Here comes energy efficient Ethernet

The Aquantia AQ1401 10GBASE-T PHY transceiver quad unit is one example of a low-power product targeting 802.3azcompliant Ethernet networks.

When most people think of the Energy Star program, the products that probably come to mind include lighting, home appliances, and consumer electronics. But as government policy makers accelerate and broaden efforts to save energy, several industries are paying closer attention, reasoning it is better come up with initiatives of their own before mandated to do so.

That's the case with the 802.3az Energy Efficient Ethernet (EEE) standard from the Institute of Electrical and Electronic Engineers (IEEE). As its name implies, it aims to reduce energy on wired Ethernet connections. It was adopted by the IEEE last September. (See “The Road to the EEE Standard,” next page.)

Ethernet, of course, is the dominant wire-line technology for communications in computer networks. It is also being considered for use in access networks and even for long-haul links.

In a nutshell, the IEEE scheme saves energy by blasting data over Ethernet lines, then putting the network controllers to sleep until they're needed again. More specifically, the .az portion takes aim at the four different standardized rates used for unshielded twisted-pair (UTP) wiring which account for the vast majority of present day Ethernet links: 10 Mbits/sec (1-BASE-T), 100 Mb its/sec (100BASE-TX), 1 Gbit/sec (1000BASE-T, and 10 Gbits/sec (10GBASE-T).

In the Energy Efficient Ethernet concept, a low-power idle mode is initiated during two active modes. When packets are sent, the Ethernet device is in the active mode. It goes into the LPI mode when packets cease coming in. It goes back into active mode when it senses the arrival of new packets. In the LPI mode, the device only sends signals during short refresh intervals to maintain alignment between a transmitter and receiver. Both active modes are framed by wake-up and sleep modes.
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Estimates are that the 802.3az EEE standard could save $400 million a year just in the U.S. alone. This figure is certain to climb as Ethernet data rates rise in that it takes more power to hit faster data rates. For example, a transceiver chip for the 1000BASE-T Ethernet physical layer (PHY) typically consumes about 0.5 W of power while a 10GBASE-T device consumes about 5 W.

Michael Bennett at the Lawrence Berkeley National Laboratory (LBNL) and chairman of the IEEE P802.3az Task Force says the new standard could account for energy savings ranging from 1.73 to 2.60 TWh/year just in U.S. residential equipment alone. That translates into $139 to $208 million a year in energy cost savings. For commercial data center equipment like servers, storage systems, switchers, routers, and so forth, energy savings could run anywhere between 1.47 to 2.21 TWh/year just in the U.S. That translates into a $118 to $177 million-a-year savings.

The official term for the energy saving technique proscribed by the IEEE P802.3az Task Force is Low Power Idle (LPI). There is a general consensus that it can be challenging to quickly turn on a dormant network card, as LPI prescribes. But it is much easier than an alternative method the task force evaluated called Adaptive Link Rate (ALR). ALR uses a two-way media-access control (MAC) frame handshake. This can be implemented in either the device driver or within the Ethernet controller.

The problem: This concept is time consuming when switching between Ethernet speeds (i.e. from higher to lower data rates and vice versa). Changing the link rate means dropping the link rate then re-establishing it, which can take several microseconds, too slow for modern networks.

The LPI concept was hatched at Intel. “You're better off sending data faster and getting to sleep quicker, which allows you to save more power over the long haul,” says Robert Hayes, Intel strategic planner for networking products. Most Ethernet data traffic comes in bursts and is thus suited for the LPI approach, he explains.

Link utilization, packet transmission time, and the distribution of packet intervals all determine the energy efficiency of the EEE concept. Task force members believe that it is reasonable to assume that EEE overhead consumes the same amount of power as packet transmission. Many studies have found that network link utilization, especially at the edge, is generally low, driven by the need for high-speed bulk data transfer or demand.

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