Asynchronous Transfer Mode (ATM) has been recommended and has been accepted by industry as the transfer mode for Broadband network. Currently, large scale effort has been undertaken both in the industry and academic environment to design and build high speed ATM networks for corporate bodies. These networks are meant to support both real-time and non-real time applications with different quality of service (QoS) requirements. The resources to support the applications QoS requirements are typically limited and therefore the need to dynamically allocate resource in a fair manner becomes inevitable. In this work, an evaluation is carried out on the performance of enterprise-wide network that its backbone is based on leased trunk. The performance of the leased trunk is evaluated when loaded with homogeneous and heterogeneous traffic. The evaluation is carried out in order to determine the exact effect of traffic overload on resources-trunk transmission capacity and buffer. The aim is to define the optimum loading level and the associated QoS parameter values. A typical network was adopted, modeled and simulated in MATLAB environment by using Simulink tool and the results obtained were analyzed by using Microsoft Excel.