Power management has traditionally focused on portable and handheld devices. This paper breaks with tradition and presents a case for managing power consumption in web servers. Web servers experience large periods of low utilization, presenting an opportunity for using power management to reduce energy consumption with minimal performance impact. We measured the energy consumption of a "typical" web server under a variety of workloads derived from access logs of real websites, including the 1998 Winter Olympics web site. Our measurements show that the CPU is the largest consumer of power for typical web servers today. We have also

Placeshifting systems stream videos from the home to a single remote user using the limited upstream capacity of the home broadband link. We analyze the behavior of two placeshifting systems each using two types of broadband networks. We show that the duration between packets did not depend on the way that the servers were sending the packets through the bottleneck link. Even though both of these systems used TCP, the duration between packets did not follow the round trip times either. Instead, it depended on the particular broadband network. Our analysis shows how the bottlenecked first mile network leads to predictable packet delivery at the remote client. Paradoxically, it also leads to shorter periods and a single packet within each data burst. We discuss the limitations imposed by this behavior on a client side energy saving mechanism. We also describe techniques that allow the placeshifting servers to better operate with client side WNIC energy saving mechanisms.

Dynamic power management saves power by shutting down idle devices. Several management algorithms have been proposed and demonstrated effective in certain applications. We quantitatively compare the power saving and performance impact of these algorithms on hard disks of a desktop and a notebook computers. This paper has three contributions. First, we build a framework in Windows NT to implement power managers running realistic workloads and directly interacting with users. Second, we define performance degradation that reflects user perception. Finally, we compare power saving and performance of existing algorithms and analyze the difference.

In a network using links with diverse properties, a packet flow that is fine tuned for some links (by selecting a proper packet size, transmission rate, encryption method, etc.) may be inappropriate for other links. Ability to change the flow properties over segments of the network allows flows with different characteristics to coexist; making it possible to adapt to diverse link properties. Application-specific adaptation mechanisms (such as proxies) do not force adaptations on every packet flowing over the link and are therefore insufficient for this purpose. We propose the concept of transformer tunnels that force adaptations on all the packets flowing through them. Transformer tunnels can coexist with proxies because the adaptations provided by both are independent of each other. Transformer tunnels provide adaptations by means of transformation functions. By attaching various transformation functions to such tunnels, we can efficiently fine tune the flow properties. We also provide an API for developing transformation functions. We have implemented transformer tunnels and have used them in our wireless network. In this paper, we present the effects on mobile hosts that use this mechanism to transform flows over the last-hop link for reducing losses during handoffs, and for improving the link utilization. 1

As mobile computing is getting popular, there is an increasing interest in techniques that can minimize energy consumption and prolong the battery life on mobile devices. Processor voltage scheduling is an effective way to reduce energy dissipation by reducing the processor speed. In this paper, we study voltage scheduling for real-time periodic tasks with non-preemptible sections. Three schemes are proposed to address this problem. The static speed algorithm derives a static feasible speed based on the Stack Resource Policy (SRP). As worst-case blocking does not always occur, the novel dual speed algorithm switches the processor speed to a lower value whenever possible. The dynamic reclaiming algorithm deploys a reservation-based approach to reclaim unused run time for redistribution. It effectively decreases the processor idle time and further reduces the processor speed. The feasibility conditions are given and proved. Simulation results show that the two dynamic algorithms can reduce processor energy consumption by up to 80 percent over the static speed scheme.

This paper discusses the potential benefits of applicationspecific power management through remote task execution. Power management is crucial for mobile devices that have to rely on battery power for extended periods of time. Image processing and understanding is a class of applications that is important in mobile environments. Image processing can be used in autonomous robot navigation, target acquisition/classification, keyboard-less input, and aerial surveillance (Micro Air Vehicles), just to mention a few. Experimental results on an image processing application, namely a human face detection and recognition system, indicate the power savings that can be achieved for this important class of applications. We discuss a compilation strategy that generates two versions of the initial application, one to be executed on the mobile device (client), and the other on a machine connected to the mobile device via a wireless network (server). The client and server codes have to be able to deal with disconnection events. Our proposed compilation strategy uses checkpointing techniques to allow the client to monitor program progress on the server, and to request checkpoint data in order to reduce the performance penalty in case of a possible server and/or network failure. The reported results have been obtained by actual power measurements on three client systems, (1) the This research was partially conducted while the first author was a visiting researcher at the Compaq's Cambridge Research Lab (CRL). Additional funding has been provided by NSF CAREER award No. 9985050. StrongARM based low-power SKIFF system developed at Compaq's Cambridge Research Laboratory, (2) Compaq's commercially available StrongARM based iPAQ H3600, and (3) a PentiumII based laptop. Initial experimen...

system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without prior written permission of the author.

The increasing speed and complexity of the microprocessor has brought about a corresponding increase in power consumption. Coupled with relatively small gains in battery capacity over recent years, the importance of intelligent battery management has become paramount. This paper presents a mechanism that takes advantage of feedback about power consumption in order to use battery energy more effectively. This feedback mechanism allows the implementation of many different energy management policies. One such policy is presented here which allows us to direct power consumption by changing performance states in the scheduler to achieve predetermined energy goals. Furthermore, our implementation in Linux can synthesize any average power usage rate with little overhead.

The power consumption of a wireless NIC is a crucial issue because wireless nodes are often constrained by the battery due to strict requirements regarding weight and size. In the paper, we present power consumption measurement results of an Aironet PC4800 PCMCIA wireless interface which is based on Intersil's PRISM I 802.11 11 MBit/s chipset. Thereby, we vary the transmission rate, the RF transmission power, and the packet size and derive the energy consumption as a further measure. The results can be used to parameterize and to validate simulation models of an IEEE 802.11 network interfaces, and to conduct further research in the field of power saving of WLANs.

In this paper we identify the most prominent problems of wireless multimedia networking and present several state-of-the-art solutions with a focus on energy efficiency. Three key problems in networked wireless multimedia systems are 1) the need to maintain a minimum quality of service over time-varying channels, 2) to operate with limited energy resources, and 3) to operate in a heterogeneous environment. We identify two main principles to solve these problems. The first principle is that energy efficiency should involve all layers of the system. Second, Quality of Service is an essential mechanism for mobile multimedia systems not only to give users an adequate level of service, but also as a tool to achieve an energy-efficient system. Due to the dynamic wireless environment, adaptability of the system will be a key issue in achieving this. Keywords -- energy efficiency, wireless networking, mobile computing, quality of service. 1 Introduction Advances in technology enabl...