Chapter 1. Introduction
intermittent bandwidth, and extremely awkward input mechanisms. Most existing HTML pages do not
work on mobiles phones, and never will.
WAP defines a completely new markup language, the Wireless Markup Language (WML), which is
simpler and much more strictly defined than HTML. It also defines a scripting language, WMLScript,
which all browsers are required to support. To make things even simpler for the phones, it even defines
its own bitmap format (Wireless Bitmap, or WBMP).
HTTP is also too inefficient for wireless use. However, by using a semantically similar binary and
compressed format it is possible to reduce the protocol overhead to a few bytes per request, instead of the
usual hundreds of bytes. Thus, WAP defines a new protocol stack to be used. However, to make things
simpler also for the people actually implementing the services, WAP introduces a gateway between the
phones and the servers providing content to the phones.
Figure 1-1. Logical position of WAP gateway (and PPG)between a phone and a content server.
The WAP gateway talks to the phone using the WAP protocol stack, and translates the requests it receives
to normal HTTP. Thus content providers can use any HTTP servers and utilize existing know-how about
HTTP service implementation and administration.
In addition to protocol translations, the gateway also compresses the WML pages into a more compact
form, to save on-the-air bandwidth and to further reduce the phone’s processing requirements. It also
compiles WMLScript programs into a bytecode format.
Kannel is not just a WAP gateway. It also works as an SMS gateway. Although WAP is the hot and
technically superior technology, SMS phones exist in huge numbers and SMS services are thus quite
useful. Therefore, Kannel functions simultaneously as both a WAP and an SMS gateway.
Overview of WAP Push
Previous chapter explained pull mode of operation: the phone iniatiates the transaction. There is,
however, situations when the server (called in this context a push initiator) should be the initiator, for
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