A useful approach for tackling general queuing models is to use the Rate In = Rate Out Principle (RIRO) to derive a set of steady state equations for a queuing system. RIRO assumes that a system can reach a state of equilibrium. In equilibrium, the tendency to move out of a certain state must equal the tendency to move towards that state. Given the steady state equations derived from this assumption, we can solve for the probability that a system is in a given state at any particular moment.

The following example assumes a system with multiple servers and "customers" that arrive in batches.