Time threshold dimensioning and overload control in FDDI networks

The author addresses the problem of selecting timer thresholds for the asynchronous priorities in a Fiber Distributed Data Interface (FDDI) system. Based on an approximate analysis for the throughput under overload, two methods are derived. The first one is designed to meet requirements for the ratios between throughputs of stations with different priorities, whereas the second method includes absolute throughput values. Additionally, a proposition for dimensioning the timer thresholds of traffic classes that are deferred under overload is presented. Some examples for the application of these methods to overload control in FDDI systems are given. The results indicate that FDDI´s timed token protocol provides the possibility to guarantee bandwidth not only for synchronous traffic, but also for asynchronous traffic. The allocation of guaranteed bandwidth can be controlled easily by selecting the timer thresholds according to the rules derived