ePDG stands for "Evolved Packet Data Gateway". It sits between PGW and untrusted (unlicensed ) non-3GPP access network and provide LTE services through untrusted non-3GPP network. ePDG is responsible to make IP sec tunnel between UE(which is located in untrusted non-3GPP network) and PGW. it is responsible for security and tunnel authentication and authorization. ePDG also acts as local mobility anchor.