A Comparison of HTB Based Channel-Aware Schedulers for 802.11 Systems

Abstract

The 802.11 commercial devices adopt a simple FIFO scheduler, which does not allow to isolate flows addressed to different destinations. This characteristic leads to the well-known performance anomaly of 802.11: when one or more STAs experiment poor radio channel conditions, they increase the time spent to transmit a single packet leading to an inefficient use of the shared medium. These phenomena have as a consequence the worsening of the performance of all the STAs sharing the wireless medium independently of their radio channel conditions. In this scenario, the paper proposes an architecture for a scheduling algorithm to implement in the AP for the downlink traffic, based on Hierarchical Token Bucket (HTB). The peculiarity of the proposed architecture is its ability to take into account, besides the transport service class required by the destination user, the channel quality experimented by the destination mobile STAtion (STA). Starting from this architecture two scheduling schemes are then proposed: the Wireless HTB (WHTB) and the Time-based WHTB (TWHTB). Both schemes are implemented in a prototype of AP. Hence, the performance of the proposed scheduling algorithms are experimentally evaluated and compared with those obtained with standard scheduling algorithm, which do not take into account information on channel quality. Finally, the paper presents a comparison in terms of implementation issues highlighted by the two proposed schemes, during the developing phase and the experimental analysis.