13.1 Architecture Overview

The WAP architectural model is similar to the Internet WWW model with a few enhancements. Figure 13-1(a) shows the communication model for WWW and WAP. The WWW model in a wired network is a client/server model, with HTTP requests being generated from the browser on the user terminal. Figure 13-1(b) shows the WAP communication model involving a proxy between user terminals and the content servers. The WAP proxy provides the WAP protocol interfaces to the user terminal over the radio with encoded communications. This model also includes a push server to push services toward the user through the proxy. Another enhancement from the WWW model not shown in Figure 13-1 is the support for telephony applications in the WAP model.

Figure 13-1. Communication models.

The use of proxy server in the WAP communication model is to optimize and enhance the communications between wireless devices and the Internet. Specifically, the WAP proxy provides the following functions:

• Protocol gateway: The proxy translates the WAP protocols to the protocols understood in the Web, like HTTP and TCP/IP. Another function of the gateway is to perform DNS lookups of the servers named by the WAP clients in the URL requests.

• Content encoders and decoders: The use of content encoders is to translate WAP content into a compact format for transfer over the radio.

• User agent profile management: User agent profiles that define the client capabilities and user preferences are composed and presented to application servers for personalized services.

• Caching proxy: Frequently, request pages can be cached for improvement in performance and network utilization.

Figure 13-2 shows the WAP protocol architecture and the corresponding protocols in the Internet Protocol stack. Different layers within the WAP protocol stack provide different features. Applications and services, based on need, can use any layer of the protocol directly. The following sections define requirements and deployment details of individual layers .

Figure 13-2. WAP architecture.

13.1.1 Wireless Datagram Protocol

The main goal of WDP is to allow application services to be designed independent of the specific transports. Hence, WDP provides a common interface to various physical transport mechanisms, called bearers . WDP is designed for narrow to medium bandwidth channels and also to be extensible to a wide variety of cellular protocols with scope for future transport definitions. It provides an abstraction to the upper-layer protocols by providing a port-based interface.

The basic capability of WDP is to provide a connectionless, unreliable datagram communication service. It provides similar functions as the UDP in the Internet stack of protocols. It allows peer-to-peer, client/server, and one-to-many applications while ensuring multivendor and multidevice interoperability at the WDP layer. WDP provides implementation capability for standard or resource-constrained devices.

13.1.2 Wireless Transport Layer Security

The WTLS layer is responsible for security features between the terminal and the counterpart application server. It must support security features like privacy, integrity, authentication, and end-to-end security between protocol endpoints. WTLS is based on transport layer security (TLS) 1.0 and is very efficient in bandwidth usage. It implements optimized handshaking and supports datagram-oriented bearers and lightweight public key certificate format. WTLS can enable long-living secure sessions and provides a choice of different algorithms.

13.1.3 Wireless Transaction Protocol

The WTP layer provides both reliable connection-oriented and transaction-oriented services for certain application services that require these features. WTP also supports the selection of underlying bearers to the upper layers of the protocol stack. WTP allows packet concatenation and supports transaction ID negotiation.

The primary characteristics of the connection-oriented WTP (WTP/C) layer is to allow negotiation of connection setup parameters. It provides a deterministic flow control appropriate for radio transmissions, retransmission of lost packets, selective retransmission, segmentation/reassembly, and port number addressing.

One of the important requirements of both WDP and WTP is to be efficient with respect to communications and, hence, provide the support for header compression with low transmission overhead with optimum handshakes. Other requirements are with respect to scalability to operate over a range of wireless networks and devices and to be modular to allow various security solutions.

The following classes of service are supported in the WTP layer.

• Class 0 provides an unconfirmed Invoke message with no result message.

• Class 1 provides a confirmed Invoke message with no result message. This service class is used for push data operations, where no response from the destination is expected.

• Class 2 provides a confirmed Invoke message with one confirmed result message. In this mode, a single request produces a single reply.

13.1.4 Wireless Session Protocol

The main requirements of the Wireless Session Protocol (WSP) are to provide HTTP 1.1 functionality, interrupt ongoing transactions if needed, provide application acknowledgment, push content from the server to client asynchronously, and exchange static client and server state during session creation. Some of the ongoing work items in the WSP group are to support QoS parameters, multicast data, ordered pipelining, chunked data transfer, and WSP management entity.

13.1.5 Wireless Application Environment

The wireless application environment (WAE) provides application-level services, tools, and languages required for authoring content and for transferring content in WAP-enabled wireless networks. One of the objectives is to leverage the existing technology and provide a universal applications platform for Web browsing and telephony services. The WAE work group within the WAP Forum focuses on the core areas of application architecture, markup languages, scripting languages, and telephony integration.

The wireless network and mobile device characteristics have a great influence on the design of markup and scripting languages. Figure 13-3 shows the client-side WAE architecture. The WAE framework involves the user agents and services and formats to support the user agents .

Figure 13-3. WAE client architecture.