The key to the success in using a distributed system for hard real-time applications is the timely execution of computation tasks which usually reside on different nodes and communicate with one another to accomplish a common goal. End-to-end deadline guarantees are not possible without a communication network which supports the timely delivery of intertask messages. The main focus of the research is to address issues related to guaranteeing synchronous message deadlines, i.e., no matter what happens (except a network failure), the messages will be transmitted before their deadlines. We have selected FDDI (Fiber Distributed Data Interface) networks for this study because they are well suited for hard real-time communications, due not only to their high bandwidth, but also to their property of bounded token rotation time. However, a bounded token rotation time alone is not sufficient to guarantee hard real-time deadlines. Synchronous capacity allocation is equally critical in achieving this performance objective. In this project, we concentrate on design and analysis of synchronous capacity allocation to guarantee message deadlines. The project consists of four related tasks: We first address the issue of optimality. An optimal scheme achieves the highest worst case utilization and hence offers the best chance to guarantee a set of synchronous messages. Then, we study localized allocation schemes which use local information to allocate synchronous capacity to a node. They have the advantage of being able to adjust the parameters on one node without disturbing the operations of others. Next, we will extend our basic model to consider variations needed in practice. For example, some applications do not require an absolute deadline guarantee, i.e., occasionally missing a deadline is acceptable. We formalize these ideas in the design and analysis of allocations schemes in order to improve their applicability. Selection of TTRT (Target Token Rotation Time) also affects the chance of guaranteeing deadlines; this is addressed as the last task in the project.
