Below is a simplified picture of the EPS architecture.
The EPS system is made up of the Evolved Packet Core (EPC) and the E-UTRAN. The EPC provides access to external data networks (e.g., Internet, Corporate Networks) and operator services (e.g., MMS,
MBMS).
It also performs functions related to security (authentication, key agreement), subscriber information, charging and inter-access mobility (GERAN/UTRAN/E-UTRAN/ I-WLAN/CDMA2000 etc.). The CN also tracks the mobility of inactive terminals (i.e., terminals in power saving state).
E-UTRAN performs all radio related functions for active terminals (i.e. terminals sending data).
Between the EPC and E-UTRAN there is an interface called S1.
An E-UTRA capable terminal is connected directly to E-UTRAN. However some parts of the terminal control-plane protocol stack is also terminated in the EPC.
