Chapter 35 -- world wide web pages and browsing

(Latest Revision: Fri May 04 11:35 2007 )

Notes On Chapter Thirty-Five -- World Wide Web Pages And Browsing

35.1 Introduction

Continue discussion of client-server applications
Hypertext
Structure of a browser
Interaction between browser and server
Browser display
Structure and use of URL's

35.2 Browser Interface

The WWW is a (distributed) collection of information
People use browsers to access the information
A browser displays information
Some items of the information are selectable - one mouses over such items and clicks. This causes the browser to display new information corresponding to the selected item.

35.3 Hypertext and Hypermedia

According to Webopedia hypertext is:
Also according to Webopedia hypermedia is:

35.4 Document Representation

Page: a hypermedia document available on the Web
Homepage: the main page for an organization or individual
HyperText Markup Language (HTML): a language used to give general guidelines for the display of hypermedia documents.
HTML allows for the details of document display to be chosen according to the capabilities and limitations of the display hardware.

35.5 HTML Format And Representation

The general form of an HTML document is illustrated by this example:


<HTML>

  <HEAD>
      <TITLE>
         text that forms the document title
      </TITLE>
  </HEAD>

  <BODY>
     body of the document appears here.
  </BODY>

</HTML>

Click here to see how your browser renders the document above. (Remember to notice the page title too.)

35.6 Example HTML Formatting Tags

Another tag:

<BR>

makes output go to a newline

35.7 Headings

This HTML code:

Hello.<BR> <H1>This Is A Heading</H1><BR>Back to normal. makes this display:

Hello.

This Is A Heading

Back to normal.

35.8 Lists

One may create bulleted "unordered lists" within the body (like the one you are looking at).
This HTML code:
```
<UL>
<LI> first bulleted item
<LI> second bulleted item
<LI> third bulleted item
</UL>
```
makes this display:
- first bulleted item
- second bulleted item
- third bulleted item

35.9 Embedding Graphic Images In A Web Page

There is no provision to include images directly in an HTML document. Instead the author places a tag like:
```
 <IMG ALIGN=BOTTOM SRC="Gif2/becoat.gif"> 
```
into the HTML document. When the browser encounters such a tag it fetches the image from the location specified by the pathname in the tag and places a copy of the image within the rendered document. The position of the image in the document corresponds to the position of the tag in the HTML document.

35.10 Identifying A Page

Web browsers do other things besides fetch and render HTML documents. The user of a browser specifies the resource s/he wants with a Uniform Resource Locator (URL). The syntax is:
```
 protocol://computer_name:port/document_name 
```
Some popular protocols used in browsers are:
- HyperText Transfer Protocol (HTTP),
- Hypertext Transfer Protocol over Secure Socket Layer (HTTPS), and
- File Transfer Protocol (FTP)
Example URL's:
```
http://www.indeed.com/
https://smallbusiness.commercebank.com/cbi/login.aspx
ftp://www.cs.csustan.edu/pub/fd3
http://www.cs.csustan.edu:80/~john/js.html
```

35.11 Hypertext Links From One Document To Another

An HTML document can also specify a link to another document using an anchor. Examples:
If you put this text in your web page:

<A HREF="http://www.nps.gov/glba/"> Visit beautiful Glacier Bay. </A>

It will display as below. The URL http://www.nps.gov/glba/ is the URL to which the browser will link when you click on the selectable text: "Visit beautiful Glacier Bay."
Visit beautiful Glacier Bay.
If you have an image file (in this case, a smiley-face with pathname Gifs2/Smiley.gif relative to the directory containing the HTML code for this document) and if you put this text in your web page:

<A HREF="http://www.pbase.com/serenab/image/26180936"> <IMG ALIGN=TOP WIDTH=50 SRC="Gifs2/Smiley.gif"> </A>

It will display as below. The URL http://www.pbase.com/serenab/image/26180936 is the URL to which the browser will link when you click on the selectable image of the smiley-face.
The quoted string after

HREF=

is the link and everything between the

<A> and the </A>

parts of the anchor become selectable.

35.12 Client-Server Interaction

The world wide web is a client-server system. Web browsers are clients. A web browser contacts a web server, requests a document, accepts the transfer, closes the connection to the server, and displays the document.

35.13 Web Document Transfer And HTTP

Web browsers and servers communicate according to the HyperText Transfer Protocol (HTTP)
The four basic operations available to HTTP clients are
- GET (to get status information and an item (e.g. an HTML document),
- HEAD (to get only status information about an item),
- POST (to add to an item stored on the server), and
- PUT (to replace an item stored on the server).

35.14 Browser Architecture

Web servers are not very complicated. Basically all they do is respond to requests to send items to clients.
Web browsers are more complicated than web servers. Parts of a web browser include:
- A controller to manage a GUI and call other modules (clients and interpreters)
- HTML interpreter to render HTML documents
- other interpreters (e.g. for various image formats)
- drivers to be used by interpreters to create output
- HTTP client to make requests from servers - using HTTP protocol
- other clients (e.g. e-mail, FTP, HTTPS) for using other client protocols,
- network interface for clients to use for communication
(See figure 35.5 below.)

35.15 Optional Clients

FTP
E-Mail Client

35.16 Caching In Web Browsers

Web browsers typically cache what they download from servers to avoid repeating downloads unnecessarily.
It is difficult to maintain an efficient caching policy for a web browser because there is no simple way to identify which items are likely to be needed again soon.
Many browsers allow the user to configure max cache size and how long to retain items in the cache.

35.17 HTTP Support For Caching

When a server sends a document to a client, it can add a cache timeout value or specify that an item should not be cached.
A client can use a HEAD request to find out if a cached item is still valid.

35.18 Alternative Transfer Protocols

HTTPS: uses encryption
HTTP 1.1: uses persistent connections
HTTP-NG: uses binary encoding for headers

35.19 Other Markup Languages

Voice Extensible Markup Language (VoiceXML, VXML): e.g. for delivery over cell phone
Extensible Markup Language (XML)

35.20 Summary

Notes On Chapter Thirty-Five -- World Wide Web Pages And Browsing

This Is A Heading