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In todayÂ¿s competitive semiconductor business environment, wafer manufacturers are facing continuous pressure to accurately predict cycle time and tool utilization, gauge the impact of changes in capacity available, assess the impact of changes in product mix and quantity, and determine action plans to improve operational performance. Discrete event simulation (DES) is a widely used approach to perform such an analysis. However, DES has some inherent shortcomings for these planning tasks. Analytical models, like queueing networks, have much shorter response times and additional advantages compared to DES. But due to the complexity of semiconductor manufacturing systems (SMS) queueing models were not able to model all the peculiarities of those. This paper provides an overview of the main features of the IBM Enterprise Production planning and Optimization System (EPOS), a queueing network based system, which closes this gap. EPOS has been in use in the 300 mm fabrication of IBM in Fishkill for more than 2 years and has turned out to be an invaluable tool to analyze the trade-offs of cycle time and capacity within this complex environment.