Multimedia in VoLTE

It’s very interesting (and well, a bit suspicious) that the main focus of most VoLTE textbooks and trainings is signalling. But from the user-point-of-view, it is the voice data, what matters. As an end-subscriber I don’t care about signalling. My only interest is the call quality. But times they are a changin and engineers are asking about how to improve the overall voice-call quality and user experience. Today we’ll go through the basics as jitter, mouth-to-ear delay, packet loss rate or MOS, needed for QoS analysis.

For real-time multimedia we used to have dedicated telephone/radio networks. That has changed and voice/video streams are transported over IP network now.

We should understand that these IP networks were originally designed for data transport. To transport data we prefer the best-effort service model, which allows an easy network scaling and simple routers’ logic. On the other hand we don’t care much if packets arrive in-order or what are the delays between particular packets. We simply wait until we receive a whole file. If any packet is lost, TCP will re-transmit it.

Packets in Data Networks

It’s a different story with the real-time communication services though. RTC applications are less sensitive to packet loss, but they are very sensitive to packet delay. Usage of IP data network as a carrier brings a lot of challenges which have to be addressed by media protocols and network elements.

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VoLTE – close encounters

If you know a bit about SIP and IMS you are probably aware that they are quite flexible. There are many ways how to do some solution and different vendors can easily come up with different implementation for the same thing.

In trainings people are asking very often – is this SIP message mandatory, what will happen if our client will not support this feature, what header attribute is the right one to identify xyz?

Well we have a huge number of specifications (from different organizations and bodies as 3GPP, IETF, OMA, GSMA, …). To avoid the confusion for VoLTE service GSMA created (yes another) document called VoLTE Service Description and Implementation Guide. This is not a classical spec, rather a guide what are the exact call flows and what message fields are relevant for the VoLTE.

We already know the registration from other posts (Registration, How to read tcpdump – RegistrationThird party registration, Sh, ..). In the VoLTE the flow looks like this:

VoLTE registration

VoLTE registration

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