In the realization of interface between subscriber and network, D-channel may deteriorate the performance of switching system in handling subscriber. Thus, D-channel must be designd in a way to interface efficiently with the system while maintaining exciting performance. This objective can be obtained by through consideration of the structure of the system. This paper shows the method to design D-channel, which has significant relation with the structure of the system whenrealizing the interface between subscriber and network, in adding ISDN function. The goal of performance is also proposed.