Related work

Mobile applications need to be capable of responding to time-varying QoS conditions. In the following sub-sections, we briefly describe popular tools and middleware that support adaptive mobile applications and contrast their approach to our work.

Comma [KID 98] provides a simple and powerful way for application developers to access the information required to easily incorporate adaptive behavior into their application. It provides easy-to-use methods to access this information, a wide variety of operators and ranges available to provide the application the information it needs when it needs it, a small library to link with to minimize the overhead placed on the client and to minimize the amount of data that needs to be transferred between the clients and the servers.

The Rover toolkit [JOS 95] offers applications a distributed-object system based on the client-server architecture. The Rover toolkit provides mobile communication support based on re-locatable dynamic objects (RDOs). A relocatable dynamic object is an object with a well-defined interface that can be dynamically loaded into a client computer from a server computer, or vice versa, to reduce client/server communication requirements.

Sumatra [RAN 96] is an extension of the Java programming environment. Policy decisions concerning when, where and what to move are left to the application. The high degree of application control allows programmers to explore different policy alternatives for resource monitoring and for adapting to variations in resources. Sumatra provides a resource-monitoring interface, which can be used by applications to register monitoring requests and to determine current values of specific resources. When an application makes a monitoring request, Sumatra forwards the request to the local resource monitor.

Mobiware [ANG 98] provides a set of open programmable CORBA interfaces and objects that abstract and represent network devices and resources, providing a toolkit for programmable signaling, adaptation management and wireless transport services. Mobile applications specify a utility function that maps the range of observed quality to bandwidth. The observed quality index refers to the level of satisfaction perceived by an application at any moment. The adaptation policy captures the adaptive nature of mobile applications in terms of a set of adaptation policies. These policies allow the application to control how it moves along its utility curve as resource availability varies.

In general, it is left to the application to decide how to react to environment changes. This argues for exporting the network state as well as available resources of the portable device to the mobile applications to be designed to be adaptive. On the other hand, the automation of adaptation to the resources was not explored. There are a number of similarities between our work and the work in Sumatra. Both Sumatra and our work use extended Java Virtual Machines for portability and the ease of use of the language especially for implementing object mobility toolkits. The main difference is that, in our work, adaptation to the change in the resources and environment is partially left to the toolkit. We try first to use available remote resources to achieve the same task; otherwise, as a last resort, we let the application do the adaptation.

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