VoIP delta spikes below 20ms, causing the jitter to change

I am trying to do some measurements on VoIP. I am using OpenSIPS, RTPProxy, and SIPp for testing. Everything works fine as expected, but I only have a question regarding the delta time.

Below is a screenshot I got from Wireshark RTP streams' analysis.

Why do I have these spikes below the 20ms? I am using in a SIPp xml scenario, where 8kulaw has the following characteristics:

8kulaw.wav: RIFF (little-endian) data, WAVE audio, ITU G.711 mu-law, mono 8000 Hz

Much appreciated!

The "RTP Stream Analysis" from wireshark is giving you hints on the quality of the stream.

Your Max Delta value is 20.15 and occurs at packet 2008.

This will indicate the time between 2 packets which in your use-case are supposed to be spaced by exactly 20ms.

So the maximum difference is very short and should definitly not affect the quality of the stream. Usually, this is used on receiver (for incoming stream): on sender, there is usually no internal latency. This probably explains why you have so short "Max Delta".

The spikes you see are pretty big, but this is mostly because the scale is very short. Not because the stream is bad.