An extended IEEE 802.5 protocol suitable for transmitting time-constrained messages in a token ring network is studied. It differs from traditional token ring protocols in that time constraints of messages are incorporated explicitly. In this protocol, the laxities of messages are mapped into priorities. The message with the highest priority is transmitted first. As a result, this protocol approximates the optimal minimum-laxity-first policy. It is found that in the worst case the protocol can send at least 50% of the messages sent by the optimal one. This ratio is independent of the number of priorities and the priority assignment function used in the protocol. On the other hand, simulation results show that the average performance of the protocol improves as the number of priorities increases and that a simple priority assignment function is sufficient to yield satisfactory performance. >
['L. Yao', 'Wei Zhao']
Performance of an extended IEEE 802.5 protocol in hard real-time systems