TCP Dynamics over IEEE 802.11e WLANs: Modeling and Throughput Enhancement

摘要

Today, IEEE 802.11 Wireless LAN (WLAN) has become a prevailing solution for broadband wireless Internet access while Transport Control Protocol (TCP) is the dominant transport protocol in the Internet. It is known that, in an infrastructure-based WLAN with multiple stations carrying long-lived TCP flows, the number of stations that are actively contending to access the channel is very small. Therefore, the aggregate TCP throughput is basically independent of the total number of stations. This phenomenon is due to the closed-loop nature of TCP flow control and the bottleneck downlink (i.e., AP-to-station) transmissions in infrastructure-based WLANs. In the emerging Enhanced Distributed Channel Access (EDCA)-based IEEE 802.11e WLANs, with a proper configuration, packet congestion at the bottleneck downlink could be alleviated since the AP and stations are allowed to use different channel access parameters. In this paper, we first conduct a rigorous, comprehensive analysis of the TCP dynamics over the 802.11e EDCA. Then, the effects of minimum contention window sizes (of both AP and stations) on the aggregate TCP throughput are evaluated via mathematical analysis and simulation. We also show that the best TCP aggregate throughput performance can be achieved via AP's contention-free access for downlink packet transmissions. Finally, some of the simplifying assumptions used in our mathematical model are evaluated via simulation, and results show that our model is reasonably accurate when the wireline delay is small and the packet loss rate is low.