Long Term Evolution (LTE)

LTE (Long Term Evolution) is initiated by 3GPP to improve the mobile phone standard to cope with future

technology evolutions and needs. 3GPP work on the Evolution of the 3G Mobile System started with the

RAN Evolution Work Shop, 2 - 3 November 2004 in Toronto, Canada.

The main targets for this evolution are increased data rates, improved spectrum efficiency, improved

coverage, reduced latency and packet-optimized system that support multiple Radio Access

Technologies.

Architecture overview

The evolved architecture comprises E-UTRAN (Evolved UTRAN) on the access side and EPC (Evolved

Packet Core) on the core side.

The figure below shows the evolved system architecture

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

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. 

LTE Interfaces

The following are LTE Interfaces : (Ref: TS 23.401 v 841)

S1-MME :- Reference point for the control plane protocol between E-UTRAN and MME.

S1-U:- Reference point between E-UTRAN and Serving GW for the per bearer user plane

tunnelling and inter eNodeB path switching during handover.

S3:- It enables user and bearer information exchange for inter 3GPP access network mobility in

idle and/or active state.

S4:- It provides related control and mobility support between GPRS Core and the 3GPP Anchor

function of Serving GW. In addition, if Direct Tunnel is not established, it provides the user plane

tunnelling.

S5:- It provides user plane tunnelling and tunnel management between Serving GW and PDN

GW. It is used for Serving GW relocation due to UE mobility and if the Serving GW needs to

connect to a non-collocated PDN GW for the required PDN connectivity.

S6a:- It enables transfer of subscription and authentication data for authenticating/authorizing user

access to the evolved system (AAA interface) between MME and HSS.

Gx:- It provides transfer of (QoS) policy and charging rules from PCRF to Policy and Charging

Enforcement Function (PCEF) in the PDN GW.

S8:- Inter-PLMN reference point providing user and control plane between the Serving GW in the

VPLMN and the PDN GW in the HPLMN. S8 is the inter PLMN variant of S5.

S9:- It provides transfer of (QoS) policy and charging control information between the Home

PCRF and the Visited PCRF in order to support local breakout function.

S10:- Reference point between MMEs for MME relocation and MME to MME information transfer.

S11:- Reference point between MME and Serving GW.

S12:- Reference point between UTRAN and Serving GW for user plane tunnelling when Direct

Tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as

defined between SGSN and UTRAN or respectively between SGSN and GGSN. Usage of S12 is

an operator configuration option.

S13:- It enables UE identity check procedure between MME and EIR.

SGi:- It is the reference point between the PDN GW and the packet data network. Packet data

network may be an operator external public or private packet data network or an intra operator

packet data network, e.g. for provision of IMS services. This reference point corresponds to Gi for

3GPP accesses.

Rx:- The Rx reference point resides between the AF and the PCRF in the TS 23.203.

SBc:- Reference point between CBC and MME for warning message delivery and control

functions.