Bill's Blog on Automation

Data Latency Can Be Variable

A recent inquirer was concerned that his measurement of data latency in Ethernet switches yielded widely varying results. He measured a latency of 7 microseconds (µs) in one test then twice that in a second test. Another device had a measured latency of 125 µs in one instance and 40 µs when measured again. The switches functioned well in all cases, but the inquirer was mystified by the differences.

The situation was that he was unaware of the profound variation in frame size that he was encountering. Latency measures the delay of the switch but also the re-transmission time of the frame. This means the longer the frame, the greater the delay. If the traffic with which you are concerned contains regularly-sized frames, you should expect very little variation in the data latency. But with random-sized frames, you get random latency.