Fueled by concern over the energy consumption of backbone networks, lot's of work has recently gone into proposals for energy-aware traffic engineering and routing. Local and network-wide policies have been developed for switching off network interfaces and concentrating traffic in as few links as allowed by SLA constraints. In this work we examine empirically the energy saving margins of such policies using extensive data from a national and a pan-European research and academic network. We analyze the dependence of such margins on several parameters, including the level of energy proportionality, QoS constraints and the geographic span of a network. Our findings reveal that with existing devices, smart powering-off can save more than 50% of currently consumed energy, and that energy-aware traffic engineering has still quite away to go before it can be made redundant by improvements in the energy proportionality of devices.
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