| Abstract |
During the past few years, wireless local area networks (WLANs) based on the IEEE 802.11 protocol have experienced a tremendous growth. This success is primarily driven by the need for universal access to the Internet and by the ability offered to users to move freely without the need of cables. One of the challenges in wireless networks is to integrate to the existing Internet infrastructure. However, the wireless path can not guarantee reliable data transfer, introducing high packet error rates and delay variability. Hence, the interaction of existing transport protocols initially designed for wireline paths, such as TCP, with wireless protocols, such as 802.11, appears to be highly challenging. TCP performs poorly in wireless environments, mainly because packet losses are misinterpreted as congestion signals by the TCP sender. Unnecessary congestion avoidance invocations lead to transmission rate throttling by the TCP sender and, hence, reduced throughput. Moreover, local retransmissions, user mobility, and handoffs, often introduce high delay variability to the packets being transmitted. High delay variability can result in spurious timeouts, i.e. timeouts that could have been avoided if the sender's retransmission timeout (RTO) was larger. Spurious timeouts significantly degrade TCP throughput, since the congestion window is unnecessarily reduced to one segment. Moreover, they result in unnecessary segment retransmissions by the TCP sender, which consume battery power if the sending node is the mobile user. In this work, we present an approach that improves TCP performance in both infrastructure and multihop wireless networks. Firstly, our approach focuses on eliminating wireless losses by applying a robust data-link layer retransmission mechanism. Secondly, we address the negative effects of high delay variability, which is caused by local retransmissions in bursty loss environments, by injecting artificial delay to TCP acknowledgements at the access point so as to indirectly influence the sender's RTO value. The proposed algorithm is implemented solely at the access point, without requiring changes at the sending and receiving nodes. Moreover, the algorithm can adapt its behaviour to network characteristics such as packet error rate, propagation delay and number of participating mobile nodes. Experiments with the NS-2 simulation tool show that the proposed adaptive delay injection method achieves significant throughput improvements and reduces unnecessary TCP retransmissions, hence reduces a mobile TCP senders battery consumption.
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Βελτίωση της απόδοσης του πρωτοκόλλου TCP σε ασύρματα δίκτυα 802.11 με εκρήξεις λαθών
