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.

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

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.

A sample WML code:
<wml>
<card id="HTML" title="HTML Tutorial">
<p>This is HTML Bigromu blog</p>
</card>
<card id="XML" title="XML Tutorial">
<p;>This is XML Bigromu blog</p>
</card>
</wml>

What Is WML?

WML (Wireless Markup Language) is the dominant language in use with wireless devices today. Essentially, WML is a subset of HTML, but has its roots in XML. Those developers with a solid base in XML should have a relatively easy time coding WML.

The current WML standard is 1.3, although many mobile devices in use today support only the WML 1.1 standard. Therefore it's prudent to stay away from 1.3-specific features, unless you know that your target market's devices are 1.3-ready.

There are several key differences between WML and standard HTML, including the following:

WML is highly structured and very particular about syntax. Several current HTML browsers allow for "messy" code such as missing tags and other formatting snafus. Such mistakes are not allowed in WML; the mobile browser will complain and generally won't display the page.

WML is case sensitive. The tags and are treated as different tags, although they accomplish the same purpose (bold text). Therefore, you must be careful to match the case of your opening tags with your closing tags (for example, This is bold will not work as expected).

Many tags have required attributes. Developers accustomed to HTML may be used to including only attributes they need-in some WML tags, you must include a few attributes, even if they are blank or default.

WML pages are structured in "decks" (see the next section), allowing for multiple pages to be defined in each WML file.
WML also has a client-side scripting language, WMLScript, to help automate particular tasks, validate input, and so on. WMLScript is a subset of JavaScript and will be covered in a later article.

How Does WAP Work?

This post will focus on the delivery of WML content to mobile devices over a cellular or related technology network. However, the delivery of many protocols and technologies takes the same route-namely, through a proxy server that bridges the gap between the wired Internet and the wireless service provider's network.


Figure 1.1 The WAP Gateway provides wireless networks with Internet access and optional content translation and filtering.

This proxy server manages the communication between the wireless client and the Internet server(s), acting as a gateway to the wired Internet. It caches content and in some cases even translates raw HTML into WAP-compatible protocols such as WML.

Many mobile devices have a built-in wireless browser. Although several different browsers are in use today among the various wireless providers, most browsers support WML, either natively or translated into HDML. A popular precursor to WML, the Handheld Device Markup Language (HDML), is still supported on several mobile platforms. However, due to the limitations of HDML (supporting only a handful of navigation tags and virtually no formatting tags), WML is becoming the most widely used mobile markup language. That said, if you plan to support a particular platform, it's best to test your code extensively on that particular device.


Note: When coding for the general public, be careful to stick to the standards and avoid using proprietary extensions to the various languages, no matter how tempting the feature set of the extensions. If you decide to provide the extensions to those who can use them, you should take the necessary server steps to identify the connecting browser and deliver code customized for that browser.

Examples of WAP use

Checking train table information
Ticket purchase
Flight check in
Viewing traffic information
Checking weather conditions
Looking up stock values
Looking up phone numbers
Looking up addresses
Looking up sport results

What is WMLScript?

WML uses WMLScript to run simple code on the client. WMLScript is a light JavaScript language. However, WML scripts are not embedded in the WML pages. WML pages only contains references to script URLs. WML scripts need to be compiled into byte code on a server before they can run in a WAP browser.

What is WML?

WML stands for Wireless Markup Language. It is a mark-up language inherited from HTML, but WML is based on XML, so it is much stricter than HTML.

WML is used to create pages that can be displayed in a WAP browser. Pages in WML are called DECKS. Decks are constructed as a set of CARDS.

WAP microbrowser

To fit into a small wireless terminal, WAP uses a Micro Browser.

A Micro Browser is a small piece of software that makes minimal demands on hardware, memory and CPU. It can display information written in a restricted mark-up language called WML.

The Micro Browser can also interpret a reduced version of JavaScript called WMLScript.