This paper is motivated through the advantages offered by employing real-time (priority-driven) scheduling (RTS) for efficiently implementing quality-of-service guarantees. For employing RTS (e.g. for conferencing), a-priori knowledge about worst-case execution times (WCET) of the decoding process is required. With our new approach (focussing of MPEG 2 with variable bitrates) the resource usage compared to fixed-bandwidth allocation can be significantly decreased. Solutions for the WCET analysis are presented for two scenarios (Video-on-Demand (VoD), live-video). A significant reduction of overestimations during WCET analysis of the live scenario can be achieved by using our new technique. In a the VoD scenario it is even possible to predict the exact execution times.