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World Wide Web

World Wide Web

:For the world's first web browser, see WorldWideWeb. WorldWideWeb] The World Wide Web ("WWW" or simply the "Web") is an information space in which the items of interest, referred to as resources, are identified by global identifiers called Uniform Resource Identifiers (URIs). The term is often mistakenly used as a synonym for the Internet, but the Web is actually a service that operates over the Internet. (Find more information at [http://www.webopedia.com/DidYouKnow/Internet/2002/Web_vs_Internet.asp this link].)
Basic terms
The World Wide Web is the combination of four basic ideas:
- hypertext, that is the ability, in a computer environment, to move from one part of a document to another or from one document to another through internal connections among these documents (called "hyperlinks");
- computer network addresses, that is the ability, on a computer network, to locate a particular computer on the network through a unique address;
- the client-server model of computing, in which client software or a client computer makes requests of server software or a server computer that provides the client with resources or services, such as data or files; and
- markup language, in which characters or codes embedded in text indicate to a computer how to print or display the text, e.g. as in italics or bold type or font. On the World Wide Web, a client program called a web browser retrieves information resources, such as web pages and other computer files, from web servers using their network addresses and displays them, typically on a computer monitor, using a markup language that determines the details of the display. One can then follow hyperlinks in each page to other resources on the World Wide Web of information whose location is provided by these hyperlinks. It is also possible, for example by filling in and submitting web forms, to send information back to the server to interact with it. The act of following hyperlinks is often called "browsing" or "surfing" the Web. Web pages are often arranged in collections of related material called "websites." The phrase "surfing the Internet" was first popularised in print by Jean Armour Polly, a librarian, in an article called Surfing the INTERNET, published in the Wilson Library Bulletin in June, 1992. Although Polly may have developed the phrase independently, slightly earlier uses of similar terms have been found on the Usenet from 1991 and 1992, and some recollections claim it was also used verbally in the hacker community for a couple years before that. Polly is famous as "NetMom" in the history of the Internet. For more information on the distinction between the World Wide Web and the Internet itself — as in everyday use the two are sometimes confused — see Dark internet where this is discussed in more detail. Although the English word worldwide is normally written as one word (without a space or hyphen), the proper name World Wide Web and abbreviation WWW are now well-established even in formal English. The earliest references to the Web called it the WorldWideWeb (an example of computer programmers' fondness for intercaps) or the World-Wide Web (with a hyphen, this version of the name is the closest to normal English usage). Curiously, the abbreviation "WWW" is fallacious as it contains more syllables than the full term "World Wide Web", and thus takes longer to say.
How the Web works
When you want to access a web page, or other "resource", on the World Wide Web, you normally begin either by typing the URL of the page into your browser, or by following a hypertext link to that page or resource. The first step, behind the scenes, is for the server-name part of the URL to be resolved into an IP address by the global, distributed Internet database known as the Domain name system or DNS. The next step is for an HTTP request to be sent to the web server working at that IP address for the page required. In the case of a typical web page, the HTML text, graphics and any other files that form a part of the page will be requested and returned to the client in quick succession. The web browser's job is then to render the page as described by the HTML, CSS and other files received, incorporating the images, links and other resources as necessary. This produces the on-screen 'page' that you see. Most web pages will, themselves, contain hyperlinks to other relevant and informative pages and perhaps to downloads, source documents, definitions and other web resources. Such a collection of useful, related resources, interconnected via hypertext links, is what has been dubbed a 'web' of information. Making it available on the Internet produced what Tim Berners-Lee first called the World Wide Web in the early 1990s [http://www.w3.org/People/Berners-Lee/FAQ] [http://www.w3.org/People/Berners-Lee/Kids].
Origins
See also: History of the Internet History of the Internet The underlying ideas of the Web can be traced as far back as 1980, when Tim Berners-Lee and Robert Cailliau built ENQUIRE (referring to Enquire Within Upon Everything, a book Berners-Lee recalled from his youth). While it was rather different from the Web we use today, it contained many of the same core ideas (and even some of the ideas of Berners-Lee's next project after the WWW, the Semantic Web). In March 1989, Tim Berners-Lee wrote "Information Management: A Proposal", which referenced ENQUIRE and described a more elaborate information management system. [http://www.w3.org/History/1989/proposal.html] He published a more formal proposal for the actual World Wide Web on November 12, 1990 [http://www.w3.org/Proposal]. Implementation began on November 13, 1990 when Berners-Lee wrote [http://www.w3.org/History/19921103-hypertext/hypertext/WWW/TheProject.html the first Web page] on a NeXT workstation. During the Christmas holiday of that year, Berners-Lee built all the tools necessary for a working Web [http://www.w3.org/People/Berners-Lee/WorldWideWeb]: the first Web browser (which was a Web editor as well) and the first Web server. On August 6, 1991, he posted a [http://groups.google.com/groups?selm=6487%40cernvax.cern.ch short summary of the World Wide Web project] on the alt.hypertext newsgroup. This date also marked the debut of the Web as a publicly available service on the Internet. The crucial underlying concept of hypertext originated with older projects from the 1960s, such as Ted Nelson's Project Xanadu and Douglas Engelbart's oN-Line System (NLS). Both Nelson and Engelbart were in turn inspired by Vannevar Bush's microfilm-based "memex," which was described in the 1945 essay "As We May Think". Berners-Lee's brilliant breakthrough was to marry hypertext to the Internet. In his book Weaving The Web, he explains that he had repeatedly suggested that a marriage between the two technologies was possible to members of both technical communities, but when no one took up his invitation, he finally tackled the project himself. In the process, he developed a system of globally unique identifiers for resources on the Web and elsewhere: the Uniform Resource Identifier. The World Wide Web had a number of differences from other hypertext systems that were then available.
- The WWW required only unidirectional links rather than bidirectional ones. This made it possible for someone to link to another resource without action by the owner of that resource. It also significantly reduced the difficulty of implementing Web servers and browsers (in comparison to earlier systems), but in turn presented the chronic problem of broken links.
- Unlike certain applications such as HyperCard or Gopher, the World Wide Web was non-proprietary, making it possible to develop servers and clients independently and to add extensions without licensing restrictions. On April 30, 1993, CERN [http://intranet.cern.ch/Chronological/Announcements/CERNAnnouncements/2003/04-30TenYearsWWW/Welcome.html announced] that the World Wide Web would be free to anyone, with no fees due.
Web standards
At its core, the Web is made up of three standards:
- the Uniform Resource Identifier (URI), which is a universal system for referencing resources on the Web, such as Web pages;
- the HyperText Transfer Protocol (HTTP), which specifies how the browser and server communicate with each other; and
- the HyperText Markup Language (HTML), used to define the structure and content of hypertext documents. Berners-Lee now heads the World Wide Web Consortium (W3C), which develops and maintains these and other standards that enable computers on the Web to effectively store and communicate different forms of information.
Java and JavaScript
Another significant advance in the technology was Sun Microsystems' Java programming language. It initially enabled Web servers to embed small programs (called applets) directly into the information being served, and these applets would run on the end-user's computer, allowing faster and richer user interaction. Eventually, it came to be more widely used as a tool for generating complex server-side content as it is requested. Java never gained as much acceptance as Sun had hoped as a platform for client-side applets for a variety of reasons, including lack of integration with other content (applets were confined to small boxes within the rendered page) and poor perfomance (particularly start up delays) of Java VMs on PC hardware of that time. JavaScript, however, is a scripting language that was developed for Web pages. The standardised version is ECMAScript. While its name is similar to Java, it was developed by Netscape and not Sun Microsystems, and it has almost nothing to do with Java, with the only exception being that like Java its syntax is derived from the C programming language. Like Java, Javascript is also object oriented but like C++ and unlike Java, it allows mixed code - both object oriented as well as procedural. In conjunction with the Document Object Model, JavaScript has become a much more powerful language than its creators originally envisioned. Sometimes its usage is expressed under the term Dynamic HTML (DHTML), to emphasise a shift away from static HTML pages.
Sociological implications
The Web, as it stands today, has allowed global interpersonal exchange on a scale unprecedented in human history. People separated by vast distances, or even large amounts of time, can use the Web to exchange — or even mutually develop — their most intimate and extensive thoughts, or alternately their most casual attitudes and spirits. Emotional experiences, political ideas, cultural customs, musical idioms, business advice, artwork, photographs, literature, can all be shared and disseminated digitally with less individual investment than ever before in human history. Although the existence and use of the Web relies upon material technology, which comes with its own disadvantages, its information does not use physical resources in the way that libraries or the printing press have. Therefore, propagation of information via the Web (via the Internet, in turn) is not constrained by movement of physical volumes, or by manual or material copying of information. And by virtue of being digital, the information of the Web can be searched more easily and efficiently than any library or physical volume, and vastly more quickly than a person could retrieve information about the world by way of physical travel or by way of mail, telephone, telegraph, or any other communicative medium. The Web is the most far-reaching and extensive medium of personal exchange to appear on Earth. It has probably allowed many of its users to interact with many more groups of people, dispersed around the planet in time and space, than is possible when limited by physical contact or even when limited by every other existing medium of communication combined. Because the Web is global in scale, some have suggested that it will nurture mutual understanding on a global scale. By definition or by necessity, the Web has such a massive potential for social exchange, it has the potential to nurture empathy and symbiosis, but it also has the potential to incite belligerence on a global scale, or even to empower demagogues and repressive regimes in ways that were historically impossible to achieve.
Publishing web pages
The Web is available to individuals outside mass media. In order to "publish" a web page, one does not have to go through a publisher or other media institution, and potential readers could be found in all corners of the globe. Unlike books and documents, hypertext does not have a linear order from beginning to end. It is not broken down into the hierarchy of chapters, sections, subsections, etc. Many different kinds of information are now available on the Web, and for those who wish to know other societies, their cultures and peoples, it has become easier. When travelling in a foreign country or a remote town, one might be able to find some information about the place on the Web, especially if the place is in one of the developed countries. Local newspapers, government publications, and other materials are easier to access, and therefore the variety of information obtainable with the same effort may be said to have increased, for the users of the Internet. Although some websites are available in multiple languages, many are in the local language only. Also, not all software supports all special characters, and RTL languages. These factors would challenge the notion that the World Wide Web will bring a unity to the world. The increased opportunity to publish materials is certainly observable in the countless personal pages, as well as pages by families, small shops, etc., facilitated by the emergence of free web hosting services.
Statistics
According to a 2001 study [http://www.brightplanet.com/technology/deepweb.asp], there were more than 550 billion documents on the Web, mostly in the "invisible Web". A 2002 survey of 2,024 million web pages [http://www.netz-tipp.de/languages.html] determined that by far the most Web content was in English: 56.4%; next were pages in German (7.7%), French (5.6%) and Japanese (4.9%). A more recent study [http://www.cs.uiowa.edu/~asignori/web-size/] which used web searches in 75 different languages to sample the Web determined that there were over 11.5 billion web pages in the publically-indexable Web as of January 2005.
Speed issues
Frustration over congestion issues in the Internet infrastructure and the high latency that results in slow browsing has lead to an alternative name for the World Wide Web: the World Wide Wait. Speeding up the Internet is an ongoing discussion over the use of peering and QoS technologies. Other solutions to reduce the World Wide Wait can be found on [http://www.w3.org/Protocols/NL-PerfNote.html W3C].
Academic conferences
The major academic event covering the WWW is the World Wide Web series of conferences, promoted by [http://www.iw3c2.org IW3C2]. There is a [http://www.iw3c2.org/Conferences/Welcome.html list] with links to all conferences in the series.
Pronunciation of "www"
Most English-speaking people pronounce the 9-syllable letter sequence www used in some domain names for websites as "double U, double U, double U" despite shorter options like "triple double U", or even "World Wide Web" being available. Some languages do not have the letter w in their alphabet (for example, Italian), which leads some people to pronounce www as "vou, vou, vou." In some languages (such as Czech and Finnish) the w is substituted by a v, so Czechs pronounce www as "veh, veh, veh" rather than the correct but much longer pronunciation "dvojité veh, dvojité veh, dvojité veh;" the same applies to Finnish, where the correct pronunciation would be "kaksoisvee, kaksoisvee, kaksoisvee." Also in Norwegian, and similarly in Swedish and Danish: Instead of the correct "dobbel-ve, dobbel-ve, dobbel-ve" it is pronounced "ve, ve, ve". The pronunciation of "ve" instead of "dobbel-ve" is also used in other abbreviations. Several other languages (e.g. German, Dutch etc.) simply pronounce the letter W as a single syllable, so this problem doesn't occur. Depending on how the domain and web server are set up, a www website can often be accessed without entering the "www.", as long as the ".com" or other appropriate top-level domain is appended. Even this is not always necessary as some browsers will automatically try adding "www." and ".com" to typed URIs if a web page isn't found without them. In English pronunciation, saying the full words "World Wide Web" takes one-third as many syllables as saying the initialism "www". According to Berners-Lee, others mentioned this fact as a reason to choose a different name, but he persisted. Another, less common way of saying "www" is w3, or double u to the power of 3, power because the 3 in w3 is superscripted. However, the use of this initialism is uncommon. One further way is used by those wishing to speed up the full pronounciations by saying "All the double-U s" In New Zealand and occasionally in Australia, "www" is often pronounced "dub-dub-dub". This is widely accepted (for example its use in TV commercials appears standard) and is more concise than some other renditions in English. In the Southern United States the two syllable pronunciation of the letter w "dub-ya" is often used, resulting in dub-ya-dub-ya-dub-ya, even when spoken by persons who would normally use the "standard English" three syllable pronunciation for a single letter w.
See also

- History of the Internet
- Semantic Web
- Media studies
- Smartphone
- List of websites
- Search engine
- Web directory
- Hypertext
- First image on the Web
- Streaming media
- Cyberzine
- Web 2.0, term often applied to perceived ongoing transition of the WWW from a collection of websites to a full-fledged computing platform serving web applications
References

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External links

- [http://dmoz.org/Computers/Internet/Web_Design_and_Development/ Open Directory - Computers: Internet: Web Design and Development]
- [http://www.adstockweb.com/www-vl/ The World Wide Web Virtual Library: Web Design] from the World Wide Web Virtual Library
- [http://www.w3.org/History/19921103-hypertext/hypertext/WWW/TheProject.html World Wide Web], the first known web page.
- [http://www.mit.edu/people/mkgray/net/ Internet Statistics: Growth and Usage ofl
- [http://www.experienced-people.co.uk/1099-webmaster-glossary/ Alternative WWW and webmaster glossary] (humour)
Standards
The following is a cursory list of the documents that define the World Wide Web's three core standards:
- Uniform Resource Locator (URL)
- RFC 1738, URL Specification (updated by RFC 3986 "Uniform Resource Identifier (URI): Generic Syntax" in January 2005)
- Hypertext Markup Language (HTML)
- [http://www.w3.org/MarkUp/draft-ietf-iiir-html-01.txt Internet Draft, HTML version 1]
- RFC 1866, HTML version 2.0
- [http://www.w3.org/TR/REC-html32 HTML 3.2 Reference Specification]
- [http://www.w3.org/TR/html4/ HTML 4.01 Specification]
- [http://www.w3.org/TR/html/ Extensible HTML (XHTML) Specification]
- HyperText Transfer Protocol (HTTP) ja:World Wide Web ko:월드 와이드 웹 simple:World Wide Web th:เวิลด์ไวด์เว็บ

WorldWideWeb
:This article is about the first web browser. For the distributed hypertext system, see World Wide Web. WorldWideWeb was the world's first web browser and WYSIWYG HTML editor. It was introduced on February 26, 1991, by Tim Berners-Lee, and ran on the NeXTSTEP platform. It was later renamed Nexus to avoid confusion with the World Wide Web, also created by Berners-Lee. WorldWideWeb was the first program which used not only the common File Transfer Protocol but also the Hypertext Transfer Protocol, invented by Berners-Lee in 1989. At the time it was written, WorldWideWeb was the only way to view the Web.
History
Berners-Lee wrote WorldWideWeb in Objective-C on a NeXT computer during the second half of 1990. He was working for CERN at this time, simultaneously writing the first web server, called httpd, as well as various other bits of related software. The first successful build was completed on Christmas Day, 1990, and successive builds circulated among Berners-Lee's colleagues at CERN before being released to the public (by way of Internet newsgroups) in August 1991. By this time several others, including Bernd Pollermann, Robert Cailliau, Jean-François Groff, and graduate student Nicola Pellow (who wrote the line-mode browser) were involved in the project. Berners-Lee and Groff later adapted many of WorldWideWeb's components into the C programming language, creating the libwww API. By 1993, other browsers, such as NCSA/Mosaic, had replaced the WorldWideWeb program. Those involved in its creation had by then moved on to other tasks, such as defining standards and guidelines for the further development of the World Wide Web - e.g. the HTML language, various communication protocols, and so on. Several versions of WorldWideWeb are still available to download from [http://browsers.evolt.org/?worldwideweb/NeXT evolt.org's browser archive].
Technical information
Since WorldWideWeb was developed on and for the NeXTSTEP platform, the program used many of NeXTSTEP's components - WorldWideWeb's layout engine was built around NeXTSTEP's Text class.
Features
WorldWideWeb was capable of displaying basic style sheets, downloading and opening any file type supported by the NeXT system (which included PostScript, movies, sounds, and so on), browsing newsgroups, and spellchecking. At first, images were displayed in separate windows, until NeXTSTEP's Text class supported Image objects. Editing pages remotely was not yet possible, as the HTTP PUT method had not yet been implemented.
Trivia
WorldWideWeb's navigation panel contained Next and Previous buttons that would automatically navigate to the next or previous link on the last page visited, i.e., if one navigated to a page from a table of links, the Previous button would cause the browser to load the previous page linked in the table. This was initially useful because most pages were headed by such a table of links; however, as the World Wide Web grew, the convention was dropped, as were these buttons from later web browsers.
See also

- List of web browsers
- Comparison of web browsers
External links

- [http://www.w3.org/People/Berners-Lee/WorldWideWeb Tim Berners-Lee: WorldWideWeb]
- [http://www.w3.org/History.html A Little History of the World Wide Web]
- [http://www.amazon.com/exec/obidos/tg/detail/-/006251587X/qid=1121965642/sr=8-1/ref=pd_bbs_1/102-0904306-7586531?v=glance&s=books&n=507846 Weaving the Web, Berners-Lee's book about the conception of the Web] Category:Web browsers Category:NeXT ja:WorldWideWeb th:เวิลด์ไวด์เว็บ (เว็บเบราว์เซอร์)

Uniform Resource Identifier

A Uniform Resource Identifier (URI), is an Internet protocol element consisting of a short string of characters that conform to a certain syntax. The string comprises a name or address that can be used to refer to a resource. It is a fundamental component of the World Wide Web.

Relationship to URL and URN

A URI can be classified as a locator, a name, or both. A Uniform Resource Locator (URL) is a URI that, in addition to identifying a resource, provides a means of acting upon or obtaining a representation of the resource by describing its primary access mechanism or network "location". For example, the URL http://www.wikipedia.org/ is a URI that identifies a resource (Wikipedia's home page) and implies that a representation of that resource (such as the home page's current HTML code, as encoded characters) is obtainable via HTTP from a network host named www.wikipedia.org. A Uniform Resource Name (URN) is a URI that identifies a resource by name in a particular namespace. A URN can be used to talk about a resource without implying its location or how to dereference it. For example, the URN urn:ISBN:0-395-36341-1 is a URI that, like an International Standard Book Number (ISBN), allows one to talk about a book, but doesn't suggest where and how to obtain an actual copy of it. The contemporary point of view among the working group that oversees URIs is that the terms URL and URN are context-dependent aspects of URI and rarely need to be distinguished. Furthermore, the term URL is increasingly becoming obsolete, as it is rarely necessary to differentiate between URLs and URIs, in general. The term web address has also replaced URL in terms of popular usage.

Syntax

The URI syntax is essentially a URI scheme name like "http", "ftp", "mailto", "urn", etc., followed by a colon character, and then a scheme-specific part. The syntax and semantics of the scheme-specific part are determined by the specifications that govern the schemes, although the URI syntax does force all schemes to adhere to a certain general syntax that, among other things, reserves certain characters for special purposes, without always saying what those purposes are. The URI syntax also enforces restrictions on the scheme-specific part, in order to, for example, provide for a degree of consistency when the part has a hierarchical structure. Percent-encoding is an often misunderstood aspect of URI syntax.

History

Naming, addressing, and identifying resources

URIs and URLs have a shared history. The idea of a URL — a short string representing a resource that is the target of a hyperlink — was implicitly introduced in late 1990 in Tim Berners-Lee's proposals for HyperText[http://infomesh.net/html/history/early/]. At the time, it was called a hypertext name or document name[http://www.w3.org/History/19921103-hypertext/hypertext/WWW/Addressing/Addressing.html] Over the next three-and-a-half years, as the World Wide Web's core technologies of HTML (the HyperText Markup Language), HTTP, and Web browsers were developed, a need to distinguish between strings that provide an address for resources and those that merely name resources emerged. Although not yet formally defined, the term Uniform Resource Locator came to represent strings used for the former purpose, and the more contentious Uniform Resource Name came to represent strings used for the latter purpose. During the debate over how to best define URLs and URNs, it became evident that the two concepts embodied by the terms were merely aspects of the fundamental, overarching notion of resource identification. So, in June 1994, the IETF published Berners-Lee's RFC 1630: the first RFC that (in its non-normative text) acknowledged the existence of URLs and URNs, and, more importantly, defined a formal syntax for Universal Resource Identifiers — URL-like strings whose precise syntax and semantics were dependent upon their scheme. In addition, this RFC attempted to summarize the syntax of URL schemes that were in use at the time. It also acknowledged, but did not standardize, the existence of relative URLs and fragment identifiers.

Refinement of specifications

In December 1994, RFC 1738 was published in order to formally define relative and absolute URLs, refine the general URL syntax, define how relative URLs were to be resolved to absolute form, and better enumerate the URL schemes that were in use at the time. The definition and syntax of URNs was not settled upon until the publication of RFC 2141 in May 1997. With the publication of RFC 2396 in 1998, the URI syntax became a separate specification, and most parts of RFCs 1630 and 1738 became obsolete. In the new RFC, the "U" in "URI" was changed to represent "Uniform" rather than "Universal", and all parts of RFCs 1630 and 1738 relating to URIs and URLs in general were revised and expanded. Only those portions of RFC 1738 that summarized existing URL schemes were not rendered obsolete by RFC 2396. In December 1999, RFC 2732 provided a minor update to RFC 2396, allowing URIs to accommodate IPv6 addresses. Some time later, a number of shortcomings discovered in the two specifications led to the development a number of draft revisions under the title rfc2396bis. This community effort, coordinated by RFC 2396 co-author Roy Fielding, culminated in the publication of RFC 3986 in January 2005. This RFC is the current version of the URI syntax recommended for use on the Internet, and it renders RFC 2396 obsolete. It does not, however, render the details of existing URL schemes obsolete; those are still governed by RFC 1738, except where otherwise superseded — RFC 2616 for example, refines the "http" scheme. The content of RFC 3986 was simultaneously published by the IETF as the full standard STD 66, reflecting the establishment of the URI generic syntax as an official Internet protocol. In August 2002, RFC 3305 pointed out that the term URL has, despite its ubiquity in the vernacular of the Internet-aware public at large, faded into near-obsolescence. It now serves only as a reminder that some URIs act as addresses because they have schemes that imply some kind of network accessibility, regardless of whether they are actually being used for that purpose. As URI-based standards such as Resource Description Framework make evident, resource identification need not be coupled with the retrieval of resource representations over the Internet, nor does it need to be associated with network-bound resources at all.

URI reference

A URI reference is another type of string that represents a URI, and, in turn, the resource identified by that URI. The distinction between a URI and a URI reference is not often maintained in informal usage, but protocol documents should not allow for ambiguity. A URI reference may take the form of a full URI, or just the scheme-specific portion of one, or even some trailing component thereof —even the empty string. An optional fragment identifier, preceded by "#", may be present at the end of a URI reference. The part of the reference before the "#" indirectly identifies a resource, and the fragment identifier identifies some portion of that resource. In order to derive a URI from a URI reference, the URI reference is converted to "absolute" form by merging it with an absolute "base" URI, according to a fixed algorithm. Unless it is an absolute URI already, the URI reference is considered to be relative to the base URI. The base URI is typically the URI that identifies the document containing the URI reference, although this can be overridden by declarations made within the document or as part of an external data transmission protocol. If a fragment identifier is present in the base URI, it is ignored during the merging process. If a fragment identifier is present in the URI reference, it is preserved during the merging process. In web document markup languages, URI references are frequently used in places where there is a need to point to other resources, such as external documents or specific portions of the same logical document.

Uses of URI references in markup languages

- In HTML, the value of the src attribute of the img element is a URI reference, as is the value of the href attribute of the a element.
- In XML, the system identifier appearing after the SYSTEM keyword in a DTD is a fragmentless URI reference;
- In XSLT, the value of the href attribute of the xsl:import element/instruction is a URI reference, as is the first argument to the document() function.

Examples of absolute URIs

- http://somehost/absolute/URI/with/absolute/path/to/resource.txt
- ftp://somehost/resource.txt
- urn:issn:1535-3613

Examples of URI references

- http://example/resource.txt#frag01
- http://somehost/absolute/URI/with/absolute/path/to/resource.txt
- /relative/URI/with/absolute/path/to/resource.txt
- relative/path/to/resource.txt
- ../../../resource.txt
- resource.txt
- /resource.txt#frag01
- #frag01
- (empty string)

URI resolution

To "resolve" a URI means either to convert a relative URI reference to absolute form, or to dereference a URI or URI reference by attempting to obtain a representation of the resource that it identifies. The "resolver" component in document processing software generally provides both services. A URI reference may be considered to be a same-document reference: a reference to the document containing the URI reference itself. Document processing software is encouraged to use its current representation of the document to satisfy the resolution of a same-document reference; a new representation should not be fetched. This is only a recommendation, and document processing software is free to use other mechanisms to determine whether obtaining a new representation is warranted. According to the current URI specification, RFC 3986, a URI reference is a same-document reference if, when resolved to absolute form, it is identical to the base URI that is in effect for the reference. Typically, the base URI is the URI of the document containing the reference. XSLT 1.0, for example, has a document() function that, in effect, implements this functionality. RFC 3986 also formally defines URI equivalence, which can be used in order to determine that a URI reference, while not identical to the base URI, still represents the same resource and thus can be considered to be a same-document reference. Same-document references were determined differently according to RFC 2396, which was made obsolete by RFC 3986 but is still used as the basis of many specifications and implementations. According to this specification, a URI reference is a same-document reference if it is an empty string or consists of only the "#" character followed by an optional fragment.

Relation to XML namespaces

XML has a concept of a namespace, an abstract domain to which a collection of element and attribute names can be assigned. An XML namespace is identified by a character string, the namespace name, which must adhere to the generic URI syntax. However, the namespace name is not considered to be a URI because the "URI-ness" of strings is, according to the URI specification, based on how they are intended to be used, not just their lexical components. A namespace name also does not necessarily imply any of the semantics of URI schemes; a namespace name beginning with "http:", for example, likely has nothing to do with the HTTP protocol. There has been much debate about this among XML professionals on the xml-dev electronic mailing list; some feel that a namespace name could be a URI, since the collection of names comprising a particular namespace could be considered to be a resource that is being identified, and since the Namespaces in XML specification says that the namespace name is a URI reference. The consensus seems to be, though, that a namespace name is just a string that happens to look like a URI, nothing more. Initially, the namespace name was allowed to match the syntax of any non-empty URI reference, but the use of relative URI references was later deprecated by an erratum to the Namespaces In XML Recommendation. A separate specification was issued for namespaces for XML 1.1, and allows IRI references, not just URI references, to be used as the basis for namespace names. In order to mitigate the confusion that began to arise among newcomers to XML from the use of URIs (particularly HTTP URLs) for namespaces, a descriptive language called RDDL was developed. An RDDL document can provide machine- and human-readable information about a particular namespace and about the XML documents that use it. XML document authors were encouraged to put RDDL documents in locations such that if a namespace name in their document was somehow dereferenced, then an RDDL document would be obtained, thus satisfying the desire among many developers for a namespace name to point to a network-accessible resource.

External links

- RFC 3986 / STD 66 (2005) - Uniform Resource Identifier (URI): Generic Syntax
- RFC 2396 (1998), RFC 2732 (1999) - obsolete, but widely implemented, versions of the generic syntax
- RFC 3987 (2005) - Internationalized Resource Identifiers (IRIs)
- [http://gbiv.com/protocols/uri/ URI Working Group] - coordination center for development of URI standards
- [http://www.w3.org/TR/webarch/#identification Architecture of the World Wide Web, Volume One -- Chapter 2: Identification] - by W3C
- The IANA's official [http://www.iana.org/assignments/uri-schemes list of registered URI schemes]. Category:URI scheme Category:Internet standards Category:Identifiers ko:URI ja:Uniform Resource Identifier

Hypertext

In computing, hypertext is a user interface paradigm for displaying documents which, according to an early definition (Nelson 1970), "branch or perform on request." The most frequently discussed form of hypertext document contains automated cross-references to other documents called hyperlinks. Selecting a hyperlink causes the computer to display the linked document within a very short period of time. A document can be static (prepared and stored in advance) or dynamically generated (in response to user input). Therefore, a well-constructed hypertext system can encompass, incorporate or supersede many other user interface paradigms like menus and command lines, and can be used to access both static collections of cross-referenced documents and interactive applications. The documents and applications can be local or can come from anywhere with the assistance of a computer network like the Internet. The most famous implementation of hypertext is the World Wide Web. The term "hypertext" is often used where the term hypermedia would be more appropriate.

History

Foreshadowing hypertext was a simple technique used in various reference works (dictionaries, encyclopedias, etc.), consisting of setting a term in small capital letters, as an indication that an entry or article existed for that term (within the same reference work). In addition to such manual cross-references, there were experiments with various methods for arranging layers of annotations around a document. The most famous example is the Talmud. The point of hypertext is to deal with the problem of information overload. All of the persons mentioned below were obsessed with the realization that humanity is simply drowning in information, so that, too often, decisionmakers keep making foolish decisions and scientists inadvertently duplicate existing work (e.g., the belated rediscovery of Gregor Mendel's work). In the early 20th century, two visionaries attacked the cross-referencing problem through proposals based on labor-intensive brute force methods. Paul Otlet proposed a proto-hypertext concept based on his monographic principle in which all documents would be decomposed down to unique phrases stored on index cards. In the 1930s, H.G. Wells proposed the creation of a World Brain. For obvious reasons like cost, neither proposal got very far. Therefore, all major histories of hypertext start with 1945, when Vannevar Bush wrote an article in The Atlantic Monthly called "As We May Think," about a futuristic device he called a Memex. He described the device as mechanical desk linked to an extensive archive of microfilms and able to display books, texts or any document from the library, and further able to automatically follow references from any given page to the specific page referenced. Most experts do not consider the Memex to be a true hypertext system. However, the story starts with the Memex because "As We May Think" directly influenced and inspired the two American men generally credited with the invention of hypertext, Ted Nelson and Douglas Engelbart. Nelson coined the word "hypertext" in 1965 and helped Andries van Dam develop the Hypertext Editing System in 1968 at Brown University; Engelbart had begun working on his NLS system in 1962 at Stanford Research Institute, although delays in obtaining funding, personnel and equipment meant that its key features were not completed until 1968. After funding for NLS slowed to a trickle in 1974, progress on hypertext research nearly came to a halt. During this time, the ZOG at Carnegie Mellon started as an artificial intelligence research project under the supervision of Allen Newell. Only much later would its participants realize that their system was a hypertext system. ZOG was deployed in 1980 on the U.S.S. Carl Vinson and later commercialized as KMS. The first hypermedia application was the Aspen Movie Map in 1977. The early 1980s saw a number of experimental hypertext and hypermedia programs, many of whose features and terminology were later integrated into the Web. However, none of these systems achieved widespread success or name recognition with consumers. Guide was the first hypertext system for personal computers, but it was not very successful. Guide was quite expensive and difficult to use, as it had originally been developed for UNIX workstations and was subsequently ported to DOS. It was immediately eclipsed by HyperCard. In August 1987, Apple Computer revealed its HyperCard application for its Macintosh line of computers at the MacWorld convention in Boston. HyperCard was an immediate hit and helped to popularize the concept of hypertext with the general public (although as Jakob Nielsen later pointed out, it was technically a hypermedia system because its hyperlinks originated only from regions on the screen). The first hypertext-specific academic conference also took place that year. Meanwhile, Nelson had been working on and advocating his Xanadu system for over two decades, and the commercial success of HyperCard stirred Autodesk to invest in his revolutionary ideas. The project limped on for four years without ever releasing a complete product, before Autodesk pulled the plug in the midst of the 1991-1992 recession. In late 1990, Tim Berners-Lee, a scientist at CERN, invented the World Wide Web to meet the demand for automatic information sharing between scientists working in different universities and institutes all over the world. Early in 1993, the National Center for Supercomputing Applications (NCSA) at the University of Illinois released a first version of their Mosaic browser to replace the two lacking existing web browsers: one that ran only on NeXTSTEP and one that was minimally user-friendly. Mosaic ran in the X Window System environment, popular in the research community, and offered usable window-based interaction. Web traffic exploded from only 500 known web servers in 1993 to over 10,000 in 1994 after the release of browser versions for both the PC and Macintosh environments. All the earlier hypertext systems were quickly overshadowed by the success of Tim Berners-Lee's World Wide Web, even though the latter lacked many features of those earlier systems such as typed links, transclusion and source tracking.

Implementations

Besides the already mentioned HyperCard and World Wide Web, there are other noteworthy implementations of hypertext, with different feature sets:
- Microsoft Word has evolved in orientation from paper to in-computer documents.
- Information Presentation Facility used for displaying help in the IBM operating systems.
- Windows Help
- Adobe's Portable Document Format supports links.
- Texinfo, the GNU help system.
- Project Xanadu
- XML with the XLink extension.
- The many implementations of wiki, like the MediaWiki system that powers Wikipedia, that aim to compensate for the lack of integrated editors in most Web browsers.

Academic Conferences

One of the top academic conferences for new research in hypertext is the annually held ACM's Conference on Hypertext and Hypermedia ([http://www.ht04.org/ HT 2004]) Although not exclusively about hypertext, the World Wide Web series of conferences, organized by [http://www.iw3c2.org IW3C2], includes many papers of interest. There is a [http://www.iw3c2.org/conferences/ list] with links to all conferences in the series.

Hypertext as Literature

The development of hypertext fiction, a branch of electronic literature, has coincided with the growth and proliferation of hypertext development software and the emergence of electronic networks. Two software programs specifically designed for literary hypertext, Storyspace and Intermedia became available in the 1990's. Storyspace v2.0, a professional level hypertext development tool, is available from Eastgate Systems, which has also published many notable hypertext fictions, including Michael Joyce's afternoon, a story, Shelley Jackson's Patchwork Girl (hypertext), and Stuart Moulthrop's Victory Garden. Important early hypertext critics and theorists include Jay David Bolter, George Landow, Stuart Moulthrop, J.Yellowlees Douglas, Robert Coover, Michael Joyce, and N. Katherine Hayles.

References

-
-
-
-
-
-
-
- [http://www.newmediareader.com/excerpts.html].
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- [http://www.cs.brown.edu/memex/HT_87_Keynote_Address.html]

External links

- [http://www.interaction-design.org/references/conferences/series/acm_conference_on_hypertext_and_hypermedia.html The ACM Conference on Hypertext and Hypermedia]
- [http://www.eliterature.org Electronic Literature Organization] (for more on hypertext literature)
- [http://www.ericdigests.org/pre-9212/hype.htm Hypertext: Behind the Hype]
- [http://www.mprove.de/diplom/text/2_hypertext.html mprove: Historical Overview of Hypertext]
- [http://xanadu.com/XUarchive/ccnwwt65.tif The first use of hypertext (?) - TIFF image]
- [http://www.pontomidia.com.br/ricardo/colinks/english.html Co-link, a Brazilian research project]
- [http://www.hotuploads.com Scripts Search Engine]
- [http://www-2.cs.cmu.edu/~amulet/papers/uihistory.tr.html A Brief History of Human Computer Interaction Technology]
- [http://nrg78.com the TAI/MAI/NAI Progession Overview and NRG's "Laws of Good Answers" presented as a Network Distributed Dense Meme Stack] Category:Computing Category:Human-computer interaction Category:Hypertext ko:하이퍼텍스트 ja:ハイパーテキスト

Client-server

Client/Server is a network application architecture which separates the client (usually the graphical user interface) from the server. Each instance of the client software can send requests to a server or application server.

Introduction

A Client/Server architecture is intended to provide a scalable architecture, whereby each computer or process on the network is either a client or a server. Server software generally, but not always, runs on powerful computers dedicated for exclusive use to running the business application. Client software on the other hand generally runs on common PCs or workstations. Clients get all or most of their information and rely on the application server for things such as configuration files, stock quotes, business application programs, or to offload computer-intensive application tasks back to the server in order to keep the client computer (and client computer user) free to perform other tasks. Properties of a server:
- Properties of a client:
- Active (Master)
- Sending requests
- Waits until reply arrives Servers can be stateless or stateful. A stateless server does not keep The interaction between client and server is often described using Unified_Modeling_Language#Sequence ---- [[

any information between requests. Example: An HTTP server for static HTML pages. A stateful server can remember information between requests. The scope of this information can be global or session. Example: Apache Tomcat.

]]_Diagram|sequence diagrams]]. Sequence diagrams are standardized in the UML. Another type of network architecture is known as a peer-to-peer architecture because each node or instance of the program is both a "client" and a "server" and each has equivalent responsibilities. Both client/server and peer-to-peer architectures are in wide use. Each has advantages and disadvantages..

Thin Clients vs. Fat Clients

Another type of client in the Client/Server architecture is known as a thin client, which is a minimal client. Thin clients utilize as few resources on the host PC as possible. A thin client's job is generally just to graphically display information from the application server. This allows a company the ease of managing their business logic for all applications at a central location.

n-Tier Architecture

Application servers usually store data on a third machine, known as the database server. This is called a three-tier architecture whereas a generic client/server architecture is two-tier. In general, an n-tier or Multi-tier architecture may deploy any number of distinct services, including transitive relations between application servers implementing different functions of business logic, each of which may or may not employ a distinct or shared database system.

Addressing

Methods of addressing in client server environments can be described as follows
- Machine process addressing; where the address is divided up as follows process@machine. Therefore 56@453 would indicate the process 56 on computer 453
- Name Server; Name servers have an index of all names and addresses of servers in the relevant domain.
- Localization packets; Broadcast messages are sent out to all computers in the distributed system to determine the address of the destination computer
- Trader; A trader is a system that indexes all the services available in a distributed system. A computer requiring a particular service will check with the trading service for the address of a computer providing such a service.

Examples

A popular client in widespread use today is the web browser which communicates with web servers over the internet to fetch and display web page content. The X Window System is a client-server architecture with an unusual property. The server is always local (near the user) and the client can be local or remote. This can be less confusing if you think of the server (the X display) as making some resource available (a windowing display system) and the client as making use of that resource.

Other Network Architectures

- multitier architecture
- Peer-to-peer Category:Computer networks Category:Software architecture

Markup language

. This enables sophisticated queries to be performed, as well as easy translation into HTML.]] A markup language combines text and extra information about the text. The extra information, for example about the text's structure or presentation, is expressed using markup, which is intermingled with the primary text. The best-known markup language in modern use is HTML (Hypertext Markup Language), one of the foundations of the World Wide Web. Historically, markup was (and is) used in the publishing industry in the communication of printed work between authors, editors, and printers.

Classes of markup languages

Markup languages are often divided into three classes: presentational, procedural, and descriptive.

Presentational markup

Presentational markup expresses document structure via the visual appearance of the whole text of a particular fragment. For example, in a word processor file, the title of a document might be preceded by several newlines and spaces, thus accomplishing leading space and centering. Punctuation may also be considered a form of presentational markup. Word-processing and desktop publishing products cannot help but support presentational markup, and for better or worse it is the first, or even only, kind learned by many users. While trivial to learn, it is the least amenable to computer processing, such as applying new rendering to the text, or searching for particular components.

Procedural markup

Procedural markup is typically also focused on the presentation of text, but is usually visible to the user editing the text file, and is expected to be interpreted by software in the order in which it appears. To format a title, a succession of formatting directives would be inserted into the file immediately before the title's text, instructing software to switch into centered display mode, then enlarge and embolden the typeface. The title text would be followed by directives to reverse these effects; in more advanced systems macros or a stack model make this less tedious. In most cases, the procedural markup capabilities comprise a Turing-complete programming language. Examples of procedural-markup systems include nroff, troff, TeX, and PostScript. Procedural markup has been widely used in professional publishing applications, where professional typographers can be expected to learn the languages required.

Descriptive Markup

Descriptive Markup applies labels to fragments of text without necessarily mandating any particular display or other processing semantics. For example, the Atom syndication language provides markup to label the "updated" time-stamp which is an assertion from the publisher as to when some item of information was last changed. While the Atom specification discusses the meaning of the "updated" timestamp, and specifies the markup used to identify it, it makes no assertions about whether or how it might be presented to a user. Software might put this markup to a variety of uses, including many not foreseen by the designers of the Atom language. SGML and XML are systems explicitly designed to support the design of descriptive markup languages. In practice, the classes of markup usually co-occur in any given system. For example, HTML contains markup elements which are purely procedural (for example b for bold) and others which are purely descriptive ( "blockquote", or the "href=" attribute). HTML also includes the PRE element, which encloses areas of presentational markup to be laid out exactly as typed. Sets of markup elements and rules for their use are commonly developed by standards bodies to support the kinds of documents used in particular industries or communities. One of the earliest of these was CALS, used by the US military for technical manuals. Industries with large-scale documentation requirements soon followed suit, developing tag-sets for aircraft, telecommunications, automotive, and computer hardware manuals. This led to delivering many such manuals solely in electronic form; some companies were able to produce printed, online, and CD-based manuals all from a single (descriptive markup) source. A notable example was Sun Microsystems, where Jon Bosak (who later founded the XML committee) decided on SGML for multi-target documentation delivery, achieving considerable cost savings. Markup languages now abound; among the more widely known are DocBook, MathML, SVG, Open eBook, TEI, and XBRL. Many are for various kinds of text documents, but specialized languages are used in many other domains. Generic Markup is another term for "Descriptive Markup". Most modern descriptive markup systems structure documents into trees, while also providing some means for embedding cross-references. Because of this, documents can be readily treated as databases, in which the database system is aware of the structure (not "blobs" as in the past). Because they do not have such strict schemas as relational databases, however, they are commonly called "semi-structured databases". In the third millennium, great interest has arisen in document structures that are not trees. For example, ancient and sacred literature commonly has a rhetorical or prose structure (stories, pericopes, paragraphs, and so on), as well as a reference structure (books, chapters, verses, lines). Since the boundaries of these units often cross, they cannot readily be encoded using tree-structured markup systems. Among the document modeling systems that support such structures are MECS (developed for encoding the works of Wittgenstein), aspects of the TEI Guidelines, LMNL, and CLIX. A primary virtue of descriptive markup is considered to be its flexibility: if the fragments of text are labeled as to "what they are" as opposed to "how they should be displayed", software may be written to process these fragments in useful ways not anticipated by the designers of the languages. For example, HTML's hyperlinks, originally designed for activation by a human following a link, are also widely used by Web search engines both in discovering new material to index and in estimating the popularity of Web resources. Descriptive markup also facilitates the simpler task of reformatting a document as needed, because the format specification is not intertwined with the content. For example, italics might be used both for emphasis, and to indicate foreign words. However, if both are merely tagged (presentationally or procedurally) as italic, this ambiguity cannot readily be sorted out. If a decision is later made not to italicize foreign words, there is nothing for it but to review all italic portions and sort them out one by one. However, if the two cases were (descriptively or generically) tagged differently to begin with, either can be reformatted without interfering with the other.

History

The term "markup" is derived from the traditional publishing practice of "marking up" a manuscript, that is, adding printer's instructions in the margins of a paper manuscript. For centuries, this task was done by specialists known as "markup men" who marked up text to indicate what typeface, font, style, and size should be applied to each part, and then handed off the manuscript to someone else for the tedious task of typesetting by hand. The idea of "markup languages" was apparently first presented by publishing executive William W. Tunnicliffe at a conference in 1967, although he preferred to call it "generic coding." Tunnicliffe would later lead the development of a standard called GenCode for the publishing industry. Book designer Stanley Fish also published speculation along similar lines in the late 1960s. Brian Reid, in his 1980 dissertation at Carnegie Mellon University, developed the theory and a working implementation of descriptive markup in actual use. However, IBM researcher Charles Goldfarb is more commonly seen today as the "father" of markup languages, because of his work on IBM GML, and then as chair of the International Organization for Standardization committee that developed SGML, the first widely-used descriptive markup system. Goldfarb hit upon the basic idea while working on an early project to help a newspaper computerize its workflow, although the published record does not clarify when. He later became familiar with the work of Tunnicliffe and Fish, and heard an early talk by Reid which further sparked his interest. It must be noted that the details of the early history of descriptive markup languages are hotly debated. However, it is clear that the notion was independently discovered several times throughout the 70s (and possibly the late 60s), and became an important practice in the late 80s. Some early examples of markup languages available outside the publishing industry can be found in typesetting tools on Unix systems such as troff and nroff. In these systems, formatting commands were inserted into the document text so that typesetting software could format the text according to the editor's specifications. It was a trial and error iterative process to get a document printed correctly. Availability of WYSIWYG ("what you see is what you get") publishing software supplanted much use of these languages among casual users, though serious publishing work still uses markup to specify the non-visual structure of texts. Another major publishing standard was TeX, created and continuously refined by Donald Knuth in the 1970s and 80s. TeX concentrated on detailed layout of text and font descriptions in order to typeset mathematical books in professional quality. This required Knuth to spend considerable time investigating the art of typesetting. However, TeX requires considerable skill from the user, so that it is mainly used in academia, where it is a de-facto standard in many scientific disciplines. A TeX macro package known as LaTeX provides a descriptive markup system on top of TeX, and is widely used. The first language to make a clear and clean distinction between structure and presentation was certainly Scribe, developed by Brian Reid and described in his 1980 doctoral thesis in [5]. Scribe was revolutionary in a number of ways, not least that it introduced the idea of styles separated from the marked up document, and of a grammar controlling the usage of descriptive elements. Scribe influenced the development of Generalized Markup Language (later SGML) and is a direct ancestor to HTML and LaTeX. In the early 1980s, the idea that markup should be focused on the structural aspects of a document and leave the visual presentation of that structure to the interpreter led to the creation of SGML. The language was developed by a committee chaired by Goldfarb. It incorporated ideas from many different sources, including Tunnicliffe's project, GenCode. Sharon Adler, Anders Berglund, and James Mason were also key members of the SGML committee. SGML specified a syntax for including the markup in documents, as well as one for separately describing what tags were allowed, and where (the DTD or schema). This allowed authors to create and use any markup they wished, selecting tags that made the most sense to them and were named in their own natural languages. Thus, SGML is properly a meta-language, and many particular markup languages are derived from it. From the late 80s on, most substantial new markup languages have been based on SGML system, including for example TEI and DocBook. SGML was promulgated as an International Standard by International Organization for Standardization, ISO 8879, in 1986. SGML found wide acceptance and use in fields with very large-scale documentation requirements. However, it was generally found to be cumbersome and difficult to learn, a side effect of attempting to do too much and be too flexible. For example, SGML made end tags (or start-tags, or even both) optional in certain contexts, because it was thought that markup would be done manually by overworked support staff who would appreciate saving keystrokes. By 1991, it appeared to many that SGML would be limited to niche uses while WYSIWYG tools (which stored documents in proprietary binary formats) would take over the vast majority of document processing. The situation changed dramatically when Sir Tim Berners-Lee, learning of SGML from co-worker Anders Berglund at CERN, used SGML syntax to create HTML. HTML resembles any other SGML-based tag language, though it began as simpler than most and a formal DTD was not developed until later. DeRose[3] argues that HTML's use of descriptive markup (and SGML in particular) was a major factor in the success of the Web, because of the flexibility and extensibility that enabled (other factors include the notion of URLs and the free distribution of browsers). HTML is likely the most used document format in the world today. Another, newer, markup language that has gained great importance is XML (Extensible Markup Language). XML was developed by the World Wide Web Consortium, in a committee created and chaired by Jon Bosak. The main purpose of XML was to simplify SGML by focusing on a particular problem — documents on the Internet [4]. XML remains a meta-language like SGML, allowing users to create any tags needed (thus it's extensible) and then describe those tags and their permitted uses. XML adoption was greatly helped because every XML document is also an SGML document, and existing SGML users and software could switch over relatively easily. However, XML mercilessly eliminated the complex features of SGML, radically easing learning and implementation. Other major contributions were to rectify some SGML problems in international settings, and to make it possible to parse and interpret documents correctly whether or not a schema is available. XML was designed primarily for semi-structured environments such as documents and publications. However, it appeared to hit a sweet spot between simplicity and flexibility, and was rapidly adopted for many other uses. XML is now a markup language of choice for interchanging relational database data; for communicating transaction data between servers; for interactive vector graphics; and for many other unanticipated uses. The newest incarnation of HTML is also based on XML: XHTML or eXtensible Hypertext Markup Language is a more rigorous and robust version that requires documents to be "well-formed" XML documents, but which uses the familiar HTML tags. The main difference between HTML and XHTML from the standpoint of coding the language is that all tags must be closed ('empty' tags such as
must either be 'closed' with a regular end-tag, or replaced by a special form:
).

Features

A common feature of many markup languages is that they intermix the text of a document with markup instructions in the same data stream or file. Here, for example, is a small section of text marked up in HTML: <h1> Anatidae </h1> <p> The family <i>Anatidae</i> includes ducks, geese, and swans, but <em>not</em> the closely-related screamers. </p> The codes enclosed in angle-brackets <like this> are markup instructions (known as tags), while the text between these instructions is the actual text of the document. The codes "h1", "p", and "em" are examples of structural markup, in that they describe the intended purpose or meaning of the text they include. Specifically, "h1" means "this is a first-level heading", "p" means "this is a paragraph", and "em" means "this is an emphasized word". A device reading such structural markup may apply its own rules or styles for presenting it, using larger type, boldface, indentation, or whatever style it prefers. The "i" instruction is an example of presentational markup. It specifies the exact appearance of the text (in this case, the use of an italic typeface) without specifying the reason for that appearance. The Text Encoding Initiative (TEI) has published extensive guidelines for how to encode texts of interest in the humanities and social sciences, developed through years of international cooperative work. These guidelines are used by countless projects encoding historical documents, the works of particular scholars, periods, or genres, and so on.

Alternative usage

While the idea of markup language was originated from text document, there is an increasing usage of markup languages in areas like vector graphics, web services, content syndication, and user interfaces. Most of these are applications of XML because it is clean, well-formatted, and extensible. The use of XML has also lead to the possibility of combining multiple markup languages into a single profile, like XHTML+SMIL and XHTML+MathML+SVG [http://www.w3.org/TR/2002/WD-XHTMLplusMathMLplusSVG-20020809/].

References

- [http://www.tei-c.org/Guidelines2/index.html TEI guidelines]
- [http://xml.coverpages.org/coombs.html Markup systems and the future of scholarly text processing] by James H. Coombs, Allen H. Renear, and Steven J. DeRose. Originally published in the November 1987 CACM, and reprinted several times in other forums, this article introduced many of the concepts now used in discussing markup languages, and lays out the basic arguments for the superior usability of descriptive markup.
- [3]DeRose, Steven J. "The SGML FAQ Book." Boston: Kluwer Academic Publishers, 1997. ISBN 0-7923-9943-9
- [4]http://www.w3.org/TR/2004/REC-xml11-20040204/ Extensible Markup Language (XML)]
- [5]Reid, Brian. "Scribe: A Document Specification Language and its Compiler." Ph.D. thesis, Carnegie-Mellon University, Pittsburgh PA. Also available as Technical Report CMU-CS-81-100. Category:Formal_languages ko:마크업 언어 ja:マークアップ言語

Web page

A Web page or webpage is a resource on the World Wide Web, usually in HTML/XHTML format (the file extensions are typically htm or html) and with hypertext links to enable navigation from one page or section to another. Web pages often use associated graphics files to provide illustration, and these too can be clickable links. A web page is displayed using a web browser. A web page can contain any of the following:
- Text
- graphics (gif, jpeg or png)
- Audio (.mid or .wav)
- Interactive multimedia content that requires a plugin such as Flash, Shockwave or VML
- applets (subprograms that run inside the page) which often provide motion graphics, interaction, and sound applet Web pages also contain content that cannot be seen in the browser:
- Scripts (usually JavaScript) which add functionality to the page - e.g. powering rollover effects or verifying form input.
- Meta tags - hidden content with information about the page, instructions for search engine robots, etc. The keywords and description meta tags help search engines categorise the page and provide information for search results.
- Cascading Style Sheets (CSS) which determine how the page is formatted
- Comments Web pages can be larger than the web browser window. They are often taller than the browser window and require vertical scrolling. Pages sometimes have additional content on the right which is not visible in smaller windows without horizontal scrolling. Pages designed to scroll horizontally are less common, partly because they sometimes do not print out well, but mainly because visitors find having them inconvenient. See also page widening. A web page can be either a single page, or a frameset made up of separate frames. Each frame contains an HTML page. Frames can cause problems with navigation and printing and can adversely affect search engine ratings. Their main merit is to allow some content (usually a navigation menu) to stay in one place while other content is scrolled. This effect is now possible using CSS and if it is required this is the recommended way to achieve it. A collection of web pages stored in a single folder or within related subfolders of a web server is known as a website. A website includes an index page. This is a file with a special name, often index.html or something similar, which is defined in the web server's configuration. When a visitor requests a web address URL which doesn't include a page (e.g. www.mysite.com) the web server will look for an index page and serve that to your browser. If no index page exists, you will see either a listing of the folder's contents, or an error page (depending on other web server settings). A consideration in designing and testing web pages is that they should meet the recommendations for correct HTML, CSS and other standards as laid down by the World Wide Web Consortium. This is so as to keep the World Wide Web free and available to all. If you do this, your web site will be suitable for many browsers and browser settings and different screen resolutions, as well as accessible to people with disabilities (e.g. sight impairment), older browsers and text-only browsers. Target audience, budget and lack of awareness of accessibility issues often mean that a site is not as accessible as it could be. Using features which only work in one browser without providing alternative content is considered bad practice; the World Wide Web Consortium works with web server and browser designers to develop web standards and encourage standards compliance.

Navigation bar

A navigation bar, linkbar, link bar or links bar is an area/a pane of a web page that contains hypertext links in order to enable navigation from one page of a website to another.

URL

Usually a web page has a more or less permanent URL, a so-called permalink, which therefore allows deep linking. Some URLs refer only to a cache area, e.g. when the page is the result of zooming and shifting a view of a map. Other URLs contain query data that define or change the information displayed on the resulting page in some major or minor way.

Graphics

For embedding (transclusion) of an image in a web page, see HTML element#Images. The graphics file format in web pages is usually JPEG for photographs and GIF or PNG for other images such as diagrams, drawings, graphs, etc. The last two formats can also be used for photographs but are not as suitable for that purpose as JPEG (JPEG is lossy while GIF and PNG are lossless). GIF is used for animations, GIF and PNG for images with transparent pixels, PNG for images with partially transparent pixels (but this is not supported by e.g. IE). All these are raster graphics. There is also the SVG format: Scalable Vector Graphics. Currently more common ways to supply vector graphics are either with a PDF file, viewed either using a plug-in of the browser or a separate viewer, or with Flash. This is useful e.g. for a map, often a combination of a vector graphics layer and text, and possibly a raster graphics layer. This gives better results when zooming in than a GIF or PNG image (JPEG would be even worse due to compression artifacts). Alternatively, on zooming in the server supplies a new image. In that case one can not download the whole map, unless perhaps piece by piece. See e.g. the links in Map#External links. Also, as an example, compare the GIF and PDF province maps in South Holland#External links. See also Map#Electronic maps.

Viewing a web page

Since most web pages are mostly text, you can view them in any application that can read text documents. However, to view a web page, as it is intended, one needs a type of software known as a user agent or better still a web browser, which is a piece of software specifically designed to view web pages. There are many different types of web browsers available with various capabilities and a wide range of supported platforms.

Creating a web page

To create a web page, one needs a general-purpose programming editor or text editor or a specialised HTML editor like Microsoft FrontPage, Macromedia Dreamweaver, Mozilla Composer and so on, and an FTP client to upload the page to the web server. One can often use tools supplied with the computer, such as the file explorer or web browser to upload a small number of web page files to a remote server. One may use a pre-made web template to create a web page. Web Templates let web page designers edit the content of a web page without having to worry about the overall asthetics. Wiki is a special way to create or modify and upload web pages without FTP-ing or upload file, only filling a text form in a web page. Wikipedia is an example of wiki technology.

Saving a web page

When saving a local copy of a web page, the web browser usually allows a choice between:
- saving the rendered text without formatting or images, and without indicating which words are links or what their destination is
- saving the HTML-file without changes, without images (view the source and save that)
- saving the HTML-file, changing relative links to absolute ones, without images
- also saving the images and adjusting the references to them accordingly; either a separate folder is made (IE, Mozilla) or the same is used (Opera);
- saving the HTML-file including all images, stylesheets, scripts etc. to a single .MHT file. This is support by Internet Explorer, Mozilla and Mozilla Firefox (the last two only if the Maf plugin has been installed). An .MHT file is based upon the MHTML standard. The common web browsers, like Mozilla, Mozilla Firefox and Internet Explorer, also allow you to print the currently viewed web page or optionally "print" to a file which can later be viewed or printed. This has an advantage in that some web pages are specially designed using Cascading Style Sheets, or a separately generated page, to show both the text and target destination of links contained within the web page. Likewise any images are contained within the single file. For a short page another possibility is saving a screenshot (only useful in special cases). This shows links, but not their destination.

Computer file

A file in a computer system is a stream (sequence) of bits stored as a single unit, typically in a file system on disk or magnetic tape. While a file is usually presented as a single stream, it most often is stored as multiple fragments of data at different places on a disk (or even multiple disks). One of the services operating systems usually perform for applications is that of organization of files in a file system. Files are created by software and usually conform to a particular file format. They are almost always assigned file names by the file system on which they are stored, so that they can be referred to at a later time. Some operating systems allow the contents of a file to be segmented into fixed and variable length records. For example, OpenVMS allows any arbitrary set of characters to be defined as the terminators to variable length record within a file. Others, like Microsoft Windows, have only one specialised subclass of file, called a text file, where a sequence of characters separate the data into lines of text (a specialised variable length record). Some operating systems, such as Unix, do not handle file records at the operating system level, instead it is done at the application level. See record-oriented filesystem. A special file is a file system object which is accessed as though it was a file, but the sequence of bits is supplied or consumed by another process (or by the operating system itself) such as a device driver or network interface. Indeed, the philosophy that "everything is a file" is one of the best known design decisions in Unix and Unix-like operating systems (such as Linux). Files are often organized hierarchically by the operating system, placing them in directories.

Notes

# A collection of bytes in RAM isn't usually known as a file, unless it's stored in a RAM disk. # Historically it was common for files to be defined as sequences of records. However this is now uncommon except on certain mainframe operating systems. On most systems, the application or a library creates the "record" abstraction from the byte stream according to the file format. #Some operating systems use an extension (or "suffix", although the extension does not have to be placed at the end of the filename -- some systems may place it before the filename, for example) to differentiate between files whose contents or data are organized in different formats. The operating system, as well as the underlying file system, may impose restrictions on the length of file extensions. For example, MS DOS limits file extensions to three characters or less. Examples of common file extensions on MS-DOS systems include .EXE for executable programs, .TXT for plain text files, and .ZIP for archives encoded and usually compressed using methods compatible with PKWARE's PKZIP archiver product. File extensions are commonly used to simplify file management tasks for users. For example, suppose you have a directory containing files of different media formats. Some are still images, some are animations, and some are digitally sampled audio waveforms. If you know that all the samples have an extension of .PCM, you could easily issue a copy command to copy all these files to a different directory, whether it be within the same file system or on a separate physical disk. Many application software programs use the presence of a header in the file formats with which they work to determine the actual type of a given file. For example, an executable program on DOS may have an extension of .COM even though the .COM extension is usually reserved for a different, more simple executable format. DOS knows that the file is an executable because of the presence of some signature bytes at the start of the file. 'Unix like' operating systems do not have any fixed extension for files, although .tar, .gz, .sh, and others are standard (see also magic numbers). In many cases, standardised file extensions are useful because they make it easier for users of different computer systems and operating systems to share files, since the format of a file's content is more clear.

External Links and References

- [http://www.dotwhat.net/ Dotwhat? - File Extension Listing] - Listing of file extensions and the programs that use them.
- [http://filext.com/ FILExt - The File Extension Source] - Site for looking up file extensions.
- [http://www.filename.info/ Filename.info] - Information about Microsoft Windows filenames.
- [http://www.2-spyware.com/files.php Exploit Files] - List of files associated with spyware and adware. Category:Computer data File Category: Inter-process communication ko:파일

Computer display

A computer display, monitor or screen is a computer peripheral device capable of showing characters and/or still or moving images generated by a computer and processed by a graphics card. Monitors generally conform to one or more display standards. Sometimes the name "display" suits better than the word "monitor", as the latter term can also ambiguously refer to a "machine-level debugger" or to a "thread synchronization mechanism". Some people also refer to computer displays as "heads", especially when talking about multiple displays connected to a single physical computer. Once an essential component of a computer terminal, computer displays have long since become standardized peripherals in their own right.

Hardware

Technologies

As with television, several different hardware technologies exist for displaying computer-generated output:
- Cathode ray tube (CRT)
- Liquid crystal display (LCD). [LCD-based monitors can receive television and computer protocols (SVGA, PAL, SECAM; NTSC).)
- Plasma display (rarely seen)
- Video projector A modern CRT display has considerable flexibility: it can often handle a range of resolutions from 640 by 480 pixels (640×480) up to 2048 by 1536 pixels (2048×1536), with 32-bit colour and a variety of refresh rates. Dot pitch measures the sharpness of a display. In general, the lower the dot pitch, (e.g. .24), the sharper the picture will appear. Early CRT-based VDUs (Visual Display Units) without graphics capabilities gained the label "glass teletypes", because of the functional similarity to their electromechanical predecessors. Black-and-white displays can only display one colour: either as on or off. Monochrome displays can show only levels of a single colour. In both cases the display usually uses green, orange (amber) or gray (white). green Colour monitors may show either digital colour (turning each of the red, green and blue signals either on or off, giving eight possible colours: black, white, red, green, blue, cyan, magenta and yellow) or analog colour (red, green and blue signals vary continuously, allowing the display of any combination). Digital monitors are sometimes known as TTLs because the voltages on the red, green and blue inputs are compatible with TTL logic chips. Most modern computer displays can show thousands or millions of different colours in the RGB colour space by combining red, green, and blue dots in varying intensities. Some display technologies (especially LCD) have an inherent misregistration of the colour planes, that is, the centers of the red, green, and blue dots do not line up perfectly. Subpixel rendering depends on this misalignment; technologies making use of this include the Apple II from 1976 [http://grc.com/ctwho.htm], and more recently Microsoft (ClearType, 1998) and XFree86 (X Rendering Extension). Moving texts can appear in italics, even when the display resolution is too low to show static italics: a fractional time delay causes an apparent corresponding shift of a fraction of a pixel. Note the sometimes disputed issue of screen emissions.

History

A trend of miniaturisation within computer displays has seen a general move away from the older, bulky CRT devices in the general direction of flat screens as found in modern laptops.

Major manufacturers

- Apple Computer
- BenQ
- Dell, Inc.
- Eizo
- Iiyama Corporation
- LG Electronics
- NEC/Mitsubishi
- Philips
- Samsung
- Sony
- ViewSonic

Configuration and usage

Multi-head

Some users use more than one monitor. The displays can operate in multiple modes. One of the most common spreads the entire desktop over all of the monitors, which thus act as one big desktop. The X Window System refers to this as "Xinerama". A monitor may also clone another monitor. Terminology:
- dualhead - Using two monitors
- triplehead - using three monitors
- display assembly - multi-head configurations actively managed as a single unit

Virtual displays

The X Window System provides configuration mechanisms for using a single hardware monitor for rendering multiple virtual displays, as controlled (for example) with the Unix DISPLAY global variable or with the -display command option.

External links

- [http://electronics.howstuffworks.com/monitor.htm How stuff works] - How Computer Monitors Work.
- [http://www.lcdmonitor.org/ LCD Monitor] - Explains how LCD monitors work and compares them with CRTs Category:Human-computer interaction Category:Display technology Category:Computer device