With the increasing sophistication of wireless communications and sensing technologies, various sensor-based applications, such as tour guiding and industrial automation, generate tremendous economic and social benefits. The potential for even greater impact has motivated extensive studies on Wireless Sensor Networks (WSNs) in recent years. Zigbee is the key of interest for number of researchers. ZigBee is a specification formalized by the IEEE802.15.4 standard for low-power, low-cost, low-data-rate wireless personal area networks. In ZigBee networks, a tree topology is often used to construct a wireless sensor network for data delivery applications.