Access Point Selection for Multi-Rate IEEE 802.11 Wireless LANs

Access Point (AP) selection is important in WLANs as it affects the throughput of the joining station (STA). In this paper, we propose a class of AP selection algorithms to maximize the joining STA's expected throughput by considering interference at STAs, and transmit opportunities (TXOPs) at APs. Specifically, we collect a binary-valued local channel occupancy signal, called busy-idle (BI) signal, at each node and require the APs to periodically broadcast their BI signal and a quantity representing their TXOPs. This enables the joining STA to estimate throughput from candidate APs before selecting one. We use NS-2 simulations to demonstrate the effectiveness of our algorithms for saturated UDP and TCP downlink traffic, and compare them with received signal power (rxpwr) algorithm, load based algorithm, and Fukuda algorithm. For a random topology consisting of 24 APs and 60 STAs, our algorithms increase the joining STAs average throughput by as much as 42% and 24% compared to rxpwr for UDP and TCP respectively. In addition, the achieved average throughput is 90% and 93% of that obtained via the optimal selection. We also show that, in contrast to rxpwr, the throughput of proposed algorithms remains close to optimal with the increase in AP or STA density.