Automatic control in TCP over wireless
Abstract: Over the last decade, both the Internet and mobile telephony has become parts of daily life, changing the ways we communicate and search for information. These two distinct tools are now slowly merging. The topic of this thesis is TCP over wireless, and the automatic control that is used both within the system, from the link-layer power control to the end-to-end congestion control. It consists of three main contributions.The first contribution is a proposed split-connection scheme for downloads to a mobile terminal. A wireless mobile terminal requests a file or a web page from a proxy, which in turn requests the data from a server on the Internet. During the file transfer, the radio network controller (RNC) sends radio network feedback (RNF) messages to the proxy. These messages include information about bandwidth changes over the radio channel, and the current RNC queue length. A novel control mechanism in the proxy uses this information to adjust the sending rate. The stability and convergence speed of the proxy controller is analyzed theoretically. The performance of the proposed controller is compared to end-to-end TCP Reno, using ns-2 simulations of realistic use cases. It is shown that the proxy control is able to reduce the response time experienced by users, and increase the utilization of the radio channel. The changes are loalized to the RNC and the proxy; no changes are required to the TCP implementation in terminal or server.The second contribution is the analysis of an uplink channel using power control and link-layer retransmissions. To be able to design the link-layer mechanisms in a systematic way, good models for the link-layer processes, and their interaction with TCP, are essential.The use of link-layer retransmissions transforms a link with constant delay and random losses into a link with random delay and almost no losses. As seen from the TCP end points, the difference between such a link and a wired one is no longer the loss rate, but the packet delay distribution. Models for the power control and link-layer retransmissions on the link are used to derive packet delay distribution, and its impact on TCP performance is investigated.The final contribution considers ways to optimize the link-layer processes. The main result is that TCP performance, over a wireless link with random retransmission delays, can be improved by adding carefully chosen artificial delays to certain packets. The artificial delays are optimized off-line and applied on-line. The additional delay that is applied to a packet depends only on the retransmission delay experienced by that same packet, and this information is available locally at the link.
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