Many problems in management science and telecommunications can be solv
ed by the analysis of a D-X/D-m/1 queueing model. In this paper, we us
e the zeros, both inside and outside the unit circle, of the denominat
or of the generating function of the model to obtain an explicit close
d-form solution for the equilibrium probabilities of the number of cus
tomers in the system. The moments of the number of customers in the qu
eue or in the system are also studied. When there are infinitely many
zeros outside the unit circle, we propose an approximation method usin
g polynomials. This method yields correct values for a finite number o
f the probabilities, the number depending on the degree of the polynom
ial approximation.