Recently the tendency of the network system development rapidly progresses toward ISDN that intergrates all presetn-day communication networks and services into a single, universal network set up on the basis of the exisiting telephone network. For this reason many countries pay worldwide attention to the researches about ISDN, especially to the researches about the exchange and the ISDN user-network interface which is one of the most important parts of network system. This paper is the first part of the two-par papers describing the ISDN user-network interface. in this paper, after surveying the architecture of ISDN exchange recommended by CCITT, the general architecture of the ISDN exchage is proposed, Based on this architecture, the switching emulator is implemented, and the necessary conditions of the ISDN exchage(LAPO, CCP etc) are also studied.