Averting speed inefficiency in rate-diverse WiFi networks through queueing and aggregation

IEEE 802.11n, the latest version of the widely used standard for wireless LANs, promises significant increases in speed by incorporating multiple enhancements at the physical layer. In this paper we demonstrate that, on the contrary, the straightforward deployment of 802.11n in conjunction with TCP over a simple, single access-point network, can dramatically underachieve the promised speeds. Part of the deficiency is due to overheads and can be improved by the technique of packet aggregation present in the standard. However more subtle problems are identified, in particular the downward equalization of throughputs that occurs under physical rate diversity, or the unreasonable portion of resources taken by uplink flows when competing with the more numerous downlink connections. These difficulties are demonstrated and their causes explained through a sequence of experiments with the ns3 packet simulator. Our analysis leads us to propose a desirable resource allocation for these situations of competition, and an architecture for control in the access-point to achieve it. Our solution involves a combination of packet aggregation, multiple queues and TCP-ACK isolation, compatible with the standard and where all the control resides at the AP. We demonstrate analytically and through extensive simulation that our method is able to provide significant enhancements in performance under a variety of traffic conditions.