Created on: 11 April 2013
Final part of the Arduino Ethernet Shield Web Server Tutorial
In this final part of the multi-part Arduino Ethernet shield web server tutorial, we summarise the technologies covered by the tutorial and provide some extra information. We also look at where to get further information on the various technologies and how to progress from here.
The first technology to get familiar with when writing code for a web server that hosts HTML web pages is the Hypertext Transfer Protocol (HTTP). HTTP is normally invisible to the user of a web browser, except for http:// in the browser address bar. HTTP is happening behind the scenes for the web surfer.
Because we are writing web server code on our Arduino, we need to know how HTTP works. When a web browser connects to an Arduino web server by a user entering the IP address in the address bar of the web browser (the IP address is set in the Arduino sketch), the web browser sends an HTTP request for the default or home page of the web server.
The Arduino web server must answer the HTTP request with the text that the protocol requires and then send the actual web page.
A good resource for learning more about HTTP is HTTP Made Really Easy by James Marshall.
Part 2 of this Arduino web server tutorial shows how HTTP is handled in the first Arduino web server sketch.
Every web page consists of a text file written in Hypertext Markup Language (HTML). HTML structures the web page, marking which portion of the text are paragraphs, headers, checkboxes, buttons, etc.
There are actually several versions of HTML and also several versions of XHTML. XHTML is also used for creating web pages, but has a stricter syntax than HTML. For example, all (X)HTML tags must use lower-case letters in XHTML, but HTML can use upper or lower-case letters.
A web page can be written in any of the standards and could be HTML or XHTML – the person browsing a website would normally not know which standard the website is using unless he looked at the source code of a web page from the site.
We have actually been using HTML 5 in this tutorial. The version of HTML or XHTML used in a web page is specified by the very first line of markup in the web page. All the examples in this tutorial have used the following line at the top of the HTML page:
This lets the browser know that the web page that it is loading is written using HTML 5.
Other HTML and XHTML standards use different markup for the first line of the web page. For example, strict and transitional XHTML use the following lines respectively (these are shown on two lines each here to fit on the page, the two lines are normally written on one line and separated by a space):
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
The first line of markup for two of the HTML standards for HTML 4.01 are shown here (strict and transitional):
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
Cascading Style Sheets (CSS) are used to style the HTML elements on a web page. This means that CSS can be used to change the font style, size and colour. The size position and colour of various HTML elements can be changed using CSS.
In our examples in this tutorial, the CSS was included as part of the HTML file and put between opening and closing style tags.
Normally a website will have a separate style sheet file (e.g. style.css) containing all the CSS. This allows the same style sheet to be used to apply styles to all the pages on the website.
Because we have mostly been using single web pages on the Arduino web server, it was easier to include the style in the web page. If more than one web page is needed using this method, then the same styles would need to be copied to the top of each page. The advantage of using a separate style sheet means that the style sheet only needs to be included in the top of each HTML page (the style sheet is included using a single line of markup).
In the relatively slow Arduino, a faster response will be obtained by including the CSS styles in the top of the web page when hosting a single web page. This is because only one HTTP request is then needed to get the page and CSS.
Ajax enables parts of the web page to be updated. This reduces the amount of data that needs be sent from the Arduino making the updates faster because the entire web page does not need to be reloaded every time new data is to be displayed on it.
Another advantage of using Ajax is that the web page does not flicker when data is updated as occurs when the entire page is refreshed.
Only modern web browsers that support the relatively new HTML 5 standard will be able to run the gauge example.
Most of the technologies used to create a web page are rather large topics and have entire books dedicated to each one.
We have used a small subset of each of these technologies that have hopefully given you a start in creating your own Arduino web server and web pages.
How far you wish to pursue each of these subjects depends on what you want to implement on your web server and how good you want to get at each technology.
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