Evolution of the core network started with Release 4. This provided the starting point for the introduction of IP into the CS domain of the core network as well as enhancements to enable real time and non real time services to be successfully integrated onto a common transport medium. In Release 8 of the 3GPP standards the functionality of the core network is made flatter with much of the intelligence required to support service delivery being placed within domains such as the IMS. Consequently the EPC comprises of the MME (Mobility Management Entity) that performs many of the mobility functions traditionally provided by the SGSN such as paging, security and Idle Mode procedures. EPC also includes the S-GW (Serving Gateway), which replaces the User Plane functionality of the SGSN, as well as the PDN-GW (Packet Data Network Gateway), which terminates the user plane within the EPC.