This paper deals with a M/M/c queueing system with waiting servers, balking,
reneging, and K-variant working vacations subjected to Bernoulli schedule
vacation interruption. Whenever the system is emptied, the servers wait for
a while before synchronously going on vacation during which services are
offered with a lower rate. We obtain the steady-state probabilities of the
system using the matrix-geometric method. In addition, we derive important
performance measures of the queueing model. Moreover, we construct a cost
model and apply a direct search method to get the optimum service rates
during both working vacation and regular working periods at lowest cost.
Finally, numerical results are provided.