WWW

URL (Uniform Resource Locator), a descriptor of a document, that is unique in the whole internet.

$\bullet$

A URL has the following form:

    PROTOCOL://HOSTNAME/LOCAL_NAME
  

• here PROTOCOL is the protocol for the transmission of the document, HOSTNAME is the domain name of the server providing the document, and LOCAL_NAME tells, where to find the document on the server.

$\diamond$

http://www.tu-harburg.de/rzt/rzt/pj/rz/kurse.html

$\triangleright$

LOCAL_NAME could simply be the directory path of the document on the server; but the server can interpret it differently. The last part is usually a file name (kurse.html). If it is left out, the server usually looks for a file index.html or index.htm at the corresponding place.

$\triangleright$

http is the protocol for the transfer of HTML documents. Other protocols allow to integrate other internet services.

$\diamond$

Other protocols:

name	meaning
ftp	file transfer
news	article in news groups
file	access to local files

$\bullet$

Basic function of a browser is the formatted display of HTML documents. References to other documents (hyperlinks) are usually underlined and/or have a different color. By clicking on a link the referenced document gets loaded and displayed.

$\triangleright$

Using a special browser function one can go back to the last document (or even to each previous documents).

$\triangleright$

One can enter a URL directly. Furthermore one can collect a list of interesting pages (bookmarks, favorites) and order it in a hierarchical way.

Homepage, front page to all web pages of an institution, company or person.

$\bullet$

Web pages can contain many kinds of multimedia elements.

$\diamond$

Pictures, sound, movies and animations, 3D models, embedded programs (Applets).

$\bullet$

Associated with each kind of multimedia element comes a special type (MIME-Typ) . The browser uses this type information to decide how to display the element:

1. It can show it itself in the document (some image formats).
2. It can use certain extension modules (plugins) to show it embedded in the document.
3. It calls external programs (``viewers'') which display it in an extra window.

$\triangleright$

Many plugins for different browsers and operating systems are distributed for free by companies, often for their own formats. Plugins for the most often used MIME types come with the browsers.

$\diamond$

Plugins for the following formats are wide spread:

format	description
Wave	audio clips
MIDI	music
Quicktime	videos, animations
RealAudio, RealVideo	sound and video in real time
VRML	3d models
PDF	structured (text) documents

$\bullet$

Pages that have been loaded, are cached in different places to reduce the time until a document with all its embedded elements is loaded. Instead they are reload from the cache, if possible. Usually three different levels of cache memory are used:

-

main memory (memory cache)

-

hard disk (disk cache)

-

on external computers (proxy cache)

$\triangleright$

Proxy caches are special machines of the internet provider (at universities usually the computing center) to hold copies of the most often read pages. They form a global network that helps bringing a requested page to the user as fast as possible.

$\triangleright$

If pages are changed on a server, then the cached versions are outdated. Pages that are changed frequently, can specify a time interval, after thast all caches have to load a new version from the original server. Usually their is a way to force the browser to get a page from the server directly, bypassing all caches. (for netscape: $<$SHIFT$>$-Reload).