A queueing system $MMPP_1$,$MMPP_2$/$G_1$,$G_2$/1/K+1 with a non-preemptive HOL and push-out priority scheme is analyzed in this thesis. This work is motivated by the study of cell loss priority control in ATM networks: ATM cells, representing the aggregate traffic, arrive at the multiplexer according to a Markov Modulated Poisson Process(MMPP). It is assumed that traffic consists of two classes with different priorities and the arrivals of the two classes are modeled as MMPPs which are independent with each other. The multiplexer is modeled as a single server queueing system. In order to accomodate different services, 2 buffer management schemes, non-preemptive Head Of the Line and push-out, are implemented. Class 1 and class 2 have different service time with general distributions. An exact solution is obtained for buffer occupancy distribution by using the classical imbedded Markov chain method. Our performance evaluation is concentrated upon the loss probabilities under assumption that the buffer has been dimensioned to satisfy the maximum admissible delay. Numerical examples describe the loss probabilities of class 1, 2 for a wide range of traffic parameters under deterministic service time distribution and show the effectiveness of the push-out and HOL scheme.