\(Q(t)\) the number of items in the system at time \(t\)

\(\rho = \lambda/\mu\) is the traffic intensity

\(Q\) is a continuous-time Markov chain, with generator \(G\) given by \[G(n, n-1) = \mu \text{ for all $n \geq 1$} \] and \[G(n, n+1) = \lambda \text{ for all $n \geq 0$ }\]