The system architecture of a network system is of fundamental importance to the performance of computer networks. However, evaluating different architectural and algorithmic choices of network systems is usually difficult. This poster presents NetSysSim, a network system simulator implemented in SystemC. NetSysSim provides a hierarchical framework of different levels of abstraction. With SystemC's modular and timing based simulation, researchers can simulate any part of the system with the desired level of details. Our design of NetSysSim allows for performance evaluation of data plane architectures, including different queuing, scheduling or transmission schemes.