ENHANCING TCP ENERGY EFFICIENCY FOR MOBILE HOSTS W. Lilakiatsakun*, ** A. Seneviratne** *Dept. of Computer Engineering Mahanakorn University of Technology, Bangkok 10530 Email:w.lila@ee.unsw.edu.au **Dept. of Electrical Engineering and Telecommunications The University of New South Wales, Sydney 2052 Australia Email:aruna@ee.unsw.edu.au ABSTRACT We present the finding of an investigation into an energy and performance enhancing technique for TCP operating in wireless networking environments. This technique, in contrast to other schemes that have been studied to date, are based on the concept of minimizing the probability of packet loss in wireless links rather than on error recovery. The scheme we propose intelligently suspends transmissions when the wireless link displays poor error characteristics and resumes when the error condition abates, to minimize packet loss without TCP modification. Through simulation, in LAN environment it is shown that intelligently suspending and resuming is possible to get better TCP performance and energy efficiency when the error is persistent, long duration of bad period and high error rate. On the other hand, in WAN environment it is worth doing in all situations that can improve energy efficiency up to 40% and improving TCP performance up to 35 %. This study will lead to improve TCP performance and energy efficiency by deploying lower layer information efficiently. 1. INTRODUCTION TCP have been widely used as a reliable transport protocol in many applications. During the past few years, a large number of studies have focused on improving TCP performance. TCP Tahoe [1], Reno [2], and Newreno [3] algorithms have been developed to deal with error conditions in various situations. However, these were designed for wired networking environments in which packets losses are primarily due to network congestion. TCP implementations assume that all packet losses are due to network congestion. Wireless link transmission characteristics are completely different to wired links, especially in terms of their error characteristics. They always suffer from packet losses due to link errors as well as network congestion. Therefore, one of the interesting aspects of using TCP in wireless networking environments is the side-effects of misinterpretation the cause of packet losses. The most common effect, shown by researchers, is the inappropriate invocation of the TCP congestion algorithm and the resulting loss in performance [10]. Several methods of overcoming this have been reported in the literature [4,5,10] introducing mechanisms that enable the transmitter to distinguish error within the wireless and wired network segments. Although these methods can improve TCP performance, but the energy issue has not been addressed. It, therefore, is not clearly presented that improving performance will improve the energy efficiency. The other proposals propose the use of a split connection where they divide a TCP connection into two connections, one between the mobile hosts and the base station and the other between the base station and the fixed host [6]. The main disadvantage of split connection schemes is the destructions of the TCP end-to-end. For example, if the base station crashes, it will not be possible to recover the connection. In addition to the above, both schemes do not take into account, the impact of the scheme on energy consumption. Energy consumption is a significant metric in mobile devices as they have to rely on battery power. In an attempt to address this, Stemm et al. [7] proposed turning off the network interfaces based on the knowledge of higher layers. By doing this however, made the mobile devices unreachable hosts whist being turned off. Tsaoussidis et al [8][9] also studied the tradeoff between energy and throughput, and proposed a new protocol to probe the network condition when a packet loss was detected and TCP will response toward error adaptively. Even though, adaptive error recovery mechanism in TCP probe can achieve better TCP throughput. But some packets will be dropped before TCP takes any responding, especially in deep fading condition TCP might pay expenditure in term of energy to retransmit those packets. Closely related work is Tom Goff et al. [14] proposal on Freeze- TCP technique to improve TCP performance for mobile environment. That is the mobile host will send the zero window advertisement (ZWA) when it senses to disconnection. However, the Freeze TCP was intended to implement on a mobile host while being as a TCP receiver. As oppose to Freeze TCP, we proposed scheme implemented on mobile host as a TCP sender. Again, the energy issue was not taken into account. In current wireless networking environments it is possible to determine the SNR of the channel, and hence infer the link status, quality and PER (packet error rate) and make it use of this information at the TCP layer. In this paper, we propose a way of achieving this under existing TCP mechanisms. The aim of this scheme is not intended to design the new TCP for wireless channel. Rather, the scheme has been designed to be additional function. The scheme basically suspends transmissions when the link state is poor, and resume when it improves. Using simulation, we show the viability of this simple scheme, and also