8.2.6.18 Energy-efficient Ethernet Support

IEEE 802.3az adds support for energy efficiency to Ethernet. These are the key features of 802.3az:

  • Allows a system’s transmit path to enter a Low-Power mode if there is nothing to transmit.
  • Allows a PHY to detect whether its link partner’s transmit path is in Low-Power mode, therefore allowing the system’s receive path to enter Low-Power mode.
  • Link remains up during lower power mode and no frames are dropped.
  • Asymmetric, one direction can be in Low-Power mode while the other is transmitting normally.
  • LPI (Low Power Idle) signaling is used to control entry and exit to and from Low-Power modes.
  • LPI signaling can only take place if both sides have indicated support for it through auto-negotiation.

These are the key features of 802.3az operation:

  • Low-power control is done at the MII (reconciliation sublayer).
  • As an architectural convenience in writing the 802.3az it is assumed that transmission is deferred by asserting carrier sense, in practice it will not be done this way. This system will know when it has nothing to transmit and only enter Low-Power mode when it is not transmitting.
  • LPI should not be requested unless the link has been up for at least one second.
  • LPI is signaled on the transmit path by asserting 0x01 on txd with tx_en low and tx_er high.
  • A PHY on seeing LPI requested on the MII will send the sleep signal before going quiet. After going quiet it will periodically transmit refresh signals.
  • The sleep, quiet and refresh periods are defined in Table 78-2 of 802.3az. For 1000BASE-X the sleep period is 20 microseconds, the quiet period 2.5 milliseconds and the refresh period 20 microseconds.
  • 1000BASE-X is required to go quiet after sleep is signaled. The easiest way to do this is to write to a control register to disable transmit in the SerDes.
  • LPI mode ends by transmitting normal idle for the wake time. There is a default time for this but it can be adjusted in software using the Link Layer Discovery Protocol (LLDP) described in Clause 79 of 802.3az.
  • LPI is indicated at the receive side when sleep and refresh signaling has been detected.