Energy Efficient Ethernet according
to IEEE 802.3az
A system’s transmit path can enter a
low power mode if there is nothing to transmit.
A PHY can detect whether its link
partner’s transmit path is in low power mode, and configure its own 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.
Note: LPI signaling can only take place
if both sides have indicated support for it through auto-negotiation.
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 MII 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 emit refresh signals.
The sleep, quiet and refresh periods are
defined in 802.3az, Table 78-2.
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 802.3az, Clause 79.
LPI is indicated at the receive side when sleep and refresh signaling has been
detected.