Local renewable DC electricity sources, such as solar, have become cost competitive. In this position paper, we argue that an Ethernet LAN with Power-over-Ethernet (PoE) is a nanogrid where the physical layer infrastructure is used for DC electricity distribution and the link-layer capabilities can be used to match electricity supply and demand. To be cost effective with respect to wiring, many of these nanogrids must be multi-drop where each drop may be a power socket. Open challenges to employing PoE as a multi-drop managed nanogrid include 1) extending LLDP for multi-drop, 2) adapting LLDP to support the use of price signals for modulating power demand, 3) achieving efficient PoE power flow in a multi-drop circuit, and 4) implementing low-cost and fair scheduling for packet transmission. We explore how multi-drop Ethernet can be implemented with fair access to all devices by employing at each drop a mini-switch with packet scheduling.

Data centers are a significant and growing component of electricity demand in the United States. This paper presents a bottom-up model that can be used to estimate total data center electricity demand within a region as well as the potential electricity savings associated with energy efficiency improvements. The model is applied to estimate 2008 U.S. data center electricity demand and the technical potential for electricity savings associated with major measures for IT devices and infrastructure equipment. Results suggest that 2008 demand was approximately 69 billion kilowatt hours (1.8% of 2008 total U.S. electricity sales) and that it may be technically feasible to reduce this demand by up to 80% (to 13 billion kilowatt hours) through aggressive pursuit of energy efficiency measures. Measure-level savings estimates are provided, which shed light on the relative importance of different measures at the national level. Measures applied to servers are found to have the greatest contribution to potential savings.

Billions of dollars of electricity are being used to keep idle or unused network hosts fully powered-on only to maintain their network presence. We investigate how a network connectivity proxy (NCP) could enable significant energy savings by allowing idle hosts to enter a low-power sleep state and still maintain full network presence. An NCP must handle ARP, ICMP, DHCP, and other low-level network presence tasks for a network host. An NCP must also be able to maintain TCP connections and UDP data flows and to respond to application messages. The focus of this paper is on how TCP connections can be kept alive during periods of host sleep by using a SOCKS-based approach called green SOCKS (gSOCKS) as part of an NCP. The gSOCKS includes awareness of the power state of a host. A prototype implementation of gSOCKS in a Linksys router shows that TCP connections can be preserved.

The rapidly increasing energy consumption by computing and communications equipment is a significant economic and environmental problem that needs to be addressed. Ethernet network interface controllers (NICs) in the US alone consume hundreds of millions of US dollars in electricity per year. Most Ethernet links are underutilized and link energy consumption can be reduced by operating at a lower data rate. In this paper, we investigate adaptive link rate (ALR) as a means of reducing the energy consumption of a typical Ethernet link by adaptively varying the link data rate in response to utilization. Policies to determine when to change the link data rate are studied. Simple policies that use output buffer queue length thresholds and fine-grain utilization monitoring are shown to be effective. A Markov model of a state-dependent service rate queue with rate transitions only at service completion is used to evaluate the performance of ALR with respect to the mean packet delay, the time spent in an energy-saving low link data rate, and the oscillation of link data rates. Simulation experiments using actual and synthetic traffic traces show that an Ethernet link with ALR can operate at a lower data rate for over 80 percent of the time, yielding significant energy savings with only a very small increase in packet delay.