Energy Efficient Ethernet (EEE)
In the office where we work, the meeting rooms have a smart feature. The lights turn on whenever someone walks in. When there is no activity for a certain duration, the lights get turned off. Saving electricity. Our planet has finite resources. This may be a tiny effort, but the impact becomes significant at a larger scale. If thousands of rooms save electricity, the saving is big. If all rooms in a nation have this smart feature, the saving is colossal. But to have the benefit, we need to have the smart feature built as a standard.
These days we are working on an ethernet product. Ethernet is the technology used to connect the devices via physical cables. The physical cables carry the signals from one device to the other. Voltage pulses are required for this transmission. The data flows in the cables only when the connected devices are sending and receiving. When there is no data flowing, the cables are idle. During the no transmission periods, some power consumption happens due to the IDLE synchronization signals being exchanged. Is there a way to reduce the power consumption?
"Just like smart lighting systems conserve electricity in buildings, having Energy Efficient Ethernet reduces power consumption in the computer networks."
Energy Efficient Ethernet (EEE)
The official answer is IEEE 802.3az, commonly known as Energy Efficient Ethernet (EEE).
With EEE, the connected devices stay in the Low Power Idle (LPI) mode during the non-transmission periods. This reduces the power usage. Here is how EEE works.
When two devices start talking to each other (the link up), EEE is negotiated via auto-negotiation. Auto-negotiation is the exchange of information about the capabilities of the devices.
During data transmission, the PHY (Physical Layer transceiver) circuitry is fully powered.
During the idle periods, traditional Ethernet keeps transmitting "idle signals" even when no data is sent, which involves power consumption.
Low Power Idle (LPI)
When no data needs to be sent, the device sends an LPI indication signal to its link partner. If both devices have nothing to send, they both agree to enter the low-power mode. The PHY enters a quiet state, where most circuitry powers down and only minimal logic remains active. This is the Low Power Idle (LPI) mode which reduces power consumption. LPI is per link, and not global.
To ensure the link stays alive, the device periodically sends refresh signals, called "refresh bursts". This prevents loss of synchronization between the devices.
When some data needs to be transmitted, the sender transmits a wake signal. The PHY powers back up and data transmission resumes. Different PHYs have different wake times. The wakeup time is typically a few microseconds, so impact on latency is minimal.
Where EEE is Useful
Many modern networking devices have the EEE functionality.
Having EEE is most useful in:
- Data centers
- Enterprise networks
- Access switches with many idle ports
Where EEE Should Be Avoided
Having EEE should be avoided with systems requiring ultra-low latency. The PHY wake up time introduces latency of a few microseconds. This is not ideal for real-time systems and High-Frequency Trading (HFT) systems.
Having EEE is not good at places where poll mode driver or Data Plane Development Kit (DPDK) is implemented. The DPDK has no issues with EEE, but the DPDK applications which expect extremely low latency are not in favor of EEE.
Having EEE may cause issues if PTP (Precision Time Protocol) is used.
Conclusion
While the power saving per link may be small, the cumulative impact across data centers and enterprise networks is significant. As networks continue to grow, such efficiency mechanisms play an important role in sustainable infrastructure design.
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