The Evolved Packet Core (EPC) is the core network of the 4G-LTE mobile network.

Unlike the 2G/3G core network, that was containing two different domains [Packet switching (PS) and Circuit switching (CS)], the 4G Evolved Packet Core network (EPC) contains only one domain which is the Packet Switching (PS) domain.

So in order for the UE to make a voice or a vdeo call while attached to the 4G network, it establishes this call through connecting to the IMS network using the 4G core network.

The 4G core network contains different nodes such as the MME, the SGW, the PGW, and the HSS.

Each of those nodes is having certain roles that enables the UE to access the requested services.