System Architecture Evolution (SAE) is the core network architecture of mobile communications protocol group 3GPP's LTE wireless communication standard.
SAE is the evolution of the GPRS Core Network, but with a simplified architecture; an all-IP Network (AIPN); support for higher throughput and lower latency radio access networks (RANs); and support for, and mobility between, multiple heterogeneous access networks, including E-UTRA (LTE and LTE Advanced air interface), and 3GPP legacy systems (for example GERAN or UTRAN, air interfaces of GPRS and UMTS respectively), but also non-3GPP systems (for example Wi-Fi, WiMAX or CDMA2000).
The 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 the Mobility Management Entity, Serving Gateway and PDN Gateway subcomponents).
The subcomponents of the EPC are:[1] [2]
The MME is the key control-node for the LTE access-network. It is responsible for idle mode User Equipment (UE) paging and tagging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the Serving Gateway 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 Home Subscriber Server). 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 HSS for roaming UEs.
The Serving Gateway 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 Packet Data Network Gateway). For idle state User Equipment, the Serving Gateway terminates the downlink data path and triggers paging when downlink data arrives for the User Equipment. 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.
The Packet Data Network Gateway (PDN Gateway, also PGW) provides connectivity from the User Equipment (UE) to external packet data networks (PDNs) by being its point of exit and entry of traffic. A piece of User Equipment may have simultaneous connectivity with more than one Packet Data Network Gateway for accessing multiple packet data networks. The PDN Gateway performs policy enforcement, packet filtering for each user, charging support, lawful interception and packet screening. Another key role of the Packet Data Network Gateway is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO).
The Home Subscriber Server is a central database that contains user-related and subscription-related information. The functions of the HSS include 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).
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.
The main function of the ePDG is to secure the data transmission with a UE connected to the EPC over untrusted non-3GPP access, e.g. VoWi-Fi. For this purpose, the ePDG acts as a termination node of IPsec tunnels established with the UE.
The 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 (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.
Complete NAS transactions consist of specific sequences of elementary procedures with EPS Mobility Management (EMM) and EPS Session Management (ESM) protocols.
The EPS (Evolved Packet System) 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:
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.
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:
The MME maintains EMM context and EPS bearer context information for UEs in the ECM-IDLE, ECM CONNECTED and EMM-DEREGISTERED states.
The MME protocol stack consists of:
MME supports the S1 interface with eNodeB. The integrated S1 MME interface stack consists of IP, SCTP, S1AP.
MME supports S11 interface with Serving Gateway. The integrated S11 interface stack consists of IP, UDP, eGTP-C.
The SGW consists of
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.
Main interfaces supported by the P-GW are:
The 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:
3GPP specified three solutions for SMS:
CSFB and SMS over SGs are seen as interim solutions, the long term being IMS.[4]
The UE can connect to the EPC using several access technologies. These access technologies are composed of:
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.
The 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, 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 | 2015 Q1 | Enhanced Small Cells operation, Carrier Aggregation (2 uplink carriers, 3 downlink carriers, FDD/TDD carrier aggregation), MIMO (3D channel modelling, elevation beamforming, massive MIMO), MTC - UE Cat 0 introduced, D2D communication, eMBMS enhancements. |
| Release 13 | 2016 Q1 | Introduced LTE-U / LTE-LAA, LTE-M, Elevation beamforming / Full Dimension MIMO, Indoor positioning, LTE-M Cat 1.4 MHz & Cat 200 kHz |
| ... | ||
| Release 18 | https://www.3gpp.org/release18 | |
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