An (S - 1, S) inventory system with fixed shelf life and constant lead times

We consider an (S - 1, S) type perishable inventory system in which the maximum shelf life of each item is fixed. An order for an item is placed at each demand time as well as at each time that the maximum shelf life of an item is reached. The order lead times are constant, and the demand process for items is Poisson. Although the resulting process is ostensibly nonregenerative, we adapt level-crossing theory for the case of an S-dimensional Markov process to obtain its stationary law. Within this framework a number of model variants are solved.