telecom • networking • design

an EventHelix.com blog


Leave a comment

Voice Over LTE (VoLTE) Originating Call

Voice over LTE (VoLTE) is the standard for voice call setup in LTE networks. When VoLTE is deployed, phones will not need to fallback to 3G for voice calls.

VoLTE uses IMS SIP signaling to set up voice calls. The following VoLTE call flow describes the IMS call setup and release. An example of sending an SMS over IMS is also included. Sample RTP and RTCP messages are also shown in the flow.
Voice over LTE Call Flow

Advertisements


Leave a comment

IMS subscriber to PSTN subscriber call flow

IP Multimedia Subsystem is an IP based signaling system for setting up and tearing down multimedia sessions. SIP based signaling is used to setup these sessions.

We have covered call flows for an IMS to IMS and PSTN to IMS calls. We now look at the call flow for a IMS to PSTN subscriber call.

The call is routed via the BGCF (Border Gateway Control Function) to the MGCF (Media Gateway Control Function). The MGCF uses one context with two terminations in IM-MGW (Media Gateway). The termination RTP1 is used towards IMS Core network subsystem entity and the bearer termination TDM1 is used for bearer towards PSTN CS network element.

The call flow is complex and it is analyzed with multiple diagrams. Some of the diagrams are presented here:

IMS to PSTN sequence diagram

A detailed call flow describing all message interactions in a IMS to PSTN call

IMS to PSTN high level flow

A high level view that abstracts out individual component details and just presents the flow between the UE, IMS Core and the PSTN.

IMS to PSTN UE collaboration diagram

Here we examine the call flow from the UE point of view. The call flow is represented as a collaboration diagram.

Link: IMS subscriber to PSTN subscriber call flow

All documents: