EPC (Evolved Packet Core) is the core network of 4G mobile network, it has traditional mobile network capabilities, such as user subscription data storage, mobility management, and data exchange, and can provide users with ultra-high-speed Internet experience. The EPC has only the PS domain but no CS domain, is based on the all-IP structure, has the control plane separated from the bearer plane, and has a flat network structure. It mainly consists of three NEs:

MME (Mobility Management Entity): As an NE for signaling control, it is mainly used for mobility management and implements session-related control processing.

SGW (Serving Gateway): It is responsible for processing and forwarding the data of local network users.

PGW (PDN Gateway): Acting as the boundary of connecting Public Data Network (PDN), it is responsible for user data processing and forwarding and the execution of user control policy.

The EPC supports multiple access modes of 3GPP and non-3GPP (such as Wi-Fi), and is an integrated architecture supporting heterogeneous networks. In this architecture, traditional CS domain services such as short message and voice services will be carried by the VoLTE, or the CS fallback solution can be adopted to complete voice services by existing CS domain.