Are Priorities Useful in an 802.5 Token Ring?

Authors:Peden, Jeffery, Department of Computer ScienceUniversity of Virginia Weaver, Alfred, Department of Computer ScienceUniversity of Virginia

The IEEE 802.5 token ring protocol defines eight packet priorities. The intent is that "high priority" packets should be delivered prior to "low priority" packets. Through a series of simulations we show that this expected behavior occurs under a unique set of circumstances: when there are very few network stations, very short data packets (but still long relative to ring latency). very short token hold times, and very high network load. In the general case, we found that priorities did not markedly influence packet delivery time. Use of the priority system generally resulted in more overhead and longer average packet delays than when all packets were carried as a single priority. We mathematically describe the features of the protocol operation which are the cause of this increased‘ delay and lack of priority discrimination.
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Source Citation:

Peden, Jeffery, and Alfred Weaver. "Are Priorities Useful in an 802.5 Token Ring?." University of Virginia Dept. of Computer Science Tech Report (1988).

University of Virginia, Department of Computer Science
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