Modelling and analysis of message exchange in field bus FIP

The authors present the message exchange service of the factory instrumentation protocol (FIP) as a two-stage parallel-series queuing system and study its performance in the low-load case. The first stage is composed of N queues in parallel. The output flows from these queues are superposed and form the input flow entering into the second stage, which is modeled by a single queue in which the service time distribution is deterministic. The exact analysis for the first stage is similar to that for TDMA (time-division multiple access). An analytic solution for the second stage seems difficult to achieve owing to the unknown characteristics of the input flow entering into this stage. Assuming a constant message length, the authors propose a two-fold approximation to obtain an approximate solution for the second stage. The concept of distance in variation (DV) is introduced, and the relationship between the various DVs is derived and used to prove this approximation. The proposition is also justified by the simulation results