Ethernet is the most widely used networking interface in the world; with virtually all network traffic passing over multiple Ethernet links. However, the majority of Ethernet links spend significant time waiting for data packets. Worse, some links, like traditional 1000BASE-T Ethernet links, consume power at near full active levels because of clock synchronization requirements during those idle periods. Indeed, the 2010 ACEEE Summer Study on Energy Efficiency in Buildings published by Lawrence Berkeley National Laboratory estimated that network devices and network interfaces account for over 10% of total IT power usage. Energy Efficient Ethernet (EEE) provides a mechanism and a standard for reducing this energy usage without impacting the vital function that these network interfaces perform in communication infrastructure.
The EEE project (IEEE 802.3az) was developed by the Institute of Electrical and Electronics Engineers (IEEE) and the initial version was published in November 2010. This version targets mainstream "BASE-T" interfaces (i.e. 10BASE-T; 100BASE-TX; 1000BASE-T; and 10GBASE-T) that operate over twisted pair copper wiring and Backplane Ethernet. Today, Vitesse offers a broad line of 10T/100TX/1000BASE-T copper PHY cores fully compliant to the EEE standard, including newly introduced 10BASE-TE.
Features of IEEE Efficient Ethernet project (IEEE 802.3az)
Backwards compatible, the new standard can be deployed in networks with the appropriate legacy interfaces and protocols. Thus, a copper PHY core supporting EEE can seamlessly support the broad range of applications already deployed on these networks. However, it was accepted that interfaces complying with the new standard might not save energy when connecting with older devices, as long as the existing functions were fully supported. As a result, this allows incremental network upgrades to increasingly benefit from EEE as the proportion of EEE equipment increases.
The standard also recognizes that some network applications may allow larger amounts of traffic disturbance and includes a negotiation mechanism to take advantage of such environments and increase the depth of energy savings.
The standard for EEE defines the signaling necessary for energy savings during periods where no data is sent on the interface, but does not define how the energy is saved, nor mandate a level of savings. This approach allows for a staged rollout of systems with minimal changes and which are compatible with future developments that extend the energy savings.
An EEE PHY can save energy during idle periods when data is not being transmitted. PHYs typically consume between 20 to 40 percent of the system power, and the static design methods allow savings of up to 50 percent of the PHY power. Therefore the expected system-level savings may be in the range of five to 20 percent.
Low Power Idle
EEE puts the PHY in an active mode only when real data is being sent on the media. Most wireline communications protocols developed since the 1990s have used continuous transmission, consuming power whether or not data was sent. The reasoning behind this was that the link should be maintained with full bandwidth signaling to be ready to support data transmission at all times. In order to save energy during gaps in the data stream, EEE uses a signaling protocol that allows a transmitter to indicate the data gap and allow the link to go idle. The signaling protocol is also used to indicate that the link needs to resume after a pre-defined delay.
The EEE protocol uses a signal, termed low power idle (LPI), that is a modification of the normal idle transmitted between data packets. The transmitter sends LPI in place of idle to indicate that the link can go to sleep. After sending LPI for a period (Ts = time to sleep), the transmitter can stop signaling altogether, so that the link becomes quiescent. Periodically, the transmitter sends some signals, so that the link does not remain quiescent for too long without a refresh. Finally, 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 can resume.
Figure 1 below describes the different EEE states.
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 generally aimed to be similar to the time taken to transmit a maximum length packet at the particular link speed. For example, the wake time for 1000BASE-T is 16.5μS, roughly the same time that it takes to transmit a 2000 byte Ethernet frame.
The refresh signal that is sent periodically while the link is idle is important for multiple reasons. First, it serves the same purpose as the link pulse in traditional Ethernet. The heartbeat of the refresh signal helps ensure that both partners know that the link is present and allows for immediate notification following a disconnection. The frequency of the refresh, which is typically greater than 100Hz, prevents any situation where one link partner can be 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. Changing the power level must not cause connectivity interruptions that would result in link flap, network reconfiguration, or client association changes.
Second, the refresh signal can be used 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.
Vitesse's EcoEthernet, Energy Effcient Solutions for Ethernet Electronics
Vitesse's EcoEthernetTM 2.0 is the latest generation of its award-winning energy saving technologies, delivering unprecedented energy-efficiency for Ethernet networks. These features include: ActiPHY automatic link-power down; PerfectReach intelligent cable algorithm; IEEE 802.3az idle power savings; temperature monitoring; smart fan control; and adjustable LED brightness. The first three are mandated in the Energy Star's Small Networking Equipment recommendation guidelines and are available in all 65nm process and below 10/100/1000BASE-T copper PHY IP cores.
Vitesse's power efficient IP cores optimize performance for the green automotive, consumer electronics, broadband access, network security, printer, smart grid, storage, and other applications. Coupled with the cost and performance gains of 65-nm CMOS or more advanced process technologies, the IP cores are a competitive differentiator for Vitesse's IP licensees.
Javad Lavasani, Sr. MTS for Vitesse's line of Gigabit Ethernet PHY devices, has more than twenty five years of experience in the telecommunications and semiconductor industries. A senior member and regular contributor to IEEE, Mr. Lavasani holds a Bachelor of Science in Physics from Tehran University and a Master of Engineering in Microwave communications from Leeds University in United Kingdom.