Medium Access Protocol for Expert Assistants in Distributed Control Systems

University of California, Santa Barbara, Electrical and Computer Engineering Department; Santa Barbara, CA

Locally Distributed Multiprocessor Systems are undergoing constant development in plant control applications. For several practical reasons, Local Area Networks based on multi-access broadcast buses are extensively used for interprocessor communication in these distributed control architectures. A new and powerful software tool has been recently introduced in the control field: the Real-Time Expert Systems for Crisis Management, also known as Expert Assistants.

The activation of an Expert Assistant in a Distributed Control System under the event of an emergency situation creates a traffic overload of high-priority messages. The response times in broadcast communication buses strongly depend on the medium access protocols. The simulation model proved that prioritized random access protocols bring about improved performance for this application. The model also tested a novel implementation known as the Stubborn protocol which provides a balanced and near-optimal performance under both normal (control) operation and transient (emergency) conditions. It is based on the following simple principle: stations make access attempts as in the usual 1-persistent CSMA/CD protocol; however, in case of collision, a station with priority higher than 0 will not interrupt transmission, but will exercise its right to extend the collision for a period proportional to its priority level. The station that persists longer will be able to effectively start transmission as soon as all others have ceased. The algorithm is complemented with tie breaking procedures.