ePDG stands for "Evolved Packet Data Gateway". It is a key network element in the 3GPP architecture.

The ePDG provides secure access to the 4G core network (EPC), for user equipment (UEs) connected to untrusted non-3GPP access networks, such as Wi-Fi or public internet.

It is responsible for authenticating the UE and authorizing its access to the EPC.

In addition, it handles the mobility management for UEs, and manages the IP address allocation, session establishment, and session continuity during UE mobility.

The ePDG receives the user's IP traffic from the untrusted non-3GPP access network and forwards it to the appropriate PGW (PDN Gateway) in the EPC, and vice versa.