Event Order with Interval Timestamp in Event Correlation Service over Wireless Ad Hoc Networks
Eiko Yoneki, Jean Bacon · 2004
Temporal ordering of these events (a happened before b) originating from dieren t devices and clocks has to be determined and real-time issues (a and b happened within a certain time interval) have to be solved. Logical time cannot be used to determine temporal ordering, because causal ordering of events in the real world must be honoured. Thus, physical time has to be used requiring clock synchronization. However, most of the synchronization algorithms rely on partitioned networks. Global infrastructures like GPS provide an accurate time base, but GPS is not suitable for resource-constrained devices. We previously used an interval timestamp for event correlation [1]. A two-part interval timestamp represents the clock uncertainty, and intervals can include the estimated network delays. Further consideration is required to support wireless ad hoc networks. We propose an event ordering algorithm for real-time event correlation using an intervalbased timestamp based on the time synchronization mechanism [2]. When a publisher node records an event in realtime it generates a timestamp using its unsynchronized local clock, which is passed to other nodes. The algorithm uses interval-based timestamps as a lower and upper bound for the exact value, and transforms them to the local time of the receiver instead of adjusting the clocks. Event delay is measured using the round trip time by exchanging another