Power Saving Policies for wireless access to TCP / IP Networks

A mobile computer operates on a finite battery power that represents one of the greatest limitation to the utility of portable computers [2,6]. Projections on progress in battery technology show that only small improvements in the battery capacity are expected in next future [16]. If the battery capacity cannot be improved, it is vital that power utilization is managed efficiently by identifying way to use less power preferably with no impact on the applications. Many researchers have focused on this problem. Strategies for power saving have been investigated at several layers including the physical-layer transmissions, the operating system, and the application levels. Specifically, by focusing on power-saving at the transmission level, some authors have proposed and analyzed policies, based on the monitoring of the transmission error rates, which avoid useless transmissions when the channel noise makes low the probability of a successful transmission ([12-15] and [21-25]). Power-saving policies at the operating system level include the study of strategies for the CPU scheduling [7, 20] and for the hard-disk management [3]. At the application-level, among the other, have been investigated policies that exploit the application semantic (e.g. for the applications involving data access, the base station can periodically broadcast the “hot spot” data, i.e. the information more frequently accessed by mobile users) or profit of tasks remote execution (user’s jobs are transferred from a mobile host to a fixed host to reduce power consumption by the mobile-host CPU) [5, 6, 8, 11]. In this paper we investigate the relationship between power saving and the data transfer from/to mobile hosts. Data transfer contributes to the power management problem as it uses significant power when data is sent and received. Communication hardware and software, involved in data transfer, can be partitioned in two classes: subnetwork protocols and internetwork protocols. Subnetwork protocols include network technologies such as LAN and WAN protocols, and are only involved in the information transfer between hosts connected to the same physical network. Internetwork protocols exploit the subnetwork services to provide a data transfer between all the computers that have a network access. The impact of network technologies on power consumption has been investigated in depth in [17]. The power saving features of the IEEE 802.11 wireless LANs have been analyzed in [10, 19]. A simple and effective mechanism for minimizing the power consumption in CSMA-based MAC protocols, such as the IEEE 802.11, has been proposed in [26, 27]. Power-saving strategies at the subnetwork level are strongly dependent on the network technology and no general solution can be developed. On the other hand, at the internetwork level the TCP/IP protocol stack is a de-facto standard, and hence power-saving strategies at this level apply to almost any information transfer through a computer network. In this work we investigate power-saving strategies at the Internet-level protocols. Specifically, we focus on transport layer protocols and we investigate the impact of the TCP protocol when a mobile host access a fixed host, see Figure 1. In our work the mobile host is connected to the Internet by an IEEE 802.11 wireless LAN. To evaluate the impact of the TCP protocol on the consumption of the battery power we first define a