You can never have too much battery life in portable devices. That's a given, but what about the wired networks? They also use too much power, and let's face it, power is money. Because so much of the data we use travels through Ethernet-based networks, we have the IEEE 802.3az standard for Energy Efficient Ethernet (EEE).
The concept behind EEE is simple: Go to low-power mode when not transmitting or receiving data. There's no need for the physical-layer (PHY) transmitters to be at full power when idle. Figure 1 shows the various PHY states specified in the 802.3az standard.
The states of Energy Efficient Ethernet are idle, sleep, refresh, and wake.
EEE uses a series of messages at the data-link layer where a PHY, when not in use for a specified time, tells other PHYs on the network that it's going into a low-power state. But as Figure 1 shows, it can't completely turn off. Instead, it partially turns off, but "wakes up" just long enough to send a refresh packet, telling other network devices of its condition, enough for the link to remain open. When data needs to flow, the transmitting PHY sends a wakeup message to the receiver to awaken it before data arrives.
Metro and long-haul networks multiplex traffic from network nodes for transport, then demultiplex traffic at the receiver. For example, 10Base-T links can be aggregated into 100Base-T links, which can be further aggregated into 1000Base-T links, and so on. If aggregation is managed properly, the active links will be aggregated into as few higher-speed links as possible, thereby letting some links go into low-power mode until needed. In Optimize Energy Efficient Ethernet (IEEE 802.3az) performance in bundled links, Pedro Reviriego and Juan Antonio Maestro describe the reasoning and the process of maximizing utilization and, therefore, efficiency.
Timing is important to ensure 802.3az PHYs meet specifications. That's because refresh packets must occur at specified intervals to keep a link open. Testing for compliance to the 802.3az standard involves timing measurement for each action in the protocol. To make the measurements, you need an oscilloscope and a test fixture (Figure 2). You can either capture the signals and perform post-processing with software, or you can use EEE oscilloscope software. The software can automate testing and provide formatted test results. Figure 3 shows a screen from the Keysight Technologies N5392B EEE test software, which supports testing for 10Base-T 100Base-T, and 1000Base-T Ethernet. The IEEE 802.3az standard also covers 10GBase-T and other copper-based Ethernet flavors.
A test fixture provides all connections needed to test Energy Efficient Ethernet, including test points for an oscilloscope.
An oscilloscope screen displaying an EEE refresh packet.
The fixture and software let you make compliance tests on the bench. For interoperability testing, a third-party test lab such as the University of New Hampshire Interoperability Lab has many compliant products.
— Martin Rowe, Senior Technical Editor