Core 4G – EPC (Evolved Packet Core)
SGW (Serving Gateway) – is the boundary between E-UTRAN and EPC. SGW transfers data from the eNodeB to the PGW using the S1-U (in communication with the eNodeB) and S5 / S8 (in communication with the PGW) interfaces. SGW is responsible for routing, forwarding, packet marking and buffering, user mobility management, and support for handover connections between two eNBs. Each terminal may only be operated by one SGW at a time.
PGW (Packet Network Data Gateway) – is the boundary between the EPC and the external packet network (Internet). PGW is responsible for assigning IP addresses to terminals, filtering / inspecting packets, supporting selected functionalities in the network and charging for their use. Each terminal can be operated by many PGWs simultaneously, thus it can access many external packet networks.
MME (Mobile Management Entity) – is the boundary between E-UTRAN and EPC and is responsible only for the control plane – it transmits the signaling that enables low-level connection management. The MME communicates with the eNodeB using the S1-MME interface and with the HSS via the S6a interface. MME is responsible for e.g. NAS layer signaling (Non Access Stratum), user authentication and authorization, support for connecting UE to the network, setting and managing bearer, selection of PGW and SGW for a given connection, selection of a different MME when switching between eNodeB or SGSN in case of connection for 2G / 3G networks, TA (Tracking Area) management, roaming support.
HSS (Home Subscriber Server) – is the unit that manages user profiles, subscriptions and security functions. HSS is responsible for e.g. user authentication when trying to connect to the network and authorization of access to selected services. The HSS also stores information about the user’s location, the MME unit it is currently registered with, and the packet networks it can connect to. The HSS operates in contact with the EPC in the control plane and communicates with the MME via interface S6a.