This paper considers optimization of resource dimensioning in data communication networks, with the aim to provide Quality of Service (QoS) while maximizing resource utilization. The concept of QoS is briefly reviewed. The Differentiated Services (DiffServ) environment is introduced. By means of simulations optimal Provisioning Factors were derived. For simulations the Markov Modulated Poisson Process (MMPP) model was used for UDP traffic, and the Multi-fractal Wavelet Model (MWM) for TCP traffic. These choices were based on network traffic phenomena like self-similarity, long range dependency and burstiness. A test bed was developed and the traffic models were used to simulate single class traffic and to derive optimal Provisioning Factors (PF) for bandwidth dimensioning for four traffic classes. These Provisioning Factors were evaluated in simulated multiclass traffic. Results conclude that 87% link bandwidth utilization can be achieved while complying with QoS parameters defined for the 4 concerned traffic classes.