  This paper presents a new condition for the existence of optimal stationary
policies in average-cost continuous-time Markov decision processes with
unbounded cost and transition rates, arising from controlled queueing systems.
This condition is closely related to the stability of queueing systems. It
suggests that the proof of the stability can be exploited to verify the
existence of an optimal stationary policy. This new condition is easier to
verify than existing conditions. Moreover, several conditions are provided
which suffice for the average-cost optimality equality to hold.
