The paper^1 deals with the problem of assuring predefined QoS for real time (RT) class of service (CoS), which we define as one of inter-provider or inter-domain classes of service in IP network. We propose an analysis and adequate formulas to obtain admissible traffic load in a case when we map two end-to-end classes of service dedicated for telephony CoS and video conference CoS into one inter-domain RT CoS. The aim of the analysis is to determine the admissible load when the target packet loss ratio and buffer size dedicated for RT CoS are known. The proposed solution accounts for the difference between packet sizes of streams generated by voice (about 100 bytes) and video conference (rather 1500 bytes) applications. We illustrate our studies by simulation results.