Learning WML - WAP Basics, Page 2

Understanding Decks

WML pages are structured within "decks," allowing several pages ("cards") to be defined in each WML file. This deck analogy allows multiple pages to be delivered to the mobile client at the same time, minimizing the loading time between related pages. However, the limited memory on most devices constrains the deck size, usually to less than 1024 bytes. Therefore, careful consideration and planning should go into any WAP application; don't start coding without investing time in planning.

Note: Remember your audience. Mobile users generally scroll through cards rapidly and will be reading on a display that's a mere handful of characters wide (usually less than 20 characters) and usually less than 10 lines high. Keep your content to a minimum, provide an intuitive navigation structure, and optimize your decks to maximize links within the deck and minimize links outside of the deck.

Visualizing a physical "deck of cards" structure can help in understanding the principles of WML. For example, suppose we have three simple cards (pages) as shown below:

Figure 1.2 - The physical card analogy to WML decks helps visualize how they work.

These cards together form a deck and are delivered to the mobile device in one file. Now suppose that each card links to the next (card one links to card two, which links to card three, and so on), and that each card also has a "back" link to take the user back to the previous card. As the user navigates the deck, the cards stack in memory as shown below:

Figure 1.3 - As the user follows the links through the deck, the cards stack up in memory.

A developer accustomed to HTML might be tempted to implement the "back" feature by providing a link to the deck, specifying the previous card. However, this would cause the mobile device to re-request the whole deck before redisplaying the card-a card it already had in memory.

Instead, you should use the tag, which tells the browser to remove the current page and display the previous page in the history list (like using the Back button on a PC browser). Of course, the content of the previous page might need to be refreshed each time it's accessed; in that case, valid techniques could include recalling the whole deck or specifying that the page not be cached. Proper navigation will be covered in future articles.

Figure 1.4 - The <prev> tag "pops" the top card off the stack (out of the history list), redisplaying the previous card in the history.

Setting Up Your Server for WML

To configure your Web server to deliver WML, you must define the related MIME types for WML content. Web servers and client browsers use MIME (Multipurpose Internet Mail Extensions) to communicate the type of data that is being sent. Before sending data, the server sends a MIME identifier to the client browser, identifying the format of the following data. The client browser can then properly decode and apply the data. Most WML applications require three MIME types, as listed in the following table.

File Extension	MIME Type Definition	Use
.wml	text/vnd.wap.wml	WML source file
.wmls	text/vnd.wap.wmlscript	WML script file
.wbmp	image/vnd.wap.wbmp	Wireless bitmap file (image)

To add MIME types to your Web server, you must have administrator access to the server. The following sections cover setting up Microsoft's Internet Information Server (IIS) and Apache for WML. If you're using another type of server, read your server's documentation for more information on adding MIME types.