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IEEE 802.15.4 standard defines the specification of the physical layer and the MAC sub-layer for low rate wireless personal area networks (LR-WPAN). The standard specifies CSMA/CA as the access method to use during Contention Access Period (CAP). Such method gives almost the same probability for all devices to access the medium. In case the traffic generated by the devices has the same priority level, this method seems to be quite fair. However, if traffic differentiation is considered, this method should be adjusted to overcome the shortcoming. In fact, as the number of applications keeps growing, it is completely sane to classify the generated traffic according to its priority level. In this paper, we propose an enhancement of IEEE 802.15.4 Beacon-enabled mode in which we adapt the probability of a device to access the medium according to its priority level. BO and SO are two parameters in IEEE 802.15.4 standard that determine the duration of Beacon Interval and Superframe Duration respectively. Our scheme proposes to allocate a different BO to each type of traffic so that the highest priority level type of traffic has the lowest BO and vice versa. Simulation results show an improvement in synchronization time, a reduction in the number of sent packets during the association phase, and a decrease in energy consumption during the synchronization of the entire network.