QoS provisioning in CSMA/iCA based medium access control protocol for WLAN

The fundamental basis of QoS differentiation in IEEE 802.11e EDCA is to differentiate durations of several channel access parameters for initiating and pursuing contention over the shared channel. More precisely, EDCA specifies the durations of those parameters in such a way that they remain inverse-proportional to the corresponding priorities of the contending packets. As such, the higher priority packets have shorter channel access parameters which privilege them to possibly win contention even in the presence of lower priority packets. As the number of lower priority packets increases, however, frequent inevitable priority inversions and inter-priority class packet collisions degrade the perceived QoS grade of the higher priority packets. We identify that occurrence probability of such problematic events would increase if the conventional CSMA/CA in EDCA is replaced by its newly proposed enhanced version known as CSMA/iCA. We find that the problem is rooted to the CSMA/iCA´s approach of tuning Contention Slot Selection Distribution (CSSD) over the contention window. In this paper, we present a simple scheme, named Push CSSD Mean Right, that slightly modifies the original CSSD tuning mechanism in CSMA/iCA in order to reduce the occurrence probability of such QoS-degrading problematic events. Via rigorous simulations in ns-2, we demonstrate the suitability of the proposed scheme in CSMA/iCA based EDCA for provisioning QoS.