JOCHEN SCHILLER - AllOnline

Support for mobility 10
Transferring data from a sender to a single receiver or many receivers is
not enough. Only applications make a communication network useful.
However, to use well-known applications from fixed networks, some additional
components are needed in a mobile and wireless communication
system. Examples are file systems, databases, security, accounting and billing
mechanisms. As mobile devices have limited energy resources, power consumption
is an important issue.
This chapter focuses on two aspects, file systems/file synchronization and
access to the world wide web (www). Some years ago, many research projects
dealt with the problems of distributed file systems. Some focused on the support
of mobile devices, low bandwidth wireless links, and disconnected
operation. The main problem for distributed, loosely coupled file systems is the
maintenance of consistency. Are all views on the file system the same? What
happens if a disconnected user changes data? When and how should the system
propagate changes to a user? Section 10.1 discusses several problems and presents
some research projects, while section 10.5 focuses on a new framework for
synchronization, SyncML.
However, the success of the www shifted the focus of many projects. A lot of
research effort was, and still is, put into the support of web browsing for mobile
users, as the web is the application driving the internet. Section 10.2 explains
some basic properties of the web and presents the hypertext transfer protocol
(HTTP) and hypertext markup language (HTML) in a short overview. For this section,
it is important to demonstrate the fundamental problems with HTTP and
HTML if used in a mobile network with only low-bandwidth wireless access. The
web has been designed for conventional computers and fixed networks. Several
new system architectures try to alleviate these problems. These architectures are
also good examples for client/server scenarios in wireless environments.
Section 10.3 which presents the wireless application protocol (WAP) version
1.x is the main part of the chapter. WAP is a common effort of many
companies and organizations to set up a framework for wireless and mobile web
access using many different transport systems. Examples are GSM, GPRS, and
UMTS as presented in chapter 4. WAP integrates several communication layers
for security mechanisms, transaction-oriented protocols, and application
support. In the current www, these features are not an integral part but add-ons.
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