Distributed Complex Event Processing (CEP) is gaining increasing interest for two reasons: (1) to scale system performance to handle higher workloads in real-time, and (2) to perform in-network processing, e.g., in mobile networks to reduce the amount of data that has to be transferred through the network. System scalability and the complexity of mobile systems are some of the major challenges when evaluating the performance of new Distributed CEP solutions. We propose an open framework for distributed CEP (DCEP-Sim) built on a well-established network simulator, i.e, ns-3. The design of DCEP-Sim is based on the engineering principles of separation of concerns and the separation of mechanisms and policies. By leveraging the ns-3 feature of object aggregation it is very easy to add new policies, e.g., placement or selection policies, and evaluate them without changing anything else in the DCEP-Sim. The fact that ns-3 includes many accurate network models implies that Distributed CEP simulation with DCEP-Sim will also be much more accurate than ad-hoc handcrafted simulations. We demonstrate in a use case how easy it is to configure performance evaluation experiments and we perform experiments to confirm that the integration of the Distributed CEP in ns-3 is good foundation for large-scale experiments. The evaluation results demonstrate that DCEP-Sim substantially reduces the effort and costs of Distributed CEP evaluation.