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The hybrid hierarchical architecture (HHA) represents a particular case of wireless hybrid network, where sensor nodes transmit their samples to an infrastructure network through multiple hops. In the HHA, gateway terminals implementing both cellular and infrastructure-less air interfaces, allow integration of the two separate paradigms characterising the wireless sensor network (WSN) and the cellular network. In this paper, in particular we study a hierarchical network where an IEEE 802.15.4 WSN, organised in a tree-based topology, is connected, through a mobile gateway, to an infrastructure network using a cellular air interface like UMTS. In such scenario, the mobile gateway receives data from sensors with an inter-arrival time distribution which depends on the WSN topology, the number of sensors distributed, and the parameters which characterise the 802.15.4 medium access control protocol, such as the superframe order, the beacon order, the number of guaranteed time slots, etc. Such distribution is analysed in this paper through simulation. The outcome of this work provides useful hints to the characterisation of the traffic generated by the mobile gateway and provided to the infrastructure network. The design of the scheduling techniques implemented at the infrastructure side requires suitable knowledge of the characteristics of such traffic.