XML Points the Way to the Wireless Future
For those who argue that XML is too verbose for wireless apps, see what standards and technologies we can expect down the road
by Mikael Hillborg
Posted July 25, 2002
Wireless applications execute on wireless devices, which are normally small handheld devices, and these have limitations. The most obvious limitation is that handheld devices are small, and a small display cannot display the same amount of information (at the same time) as a large one. Small devices usually also have a limited amount of memory available, which makes it impossible to incorporate all the fine features that are part of a full-fledged desktop computer. Therefore, technologies such as Java 2 Micro Edition (J2ME), Connected Limited Device Configuration (CLDC), and Mobile Information Device Profile (MIDP) have been developed.
There's substantial support for XML in many Java configurations, including the CLDC. For example, the kXML parser and NanoXML are available. These parsers can be used by a MIDlet, which requests XML documents from a server, parses the information therein, and uses it for arbitrary purposes.
Some might argue that XML is too verbose for wireless applications. However, WBXML (Wireless Binary XML) is used by WAP gateways to compress XML documents into a stream of binary tokens. The technology uses a table lookup algorithm to store common strings. Thus, frequently occurring strings, such as <p> or http://www can be referred to as an index in the table saving space and reducing the necessary bandwidth for the wireless application.
You might develop a MIDlet, which creates an HttpConnection object, to retrieve information from the server. It retrieves an XML document such as a Multimedia Messaging presentation or an XHTML Basic document, which contains ECML fields for e-commerce. The MIDlet and the XML parser retrieve some of the fields and use them for arbitrary purposes. The MIDlet could display the MMS presentation or it could use your personal settings, stored on the device or on a server, to supply default values for the ECML fields in the XHTML Basic document.
What can you expect from XML-based standards and technologies in the future? As you will see, they might play important roles for not only 2.5G and 3G applications, but also wireless applications in general.
Location Registers
Databases are an essential part of wireless applications and systems. In GSM networks, Home Location Registers (HLRs) and Visiting Location Registers (VLRs) store the current location of each subscriber and the services they're allowed to use. Content providers might choose to store the content in databases and generate personalized instance documents. The benefit of storing the content in a database, rather than an ordinary file system, is that the content can be put together using smaller units or modules. This also makes it possible to store generic XML documents, which are transformed to a version and format that fits the end user's device and software. One problem is this: Many databases today aren't prepared for XML-based content, and they certainly aren't made for queries via XML instance documents. Some manufacturers have added extensions to their original database management system, which make it possible to work with XML content. The query language used is still SQL, however. This is about to change, because many database manufacturers have seemed to adopt a new discussed technology to query databases using an XML syntax. XQuery is still a draft only, but it probably will be used in future versions of some database engines because manufacturers have expressed their support for the initiative.
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