The IEEE token bus-A performance bound on GM MAP

All versions of the General Motors manufacturing automation protocol (GM MAP) specify that MAP´s lower-layer services are to be provided by the IEEE 802.4 token bus. An understanding of 802.4 and its performance aspects is therefore a prerequisite to predicting the performance of MAP. It is shown how total bus capacity is divided among data throughput, token traffic, and propagation delays. The relative contributions of access delay and queuing delay to total message delivery time are discussed. The effects on message delivery times of message size and the number of active stations are also reported. As the token cycle time increases beyond the target rotation time for each of the asynchronous access classes, service to the lower-priority classes is curtailed; a formula that can be used to identity the offered load at which the transition from normal to curtailed service begins is presented