WAP: The Web for Cellphones and PDAs (2/4) - exploring XML

WAP specialties

Why do we need yet another set of standards? Because adaptations for wireless devices were necessary to accomodate their own set of special characteristics, namely:

Limited bandwidth

High latency

Limited device memory and processing power

Limited screen real estate

How do the WAP components compare with their Internet siblings?

WML is quite limited compared to HTML - it only supports very basic text formatting and layout. This is deemed adequate for cellphone displays, which are usually monochrome and have about 6 lines with 20 characters each.

Input methods are limited to edit fields and option selections, taking into account the limited suitability of keypads for large data entry tasks.

WMLScript's object model is much simpler than DHTML's, adapting to the limited processing power and memory available on cellphones.

WML can aggregate multiple pages, dubbed "cards", into so-called "decks" sent to the client browser in one batch. This helps to minimise server roundtrips on high-latency cellular networks.

Both WML and WMLScript have an equivalent byte code representation, for reduced network load and memory consumption, also eliminating the need for a WML parser in the device.

WAP pages are accessed with URLs just like Web pages, and their content can either be original WML or translated from HTML, a function which is usually incorporated into the WAP gateway. As we will see later, the whole request-response process is very similar to that of the Web, with the gateway acting as a proxy between the telecom and computer networks.

Let's look at an example.