Green Ethernet
HMG-838PT & HMG-838EPT Web Configuration 3-2
Port Power Savings Configuration:
Optimize EEE for: Enables/disables the EEE function for this switch. The two options are:
Power: The EEE function is enabled. This is the default setting.
Legacy: EEE is not enabled.
Port Configuration:
ActiPHY™: ActiPHY™ works by lowering the power for a port when there is no link. The port is power up for short
moment in order to determine if an Ethernet cable is inserted. For ports with no cable connection, the PHY
remains powered down to save energy.
PerfectReach™: PerfectReach™ is another power saving mechanism. PerfectReach™ works by determining the
cable length and lowering the Ethernet transmit power for ports with short cables.
EEE (Energy-Efficient Ethernet): EEE is a power saving option that reduces the power usage when there is low
or no traffic utilization. EEE was developed through the IEEE802.3az task force of the Institute of Electrical and
Electronic Engineers (IEEE). EEE works by powering down circuits when there is no traffic. When a port gets data
to be transmitted all circuits are powered up. The time it takes to power up the circuits is called wakeup time. The
default wakeup time is 30 us for 100Mbit links. EEE devices must agree upon the value of the wakeup time in
order to make sure that both the receiving and transmitting device has all circuits powered up when traffic is
transmitted. The devices can exchange wakeup time information using the LLDP (Link Layer Discovery Protocol)
protocol. EEE works for ports in auto-negotiation mode, where the port is negotiated to either 10 or 100 Mbit full
duplex modes. For ports that are not EEE-capable the corresponding EEE checkboxes are grayed out and thus
impossible to enable EEE for.
When a port is powered down for saving power, outgoing traffic is stored in a buffer until the port is powered up
again. Because there are some overhead in turning the port down and up, more power can be saved if the traffic
can be buffered up until a large burst of traffic can be transmitted. Buffering traffic will give some latency in the
traffic. For traffic that should not be held back, urgent queues may be assigned to reduce latency yet still result in
overall power saving.
EEE Urgent Queues: It is possible to minimize the latency for specific frames, by mapping the frames to a
specific queue (done with QOS), and then mark the queue as an urgent queue. When an urgent queue gets data
to be transmitted, the circuits will be powered up at once and the latency will be reduced to the wakeup time.
Queues set will activate transmission of frames as soon as data is available. Otherwise the queue will postpone
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