The world of mobile communications has undergone dramatic changes in recent years. Since the Personal Digital Cellular (PDC) system of digital mobile communications became commercially available in Japan in 1993, the number of subscribers has increased to the extent that almost 30% of the Japanese population are subscribed. Furthermore, the increasing sophistication of communications, such as the growing popularity of the Internet and expansion of our spheres of existence, have led to demand for new applications, such as use on a global scale. Many new technologies are also being introduced, such as the IMT-2000 system and the IN system which are slated to be introduced at the end of 2000. Based on the above and in view of the increasing scale and sophistication of mobile communications networks, the time has come to reexamine the configuration within networks of SCPs, which perform subscriber data management and service control. This paper focuses on the functions of subscriber data management and service control, classifies data necessary for mobile networks and using the signal number of common networks as a guide considers the optimum arrangement of data and control nodes. The paper also examines reliability measures for SCPs to ensure the reliability of increasingly large-scale networks.