The IMS: IP Multimedia Concepts And Services, Second Edition

3.14. Connectivity between traditional CS users and IMS users

For the time being, most users utilize traditional CS UE: that is, fixed line telephones and all kinds of cellular terminals. Therefore, it is desirable for the IMS to interwork with legacy CS networks to support basic voice calls between IMS users and CS network users. This requires interworking both at the user plane and the control plane because the used protocols are different in both planes. Control-plane interworking is tasked to the MGCF. It performs mapping from SIP signalling to the BICC or ISUP used in CS legacy networks, and vice versa. The IMS Media Gateway (IMSMGW), in turn, translates protocols at the user plane. It terminates the bearer channels from the CS (PSTN/ISDN/GSM) networks as well as media streams from IP or ATM-based PS networks and provides the translation between these terminations. Additional functions, such as codec interworking, echo cancellation and continuity check, can also be provided. The terminations are controlled by the MGCF. Network configurations for handling both IMS and CS-originated calls are explained next.

3.14.1. IMS-originated session toward a user in the CS core network

When an IMS user initiates a session she does not need to bother about whether the called user is an IMS user or a CS user. She simply makes a call and the IMS takes care of finding the called party. The session request from the calling user will always arrive at the S-CSCF serving the calling user, based ...