5 Functional entities

24.2063GPPStage 3TSVoice call continuity between Circuit Switched (CS) and IP Multimedia Subsystem (IMS)

5.1 Introduction

This clause associates the functional entities described for the IM CN subsystem and for the CS domain, with the VCC roles described in the stage 2 architecture document (see 3GPP TS 23.206 [3]).

5.2 User Equipment (UE)

To be compliant with this document, a UE shall implement the role of a VCC UE (see subclause 6.2, subclause 7.2, subclause 8.2, subclause 9.2 and subclause 10.2).

5.3 Media Resource Function Controller (MRFC)

There are no roles specific to VCC associated with the MRFC.

5.4 Application Server (AS)

The VCC application provides the following roles:

a) Domain Transfer Function (DTF) as defined in 3GPP TS 23.206 [3] subclause;

b) Domain Selection Function (DSF) as defined in 3GPP TS 23.206 [3] subclause;

c) CS Adaptation Function (CSAF) as defined in 3GPP TS 23.206 [3] subclause;

d) CAMEL service function as defined in 3GPP TS 23.206 [3] subclause

An AS implementing the VCC application shall implement one or more of the roles DTF, DSF or CSAF. The CSAF cooperates with a CAMEL Service Function as defined in 3GPP TS 23.206 [3] subclause in order to provide VCC functionality. All four roles can be co-located.

5.5 Media Gateway Control Function (MGCF)

In order to support VCC for any call, the MGCF has to provide signalling interworking and control of the media between the CS domain and the IM CN subsystem. The VCC application can only be configured to operate where appropriate interworking is provided, e.g. a voice call represented by SDP in the IM CN subsystem has an equivalent coding in the transmission media requirement and optionally the ISUP USI parameter in the CS domain.

As the procedures for call termination in the CS domain may involve an MGCF provided by another network operator, the provision of appropriate interworking can extend to peering agreements between operators.