System Architecture Evolution (SAE) is the core network architecture of 3GPP's LTE wireless

communication standard.

SAE is the evolution of the GPRS Core Network, with some differences:

simplified architecture

all-IP Network (AIPN)

support for higher throughput and lower latency radio access networks (RANs)

support for, and mobility between, multiple heterogeneous access networks, including E-

UTRA (LTE and LTE Advanced air interface), 3GPP legacy systems (for

exampleGERAN or UTRAN, air interfaces of GPRS and UMTS respectively), but also non-3GPP

systems (for example WiFi, WiMAX or cdma2000)

Contents

1SAE Architecture 2Non Access Stratum (NAS) protocols

o 2.1EMM (EPS Mobility Management)o 2.2ESM (EPS Session Management)

3EPC protocol stacko 3.1MME (Mobility Management Entity) protocolso 3.2SGW (Serving Gateway) protocolso 3.3PGW (Packet Data Network Gateway) protocols

4Support of voice services and SMS 5Multiple access networks 63GPP releases 7Reading 8References 9External links

SAE ArchitectureThe SAE has a flat, all-IP architecture with separation of control plane and user plane traffic.

The main component of the SAE architecture is the Evolved Packet Core (EPC), also known

as SAE Core. The EPC will serve as the equivalent of GPRS networks (via theMobility

Management Entity, Serving Gateway and PDN Gateway subcomponents).

EPC nodes and interfaces

The subcomponents of the EPC are

MME (Mobility Management Entity): The MME is the key control-node for the LTE access-

network. It is responsible for idle mode UE (User Equipment) paging and tagging procedure

including retransmissions. It is involved in the bearer activation/deactivation process and is also

responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover

involving Core Network (CN) node relocation. It is responsible for authenticating the user (by

interacting with the HSS). The Non Access Stratum (NAS) signaling terminates at the MME and

it is also responsible for generation and allocation of temporary identities to UEs. It checks the

authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN)

and enforces UE roaming restrictions. The MME is the termination point in the network for

ciphering/integrity protection for NAS signaling and handles the security key management.

Lawful interception of signaling is also supported by the MME. The MME also provides the

control plane function for mobility between LTE and 2G/3G access networks with the S3

interface terminating at the MME from the SGSN. The MME also terminates the S6a interface

towards the home HSS for roaming UEs.

SGW (Serving Gateway): The SGW routes and forwards user data packets, while also acting as

the mobility anchor for the user plane during inter-eNodeB 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 PGW). For idle state UEs, the SGW terminates the downlink

data path and triggers paging when downlink data arrives for the UE. It manages and stores UE

contexts, e.g. parameters of the IP bearer service, network internal routing information. It also

performs replication of the user traffic in case of lawful interception.

PGW (PDN Gateway): The PDN Gateway provides connectivity from 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 PGW for accessing multiple PDNs. The PGW

performs policy enforcement, packet filtering for each user, charging support, 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).

HSS (Home Subscriber Server): The HSS is a central database that contains user-related and

subscription-related information. The functions of the HSS include functionalities such as mobility

management, call and session establishment support, user authentication and access

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

Center (AuC).

ANDSF (Access Network Discovery and Selection Function): The ANDSF provides

information to the UE about connectivity to 3GPP and non-3GPP access networks (such as Wi-

Fi). The purpose of the ANDSF is to assist the UE to discover the access networks in their

vicinity and to provide rules (policies) to prioritize and manage connections to these networks.

ePDG (Evolved Packet Data Gateway): The main function of the ePDG is to secure the data

transmission with a UE connected to the EPC over an untrusted non-3GPP access. For this

purpose, the ePDG acts as a termination node of IPsec tunnels established with the UE.

Non Access Stratum (NAS) protocolsThe Non-Access Stratum (NAS) protocols form the highest stratum of the control plane between the

user equipment (UE) and MME.[3] NAS protocols support the mobility of the UE and the session

management procedures to establish and maintain IP connectivity between the UE and a PDN GW.

They define the rules for a mapping between parameters during inter-system mobility with 3G

networks or non-3GPP access networks. They also provide the NAS security by integrity protection

and ciphering of NAS signaling messages. EPS 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.

Complete NAS transactions consist of specific sequences of elementary procedures with EPS

Mobility Management (EMM) and EPS Session Management (ESM) protocols.

EMM (EPS Mobility Management)

The EPS Mobility Management (EMM) protocol provides procedures for the control of mobility when

the User Equipment (UE) uses the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). It

also provides control of security for the NAS protocols.

EMM involves different types of procedures such as:

EMM common procedures — can always be initiated while a NAS signalling connection exists.

The procedures belonging to this type are initiated by the network. They include GUTI

reallocation, authentication, security mode control, identification and EMM information.

EMM specific procedures — specific to the UE only. At any time only one UE-initiated EMM

specific procedure can run. The procedures belonging to this type are attach and combined

attach, detach or combined detach, normal tracking area update and combined tracking area

update (S1 mode only) and periodic tracking area update (S1 mode only).

EMM connection management procedures — manage the connection of the UE with the

network:

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 procedure: Initiated by the network and used to request the establishment of a NAS

signalling 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.

The UE and the network execute the attach procedure, the default EPS bearer context activation

procedure in parallel. During the EPS attach procedure the network activates a default EPS bearer

context. The EPS session management messages for the default EPS bearer context activation are

transmitted in an information element in the EPS mobility management messages. The UE and

network complete the combined default EPS bearer context activation procedure and the attach

procedure before the dedicated EPS bearer context activation procedure is completed. The success

of the attach procedure is dependent on the success of the default EPS bearer context activation

procedure. If the attach procedure fails, then the ESM session management procedures also fails.

ESM (EPS Session Management)

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

contexts. Together with the bearer control provided by the Access Stratum, it provides the control of

user plane bearers. The transmission of ESM messages is suspended during EMM procedures

except for the attach procedure.

EPS Bearer: Each 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. An 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. 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.

Types of ESM procedures: ESM involves different types of procedures such as:

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.

The MME maintains EMM context and EPS bearer context information for UEs in the ECM-IDLE,

ECM CONNECTED and EMM-DEREGISTERED states.

EPC protocol stackMME (Mobility Management Entity) protocols

The MME protocol stack consists of:

1. S1-MME stack to support S1-MME interface with eNodeB

2. S11 stack to support S11 interface with Serving Gateway

MME supports the S1 interface with eNodeB. The integrated S1 MME interface stack consists

of IP, SCTP, S1AP.

SCTP (Stream Control Transmission Protocol) is a common transport protocol that uses the

services of Internet Protocol (IP) to provide a reliable datagram delivery service to the adaptation

modules, such as the S1AP. SCTP provides reliable and sequenced delivery on top of the

existing IP framework. The main features provided by SCTP are:

Association setup: An association is a connection that is set up between two endpoints for

data transfer, much like a TCP connection. A SCTP association can have multiple addresses

at each end.

Reliable Data Delivery: Delivers sequenced data in a stream (Elimination of head-of-line

blocking): SCTP ensures the sequenced delivery of data with multiple unidirectional streams,

without blocking the chunks of data in other direction.

S1AP (S1 Application Part) is the signaling service between E-UTRAN and the Evolved Packet

Core (EPC) that fulfills the S1 Interface functions such as SAE Bearer management functions,

Initial context transfer function, Mobility functions for UE, Paging, Reset functionality, NAS

signaling transport function, Error reporting, UE context release function, Status transfer.

MME supports S11 interface with Serving Gateway. The integrated S11 interface stack consists

of IP, UDP, eGTP-C.

SGW (Serving Gateway) protocols

The SGW consists of

1. S11 control plane stack to support S11 interface with MME

2. S5/S8 control and data plane stacks to support S5/S8 interface with PGW

3. S1 data plane stack to support S1 user plane interface with eNodeB

4. S4 data plane stack to support S4 user plane interface between RNC of UMTS and SGW of

eNodeB

SGW supports S11 interface with MME and S5/S8 interface with PGW. The integrated control plane

stack for these interfaces consists of IP, UDP, eGTP-C.

SGW supports the S1-U interface with eNodeB and S5/S8 data plane interface with PGW. The

integrated data plane stack for these interfaces consists of IP, UDP, eGTP-U.

PGW (Packet Data Network Gateway) protocols

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

PGW supports S5/S8 interface with Serving Gateway. The integrated control plane stack for the

S5/S8 interfaces consists of IP, UDP, eGTP-C.

The integrated data plane stack for the S5/S8 interface consists of IP, UDP, eGTP-U.

Support of voice services and SMSThe EPC is a packet-only core network. It does not have a circuit-switched domain, which is

traditionally used for phone calls and SMS.

3GPP specified two solutions for voice:

IMS : A solution for IMS Voice over IP was specified in Rel-7.

Circuit-Switched fallback (CSFB): in order to make or receive calls, the UE changes its radio

access technology from LTE to a 2G/3G technology that supports circuit-switched services. This

feature requires 2G/3G coverage. A new interface (called SGs) between the MME and

the MSC is required. This feature was developed in Rel-8.

3GPP specified two solutions for SMS:

IMS : A solution for SMS over IP was specified in Rel-7.

SMS over SGs: this solution requires the SGs interface introduced during the work on CSFB.

SMS are delivered in the Non Access Stratum over LTE. There is no inter-system change for

sending or receiving SMS. This feature was specified in Rel-8.

CSFB and SMS over SGs are seen as interim solutions, the long term being IMS.[4]

Multiple access networksThe UE can connect to the EPC using several access technologies. These access technologies are

composed of:

3GPP accesses: these access technologies are specified by the 3GPP. They

include GPRS, UMTS, EDGE, HSPA, LTE and LTE Advanced.

non-3GPP accesses: these access technologies are not specified by the 3GPP. They include

technologies such as cdma2000, WiFi or fixed networks. 3GPP specifies two classes of non-

3GPP access technologies with different security mechanisms:

trusted accesses, that the network operator consider trustable from a security stand point

(for example: a cdma2000 network). Trusted non-3GPP accesses interface directly with the

network.

untrusted accesses, that the network operator doesn't consider trustable from a security

stand point (for example, a connection over a public WiFi hotspot). Untrusted non-3GPP

accesses are connected to the network via an ePDG, which provide additional security

mechanisms (IPsec tunneling).

It is up to the network operator to decide whether a non-3GPP access technology is trusted or

untrusted.

It is worth noting that these trusted/untrusted categories do not apply to 3GPP accesses.

3GPP releasesThe 3GPP delivers standards in parallel releases, which compose consistent sets of specifications

and features.

Version[5] Released[6] Info[7]

Release 7 2007 Q4 Feasibility study on All-IP Network (AIPN)

Release 8 2008 Q4

First release of EPC. SAE specification: high level

functions, support of LTE and other 3GPP accesses,

support of non-3GPP accesses, inter-system mobility,

Single Radio Voice Call Continuity (SRVCC), CS fallback.

Earthquake and Tsunami Warning System (ETWS).

Support of Home Node B / Home eNode B.

Release 9 2009 Q4

LCS control plane for EPS. Support of IMS emergency

calls over GPRS and EPS. Enhancements to Home Node

B / Home eNode B. Public Warning System (PWS).

Release 10 2011 Q1

Network improvements for machine-type communications.

Various offload mechanisms (LIPA[disambiguation

needed], SIPTO, IFOM).

Release 11 2012 Q3

Further improvements for machine-type communications.

Simulation of USSD in IMS. QoS control based on

subscriber spending limits. Further improvements to LIPA

and SIPTO. Single Radio Video Call Continuity

(vSRVCC). Single Radio Voice Call Continuity from

UTRAN/GERAN to HSPA/E-UTRAN (rSRVCC). Support

of interworking with Broadband Forum accesses.

Release 12 2014 Q2

Work in progress (as of October 2012). LIPA Mobility and

SIPTO at the Local Network. IMS-based telepresence.

Service and Media Reachability for Users over Restrictive

Firewalls (SMURFs).