Chapter 27 - Voice Call Continuity pot

Chapter 27

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 [27] and 24.206 [26], 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 access with higher bandwidth, perhaps Wireless LAN,

or perhaps cellular High Speed Packet Access. He can then move the CS audio stream over

the packet data IP-CAN, and perhaps enrich the session with video or some other data stream.

The goal of VCC is thus: maintain existing audio calls at any cost when the user moves

between areas with different connectivity characteristics. This is achieved by handing the

audio stream from CS to IMS or vice versa. The feature can be implemented transparently

from the point of view of the user. However, the feature requires VCC-enabled dual-mode

IMS/CS terminals and network support.

Because of the interaction between IMS and CS networks, this chapter assumes that the

reader has basic knowledge of the GSM architecture and its evolution into 3G networks.

27.1 Overview of Voice Call Continuity

Figure 27.1 presents a high-level overview of the VCC architecture. The figure shows

two VCC-enabled dual-mode IMS/CS terminals. Both can have access to packet-switched

accesses, which leads to an IMS core network, and CS accesses, which leads to the CS core

ıa- ´ Martın´

The 3G IP Multimedia Subsystem (IMS): Merging the Internet and the Cellular Worlds Third Edition

Gonzalo Camarillo and Miguel A. Garc

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