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We study the problem of guaranteeing synchronous message deadlines in token ring networks where the timed token medium access control protocol is employed. Synchronous bandwidth, defined as the maximum time for which a node can transmit its synchronous messages every time it receives the token, is a key parameter in the control of synchronous message transmission. To ensure the transmission of synchronous messages before their deadlines, synchronous capacities must be properly allocated to individual nodes. We address the issue of appropriate allocation of the synchronous capacities. Several synchronous bandwidth allocation schemes are analyzed in terms of their ability to satisfy deadline constraints of synchronous messages. We show that an inappropriate allocation of the synchronous capacities could cause message deadlines to be missed, even if the synchronous traffic is extremely low. We propose a scheme, called the normalized proportional allocation scheme, which can guarantee the synchronous message deadlines for synchronous traffic of up to 33% of available utilization.

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In the design of real-time systems, tasks are often assigned priorities. Preemptive priority driven schedulers are used to schedule tasks to meet the timing requirements. Priority inversion is the term used to describe the situation when a higher priority task's execution is delayed by lower priority tasks. Priority inversion can occur when there is contention for resources among tasks of different priorities. The duration of priority inversion could be long enough to cause tasks to miss their deadlines. Priority inversion cannot be completely eliminated. However, it is important to identify sources of priority inversion and minimize the duration of priority inversion. IN the paper we present a comprehensive review of the problem of and solutions to unbounded priority inversion.

Space Station Freedom (SSF) faces several challenges which are unique in the NASA history of manned and unmanned spacecraft systems. The challenges include a 30 year life, and evolution in capability of the station as a science platform and potentially a transportation node for manned missions to the solar system. The Data Management System (DMS) is a key element of SSF. DMS is the computational infrastructure of SSF, responsible for integrating information onboard into a cooperative whole. Its primary role is to provide integrated data processing and communication services for both the core function and the payloads. In this paper, we focus on some real-time issues in the design of DMS.