Deference Mechanisms Significantly Increase the MAC Delay of Slotted CSMA/CA

Slotted CSMA/CA is an algorithm proposed by IEEE 802.15.4 in order to deal with the contention of energy constrained nodes. Slotted CSMA/CA is used during a limited time interval, included in a superframe. Two specific mechanisms defer the transmissions that would occur towards the end of the contention period. Multiple models of the average delay or throughput of slotted CSMA/CA have been proposed recently, but they do not take into account the deference mechanisms. In this paper, we show that the deference mechanisms occurring at the end of the superframe have a significant impact on the performance of slotted CSMA/CA. We prove this by giving the distribution of the delay in a simple scenario, obtained by simulation. Then, we compute the average delay t_MAC of traversing the MAC layer, as the MAC parameters or the frame size vary. We show that for either reactive WPANs and energy-efficient WPANs, the impact of the deference mechanisms is important.