This chapter introduces Wireless Markup Language (WML) and provides examples of programming wireless applications using presentation factories.

Wireless Application Protocol (WAP) devices include WML browsers that can display some content described in WML and WMLScript, which is part of the WAP specification. WMLScript can be used to add programming support to the client. It is similar to JavaScript and can be used in a similar manner. You can use WMLScript to validate user input, but most importantly, you can use it to generate messages and dialogs locally, so error messages and confirmations can be viewed faster.

It is also possible to access facilities of the user agent; for example, WMLScript allows the programmer to make phone calls, access the SIM (subscriber identity module) card, or configure the user agent after it has been deployed. (Keep in mind that data and voice are separate.)

Memory and CPU resources are limited and often prohibit the use of WMLScript, although WMLScript may be necessary to program proprietary telephony features for specific devices. For example, some devices allow your program to:

• Send DTMF (Dual Tone Multi-Frequency. The technical term describing Touch Tone dialing with combination of two tones, one low frequency and one high frequency.) tones

• Modify the device's phone book

• Use the phone book to start voice call or reject a call

Note that such programs would hardly work for a variety of models. The compatibility problem is one of the serious problems WAP developers face.