Stability conditions for retrial queueing systems with regenerative input flow

We consider two classes of multiserver retrial queueing systems. For the first class, the rate of retrial requests depends on the number of customers on the orbit, and for the second class, the rate is constant. The input flow is supposed to be a regenerative one and service time has an arbitrary distribution. Based on the synchronization of the input flow and an auxiliary service process, we establish necessary and sufficient stability conditions for models of both classes.