A five-layer simulation model of OSI protocols is described and applied to predict transport performance on a local area network (LAN). Emphasis is placed on time-critical applications typical of a small, flexible manufacturing system. The results predict that, with current technology, OSI protocols can provide 1.5 Mbps throughput, one-way delays between 6 and 10 ms, and response times between 15 and 25 ms. The results also indicate that CSMA/CD is a reasonable access method for time-critical applications on small, factory LANS, if loads of less than 40% are anticipated. For loads between 40% and 70%, a token passing access method provides better performance for time-critical applications.