IMS user to IMS user call flow

Voice in LTE network (VoLTE) is carried over an IP Multimedia Subsystem (IMS). Understanding IMS basics will understanding VoLTE signaling interactions.

A detailed IMS subscriber to IMS subscriber flow is presented here. The preconditions for this call flow are:

• The calling IMS subscriber is currently roaming outside the home network.
• The called user is registered in the home network.
• The caller and the called user require resource reservation before the call can be setup.

The major steps in the call flow are:

• IMS routing of the initial SIP INVITE.
• IMS routing of the first response to the INVITE.
• Resource allocation via PDP context activation.

IMS originating and IMS terminating call

The complex IMS signaling flow is analyzed with multiple flows that focus on different aspects of message interaction:

• IMS to IMS Call Flow
• IMS to IMS Call Flow Poster (11x17)
• IMS to IMS Call Flow Overview
• IMS to IMS call flow with focus on UE and Network Interactions
• IMS to IMS Call Flow that focuses on interactions between the UE, the visited and the home IMS networks.
• Origination UE Role Sequence Diagram
• Originating UE Role Collaboration Diagram
• Originating P-CSCF Role Sequence Diagram
• Originating P-CSCF Role Collaboration Diagram
• Originating S-CSCF Role Sequence Diagram
• Originating S-CSCF Role Collaboration Diagram
• Terminating I-CSCF Role Sequence Diagram
• Terminating I-CSCF Role Collaboration Diagram
• Terminating S-CSCF Role Sequence Diagram
• Terminating S-CSCF Role Collaboration Diagram
• Terminating P-CSCF Role Sequence Diagram
• Terminating P-CSCF Role Collaboration Diagram
• Terminating UE Role Sequence Diagram
• Terminating UE Role Collaboration Diagram