Queueing theory offers a large variety of techniques that can be used in performance modelling of computer systems and data communication networks. The diversity of assumptions causes that the numerical results, obtained for the same system but by means of different techniques, can often be numerically very different. The project is aimed at finding common denominators for numerical results obtained with the assumption of infinite buffer capacity and that obtained with the assumption of finite buffer capacity. To be precise, this project investigates the traffic intensity regions where the approximation of queueing systems with finite buffer capacity by the queueing systems with infinite buffer capacity can be done with some amount of safety margin. The investigation includes both queueing systems with single arrivals (M/M/1, M/D/1), and a queueing system with batched arrivals (M⁽ᵇ⁾ /M/1), and two the most important characteristics of queueing systems are considered, the probability of overflow and average waiting time in the system.