System Architecture Evolution

System Architecture Evolution (aka 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:

Contents

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 (EPC), also known as SAE Core. The EPC will serve as equivalent of GPRS networks (via the Mobility Management Entity, Serving Gateway and PDN Gateway subcomponents).

The subcomponents of the EPC are[1][2]:

Non Access Stratum (NAS) protocols

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. 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 involves different types of procedures such as:

  1. EMM common procedures -- can always be initiated whilst 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, EMM information.
  2. 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), Periodic tracking area update (S1 mode only).
  3. EMM connection management procedures -- manage the connection of UE with 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 NAS messages.

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.

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:

  1. 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.
  2. 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

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 S11 Interface support MME supports S11 interface with Serving Gateway. The integrated S11 interface stack consists of IP, UDP, eGTP-C.

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.  

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 SMS

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 two solutions for SMS:

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

Multiple access networks

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.

3GPP releases

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 (to be confirmed) Work in progress (as of September 2011). 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). Support of interworking with Broadband Forum accesses.

Reading

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