Recent studies have shown that the total energy consumed by the Internet has followed an increasing trend over the years. A considerable part of this energy is wasted due to an inefficient utilization of edge devices, i.e., Personal Computers (PCs) and other user equipment in homes and offices. PCs are often left on even when they are not used (e.g., overnight or during weekends), while other edge devices, such as printers, IP phones, and displays, are typically kept always on, especially in

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... lic buildings. A number of solutions have been proposed for eliminating, or reducing, energy wastes due to edge devices. In this chapter we propose a general taxonomy for their classification. Then, according to the introduced taxonomy, we survey the main proposals appeared in the literature. Finally, we highlight some open issues. energy can be huge. Moreover, edge devices are typically used with little or no attention to the energy problem. For example, many PCs are left on, even overnight and during the weekend, due to laziness, omissions or to maintain network connectivity (e.g., for Peer-to-Peer file sharing). In addition, users often do not use energy saving policies (e.g., automatic hibernation after a certain period of inactivity). Other edge devices, such as printers, IP phones, and displays, are typically kept always on, especially in offices and 3 public buildings. Every year, the estimated overall energy consumption due to edge devices in USA is in the order of tens of TWh, causing an expense of billions of dollars. The need for specific solutions to the problem is, thus, quite apparent. In the next sections we will consider the main approaches to power management of PCs and other user equipment connected to the Internet. Specifically, we will introduce a general taxonomy to classify the proposed solutions. Then, according to the introduced taxonomy, we will survey the main proposals presented in the literature. Obviously, most of the proposed solutions refer to PCs. However, some of them could be extended to other edge devices as well. GENERAL APPROACHES TO POWER MANAGEMENT OF EDGE DEVICES In order to identify possible approaches to energy efficiency in edge devices, it is necessary to determine preliminarily the cause of energy waste. One of the fundamental causes is the fact that many users leave their PC always on (especially in their workplace), due to laziness and/or omissions. This clearly emerges, for example, in the PC Energy Report [9] about the energy consumption of PCs used at work, issued by the UK National Energy Foundation. This report highlights that about 21% of PCs used at work are almost never turned off (during nights and weekends), thus resulting in a waste of energy equal to approximately 1.5 TWh per year (corresponding to 700,000 tons of CO2). In order to reduce this energy waste due to laziness and omissions, PCs and other edge devices could be forcedly turned off at a certain time, employing common solutions, such as Nightwatchman [9] , which is already used in many environments. There are cases, however, in which PCs are deliberately left on for the execution of certain network activities, such as remote connection or P2P file sharing. Since the PC is