Clock Synchronization Issues in Multi-Cluster Time-Triggered Networks

Abstract

We address the issue of establishing and maintaining a system-wide common time base in fault-tolerant multi-cluster time-triggered systems.

We propose an approach how to synchronize system nodes among several clusters using the fault-tolerant mid-point algorithm. Before executing clock synchronization each node measures the clock deviation values and stores them in a convenient data structure. From these values the clock synchronization algorithm calculates a correction term which should be added or

subtracted from the local clock. For distributed real-time systems that are structured in a set of clusters the set of clock deviations can be subdivided into a set of local clock deviations and a set of global clock deviations. Local clock deviation values (respectively global clock deviation values) of a specific node are captured by building the time difference between the observed and expected arrival time of synchronization messages sent by a node belonging to the same cluster (respectively to another cluster).

In order to receive messages from other clusters the clock deviation between the sender and the receivers should be bounded. We derive the lower bound of the network precision of a multi-cluster system that executes the FlexRay protocol and will show that it depends mainly on the transmission delays and measurement errors. Further, we inquire about the amount of the minimum time gap between two successive messages that could be exchanged via the FlexRay System. This time gap is an important parameter for developing a correct configuration of multi-cluster systems.