Energy-Efficient Ethernet (EEE) is a set of enhancements to the Ethernet family of computer networking standards that allow for less power consumption during periods of low data activity.  The intention is to reduce power consumption while retaining compatibility with existing equipment.  The Institute of Electrical and Electronics Engineers (IEEE), through the IEEE 802.3az task force developed the standard. 
To save energy, in 100 Mbit/s and faster Ethernet data links can be put into sleep mode when no data is being sent.  By sending a low-power-idle (LPI) indication signal for a specified time the transmit chips in the system can be turned off.  LPI is sent periodically to refresh the sleep mode.  When there is data to transmit a normal idle signal can be sent to wake the transmit system.  The data link layer is considered to be always operational, as the receive signal circuit remains active even when the transmit path is in sleep mode.