Self-adaptive implicit contention window adjustment mechanism for QoS optimization in wireless sensor networks

Synchronous contention-based medium access control (MAC) protocols play a vital role in the delay guarantee essentially required by many recent delay-sensitive applications. Great efforts have been made to improve the end-to-end delay by means of optimizing the medium access performance. However, their effectiveness is still constrained by the contradiction between reducing the collision probability and idle listening duration. Instead of traditional tradeoff strategies, developing a way to further alleviate the contradiction is definitely beneficial for further improving the medium access performance. To this end, a self-adaptive implicit contention window adjustment mechanism for SMAC (Sensor MAC) is proposed based on the investigation of the medium competition behavior impacting the medium access delay and delay jitter, from the perspective of any given contending node. We mathematically derive the expected medium access delay for revealing the strengths and weaknesses of typical dynamic contention window mechanisms. Moreover, it is discovered that their delay-oriented optimizing window only offers fairly limited improvement in the medium access performance since the uniformly random slot-selection mechanism results in high delay jitter and the contradiction between reducing the collision probability and idle listening duration. In order to overcome this drawback, furthermore, an optimization model of the node contention priority distribution is proposed to alleviate the aforementioned contradiction by means of optimizing slot-selection mechanism. Finally, a self-adaptive implicit contention window adjustment (SICA) mechanism is designed integrating delay-oriented optimizing contention window, contention priority optimization model with the estimation method of contenders' number. Through simulations in OMNeT++, the results demonstrate that the proposed SMAC-SICA outperforms its peers in terms of delay guarantee, energy efficiency and throughput.