Extension of prioritized data analysis in the Foundation Fieldbus to three classes

Abstract

The Foundation Fieldbus data link layer provides both a token-passing service required for sporadic data and a scheduled service for periodic data. It has three priority levels: urgent, normal, and time-available. Modeling the virtual token rotation time (VTRT), which is the time required to receive tokens for the transmission of one's own data, is difficult due to the interactive data delay driven by the three priority levels. Therefore, a simplified mathematical model consisting of just two priority levels has been considered. In a previous research, the VTRT probability density function was defined using the probability of a token generation; however in the present study, a simpler expression was successfully developed in a system of three priority levels by tracking the token's priority changes generated by the link active scheduler. These priority changes are tracked by using the state transition probability and the state transition diagram of a token. Therefore, the success of this new mathematical model makes this research important. Furthermore, we found the existence and the cause of a fluctuating state, both of which are useful to a system designer for setting network parameters. The validity of the analytical model was verified in a simulation experiment. Copyright (C) 2011 John Wiley & Sons, Ltd.