The efficient use of energy in communications is an area of growing interest. Many existing standards in the area of wire-line communications are not designed to make efficient use of energy. For example, many standards specify that the transmitter and receiver operate at full power even when no data is being sent. This is the case in many Ethernet standards, resulting in a considerable waste of energy. This has triggered efforts to develop new standards, such as Energy Efficient Ethernet, which have the aim of reducing energy consumption when no data is being transmitted over a link.

Further energy savings are possible in many existing standards when data is being transmitted and channel conditions are better than the worst case for which the receiver was designed. In these cases, receivers operate with a Signal to Noise Ratio (SNR) that is above the required level. The idea presented in this work is to trade the excess SNR for lower power consumption by reducing processing in the receiver. This technique can be applied to existing standards, as it only requires modifications to the receiver and is transparent to the remote transmitter device.

To illustrate the approach, a 1000BaseT receiver is studied and a number of techniques are proposed to reduce the energy consumption of the receiver. The results show that the potential energy savings are significant--up to 50% in some cases.

Introduction
Communications equipment globally consumes a vast amount of energy. The core of the Internet alone consumes 6 TWh per year¹. This increase in energy consumption has arisen due to both an increase in the number of devices and the lack of focus on energy efficiency in the design of wire-line communications devices.

There are hundreds of millions of installed Ethernet links. When powered on, each link consumes a substantial amount of energy, even if no data is being transmitted². This leads to the waste of over 3 TWh per year². The cause of this waste is the lack of energy efficiency criteria in the design of the original Ethernet standards. The standards specify that master and slave devices must send idle pulses to maintain the link even in the absence of data. This issue is now being addressed by the IEEE 802.3az Task Force, Energy Efficient Ethernet. The Task Force is set to agree a standard by 2010 that will introduce energy efficiency enhancements to existing Ethernet standards³.

The Energy Efficient Ethernet Task Force and other similar groups aim to ensure energy efficiency operation while maintaining backward interoperability. By definition, standards give designers considerable freedom in implementation with the aim of promoting competition and innovation. Thus, energy efficiency needs to be considered a priority in the design process as well as in the standardization process. Traditionally, designers aim to reduce power consumption under worst case conditions. This has been driven by the need to minimize the cost of packaging the device and cooling system.

For example, a plastic package may be sufficient for a low power consumption device while a higher consumption device might need a ceramic package and a heat sink. Additionally, a system that uses devices with higher power consumption may need forced ventilation to avoid overheating, increasing costs and lowering reliability. While worst-case power consumption is still important for cost reasons, today's designer should have the additional goal of reducing energy consumption in all cases.

When the channel is better than worst case, the receiver typically operates above the required Signal to Noise Ratio (SNR) and achieves a lower than required Bit Error Rate (BER). In this situation, it makes sense from an energy efficiency point of view, to trade excess SNR for lower power consumption by reducing the processing done in the receiver. Similar energy efficient techniques are commonly applied in devices intended for battery operated systems, such as laptops and mobile phones. For these devices, considerable design effort is devoted to reducing energy consumption so as to extend battery life. However, since it is a wired standard, these techniques are not commonly applied to Ethernet devices. Designers assume that, in most cases, Ethernet devices will operate from a mains power supply and energy savings for environmental reasons have not been a priority. The idea of energy scalable receivers is explored in this article and applied to the design of a 1000BaseT (Gigabit) Ethernet receiver. The results show that the proposed approach can provide substantial energy savings in a large number of cases.

The rest of the article is structured as follows, in section II related work in the area of energy efficient communications is reviewed then a brief description of 1000BaseT receivers is provided in section III. In section IV energy efficient receiver scaling techniques for 1000BaseT are proposed and their effectiveness is discussed. Finally, conclusions are presented in section V.

II. Energy Efficient Communications
Energy efficiency in the Internet is receiving increasing attention. Early works such as [1], proposed reducing the energy consumption of routers and switches by using low power modes of operation when there is little or no network activity. This idea has also been applied to reduce the power consumption of network nodes and end user devices. In [4], a method was proposed to offload some packet processing functions to the Network Interface Card (NIC) so that the PC processor can enter low power modes for extended periods of time. In [5], the use of low-power modes in different elements of LAN switches was studied. Similarly, a proposal was made in [6] to modify TCP in order to put the connection into a sleep mode when there is no activity. The energy efficiency in TCP was considered from a different perspective in [7]. In that work, the energy cost of implementing TCP was studied and proposals were made to reduce the energy costs associated with TCP processing in a PC.

The idea of entering low power modes when there is little activity has also been applied to Ethernet. The authors of [2] propose a method to reduce the speed of Ethernet links when there is little traffic. This speed reduction results in considerable power savings. In the case of switching from 1Gbps to 100Mbps, power savings were estimated to be over 50%. The Energy Efficient Ethernet Taskforce is working on modifications to existing standards that would reduce power consumption by allowing the physical layer device (PHY) to enter a low power mode when there is little traffic [4]. These modifications to the PHY allow for much faster wake ups times than the link layer speed changes proposed in [2]. In [8], the encodings used in two different Ethernet standards, 100BaseTx and 1000BaseT, are analyzed in terms of their energy efficiency. Unfortunately, the analysis does not consider the complexity of Ethernet physical layer devices and the results are of limited interest.

To summarize, most previous work has focused on energy efficiency at higher layers while work at the physical layer has concentrated on using low power states when there is little traffic [2]. Improving the energy efficiency of Ethernet devices when the link is active and traffic is flowing seems to be a promising line of enquiry that is complementary to existing work.