Interaction between sensornet nodes and the physical environment in which they are embedded implies realtime requirements. Application tasks must be executed in the correct place, and in the correct order, for correct application behaviour. Sensornets generally have no global clock, and incur unacceptable cost if traditional synchronisation protocols are implemented. We present a lightweight primitive which generates a periodic sequence of synchronisation events which are coordinated across large sensornets structured into clusters or cells. Two biologically-inspired mechanisms are combined; desynchronisation within cells, and synchronisation between cells. This hierarchical coordination provides a global basis for local application-driven timing decisions at each node.
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