Design and evaluation of the mod service system based on Petri net theory

A design for a distributed multimedia-on-demand (MOD) service system is presented. The MOD system is designed to store, retrieve, and communicate objects comprised of mixed data types including images, text, video, and audio, in real-time. In the MOD system, such data types represent the main concept of movie files. To illustrate how to model and analyze the important multimedia aspects of the MOD system, we use the Real-Net (R-net) modeling technique. We choose R-net as an extension to the Time Petri Net technique due to its ability to specify hard real-time process interaction, represent the synchronization of multimedia entities, describe sequential and parallel multimedia activities, and illustrate the interprocess timing relationships as required for multimedia presentation. Based on modular techniques, we build two R-net performance models for describing the dynamic behavior of the MOD service system. One of them incorporates the Earliest Deadline First (EDF) disk scheduling algorithm. The other model incorporates the Scan-EDF algorithm. These algorithms help us to illustrate how the real-time user requests can be satisfied within the specified deadline times. Since R-nets are amenable to analysis including Markov process modeling, the interesting performance measures of the MOD service system such as the request response time, the disk scheduling algorithm time, and the actual retrieval time can be easily obtained.