If you want to humble yourself as a VoLTE engineer at a point in time when you think you have more or less understood how VoLTE works, make a trace of a VoLTE conference call with a few participants and try to analyze that. You get back down to earth pretty quickly as complexity of conference calls in IMS VoLTE is staggering. But don’t despair, here are two tips how to figure this out:
The first tip is to head over to Karel Berkovec’s blog and have a look at his post on VoLTE conference calls he’s recently published. Excellent work, thanks very much for this, I’ve bookmarked the post for whenever I need a refresher.
And my second tip is to make the SIP Call-ID parameter a column in Wireshark and then colorize the trace based on the different Call-IDs. This can be done by right-clicking on a Call-ID, e.g. in a SIP Invite message and selecting ‘Colorize with Filter’. Then assign a color to this particular Call-ID. Repeat this for each call leg and suddenly it becomes clear which SIP message goes to which destination and to the conference bridge. Without colors it’s more than likely that you are totally lost when looking at a trace.