The 2nd generation/3rd generation (2G/3G, 2nd Generation/3rd Generation) mobile communication networks have been deployed for many years, and may be regarded to have full coverage to some extent. With the development of the system architecture evolution/long term evolution (SAE/LTE, System Architecture Evolution) network, high speed packet access (HSPA, High Speed Packet Access) network, enhanced HSPA (HSPA+) network, and worldwide interoperability for microwave access (WiMAX, Worldwide Interoperability for Microwave Access) network, these networks have covered some urban areas and heavy traffic areas, and coexist with the 2G/3G networks among the current communication networks. For better description, networks providing only packet switched (PS) domain, for example, SAE/LTE, HSPA, HSPA+, and WiMAX networks, are hereinafter referred to as PS domain-only networks.
With respect to different network bearer mechanisms, mobile communication services may be categorized into the following services:
1. services carried in Circuit Switched (CS, Circuit Switched) domain, for example, voice telephone service, short message service, and supplementary service.
2. services carried in Packet Switched (PS, Packet Switched) domain, including a data service and a session service, where the session service is also known as a voice over Internet Protocol (VoIP, Voice over Internet Protocol) service.
Voice services may be carried in the CS domain or PS domain. In an LTE network, different solutions are proposed to provide voice services according to different network deployment scenarios and requirements of operators. For an LTE network without an IP multimedia subsystem (IMS), a circuit switched fallback (CSFB, Circuit Switched Fall-Back) technology is used. The technology is described in detail in the following.
An operator may use a 2G/3G network to provide voice services for users, and use an LTE network to provide data services for users. A user equipment (UE, User Equipment) is a multi-mode terminal. However, due to complexity of implementation and high cost, most UEs cannot receive and send LTE signals and 2G/3G signals simultaneously. There is two applying scenarios in the following: 1. A UE performs a data service or is in an idle state in an LTE network, and wants to initiate a CS voice call. And the radio module of the UE needs to be tuned from the LTE network to the 2G/3G network, and then the UE initiates a voice call in the 2G/3G network. This call is referred to as a UE-initiated call. 2. A UE performs a data service or is in an idle state in an LTE network, and a user wants to call the UE. And the LTE network needs to notify the UE that a CS call arrives, and the UE can tune the radio module to the 2G/3G network to receive the CS call according to the notification. This call is referred to as a UE-called call.