· Modelling the fundamental performance limits of Wireless Sensor Networks (WSNs) is of paramount importance to understand the behaviour of WSN under worst-case conditions and to make the appropriate design choices. In that direction, this paper contributes with a methodology for modelling cluster-tree WSNs with a mobile sink. We propose closed-form recurrent expressions for computing the worst-case end-to-end delays, buffering and bandwidth requirements across any source-destination path in the cluster-tree assuming error free channel. We show how to apply our theoretical results to the specific case of IEEE 802.15.4/ZigBee WSNs. Finally, we demonstrate the validity and analyze the accuracy of our methodology through a comprehensive experimental study, therefore validating the theoretical results through experimentation.