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Analysis of IEEE 802.15.4 CSMA/CA scheme with homogeneous or saturated traffic has received considerable attention recently since a great convenience is offered to applications of low-power and low-rate wireless sensor networks (WSNs) which almost touch upon all aspects of our life. In this paper, we propose a stochastic analysis approach to evaluate the delay performance of CSMA/CA scheme with heterogeneous buffered traffic which is more realistic in our applications. We adopt two modified semi-Markov chain with a macro-Markov chain to present the CSMA/CA process and the theory of queues to character the buffered condition, which are used to evaluate characteristics of our time-critical networks. Packet delay of unsaturated, unacknowledged IEEE 802.15.4 beacon enabled networks is predicted based on these models. Moreover, the impacts of different parameters such as the packet arrival rate, packet size, node's buffer size and the number of different kinds of nodes are presented in this paper. Comprehensive simulation results are consistent with the predictions of these models.