LTE (Long Term Evolution) Introduction

LTE Introduction

Contents

Ⅰ. LTE Introduction

Ⅱ. LTE Protocol Layer

Ⅲ. SAE Architecture

Ⅳ. Non Access Stratum(NAS) Protocols

Ⅴ. EPC Protocol Stacks 32

2

6

17

26

1

I. LTE Introduction (1/4)

Long Term Evolution (LTE) is the latest step in moving forward from the cellular 3G services to 4G services (e.g. HSPA to LTE or CDMA to LTE).

• LTE is based on standards developed by the 3rd Generation Partnership Project (3GPP).

• LTE may also be referred more formally as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network(E-UTRAN).

• Even though 3GPP created standards for GSM/UMTS family, the LTE standards are completely new, with exceptions where it made sense.

• The following are the main objectives for LTE.

Increased downlink and uplink peak data rates. Scalable bandwidth Improved spectral efficiency All IP network A standard’s based interface that can support a multitude of user types.

2


The original study item on Long Term Evolution (LTE) of 3GPP Radio Access Technology was initiated with the aim to ensure that 3GPP RAT is competitive in the future (next 10 years).

• Focus of the study was on enhancement of the radio-access technology (UTRA) and optimization & simplification of radio access network (UTRAN).

• The key driving factors for LTE are: Efficient spectrum utilization Flexible spectrum allocation Reduced cost for the operator Improved system capacity and coverage Higher data rate with reduced latency

• Targets set for LTE are listed below [3GPP TR 25.913]: Increased peak data rate:100Mbps for DL with 20MHz (2 Rx Antenna at UE), 50Mbps for UL with 20MHz Improved spectral efficiency: 5bps/Hz for DL and 2.5bps/Hz for UL Improved cell edge performance (in terms of bit rate) Reduced latency

3


LTE uses a simplified single node architecture consisting of the eNBs (E-UTRAN Node B) and MME / S-GW and PDN-GW.

• The eNB communicates with the Evolved Packet Core (EPC) using the S1 interface; specifically with the MME (Mobility Management Entity) and the UPE (User Plane Entity) identified as S-GW (Serving Gateway) using S1-C and S1-U for control plane and user plane respectively.

• The MME and the UPE are preferably implemented as separate network nodes so as to facilitate independent scaling of the control and user plane.

• Also the eNB communicates with other eNB using the X2 interface (X2-C and X2-U for control and user plane respectively).

4


LTE supports an option of Multicast/Broadcast over a Single Frequency Network (MBSFN), where a common signal is transmitted from multiple cells with appropriate time synchronization.

• The eNB being the only entity of the E-UTRAN supports all the functions in a typical radio network such as Radio Bearer control, Mobility management, Admission control and scheduling.

• The Access Stratum resides completely at the eNB.

5

II. LTE Protocol Layer 1 - DL(OFDM)

1 Physical resource block consists out of 2 Slots = 1 Subframe = 1 Transmission time

interval (TTI). Each TTI = 1 ms, a user can be allocated to a different PRB.

1 PRB is mapped to 12 sub carriers. The scalability of LTE derives from the fact that

subchannels can be added; 73 subchannels for 1.4 MHz upto 1201 subchannels for 20

MHz.

Subchannels / Tones (each 15 kHz)

1 TTI = 1ms

1 PRB (Physical Resource Block) = 12 Subcarriers = 180 kHz

1 PRB = 2 Slots = 2 * 0.5 ms

1.4 MHz = 72 Tones 20 MHz = 1200 Tones User 1

User 2

User 3

User ..

6

II. LTE Protocol Layer 1 - UL(SC-FDMA)

PRB‘s are grouped to bring down Peak to Average Power Ratio (PAPR) > better

power efficiency at the terminal.

Subchannels / Tones (each 15 kHz)

1 TTI = 1ms

1 PRB (Physical Resource Block) = 12 Subcarriers = 180 kHz

1 PRB = 2 Slots = 2 * 0.5 ms

1.4 MHz = 72 Tones 20 MHz = 1200 Tones User 1

User 2

User 3

User ..

Special subframe containing guard period (switching from DL -> UL)

7

II. LTE Protocol Layer 2 - MAC/RLC/PDCP (1/4)

LTE Layer 2 protocol stack consists of MAC(Medium Access Control), RLC(Radio Link Control) and PDCP(Packet Data Convergence Control) sublayer.

• The MAC sublayer offers a set of logical channels to the RLC sublayer that it multiplexes into the physical layer transport channels.

• It also manages the HARQ error correction, handles the prioritization of the logical channels for the same UE and the dynamic scheduling between UEs, etc.

8

//upload.wikimedia.org/wikipedia/commons/1/1e/EUTRAN_protocol_stack.op.svg


• The RLC sublayer transports the PDCP's PDUs.

• It can work in 3 different modes depending on the reliability provided.

• Depending on this mode it can provide: ARQ error correction, segmentation/concatenation of PDUs, reordering for in-sequence delivery, duplicate detection, etc.


9



• For RRC layer, PDCP sublayer provides transport of its data with ciphering and integrity protection.

• And for the IP layer, it provides transport of the IP packets, with ROHC header compression, ciphering, and depending on the RLC mode in-sequence delivery, duplicate detection and retransmission of its own SDUs during handover.


10



11

II. LTE Protocol Layer 3 - RRC

• Between other layers RRC takes care of the broadcasted system information related to the access stratum and transport of the Non-Access Stratum (NAS) messages, paging, establishment and release of the RRC connection, security key management, handover, UE measurements related to inter-system (inter-RAT) mobility, QoS(Quality of Service), etc.

LTE Layer 3 protocol stack consists of RRC(Radio Resources Control) sublayer.

12


III. SAE Architecture

System Architecture Evolution (SAE) is the core network architecture of 3GPP LTE standard and is the evolution of the GPRS (General Packet Radio Service) core network with some differences.

• Simplified Architecture compared to GPRS

• All IP Network

• Higher throughput and lower latency RANs (Radio Access Networks)

• Support for mobility management between multiple heterogeneous access networks

including;

E-UTRA (LTE, LTE Advanced)

3GPP legacy systems (GERAN of GPRS, UTRAN of UMTS)

Non-3GPP systems (WiMAX, CDMA2000 etc.)

13

III. SAE Architecture

The SAE has a flat, All IP Network architecture with separation of control plane & user plane traffic and its main component is the EPC (Evolved Packet Core).

• EPC is the main component of SAE and also known as SAE Core

• EPC serves as equivalent of GPRS networks via MME (Mobility Management Entity), SGW (Serving Gateway) and PGW (PDN Gateway) sub-component.

• EPC sub-components consist of MME, SGW, PGW, HSS (Home Subscriber Server), ePDG (Evolved Packet Data Gateway) and ANDSF (Access Network Discovery & Selection Function).

14

http://en.wikipedia.org/wiki/File:Evolved_Packet_Core_Diagram.svg

III. SAE Architecture – MME (1/2)

MME (Mobility Management Entity) is the key control component for LTE access network in SAE architecture.

• Be responsible for idle mode UE (User Equipment) tracking and paging procedure including retransmissions

• Involved in the bearer activation /deactivation process

• Choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation.

• The Non Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs.

15


III. SAE Architecture – MME (2/2)

MME (Mobility Management Entity) is the key control component for LTE access network in SAE architecture.

• Authenticating the user (by interacting with the HSS)

• Check the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforce UE roaming restrictions

• Lawful Interception (LI) of signaling is also supported by the MME.

• Terminate the S6a interface towards the home HSS for roaming UEs

• Provide the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN

• Termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management

16


III. SAE Architecture – SGW

SGW (Serving Gateway) controls routing and forwarding of user data packet for LTE access network in SAE architecture.

• Acting as the mobility anchor for the user plane during inter-eNodeB handovers

• Also doing as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PGW)

• Terminate the downlink data path for idle state UEs

• Trigger paging when downlink data arrives for the UE

• Manages & store UE contexts, e.g. parameters of the IP bearer service, network internal routing information

• Perform replication of the user traffic in case of lawful Interception (LI)

17


III. SAE Architecture – PGW

PGW (PDN Gateway) provides connectivity from the UE to external packet data networks by being the point of exit & entry of traffic for the UE in SAE architecture.

• A UE may have simultaneous connectivity with more than one PGW for accessing multiple PDNs.

• The PGW performs policy enforcement, packet filtering for each user, charging support, LI (Lawful Interception) and packet screening.

• Another key role of the PGW is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO).

18


III. SAE Architecture – HSS

HSS (Home Subscriber Server) is a central database that contains user-related and subscription-related information in SAE architecture.

• Store user information and service profile information

• Support mobility management

• Provide call and session establishment interworking with MME

• User authentication and access authorization

• The HSS is based on pre-Rel-4 Home Location Register (HLR) and Authentication Center (AuC).

19


III. SAE Architecture – ANDSF

ANDSF (Access Network Discovery & Selection Function) provides information to the UE about connectivity to 3GPP & non-3GPP access networks in SAE architecture.

• Assist the UE to discover the access networks in their vicinity

• Provide UE with rules or policies to prioritize & manage connections to these access networks

20


III. SAE Architecture – ePDG

ePDG (Evolved Packet Data Gateway) provides security feature with Access Network Discovery & Selection Function) provides information to the UE about connectivity to 3GPP & non-3GPP access networks in SAE architecture.

• Secure the data transmission with a UE connected to the EPC over an untrusted non-3GPP access.

• Act as a termination node of IPsec (IP Security) tunnels established with the UE.

21


IV. Non Access Stratum(NAS) Protocols

Non Access Stratum (NAS) protocols form the highest stratum of the control plane between the UE and MME. including the following:

• Support the mobility of the UE and the session management procedures to establish and maintain IP connectivity between the UE and a PDN GW

• Define the rules for a mapping between parameters during inter-system mobility with 3G networks or non-3GPP access networks

• Provide the NAS security by integrity protection and ciphering of NAS signaling messages

• Consist of specific sequences of elementary procedures with EPS Mobility Management (EMM) and EPS Session Management (ESM) protocols for complete NAS transactions

* EPS (Evolved Packet System) provides the subscriber with a "ready-to-use" IP connectivity and an "always-on" experience by linking between mobility management and session management procedures during the UE attach procedure.

22

IV. NAS Protocols – EPS Mobility Management (1/3)

• EPS Mobility Management (EMM) connection management procedures handle the connection of the UE with the network.

• EPS Mobility Management (EMM) common procedures handle GUTI (Globally Unique Temporary Identity) reallocation, authentication, security mode control, identification.

• EPS Mobility Management (EMM) specific procedures handle attach, detach & tracking area update.

EPS Mobility Management (EMM) protocol consists of different types of procedures such as connection management, common & specific procedures.

23



• EPS Mobility Management (EMM) connection management procedures handle the connection of the UE with the network through;

Service Request: Initiated by the UE and used to establish a secure connection to the network or to request the resource reservation for sending data or both

Paging: Initiated by the network and used to request the establishment of a NAS signaling connection or to prompt the UE to re-attach if necessary as a result of a network failure

Transport of NAS messages: Initiated by the UE or the network and used to transport SMS messages

Generic transport of NAS messages: Initiated by the UE or the network and used to transport protocol messages from other applications

24



• EPS Mobility Management (EMM) Common Procedures

Always can be initiated while a NAS signaling connection exist

Be initiated by network

Include GUTI (Globally Unique Temporary Identity) reallocation, authentication, security mode control, identification and EMM information

• EPS Mobility Management (EMM) Specific Procedures

Only specific to the UE

At any time only one UE initiated EMM specific procedure can run

Include attach & combined attach, detach & combined detach, normal tracking area update & combined tracking area update (S1 mode only) & periodic tracking area update (S1 mode only)

25

IV. NAS Protocols – EPS Session Management (1/2)

• ESM provides the control of user plane bearers. Transmission of ESM message is suspended during EMM procedures except for

the attach procedure.

• EPS bearer context represents an EPS bearer between the UE and a PDN.

• EPS bearer contexts can remain activated even if the radio and S1 bearers constituting the corresponding EPS bearers between UE and MME are temporarily released.

• EPS bearer context can be either a default bearer context or a dedicated bearer context. A default EPS bearer context is activated when the UE requests a connection to

a PDN. The first default EPS bearer context, is activated during the EPS attach

procedure. Additionally, the network can activate one or several dedicated EPS bearer

contexts in parallel.

• Generally, ESM procedures can be performed only if an EMM context has been established between the UE and the MME, and the secure exchange of NAS messages has been initiated by the MME by use of the EMM procedures.

EPS Session Management (ESM) protocol provides procedures for the handling of EPS bearer contexts.

26

IV. NAS Protocols – EPS Session Management (2/2)

• Once the UE is successfully attached, the UE can request the MME to set up connections to additional PDNs.

• For each additional connection, the MME activates a separate default EPS bearer context.

• A default EPS bearer context remains activated throughout the lifetime of the connection to the PDN.

• EPS bearer contexts procedures Initiated by the network and are used for the manipulation of EPS bearer

contexts, including default EPS bearer context activation, dedicated EPS bearer context activation, EPS bearer context modification, EPS bearer context deactivation.

• Transaction related procedures Initiated by the UE to request for resources, i.e. a new PDN connection or

dedicated bearer resources, or to release these resources. They include PDN connectivity procedure, PDN disconnect procedure, bearer

resource allocation procedure, bearer resource modification procedure.

EPS Session Management (ESM) protocol provides procedures for the handling of EPS bearer contexts.

27

V. EPC Protocol Stacks

Below picture is for protocol stacks of E-UTRAN and MME through S1-MME stack and S-GW through S1 interface.

• MME provides S1-MME stack to support interface with eNodeB and S11 stack to support S11 interface with Serving Gateway.

• SGW provides S11 stack to support S11 interface with MME and S5/S8 stack to support interface with PGW.

• PGW consists of S5/S8 control and data plane stacks to support S5/S8 interface with SGW.

28


Below picture is for user plane protocol stacks of E-UTRAN and S-GW and P-GW through S1-U and S5/S8.

L1

MAC

RLC

PDCP

IP

App

L1

MAC

RLC

PDCP

L1

L2

UDP/IP

GTP-U

Relay

LTE-Uu S1-U S5/S8

UE eNB S-GW P-GW

L1

L2

UDP/IP

GTP-U

L1

L2

UDP/IP

GTP-U

Relay

L1

MAC

UDP/IP

GTP-U

IP

SGi

NAS : Non Access Stratum S1-AP : S1 Application Protocol GTP-C : GPRS Tunneling Protocol for Control Plane GTP-U : GPRS Tunneling Protocol for User Plane

29


Below picture is for control plane protocol stacks of E-UTRAN and MME and S-GW and P-GW through S11-MME and S11 and S5/S8a interface.

L1

MAC

RLC

PDCP

RRC

NAS

L1

MAC

RLC

PDCP

RRC

L1

L2

IP

SCTP

S1-AP

Relay

L1

L2

IP

SCTP

S1-AP

L1

L2

IP

UDP

GTP-C

L1

L2

IP

UDP

GTP-C

L1

L2

IP

UDP

GTP-C

L1

L2

IP

UDP

GTP-C

NAS

LTE-Uu S1-MME S11 S5/S8a UE eNB MME S-GW P-GW

NAS : Non Access Stratum S1-AP : S1 Application Protocol GTP-C : GPRS Tunneling Protocol for Control Plane GTP-U : GPRS Tunneling Protocol for User Plane

30