THE TIMED-TOKEN PROTOCOL FOR REAL-TIME COMMUNICATIONS

Most real-time applications such as industrial process control depend on digital computers. Typically, sensory data is periodically transmitted over a communication network to a digital computer controlling the process. Such systems often have stringent timing requirements, where a failure could lead to catastrophe. The key to success in such systems is the timely execution of data-processing tasks that usually reside on different nodes and communicate with one another to accomplish a common goal. This article outlines a local bandwidth allocation scheme using the timed-token protocol that guarantees the meeting of synchronous-message deadlines. The authors discuss ways of setting network paramaters and of properly allocating synchronous bandwidth, target token rotation time, and buffer size. They also discuss ways of measuring the impact of these paramaters on network performance. This local allocation scheme has several advantages. Because the synchronous-bandwidth allocation method uses only information local to a node, system designers can create or modify synchronous-message streams locally without reinitializing the network. Furthermore, the method is valid regardless of the behavior of asynchronous messages, because it assumes the worst possible impact of asynchronous traffic.