Friday, December 15, 2017
Introduction to 4G: LTE Network Architecture

Introduction to 4G: LTE Network Architecture

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INTRODUCTION TO 4G MOBILE TECHNOLOGIES:

LTE (LONG TERM EVOLUTION), NETWORK ARCHITECTURE

 

Mobile telephony standards have been gradually adopting packet switched technologies since the introduction of 2.5G GPRS networks back in the nineties. But the continuous growth in demand for data services has forced the mobile networking standardisation processes to move away from legacy circuit switched technologies and to focus primarily on implementing efficient wider bandwidth data carrying capabilities. This has finally culminated in the introduction of the all IP based Fourth Generation Long Term Evolution (4G LTE) standard by 3GPP standardisation body; and this new technology is already being deployed worldwide and going through several feature additions such as ‘LTE-Advanced’. Here in this first article of this series, we will take a look the overall architecture of a basic LTE network including the network elements and protocol stacks.

The overall standardisation body which oversees the development and maintenance of all public mobile telecommunications technologies worldwide is the 3GPP (3rd Generation Partnership Project), based in Sophia-Antipolis, France. 3GPP is an umbrella organisation uniting six telecommunications standard development organizations (the ‘organisation partners’) from across the globe. These are ARIB from Japan, ATIS from USA, CCSA from China, ETSI from Europe, TTA from South Korea and TTC from Japan.

3GPP technical standards are organised based on ‘Releases’. A 3GPP ‘Release’ is either a set of new features to be added to an existing technology or may introduce a brand new technology. 3GPP Release-8 (frozen in December 2008) introduced for the first time the 4Th Generation LTE (Long Term Evolution) technology standards. LTE brought with it an all-IP based network design, new air-interface radio communication based on OFDMA (Orthogonal Frequency Division Multiple Access) and SC-FDMA (Single Carrier Frequency Division Multiple Access) among many other new features. Number of network elements in LTE was reduced and a very strict latency requirement was introduced to cater for high data rate planned in this release (peak rate of 100 Mbps in the downlink and 50 Mbps in the uplink in Release-8). Since December 2008, further Releases have already been made, introducing LTE-Advanced with Carrier Aggregation in Release-10 with peak data rate of 3.0 Gbps in downlink and 1.5 Gbps in the uplink. At the time of writing of this article, Release-12 is on its way to the final freeze and Release-13 is already planned in 2014-2015 time-frame.

 

INTRODUCTION

LTE network is divided in two parts in a similar fashion as the legacy 3G and 2G networks are. These parts are the access network called EUTRAN (Evolved UMTS Terrestrial Radio Access Network, or in short Evolved UTRAN) and the Core-Network called the EPC (Evolved Packet Core). As the name suggests, EUTRAN is solely concerned with providing mechanisms related to radio access to the network for the mobile terminals or UEs (User Equipment). EPC is on the other hand is an all IP ‘Core network’ with many new nodes introduced in LTE. The interface between EUTRAN and EPC is the S1-interface connecting an eNB with a MME (Mobility Management Entity, see Fig. 1). A new feature introduced in LTE is to provide direct connectivity between the base stations (eNB) by introducing the inter-eNB X2-interface and the corresponding X2-AP protocol. Interconnected eNBs over X2 links add new features such as enhanced mobility and inter-cell interference management and the most advanced configuration feature in LTE: SON (Self Organising Network) functionalities.

EUTRAN is LTE consists of only one type of network node: eNodeB or in short eNB (evolved Node-B). The eNB node is an equivalent of a combination of the RNC and Node-B nodes from the legacy 3G architecture.

 

LTE EPC Network Elements
LTE EPC Network Elements

Figure 1: Simplified architecture of a basic non-roaming LTE network deployment

As in the legacy 3G networks, the LTE networks is also logically sub-divided into two ‘planes’ and into two ‘strata’. The two planes are known as the Control-Plane or the C-Plane and the User-Plane or U-Plane. The two strata are known as the ‘Access Stratum’ or AS and the ‘Non-Access Stratum’ (NAS).

The C-Plane comprises of protocol stack to perform control signalling for all aspects of radio access over the air-interface through the ‘Layer-3’ Access Stratum protocol known as the RRC (Radio Resource Control). The C-Plane also contains a NAS protocol suite to perform mobility management, session management functions between the mobile terminal (UE) and the MME.

The AS protocol stacks are concerned only with the radio access part and terminated at the UE and at the eNB. The AS protocol stacks are describes in a later section. The NAS protocols are terminated at the UE and at the MME. NAS protocol message PDUs are transferred either piggybacked on RRC PDUs or using dedicated NAS transfer messages.

 

LTE EPC

The network nodes in a basic LTE network deployment are shown in Fig.1 above. In this scenario, the LTE network is connected neither to any other 3GPP RAN/Core-Networks (such as UMTS, GERAN) nor to any non-3GPP networks (such as Wi-Max, HRPD etc.). The EPC network is comprised of several logical nodes; some of them may be co-located with other nodes in a real network deployment. In this non-roaming scenario, the following EPC logical nodes are depicted:

  • MME – Mobile Management Entity
  • SGW – Serving Gateway
  • PGW – PDN Gateway
  • HSS – Home Subscriber Server
  • PCRF – Policy and Charging Rules Function

 

Mobile Management Entity (MME)

The MME is the access node connecting the EUTRAN to the EPC and terminates the NAS protocols which are running between the UE and the EPC. The MME performs among other functions bearer management including bearer establishment and release and maintenance using the ESM Session Management protocol layer in NAS. The MME also performs connection management procedures using the ECM layer as well as mobility management procedures using the EMM layer. NAS security mode related procedures are also performed by the MME.

The MME creates and thereafter maintains a UE context for a UE when it performs an attach procedure in the network and also assigns a temporary identity (S-TMSI: SAE Temporary Mobile Subscriber Identity) to each such UE during the Attach/Registration procedure. The S-TMSI is then used during further signalling procedures instead of the permanent subscriber Identity (IMSI).

Serving Gateway (SGW)

The SGW acts as the local mobility anchor for an UE connected to EPC and IP packets to and from the UE are routed via the SGW handling the particular UE. It also performs data buffering for the UE, maintains bearer context information for UEs in IDLE state. Charging related information such as transmitted data volume is also collected by the SGW in the visited network.

PDN Gateway (PGW)

The PGW allocates IP addresses to every UE, performs IP packet filtering to different bearers based on QoS profiles using ‘Traffic Flow Templates’ (TFT) on Guaranteed Bit Rate (GBR) bearers. PGW is also the edge node of LTE facing the external IP networks and acts as an edge router for LTE.

Home Subscriber Server (HSS)

The HSS is a database containing all the permanent subscriber data including subscriber identities such as the IMSI (ID number of the SIM card) and the corresponding MSISDN as well as QoS profiles, and allowed Packet Data Network (PDN) address details such as the APN name, subscribed IP address or DNS names. The HSS also holds dynamic mobility related information such as the address of the MME to which the UE is currently attached. The HSS also includes or connected to authentication servers such as an AUC (Authentication Centre) which performs ‘Authentication Vector’ generation for LTE AKA (Authentication and Key Agreement) procedure for security and integrity mechanisms used in LTE.

Policy and Charging Rules Function (PCRF)

The PCRF performs QoS authorisations including bit rates for EPS bearers based on individual subscriber profiles and also performs decision making functions for the flow based charging for the Policy Control Enforcement Function logical node (PCEF: co-located in the PGW) to control how each data flow should be treated. Network operators can configure the subscriber profiles to include rate plans, admission control criteria, user data quota management etc. The PCRF is connected to the SGW using Gx interface and to the PGW using the Rx interface. 3GPP specified ‘Diameter Routing Agent’ (DRA) implements the Diameter protocol which is used over both the Gx and Rx interfaces.

 

LTE EUTRAN

The Radio Access Network part of LTE (EUTRAN) is comprised solely of one network element, namely the eNB. Since there is no centralised controller in an EUTRAN to control the network of eNBs (as opposed to the RNC in UMTS or BSC in GERAN), the EUTRAN is said to have a flat architecture. Elimination of a single ‘Radio Controller’ node in EUTRAN and instead distributing its functions amongst the eNBs reduces cost (since a single radio controller needs to be a high-availability, fault-tolerant and CPU intensive node), reduces latency and also eliminates the risk of having a single point of failure in the network.

But on the other hand, the eNBs can be (and usually is in a commercial deployment) interconnected via the X2-interface to each other, which is used for many purposes including handover from one eNB to another, inter-cell interference management functions and probably more importantly a new feature called ‘Automatic Neighbour Relation Function’ (ANRF).

ANRF makes it possible for one eNB to automatically determine its useful neighbour eNBs by instructing the mobile terminal (UE) to start measuring the received signal power and send that information to the eNB along with the ‘Physical Cell Identity’ (PCI) and the ‘Cell Global Identity’ (CGI) of that measured cell in a report message. Based on these measurement reports, an eNB can initiate an X2 link setup procedure to connect to that eNB and exchange application level configuration data for the X2-interface. This way, eNBs can discover each other and set up and configure the X2 link between themselves automatically with minimal configuration effort from the network operator. This is part of the ‘Self Organising Network’ (SON) feature introduced in LTE. It is worth mentioning that the network operator still has the option to configure the X2-links between eNBs manually if it so chooses.

An eNB is responsible for not only the radio access protocol terminations but also functionalities such as Radio Admission Control, Radio Resource Management (RRM), User-Plane data routing towards the Serving-Gateway (SGW), Measurement reporting management, IP header compression and encryption etc.

 

LTE PROTOCOL OVERVIEW

LTE C-PLANE PROTOCOL STACKS

The eNB is the contact point for the mobile terminals (UE) to access the EUTRAN to connect to the EPC. The interface between an UE and an eNB is known as the air-interface or more formally as the LTE-Uu interface. The air-interface in LTE is used by the ‘Access Stratum’ (AS) protocols and provides a link between the UE and the MME/EPC to communicate using the ‘Non-Access Stratum’ (NAS) protocols. The C-Plane protocols are distributed over two major interfaces: LTE-Uu and the S1-MME. Figure 2 below depicts the air-interface and the S1-MME interface C-Plane protocol stacks.

 

LTE AIR-INTERFACE (LTE-Uu)

The physical layer (PHY) in LTE-Uu is designed in LTE for OFDM and SC-FDMA radio transmission techniques and support several different RF bandwidths (1.4 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz). LTE specification supports many different frequency bands and bandwidth combinations although not all bands are available in every country (for reference, see 3GPP TS 36.101). The other two layers (MAC/RLC and RRC) on the air-interface are named exactly the same way as in the legacy 3G network and they perform similar functions. PDCP sits between RRC and RLC layers in LTE to perform IP header compression and related functions.

The RRC (Radio Resource Control) layer performs all radio resource related functions between the UE and the EUTRAN and is terminated at the UE and at the eNB. The RRC layer provides both a common and up to two dedicated ‘Signalling Radio Bearer’s (SRB) for transporting RRC signalling messages as well as carrying piggybacked NAS messages to the MME. The RRC layer in LTE is also responsible for providing Access Stratum (AS) security functions such as Integrity protection of signalling PDUs (to make sure that the signalling messages between the UE and the RUTRAN are not tampered with) and ciphering of both the C-Plane signalling PDUs and U-Plane data PDUs transported over the air-interface using the RRC ‘Data Radio Bearers’ (DRB).

The RRC layer is also responsible for configuring the lower layers (RLC, MAC and PHY) depending on the requirements. The RRC layer in the eNB is the master entity which translates the EUTRAN bearer requirements (such as data rate, delay budget etc.) and translates them to suitable parameters for RRC bearers, RLC PDUs, MAC and PHY configurations etc. and sends these configuration informations to the UE via RRC procedures such as ‘RRC Connection Establishment’ or ‘RRC Connection Reconfiguration’. The RRC layer in the UE is then responsible for extracting the protocol information elements (IEs) from the RRC message and then it applies the new configurations to the local RRC layer as well to the RLC, MAC and PHY layers as applicable.

In the eNB, the RRC layer is responsible for ‘System Information broadcast’, which is a cell-wide broadcast carrying vital system and access related information for the UEs. The system informations are broadcast using the BCCH (Broadcast Control Channel) logical channel which is mapped to the BCH (Broadcast Channel) transport channel at the MAC layer.

LTE Control-Plane Protocol Stack

Figure 2: E-UTRAN Control-Plane protocol stacks

The RLC (Radio Link Control) layer in LTE is very similar to the RLC layer in the legacy 3G network. The RLC layer provides three different modes of transmission: Transparent Mode (RLC-TM), Unacknowledged Mode (RLC-UM) and Acknowledged Mode (RLC-AM).

The RLC Transparent Mode provides data transmission for binary data which are octet-aligned but the data is neither segmented nor concatenated by RLC layer and no header information is added by RLC.

RLC Unacknowledged Mode provides unreliable transmission for octet-aligned data and is used for Dedicated Traffic Channel (DTCH logical channel) and for the Multi-Cast Logical Channels (MCCH, MTCH). In this mode, data from PDCP can be segmented by RLC layer before transmission and the receiving side uses a concatenation mechanism. RLC-UM mode does not provide any mechanism to inform the sender about the successful reception of any transmitted RLC-UM PDU.

RLC Acknowledged Mode provides a reliable transmission mechanism for octet-aligned data for RRC layer dedicated logical channels (DCCH, DTCH). An RLC-AM entity on either UE or eNB side contains both a transmitting side as well as a receiving side. RLC-AM provides segmentation/concatenation of RLC SDUs on the transmitting side, re-ordering and re-assembly on the receiving side and a reliable delivery mechanism based on a mechanism using PDU acknowledgement with re-transmission (ARQ-Automatic Repeat Request) for unacknowledged PDUs.

The MAC (Medium Access Control) layer in LTE-Uu provides data transfer using transport channels between itself and the LTE Physical layer. The MAC layer also performs radio resource allocation functions (when ordered by the RRC layer), error correction using HARQ (Hybrid-ARQ) mechanism, multiplexing and de-multiplexing of MAC SDUs onto/from transport blocks delivered to/by the physical layer. In the eNB, the MAC layer also performs data scheduling function for both uplink and downlink directions and priority handling between mobiles (UEs). Apart from these, the initial network access (Random Access procedure) by a mobile is also performed by the MAC layer.

S1-MME INTERFACE

On the S1-MME interface, layer 1 and layer 2 protocols depend on the particular transmission technology in use. The 3GPP specification allows for any suitable point-to-point and point-to-multipoint technologies. LTE uses an all-IP backhaul network and ‘Synchronised Ethernet’ (ITU SyncE) along with ‘Precision Time protocol’ (PTP – IEEE 1588 standard) provide synchronisation over LTE backhaul networks.

The S1-MME interface in the C-Plane uses SCTP (Stream Control Transmission Protocol, IETF RFC-4960) as the transmission protocol and S1-AP (S1-Application Part) protocol as defined by the 3GPP. S1-AP is the application protocol connecting the EUTRAN (eNB) and the EPC (MME) and provides interface management functions, mobility management functions (handover preparation, notification etc.), bearer (EUTRAN Radio Access Bearer or E-RAB) management functions, UE context management (UE context creation/transfer/release/modification) functions and NAS signalling transport to EPC, location reporting functions, eNB and MME configuration transfer functions among others.

 

LTE U-PLANE PROTOCOL STACKS

Figure-3 below depicts the air-interface as well as the EPC U-Plane protocol stacks. On the air-interface, difference between the C-Plane and the U-Plane stacks are minimal. The RRC protocol is absent in the U-Plane and the IP packets carrying the user data are transported by the PDCP layer between the UE and the eNB.

In the U-Plane, the application layer IP packets are transferred between a UE and eNB over the air-interface using PDCP protocol with the IP data as PDCP payload.

At the eNB, the user IP packets are then unpacked from the PDCP PDUs and then tunnelled through the EPC using GTP (GPRS Tunnelling Protocol) to the SGW and then finally to the PGW. At the PGW the user IP packets are then taken out and sent out to the external IP network (PDN).

It is possible for a LTE network deployment to have more than one PDN Gateways and server different purposes. For example, a LTE network offering IMS (IP Multimedia Sub-system) services for Voice-over-LTE (VoLTE) voice call services will have to add many other network elements and may use a separate PGW for the IMS traffic while a separate PGW for internet traffic. An UE can have bearer paths established to more than one PGWs simultaneously. The PGW performs data filtering using Traffic Flow Templates and assigns individual IP packets onto its corresponding S5/S8 bearer. At the PGW, the GTP finally terminates and the uplink application level IP packet is finally forwarded to the external network/internet. In the downlink direction, incoming IP packets are put onto GTP PDUs for tunnelling to the eNB at the PGW.

 

LTE U-Plane Ptotocol Stack

Figure 3: E-UTRAN User-Plane protocol stacks

The application layer data (HTTP, SIP/SDP, SMTP/POP etc.) using either TCP/UDP over IP is carried over the U-Plane from the UE to the PDN Gateway (PGW) using GTP (GPRS Tunnelling Protocol) which terminates between eNB and SGW and also between SGW and PGW. The incoming application layer data packets are also carried from PGW to the UE using GTP. Several different logical bearer types are defined between EUTRAN/EPC nodes to keep track of data flows of individual UEs and also individual data flows for different services used by a particular UE.

Data flows for different services originated at one single UE (for internet data, VoIP voice data etc.) are mapped to RRC Data Radio Bearers (DRB). The DRBs run between the UE and the eNB at the RRC protocol level.

Between the eNB and the SGW exists ‘S1-Terminal Endpoint ID’ (S1 TE-ID) which are mapped in the eNB to each and every DRB used by a particular UE. GTP PDUs for a particular data flow are then forwarded to the SGW in the uplink direction using the RB-ID to S1-TEID mapping at the eNB. An exactly opposite mapping is performed at the eNB for GTP PDUs in the downlink direction.

Between SGW and PGW, either a S5 or S8 (depending or whether it’s a roaming or non-roaming deployment) TE-ID is used to keep track of data flows. The GTP PDUs are forwarded by the SGW using mappings kept between S1-TE IDs and their corresponding S5/S8 TE-IDs. A reverse mapping is performed by the SGW to forward downlink GTP PDUs to the eNB.

LTE EPS bearer mappings

Figure 4: E-UTRAN data transfer and bearer mappings

The reverse procedure is applied for the IP packets arriving from the external IP PDN for an UE. Incoming IP packets are filtered at the PGW based on applicable packet filters using Downlink ‘Traffic Flow Templates’ (DL-TFT) and put onto the corresponding EPS bearer using the corresponding GTP tunnel (depending on the QoS profiles in use) to be transported to the SGW and then to the eNB, where the tunnelling terminates. The eNB will then forward the user IP packets using PDCP over the air-interface on the Data Radio Bearer (DRB) corresponding to the EPS bearer which the IP data arrived onto.

 

Abbreviations

APN              Access Point Name

AS                 Access Stratum

DCCH            Dedicated Control Channel

DNS              Domain Naming Service

DRA              Diameter Routing Agent

DTCH            Dedicated Traffic CHannel

ECM              Evolved Connection Management

EMM             Evolved Mobility Management

EPC              Evolved Packet Core

ESM             Evolved Session Management

EUTRAN      Evolved UTRAN

GBR            Guaranteed Bit-Rate

GTP              GPRS Tunnelling Protocol

HSS             Home Subscriber Server

IMSI            International Mobile Subscriber Identity

LTE              Long Term Evolution

RNC             Radio Network Controller

MAC            Medium Access Control

MCCH          Multicast Control CHannel

MIMO           Multiple Input Multiple Output

MME             Mobility Management Entity

MTCH            Multicast Traffic CHannel

OFDM(A)       Orthogonal Frequency Division Multiplexing/ Multiple-Access

PCRF               Policy and Charging Rules Function

PDCP               Packet Data Convergence Protocol

PDN                 Packet Data Network

PDU                 Protocol Data Unit

PGW                PDN Gateway

PHY                 PHYsical layer

QCI                  QoS Class Identifier

QoS                 Quality of Service

RAN                 Radio Access Network

RLC                  Radio Link Control

RRC                 Radio Resource Control

NAS                  Non-Access Stratum

S1-AP               S1-Application Part

S1-U                 S1 protocol on the U-plane

SC-FDMA         Single-Carrier Frequency Division Multiple Access

SCTP               Stream Control Transmission Protocol

SGW                Serving Gateway

SON                 Self-Organising Network

TCP                 Transmission Control Protocol

UDP                 User Datagram Protocol

UMTS               Universal Mobile Telephony System

UTRAN            UMTS Terrestrial Radio Access Network

X2-AP              X2 Application Part

 

References:

3GPP TS 36.300 V10.11.0 (2013-09)
Evolved Universal Terrestrial Radio Access (EUTRA) and Evolved Universal Terrestrial Radio Access Network
(E-UTRAN); Overall description; Stage 2 (Release-10)
http://www.3gpp.org/ftp/specs/2013-12/Rel-10/36_series/36300-ab0.zip

3GPP TS 36.101 V11.4.0 (2013-03)
Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA);
User Equipment (UE) radio transmission and reception (Release 11)
http://www.3gpp.org/ftp/specs/2013-12/Rel-10/36_series/36300-ab0.zip

ETSI: Cellular History
http://www.etsi.org/technologies-clusters/technologies/mobile/cellular-history

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