Reliable data transmissions in sensor networks are important for its longevity and require research attention. Since, fault tolerance and reliability become vital for embedded systems, estimating failure scenarios aid WSN applications to establish robust backup mechanisms in case of failures in the network. The article aims to model failure in IEEE 802.15.4 Zigbee networks in order to estimate the longevity of the network in context to the network connectivity. Ideally, IEEE 802.15.4 can manage two types of networks, the star topology or the peer-to-peer topology. On combining these two we get a mesh network. Hence, the article presents the failure conditions for both star and mesh based network topology to the point where the network is disconnected owing to varied failure conditions.