Method and apparatus for providing voice service to user equipment in hybrid mobile communication system

A combo device of a session management function (SMF) and a packet data network gateway (PGW) control plane function (PGW-C) for controlling fallback of a user equipment (UE) in a hybrid mobile communication system in which a first mobile communication system and a second mobile communication system interwork, the combo device comprising:

• a transceiver configured to communication with a network function of the hybrid mobile communication system;
• a memory;
• and
• a processor configured to:
• receive, from an access and mobility management function (AMF) of the first mobile communication system, a reactivation request for a packet data network (PDN) connection of the second mobile communication system for the UE, the reactivation request comprising a PDN connection context,
• retrieve a corresponding protocol data unit (PDU) session based on the PDN connection context included in the reactivation request,
• modify evolved packet system (EPS) bearer identity (EBI) information for an internet protocol (IP) multimedia subsystem (IMS) voice allocated to the UE based on information supported in the first mobile communication system,
• establish a core network (CN) tunnel for the PDU session for the UE to a user plane function (UPF) and a PGW user plane function (PGW-U) based on the modified EBI information,
• update session management (SM) context based on the established CN tunnel, and
• transmit, to the AMF, a response message comprising the updated SM context as a response to the reactivation request.

A combo device of a session management function (SMF) and a packet data network gateway (PGW) control plane function (PGW-C) for controlling fallback of a user equipment (UE) in a hybrid mobile communication system in which a first mobile communication system and a second mobile communication system interwork, the combo device comprising:

• a transceiver configured to communication with a network function of the hybrid mobile communication system;
• a memory;
• and
• a processor configured to:
• receive, from an access and mobility management function (AMF) of the first mobile communication system, a reactivation request for a packet data network (PDN) connection of the second mobile communication system for the UE, the reactivation request comprising a PDN connection context,
• retrieve a corresponding protocol data unit (PDU) session based on the PDN connection context included in the reactivation request,
• modify evolved packet system (EPS) bearer identity (EBI) information for an internet protocol (IP) multimedia subsystem (IMS) voice allocated to the UE based on information supported in the first mobile communication system,
• establish a core network (CN) tunnel for the PDU session for the UE to a user plane function (UPF) and a PGW user plane function (PGW-U) based on the modified EBI information,
• update session management (SM) context based on the established CN tunnel, and
• transmit, to the AMF, a response message comprising the updated SM context as a response to the reactivation request,
• wherein the response message further comprises an indicator indicating that an EPS fallback procedure is available to be performed after a procedure of registering the UE in the first mobile communication system is completed.