Thin Client Application Models | Mobile and Wireless Design Essentials

Wireless Application Protocol (WAP) Overview

The Wireless Application Protocol (WAP) is a worldwide standard for the delivery and presentation of wireless information to mobile phones and other wireless devices. The idea behind WAP is simple: simplify the delivery of Internet content to wireless devices by delivering a comprehensive, Internet-based, wireless specification. The WAP Forum released the first version of WAP in 1998. Since then, it has been widely adopted by wireless phone manufacturers, wireless carriers, and application developers worldwide. Many industry analysts estimate that 90 percent of mobile phones sold over the next few years will be WAP-enabled.

The driving force behind WAP is the WAP Forum component of the Open Mobile Alliance. The WAP Forum was founded in 1997 by Ericsson, Motorola, Nokia, and Openwave Systems (the latter known as Unwired Planet at the time) with the goal of making wireless Internet applications more mainstream by delivering a development specification and framework to accelerate the delivery of wireless applications. Since then, more than 300 corporations have joined the forum, making WAP the de facto standard for wireless Internet applications. In June 2002, the WAP Forum, the Location Interoperability Forum, SyncML Initiative, MMS Interoperability Group, and Wireless Village consolidated under the name Open Mobile Alliance to create a governing body that will be at the center of all mobile application standardization work.

The WAP architecture is composed of various protocols and an XML-based markup language called the Wireless Markup Language (WML), which is the successor to the Handheld Device Markup Language (HDML) as defined by Openwave Systems. WAP 2.x contains a new version of WML, commonly referred to as WML2; it is based on the eXtensible HyperText Markup Language (XHTML), signaling part of WAP's move toward using common Internet specifications such as HTTP and TCP/IP.

In the remainder of this section we will take a look at the WAP programming model and the various components that comprise the WAP architecture. Where it is applicable, we will supply information on both the WAP 1.x and 2.x specifications. (More information on the leading markup languages used in wireless Internet applications is provided in Chapter 13.)

WAP Programming Model

The WAP programming model is very similar to the Internet programming model. It typically uses the pull approach for requesting content, meaning the client makes the request for content from the server. However, WAP also supports the ability to push content from the server to the client using the Wireless Telephony Application Specification (WTA), which provides the ability to access telephony functions on the client device.

Content can be delivered to a wireless device using WAP in two ways: with or without a WAP gateway. Whether a gateway is used depends on the features required and the version of WAP being implemented. WAP 1.x requires the use of a WAP gateway as an intermediary between the client and the wireless application server, as depicted in Figure 11.6. This gateway is responsible for the following:

Translating requests from the WAP protocol to the protocols used over the World Wide Web, such as HTTP and TCP/IP.
Encoding and decoding regular Web content into compact formats that are more appropriate for wireless communication.
Allowing use of standard HTTP-based Web servers for the generation and delivery of wireless content. This may involve transforming the content to make it appropriate for wireless consumption.
Implementing push functionality using WTA.

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Figure 11.6: WAP Programming model using a wireless gateway (or proxy).

Note

The WAP gateway is often called the WAP proxy in the WAP 2.x documents available from the OMA. In this chapter we continute to refer to it as the WAP gateway; just be aware that both terms are used to refer to the same technology.

When developing WAP 2.x applications, you no longer are required to use a WAP gateway. WAP 2.x allows HTTP communication between the client and the origin server, so there is no need for conversion. This is not to say, however, that a WAP gateway is not beneficial. Using a WAP gateway will allow you to optimize the communication process and facilitate other wireless service features such as location, privacy, and WAP Push. Figure 11.7 shows the WAP programming model without a WAP gateway: Note that removing it makes the wireless Internet application architecture nearly identical to that used for standard Web applications.

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Figure 11.7: WAP programming model without gateway.

Both WAP programming models require the same core set of steps to process a wireless Internet request. These steps are based on the common pull model used for Internet applications; that is, a request/response method for communication. If you are interested in more details about how a wireless request is processed, refer back to the previous section of this chapter entitled Processing a Wireless Request.

WAP Components

The WAP architecture comprises several components, each serving a specific function. These components include a wireless application environment, session and transaction support, security, and data transfer. The exact protocols used depend on which version of WAP you are implementing. WAP 2.x is based mainly on common Internet protocols such as HTTP and TCP/IP, while WAP 1.x uses proprietary protocols developed as part of the WAP specification. We will investigate each component and its related function.

To begin, we will look at how WAP conforms to the Open Systems Interconnection (OSI) model as defined by the International Standards Organization (ISO). The OSI model consists of seven distinct layers, six of which are depicted in Figure 11.8 as they relate to the WAP architecture. The physical layer is not shown; it sits below the network layer and defines the physical aspects such as the hardware and the raw bit-stream. For each of the other six layers, WAP has a corresponding layer, which will now be described in more depth.

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Figure 11.8: WAP architecture and its relationship to the OSI model.

Wireless Application Environment (WAE)

The Wireless Application Environment (WAE) is the application layer of the OSI model. It provides the required elements for interaction between Web applications and wireless clients using a WAP microbrowser. These elements are as follows:

A specification for a microbrowser that controls the user interface and interprets WML and WMLScript.
The foundation for the microbrowser in the form of the Wireless Markup Language (WML). WML has been designed to accommodate the unique characteristics of wireless devices, by incorporating a user interface model that is suitable for small form-factor devices that do not have a QWERTY keyboard.
A complete scripting language called WMLScript that extends the functionality of WML, enabling more capabilities on the client for business and presentation logic.
Support for other content types such as wireless bitmap images (WBMP), vCard, and vCalendar.

WAP 2.x extends WAE by adding the following elements:

A new markup language specification called WML2 that is based on XHTML-Basic. Backward compatibility with WML1 has been maintained.
Support for stylesheets to enhance presentation capabilities. Stylesheet support is based on the Mobile Profile of Cascading Style Sheets (CSS) from the W3C, and supports both inline and external style sheets.

Note

WAP 2.x WAE has backward compatibility to WML1. This is accomplished either via built-in support for both languages or by translating WML1 into WML2 using eXtensible Stylesheet Language Transformation (XSLT). The method used depends on the implementation by the device manufacturer.

WAP Protocol Stack

The WAP protocol stack has undergone significant change from WAP 1.x to WAP 2.x. The basis for the change is the support for Internet Protocols (IPs) when IP connectivity is supported by the mobile device and network. As with other parts of WAP, the WAP 2.x protocol stack is backward-compatible. Support for the legacy WAP 1.x stack has been maintained for non-IP and low-bandwidth IP networks that can benefit from the optimizations in the WAP 1.x protocol stack.

We will take a look at both WAP 1.x and WAP 2.x, with a focus on the technologies used in each version of the specification.

WAP 1.x

The protocols in the WAP 1.x protocol stack have been optimized for low-bandwidth, high-latency networks, which are prevalent in pre-3G wireless networks. The protocols are as follows:

Wireless Session Protocol (WSP). WSP provides capabilities similar to HTTP/1.1 while incorporating features designed for low-bandwidth, high-latency wireless networks such as long-lived sessions and session suspend/resume. This is particularly important, as it makes it possible to suspend a session while not in use, to free up network resources or preserve battery power. The communication from a WAP gateway to the microbrowser client is over WSP.
Wireless Transaction Protocol (WTP). WTP provides a reliable transport mechanism for the WAP datagram service. It offers similar reliability as Transmission Control Protocol/Internet Protocol (TCP/IP), but it removes characteristics that make TCP/IP unsuitable for wireless communication, such as the extra handshakes and additional information for handling out-of-order packets. Since the communication is directly from a handset to a server, this information is not required. The result is that WTP requires less than half of the number of packets of a standard HTTP-TCP/IP request. In addition, using WTP means that a TCP stack is not required on the wireless device, reducing the processing power and memory required.
Wireless Transport Layer Security (WTLS). WTLS is the wireless version of the Transport Security Layer (TLS), which was formerly known as Secure Sockets Layer (SSL). It provides privacy, data integrity, and authentication between the client and the wireless server. Using WTLS, WAP gateways can automatically provide wireless security for Web applications that use TLS. In addition, like the other wireless protocols, WTLS incorporates features designed for wireless networks, such as datagram support, optimized handshakes, and dynamic key refreshing.
Wireless Datagram Protocol (WDP). WDP is a datagram service that brings a common interface to wireless transportation bearers. It can provide this consistent layer by using a set of adapters designed for specific features of these bearers. It supports CDPD, GSM, CDMA, TDMA, SMS, FLEX (a wireless technology developed by Motorola), and Integrated Digital Enhanced Network (iDEN) protocols.

WAP 2.x

One of the main new features in WAP 2.x is the use of Internet protocols in the WAP protocol stack. This change was precipitated by the rollout of 2.5G and 3G networks that provide IP support directly to wireless devices. To accommodate this change, WAP 2.x has the following new protocol layers:

Wireless Profiled HTTP (WP-HTTP). WP-HTTP is a profile of HTTP designed for the wireless environment. It is fully interoperable with HTTP/1.1 and allows the usage of the HTTP request/response model for interaction between the wireless device and the wireless server.
Transport Layer Security (TLS). WAP 2.0 includes a wireless profile of TLS, which allows secure transactions. The TLS profile includes cipher suites, certificate formats, signing algorithms, and the use of session resume, providing robust wireless security. There is also support for TLS tunneling, providing end-to-end security at the transport level. The support for TLS removes the WAP security gap that was present in WAP 1.x.
Wireless Profiled TCP (WP-TCP). WP-TCP is fully interoperable with standard Internet-based TCP implementations, while being optimized for wireless environments. These optimizations result in lower overhead for the communication stream.

Note

Wireless devices can support both the WAP 1.x and WAP 2.x protocol stacks. In this scenario, they would need to operate independently of each other, since WAP 2.x provides support for both stacks.

Other WAP 2.x Services

In addition to a new protocol stack, WAP 2.x introduced many other new features and services. These new features expand the capabilities of wireless devices and allow developers to create more useful applications and services. The following is a summary of the features of interest:

WAP Push. WAP Push enables enterprises to initiate the sending of information on the server using a push proxy. This capability was introduced in WAP 1.2, but has been enhanced in WAP 2.x. Applications that require updates based on external information are particularly suited for using WAP Push. Examples include various forms of messaging applications, stock updates, airline departure and arrival updates, and traffic information. Before WAP Push was introduced, the wireless user was required to poll the server for updated information, wasting both time and bandwidth.
User Agent Profile (UAProf). The UAProf enables a server to obtain information about the client making the request. In WAP 2.x, it is based on the Composite Capabilities/Preference Profiles (CC/PP) specification as defined by the W3C. It works by sending information in the request object, allowing wireless servers to adapt the information being sent according to the client device making the request.
External Functionality Interface (EFI). This allows the WAP applications within the WAE to communicate with external applications, enabling other applications to extend the capabilities of WAP applications, similar to plug-ins for desktop browsers.
Wireless Telephony Application (WTA). The WTA allows WAP applications to control various telephony applications, such as making calls, answering calls, putting calls on hold, or forwarding them. It allows WAP WTA-enabled cell phones to have integrated voice and data services.
Persistent storage interface. WAP 2.x introduces a new storage service with a well-defined interface to store data locally on the device. The interface defines ways to organize, access, store, and retrieve data.
Data synchronization. For data synchronization, WAP 2.x has adopted the SyncML solution. As outlined in Chapter 10, "Enterprise Integration through Synchronization," SyncML provides an XML-based protocol for synchronizing data over both WSP and HTTP.
Multimedia Messaging Service (MMS). MMS is the framework for rich-content messaging. Going beyond what is possible for SMS, MMS can be used to transmit multimedia content such as pictures and videos. In addition, it can work with WAP Push and UAProf to send messages adapted specifically for the target client device.

WAP Benefits

The WAP specification is continually changing to meet the growing demands of wireless applications. The majority of wireless carriers and handset manufacturers support WAP and continue to invest in the new capabilities it offers. Over the years WAP has evolved from using proprietary protocols in WAP 1.x to using standard Internet protocols in WAP 2.x, making it more approachable for Web developers. The following are some of the key benefits that WAP provides:

WAP supports legacy WAP 1.x protocols that encode and optimize content for low-bandwidth, high-latency networks while communicating with the enterprise servers using HTTP.
WAP supports wireless profiles of Internet protocols for interoperability with Internet applications. This allows WAP clients to communicate with enterprise servers, without requiring a WAP gateway.
WAP allows end users to access a broad range of content over multiple wireless networks using a common user interface, the WAP browser. Because the WAP specification defines the markup language and microbrowser, users can be assured that wireless content will be suitable for their WAP-enabled device.
WAP uses XML as the base language for both WML and WML2 (which uses XHTML), making it easy for application developers to learn and build wireless Internet applications. It also makes content transformation easier by incorporating support for XSL stylesheets to transform XML content. Once an application is developed using WML or WML2, any device that is WAP-compliant can access it.
WAP has support for WTA. This allows applications to communicate with the device and network telephony functions. This permits the development of truly integrated voice and data applications.
Using UAProf, the information delivered to each device can be highly customized. (Chapter 13 provides more details on how this information can be used to deliver user-specific content.)
WAP works with all of the main wireless bearers, including CDPD, GSM, CDMA, TDMA, FLEX, and iDEN protocols. This interoperability allows developers to focus on creating their applications, without having to worry about the underlying network that will be used.

At present, all major wireless carriers support the WAP specification. This universal support is expected to continue as WAP evolves, providing a robust, intuitive way to extend Web content to wireless devices.