In this section we focus on an Internet application by which multimedia information is disseminated over the Internet. It is based on the concept of hypertext, a term that originally referred to text documents that contained links, called hyperlinks, to other documents. Today, hypertext has been expanded to encompass images, audio, and video, and because of this expanded scope it is sometimes
referred to as hypermedia.
When using a GUI, the reader of a hypertext document can follow the hyperlinks associated with it by pointing and clicking with the mouse. For example, suppose the sentence “The orchestra’s performance of ‘Bolero’ by Maurice Ravel was outstanding” appeared in a hypertext document and the name Maurice Ravel was linked to another document—perhaps giving information about the
composer. A reader could choose to view that associated material by pointing to the name Maurice Ravel with the mouse and clicking the mouse button. Moreover, if the proper hyperlinks are installed, the reader might listen to an audio recording of the concert by clicking on the name Bolero.
In this manner, a reader of hypertext documents can explore related documents or follow a train of thought from document to document. As portions of various documents are linked to other documents, an intertwined web of related information is formed. When implemented on a computer network, the documents within such a web can reside on different machines, forming a networkwide
web. The web that has evolved on the Internet spans the entire globe and is known as the World Wide Web (also referred to as WWW, W3, or the Web). A hypertext document on the World Wide Web is often called a Web page.
A collection of closely related Web pages is called a Web site. The World Wide Web had its origins in the work of Tim Berners-Lee who realized the potential of combining the linked-document concept with internet technology and produced the first software for implementing the WWW in December of 1990.
Web Implementation
Software packages that allow users to access hypertext on the Internet fall into one of two categories: packages that play the role of clients, and packages that play the role of servers. A client package resides on the user’s computer and is charged with the tasks of obtaining materials requested by the user and presenting these materials to the user in an organized manner. It is the client that
provides the user interface that allows a user to browse within the Web. Hence the client is often referred to as a browser, or sometimes as a Web browser.
The server package (often called a Web server) resides on a computer containing hypertext documents to be accessed. Its task is to provide access to the documents under its control as requested by clients. In summary, a user gains access to hypertext documents by means of a browser residing on the user’s computer. This browser, playing the role of a client, obtains the documents by soliciting the services of the Web servers scattered throughout the Internet. Hypertext documents are normally transferred between browsers and Web servers using a protocol known as the Hypertext Transfer Protocol (HTTP).
In order to locate and retrieve documents on the World Wide Web, each document is given a unique address called a Uniform Resource Locator (URL). Each URL contains the information needed by a browser to contact the proper server and request the desired document. Thus to view a Web page, a person first provides his or her browser with the URL of the desired document and then
instructs the browser to retrieve and display the document.
A typical URL is presented in Figure 4.8. It consists of four segments: the protocol to use to communicate with the server controlling access to the document, the mnemonic address of the machine containing the server, the directory path needed for the server to find the directory containing the document, and the name of the document itself. In short, the URL in Figure 4.8
tells a browser to contact the Web server on the computer known as ssenterprise.aw.com using the protocol HTTP and to retrieve the document named Julius_Caesar.html found within the subdirectory Shakespeare within the directory called authors.
Sometimes a URL might not explicitly contain all the segments shown in Figure 4.8. For example, if the server does not need to follow a directory path to reach the document, no directory path will appear in the URL. Moreover,
sometimes a URL will consist of only a protocol and the mnemonic address of a
computer. In these cases, the Web server at that computer will return a predetermined document, typically called a home page, that usually describes the information available at that Web site. Such shortened URLs provide a simple means of contacting organizations. For example, the URL http://www. google.com will lead to the home page of Google, which contains hyperlinks to
the services, products, and documents relating to the company.
To further simplify locating Web sites, many browsers assume that the HTTP protocol should be used if no protocol is identified. These browsers correctly retrieve the Google home page when given the “URL” consisting merely of www.google.com.
HTML
A traditional hypertext document is similar to a text file because its text is encoded character by character using a system such as ASCII or Unicode. The distinction is that a hypertext document also contains special symbols, called tags, that describe how the document should appear on a display screen, what multimedia resources (such as images) should accompany the document, and which
items within the document are linked to other documents. This system of tags is known as Hypertext Markup Language (HTML). Thus, it is in terms of HTML that an author of a Web page describes the
information that a browser needs in order to present the page on the user’s screen and to find any related documents referenced by the current page. The process is analogous to adding typesetting directions to a plain typed text (perhaps using a red pen) so that a typesetter will know how the material should appear in its final form. In the case of hypertext, the red markings are replaced
by HTML tags, and a browser ultimately plays the role of the typesetter, reading the HTML tags to learn how the text is to be presented on the computer screen.
The HTML encoded version (called the source version) of an extremely simple Web page. Note that the tags are delineated by the symbols and . The HTML source document consists of two sections—a head (surrounded by the head and /head tags) and a body (surrounded by the body and /body tags). The distinction between the head and body of a Web page is similar to that of the head and body of an interoffice memo. In both cases, the head contains preliminary information about the document (date, subject,
etc. in the case of a memo). The body contains the meat of the document, which in the case of a Web page is the material to be presented on the computer screen when the page is displayed.
The head of the Web page displayed in contains only the title of the document (surrounded by “title” tags). This title is only for documentation purposes; it is not part of the page that is to be displayed on the computer screen. The material that is displayed on the screen is contained in the body of the document.
The first entry in the body of the document is a level-one heading (surrounded by the h1 and /h1 tags) containing the text “My Web Page.” Being a level-one heading means that the browser should display this text prominently on the screen. The next entry in the body is a paragraph of text
(surrounded by the p and /p tags) containing the text “Click here for another page.” the page as it would be presented on a computer screen by a browser.
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