The wireless markup language (WML) (WAP Forum, 2000j) is based on the standard HTML known from the www and on HDML (King, 1997). WML is specified as an XML (W3C, 1998a) document type. When designing WML, several constraints of wireless handheld devices had to be taken into account. First of all, the wireless link will always have only a very limited capacity compared to a wire. Current handheld devices have small displays, limited user input facilities, limited memory, and only low performance computational resources. While the bandwidth argument will remain for many years, it currently seems that the gap between mobile and fixed devices regarding processing power is getting narrower.

Today‘s CPUs in PDAs have performance figures close to desktop CPUs just a few years ago. WML follows a deck and card metaphor. A WML document is made up of multiple cards. Cards can be grouped together into a deck. A WML deck is similar to an HTML page, in that it is identified by a URL and is the unit of content transmission. A user navigates with the WML browser through a series of WML

 cards, reviews the contents, enters requested data, makes choices etc. The WML browser fetches decks as required from origin servers. Either these decks can be static files on the server or they can be dynamically generated.

 It is important to note that WML does not specify how the implementation of a WML browser has to interact with a user. Instead, WML describes the intent of interaction in an abstract manner. The user agent on a handheld device has to decide how to best present all elements of a card. This presentation depends much on the capabilities of the device.

WML includes several basic features:

 ● Text and images: WML gives, as do other mark-up languages, hints how text and images can be presented to a user. However, the exact presentation of data to a user is up to the user agent running on the handheld device. WML only provides a set of mark-up elements, such as emphasis elements (bold, italic, etc.) for text, or tab columns for tabbing alignment.

 ●  User interaction: WML supports different elements for user input.Examples are: text entry controls for text or password entry, option selections or controls for task invocation. Again, the user agent is free to choose how these inputs are implemented. They could be bound to, e.g., physical keys, soft keys, or voice input.

 · Navigation: As with HTML browsers, WML offers a history mechanism with navigation through the browsing history, hyperlinks and other intercard navigation elements.

 ● Context management: WML allows for saving the state between different decks without server interaction, i.e., variable state can last longer than a single deck, and so state can be shared across different decks. Cards can have parameters defined by using this state without access to the server

over the narrow-band wireless channel.

 The following paragraph gives a simple example of WML; the reader is referred to the standard or Singhal (2001) for a full reference and in-depth discussion of the language.

First, a reference to XML is given where WML was derived from. Then, after the keyword wml the first card is defined. This first card of the deck ―displays‖ a text after loading (―displaying‖ could also mean voice output etc.). As soon as a user activates the do element (a button or voice command), the user agent displays the second card. On this second card, the user can select one out of three pizza options. Depending on the choice of the user, PIZZA can have one of the values Mar, Fun, or Vul. If the user proceeds to the third card without choosing a pizza, the value Mar is used as default. Again, describing these options with WML does not automatically mean that these options are displayed as text. It could also be possible that the user agent reads the options through a voice output and the user answers through a voice input. WML only describes the intention of a choice. The third card finally outputs the value of PIZZA.

<?xml version=”1.0″?>

 <!DOCTYPE wml PUBLIC “-//WAPFORUM//DTD WML 1.1//EN” “http://www.wapforum.org/DTD/wml_1.1.xml”>

<wml>

 <card id=”card_one” title=”Simple example”> <do type=”accept”>

 <go href=”#card_two”/> </do>

<p>

 This is a simple first card! <br/>

On the next one you can choose …

 </p>

</card>

 <card id=”card_two” title=”Pizza selection”> <do type=”accept” label=”cont”>

 <go href=”#card_three”/> </do>

<p>

 … your favourite pizza!

 <select value=”Mar” name=”PIZZA”> <option value=”Mar”>Margherita</option> <option value=”Fun”>Funghi</option> <option value=”Vul”>Vulcano</option> </select>

 </p>

</card>

 <card id=”card_three” title=”Your Pizza!”> <p>

 Your personal pizza parameter is <b>$(PIZZA)</b>! </p>

</card>

 </wml>

WML may be encoded using a compact binary representation to save bandwidth on the wireless link. This compact representation is based on the binary XML content format as specified in WAP Forum (2000k). The binary coding of WML is only one special version of this format; the compact representation is valid in general for XML content. The compact format allows for transmission without loss of functionality or of semantic information. For example, the URL prefix href=_http://, which is very common in URLs, will be coded as 4B. The code for the select keyword is 37 and option is 35. These single byte codes are much more efficient than the plain ASCII text used in HTML and today‘s www.