WTA: Enabling Telephony Functions on the Web

Once you can view data on your cell phone, it makes sense to have the phone's micro browser interact with the telephone network. The simplest example of this is adding a hyperlink to a document that dials a phone number when the user selects it. The browser, in this case, uses the Wireless Telephony Applications (WTA) interface to dial a phone number. However, WTA also enables more sophisticated processing, including handling telephone network events from within WMLScript (a modified version of JavaScript).

WTA requires a WTA User Agent on the cell phone and a WTA server that resides somewhere on the telephone network. The WAP documents don't specify how the WTA server interacts with the phone network, since that varies depending on the phone system you use. This makes sense—HTTP doesn't specify how a Web server retrieves files. It only notes that the client can ask for a file and receive it. The same thing holds true for WTA. It specifies that you can dial a number, for example, but it isn't concerned with how that actually occurs.

To simplify interactions with WTA, the specification calls for a WTA library that handles common tasks for applications. Applications use a super set of WML known as WTA-WML. These documents start with the wtai:// protocol identifier.

The phone has storage for some WTA-WML code in a place known as the repository. The repository stores resources (scripts, usually) in groups called channels. You can assign channels to particular events. For example, you might want some script to run when the phone receives an incoming call. Another channel might notify you that e-mail or voice mail is waiting for pick-up.

Of course, you don't want a WML script dialing a 900 number and running up your phone bill. To prevent this, WTA has a robust security feature that lets you trust some applications more than others.

More information is available online in the WTA specification at Although the specification is a bit dry, the example applications in Annex A are quite informative.

—Al Williams