A Study of Preemptable vs. Nonpreemptable Token Reservation Access Protocols

Author:Strayer, W, Department of Computer ScienceUniversity of Virginia

LAN protocols based on token reservation, like those found in the IEEE 802.5 Token Ring and the SAE High Speed Ring Bus LAN specifications, provide service to messages in nearly priority order. Token Reservation fails when higher priority messages enter the system too late to be included in the priority bidding scheme or after a lower priority message successfully captures the token. Although allowing preemption in the media access protocol could reduce or eliminate these failures, no preemptable protocols exist among the extant LAN protocols. We seek to determine if the performance difference between a preemptable and a non-preemptable protocol is noticeable or significant; since no preemptable protocols exist there is no basis for comparison. Here we present an investigation of the effectiveness of token reservation with respect to providing service in priority order. We describe a preemptable token reservation protocol, and implement it, along with the non-preemptable token reservation protocol, in a simulator. We compare them using conventional metrics (such as throughput and delay), as well as introducing a new metric, the priority inversion ratio, to quantify the effectiveness of the protocols for providing priority ordered service. We show that preemption in a token reservation protocol can provide better service to high priority messages.
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Source Citation:

Strayer, W. "A Study of Preemptable vs. Nonpreemptable Token Reservation Access Protocols." University of Virginia Dept. of Computer Science Tech Report (1990).

University of Virginia, Department of Computer Science
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