Voice Call Continuity
This chapter is devoted to the Voice Call Continuity (VCC) feature in the IMS. VCC, which is specified in 3GPP TS 23.206  and 24.206 , allows smooth transitions of voice calls executed over the IMS and Circuit-Switched (CS) calls and vice versa. The feature allows a double IMS/CS terminal to initiate or receive a voice call in either the IMS or CS domain and move it to the other domain during the duration of the call.
VCC considers voice calls exclusively. At the time of writing, 3GPP is standardizing an enhanced version of VCC that includes other types of media streams different than audio. This is known as the Multimedia Session Continuity (MMSC), which it is not considered in this chapter.
VCC fills an important gap in the transition of CS to IMS. Consider a user that is accessing the IMS over a narrow bandwidth packet data channel. For example, this narrow bandwidth channel can be a regular GSM cellular access over GPRS. The GPRS connectivity is used for signaling and, thus, for SIP signaling towards the IMS. Voice over IP (e.g., RTP packets) will not have the appropriate quality of service, mostly due to the delays and lack of bandwidth. However, CS connections are available, and working reasonably well. So, with the VCC feature enabled, the user can establish audio communications that use a regular CS bearer. The session is controlled with SIP in the IMS. Assume now that, at a later time, the user initiates another packet data IP-CAN ...