starts out empty and idle , that is, it’s empty of entities and all resources are idle . So for a while at the beginning, the model could be uncharacteristically uncongested (in comparison to long-run steady-state conditions) until it starts f lling up with application entities. As you’ve no doubt noticed, there’s a f eld called Warm-up Period in Run . Setup . Replication Parameters and, as its name suggests, Arena “warms up” your model this long before starting to collect output statistics (this Warm-up Period is taken out of the beginning of your Replication Length, so statistics are collected for the last Replication Length - Warm-up Period time units). The Warm-up Period defaults to zero, in which case statistics are collected from the very beginning of the run, but you could f ll in a positive value. Try warming up for 40 hours, and repeat the original Model