WLC47-4: Analysis of Unfairness between TCP Uplink and Downlink Flows in Wi-Fi Hot Spots

Abstract

This paper focuses on the unfairness problem between TCP uplink and downlink flows in the 802.11 Wi-Fi hot spots and shows that the service is prone to be unfair. The cause of unfairness is analyzed from two aspects: TCP-induced asymmetry and MAC-induced asymmetry. Due to the asymmetric behavior of TCP congestion control with a cumulative acknowledgment mechanism between uplink and downlink flows, the service is biased toward the uplink flow and the downlink flow tends to starve. The contention-based channel access mechanism of 802.11 MAC exacerbates this unfairness problem because it intends to provide fair access opportunity only to the sending stations. Next, the analysis of the interaction between congestion control of TCP and contention control of MAC reveals interesting and counter-intuitive results: (i) Even when a station has a sufficiently large amount of traffic to send, it does not always participate in the MAC-layer contention, its opportunity for MAC-layer contention is controlled by the TCP congestion control, (ii) The aggregate throughput remains almost constant with respect to the number of stations sending/receiving TCP traffic. (Hi) Both TCP-induced unfairness and MAC-induced unfairness can be resolved if packet loss due to buffer overflow in an access point does not occur.