The latency test measures the time required for a frame to travel from the originating device through the network to the destination device (also known as end-to-end testing). This test can also be configured to measure the round-trip time; i.e., the time required for a frame to travel from the originating device to the destination device and then back to the originating device.
When the latency time varies from frame to frame, it causes issues with real-time services. For example, latency variation in VoIP applications would degrade the voice quality and create pops or clicks on the line. Long latency can also degrade Ethernet service quality. In client-server applications, the server might time out or poor application performance can occur. For VoIP, this would translate into long delays in the conversation, producing a “satellite call feeling”.
The test procedure begins by measuring and benchmarking the throughput for each frame size to ensure the frames are transmitted without being discarded (i.e., the throughout test). This fills all device buffers, therefore measuring latency in the worst conditions. The second step is for the test instrument to send traffic for 120 seconds. At mid-point in the transmission, a frame must be tagged with a time-stamp and when it is received back at the test instrument, the latency is measured. The transmission should continue for the rest of the time period. This measurement must be taken 20 times for each frame size, and the results should be reported as an average.