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1.3 The World Wide Web
The World Wide Web (often referred to with the acronyms WWW or W3) has been the late informative service coming up on Internet. But the success of the “world net “ has been at so an extent that nowadays the sizeable part of users coincides with the same net.
The history of the World Wide Web begins on May 1990, when Tim Berners Lee, a researcher at the CERN of Geneva, submits a proposal having the purpose to develop an information publication and finding system, distributed on a geographical net, connecting the physicists’ international community. The decisive impulse to its development comes only at the beginning of 1993, by the National Center for Supercomputing Applications (NCSA) of the University of Illinois. The success of the World Wide Web has naturally risen the interest of a lot of new telematic authors and publishers, and such interest has determined more than exponential growth rates.
The features making of the World Wide Web a real revolution inside the telematic world, can be summed up in the following points:
* its planetary diffusion
* interfaces easy to use
* its hypertext organization
* the possibility to transmit and to receive multimedia information
* the information supply management simpleness
By the point of view of the final user, Web presents itself as an unlimited universe of a multitude of documents, integrated and interconnected by the means of net dynamic connections. Not only: Web’s client is able to enter automatically the whole resources and services being on Internet: gopher, FTP, telnet connections, newsgroup. Among the more innovative aspects of the World Wide Web, the most remarkable are surely the hypertext organisation and the possibility to transmit integrally multimedia information. By the point of view of logic a hypertext is an information organization system (textual, but not only) in a no sequential structure, but rather reticular. It is constituted by a set of informative units (the knots) and a set of links that allow passing from a block to another or more. If information that are linked among them in the net are not only textual documents, but more in general information carried by different media (text, images, sounds, video), the hypertext becomes multimedia and it is defined hypermedia. By the point of view of the concrete implementation, a digital hypertext presents itself as an electronic document where some fragments of text or images on the screen, highlighted by graphics artifices (icons, colours, characters kind and style), represent the different links available on the page. The operating of the World Wide Web does not differ too much from that of other Internet applications. Also this case the system founds on a client and a server interaction. The communication protocol used by the two interaction modulus is called Hyper Text Transfer Protocol (HTTP). The sole and important specific difference is the presence of a special format where documents introduced on Web must be memorized, called Hyper Text Markup Language (HTML). Web’s clients are interface instruments between the user and the system; the main functions they carry out are:
* receiving the user’s commands
* requiring servers documents
* format interpretation and its presentation to the user In the telematic slang those programs are called Browsers, from “to browse”, to run through, since they just let run through documents. In the moment a user activates a connection, acting over a link or typing the address of a document, the client sends a requirement to a certain server pointing out the files it intends to receive. The Server Web, or more exactly the HTTP, instead leads with the management, finding and delivering of the single documents required by clients. Naturally it is able to manage more requirements at the same time. The Server has also other functions, in detail the interaction with other programs that allow producing documents in a dynamic way. The Web Server can carry out these operations by the means of the Common Gateway Interface (CGI), that is a series of standard commands thanks to whom it can communicate with applications or programs (for example carry out an automatic research in a Data Base) and produce instantly Web documents corresponding to the completed operation (for example, containing the results of a research). As we have said previously the main instrument to navigate on the World Wide Web pages is the browser, that is a program able to require the page we want, to reach the remote server where it is stocked, to receive it and to visaulize it correctly. The most widespread browsers belong to the graphic browser family and particularly Netscape Navigator and Microsoft Internet Explorer represent the two peak products in this area. In general the browsers, beyond the HTML format, can visualise autonomously the texts in the no marked ASCII simple format, and at least two formats of the most widespread graphic files on Internet: the GIF and JPEG, integrating images inside the document. If the file received from the net is formatted the way the browser does not know how to interpret, since it is associated to other programme available in the system, it can start-off directly some support applications able to interpret it. The sizeable part of the browsers have further functions making more efficient the on line access to pages, specially for who uses a telephone line:
* advanced management of text and images
* local stock memory employment, called cache
The first characteristic refers to the way the browser manages the files sent by the remote server, and the priority in page video composition. The HTML files are simple files in ASCII format. So a textual document on Web, even if too long, has a very contained dimension, expressed in byte (basic element to warrant the answer speed). The graphic files, instead, even if employing compression algorithms, occupy too much space and so they require more time to be transmitted. So when a Web page is delivered, the text file arrives faster than graphics files eventually annexed. The second characteristic has the function to create a sort of a local duplicate of little sections of the Web. The employ of the cache allows fastening an eventual access to pages previously opened, or to files already loaded. Anytime the browser receives a pages from the net, it makes a copy of all files composing it on the local fix disk. If then the user attempts to access another time to this page, the program loads the files memorised in the cache, rather than requiring them again to the remote server. The availability and the size of the cache memory are changeable by the means of the browser configuration commands.
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