How Does WAP Fit Together?

All of these elements are brought together to provide an environment that facilitates mobile devices interacting with applications that reside on remote servers. The WAE is deployed on the client device to provide an environment in which the user interface can function. Sessions from the client device through to the WAP gateway are established and managed using Wireless Session Protocol (WSP)/Wireless Transport Protocol (WTP) and the underlying protocol layers.

The WAP gateway itself includes a variety of functionality, including protocol adapters to allow translation between WSP and the various protocols that could be used to access the network services, and encoding and compilation of the WML and WMLScript from text to binary formats. The binary format is used to minimize the amount of data that has to be sent over the network, and also to reduce the amount of processing that the mobile device needs to perform. Although WSP is used almost exclusively by the mobile device to request content from the gateway, the protocol that is used by the back-end service is the one encoded into the URL. The only protocol supported for access to network services at the moment is HTTP.

The WAP gateway communicates with an ordinary web server, using HTTP. What happens from the web server through to the application is no different from Internet access from fixed-wire devices, so the content could be static WML pages, or dynamic content generated using servlets, ASP, CGI, or any other server-side web technology. All content is referenced using standard URLs.

How Does WAP Stack Up?

The differences between the WAP protocol stack and a typical Internet protocol stack is the most important part of enabling wireless access for mobile devices. The WAP protocol stack does not map directly onto other stacks, although some comparison is possible. This is illustrated in the diagram below:

The kind of functionality that is provided by HTML and Java in the Internet world is incorporated in WAP in the WAE and, to some extent, the WSP. WSP and WTP between them cover the functionality that is provided by HTTP.

If security is required in the fixed-wire world, Transport Layer Security (TLS) is usually used, and in the wireless world there is the Wireless Transport Layer Security (WTLS) equivalent. The transport layer in the fixed-wire world is usually either TCP or UDP over IP, and in the wireless world it is UDP over IP where possible, or the Wireless Datagram Protocol (WDP) is provided for networks that cannot support IP at the network layer. Underlying WDP there is a large number of over-the-air bearers that are supported.