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Energy efficiency advances

The EEE standard is an important first step to improve the overall energy efficiency of IT networks.


November 1, 2010  


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In writing the Standards Update column, it is sometimes challenging to choose a topic that is relevant, informative and up to date with the latest developments in the industry.

For this issue, I chose the topic of energy efficiency.

It is relevant in the sense that there is a lot more involved to meeting energy efficiency objectives for networking than appears on the surface. It is informative from the perspective of the type of technologies that are used for energy efficient networks. And finally, a new standard for energy efficient networking has just been published.

A task group in IEEE 802.3 has been hard at work since 2008 to develop an Energy Efficient Ethernet (EEE) Standard. EEE is a protocol to transition to and from a state of lower power consumption in response to changes in network demand.

EEE is designed to ensure that the link status is not changed as a result of the transition; that no frames are dropped or corrupted during the transition; and that the transition time to and from a state of lower power consumption are transparent to upper layer protocols and applications.

In September of this year the IEEE 802.3az-2010 Energy Efficient Ethernet (EEE) standard was ratified.

This standard impacts the whole family of Ethernet standard interfaces, including 100BASE-TX, 1000BASE-T and 10GBASE-T.

There is a new Clause 78 and associated changes in other clauses of the IEEE 802.3 document to support Ethernet operation in the Low Power Idle (LPI) mode.

When the LPI mode is enabled, the network devices connected at both ends of the link can save power during periods of low link utilization. EEE-enabled networks can save as much as 80% on energy consumption when in the low power state. The overall energy saving will depend greatly on the traffic utilization of the link

Although EEE is only concerned with one aspect of energy efficiency for networking at the Physical Layer (PHY), even greater power savings can be realized when higher layer standards can leverage IEEE 802.3az and begin to turn off additional system components.

There are efforts underway in standards like Energy Star to include EEE as part of the overall system of power saving capabilities.

So how does it work? Currently, legacy Ethernet interfaces for 100 Mb/s and higher have an active idle state where the bulk of the circuitry remains powered up whether data is transmitted on the link or not. LPI provides for a lower consumption energy state that can be employed during periods of low link utilization (high idle time) that is common in many Ethernet networks. During Auto-Negotiation, both link partners indicate their EEE capabilities. If EEE is not supported by both, the local device and the link partner, all EEE functionality is disabled.

When the LPI assert condition is detected, the local PHY signals to its link partner that it is entering LPI mode. The local PHY transmitter goes quiet only after the local PHY receives a sleep signal from the remote PHY.

In LPI mode, the transmit function of the local PHY is enabled periodically to transmit refresh signals that are used by the link partner to update adaptive filters and timing circuits in order to maintain link integrity.

This quiet-refresh cycle continues until the link partner initiates transition back to normal mode by transmitting the wake signal. The PHY then enters the normal operating state.

What are the conditions that determine when the physical layer enters and exits the low power state?

According to the EEE standard, the conditions under which the LPI Client decides to send LPI, and what actions are taken by the LPI Client when it receives LPI from the link partner, are implementation specific and beyond the scope of this standard.

The control policy and the level of integration play a key role in the overall efficiency attained and any performance impact on the network.

What are the drawbacks of EEE enabled networks? It really depends on the application.

When a frame arrives for transmission and the link is in the low power mode, it has to wait until the link returns to normal mode before transmission can begin. This additional latency is on the order of microseconds.

This latency increase will not be an issue for most LANs configurations, but can compromise the performance in low latency environments such high performance computing.

The importance of energy efficiency is paramount in all sectors of our society because of the high energy costs and the green initiative to reduce carbon emissions.

The EEE standard is an important first step to improve energy efficiency of IT networks. CNS

Paul Kish is Director, Systems and Standards at Belden. The information presented is the author’s view and is not official TIA correspondence.