IP Multimedia Subsystem is the new IP based signaling system for setting up multimedia sessions. We have already covered the call flow for an IMS subscriber calling another IMS subscriber. Here we will look at the call flow of a regular PSTN subscriber calling an IMS user.
PSTN to IMS call flow
This call flow covers the handling of a CS network originated call with ISUP. In the diagram the MGCF requests seizure of the IM CN subsystem side termination and CS network side bearer termination. When the MGCF receives an answer indication, it requests the IM-MGW to both-way through-connect the terminations. Originating and terminating end initiated call releases are also covered.
The following sequence is covered:
- ISUP IAM Handling and Initial IM-MGW and MGCF (Mn) Interactions
- Initial Handshake between MGCF and IMS CSCF Servers
- Mn Interactions for Codec selection
- ISUP ACM related interactions on Mn interface
- IMS Answer to ISUP ANM Handling
- Conversation Mode
- Call Release:
- Calling PSTN Subscriber Initiated Call Release
- Called Subsciber Initiates Call Release