At the end of each step a new state is calculated using
the model equations. The implication is that events
(corresponding to a change of state) occur at the
beginning of each period –the lead time for an item in a
production resource is considered small compared to the
size of the time bucket –the main states are the states of
the set of resources. They describe the activities in which
resources are implicated in a given time period.
(Thierry et.al.2010).