TWHTB: A Transmission Time Based Channel-Aware Scheduler for 802.11 systems

802.11 WLANs suffer severe performance degradation as soon as at least one of the stations in the Basic Service Set (BSS) experiments unfavorable radio channel conditions. This degradation is due to a so-called anomaly which manifests when one or some of the stations adopt a reduced transmission data rate, as well as to the reduced efficiency of radio channel exploitation due to retransmissions. The plain FIFO scheduler, typically implemented in commercial devices, is the main cause of the aforementioned anomaly. To overcome such problem, and optimize radio resource utilization, a channel quality aware scheduler has been designed and integrated in a prototypal Access Point (AP). The developed scheduler is based on the Hierarchical Token Bucket (HTB), and aims at granting the configured transmission share towards all the STAs served by the AP, irrespective of their channel quality. This goal is achieved limiting the frame transmission rate towards the STAs experimenting bad channel conditions. Indeed, in these channel conditions, the received frame will be likely corrupted. As a consequence, retransmission and reduction of transmission data rate will be needed, leading to a higher occupation of the channel. Lastly, the performance of the presented prototype are experimentally evaluated and compared with those obtained with standard scheduling algorithm, which do not take into account information on channel quality.