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IEEE P802.3az Energy Efficient Ethernet: Small network power savings add up

Much energy is wasted while the network is idle. The IEEE P802.3az Energy Efficient Ethernet project aims to deliver major energy savings by combining many small network power savings, both economically and environmentally beneficial.

The new IEEE P802.3az Energy Efficient Ethernet project aims to deliver major energy savings by combining many small savings.

This initiative is based on the work of Mike Bennett and Bruce Nordman of the Lawrence Berkeley National Laboratory and Professor Ken Christensen of the University of South Florida. They observed that much energy is wasted while the network is idle.

The facts:
1. Most desktop PCs now ship with a 1 Gbps interface, an increase from the 100 Mbps interface that was standard a few years ago.
2. A typical 1 Gbps interface consumes roughly 2W more than a 100 Mbps interface.
3. Ethernet interfaces continue to consume power at this level even when the network is idle.
4. The 1 Gbps interface in the switch port at the other end of the link also consumes 2W more than a 100 Mbps interface.
5. The upgrade from 100 Mbps to 1 Gbps thus consumes an extra 4W for each connected PC.
6. The IEEE project task force has proposed that interfaces reduce speed while idle or lightly loaded. Proposed is a method by which interfaces negotiate a speed reduction when load drops and an increase when load increases.

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Power savings calculation

Bennett, Nordman and Christensen point to studies showing that, on average, a desktop PC's network interface is active only 5% of the time. Further, they estimate that two-thirds of PCs are left on 24/7. It is estimated that there will be 80 million desktop PCs in the U.S. by 2012, and half could have efficient Ethernet interfaces. A $78 million savings is estimated for the U.S. based on 40 million PCs and a $.08 per kWh electricity cost. Total worldwide savings are estimated at approximately $235 million, based on equivalent market size in Europe and the rest of the world.

The estimate and proposal were completed in 2006. Energy savings may actually be much higher than the proposal projects. The latest U.S. government figures show significantly higher electricity prices in parts of the U.S. The figure for 2007 was approximately $.15 per kWh in New York and $.13 per kWh in California. Reduced cooling load was not included in the calculations but would add to the savings.

Negotiation mechanism and policy

The P802.3az Energy Efficient Ethernet project task force proposes the creation of a series of new MAC level commands and responses. The interface that senses a need to change rate, either up or down, sends a command requesting the change to its partner at the opposite end of the link. The partner may respond either positively or negatively.

The rate change must be done quickly. The goal is to resynchronize at the new speed in 1 msec or less. A longer delay could result in lost packets, as buffers overflow in the transmitting interface.

Efficient interfaces will be introduced as equipment incorporating the proposed standard is installed. In many cases, equipment at both ends of a link will not be upgraded at the same time. The standard will include a way for an upgraded interface to determine at link initialization time whether the opposite interface also supports the standard.

The IEEE P802.3az task force is concentrating on the MAC level mechanisms, but decisions about when to change rate will have an important impact on how much energy is saved. Each vendor will determine its rate change policy. Rate changes could be based on the number of packets buffered for transmission, on average data rate over time, or on some combination of factors.

Energy savings for optical fiber

The task force has focused so far on copper interfaces only, but a proposal has been made to include optical fiber. It is not practical to reduce interface rate on optical fiber, but it is possible to save energy by pausing the link for short periods of time.

Energy saving on optical fiber has become an important goal for service providers deploying fiber to the premises (FTTP) services such as Verizon FIOS. These services will reach millions of subscribers, so small energy savings for each subscriber will translate into major savings overall.

Much work by the Energy Efficient Ethernet task force remains to be done. Even after the standard is completed and accepted, several years will pass before savings are realized. New interface devices must be designed and then deployed in sufficient numbers.

Consider that the cost of power and the importance of reducing its use will probably increase in the future. Actual savings may ultimately far exceed the 2006 study projections.

About the author:
David B. Jacobs of The Jacobs Group has more than 20 years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software startups.


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