Doc. Code
eRAN2.2
Overview
Issue
01
Date
2011-09-30
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eRAN2.2
Overview
About This Document
About This Document
Change History
The change history describes the changes in each release, and the latest document contains all
change histories in earlier documents.
01 (2011-09-30)
First official release.
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eRAN2.2
Overview
Contents
Contents
About This Document ................................................................................................................... iii
1 Overview of LTE Networks ........................................................................................................ 1
2 Overview of Interfaces ................................................................................................................. 3
2.1 S1 Interface ...................................................................................................................................................... 3
2.2 X2 Interface ...................................................................................................................................................... 3
2.3 Uu Interface ...................................................................................................................................................... 3
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eRAN2.2
Overview
1 Overview of LTE Networks
1
Overview of LTE Networks
The Long Term Evolution (LTE) system uses a distributed network architecture in which
various functions are carried on different network element (NEs). In the architecture, each NE
properly exchanges information with another for communication. In this way, the distributed
NEs can complete each task. In addition, all NEs or subsystems must connect to each other
through different interfaces based on the specified protocol.
In the communications system, the information exchanged between the NEs is called
signaling. The signaling system instructs all the NEs in a network to complete each process.
In LTE networks, signaling messages must comply with the specifications defined in the
protocol for the interfaces.
An interface is the connection point between two neighboring NEs, and a protocol defines the
rules that each interface, also called connection point, must comply with during information
exchange. Different protocols are defined for different network layers, and the application
layer protocol defines specifications for interfaces.
The LTE radio access network is called evolved UMTS terrestrial radio access network
(E-UTRAN), which contains an NE called eNodeB. UMTS is short for Universal Mobile
Telecommunications System.
As an open NE, the eNodeB and its external interfaces must comply with open protocols.
Figure 1-1 shows the interfaces between the NEs in an LTE network.
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eRAN2.2
Overview
1 Overview of LTE Networks
Figure 1-1 Interfaces between the NEs in an LTE network
MME / S-GW
MME / S-GW
S1
S1
S1
S1
X2
E-UTRAN
eNB
eNB
X2
X2
eNB
Table 1-1 lists the interfaces between the eNodeB and different NEs and the protocols that the
interfaces comply with.
Table 1-1 Interfaces between the eNodeB and different NEs and the protocols that the interfaces
comply with
NE
Interface
Signaling and Protocol
eNodeB-MME/S-GW
S1
S1AP, GTPU
eNodeB-eNodeB
X2
X2AP
eNodeB-UE
UU
RRC
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eRAN2.2
Overview
2 Overview of Interfaces
2
Overview of Interfaces
2.1 S1 Interface
The S1 interface connects the eNodeB and the mobility management entity (MME) or serving
gateway (S-GW) and is classified into two planes: user plane and control plane.
The S1 interface transmits information about the following items based on the S1 Application
Part (S1AP) protocol:

E-UTRAN management

Paging processing

Mobility management
The S1 interface carries and transmits the service flows based on the GPRS Tunneling
Protocol-User (GTPU) protocol. GPRS is short for general packet radio service.
2.2 X2 Interface
The X2 interface connects two eNodeBs. It transmits information about the following items
based on the X2 Application Part (X2AP) protocol:

Paging control after a handover between eNodeBs

Information update between eNodeBs
2.3 Uu Interface
The Uu interface is an air interface between the eNodeB and the user equipment (UE). It
transmits the following items based on the radio resource control (RRC) protocol:

Broadcast common control information

Paging messages transmit to UEs in RRC_IDLE state, for example, to UEs in earthquake
and tsunami warning system (ETWS) scenarios

Dedicated control information for specified UEs
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