The addition of ISDN function in an exiting electronic switching system results in structural changes in the user-network interfaces module. The changes may affect the performance of the system. Thus it is one of significant design consigerations not to cause the performance degradation of the system. In order to estimate the prospective performance of the interface module for the ISDN function to be implementation on TDX-IB switching system, the effect of the ISDN user-network interface on the system performance is analyzed to the satisifcation of the goal using an analytical method. And the performance of the proposal system is also analyzed by the measurement using ISDN call simulations, followed by the comparision with the existing system.