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Energy-efficient Ethernet: A greener choice for 2010

Sanjay Kasturia | March 4, 2010
Data center managers and equipment vendors looking for greener alternatives will begin to benefit this year from a major initiative aimed at reducing the power consumed by Ethernet equipment.

This capability is reminiscent of the Wake-on-LAN standard (which defines magic packets that can be sent to remotely wake up a computer in a sleep mode), however, EEE signaling has much less latency, on the order of 10 microseconds.

Low power idle mode

LAN links generally average less than 10 per cent utilization and even at peak times, the utilization does not reach 100 per cent. When data is not being sent, the lower-speed PHYs (10Mbps and 100Mbps) would stop transmitting and hence would automatically consume less power. However, the higher-speed PHYs (1000BASE-T and 10GBASE-T), the ones that are relevant to data centers, continue to transmit actively when there is no data to send, and thereby continue to consume power when idle.

For the lower speed standards, the new EEE standard (IEEE 802.3az) specifies the transport of LPI signaling from the system on one side of the link to the system on the other side and provides only small incremental savings in the PHY power itself, but it enables rapid adjustments in power modes of the connected devices.

For 1000BASE-T and 10GBASE-T transceivers new LPI modes have been defined. Key features are:

1. They allow powering down the transmitters and three of the four receivers in a link when there is no data to send.

2. They include a refresh cycle that requires transmission of short training sequences in LPI mode so the PHY parameters (clock tracking at the slave, receiver equalizer coefficients, echo canceller coefficients, crosstalk canceller coefficients, etc.) can be updated and kept current.

3. They include the definition of an alert signal that can be used to rapidly wake up a PHY from sleep in the LPI mode.

4. They can be initiated either from the local system by signaling from the MAC or station management or from the remote system over the PHY link.

Because of these features, the LPI-to-active state transition can be made in less than 0.001 per cent of the time it takes for the initial link-up of the PHY. During the sleep-to-wake transition, EEE requires that data transmission be held off for the PHY wake time so no data is lost.

The EEE standard also provides a mechanism to hold off transmission for longer periods than the minimum specified to allow for extended system wake times, and this can be coordinated across the network using the IEEE802.1AB protocol. This allows much finer-grained and more flexible cycling between normal and LPI modes. This capability will be a powerful tool for power savings in future data centers.

The EEE standard has been through three cycles of working group balloting and has maintained an approval rating greater than 80%. It is slated to get approval to go to sponsor ballot next month. Full ratification is anticipated before year-end.


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