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In this paper, we propose two mechanisms, i.e., 1) backoff exponent differentiation (BED) and 2) contention window differentiation (CWD), to provide multilevel differentiated services for IEEE 802.15.4 sensor networks. The beacon-enabled mode with the slotted carrier-sense multiple-access mechanism with collision avoidance (CSMA/CA) algorithm is considered for nonsaturation. A mathematical model based on a discrete-time Markov chain is presented and analyzed to measure the performance of the proposed mechanisms. Numerical results show the effects of varying parameters. It shows that BED is better for service differentiation and channel utilization, regardless of the number of devices and the packet-arrival rate. It also shows that the throughput depends on the backoff exponent (BE), contention window (CW), and packet size and that the relative delay for successfully transmitted packets for BED is longer than that for CWD, which enables a higher relative gain for CWD. In addition, we obtain the number of unit backoff periods required to process all of the head packets of devices. Based on the numerical results, we obtain a criterion that provides guidelines for choosing a mechanism that is suitable for the objective function and determining the optimal length of the superframe to process the packets in the system.