Examples of XML Vocabularies

WML is an XML vocabulary created to serve the needs of a certain group of users: the wireless. As we discussed in a previous article, wireless presents a number of special needs, both in the presentation of content and the type of content.

Those clever chaps over at the WAP Forum (http://www.wapforum.org/) created WML, part of the WAP specification, as a language for describing wireless-specific content.

But why bother creating a new vocabulary? Although XML code itself doesn't express how data is to be displayed, Extensible Style Language (XSL) does just that, allowing content to be targeted to different display devices. But as we saw in the previous article, delivering content to wireless devices involves more than simply formatting data for a smaller screen. Some special tags were needed to handle the various challenges of wireless.

One construct that's unique to WML is the DECK and CARD set of elements. One way that cell phones and other handheld devices get around the small screen size is by arranging content into groups of tiny pages, like a (small) deck of cards. Instead of a document consisting of a single page (as in HTML), a document can contain several small pages organized in a deck, which the user can page through. This works well for small devices, because it allows you to squeeze in more content despite the small screen. Also, because the deck is downloaded as a whole, it's not necessary to reconnect to the server when the user moves from one card to another.

Another XML vocabulary that seems destined for widespread acceptance is the Synchronized Multimedia Integration Language (SMIL) (http://www.w3.org/TR/REC-smil/). SMIL is particularly interesting because it accommodates a completely different media model than that of the traditional Web page. Content marked up in HTML uses what we may call a "random-access media model", meaning that the user proceeds through it at her own pace, and can jump around to any point as desired. But a multimedia presentation such as a movie uses a linear media model - once it begins, it proceeds at a constant speed, with events occurring in a pre-determined sequence.

In fact, a SMIL presentation can include bits of media in just about any format, which do their thing at precisely-determined moments, for example a sound effect which must be linked to a corresponding visual event. This requires the use of a "timeline", which specifies a precise time at which each event is to begin and/or end. This concept is of course nothing new, and is used by all audio and video editing software. But SMIL allows hyperlinks to be included in a linear multimedia presentation. For example, a movie can contain clickable areas that link to related information, or a music player can allow the user to choose pictures to be displayed as a song plays.

For more information about SMIL, see my previous article Web Audio 2000.

Another useful XML vocabulary is the Resource Description Framework (RDF) Schemas which provides a way to include metadata (data that describes other data) in a Web page or other file. Using RDF and a metadata standard such as the Dublin Core, a file may include information about its author, date, subject matter and so forth, making it possible to search for information in a much more powerful way than that provided by the traditional search engine. See " Are search engines dead?" for an ode to the wonders of RDF.

Building Languages with XML
Building Languages with XML
How to Create a Vocabulary