Equation ( 1.4) is valid for any queueing system and can be used to prove Little's formula for the G /G /1 queue in a somewhat different manner than that used in Section 1.5.1. The approach is to plot the cumulative count of arrivals on the same graph as the cumulative count of departures. Then it can be seen that the area between these two step functions from the beginning of a busy period to the beginning of the next (a busy cycle) is the accumulated total of the system waiting times of all the customers who have entered into the system during this busy cycle. Use this argument to derive an empirical version of Little's formula over a busy cycle.