What Is Energy-Efficient Ethernet(EEE)?

Energy-Efficient Ethernet (EEE) is a set of enhancements to the twisted-pair and backplane Ethernet family of computer networking standards that reduce power consumption during periods of low data activity. The intention is to reduce power consumption by 50% or more, while retaining full compatibility with existing equipment.

The Institute of Electrical and Electronics Engineers (IEEE), through the IEEE 802.3az task force, developed the standard. The first study group had its call for interest in November 2006, and the official standards task force was authorized in May 2007. The IEEE ratified the final standard in September 2010. Some companies introduced technology to reduce the power required for Ethernet before the standard was ratified, using the name Green Ethernet.

Some energy-efficient switch integrated circuits were developed before the IEEE 802.3az Energy-Efficient Ethernet standard was finalized.

Energy-Efficient Ethernet Overview

  • Energy Efficient Ethernet (EEE) is a method to reduce energy used by an Ethernet device during periods of low link utilization
  • Specified in IEEE 802.3az-2010™
  • The premise for EEE is that Ethernet links have idle time and thus opportunity to save energy
  • Specified for copper interfaces
    • “BASE-T’s’
    • Backplane (except 40G)
  • The method is called Low Power Idle (LPI)

Energy-Efficient Ethernet Concepts

The power reduction is accomplished in a few ways. In Fast EthernetGigabit Ethernet and 10 Gigabit Ethernet links, constant and significant energy is used by the physical layer as transmitters are active regardless of whether data is being sent. If they could be put into sleep mode when no data is being sent, that energy could be saved. When the controlling software or firmware decides that no data needs to be sent, it can issue a low-power idle (LPI) request to the Ethernet controller physical layer PHY. The PHY will then send LPI symbols for a specified time onto the link, and then disable its transmitter. Refresh signals are sent periodically to maintain link signaling integrity. When there is data to transmit, a normal IDLE signal is sent for a predetermined period of time. The data link is considered to be always operational, as the receive signal circuit remains active even when the transmit path is in sleep mode.

What is Low Power Idle(LPI)?

  • Concept: Transmit data as fast as possible, return to Low-Power Idle
  • Saves energy by cycling between Active and Low Power Idle
    • Power reduced by turning off unused circuits during LPI
    • Energy use scales with bandwidth utilization

LPI Overview

LPI Overview
  • LPI – PHY non-essential circuits shut down during idle periods
  • During power-down, maintain coefficients and sync to allow rapid return to Active state
  • Wake times (Tw_PHY ) for Twisted-Pair PHYs:
    • 100BASE-TX: <= 30 usec
    • 1000BASE-T: <= 16.5 usec
    • 10GBASE-T: <= ~8 usec (2 modes)

802.3az EEE Overview

802.3az EEE is designed to save power when there is no traffic on the link. In Green Ethernet, power is reduced when the port is down. With 802.3az EEE, power is reduced when the port is up, but there is no traffic on it.

802.3az EEE is not supported on the Out Of Band port.

The remote link partner status can be displayed only when the link speed is 1G or 10G.

When using 802.3az EEE, systems on both sides of the link can disable portions of their functionality and save power during periods of no traffic.

802.3az EEE supports IEEE 802.3 MAC operation at 100 Mbps and 1000 Mbps:

LLDP is used to select the optimal set of parameters for both devices. If LLDP is not supported by the link partner, or is disabled, 802.3az EEE still be operational, but it might not be in the optimal operational mode.

The 802.3az EEE feature is implemented using a port mode called Low Power Idle (LPI) mode. When there is no traffic and this feature is enabled on the port, the port is placed in the LPI mode, which reduces power consumption dramatically.

Both sides of a connection (device port and connecting device) must support 802.3az EEE for it to work. When traffic is absent, both sides send signals indicating that power is about to be reduced. When signals from both sides are received, the Keep Alive signal indicates that the ports are in LPI status (and not in Down status), and power is reduced.

For ports to stay in LPI mode, the Keep Alive signal must be received continuously from both sides.


An Overview of Energy Efficient Ethernet


  1. Energy-Efficient Ethernet -Wikepedia
  2. 802.3az EEE Overview-Cisco
  3. An Overview of Energy-Efficient Ethernet 1 – ieee802.org
  4. An Overview of Energy-Efficient Ethernet 2 – ieee802.org
  5. IEEE 802.3az Energy Efficient Ethernet – ieee802.org
  6. How to Use Energy Efficient Ethernet (IEEE 802.3az) With Texas Instruments Ethernet PHYs