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Introduction to WAP
WAP supports the delivery of Web content over wireless networks

The name "Wireless Application Protocol" (WAP) is misleading. WAP is not actually a protocol at all, in the sense that HTTP and IP are protocols. In fact, WAP involves multiple protocols and a complete network architecture for delivery of wireless content.

 More of this Feature
• Part 2: WAP and WML
• Part 3: WAP Applications and Future
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"How long will it be before mobile wireless networks with WML-based content offer something really interesting to the general public? One year? Five years? Never??"
  Related Resources
• Wireless Networking Directory
 Elsewhere on the Web
• WAP Forum Home Page
What Exactly Is WAP?

In 1997, several companies organized an industry group called the WAP Forum. This group produces the WAP specification, a (long and detailed) series of technical documents that define standards for implementing wireless network applications. Hundreds of industry firms have given strong backing to the WAP Forum, so the technology should become widely adopted, and it is already well-hyped.

WAP specifies an architecture based on layers that follows the OSI model fairly closely. The WAP model, or stack as it is commonly known, is illustrated below.

The WAP Model (Stack)

The WAP Model
Application Layer

WAP's application layer is the Wireless Application Environment (WAE). WAE directly supports WAP application development with Wireless Markup Language (WML) instead of HTML and WMLScript instead of JavaScript. WAE also includes the Wireless Telephony Application Interface (WTAI, or WTA for short) that provides a programming interface to telephones for initiating calls, sending text messages, and other networking capability.

Session Layer

WAP's session layer is the Wireless Session Protocol (WSP). WSP is the equivalent to HTTP for WAP browsers. WAP involves browsers and servers just like the Web, but HTTP was not a practical choice for WAP because of its relative inefficiency on the wire. WSP conserves precious bandwidth on wireless links; in particular, WSP works with relatively compact binary data where HTTP works mainly with text data.

Transaction, Security, and Transport Layers

These three protocols can be thought of as "glue layers" in WAP:

  • Wireless Transaction Protocol (WTP)
  • Wireless Transaction Layer Security (WTLS)
  • Wireless Datagram Protocol (WDP)

WTP provides transaction-level services for both reliable and unreliable transports. It prevents duplicate copies of packets from being received by a destination, and it supports retransmission, if necessary, in cases where packets are dropped. In this respect, WTP is analogous to TCP. However, WTP also differs from TCP. WTP is essentially a pared-down TCP that squeezes some extra performance from the network.

WTLS provides authentication and encryption functionality analogous to Secure Sockets Layer (SSL) in Web networking. Like SSL, WTLS is optional and used only when the content server requires it.

WDP implements an abstraction layer to lower-level network protocols; it performs functions similar to UDP. WDP is the bottom layer of the WAP stack, but it does not implement physical or data link capability. To build a complete network service, the WAP stack must be implemented on some low-level legacy interface not technically part of the model. These interfaces, called bearer services or bearers, can be IP-based or non-IP based.

Bearer Interfaces

WAP supports dial-up networking using IP and Point-to-Point Protocol (PPP) as the bearer interface underneath WDP. It also supports Short Message Service (SMS) and General Packet Radio System (GPRS). SMS passes text and binary data between digital phones. GPRS is a relatively new technology that implements faster, "always-on" connections for wireless devices; GPRS actually runs on top of IP.

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