Figure shows the network architecture of the Evolved Packet Core (EPC). The EPC consists of three main nodes: the Mobility Management Entity (MME), the Serving Gateway (SGW) and the Packet Data Network Gateway (PGW). The MME may be co-located with the SGW, and the SGW may be co-located with the PGW. Hence, the standard allows a completely collapsed ‘one-node’ core network or a distributed (easily scalable) core network, or any possible ‘combination’ in-between.
The MME connects to the E-UTRAN via the S1-MME interface and is present solely in the CP. It is responsible for handling mobility and security procedures, such as network Attach, Tracking Area updates (similar to Location/Routing Area updates) and authentication. The MME also connects to the SGSN via the S3-interface.
The SGW connects to the E-UTRAN via the S1-U interface and is present solely in the UP. Its prime responsibility is routing and forwarding of user IP-packets. It acts as a UP anchor when the UE moves between 3GPP radio access technologies (S4-interface).
The PGW connects to the SGW via the S5-interface and to external packet data networks (or IMS) via the SGi-interface. It is responsible for the enforcing of QoS and charging policies. It also acts as a UP anchor when the UE moves between 3GPP and non-3GPP radio access (S2-interface).
It should be noted that additional network nodes/functions, not shown in figure , might be present as well. For example, a Packet Data Gateway (PDG) is needed for non-trusted IP access and a Policy and Charging Rules Function (PCRF) is required for IMS controlled QoS and charging mechanisms.