The problems of determining appropriate QOS(Quality of service) requirements of B-ISDN services and finding the corresponding NP(Network Performance) values satisfying the given QOS requirements are very important problems which are directly related to the successful operation of the ATM network, but have not been studied with enough attention yet. The QOS(NP) parameter definition, QOS measurements, and QOS managements are three major problems that should be solved. In this paper, we first introduce the basic concepts of QOS and NP and their relationships. Then we discuss more comprehensive concepts of QOS based on the layered structure of ATM services and the reference time, which can provide a systematic approach for QOS and NP mapping problems. We also provide examples of determining basic QOS(NP) parameters in the ATM layer and a specific example of QOS mapping based on the file transfer in ATM. The results of this paper can be used to establish the comprehensive QOS management model.