System Architecture Evolution


System Architecture Evolution 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 ; support for higher throughput and lower latency radio access networks ; and support for, and mobility between, multiple heterogeneous access networks, including E-UTRA, and 3GPP legacy systems, but also non-3GPP systems.

SAE Architecture

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, also known as SAE Core. The EPC will serve as the equivalent of GPRS networks.

Evolved Packet Core (EPC)

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 User Equipment 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 node relocation. It is responsible for authenticating the user. The Non Access Stratum 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 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.

SGW (Serving Gateway)

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. 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.

PGW (Packet Data Network Gateway)

The Packet Data Network Gateway provides connectivity from the User Equipment to external packet data networks 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.

HSS (Home Subscriber Server)

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 and Authentication Center.

ANDSF (Access Network Discovery and Selection Function)

The ANDSF provides information to the UE about connectivity to 3GPP and non-3GPP access networks. The purpose of the ANDSF is to assist the UE to discover the access networks in their vicinity and to provide rules 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 untrusted non-3GPP access, e.g. Wi-Fi calling. For this purpose, the ePDG acts as a termination node of IPsec tunnels established with the UE.

Non Access Stratum (NAS) protocols

The Non-Access Stratum protocols form the highest stratum of the control plane between the user equipment and MME. 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 and EPS Session Management protocols.

EMM (EPS Mobility Management)

The EPS Mobility Management protocol provides procedures for the control of mobility when the User Equipment uses the Evolved UMTS Terrestrial Radio Access Network. 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 and periodic tracking area update.
  • 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 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 option 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 stack

MME (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 is a common transport protocol that uses the services of Internet Protocol 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 : SCTP ensures the sequenced delivery of data with multiple unidirectional streams, without blocking the chunks of data in other direction.
  • S1AP is the signaling service between E-UTRAN and the Evolved Packet Core 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.