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The invisible link

© Wai Seto

The Internet has been known to be good at connecting people and machines together. Traditional connection media are baseband (single frequency signal) or broardband (multiple frequency signals). These technologies work well, if the people or machines are stationary. However, the fact is we don't want to be stationary. We want to be mobile, and keep moving. Therefore we are searching for the invisible link, no physical media attached. WAP provides such an invisible link for the Internet. In this issue, we explore how this invisible link works over thin air.

ABC of WAP Last article we have looked at how WAP works in general. A quick recap is that contents are generated by WML, Wireless Markup Language. Microbrowsers are used on small wireless devices to read WML contents and navigate through the contents. Behind the scene, there is a WAP Server (or Gateway) that handles traffic between the devices and content servers (web servers on the Internet).

This time we are going to look at how the wireless devices communicate with WAP Gateway. When it comes to network communication, the buzz word 'protocol' should be the first thing we think about. In short, protocol is a set of rules that dictates the way of communications between two parties.

WSP, Wireless Session Protocol, is defined in WAP to provide a session service to wireless applications for cilient/server communications. Same to all other components under WAP, there are two major considerations when these components are being design, latency and low-bandwidth. Before we dig deep into this protocol, a quick mental note should be made. WSP is designed to provide (not limited to) browsing capability. At first sight, it can be looked at HTTP's sibling. (only smarter) This protocol provide the following features to wireless applications.

1. It handles the establishing and releasing of reliable sessions between client (e.g. mobile phones) and server (WAP gateway).

2. The protocol provides client and server capability negotiation. Elements such as Protocol Data Unit size and maximum outstanding requests are agreed on a session bases.

3. WSP also defines compact encoding for content exchange between client and server. The intention of encoding is to actually minimize data that have to go over the link. This is a big improvement on content size over HTTP. (HTTP transmit all headers and contents as plain text).

4. Another feature that makes WSP efficient over wireless links is the suspend and resume session capability. WSP allows session to last over weeks and months. This implies that session setup and negotiation can be done once and be reused overtime. Current HTTP sessions are created for every connection, and are shutdown down after a HTML page is loaded.

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