THE IMS: IP Multimedia Concepts and Services, 3rd Edition

13

An example IMS Voice Call Continuity Procedures

13.1 Overview

This chapter provides a detailed description of how a specific voice call continuity (VCC) example works. It handles anchoring of CS and IMS calls as well as domain transfers from CS to IMS domains and vice versa. VCC is generally described in Section 3.20.

VCC allows circuit switched voice calls to be transferred to IMS multimedia telephony sessions and vice versa. CS and IMS are seen as two different communication domains, therefore the procedure of handing over a call from one domain to another is called domain transfer.

In order to allow domain transfer, every call that is set up by the user in either CS or IMS domain, will be checked whether it could be subject to such a domain transfer. If the call could be subject to domain transfer, it will be routed to the VCC Application Server (VCC-AS), which then anchors this call. As the VCC-AS is located in the IMS domain this means, that all CS calls that can be subject to domain transfer, need to be intworked into IMS.

In this example we will look at the following scenario:

• Tobias dials the telephone number of his sister Theresa in order to set up a CS voice call (Section 13.3.1);
• during the routing of the call, the CS domain of Tobias's network is made aware that the call can be subject to VCC domain transfer, therefore the call gets interworked at the MGCF into Tobiases IMS domain (Section 13.3.3);
• the call gets anchored at the VCC-AS within Tobiases IMS domain ...