An Efficient Priority Mechanism for Token-Ring Networks

In a token-ring Local Area Network it is important to have minimum delay at each station. One-bit-delay is the minimum possible delay a ring station may have. It can be achieved only if every received bit is transmitted with no change or its outgoing value is determined independently of its incoming value and the incoming values of subsequent bits. The Paper introduces the distributed priority mechanism for token-rings as approved by the IEEE-802.5 standard. In this scheme, the token is accompanied by a priority field P and a reservation field R, that work together in an attempt to match the service priority of the ring to that of the most urgent waiting message. It is shown that due to the computation restrictions imposed by the one-bit-delay requirements, the scheme may require up to 7 round-trips in ordor to reduce P to R. This may lead to loss of bandwidth and starvation at stations with low priority data. The paper presents a new priority mechanism and proves its correctness. The new mechanism retains the desired properties of the standard protocol: It ensures fairness and can be executed by the stations with one-hit-delay. In the new mechanism at most one round-trip is required in order to reduce P to R. This increases the ring throughput and enables low-priority PDUs to get service when PDUs with higher priorities do not exist.