Performance analysis of IEEE 802.11 multirate WLANs: time based fairness vs throughput based fairness

This paper investigates the issue of fairness in a heterogeneous environment in which stations in IEEE 802.11 based wireless LANs use multiple data rates for transmission. CSMA/CA provides throughput based fairness and hence the saturation throughput of any station is limited by the saturation throughput of the station with the lowest bit rate in the network. We consider a new fairness notion called time based fairness in which each competing node receives an equal share of the wireless channel occupancy time. We demonstrate that the time based fairness can achieve larger aggregate throughput while still guaranteeing that no node receives worse channel access than it would in a single rate wireless LAN. We propose and analyze two mechanisms to achieve time based fairness; and develop an analytical model for computing saturation throughput and saturation delay for competing nodes in a 802.11 based multirate wireless LAN. Using Jain's fairness index, optimal MAC parameters required to achieve maximum fairness between slow and fast stations in terms of channel occupancy time are obtained. The impact of these mechanisms on throughput and frame delay of slow and fast stations is also explored.