Latency

VoIP latency is a metric of the difference in time between when one caller speaks and when the other caller hears what the first has said. Excessive network latency can cause noticeable gaps and synchronization loss in transmitted conversations, particularly when VoIP is used with other types of data, as in a videoconference. If these gaps become large enough, callers may find that they will inadvertently interrupt each other while conversing.

IP SLA tests measure latency by sequentially applying four different timestamps to a single test packet, as follows:

1. Timestamp T1 is applied to a test packet as it leaves the source router.

2. Timestamp T2 is applied as the test packet arrives at the target router.

3. Timestamp T3 is applied as the test packet leaves the target router to return to the source.

4. Timestamp T4 is applied when the test packet returns to the source.

IP SLA tests then provide four separate measures of latency by computing differences among the four timestamps, as follows.

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