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The lowdown on powering Ethernet (Part 1)

30 Jun 2015  | Jones Mike

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Analysing power consumption in Ethernet circuitry reveals it to be far from efficient. This is in part because Ethernet consumes similar energy during both traffic and idle periods, and in fact, idle periods typically account for more than 97 per cent of the time. This was key to determining where improvements could be made to reduce power consumption, when investigated by an IEEE task force; resulting in the standardisation of IEEE 802.3az, or Energy Efficient Ethernet.

Energy Efficient Ethernet shows great promise to universally succeed where earlier attempts to reduce idle period power have somewhat failed with methods such as Wake-on-LAN. Complimenting Energy Efficient Ethernet, additional power savings can also be made both during normal traffic and link down. This paper outlines where the current is consumed and how to design for the lowest power consumption, both in operation (Part 1) and standby (Part 2), since calculating the power consumption of an Ethernet circuit is not always straightforward.

The first step towards achieving low power Ethernet designs is understanding where the power is dissipated. In any Ethernet device, the major power dissipation is from the PHY transceiver (figure 1).


Figure 1: Power dissipation from the PHY transceiver: power is often consumed both internally to the PHY and externally in the transformer.


In the case where Ethernet datasheets publish the device only current consumption, calculating the total circuit current consumption requires the designer to add typically around 40mA per 100Base-TX or 70mA per 10Base-T PHY for dissipation in the transformers. As a result, a lower device only consumption at 10Base-T will rarely equate to lower total circuit current consumption, relative to 100Base-TX mode. Micrel's latest PHY technology employs voltage mode techniques which not only reduces power consumption typically by 50% but dissipates all current within the PHY itself; resulting in the total power consumption = device power consumption (no dissipation externally in transformer).

A designer must consider the following two modes: Normal Operation and Standby, when trying to further reduce power consumption.


Ethernet power consumption (normal operation)
So, what is the definition of normal operation for an Ethernet network? Is it 100 per cent utilisation, 50 per cent utilisation, or 10 per cent utilisation? The reality is long quiet periods followed by relatively short bursts of traffic (figure 2). During these quiet periods, Ethernet power consumption might be expected to significantly drop, however, this turns out to not necessarily be the case.


Figure 2: Ethernet Network Utilisation—Typical.


Idle period
1000Base-TX and 100Base-TX are both designed so that the link partners are continually synchronised to each other. To enable this, when no traffic is being transmitted the PHY will automatically send out IDLE symbols (11111 5B code). As a consequence, during any quiet period the PHY transmitter is still operating in a manner similar to full traffic – meaning it consumes a similar amount of power.

It is strongly advisable with multi-port Ethernet devices, to disable any unused port (PHY), since simply by connecting to a link partner, around 40mA current is consumed even with no traffic present. The port can usually be disabled via the internal register map (Register 0h bit 11 of the IEEE Defined PHY Registers) and will typically save a further 15-20mA of device current.

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