This paper deals with the quality of service in IP Multimedia Subsystem networks. The evaluation of Quality o Service (QoS) is done through admission control methods. We analyse various methods, which are based on the traffic measurement. The approach that can be applied in finding the suitable admission control method while considering selected QoS parameters is described. Different important QoS parameters can be defined, however we decided to focus on the loss parameter. In our simulations we have used Simple Sum method. Based on the simulation results, we have used new algorithm. The main difference is that it uses the mechanism of time window for achieving more accurate traffic estimate. Algorithm Measured Sum with implementation of time window T has better ability to predict the traffic fluctuation. Compared to the Simple Sum algorithm it achieves lower link utilisation, but also lower level of loss ratio. *