Deep Dive: Energy Efficient Ethernet

By Pallab Chatterjee
In late September, the IEEE ratified the 802.3az Energy Efficient Ethernet (EEE) specification. The standard, and associated test certification specification, was supported by co-development of over 20 commercial products from multiple vendors, of which 13 were release to market simultaneous with the ratification.

Wael Diab, from the office of the CTO at Broadcom, and vice-vhair of the 802.3 working group, indicated that the importance of the specification is high, which why there was a simultaneous release. The adoption of the product is in progress now.

For lower data rates (10Mb and below) there is a power-down mode for the Ethernet connection that is based on a “wake on LAN” methodology. For 100Mb, 1000Mb (1G), and 10Gb data rates, the original specification has the port, both transmit and receive, on at all times. As a result, there is a lot of wasted power dissipation in a product such as a multi-port switch.

In most cases, on a 24/7 schedule, any given port has significant periods of being idle. However, there is only an “on” state so the power usage does not change. The EEE specification identifies a tweak that can be made tot the PHY so that there is both a low-power idle state and deep low-power sleep state added. The worst-case “wake” time from these states is 5uS, so the overall switch throughput is not significantly upset. Advanced features of the spec include a “blank” mode where both the transmit and receive ends of the port connection are powered down, not just the receive side. The specification was made possible, as the packet based data traffic going through these high bandwidth connection is typically less than 10% of the time, with 90% idle.

Broadcom has created a series of products that implement the specification and the EEE parts are available as full stand alone controllers, and as IP in the form of a 10G PHY. All the products meet or exceed the 802.3az specification (http://standards.ieee.org/announcements/2010/8023az.html) . Their designs use the idle state to turn off the additional control logic subsystem for the port as well as the associated L2 Cache. To enable customers to get to market quickly, they made the PHY external from the MAC and created a signal control path between them. This allows for the use of the new PHY is an existing system, as it does not require the custom part containing the MAC from the original 802.3 specification to be respun. Developers can simply drop in the new EEE PHY part. The parts are designed to support 100M, 1000M and 10G applications.

The specification is smart in that it is application-independent. If the data transfers are standard short blocks—extended length video style blocks or new Advanced Format (AF) larger block size data—the power-save modes are the same. The PHY does not care about the application or the data size. The transmitter side of the port pairs controls whether the connections are in active, idle or sleep mode.

Based on using the PHY or new ICs in systems applications, the control logic has “knobs” that are able to support tweaking of the idle state. The control hardware supports this optimization and the resulting shifts in the power waveforms, in real time, so it can be tuned to make sure there is no change of state in the power-up and down sequence. The idle state control is set from a policy engine rather than a straight sense configuration.

In addition to Broadcom, Realtek, Intel and others are producing the product, and the end systems from providers like Netgear will be available starting this month.