Real-Time IEEE 802.5 Token Ring Implementation Studies

Previous work [15] developed the algorithms, models and methodologies to realize highly responsive, deterministic communications services utilizing the IEEE 802.5 Token Ring Standard. Simulation studies were used to validate the timing determinism property, to demonstrate greatly enhanced asynchronous response limes, and to introduce guaranteed asynchronous alert class message service. This work reports the results of implementation studies utilizing commercially available IEEE 802.5 chip sets and boards. Serious implementation flaws were discovered that prevent the full potential of the IEEE 802.5 Token Ring Standard from being realized for real-time applications. Specifically, unbounded delays can be introduced which not only destroy the desired response time determinism offered by algorithmic scheduling, but also negate asynchronous alert class guarantees and unnecessarily increase other asynchronous response times. This work analyzes the scheduling properties of current implementations and proposes modifications to those implementations to allow them to realize the full potential offered by the IEEE 802.5 Standard for real-time applications. This work also documents end to end message delivery time measurements from the ARTS hardware testbed.