WAP - Wireless Application Protocol

This section discusses the background of the WAP development. It starts with an overview of the reasons for introduction of the protocol, followed by some typical problems it solves. Finally a short overview of the WAP specification is given.

5.2.1 Reasons for development

The Wireless Application Protocol is a world standard for the presentation and delivery of wireless information and telephony services on mobile phones and other wireless

terminals. Wireless data networks present a more constrained communication environment compared to wired networks. Because of limitations of power, available spectrum and mobility, wireless data networks tend to have less bandwidth, more latency, less connection stability and less predictable availability.

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Furthermore, as bandwidth increases, the handset's power consumption also increases which further burdens the already limited battery life of a mobile device. Therefore, even as wireless networks gain higher bandwidth, battery capacity and size will always limit the power of a handset, thus challenging the amount of data throughput. WAP has to

overcome these network limitations and still deliver a satisfactory user experience.

Handheld wireless devices present a more constrained computing environment than desktop computers. Because of limitations of battery life and size, handheld devices tend to have restricted power consumption, less powerful CPUs, less memory, both ROM and RAM, smaller displays and different input devices (e,g. phone keypad, voice input).

Therefore, handheld devices require a completely different user interface than the

traditional desktop GUI. These conditions are not likely to change much in the near future.

Most important of all, wireless subscribers have a different set of desires and needs than desktop or even laptop Internet users. WAP-enabled devices are companion products that have to deliver timely information and accept transactions and inquiries when the user is moving around. WAP services provide information access and delivery when the full screen environment is either not available or not necessary.

5.2.2 Considerations

The WAP specification addresses these issues by using existing industry standards as the basis for its architecture and design where possible, and adding new extensions where needed. For example, a WAP Gateway is required to communicate with other Internet nodes using the standard HTTP 1.1 protocol. Furthermore, the specification calls for wireless handsets to use the standard URL addressing scheme to request services.

The WAP standards are developed in such a way that they complement existing standards. For example, the WAP specification does not specify how data should be transmitted over the air interface. Instead, WAP is intended to sit on top of existing bearer channel standards so that any bearer standard can be used with the WAP protocols. By making minimal demands on the air interface itself, the WAP specification can operate on a large number of air interfaces. It defines a protocol stack that can even operate on high latency, low bandwidth networks such as Short Message Service (SMS), or GSM Unstructured Supplementary Service Data (USSD) channel. Being air interface independent also makes the speCification easy to extend to new networks as they develop. As air interfaces become more sophisticated, the services they provide can be designed to comply with the WAP specification.

In addition to being air interface independent, the WAP specification is also independent of any particular device. It specifies the bare minimum functionality a device must have, and has been designed to accommodate any functionality above that minimum. Device independence offers benefits similar to bearer independence: applications developed for a standard can operate on a wide variety of devices that implement the speCification.

Network operators and service providers can choose any standard compliant device that meets their requirements and provide their customers with a consistent user interface for their services across multiple vendors' handsets. Application developers do not have to write separate versions of their code for different devices. WAP not only enables air interface independent, device independent and interoperable solutions. It also ensures that these solutions are fast, reliable and secure.

5.2.3 Specification

One of the key elements of WAP is the definition of the WAP Programming Model, seen in Figure 5-1, which is based on the existing WWW Programming Model. This provides several benefits to the application developer community, including a familiar programming model, a proven architecture and the ability to leverage existing tools (e.g. Web servers, XML tools). Optimisations and extensions have been made in order to match the characteristics of the wireless environment. Wherever possible, existing standards have been adopted or have been used as the starting point for WAP technology.

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Figure 5-1 The WAP Programming Model

The mark-up language, which adheres to XML standards, is designed to enable powerful applications within the constraints of handheld devices. The Wireless Mark-up Language (WML) and WMLScript do not assume that a QWERTY-keyboard or a mouse are

available for user input, and are designed for small screen displays. Unlike the flat structure of HTML documents, WML documents are divided into a set of well-defined units of user interactions. One such unit is called a card, and services are created by letting the user navigate back and forth between cards from one or several WML documents. WML provides a small, telephony aware, set of mark-up tags that is more efficient and appropriate than HTML within handheld devices.

From the WAP Gateway, all WML content is accessed over the Internet using standard HTTP 1.1 requests, so traditional Web servers, tools and techniques can be used. The WAP Gateway encodes and decodes all requests and responses from efficient bytecode on the mobile side into standard text-based packets on the Internet side.

The micro-browser in the wireless terminal controls the user interface, analogous to a standard web browser. Its specification defines how WML and WMLScript should be interpreted in the handset and presented to the user. The micro-browser specification has been designed for wireless handsets so that the resulting code will be compact and efficient, yet still provides a flexible and powerful user interface.

The protocol stack is lightweight to minimise bandwidth requirements, guaranteeing that a variety of wireless networks can run WAP applications. For instance, a typical handset session, consisting of three requests and three responses, requires only seven packets to be exchanged with a total of 14% overhead. In comparison, the same Internet session would require the exchange of 17 packets with a total of 65% overhead. The protocol stack is shown in Figure 5-2.

Figure 5-2 The WAP Protocol Stack

From the protocol stack it is clear that WAP is a flexible system, designed to grow with user demand and network technology. For instance, the transport layer security protocol (WTLS) is designed to be replaced by its internet-peer, TLS, as bandwidth increases.

TLS is the successor of SSL, see Appendix A.

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The WAE architecture includes all elements of the WAP architecture related to application specification and execution. WAE specifies a micro-browser environment, which is the WAP variety of a web browser and which contains four elements. First of all it consists of the Wireless Mark-up Language (WML), which is a mark-up language similar to HTML, but optimised for use in hand-held mobile terminals. Secondly it contains WMLScript, a lightweight scripting language similar to JavaScript™. It also includes a set of well-defined data formats, including images, phone book records and calendar information, together called Content Formats. Finally it incorporates the Wireless Telephony Application (WTA) framework.

The WTA framework provides WMLScript applets with access to telephony functionality such as call control, phone book access and messages. This allows operators to develop secure telephony applications integrated into WMLIWMLScript services. For example, services such as Call Forwarding may provide a user interface that prompts the user to make a choice between accepting a call, forwarding it to another person or to a voicemail box.

Standard HTTP has no support for 'push' technology which means that it does not allow a WVVW server to push information to a client, i.e. a web browser, without the client's request for this information. For mobile applications a push feature can be very valuable, as it allows services to alert terminals of time-sensitive information changes. WAP enables this service with a newly developed protocol called Push Access Protocol (PAP).

More information on the architecture of WAP can be found in Appendix E. This appendix also contains descriptions of WAE (Wireless Application Environment), WTA (Wireless Telephony Application) and the WAP Push Architecture. The next section gives an overview of the WT A functionality.