This paper presents a simulation study on complex On-Line Transaction Processing systems using TPC-C workload. The impacts on the response time of Inter-Connection Network (ICN) in the system are studied. From the simulation results, we find anomalous cases where with the ICN that employs CSMA/CD protocol, increasing the number of database servers degrades the performance of the whole system. The explanation of the structure underlying this anomalous phenomenon is given after having investigated the behavior of the components in the system. q 2004 Elsevier B.V. All rights reserved.