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Issue No. 02 - March/April (2005 vol. 4)
ISSN: 1536-1233
pp: 209-221
Zhenghua Fu , IEEE Computer Society
Haiyun Luo , IEEE Computer Society
Petros Zerfos , IEEE Computer Society
Songwu Lu , IEEE Computer Society
Lixia Zhang , IEEE Computer Society
Mario Gerla , IEEE Computer Society
This paper studies TCP performance in a stationary multihop wireless network using IEEE 802.11 for channel access control. We first show that, given a specific network topology and flow patterns, there exists an optimal window size W^{\ast} at which TCP achieves the highest throughput via maximum spatial reuse of the shared wireless channel. However, TCP grows its window size much larger than W^{\ast}, leading to throughput reduction. We then explain the TCP throughput decrease using our observations and analysis of the packet loss in an overloaded multihop wireless network. We find out that the network overload is typically first signified by packet drops due to wireless link-layer contention, rather than buffer overflow-induced losses observed in the wired Internet. As the offered load increases, the probability of packet drops due to link contention also increases, and eventually saturates. Unfortunately, the link-layer drop probability is insufficient to keep the TCP window size around W^{\ast}. We model and analyze the link contention behavior, based on which we propose Link RED that fine-tunes the link-layer packet dropping probability to stabilize the TCP window size around W^{\ast}. We further devise Adaptive Pacing to better coordinate channel access along the packet forwarding path. Our simulations demonstrate 5 to 30 percent improvement of TCP throughput using the proposed two techniques.
TCP performance, multihop networks, congestion and contention control.

Z. Fu, H. Luo, L. Zhang, P. Zerfos, S. Lu and M. Gerla, "The Impact of Multihop Wireless Channel on TCP Performance," in IEEE Transactions on Mobile Computing, vol. 4, no. , pp. 209-221, 2005.
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