Effect of the Token Holding Time on MAP Performance.

The Manufacturing Automation Protocol (MAP), which is based on the seven layer OSI/ISO communications model, specifies the LAN protocol to be the IEEE 802.4 Token Bus standard (the timed token scheme). To provide a priority-based network that gives time-critical messages high priority, this timed token scheme uses two types of timers: token holding and token rotation timers. In this paper we extend some previous results on this scheme to non-exponential message lengths, and to account for explicit token holding timer settings. Since the service discipline is non-preemptive, the station will keep the token for an additional period of time after the expiration of the timer, to complete transmission of the current message. New results on this excess token holding time were derived and used in the approximate expressions for the mean message delay. Comparison with Performance Analysis Workstation (PAW) simulations using various message length distributions, with coefficients of variation less and larger than unity, showed good accuracy for the analytical approximations over wide ranges of utilization in two different network configurations. These approximations should be useful for MAP network design and operation in terms of choice of timer values.