Next Generation Wireless Applications: Creating Mobile Applications in a Web 2.0 and Mobile 2.0 World, 2nd Edition

7.5 WAP DATA TRANSMISSION PROTOCOLS

In this section, we shall look at the WAP protocols that enable us to implement a link–fetch–response communications paradigm, like HTTP, but in a manner that works well on a mobile RF link. Before we look at the details of the protocols, let's first talk a little about protocol stacks and how data communications are implemented in software terms. This enables us to better understand the concept of service layers within a communications protocol.

7.5.1 Protocol Stack Paradigm

In our discussion of HTTP, we saw that the protocol consisted of a set of communications primitives based upon request methods, such as GET. However, we have just learnt that the underlying mechanism of TCP/IP is a bit more complex. Perhaps, and most probably, a single GET request may not make it in one attempt to our origin server. It may need to be divided up into segments and sent one piece at a time, allowing for retries and handshaking, as shown in Figure 7.10.

Figure 7.10 Segmenting a HTTP GET request into packets.

The exact size of the packets may not be as we have shown them in the figure, nor is it likely that the packets will be delineated neatly along the boundaries of HTTP headers as shown; this is just to illustrate the principle. The key point is that the user agent is not responsible for this segmentation process, or the underlying flow control. The user agent ...