WEBFor Poisson arrivals, the arrivals in any future increment of time is independent of those in past increments and for many systems of interest, independent of the present state S(t) (true for M/M/1, M/M/m, and M/G/1). For such systems, P{S(t)=s|A(t+δ)-A(t)=1} = P{S(t)=s} (where A(t)= # arrivals since t=0)