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Design Choice: Determining the Degrees of Freedom in degradation trajectories

The OI interfaces of the components will normally provide a discrete set of implementations upon some variation. Imagine a simple speech tool which provided a set of encodings to use for speech. As the degree of compression in the encodings gets higher, the bandwidth requirements decrease but the quality of the resultant speech is decreased, thereby decreasing the utility of the application.

For more complex applications, each component may provide other choices of implementation which are orthogonal to each other in that they can reduce resource usage in different ways. If we can add redundancy to the speech encodings, using some of our bandwidth to provide data to repair lost packets at the expense of greater latency, then we provide another set of choices of about how to use valuable bandwidth.

If we regard each component as supplying an OI interface which can be adjusted independently of the other components, the designer thus has a choice over each component of an implementation. The number of independent axes which contain implementation choices is the degree of freedom of the application. Adjusting the Open Implementation of a component can be viewed as placing the component at some new point in the space of (implementation choice, resource usage, application utility) space. As we select a new set of implementation choices to decrease the resource usage of a component, we generally decrease the utility of the overall application.

Thus for each of the possible implementations of the component, we could theoretically measure the utility of the resultant application. The designer must understand this space of possible implementations, for it is from this set of implementation choices that the designer chooses a path of degradation in the face of resource constraints.

Malcolm McIlhagga
Thursday June 11 16:17:19 BST 1998