The high-level network architecture of LTE is comprised of following three main components:

• The User Equipment (UE).
• The Evolved UMTS Terrestrial Radio Access Network (E-UTRAN).
• The Evolved Packet Core (EPC).

The E-UTRAN (The access network)

• The architecture of evolved UMTS Terrestrial Radio Access Network (E-UTRAN) has been illustrated below.
• The E-UTRAN handles the radio communications between the mobile and the evolved packet core and just has one component, the evolved base stations, called eNodeB or eNB.
• Each eNB is a base station that controls the mobiles in one or more cells. The base station that is communicating with a mobile is known as its serving eNB.
• Each eBN connects with the EPC by means of the S1 interface and it can also be connected to nearby base stations by the X2 interface, which is mainly used for signalling and packet forwarding during handover.

The Evolved Packet Core (EPC) (The core network)

• The architecture of Evolved Packet Core (EPC) has been illustrated below.

• The Home Subscriber Server (HSS) component has been carried forward from UMTS and GSM and is a central database that contains information about all the network operator's subscribers.

• The Packet Data Network (PDN) Gateway (P-GW) communicates with the outside world ie. packet data networks PDN, using SGi interface. Each packet data network is identified by an access point name (APN). The PDN gateway has the same role as the GPRS support node (GGSN) and the serving GPRS support node (SGSN) with UMTS and GSM.

• The serving gateway (S-GW) acts as a router, and forwards data between the base station and the PDN gateway.

• The mobility management entity (MME) handles the signaling of message and security, tracking of the user equipment.

• The Policy Control and Charging Rules Function (PCRF) is a component which is not shown in the above diagram but it is responsible for policy control decision-making, as well as for controlling the flow-based charging functionalities in the Policy Control Enforcement Function (PCEF), which resides in the P-GW.