1. Introduction

This document presents the LTE network architecture as the first technical document of “LTE” area. The LTE network called EPS (Evolved Packet System) is an end-to-end (E2E) all IP network; EPS is divided into two parts - LTE part which deals with the technology related to a radio access network (E-UTRAN) and EPC part which deals with the technology related to a core network. An E2E all IP network means that all traffic flows – from a UE all the way to a PDN which connects to a service entity – are transferred based on IP protocol within EPS.

In order for LTE services to be provided to a user over the LTE network, an E2E LTE network reference model (NRM) is generally comprised of three additional domains - BSS/OSS domain for subscriber, application domain for providing services, and IP transport network domain for sending IP packets – in addition to basic EPS domain. The scope of this document is focused on the basic EPS domain. Other EPS domain features, such as the architectures for LTE interworking with 3GPP/non-3GPP and roaming, are out of the scope of this document and will be described in other technical documents later.

The remainder of this document is organized as follows. In Chapter 2, the LTE network reference model is defined and the EPS entities and interfaces are explained. Chapter 3 will describe the protocol stacks and then Chapter 4 will explain traffic flows over the LTE network.

2. LTE Network Reference Model

Figure 1 shows an LTE network reference model, consisting of LTE entities (UE and eNB) and EPC entities (S-GW, P-GW, MME, HSS, PCRF, SPR, OCS and OFCS). A PDN is an internal or external IP domain of the operator that a UE wants to communicate with, and provides the UE with services such as the Internet or IP Multimedia Subsystem (IMS). In the following, Table 1 and Table 2 show the functions of the LTE and EPC entities. Table 3 lists the reference points of the LTE network reference model and gives a description of interfaces between EPS entities. 

Figure 1. LTE network reference model

Table 1. LTE entities

Table 2. EPC entities

Table 3. LTE interfaces

3. LTE Protocol Stacks

Based on the EPS entities and interfaces defined in Chapter II, the LTE protocol stacks for the user plane and control plane are described in Chapter III.

3.1 User plane protocol stacks

Figure 2 shows the user plane protocol stacks for the LTE network reference model shown in Figure 1. The functions of the key layers of the protocol stacks are briefly described below.

Figure 2. LTE user plane protocol stacks

1) LTE-Uu interface

• PDCP: The PDCP protocol supports efficient transport of IP packets over the radio link. It performs header compression, Access Stratum (AS) security (ciphering and integrity protection) and packet re-ordering/retransmission during handover.
• RLC: In the transmitting side, the RLC protocol constructs RLC PDU and provides the RLC PDU to the MAC layer. The RLC protocol performs segmentation/concatenation of PDCP PDUs during construction of the RLC PDU. In the receiving side, the RLC protocol performs reassembly of the RLC PDU to reconstruct the PDCP PDU. The RLC protocol has three operational modes (i.e. transparent mode, acknowledged mode and unacknowledged mode), and each offers different reliability levels. It also performs packet (the RLC PDU) re-ordering and retransmission.
• MAC: The MAC layer lies between the RLC layer and PHY layer. It is connected to the RLC layer through logical channels, and to the PHY layer through transport channels. Therefore, the MAC protocol supports multiplexing and de-multiplexing between logical channels and transport channels. Higher layers use different logical channels for different QoS metrics. The MAC protocol supports QoS by scheduling and prioritizing data from logical channels. The eNB scheduler makes sure radio resources are dynamically allocated to UEs and performs QoS control to ensure each bearer is allocated the negotiated QoS.

2) S1-U/S5/X2 interface

• GTP-U: GTP-U protocol¹ is used to forward user IP packets over S1-U, S5 and X2 interfaces. When a GTP tunnel is established for data forwarding during LTE handover, an End Marker packet is transferred as the last packet over the GTP tunnel. 

3.2 Control plane protocol stacks

Figure 3 shows the control plane protocol stacks for the LTE network reference model. The functions of the key layers of the protocol stacks are briefly described below.

Figure 3. LTE control plane protocol stacks

1) LTE-Uu Interface

• NAS²: NAS protocol performs mobility management and bearer management functions.
• RRC: RRC protocol supports the transfer of the NAS signaling. It also performs functions required for efficient management of the radio resources. The main functions are as follows:
  • Broadcasting of system information
  • Setup, reconfiguration, reestablishment and release of the RRC connection
  • Setup, modification and release of the radio bearer
• Same as in user plane
• Same as in user plane
• Same as in user plane

2) X2 interface

• X2AP: X2AP protocol supports UE mobility and SON functions within the E-UTRAN. To support UE mobility, the X2AP protocol provides functions such as user data forwarding, transfer of SN status and UE context release. For SON functions, eNBs exchange resource status information, traffic load information and eNB configuration update information, and coordinate each other to adjust mobility parameters using the X2AP protocol.

3) S1-MME interface

• S1AP: S1AP protocol supports functions such as S1 interface management, E-RAB management, NAS signaling transport and UE context management. It delivers the initial UE context to the eNB to setup E-RAB(s) and manages modification or release of the UE context thereafter.

4) S11/S5/S10 interfaces 

• GTP-C: GTP-C protocol supports exchange of control information for creation, modification and termination for GTP tunnels. It creates data forwarding tunnels in case of LTE handover.

5) S6a interface

• Diameter: Diameter protocol supports exchange of subscription and subscriber authentication information between the HSS and MME.

6) Gx interface

• Diameter: Diameter protocol supports delivery of PCC rules from the PCRF to the PCEF (P-GW).

7) Gy interface

• Diameter: Diameter protocol supports exchange of real-time credit control information between the P-GW and OCS.

8) Gz interface

• GTP’: GTP’ protocol supports CDR transfer from the P-GW to the OFCS.