Due to its timed-token medium access control protocol, fibre distributed data interface is suitable for transmitting real-time and non-real-time data. Several protocol parameters should be properly selected to satisfy the time constraints of the synchronous messages and to provide the maximum usage of the asynchronous transmission bandwidth. In this study the effect of asynchronous threshold values on the behaviour of the ring is considered, and two methods are developed to calculate suitable threshold values for different purposes. In the first method the proper asynchronous threshold values are calculated to share out the asynchronous bandwidth between nodes in given proportions. In the second method, asynchronous data transmissions of nodes are limited by using appropriate asynchronous threshold values. In this way the actual target token rotation time can be reduced to facilitate achieving the real-time requirements of synchronous messages.