Currently Being Moderated

Introduction

A friend who labels himself as “technologically adverse” recently took a big step and reluctantly upgraded from an “antique” flip-phone he had for 3 years to a new “high-tech” smartphone. He has joined the estimated 60 million other smartphone users in the US who can access their email, check stocks, listen to streaming music, watch streaming videos, etc. with the flick of a finger.

 

So what does this have to do with IEEE802.3az: Energy Efficient Ethernet or “EEE” (pronounced “triple-E”)? As the computing continuum of wireless and connected devices increases, consequently the datacenter must also scale to meet the increased data demand. The datacenter and its networking equipment is not the only target market; so is networked equipment. Consumer and enterprise computers, consumer electronics like set-top boxes, routers and other Ethernet applications are part of the ecosystem.

 

As the world becomes increasingly connected, there is the tradeoff of increased demand for power. In order to help curb this need, EEE is seen as an incremental step to reduce power consumption. A Lawrence Berkeley Labs researcher estimates that adoption of EEE in the networking and networked device markets would result in energy savings of $400 million or more per year in the US (Merritt, 2008).

 

What is EEE?

Energy Efficient Ethernet allows for an Ethernet device operating over twisted-pair cabling (100BASE-TX, 1000BASE-T and 10GBASE-T) or electrical backplanes (1000BASE-KX, XAUI, 10GBASE-KX4 or 10GBASE-KR) to dynamically scale power consumption based on traffic load. This ability is the value proposition of 802.3az and is accomplished by a communication protocol called Low Power Idle (LPI) which is negotiated between a remote (server) and local (client) PHY at power up. LPI with be further explained in the next section.

 

Once LPI has been negotiated, the local PHY is allowed to enter a link state referred to as simply “LPI”. Conversely a non-EEE compliant PHY will remain in an idle state and, depending on the link speed, will consume up to 10 times more power than an EEE compliant PHY. But there is one caveat. In order to utilize EEE, the local and remote PHYs must both be EEE compliant.

 

The Intel® 82579 Gigabit Ethernet PHY is an 802.3az compliant part with the following power characteristics:

 

System State

Link State

Device Power (mW)

S0

1000Mbps Active

619

1000Mbps Idle

509

1000Mbps LPI

49

100Mbps Active

315

100Mbps Idle

207

100Mbps LPI

53

Figure 1: 82579 Power Measurements

 

 

How does it Work?

The power savings from a purely percentage basis is significant, but how does EEE work? Let’s start off with the example of traffic flowing between a remote (server) and local (client) PHYs with network and switching equipment between them. As stated before, both devices will advertise and handshake the capability to support 802.3az at power up. Once the handshake is completed, the remote PHY knows it can initiate a LPI signature (figure 2) to the local PHY.

 

Picture.jpg

Figure 2: LPI Signature

 

From figure 2, the LPI sleep and wake signals are initiated by the remote PHY between periods of activity. Once the local PHY is asleep, periodic refreshes are sent to the client in order to update adaptive filters and timing circuits in order to maintain link integrity. It is during the low-power state where power savings are realized at the local PHY because it is allowed to switch off part of the receive and transmit circuitry. Once there is data to be transmitted, the remote PHY sends a wake signal and data packets are then transmitted.

 

AreThere Any Drawbacks?

Performance using EEE with respect to latency is impacted. The total amount of latency for a sleep plus a wake is about 7.36µs. By comparison, at 10Gbps it takes 1.2µs to send a 1500 byte frame. The latency is created by  the overhead of sleep and wake transitions does not make this ideal for certain applications such as financial service servers.

 

Another drawback is the inability for EEE compliant PHYs to utilize power saving technology when linked with non-EEE compliant PHY. First generation silicon that supports EEE was released in 2010 and will be fully functional  with non-EEE compliant silicon, but in order for LPI to be utilized there must be EEE compliant infrastructure to support the technology.

 

Summary

Smart energy use has become a focal point of customer sentiment, international laws and regulations, and corporate environmental stewardship and will continue to be a focal point of continuous improvement. Energy Efficient Ethernet is another step in allowing Ethernet connected devices to be more energy efficient. This technology may not be appropriate for every application, but there is a clear economic, social and environmental benefit.

 

Bibliography

Merritt, R. (2008, 5 8). Energy-efficient Ethernet standard gains traction. Retrieved 4 4, 2011, from EE Times: http://www.eetimes.com

 

Note about Edit on 5/9/2011: Earlier I fixed a typo by adding an "e" changing "servic" to "service". I neglected to add a note about the change I made. I am adding this not now so no one has to spend a lot of time trying to figure out what changed.

Mark H

Comments

Filter Blog

By author: By date:
By tag: