“…In this case, U is said to possess a PH-type distribution, which is characterized by the matrix pair (α, T ). As in [4], we assume that the service speed is represented by a function c(x) > 0 when the current workload takes the value x > 0, and c(0) = 0. When c(x) = c, then the service time has a phase-type distribution with characterizing pair (α, cT ), but otherwise there is no direct relationship between the job size and the service time.…”