Measuring Ethernet Latency After Energy Efficient Idle

An Ethernet interface may deliver steady traffic with consistent timing yet add delay to the first packet after an idle interval. Energy Efficient Ethernet can change the operating state between bursts. Evaluate that transition separately from ordinary forwarding latency before using a continuous traffic measurement as the complete timing budget.

Identify the actual low power mode

The Microchip KSZ8441 documentation describes low power idle, refresh, and wake behavior associated with Energy Efficient Ethernet. It distinguishes that mechanism from simply removing power from a PHY or waiting for an entirely new link to establish.

Record EEE capability, advertisement, negotiated state, and the selected speed at both ends. An enabled software option does not establish that the partners actually entered the expected low power state.

Define the timing reference points

Choose where packet latency begins and ends, such as hardware timestamps at defined ingress and egress points. A software timestamp can include scheduling and queue delays that are unrelated to PHY wake behavior.

Measure a steady burst first, then introduce controlled idle intervals before the next packet. Keep packet length and offered traffic otherwise unchanged. Compare the first packet with later packets in the same burst.

Vary the idle and traffic pattern

Sweep idle duration across the range relevant to the application. Very short gaps may not trigger the same state as a long quiet period. Include traffic in each direction because transmit and receive behavior may be managed independently.

Check for packet coalescing or buffering in the MAC, switch, and host. Those functions can add delay that resembles wake latency. Use device status or supported counters to correlate the measurement with the intended low power transition.

Compare configurations deliberately

Repeat with EEE enabled and disabled using supported settings, while leaving other configuration unchanged. Record power as well as timing if the purpose is to choose an operating mode. A lower idle power result and a tighter first packet deadline may represent a real application tradeoff.

Include the actual link partners and operating modes used in the product rather than assuming every switch implements the same buffering policy.

Save timestamp boundaries, idle pattern, negotiated state, packet latency distribution, and power conditions. Define acceptance for the first packet after idle separately where necessary. A successful timing result explains the observed state transition and its system impact; a high sustained throughput figure does not establish the latency of a sparse control message arriving after the interface has been quiet.