A virtualized evolved packet core (vEPC) is a mobile-core network system that accommodates LTE access systems.
Server Manager represents single data model normalizing vEPC infrastructure from physical, virtual to Serving GW, PDN GW and MME layers supporting network services for 4G, LTE, and VoLTE.
The gateways (Serving GW and PDN GW) deal with the user plane. They transport the IP data traffic between the User Equipment (UE) and the external networks.
The Serving GW is the point of interconnect between the radio-side and the EPC. As its name indicates, this gateway serves the UE by routing the incoming and outgoing IP packets. It is the anchor point for the intra-LTE mobility (i.e. in case of handover between eNodeBs) and between LTE and other 3GPP accesses. It is logically connected to the other gateway, the PDN GW.
The PDN GW is the point of interconnect between the EPC and the external IP networks. These networks are called PDN (Packet Data Network), hence the name. The PDN GW routes packets to and from the PDNs.
The PDN GW also performs various functions such as IP address / IP prefix allocation or policy control and charging.
3GPP specifies these gateways independently but in practice they may be combined in a single "box" by network vendors.
The MME (for Mobility Management Entity) deals with the control plane. It handles the signalling related to mobility and security for E-UTRAN access.
The MME is responsible for the tracking and the paging of UE in idle-mode. It is the termination point of the Non-Access Stratum (NAS).