open water.

PADI courses target four progressive experience demographics, designated by PADI as "Youth", "Beginner", "Continuing Education" and "Professional". Options are available for students or instructors to attain additional certifications across a range of categories, including "Essentials", "Safety Focus", "Advanced Skills", "Cold Water", "Conservation", "Photography", "Discovery", "Freediving" and "Technical Diving".

For the youth demographic, the Padi Seal Team and Bubblemaker Programs are provided for students ages 8+.

As of 2023, the courses in "Beginner", "Continuing Education" and "Professional" categories include:

Workplace programs
PADI offers a specialty program called Public Safety Diver for divers who are either employed in or serve as volunteers in the public safety diving sector principally within the United States. This certification is not recognised in some countries which have national standards for occupational diving qualifications and require registration with a ntional body.

First aid programs
PADI, via its subsidiary, Emergency First Response, Corp, distributes the following programs in cardiopulmonary resuscitation (CPR) and first aid for both divers and non-divers: These may not be recognised by health and safety authorities in some jurisdictions where workplace first aid certification must be through a nationally or state accredited training provider.
 * Primary Care (CPR)
 * Secondary Care (First Aid)
 * Care for Children
 * Region-specific workplaces courses for countries including Australia, the United Kingdom and the United States.

Accreditation and memberships
PADI courses are recognized, recommended and cited by the institutions and organizations listed below for both recreational diving and vocational training.

PADI courses are recommended for college credit in the US by ACE. PADI is a member of the United States Recreational Scuba Training Council (RSTC).

Recognition's and equivalencies has been established between PADI and Confédération Mondiale des Activités Subaquatiques, the Colombian Navy, and Fédération Française d'Études et de Sports Sous-Marins (FFESSM). PADI is also a registered training organisation in Australia. As of 2012, PADI rescue diver and divemaster programs are included on the United Kingdom's Health and Safety Executive list of approved diving qualifications.

Those PADI courses aligning with standards published by the International Organization for Standardization (ISO) for 'Recreational diving services' were audited by the European Underwater Federation (EUF) Certification Body in 2004 and 2009, and were certified at both times as complying with these standards.

The relevant certifications may include: Most PADI training programes are not directly covered by ISO standards.
 * EN 14153-1 / ISO 24801-1 - Part 1: Level 1 "Supervised Diver" (PADI equivalent – Scuba Diver)
 * EN 14153-2 / ISO 24801-2 - Part 2: Level 2 "Autonomous Diver" (PADI equivalent – Open Water Diver)
 * EN 14153-3 / ISO 24801-3 - Part 3: Level 3 "Dive Leader" (PADI equivalent – Divemaster)
 * EN 14413-1 / ISO 24802-1 - Part 1: Level 1 Instructor (PADI equivalent – Assistant Instructor)
 * EN 14413-2 / ISO 24802-2 - Part 2: Level 2 Instructor (PADI equivalent – Open Water Scuba Instructor)
 * ISO 11107 Enriched air nitrox (EAN) diving (PADI equivalent – Enriched Air Diver)
 * ISO 11121 introductory training programmes to scuba diving. (PADI equivalent – Discover Scuba Diving)

PADI is a member of the following member councils of the World Recreational Scuba Training Council – the RSTC Canada, the RSTC Europe and the C-Card Council (Japan).

Business strategy
Padi is recognised as the largest recreational diver training agency with the largest rate of issue of diver certification, and the largest membership of professional instructors and dive leaders in recreational diving. They are very distinctly a for profit otganisation focused on efficiency of training of the largest number of customers. This strategy appears to work for them.

Marketing
PADI marketing puts an emphasis on convenience, fun and continued attendance on PADI courses. Risk management is largely by limitations on the certified diver's recommended range of acceptable environmental and equipment options and providing profitable additional training programmes to extend these options by small increments.

Affiliates and sponsorships
Since 2009, PADI and the Boy Scouts of America (BSA) have maintained a mutual support partnership. In Canada, PADI sponsors the Scouts Canada Scuba Program. Affiliates include:

 * Emergency First Response provides CPR and First Aid training both for the lay person and in the workplace.
 * Current Publishing Corporation develops marine science programs for high school and upper level educational facilities.
 * Diving Science and Technology Corporation (DSAT) is the development arm for the Recreational Dive Planner and PADI's Tec-Rec program.

Citations in professional literature
PADI's instructional methodology is cited in EDUCAUSE's 2012 book, Game Changers: Education and Information Technologies regarding badges as "a symbol or indicator of an accomplishment, skill, quality, or interest. From the Boy and Girl Scouts to PADI diving instruction, to the more recently popular geolocation game Foursquare, badges have been successfully used to set goals, motivate behaviors, represent achievements, and communicate success in many contexts."

PADI's environmental emphasis is cited in the 2007 book, New Frontiers in Marine Tourism, in its section, Dive Tourism, Sustainable Tourism and Social Responsibility: A Growing Agenda – Environmental management and education: the case of PADI, (Chapter Seven). "PADI, as well as other diver certification organisations and individual businesses, has put significant resources into conservation and developed public awareness programs".

New Frontiers in Marine Tourism also cites in the section entitled Student Scholarships and Social Responsibility: A Growing Agenda for PADI, that "The PADI Scholarship program … is a good example of the way that various disparate parts of an industry, each with limited resources, can pool their efforts to help more people from developing countries to enter the diving profession… PADI recognizes that good relations with the involvement of local people is essential both to business development and to environmental protection. The scholarship scheme makes entry into the dive business more possible for some students who have the backing of their dive center."


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Barbara Stanwyck

Barbara Stanwyck (born Ruby Catherine Stevens; July 16, 1907 – January 20, 1990) was an American actress, model and dancer. A stage, film, and television star, during her 60-year professional career she was known for her strong, realistic screen presence and versatility. She was a favorite of directors, including Cecil B. DeMille, Fritz Lang, and Frank Capra, and made 85 films in 38 years before turning to television.

Orphaned at the age of four and partially raised in foster homes, she always worked. One of her directors, Jacques Tourneur, said of her, "She only lives for two things, and both of them are work." She made her debut on stage in the chorus as a Ziegfeld girl in 1923 at age 16, and within a few years was acting in plays. Her first lead role, which was in the hit Burlesque (1927), established her as a Broadway star.

In 1929, she began acting in talking pictures. Frank Capra chose her for his romantic drama Ladies of Leisure (1930). This led to additional leading roles which raised her profile, such as Night Nurse (1931), Baby Face (1933), and the controversial The Bitter Tea of General Yen (1933). In 1937, she played the title role in Stella Dallas, for which she earned her first Academy Award nomination for best actress. In 1939, she starred in Union Pacific. In 1941, she starred in two screwball comedies: Ball of Fire with Gary Cooper, and The Lady Eve with Henry Fonda. She received her second Academy Award nomination for Ball of Fire, and in the decades since its release The Lady Eve has come to be regarded as a comedic classic, with Stanwyck's performance called one of the best in American comedy. Other successful films during this period are Meet John Doe (1940) and You Belong to Me (1941), reteaming her with Cooper and Fonda, respectively. By 1944, Stanwyck had become the highest-paid actress in the United States. She starred with Fred MacMurray in the seminal film noir Double Indemnity (1944), playing the wife who persuades an insurance salesman to kill her husband, for which she received her third Oscar nomination. In 1945, she starred as a homemaker columnist in the hit romantic comedy Christmas in Connecticut. The next year, she portrayed the title tragic femme fatale in The Strange Love of Martha Ivers. She garnered her fourth Oscar nomination for her performance as an invalid wife in the noir-thriller Sorry, Wrong Number (1948). Stanwyck's film career declined by the start of the 1950s, despite having a fair number of leading and major supporting roles in the decade, the most successful being Executive Suite (1954).

She transitioned to television by the 1960s, where she won three Emmy Awards, for The Barbara Stanwyck Show (1961), the Western series The Big Valley (1966), and the miniseries The Thorn Birds (1983).

She received an Honorary Oscar in 1982, the Golden Globe Cecil B. DeMille Award in 1986 and several other honorary lifetime awards. She was ranked as the 11th greatest female star of classic American cinema by the American Film Institute.

Early life
Stanwyck was born Ruby Catherine Stevens on July 16, 1907, in Brooklyn, New York. She was the fifth – and youngest – child of Kathryn Ann (née McPhee) and Byron E. Stevens, both working-class parents. Her father, of English descent, was a native of Lanesville, Massachusetts, and her mother, of Scottish descent, was an immigrant from Sydney, Nova Scotia. She had three older sisters, Laura Mildred (Smith), Viola (Merkent), Mabel (Munier) and one older brother, Malcolm Byron (known as "Bert”).

When Ruby was four, her mother died of complications from a miscarriage after she was knocked off a moving streetcar in 1911 by a drunkard. Two weeks after the funeral, her father joined a work crew digging the Panama Canal and was never seen again by his family.

Ruby and her older brother, Malcolm Byron (later nicknamed "By") Stevens, were raised by their eldest sister Laura Mildred (known as Mildred; later Mildred Smith), who died of a heart attack at age 45. When Mildred got a job as a showgirl, Ruby and Byron were placed in a series of foster homes (as many as four in a year), from which young Ruby often ran away. She attended various public schools in Brooklyn, where she received uniformly poor grades and routinely picked fights with the other students.

Ruby toured with Mildred during the summers of 1916 and 1917, and practiced her sister's routines backstage. Watching the movies of Pearl White, whom Ruby idolized, also influenced her drive to be a performer. At the age of 14, she dropped out of school, taking a package wrapping job at a Brooklyn department store. Ruby never attended high school, "although early biographical thumbnail sketches had her attending Brooklyn's famous Erasmus Hall High School."

Soon afterward, she took a filing job at the Brooklyn telephone office for $14 a week, which allowed her to become financially independent. She disliked the job; her real goal was to enter show business, even as her sister Mildred discouraged the idea. She then took a job cutting dress patterns for Vogue magazine, but customers complained about her work and she was fired. Ruby's next job was as a typist for the Jerome H. Remick Music Company; she reportedly enjoyed the work, but her continuing ambition was show business, and her sister finally gave up trying to dissuade her.

Ziegfeld girl and Broadway success
In 1923, a few months before her 16th birthday, Ruby auditioned for a place in the chorus at the Strand Roof, a nightclub over the Strand Theatre in Times Square. A few months later, she obtained a job as a dancer in the 1922 and 1923 seasons of the Ziegfeld Follies, dancing at the New Amsterdam Theater. "I just wanted to survive and eat and have a nice coat", Stanwyck said. For the next several years, she worked as a chorus girl, performing from midnight to seven in the morning at nightclubs owned by Texas Guinan. She also occasionally served as a dance instructor at a speakeasy for gays and lesbians owned by Guinan. One of her good friends during those years was pianist Oscar Levant, who described her as being "wary of sophisticates and phonies".

Billy LaHiff, who owned a popular pub frequented by showpeople, introduced Ruby in 1926 to impresario Willard Mack. Mack was casting his play The Noose, and LaHiff suggested that the part of the chorus girl be played by a real one. Mack agreed, and after a successful audition gave the part to Ruby. She co-starred with Rex Cherryman and Wilfred Lucas. As initially staged, the play was not a success. In an effort to improve it, Mack decided to expand Ruby's part to include more pathos. The Noose re-opened on October 20, 1926, and became one of the most successful plays of the season, running on Broadway for nine months and 197 performances. At the suggestion of David Belasco, Ruby changed her name to Barbara Stanwyck by combining the first name of the title character in the play Barbara Frietchie with the last name of the actress in the play, Jane Stanwyck; both were found on a 1906 theater program.

Stanwyck became a Broadway star soon afterward, when she was cast in her first leading role in Burlesque (1927). She received rave reviews, and it was a huge hit. Film actor Pat O'Brien would later say on a 1960s talk show, "The greatest Broadway show I ever saw was a play in the 1920s called 'Burlesque'." Arthur Hopkins described in his autobiography To a Lonely Boy, how he came to cast Stanwyck: After some search for the girl, I interviewed a nightclub dancer who had just scored in a small emotional part in a play that did not run [The Noose]. She seemed to have the quality I wanted, a sort of rough poignancy. She at once displayed more sensitive, easily expressed emotion than I had encountered since Pauline Lord. She and [Hal] Skelly were the perfect team, and they made the play a great success. I had great plans for her, but the Hollywood offers kept coming. There was no competing with them. She became a picture star. She is Barbara Stanwyck. He also called Stanwyck "the greatest natural actress of our time," noting with sadness, "One of the theater's great potential actresses was embalmed in celluloid."

Around this time, Stanwyck was given a screen test by producer Bob Kane for his upcoming 1927 silent film Broadway Nights. She lost the lead role because she could not cry in the screen test, but was given a minor part as a fan dancer. This was Stanwyck's first film appearance.

While playing in Burlesque, Stanwyck was introduced to her future husband, actor Frank Fay, by Oscar Levant. Stanwyck and Fay were married on August 26, 1928, and soon moved to Hollywood.

Film career
Stanwyck's first sound film was The Locked Door (1929), followed by Mexicali Rose, released in the same year. Neither film was successful; nonetheless, Frank Capra chose Stanwyck for his film Ladies of Leisure (1930). Her work in that production established an enduring friendship with the director and led to future roles in his films. Other prominent roles followed, among them as a nurse who saves two little girls from the villainous chauffeur (Clark Gable) in Night Nurse (1931). In Edna Ferber's novel brought to screen by William Wellman, she portrays small town teacher and valiant Midwest farm woman Selena in So Big! (1932). She followed with a performance as an ambitious woman "sleeping" her way to the top from "the wrong side of the tracks" in Baby Face (1933), a controversial pre-Code classic. In The Bitter Tea of General Yen (1933), another controversial pre-Code film by director Capra, Stanwyck portrays an idealistic Christian caught behind the lines of Chinese civil war kidnapped by warlord Nils Asther. A flop at the time, though it received some critical success, the lavish film is "dark stuff, and it's difficult to imagine another actress handling this ... philosophical conversion as fearlessly as Ms. Stanwyck does. She doesn't make heavy weather of it."

Regarding her pre-Code work, Mick LaSalle, movie critic for the San Francisco Chronicle said "If you've never seen Stanwyck in a pre-Code film, you've never seen Stanwyck". (The Code began to be enforced seriously beginning in July 1934.) Never in her career, including "Double Indemnity," was she ever as hard-boiled as she was in the early 1930s. She had a wonderful quality of being both incredibly cool and yet blazingly passionate. Her cynicism was profound, and then, without warning, she would explode into shrieking, sobbing."

In Stella Dallas (1937) she plays the self-sacrificing title character who eventually allows her teenage daughter to live a better life somewhere else. She landed her first Academy Award nomination for Best Actress when she was able to portray her character as vulgar, yet sympathetic as required by the movie. Next, she played Molly Monahan in Union Pacific (1939) with Joel McCrea. Stanwyck was reportedly one of the many actresses considered for the role of Scarlett O'Hara in Gone with the Wind (1939), although she did not receive a screen test. In Meet John Doe she plays an ambitious newspaperwoman with Gary Cooper (1941).

In Preston Sturges's romantic comedy The Lady Eve (1941), she plays a slinky, sophisticated con-woman who "gives off an erotic charge that would straighten a boa constrictor", while falling in love with her intended mark, the guileless, wealthy herpetologist, played by Henry Fonda. Film critic David Thomson described Stanwyck as "giving one of the best American comedy performances", and she was reviewed as brilliantly versatile in "her bravura double performance" by The Guardian. The Lady Eve is among the top 100 movies of all time on Time and Entertainment Weekly's lists, and is considered to be both a great comedy and a great romantic film with its placement at #55 on the AFI's 100 Years...100 Laughs list and #26 on its 100 Years...100 Passions list.

Next, she was the extremely successful, independent doctor Helen Hunt in You Belong to Me (1941), also with Fonda. Stanwyck then played nightclub performer Sugarpuss O'Shea in the Howard Hawks directed, but Billy Wilder written comedy Ball of Fire (1941). In this update of the Snow White and Seven Dwarfs tale, she gives professor Bertram Potts (played by Gary Cooper) a better understanding of "modern English" in the performance for which she received an Academy Award nomination for Best Actress.

"That is the kind of woman that makes whole civilizations topple." -- Kathleen Howard of Stanwyck's character in Ball of Fire.

In Double Indemnity (1944), the seminal film noir thriller directed by Billy Wilder, she plays the sizzling blonde tramp /"destiny in high heels" who lures an infatuated insurance salesman (Fred MacMurray), into killing her husband. Stanwyck brings out the cruel nature of the "grim, unflinching murderess", marking her as the "most notorious femme fatale" in the film noir genre. Her performance as the "insolent, self-possessed wife is one of the screen's definitive studies of villainy. Double Indemnity is usually considered to be among the top 100 films of all time, though it did not win any of its seven Academy Award nominations. It is the #38 film of all time on the American Film Institute's list, as well as the #24 on its 100 Years...100 Thrills list and #84 on its 100 Years...100 Passions list.

She plays a columnist touted as the "greatest cook in the country" caught up in white lies while trying to pursue a romance in the comedy Christmas in Connecticut (1945). It was a hit upon release and remains a treasured holiday classic today. In 1946 she was "liquid nitrogen" as Martha, a manipulative murderess, starring with Van Heflin and newcomer Kirk Douglas in The Strange Love of Martha Ivers. Stanwyck was also the vulnerable, invalid wife that overhears her own murder being plotted in Sorry, Wrong Number (1948) and the doomed concert pianist in The Other Love (1947). In the latter film's soundtrack, the piano music is actually being performed by Ania Dorfmann, who drilled Stanwyck for three hours a day until the actress was able to synchronize the motion of her arms and hands to match the music's tempo, giving a convincing impression that it is Stanwyck playing the piano.

Pauline Kael, a longtime film critic for The New Yorker, admired the natural appearance of Stanwyck's acting style on screen, noting that she "seems to have an intuitive understanding of the fluid physical movements that work best on camera". In reference to the actress's film work during the early sound era, Kael observed that the "[e]arly talkies sentimentality ... only emphasizes Stanwyck's remarkable modernism." Stanwyck was known for her accessibility and kindness to the backstage crew on any film set. She knew the names of many of their wives and children. Frank Capra said of Stanwyck: "She was destined to be beloved by all directors, actors, crews and extras. In a Hollywood popularity contest, she would win first prize, hands down." While working on 1954's Cattle Queen of Montana (also starring Ronald Reagan) on location in Glacier National Park, she performed some of her own stunts, including a swim in the icy lake. At the age of 50, she performed an extremely difficult stunt in Forty Guns. The scene called for her character to fall from and be dragged by a horse, and the stunt was so dangerous that the film's professional stuntman refused to perform it. She was later named an honorary member of the Hollywood Stuntmen's Hall of Fame.

William Holden and Stanwyck were longtime friends and when they were presenting the Best Sound Oscar for 1977, he paused to pay a special tribute to her for saving his career when Holden was cast in the lead for Golden Boy (1939). After a series of unsteady daily performances, he was about to be fired, but Stanwyck staunchly defended him, successfully standing up to the film producers. Shortly after Holden's death, Stanwyck recalled the moment when receiving her honorary Oscar: "A few years ago, I stood on this stage with William Holden as a presenter. I loved him very much, and I miss him. He always wished that I would get an Oscar. And so, tonight, my golden boy, you got your wish."

Television career
As Stanwyck's film career declined during the 1950s, she moved to television. In 1958, she guest-starred in "Trail to Nowhere", an episode of the Western anthology series Dick Powell's Zane Grey Theatre, playing a wife who kills a man to avenge her husband. In 1961, she hosted an anthology drama series titled The Barbara Stanwyck Show that was not a ratings success but earned her an Emmy Award. The show ran for a total of 36 episodes. During this period, she also guest-starred on other television series such as The Untouchables and four episodes of Wagon Train.

She stepped back into film for the 1964 Elvis Presley film Roustabout, in which she plays a carnival owner.

The Western television series The Big Valley, which was broadcast on ABC from 1965 to 1969, made Stanwyck one of the most popular actresses on television, winning her another Emmy. She was billed in the series' opening credits as Miss Barbara Stanwyck for her role as Victoria, the widowed matriarch of the wealthy Barkley family.

In 1983, Stanwyck won an Emmy for The Thorn Birds, her third such award. In 1985, she made three guest appearances in the primetime soap opera Dynasty prior to the launch of its short-lived spinoff series The Colbys, in which she starred alongside Charlton Heston, Stephanie Beacham and Katharine Ross. Unhappy with the experience, Stanwyck remained with the series for only the first season, and her role as Constance Colby Patterson would be her last. It was rumored that Earl Hamner Jr., former producer of The Waltons, had initially wanted Stanwyck for the role of Angela Channing in the 1980s soap opera Falcon Crest, and she turned it down, with the role going to her friend Jane Wyman, but Hamner assured Wyman that it was only a rumor.

Marriages and relationships
While playing in The Noose, Stanwyck reportedly fell in love with her married co-star Rex Cherryman. When Cherryman took ill in early 1928, his doctor advised him to take a sea voyage, so Cherryman set sail for Le Havre intending to continue on to Paris, where he and Stanwyck had arranged to meet. While at sea he contracted septic poisoning and died shortly after arriving in France at the age of 31.

On August 26, 1928, Stanwyck married her Burlesque co-star Frank Fay. She and Fay later claimed that they had disliked each other at first, but became close after Cherryman's death. Fay was Catholic so Stanwyck converted for their marriage. She was reportedly unable to have children, and one biographer alleges the cause of her infertility was a botched abortion at the age of 15 that resulted in complications. After moving to Hollywood, the couple adopted a ten-month-old boy on December 5, 1932. They named him Dion, later amending the name to Anthony Dion, nicknamed Tony. The marriage was troubled; Fay's successful Broadway career did not translate to the big screen, whereas Stanwyck achieved Hollywood stardom. Fay was reportedly physically abusive to Stanwyck, especially when he was inebriated. Some claim that the marriage was the basis for dialogue written by William Wellman, a friend of the couple, for A Star Is Born (1937) starring Janet Gaynor and Fredric March. The couple divorced on December 30, 1935. Stanwyck won custody of their son, whom she raised with a strict, authoritarian hand and demanding expectations. Stanwyck and her son became estranged after his childhood, meeting only a few times after he became an adult. He died in 2006. Wrote Richard Corliss, the child "resembled her in just one respect: both were, effectively, orphans."

In 1936, while making the film His Brother's Wife (1936), Stanwyck became involved with her co-star, Robert Taylor. Rather than a torrid romance, their relationship was more one of mentor and pupil. Stanwyck served as support and adviser to the younger Taylor, who had come from a small Nebraska town; she guided his career and acclimated him to the sophisticated Hollywood culture. The couple began living together, sparking newspaper reports. Stanwyck was hesitant to remarry after the failure of her first marriage, but their 1939 marriage was arranged with the help of Taylor's studio, Metro-Goldwyn-Mayer, a common practice in Hollywood's golden age. Louis B. Mayer had insisted that Stanwyck and Taylor marry and went as far as presiding over arrangements at the wedding. Stanwyck and Taylor enjoyed time together outdoors during the early years of their marriage and owned acres of prime West Los Angeles property. Their large ranch and home in the Mandeville Canyon section of Brentwood, Los Angeles, is still referred to by the locals as "the old Robert Taylor ranch".

Stanwyck and Taylor mutually decided in 1950 to divorce and, at his insistence, she proceeded with the official filing of the papers. There have been many rumors regarding the cause of the divorce, but after World War II Taylor attempted to create a life away from the entertainment industry, and Stanwyck did not share that goal. Taylor allegedly had extramarital affairs, and unsubstantiated rumors suggested that Stanwyck had also. After the divorce, they remained friendly and acted together in Stanwyck's last feature film, The Night Walker (1964). She never remarried. According to her friend and Big Valley co-star Linda Evans, Stanwyck cited Taylor as the love of her life. She took his death in 1969 very hard, and took a long break from film and television work.

Stanwyck was one of the best-liked actresses in Hollywood and maintained friendships with many of her fellow actors (as well as crew members of her films and TV shows), including Joel McCrea and his wife Frances Dee, George Brent, Robert Preston, Henry Fonda (who had a longtime crush on her), James Stewart, Linda Evans, Joan Crawford, Jack Benny and his wife Mary Livingstone, William Holden, Gary Cooper, and Fred MacMurray. During filming of To Please a Lady, Stanwyck refused to leave her African-American maid Harriet Coray in a hotel only for African-American people and insisted that Harriet stay in the same hotel as she did. After much pressure from Stanwyck, Coray was allowed to stay in the best hotel in Indianapolis with Stanwyck and the rest of the cast and crew.

Stanwyck, at age 45, had a four-year romantic affair with 22-year-old actor Robert Wagner that had begun on the set of Titanic (1953) before Stanwyck ended the relationship. The affair is described in Wagner's 2008 memoir Pieces of My Heart.

Political views
A conservative Republican, Stanwyck opposed the presidency of Franklin D. Roosevelt. She felt that if someone from her disadvantaged background had risen to success, others should be able to prosper without government intervention or assistance. For Stanwyck, "hard work with the prospect of rich reward was the American way." She became an early member of the Motion Picture Alliance for the Preservation of American Ideals (MPA) after its founding in 1944. The mission of this group was to "combat ... subversive methods [used in the industry] to undermine and change the American way of life." It opposed both communist and fascist influences in Hollywood. She publicly supported the investigations of the House Un-American Activities Committee, and her husband Robert Taylor testified as a friendly witness. Stanwyck supported Thomas E. Dewey in the 1944 and 1948 United States presidential elections.

A fan of Objectivist author Ayn Rand, Stanwyck persuaded Warner Bros. head Jack L. Warner to purchase the rights to The Fountainhead before it became a bestseller, and she wrote to Rand of her admiration of Atlas Shrugged.

Religion
Stanwyck was originally a Protestant, and was baptized in June 1916 by the Reverend J. Frederic Berg of the Protestant Dutch Reformed Church. She converted to Roman Catholicism when she married first husband Frank Fay but does not appear to have remained an adherent after the marriage ended.

Brother
Stanwyck's older brother, Malcolm Byron Stevens (1905–1964), became an actor, using the name Bert Stevens. He appeared mostly in supporting roles, often uncredited. He appeared in two films that starred Stanwyck: The File on Thelma Jordon and No Man of Her Own, both released in 1950. He appeared in two episodes of Alfred Hitchcock Presents: (Season 6 Episode 8 (1960): "O Youth and Beauty!" as uncredited Club Member) and (Season 7 Episode 21 (1962): "Burglar Proof" as uncredited Demonstration Guest). He also appeared in one episode of The Alfred Hitchcock Hour: (Season 2 Episode 25 (1964): "The Ordeal of Mrs. Snow" as uncredited Carl the Butler).

Later years and death
Stanwyck's retirement years were active, with charity work outside the limelight.

In 1981, in her home in the exclusive Trousdale section of Beverly Hills, she was awakened during the night by an intruder who struck her on the head with his flashlight, forced her into a closet, and absconded with $40,000 in jewels.

In 1982, while filming The Thorn Birds, Stanwyck inhaled special-effects smoke on the set that may have caused her to contract bronchitis, which was compounded by her cigarette-smoking habit. She began smoking at the age of nine and stopped just four years before her death.

Stanwyck died on January 20, 1990, at the age of 82, from congestive heart failure and chronic obstructive pulmonary disease at Saint John's Health Center in Santa Monica, California. She had indicated that she wanted no funeral service. In accordance with her wishes, her remains were cremated and the ashes scattered from a helicopter over Lone Pine, California, where she had made some of her Western films.

title: Barbara Stanwyck


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List of comparative military ranks

This article is a list of various nations' armed forces ranking designations. Comparisons are made between the different systems used by nations to categorize the hierarchy of an armed force compared to another. Several of these lists mention NATO rank reference codes. These are used for easy comparison among NATO countries. Links to comparison charts can be found below.

Albania

 * Military ranks of Albania

Angola

 * Military ranks of Angola

Argentina

 * Military ranks of Argentina

Australia

 * Australian Defence Force ranks
 * Royal Australian Navy ranks and uniforms
 * Australian Army officer rank insignia; Australian Army other ranks insignia
 * Ranks of the RAAF

Austria

 * Ranks of the Austrian Bundesheer

Azerbaijan

 * Military ranks of Azerbaijan

Bahrain

 * Military ranks of Bahrain

Bangladesh

 * Ranks of Bangladesh Army
 * Ranks and insignia of Bangladesh Navy
 * Bangladesh Air Force rank insignia

Belarus

 * Military ranks of Belarus

Belgium

 * Belgian military ranks

Bhutan

 * Military ranks of Bhutan

Bolivia

 * Military ranks of Bolivia

Bosnia and Herzegovina

 * Military ranks and insignia of Bosnia and Herzegovina

Botswana

 * Military ranks of Botswana

Brazil

 * Military ranks of Brazil

Bulgaria

 * Military ranks of Bulgaria

Canada

 * Canadian Armed Forces ranks and insignia

Chile

 * Military ranks and insignia of Chile

China

 * Ranks of the People's Liberation Army Ground Force
 * Ranks of the People's Liberation Army Navy
 * Ranks of the People's Liberation Army Air Force
 * Ranks of the People's Armed Police

Colombia

 * Military ranks of the Colombian Armed Forces

Congo

 * Military ranks of the Democratic Republic of the Congo

Congo-Brazzaville

 * Military ranks of Republic of the Congo

Croatia

 * Croatian military ranks

Cuba

 * Cuban military ranks

Czechia

 * Czech military ranks

Denmark

 * Danish Army ranks
 * Danish Navy ranks
 * Danish Air Force ranks

Egypt

 * Military ranks of Egypt

Ecuador

 * Military ranks of Ecuador

Estonia

 * Military ranks of Estonia

Ethiopia

 * Military ranks of Ethiopia

Finland

 * Finnish military ranks

France

 * French Army ranks
 * French Navy ranks
 * French Air Force ranks
 * French Gendarmerie ranks

Gabon

 * Military ranks of the Armed Forces of Gabon

Georgia

 * Georgian military ranks

Germany

 * Rank insignia of the German Bundeswehr
 * Ranks of the German Bundeswehr

Greece

 * Greek military ranks
 * Hellenic Army: officer, other

Hungary

 * Hungarian military ranks

India

 * Indian Armed Forces comparative ranks
 * Indian Army ranks and insignia
 * Indian Navy ranks and insignia
 * Indian Air Force ranks and insignia

Indonesia

 * Indonesian military ranks

Iran

 * Rank insignia of the Iranian military

Iraq

 * Military ranks of Iraq

Ireland

 * Irish Defence Forces rank insignia

Israel

 * Israel Defense Forces ranks and insignia

Italy

 * Italian Army ranks
 * Italian Navy ranks
 * Italian Air Force ranks
 * Italian Carabinieri ranks
 * Italian Finance Guard ranks

Japan

 * Japanese military ranks

Jordan

 * Jordanian military ranks

Kenya

 * Military ranks of Kenya

Laos

 * Military ranks of the Lao People's Armed Forces

Lebanon

 * Military ranks of Lebanon

Lithuania

 * Lithuanian military ranks and insignia

Malaysia

 * Malaysian military ranks

Mexico

 * Military ranks of Mexico

Mongolia

 * Mongolian military ranks

Myanmar

 * Military ranks of Myanmar

Nepal

 * Military ranks of Nepal

Netherlands

 * Military ranks of the Netherlands armed forces

New Zealand

 * New Zealand military ranks

Nicaragua

 * Nicaragua military ranks

Nigeria

 * Military ranks of Nigeria

North Korea

 * Military ranks of North Korea
 * Comparative military ranks of Korea

Norway

 * Military ranks and insignia of Norway

Pakistan

 * Pakistan Army ranks and insignia
 * Pakistan Navy ranks and insignia
 * Pakistan Air Force ranks and insignia

Peru

 * Military ranks of Peru

Philippines

 * Military ranks of the Philippines

Poland

 * Polish Armed Forces rank insignia

Portugal

 * Military ranks of Portugal

Romania

 * Romanian Armed Forces ranks and insignia

Russia

 * Army ranks and insignia of the Russia
 * Naval ranks and insignia of Russia
 * Aerospace Forces ranks and insignia of the Russian Federation

Saudi Arabia

 * Saudi Arabian military ranks

Serbia

 * Military ranks of Serbia

Singapore

 * Singapore Armed Forces ranks

Slovakia

 * Military ranks of Slovakia

Slovenia

 * Slovenian military ranks

Somalia

 * Military ranks of Somalia

Somaliland

 * Military ranks of Somaliland

South Africa

 * South African military ranks

South Korea

 * Military ranks of South Korea
 * Comparative military ranks of Korea

Spain

 * Military ranks of Spain

Sri Lanka

 * Sri Lanka Army rank insignia
 * Sri Lanka Navy rank insignia
 * Sri Lanka Air Force rank insignia

Sweden

 * Military ranks of the Swedish Armed Forces

Switzerland

 * Military ranks of Switzerland

Syria

 * Military ranks of Syria

Republic of China (Taiwan)

 * Republic of China Armed Forces rank insignia

Tajikistan

 * Military ranks of Tajikistan

Tanzania

 * Military ranks of Tanzania

Thailand

 * Military ranks of the Thai armed forces

Tunisia

 * Military ranks of Tunisia

Turkey

 * Military ranks of Turkey

Ukraine

 * Military ranks of Ukraine

United Arab Emirates

 * Military ranks of United Arab Emirates

United Kingdom

 * Royal Navy officer rank insignia; Royal Navy ratings rank insignia
 * British Army officer rank insignia; British Army other rank insignia
 * Royal Air Force officer ranks; Royal Air Force other ranks
 * United Kingdom and United States military ranks compared

United States

 * United States uniformed services: comparative ranks of officers, warrant officers, and enlisted servicemen
 * United States Air Force: enlisted, officer
 * United States Army: enlisted, warrant officer, officer
 * United States Coast Guard: enlisted, warrant officer, officer
 * United States Marine Corps: enlisted, warrant officer, officer
 * United States Navy: enlisted, warrant officer, officer
 * United States Public Health Service Commissioned Corps: officer
 * United States Space Force: enlisted, officer
 * National Oceanic and Atmospheric Administration Commissioned Officer Corps: officer
 * United Kingdom and United States military ranks compared

Venezuela

 * Venezuelan military ranks

Vietnam

 * Vietnamese military ranks and insignia

Zambia

 * Military ranks of Zambia

Zimbabwe

 * Military ranks of Zimbabwe

Africa

 * Comparative army officer ranks of Africa
 * Comparative army enlisted ranks of Africa
 * Comparative air force officer ranks of Africa
 * Comparative air force enlisted ranks of Africa
 * Comparative navy officer ranks of Africa
 * Comparative navy enlisted ranks of Africa

Americas

 * Comparative army officer ranks of the Americas
 * Comparative army enlisted ranks of the Americas
 * Comparative air force officer ranks of the Americas
 * Comparative air force enlisted ranks of the Americas
 * Comparative navy officer ranks of the Americas
 * Comparative navy enlisted ranks of the Americas

Asia

 * Comparative army officer ranks of Asia
 * Comparative army enlisted ranks of Asia
 * Comparative air force officer ranks of Asia
 * Comparative air force enlisted ranks of Asia
 * Comparative navy officer ranks of Asia
 * Comparative navy enlisted ranks of Asia

Europe

 * Comparative army officer ranks of Europe
 * Comparative army enlisted ranks of Europe
 * Comparative air force officer ranks of Europe
 * Comparative air force enlisted ranks of Europe
 * Comparative navy officer ranks of Europe
 * Comparative navy enlisted ranks of Europe

Oceania

 * Comparative army officer ranks of Oceania
 * Comparative army enlisted ranks of Oceania
 * Comparative air force officer ranks of Oceania
 * Comparative air force enlisted ranks of Oceania
 * Comparative navy officer ranks of Oceania
 * Comparative navy enlisted ranks of Oceania

NATO

 * Ranks and insignia of NATO
 * Ranks and insignia of NATO armies officers
 * Ranks and insignia of NATO armies enlisted
 * Ranks and insignia of NATO Air Forces Officers
 * Ranks and insignia of NATO Air Forces Enlisted
 * Ranks and insignia of officers of NATO navies
 * Ranks and insignia of NATO Navies Enlisted

Commonwealth of Nations

 * Comparative army officer ranks of the Commonwealth
 * Comparative army enlisted ranks of the Commonwealth
 * Comparative air force officer ranks of the Commonwealth
 * Comparative air force enlisted ranks of the Commonwealth
 * Comparative navy officer ranks of the Commonwealth
 * Comparative navy enlisted ranks of the Commonwealth

Miscellaneous

 * Imperial Roman legion's ranks
 * Confederate Army during the civil war
 * Confederate Navy during the civil war
 * Military ranks of the Ottoman Empire
 * Rank insignia of the Austro-Hungarian armed forces
 * Ranks in the Austro-Hungarian Navy
 * Royal Navy during the 18th and 19th centuries
 * South Vietnamese military ranks and insignia
 * United States (Union) Army during the civil war
 * United States (Union) Navy during the civil war

First and Second World War comparisons

 * Comparative military ranks of World War I
 * Comparative officer ranks of World War II
 * World War II German Army ranks and insignia
 * Military ranks of the Luftwaffe (1935–45)
 * Corps colours of the Luftwaffe (1935–45)
 * Uniforms and insignia of the Kriegsmarine
 * Japan - army ranks of the Japanese Empire during World War II
 * Japan - naval ranks of the Japanese Empire during World War II
 * United States Army enlisted rank insignia of World War I
 * United States Army enlisted rank insignia of World War II

title: List of comparative military ranks


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Measurable space

In mathematics, a measurable space or Borel space is a basic object in measure theory. It consists of a set and a σ-algebra, which defines the subsets that will be measured.

It captures and generalises intuitive notions such as length, area, and volume with a set $$X$$ of 'points' in the space, but regions of the space are the elements of the σ-algebra, since the intuitive measures are not usually defined for points. The algebra also captures the relationships that might be expected of regions: that a region can be defined as an intersection of other regions, a union of other regions, or the space with the exception of another region.

Definition
Consider a set $$X$$ and a σ-algebra $$\mathcal F$$ on $$X.$$ Then the tuple $$(X, \mathcal F)$$ is called a measurable space.

Note that in contrast to a measure space, no measure is needed for a measurable space.

Example
Look at the set: $$X = \{1,2,3\}.$$ One possible $$\sigma$$-algebra would be: $$\mathcal {F}_1 = \{X, \varnothing\}.$$ Then $$\left(X, \mathcal{F}_1 \right)$$ is a measurable space. Another possible $$\sigma$$-algebra would be the power set on $$X$$: $$\mathcal{F}_2 = \mathcal P(X).$$ With this, a second measurable space on the set $$X$$ is given by $$\left(X, \mathcal F_2\right).$$

Common measurable spaces
If $$X$$ is finite or countably infinite, the $$\sigma$$-algebra is most often the power set on $$X,$$ so $$\mathcal{F} = \mathcal P(X).$$ This leads to the measurable space $$(X, \mathcal P(X)).$$

If $$X$$ is a topological space, the $$\sigma$$-algebra is most commonly the Borel $\sigma$-algebra $$\mathcal B,$$ so $$\mathcal{F} = \mathcal B(X).$$ This leads to the measurable space $$(X, \mathcal B(X))$$ that is common for all topological spaces such as the real numbers $$\R.$$

Ambiguity with Borel spaces
The term Borel space is used for different types of measurable spaces. It can refer to
 * any measurable space, so it is a synonym for a measurable space as defined above
 * a measurable space that is Borel isomorphic to a measurable subset of the real numbers (again with the Borel $$\sigma$$-algebra)

title: Measurable space


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Wikipedia talk:PHP script new features

What is the intended behaviour of "User contributions"? The current behaviour was introduced over a month ago as the result of a feature request. Prior to that, it worked reasonably sensibly. --Zundark, 2002 Mar 8

I think it is intended to list all the articles that a user has made a non-minor edit to. When fulfilling the above request to exclude minor edits, a bug was introduced. I just fixed it in CVS, so we should see the expected behavior with the next update. AxelBoldt

I like the triple and quadruple tilde features. Ed Poor
 * I added this on Ed Poor, Thursday, March 28, 2002

Parenthetical hiding? Namespace cropping? Holy cow, how long have these been in place without me knowing about it? -- Stephen Gilbert 12:54 Dec 1, 2002 (UTC)

title: Wikipedia talk:PHP script new features


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X.25

X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts and finalized in a publication known as The Orange Book in 1976.

The protocol suite is designed as three conceptual layers, which correspond closely to the lower three layers of the seven-layer OSI Reference Model, although it was developed several years before the OSI model (1984). It also supports functionality not found in the OSI network layer. An X.25 WAN consists of packet-switching exchange (PSE) nodes as the networking hardware, and leased lines, plain old telephone service connections, or ISDN connections as physical links.

X.25 was a popular with telecommunications companies for their public data networks from the late 1970s to 1990s, which provided worldwide coverage. It was also used in financial transaction systems, such as automated teller machines, and by the credit card payment industry. However, most users have since moved to the Internet Protocol Suite (TCP/IP). X.25 is still used, for example by the aviation industry.

History
The CCITT (later ITU-T), the organization responsible for international standardization of telecom services, began developing a standard for packet-switched data communication in the mid-1970s based upon a number of emerging data network projects. Participants in the design of X.25 included engineers from Canada, France, Japan, the UK, and the USA representing a mix of national PTTs (France, Japan, UK) and private operators (Canada, USA). In particular, the work of Rémi Després, contributed significantly to the standard, which was based on a virtual circuit service. A few minor changes, which complemented the proposed specification, were accommodated to enable Larry Roberts to join the agreement. Various updates and additions were worked into the standard, eventually recorded in the ITU series of technical books describing the telecommunication systems. These books were published every fourth year with different-colored covers. The X.25 specification is part of the larger set of X-Series.

How the CCITT standardized virtual circuits
The CCITT appointed a special Rapporteur on packet switching, Halvor Bothner-By, who held an initial meeting in January 1974. This resulted in a question, to be answered by study group (SG) VII for the next CCITT plenary in 1976, which was “Should the packet-node of operation be provided on public data networks and, if so, how should it be implemented?”. A list of packet switching networks “to be considered” was provided: ARPANET (of the ARPA in the USA), EIN (of the European COST), EPSS (of the British Post Office Telecommunications), RCP (of the French PTT), CYCLADES (of IRIA in France), the NPL network (of the NPL in the UK), the SWIFT network (of the international SWIFT society), and the SITA network (of the international SITA company).

The second Rapporteur meeting, hosted in Oslo by the Norwegian Telecommunications Administration in November 1974, gathered 24 participants, including representatives of other international organizations (ISO, IFIP, ECMA). A document submitted by France “with the active support of a number of European administrations” served as “main basis for discussion in this meeting”. It was then “agreed that two types of services should be considered, a ‘datagram’ service and a ‘virtual call’ service”.

At the third meeting, focus had moved from whether there should be packet-mode networks to whether there could be “a standard for the interface between the network and the computers”.

Starting in January 1975, several bilateral and multilateral meetings took place between network operators having commitments for a packet switching service, in view to draft a common interface specification. Meetings started between the Canadian DATAPAC and the French TRANSPAC, continued with the startup Telenet of the USA, and continued with the BPO of the UK.

In March 1975, Halvor Bothner-By produced a list of recommendations to be created, or simply updated, for a packet switching standard to become possible. It was used as a framework at a drafting meeting in Ottawa between engineers of the four operators wishing to have a standard as soon as possible in the USA, Canada, France, UK and Japan. They prepared contributions to be submitted to SG VII in their name by administrations having voting right in CCITT. One contribution was a X.2x interface specification, the first version of what will become X.25).

The fourth Rapporteur meeting, in May 1975 in Geneva, had 45 participants and 27 new documents. The Rapporteur asked whether packet switching recommendations should be issued "with a view to making international interworking possible”, "the French administration answered in the affirmative and Canada strongly supported the French proposal". No firm conclusion was however obtained yet.

The fifth Rapporteur meeting, in September 1975 in Geneva, had about 60 participants. After discussions on the proposed virtual circuit interface, numerous issues were left unresolved. Concerning datagrams, ‘It was proposed by Larry Roberts of the US delegation and supported by representatives from France and Canada respectively that the datagram classification be changed from “E” to “A”’, i.e., from essential "to be available internationally” to additional that "may be available in certain countries and internationally”. The Rapporteur's last report expressed doubts “that a standard would be ready for adoption by SG VII”.

At the last meeting of the full SG VII before the CCITT plenary of September 1976, the available draft X.25 raised numerous clarification questions and/or and technical objections. SG VII's chairman Vern MacDonald appointed an editor and provided a meeting facility for the weekend. After intense work during it, all issues had been dealt with. For an approval by the full study group, a challenge remained: copies of the updated X.25 draft had to be available in two languages. To get them in due time, Anton Rybzynski of DATAPAC and Paul Guinaudeau of TRANSPAC spent a full night to handwrite all negotiated amendments, and to assemble them with paste and scissors into clean documents. COM VII then reviewed distributed copies, and unanimously approved them for submission to the forthcoming CCITT plenary. At this plenary of September 1976, the X.25 recommendation and the other 10 of SG VII were unanimously approved.

As requested by the USA, an optional datagram service was added to the revised X.25 of 1980, together with an alignment of its link layer, now called LAPB, with a recent evolution of HDLC in ISO. In absence of any public network operator implementing this option, datagrams were finally deleted from X.25 in its update of 1984.

Worldwide public data networks
Publicly accessible X.25 networks, commonly called public data networks, were set up in many countries during the late 1970s and 1980s to lower the cost of accessing various online services. Examples include Iberpac, TRANSPAC, Compuserve, Tymnet, Telenet, Euronet, PSS, Datapac, Datanet 1 and AUSTPAC as well as the International Packet Switched Service. Their combined network had large global coverage during the 1980s and into the 1990s.

Beginning in the early 1990s, in North America, use of X.25 networks (predominated by Telenet and Tymnet) started to be replaced by Frame Relay services offered by national telephone companies. Most systems that required X.25 now use TCP/IP, however it is possible to transport X.25 over TCP/IP when necessary.

X.25 networks are still in use throughout the world. A variant called AX.25 is used widely by amateur packet radio. Racal Paknet, now known as Widanet, remains in operation in many regions of the world, running on an X.25 protocol base. In some countries, like the Netherlands or Germany, it is possible to use a stripped version of X.25 via the D-channel of an ISDN-2 (or ISDN BRI) connection for low-volume applications such as point-of-sale terminals; but, the future of this service in the Netherlands is uncertain.

X.25 is still used in the aeronautical business (especially in Asia) even though a transition to modern protocols like X.400 is without option as X.25 hardware becomes increasingly rare and costly. As recently as March 2006, the United States National Airspace Data Interchange Network has used X.25 to interconnect remote airfields with air route traffic control centers.

France was one of the last remaining countries where commercial end-user service based on X.25 operated. Known as Minitel it was based on Videotex, itself running on X.25. In 2002, Minitel had about 9 million users, and in 2011 it accounted for about 2 million users in France when France Télécom announced it would shut down the service by 30 June 2012. As planned, service was terminated 30 June 2012. There were 800,000 terminals in operation at the time. An X.25 service was still purchasable from BT in the United Kingdom in 2019.

Architecture
The general concept of the X.25 was to create a universal and global packet-switched network. Much of the X.25 system is a description of the rigorous error correction needed to achieve this, as well as more efficient sharing of capital-intensive physical resources.

The X.25 specification defines only the interface between a subscriber (DTE) and an X.25 network (DCE). X.75, a protocol very similar to X.25, defines the interface between two X.25 networks to allow connections to traverse two or more networks. X.25 does not specify how the network operates internally – many X.25 network implementations used something very similar to X.25 or X.75 internally, but others used quite different protocols internally. The ISO protocol equivalent to X.25, ISO 8208, is compatible with X.25, but additionally includes provision for two X.25 DTEs to be directly connected to each other with no network in between. By separating the Packet-Layer Protocol, ISO 8208 permits operation over additional networks such as ISO 8802 LLC2 (ISO LAN) and the OSI data link layer.

X.25 originally defined three basic protocol levels or architectural layers. In the original specifications these were referred to as levels and also had a level number, whereas all ITU-T X.25 recommendations and ISO 8208 standards released after 1984 refer to them as layers. The layer numbers were dropped to avoid confusion with the OSI Model layers.

 * Physical layer: This layer specifies the physical, electrical, functional and procedural characteristics to control the physical link between a DTE and a DCE. Common implementations use X.21, EIA-232, EIA-449 or other serial protocols.
 * Data link layer: The data link layer consists of the link access procedure for data interchange on the link between a DTE and a DCE. In its implementation, the Link Access Procedure, Balanced (LAPB) is a data link protocol that manages a communication session and controls the packet framing. It is a bit-oriented protocol that provides error correction and orderly delivery.
 * Packet layer: This layer defined a packet-layer protocol for exchanging control and user data packets to form a packet-switching network based on virtual calls, according to the Packet Layer Protocol.

The X.25 model was based on the traditional telephony concept of establishing reliable circuits through a shared network, but using software to create "virtual calls" through the network. These calls interconnect "data terminal equipment" (DTE) providing endpoints to users, which looked like point-to-point connections. Each endpoint can establish many separate virtual calls to different endpoints.

For a brief period, the specification also included a connectionless datagram service, but this was dropped in the next revision. The "fast select with restricted response facility" is intermediate between full call establishment and connectionless communication. It is widely used in query-response transaction applications involving a single request and response limited to 128 bytes of data carried each way. The data is carried in an extended call request packet and the response is carried in an extended field of the call reject packet, with a connection never being fully established.

Closely related to the X.25 protocol are the protocols to connect asynchronous devices (such as dumb terminals and printers) to an X.25 network: X.3, X.28 and X.29. This functionality was performed using a packet assembler/disassembler or PAD (also known as a triple-X device, referring to the three protocols used).

Relation to the OSI Reference Model
Although X.25 predates the OSI Reference Model (OSIRM), the physical layer of the OSI model corresponds to the X.25 physical layer, the data link layer to the X.25 data link layer, and the network layer to the X.25 packet layer. The X.25 data link layer, LAPB, provides a reliable data path across a data link (or multiple parallel data links, multilink) which may not be reliable itself. The X.25 packet layer provides the virtual call mechanisms, running over X.25 LAPB. The packet layer includes mechanisms to maintain virtual calls and to signal data errors in the event that the data link layer cannot recover from data transmission errors. All but the earliest versions of X.25 include facilities which provide for OSI network layer Addressing (NSAP addressing, see below).

User device support
X.25 was developed in the era of computer terminals connecting to host computers, although it also can be used for communications between computers. Instead of dialing directly “into” the host computer – which would require the host to have its own pool of modems and phone lines, and require non-local callers to make long-distance calls –  the host could have an X.25 connection to a network service provider. Now dumb-terminal users could dial into the network's local “PAD” (packet assembly/disassembly facility), a gateway device connecting modems and serial lines to the X.25 link as defined by the X.29 and X.3 standards.

Having connected to the PAD, the dumb-terminal user tells the PAD which host to connect to, by giving a phone-number-like address in the X.121 address format (or by giving a host name, if the service provider allows for names that map to X.121 addresses). The PAD then places an X.25 call to the host, establishing a virtual call. Note that X.25 provides for virtual calls, so appears to be a circuit switched network, even though in fact the data itself is packet switched internally, similar to the way TCP provides connections even though the underlying data is packet switched. Two X.25 hosts could, of course, call one another directly; no PAD is involved in this case. In theory, it doesn't matter whether the X.25 caller and X.25 destination are both connected to the same carrier, but in practice it was not always possible to make calls from one carrier to another.

For the purpose of flow-control, a sliding window protocol is used with the default window size of 2. The acknowledgements may have either local or end to end significance. A D bit (Data Delivery bit) in each data packet indicates if the sender requires end to end acknowledgement. When D=1, it means that the acknowledgement has end to end significance and must take place only after the remote DTE has acknowledged receipt of the data. When D=0, the network is permitted (but not required) to acknowledge before the remote DTE has acknowledged or even received the data.

While the PAD function defined by X.28 and X.29 specifically supported asynchronous character terminals, PAD equivalents were developed to support a wide range of proprietary intelligent communications devices, such as those for IBM System Network Architecture (SNA).

Error control
Error recovery procedures at the packet layer assume that the data link layer is responsible for retransmitting data received in error. Packet layer error handling focuses on resynchronizing the information flow in calls, as well as clearing calls that have gone into unrecoverable states:

 * Level 3 Reset packets, which re-initializes the flow on a virtual call (but does not break the virtual call).
 * Restart packet, which clears down all virtual calls on the data link and resets all permanent virtual circuits on the data link.

Addressing and virtual circuits

X.25 supports two types of virtual circuits; virtual calls (VC) and permanent virtual circuits (PVC). Virtual calls are established on an as-needed basis. For example, a VC is established when a call is placed and torn down after the call is complete. VCs are established through a call establishment and clearing procedure. On the other hand, permanent virtual circuits are preconfigured into the network. PVCs are seldom torn down and thus provide a dedicated connection between end points.

VC may be established using X.121 addresses. The X.121 address consists of a three-digit data country code (DCC) plus a network digit, together forming the four-digit data network identification code (DNIC), followed by the national terminal number (NTN) of at most ten digits. Note the use of a single network digit, seemingly allowing for only 10 network carriers per country, but some countries are assigned more than one DCC to avoid this limitation. Networks often used fewer than the full NTN digits for routing, and made the spare digits available to the subscriber (sometimes called the sub-address) where they could be used to identify applications or for further routing on the subscribers networks.

NSAP addressing facility was added in the X.25(1984) revision of the specification, and this enabled X.25 to better meet the requirements of OSI Connection Oriented Network Service (CONS). Public X.25 networks were not required to make use of NSAP addressing, but, to support OSI CONS, were required to carry the NSAP addresses and other ITU-T specified DTE facilities transparently from DTE to DTE. Later revisions allowed multiple addresses in addition to X.121 addresses to be carried on the same DTE-DCE interface: Telex addressing (F.69), PSTN addressing (E.163), ISDN addressing (E.164), Internet Protocol addresses (IANA ICP), and local IEEE 802.2 MAC addresses.

PVCs are permanently established in the network and therefore do not require the use of addresses for call setup. PVCs are identified at the subscriber interface by their logical channel identifier (see below). However, in practice not many of the national X.25 networks supported PVCs.

One DTE-DCE interface to an X.25 network has a maximum of 4095 logical channels on which it is allowed to establish virtual calls and permanent virtual circuits, although networks are not expected to support a full 4095 virtual circuits. For identifying the channel to which a packet is associated, each packet contains a 12 bit logical channel identifier made up of an 8-bit logical channel number and a 4-bit logical channel group number. Logical channel identifiers remain assigned to a virtual circuit for the duration of the connection. Logical channel identifiers identify a specific logical channel between the DTE (subscriber appliance) and the DCE (network), and only has local significance on the link between the subscriber and the network. The other end of the connection at the remote DTE is likely to have assigned a different logical channel identifier. The range of possible logical channels is split into 4 groups: channels assigned to permanent virtual circuits, assigned to incoming virtual calls, two-way (incoming or outgoing) virtual calls, and outgoing virtual calls. (Directions refer to the direction of virtual call initiation as viewed by the DTE – they all carry data in both directions.) The ranges allowed a subscriber to be configured to handle significantly differing numbers of calls in each direction while reserving some channels for calls in one direction. All international networks are required to implement support for permanent virtual circuits, two-way logical channels and one-way logical channels outgoing; one-way logical channels incoming is an additional optional facility. DTE-DCE interfaces are not required to support more than one logical channel. Logical channel identifier zero will not be assigned to a permanent virtual circuit or virtual call. The logical channel identifier of zero is used for packets which don't relate to a specific virtual circuit (e.g. packet layer restart, registration, and diagnostic packets).

Billing
In public networks, X.25 was typically billed as a flat monthly service fee depending on link speed, and then a price-per-segment on top of this. Link speeds varied, typically from 2400 bit/s up to 2 Mbit/s, although speeds above 64 kbit/s were uncommon in the public networks. A segment was 64 bytes of data (rounded up, with no carry-over between packets), charged to the caller (or callee in the case of reverse charged calls, where supported). Calls invoking the Fast Select facility (allowing 128 bytes of data in call request, call confirmation and call clearing phases) would generally attract an extra charge, as might use of some of the other X.25 facilities. PVCs would have a monthly rental charge and a lower price-per-segment than VCs, making them cheaper only where large volumes of data are passed.

X.25 details
The network may allow the selection of the maximal length in range 16 to 4096 octets (2n values only) per virtual circuit by negotiation as part of the call setup procedure. The maximal length may be different at the two ends of the virtual circuit.
 * Data terminal equipment constructs control packets which are encapsulated into data packets. The packets are sent to the data circuit-terminating equipment, using LAPB Protocol.
 * Data circuit-terminating equipment strips the layer-2 headers in order to encapsulate packets to the internal network protocol.

X.25 facilities
X.25 provides a set of user facilities defined and described in ITU-T Recommendation X.2. The X.2 user facilities fall into five categories:

 * Essential facilities;
 * Additional facilities;
 * Conditional facilities;
 * Mandatory facilities; and,
 * Optional facilities.

X.25 also provides X.25 and ITU-T specified DTE optional user facilities defined and described in ITU-T Recommendation X.7. The X.7 optional user facilities fall into four categories of user facilities that require:

 * Subscription only;
 * Subscription followed by dynamic invocation;
 * Subscription or dynamic invocation; and,
 * Dynamic invocation only.

X.25 protocol versions
The CCITT/ITU-T versions of the protocol specifications are for public data networks (PDN). The ISO/IEC versions address additional features for private networks (e.g. local area networks (LAN) use) while maintaining compatibility with the CCITT/ITU-T specifications.

The user facilities and other features supported by each version of X.25 and ISO/IEC 8208 have varied from edition to edition. Several major protocol versions of X.25 exist:

 * CCITT Recommendation X.25 (1976) Orange Book
 * CCITT Recommendation X.25 (1980) Yellow Book
 * CCITT Recommendation X.25 (1984) Red Book
 * CCITT Recommendation X.25 (1988) Blue Book
 * ITU-T Recommendation X.25 (1993) White Book
 * ITU-T Recommendation X.25 (1996) Grey Book

The X.25 Recommendation allows many options for each network to choose when deciding which features to support and how certain operations are performed. This means each network needs to publish its own document giving the specification of its X.25 implementation, and most networks required DTE appliance manufacturers to undertake protocol conformance testing, which included testing for strict adherence and enforcement of their network specific options. (Network operators were particularly concerned about the possibility of a badly behaving or misconfigured DTE appliance taking out parts of the network and affecting other subscribers.) Therefore, subscriber's DTE appliances have to be configured to match the specification of the particular network to which they are connecting. Most of these were sufficiently different to prevent interworking if the subscriber didn't configure their appliance correctly or the appliance manufacturer didn't include specific support for that network. In spite of protocol conformance testing, this often lead to interworking problems when initially attaching an appliance to a network.

In addition to the CCITT/ITU-T versions of the protocol, four editions of ISO/IEC 8208 exist:

 * ISO/IEC 8208:1987, First Edition, compatible with X.25 (1980) and (1984)
 * ISO/IEC 8208:1990, Second Edition, compatible with 1st Ed. and X.25 (1988)
 * ISO/IEC 8208:1995, Third Edition, compatible with 2nd Ed. and X.25 (1993)
 * ISO/IEC 8208:2000, Fourth Edition, compatible with 3rd Ed. and X.25 (1996)

Legacy
The X.25 protocol had a lot of overhead to deal with data loss, since circuits back then ran over poor-grade cabling and had a lot of single-bit errors to deal with. As circuits became more and more reliable, the overhead was no longer needed, and less expensive Frame Relay took over. Frame Relay has its technical base in X.25, but does not attempt to correct errors.

The world-wide public data networks based on X.25 helped grow IP as a protocol riding on top.

X.25 was also available in niche applications such as Retronet that allow vintage computers to use the Internet.

title: X.25