LTE Architecture Overview

The evolved architecture comprises E-UTRAN (Evolved UTRAN) on the access side and EPC (Evolved Packet Core) on the core side. In E-UTRAN, eNBs provide the E-UTRA user plane protocols (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol which terminates towards the UE.

The eNBs are interconnected with each other by means of the X2 interface. The eNBs are connected by the S1 interface to the EPC (Evolved Packet Core). The eNB connects to the MME (Mobility Management Entity) by means of the S1-MME interface and to the Serving Gateway (S-GW) by means of the S1-U interface. The S1 interface supports a many-to-many relation between MMEs / Serving Gateways and eNBs.

LTE Network Elements

eNB

eNB interfaces with the UE and hosts the PHYsical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers. It also hosts Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL QoS, cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL user plane packet headers.

Mobility Management Entity

Mobility management entity manages and stores UE context (for idle state: UE/user identities, UE mobility state, user security parameters). It generates temporary identities and allocates them to UEs. It checks the authorization whether the UE may camp on the TA or on the PLMN. It also authenticates the user.

Serving Gateway

The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW).

Packet Data Network Gateway

The PDN GW provides connectivity to the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PDN GW for accessing multiple PDNs. The PDN GW performs policy enforcement, packet filtering for each user, charging support, lawful Interception
and packet screening.

The figure below shows the evolved system architecture:




Here's a video briefing about the LTE architecture: