Abstract: The realtime capability of distributed computer system (DCS) is determined by temporal characteristics of communication network (fieldbus). With the increasing application of DCS in industrial control areas, the research on fieldbus's capability of handling realtime traffic of message is becoming more important. According to their realtime capability, fieldbuses are classified as event-driven sector and time-driven sector, and the latter directly supports realtime applications through applying token. Further, according to mechanisms to manage token, time driven fieldbuses are sub-classified into centralized token, distributed token and virtual token. For general communication network, throughput and average transmission delay of message is the most important performance metrics, and analytical method for performance evaluation focuses on statistical method, i.e. queue network and stochastic process. However, for fieldbusled, concern focuses on satisfying the temporal constraint of individual message, and its evaluation approach mainly focuses on the principle and methodology for realtime schedule. In this paper, authors primarily givea detailled expound on traffic characteristics of tasks in DCS, realtime requirements and traffic model of fieldbus, and current results of realtime schedule related to temporal characteristics of fieldbus. In next paper, the authors primarily focus on how analyzing and improving FF and WorldFIP, two well known and popular fieldbuses, through applying principle and methodology for realtime schedule, and propose some imperative problems needed be solved, such as general performance evaluation architecture.