As part of the 802.az standard, Energy Efficient Ethernet (EEE) is targeted at saving energy in Ethernet networks for a select group of physical layer devices, or PHYs. The PHYs use EEE during idle periods to reduce power consumption. If you do not utilize EEE, the PHY is fully powered up even when there is no traffic being sent. Enabling EEE reduces significant power consumption on the switch. Within EXOS, a PHY/switch combination, or a PHY with autogrEEEN capability is provided to allow EEE to work. In a typical setup, the PHY and switch communicate when to enter or exit low power idle (LPI) mode.
AutoGrEEEn technology implements the EEE standard directly into PHYs, and enables the EEE mode when interfacing with non-EEE enabled MAC devices,. This allows you to make existing network equipment EEE-compliant by simply changing the PHY devices. EEE is currently only implemented for copper ports
Previously, most wireline communications protocols used continuous transmission, consuming power whether or not data was sent. EEE puts the PHY in an active mode only when real data is sent on the media. To save energy during gaps in the data stream, EEE uses a signaling protocol that allows a transmitter to indicate the data gap and to allow the link to go idle. This signaling protocol is also used to indicate that the link needs to resume after a pre-defined delay.
The EEE protocol uses an LPI signal that the transmitter sends to indicate that the link can go idle. After sending LPI for a period (Ts = time to sleep), the transmitter stops signaling altogether, and the link becomes quiescent. Periodically, the transmitter sends some signals so that the link does not remain quiescent for too long without a refresh. When the transmitter wishes to resume the fully functional link, it sends normal idle signals. After a pre-determined time (Tw = time to wake), the link is active and data transmission resumes.
The EEE protocol allows the link to be re-awakened at any time; there is no minimum or maximum sleep interval. This allows EEE to function effectively in the presence of unpredictable traffic. The default wake time is defined for each type of PHY, and is designed to be similar to the time taken to transmit a maximum length packet at the particular link speed.
The refresh signal serves the same purpose as the link pulse in traditional Ethernet. The heartbeat of the refresh signal ensures that both partners know that the link is present, and allows for immediate notification following a disconnection. The frequency of the refresh prevents any situation where one link partner is disconnected and another inserted without causing a link fail event. This maintains compatibility with security mechanisms that rely on continuous connectivity and require notification when a link is broken.
The maintenance of the link through refresh signals also allows higher layer applications to understand that the link is continuously present, preserving network stability. You can also use the refresh signal to test the channel, and create an opportunity for the receiver to adapt to changes in the channel characteristics. For high speed links, this is vital to support the rapid transition back to the full speed data transfer without sacrificing data integrity. The specific makeup of the refresh signal is designed for each PHY type to assist the adaptation for the medium supported.
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