 For all "n", we have |"E""n" − "L""n"| ∈ {0, 1}, that is, the number of times the system leaves a state differs by at most 1 from the number of times it enters that state, since it will either return into that state at some time in the future ("E""n" = "L""n") or not (|"E""n" − "L""n"| = 1).When the system arrives at a steady state, the arrival rate should be equal to the departure rate.A common basic queuing system is attributed to Erlang, and is a modification of Little's Law.