Why can we not simply use Internet protocols over mobile networks? The answer lies primarily in the differences between the two networks. Unlike the Internet, most mobile networks do not support IP addresses, and many impose limits on packet size; extra network roundtrips are very costly, and simple recovery from interruptions is imperative (given the realities of current cellular coverage). To provide these and other features, the WAP protocols define an entirely new layered model, which is loosely based on Internet protocols (see Figure 5).
Web Versus WAP Protocols
WAP defines a Wireless Session Protocol (WSP) that provides HTTP-like functionality but with added mobile features, including session support and push messaging. The Wireless Transaction Protocol (WTP) provides TCP/IP-like functionality with added support for transactions and reduced handshakes. The Wireless Datagram Protocol (WDP) provides a low level, UDP-like protocol that maps to the underlying mobile bearer (or IP) network layer. An optional security layer, similar to the Transport Layer Security (TLSformerly named Secure Sockets Layer or SSL), is provided by Wireless Transport Layer Security (WTLS).