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System Architecture Evolution

System Architecture Evolution (SAE) is the core network architecture defined by the 3rd Generation Partnership Project () for its Long-Term Evolution () wireless communication standard, representing the evolution of the General Packet Radio Service (GPRS) core network used in and systems. Introduced to support the growing demand for , SAE features a simplified, all-IP based flat architecture that enables higher data rates, reduced latency, and efficient across multiple access technologies, including Evolved Universal Terrestrial Radio Access (), GERAN, UTRAN, and non-3GPP networks like and . The development of SAE began with feasibility studies in 3GPP Release 7 (completed in 2007), focusing on an All-IP Packet Switched (AIPN) domain, and was fully specified in Release 8, with the first Evolved Packet Core (EPC) architecture approved in December 2008. The EPC, the primary component of SAE, separates control and user planes and includes key logical elements such as the Mobility Management Entity (MME) for signaling, Serving Gateway (SGW) and Packet Data Network Gateway (PGW) for user data routing, and supporting functions like the Home Subscriber Server (HSS). SAE has continued to evolve through subsequent 3GPP releases, incorporating enhancements for (QoS), (VoLTE), and interworking with emerging technologies, paving the way for integration with New Radio (NR) in Release 15 and beyond as of 2025.

Introduction and Historical Context

Overview of SAE

System Architecture Evolution (SAE) represents the 3GPP's strategic framework for advancing the 3G Universal Mobile Telecommunications System (UMTS) architecture toward a streamlined, all-IP packet-switched core network designed to accommodate high-speed data transmission and applications. This evolution emphasizes a flat that eliminates legacy circuit-switched elements, enabling efficient packet handling across diverse services. The primary objectives of SAE include achieving a simplified architecture to reduce operational complexity, minimizing latency for enhanced in applications, bolstering for efficient device transitions, and facilitating seamless handovers between heterogeneous access networks such as and non-3GPP systems. These goals address the limitations of prior generations by prioritizing an IP-centric design that supports scalable and unified bearer for both and . At its core, introduces the Evolved Packet System (), an integrated framework that merges the Evolved Packet Core ()—serving as the central control and data plane for packet routing—with the Evolved Universal Terrestrial (E-UTRAN) to form a cohesive all-IP ecosystem. This combination ensures optimized end-to-end connectivity while maintaining with existing infrastructures. SAE development commenced within Release 8, with initial studies starting around 2005 and normative specifications developed and finalized in Release 8 from 2006 to 2008, culminating in the release freeze in 2008. The first commercial deployments of SAE-enabled networks occurred in late 2009, marking the practical realization of these advancements in operator environments.

Evolution from Previous Generations

The second generation (2G) mobile networks, exemplified by the Global System for Mobile Communications (GSM), primarily utilized a circuit-switched core network for voice services, dedicating fixed channels for the duration of calls, which ensured reliable but inefficient resource allocation for data. The introduction of General Packet Radio Service (GPRS) added packet-switched capabilities for data transmission, but this required separate core network domains: the circuit-switched domain managed by the Mobile Switching Center (MSC) for voice, and the packet-switched domain handled by the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN) for IP data. This bifurcation led to key limitations, including the inability to efficiently support simultaneous voice and data sessions without complex workarounds, low data throughput typically under 100 kbps, and suboptimal handling of bursty IP traffic due to the legacy circuit-switched overhead. The third generation (3G) Universal Mobile Telecommunications System (UMTS) marked significant progress with the UMTS Terrestrial Radio Access Network (UTRAN), supporting higher data rates up to 384 kbps in Release 99 and beyond through enhancements like High-Speed Packet Access (HSPA). However, UMTS retained the dual-domain core architecture, with the circuit-switched (CS) domain optimized for real-time voice via the MSC Server and , and the packet-switched (PS) domain for data via the SGSN and GGSN. This separation increased system complexity, as services had to navigate distinct paths, leading to inefficiencies for IP-centric applications; for instance, (VoIP) suffered from suboptimal integration with the CS domain, while PS domain latency for transitions from idle to active states typically around 500–2000 ms, hindering responsive multimedia and web services. The evolution toward System Architecture Evolution (SAE) was driven by the explosive growth of IP-based services in the mid-2000s, including mobile internet, video streaming, and multimedia messaging, which exposed the limitations of hybrid CS/PS architectures in supporting seamless of and . Traditional systems struggled with for all-IP traffic, high operational costs from maintaining dual domains, and poor for packet-optimized services, prompting to pursue a unified, flat all-IP core network. A pivotal event was the 2005 outlined in 3GPP 23.882, which evaluated architectural options to achieve lower latency, simplified signaling, and enhanced support for (IMS)-based services, ultimately recommending an evolved packet core to replace the fragmented / designs. To quantify these motivations, set ambitious performance targets compared to , aiming for latency under 100 ms (from camped to active state) versus UMTS/HSPA's typical 500–2000 ms, and user plane one-way below 5 ms to enable applications, thereby addressing the inefficiencies of prior generations' higher delays and domain-specific bottlenecks.

SAE Core Network Architecture

Evolved Packet Core (EPC) Design Principles

The Evolved Packet Core () embodies core design principles centered on a fully packet-switched, all- that supports connectivity without reliance on circuit-switched domains, enabling efficient delivery of data and voice services over and compatible technologies. This packet-switched foundation, as defined in 3GPP specifications, eliminates legacy hierarchies from prior generations like , focusing instead on streamlined -based transport for both user data and signaling. A key principle is the adoption of a flat , which reduces the number of network nodes and hierarchies to minimize and operational complexity while enhancing overall efficiency. Complementing this is the logical separation of the and user plane, allowing independent scaling and optimization of signaling functions from data forwarding paths. The handles , session control, and , while the user plane focuses on bearer-level data transport using protocols like GTP-U over /. This separation promotes flexibility, as control elements can be centralized or distributed without impacting data throughput. At a high level, the EPC structure routes user plane traffic directly from the eNodeB in the E-UTRAN through the Serving Gateway (SGW) to the Packet Data Network Gateway (PGW), and onward to external Packet Data Networks (PDNs), supporting IP, non-IP, and Ethernet PDN types with minimal intermediaries. In contrast, the control plane operates via the Mobility Management Entity (MME), which interfaces with radio access elements to orchestrate attachment, handover, and bearer establishment. High-level architectural diagrams typically illustrate this as two parallel paths: a direct, low-hop user data flow and a signaling overlay for control, emphasizing the flat topology without deep node nesting. Central to this structure are standardized interfaces that ensure and simplicity, including the S1 interface to the RAN (split into S1-MME for control signaling via S1-AP and S1-U for user data via GTP-U), S5/S8 between gateways for interworking and roaming (using GTP or PMIP), and S11 between the and SGW for control coordination. These interfaces form a modular framework that avoids proprietary dependencies and supports seamless integration. The resulting benefits include exceptional , capable of handling billions of devices through mechanisms like load balancing across multiple instances of core elements and efficient resource pooling, as well as robust support for heterogeneous access networks such as E-UTRAN, GERAN, WLAN, and non-3GPP technologies. This design facilitates global , optimized , and adaptability to diverse traffic patterns, laying the groundwork for high-performance services.

EPC Logical Components and Interfaces

The Evolved Packet Core (EPC) architecture is structured around logical components that separate control plane functions, user plane handling, and subscriber data management to ensure efficient packet-switched operations in LTE networks. The control plane comprises the Mobility Management Entity (MME) and the Policy and Charging Rules Function (PCRF), which facilitate signaling for mobility management and policy enforcement across the network. The user plane includes the Serving Gateway (SGW) and the Packet Data Network Gateway (PGW), which manage the forwarding and routing of user data packets. Subscriber data management is handled by the Home Subscriber Server (HSS), a centralized repository for user profiles and authentication keys. These logical components interconnect through defined reference points that standardize communication pathways within the EPC and with external entities. The S1-MME reference point connects the to the for signaling, enabling session establishment and mobility coordination. The S1-U reference point links the to the SGW for user plane data transfer, supporting direct tunneling of packets. The S6a reference point interfaces the with the HSS for , , and updates. The SGi reference point provides the PGW's connection to external Packet Data Networks (PDNs), allowing access to services like the or IMS. Specific protocols underpin these reference points to ensure reliable data and signaling exchange. The GPRS Tunneling Protocol (GTP) is employed for user plane tunneling on interfaces like S1-U, encapsulating user traffic to maintain end-to-end connectivity during mobility. Diameter protocol supports authentication and signaling on the S6a interface, facilitating secure subscriber data retrieval and updates. The Non-Access Stratum (NAS) protocol operates between the User Equipment (UE) and MME via the S1-MME reference point, handling higher-layer control messages for procedures such as attach and detach. The EPC's reference points and protocols collectively enable key architectural features like and , as illustrated in the non-roaming architecture (3GPP TS 23.401, Figure 4.2.1-1) and roaming architecture (Figure 4.2.2-1). In roaming scenarios, interfaces such as S6a and SGi support home-routed traffic, where user data traverses the home network's PGW for application, while S1-MME and S1-U facilitate seamless between eNodeBs by maintaining session continuity without service interruption. This design promotes and across operator networks.

Key EPC Network Elements

Mobility Management Entity (MME)

The serves as the central node in the , responsible for managing access, mobility, and session control within the /E-UTRAN environment. It handles non-access stratum () signaling termination and security, ensuring secure communication between the UE and the core network while coordinating with base stations (eNodeBs) via the S1-MME . As a non-user-plane entity, the MME focuses exclusively on signaling to minimize and optimize resource use, interacting with gateways like the Serving Gateway (SGW) over the S11 for bearer coordination. This design enables efficient support for high-mobility scenarios in networks. Key functions of the encompass NAS signaling termination, which involves processing messages for connection establishment and maintenance; , including tracking area updates and signaling to maintain location and seamless transitions; through the Evolved Packet System Authentication and Key Agreement (EPS-AKA) procedure using vectors from the Home Subscriber Server (HSS); and bearer activation for default and dedicated bearers to enable data sessions with appropriate (QoS) parameters. The also manages contexts in ECM-IDLE and ECM-CONNECTED states, storing essential information such as location on a tracking area list granularity and supporting between 3GPP accesses via inter-CN node signaling. These functions position the as the primary orchestrator of operations, ensuring network efficiency and without involvement in user data routing. The executes core procedures such as attach and detach to initiate or terminate network access, with the attach process involving an Attach Request message from the followed by an Attach Accept from the after and bearer setup. Idle mode tracking relies on tracking area lists assigned to the , allowing the to monitor location without constant signaling, while paging procedures notify UEs in ECM-IDLE state across registered tracking areas to re-establish connectivity for incoming sessions. signaling supports intra-LTE and inter-RAT by transferring contexts between MMEs or to legacy nodes, ensuring minimal disruption. via EPS-AKA occurs during attach or tracking area updates, verifying UE identity against HSS data and deriving session keys for NAS security. Bearer activation integrates with these procedures, establishing EPS bearers during attach or service requests to support PDN connectivity. For scalability, the employs pooling mechanisms where multiple MME instances serve a common MME Pool Area, enabling load balancing across cells through weight factors and rebalancing to distribute traffic and enhance redundancy. This architecture allows dynamic selection of MMEs during handovers or attaches, reducing single-point failures and supporting dense deployments. In deployment, the MME acts as a centralized control node, interfacing with eNodeBs for signaling and HSS for subscriber data, with its control-plane-only role contributing to low-latency operations by offloading user plane handling to gateways.

Serving Gateway (SGW) and Packet Data Network Gateway (PGW)

The Serving Gateway (SGW) and Packet Data Network Gateway (PGW) form the primary user plane elements in the Evolved Packet Core () of System Architecture Evolution (), handling the transport, routing, and external of user data packets while supporting and charging requirements. The SGW serves as a local anchor point within the , interfacing with eNodeBs to manage intra-LTE , whereas the PGW acts as the network's toward external Packet Data Networks (PDNs), ensuring IP-level and application. Together, they enable seamless data session establishment and , utilizing GPRS Tunneling Protocol (GTP) for efficient user plane encapsulation. The SGW anchors the user plane locally for intra-LTE handovers by switching data paths between eNodeBs without interrupting service, forwarding packets via the and buffering downlink data during UE idle states to prevent loss. It routes user plane traffic toward the PGW over the and supports by duplicating intercepted traffic streams for authorized entities, as required by national regulations. Additionally, the SGW collects basic charging-related data, such as type and location, forwarding it to the PGW or charging functions to enable accurate billing. The PGW provides external PDN connectivity by routing IP packets between the EPC and external networks via the SGi interface, allocating IPv4 or addresses (or prefixes) to the UE upon session initiation. It enforces subscriber-specific policies for data handling and generates Charging Data Records (CDRs) to track usage volumes, service types, and durations for offline and online charging systems. As the mobility anchor for inter-system handovers, the PGW maintains session continuity across 3GPP and non-3GPP accesses, updating routes dynamically during mobility events. Key procedures involving the SGW and PGW include default and dedicated bearer setup, initiated during attach or PDN connectivity requests under control. In default bearer establishment, the SGW creates GTP tunnels to the and PGW, relaying Create Session Request messages to allocate resources and assign Tunnel Endpoint Identifiers (TEIDs) for user plane encapsulation. Dedicated bearers for enhanced (QoS) follow a similar process via Modify Bearer exchanges, allowing the PGW to apply traffic flow templates (TFTs) for differentiated handling of application-specific data streams. GTP tunneling, primarily via GTP-U over /, ensures reliable user plane transport between the SGW and PGW, with the SGW managing path switches during handovers by sending end marker packets to delineate old and new paths. In combined operation, the S5/S8 interface between the SGW and PGW supports inter-operator roaming by enabling GTP-based tunneling across home and visited networks, with the PGW serving as the central edge gateway for all external PDN interactions. This setup allows the SGW to remain in the visited network for local anchoring while the PGW in the home network handles subscription-based routing and charging, ensuring minimal latency for roaming users.

Supporting Elements: HSS, ANDSF, and ePDG

The Home Subscriber Server (HSS) functions as the central subscriber database in the System Architecture Evolution (SAE) Evolved Packet Core (EPC), storing essential user data such as International Mobile Subscriber Identity (IMSI), Mobile Station International Subscriber Directory Number (MSISDN), International Mobile Equipment Identity (IMEI), authentication vectors, location information, and subscription profiles including Packet Data Network (PDN) contexts, Quality of Service (QoS) parameters like QoS Class Identifier (QCI) and Allocation and Retention Priority (ARP), and Access Point Name (APN) configurations. It provides authentication vectors, including Random (RAND), expected response (XRES), Authentication Token (AUTN), and key derivation parameters, to support EPS Authentication and Key Agreement (AKA) procedures during initial attachment and mobility events. The HSS maintains UE location data, such as Tracking Area Identity (TAI) and E-UTRAN Cell Global Identifier (ECGI), updating it via procedures like Update Location and Cancel Location to facilitate paging and mobility management. Access to the HSS is enabled through the S6a interface using the Diameter protocol, allowing the Mobility Management Entity (MME) to retrieve and update subscriber information for authorization and service provisioning. The Access Network Discovery and Selection Function (ANDSF) operates as a policy server in the architecture, assisting () in selecting between and non- accesses like and by delivering discovery rules and offload policies. It provides Inter-System Mobility Policy (ISMP) to prioritize networks based on criteria such as access technology, location (e.g., PLMN, TAC, or geolocation), and time validity, enabling efficient decisions. For traffic management, the ANDSF supplies Inter-System Routing Policy (ISRP), which defines rules for distributing IP flows across accesses using mechanisms like IP Flow Mobility (IFOM), Multi-Access PDN Connectivity (MAPCON), or non-seamless WLAN offload, including flow-based or service-based routing with priority assignments. Policies are provisioned to the via the () Device Management (DM) protocol, supporting client-initiated sessions with Management Object (MO) identifiers like urn:oma:mo:ext-3gpp-andsf:1.0 for dynamic updates based on UE context. The evolved Packet Data Gateway (ePDG) serves as the entry point for untrusted non-3GPP accesses, such as public , into the , ensuring secure interworking by establishing tunnels to the PDN Gateway (PGW). It handles IKEv2-based Security Associations (), either as a single SA for all bearers or separate SAs per dedicated bearer, routing uplink and downlink packets using Traffic Flow Templates (TFTs) and enforcing QoS via QCI or Code Point (DSCP) mapping. The ePDG connects to the PGW over the S2b interface using Proxy Mobile (PMIPv6) or GTP for mobility anchoring and bearer management, while the SWu interface manages the tunnel with the , and SWm supports signaling for authentication. It allocates IP addresses, resolves APNs, and supports PDN connectivity establishment, including emergency sessions, by interacting with policy elements for authorization. These elements integrate to enable seamless multi-access in , where the HSS supplies subscriber profiles and data to the ePDG via the Server during non-3GPP attachment, ensuring consistent authorization across accesses. The ANDSF complements this by providing discovery and routing policies that guide selection of or , facilitating handovers where the ePDG maintains tunnels and the HSS updates location information (e.g., via Update Location over S6a) to preserve session continuity without service interruption. This coordination supports preservation and bearer recreation during transitions between and untrusted non-3GPP networks, enhancing mobility for multi-access scenarios.

Protocols and Signaling in SAE

Non-Access Stratum (NAS) Protocols

The Non-Access Stratum (NAS) protocols in the () facilitate control plane signaling between the () and the core network, specifically the (), independent of the . These protocols are layered above the (RRC) sublayer and comprise two main sublayers: the EPS Mobility Management (EMM) sublayer for handling UE-network attachment and mobility, and the EPS Session Management (ESM) sublayer for managing data sessions and bearer contexts. The EMM sublayer oversees the UE's registration and mobility within the EPS, transitioning the UE between states such as EMM-REGISTERED and EMM-DEREGISTERED. Key procedures include EPS attach, which allows the UE to register with the , establish an EMM context, and often initiate a default bearer for packet (PDN) connectivity; this is triggered by the UE sending an ATTACH REQUEST message, followed by an ATTACH ACCEPT or REJECT from the MME. Detach procedures enable the UE or to terminate connectivity, releasing the EMM context and associated bearers via DETACH REQUEST and ACCEPT messages, with options indicating whether re-attachment is required. Tracking area update (TAU) procedures update the UE's location when it moves outside its assigned tracking area identity (TAI) list or upon expiry of periodic timers, using TRACKING AREA UPDATE REQUEST and ACCEPT messages to maintain efficient paging and support. Authentication verifies the UE's identity through the EPS Authentication and Key Agreement () process, involving AUTHENTICATION REQUEST and RESPONSE messages with parameters like RAND and AUTN for mutual authentication between UE and . Security context establishment follows authentication, activating and for NAS signaling via SECURITY MODE COMMAND and COMPLETE messages, selecting appropriate NAS security algorithms to protect subsequent communications. The ESM sublayer manages the activation, modification, and deactivation of bearers to support IP-based and (QoS) requirements. PDN connectivity procedures establish a connection to an external PDN, activating a default bearer through the PDN CONNECTIVITY REQUEST message from the and an ACTIVATE DEFAULT BEARER CONTEXT REQUEST from the , enabling initial data transfer. Bearer procedures create dedicated bearers for specific services, using BEARER REQUEST and ACTIVATE DEDICATED BEARER CONTEXT REQUEST messages to assign resources beyond the default bearer. Bearer resource modification adjusts parameters of existing bearers, such as QoS profiles, via BEARER RESOURCE MODIFICATION REQUEST and MODIFY BEARER CONTEXT REQUEST messages, allowing dynamic adaptation to traffic needs without full re-establishment. These procedures are transported via the , as detailed in the Mobility Management Entity () section. Error handling in NAS protocols ensures robust operation amid failures, using cause values in messages to indicate reasons for rejection or termination, such as (#22 in EMM) or insufficient resources (#26 in ESM), prompting the UE to store forbidden PLMNs or TAIs and adjust behavior accordingly. Timers supervise procedures to manage retries and prevent overload; for instance, T3410 (default 15 seconds) governs EPS attach retransmissions, while T3412 (default 54 minutes) controls periodic and attach retry intervals after rejection, with extended values supported for network flexibility. Other timers include T3482 (8 seconds, up to 4 retries) for PDN requests and T3346 for back-off during , ensuring orderly recovery without excessive signaling.

EPC Internal Protocol Stack

The Evolved Packet Core () employs distinct protocol stacks for signaling and user plane data between its internal network elements, enabling efficient , session control, and in an all- architecture. The stack facilitates signaling exchanges, such as bearer establishment and location updates, while the user plane stack handles the encapsulation and tunneling of user data packets. These stacks are optimized for low and , building on IP layers to minimize complexity compared to prior generations.

Control Plane Protocol Stack

In the EPC, control plane communications rely on application-specific protocols layered over reliable transport mechanisms. For the S1-MME interface between the Mobility Management Entity () and the eNodeB, the stack consists of the S1 Application Protocol (S1AP) atop (SCTP), which runs over (IP) and a such as Ethernet; S1AP handles procedures like initial UE context setup and handover signaling. The SCTP layer ensures reliable, ordered delivery of S1AP messages with multi-homing support for enhanced availability. For the S11 interface connecting the to the Serving Gateway (SGW), the uses GTP-Control (GTP-C, version 2) over () and , with Ethernet as the underlying layer; this stack supports bearer activation, modification, and deletion through messages like Create Session Request. GTP-C messages carry information elements for tunnel endpoint identification and sequence numbering to maintain session integrity during mobility events. The S6a interface between the MME and Home Subscriber Server (HSS) employs the Diameter protocol over SCTP (or TCP) and IP, layered on Ethernet; Diameter facilitates authentication and authorization via commands such as Authentication-Information-Request, which include Attribute-Value Pairs (AVPs) like RAND (random challenge), AUTN (authentication token), and Authentication-Info for EPS-AKA procedures. These AVPs encode subscriber-specific data, ensuring secure vector generation and verification without exposing permanent keys. The (NAS) protocol serves as the uppermost layer in these stacks for mobility and session management signaling between the and , though its details are covered separately.

User Plane Protocol Stack

The user plane in the focuses on efficient packet tunneling between gateways and the (RAN). For interfaces such as S1-U (between SGW and ) and S5/S8 (between SGW and PDN Gateway, PGW), the stack utilizes GTP-User (GTP-U) over and , with Ethernet at the ; GTP-U encapsulates IP packets from the , enabling seamless forwarding across the core. In the RAN side, packets are processed below the EPC stack through (PDCP) for header compression and ciphering, and (RLC) for segmentation, but the core elements handle GTP-U termination. GTP-U headers include a 32-bit Tunnel Endpoint Identifier (TEID) to uniquely identify tunnels per bearer, and an optional 32-bit sequence number for in-order delivery and duplication detection, particularly useful in scenarios. The header structure is compact, typically adding 8 octets when sequence numbering is enabled, supporting high-throughput data paths without mandatory acknowledgments.

Performance Aspects

The EPC protocol stacks achieve header overhead reduction compared to previous systems like the Packet Switched Core, where multi-layered protocols (e.g., GTPv1 over ) added significant encapsulation; SAE's all- design and streamlined GTP headers limit user plane overhead to approximately 40 octets (including //GTP-U), enhanced further by Robust Header (ROHC) in the RAN for //RTP flows. This optimization supports peak data rates exceeding 100 Mbps while minimizing signaling load, as GTP-C and leverage efficient AVP encoding to avoid redundant fields present in legacy SS7-based protocols.

Service Support in SAE

Voice and SMS over EPC

The Evolved Packet Core (EPC) architecture, being entirely packet-switched, lacks a traditional circuit-switched (CS) domain for voice and short message service (SMS), necessitating adaptations to deliver these services over the packet-switched (PS) domain. This is achieved primarily through integration with the (IMS), which enables (VoLTE) and SMS over IP, while fallback mechanisms ensure compatibility with legacy CS networks. VoLTE relies on IMS for call control and media transport, using () signaling to establish and manage sessions. The registers with the IMS core via the Proxy-Call Session Control Function (P-CSCF), which routes messages to the Serving-CSCF (S-CSCF) for authentication and service authorization against the Home Subscriber Server (HSS). Media streams, carried via () and (), are transported over dedicated bearers established by the , ensuring low-latency and guaranteed bitrate for voice traffic. For VoLTE call setup, the originating sends a INVITE message to the P-CSCF, which forwards it through the IMS core to the terminating , including (SDP) offers for negotiation and media parameters. Upon acceptance, the establishes a dedicated bearer for the voice media path, while IMS handles signaling continuity. VoLTE calls are prioritized with dedicated emergency bearers and dynamic , bypassing standard registration if needed, to ensure rapid connection to public safety answering points. SMS in EPC supports multiple delivery paths to accommodate IMS-capable and devices. SMS over IP leverages IMS with an IP Short Message Gateway (IP-SM-GW) for interworking between SIP-based messaging and SMS centers (SMS-SC). For mobile-originated (MO) SMS, the UE encapsulates the SMS payload in a SIP MESSAGE request to the IP-SM-GW, which converts it to the Mobile Application Part (MAP) protocol and routes it to the SMS-GMSC. Mobile-terminated (MT) SMS follows the reverse path, with the IP-SM-GW delivering the message via SIP to the IMS-registered UE. For non-IMS devices or legacy support, SMS over SGs allows the Mobility Management Entity () to interface with the CS domain's MSC/VLR using the SGs Application Part (SGsAP) protocol, emulating SGSN behavior to deliver without full CS fallback. Additionally, payloads can be transported over the Non-Access Stratum () directly to the , which then forwards them to the SMS function for routing, providing an efficient PS-only option especially for control-plane optimized devices. Bearer management for these procedures occurs via the Evolved Session Management (ESM) sublayer of . To address coverage gaps where IMS or PS voice is unavailable, Single Radio Voice Call Continuity (SRVCC) provides seamless handover of VoLTE sessions from EPC to legacy 2G/3G CS networks. During handover, the MME coordinates with the MSC Server enhanced for SRVCC via the Sv interface, transferring the IMS session using a Session Transfer Number for SRVCC (STN-SR) and splitting the PS bearer into CS voice and residual PS data paths. The UE retunes to the target UTRAN/GERAN cell, completes the CS handover, and the IMS core updates the session to route media via the CS domain. These adaptations overcome the absence of a domain in by fully leveraging infrastructure for and , with IMS providing the unified service framework and SRVCC ensuring service continuity during mobility events.

(QoS) Mechanisms

The (QoS) mechanisms in the Architecture Evolution (), also known as the Evolved Packet Core (), enable differentiated treatment of flows to support diverse service requirements such as low for and high throughput for data. employs a bearer-based QoS model where Evolved Packet System () bearers serve as the fundamental units for applying QoS policies across the network, from the () through the to the Packet Data Network Gateway (PGW). In this model, each establishes a default bearer upon initial attachment, which is typically non-Guaranteed Bit Rate (non-) and associated with () 9 for best-effort traffic and IMS signaling, ensuring basic connectivity without resource reservations. Dedicated bearers can then be activated as needed for specific services, allowing granular QoS application; for instance, 1 is used for conversational voice traffic, providing low packet delay budget (100 ms) and low packet error loss rate (10^-2), while 5 supports non-conversational video with a 150 ms delay budget. The is a scalar value from 1 to 9 (with extensions in later releases), standardizing bearer treatment by defining resource type ( or non-), priority level, packet delay budget, and packet error loss rate, which guide scheduling and queueing in network elements like the and PGW. Policy and Charging Control (PCC) forms the core of dynamic QoS management in SAE, with the Policy and Charging Rules Function (PCRF) acting as the central decision point for authorizing and provisioning QoS rules based on subscriber profiles, service data flows, and application requirements. The PCRF communicates with the PGW, functioning as the Policy and Charging Enforcement Function (PCEF), over the Gx reference point to install, modify, or remove PCC rules that map IP flows to bearers and enforce QoS parameters. Additionally, the Rx reference point allows Application Functions (AFs), such as those in the IP Multimedia Subsystem (IMS), to provide session and media information to the PCRF, enabling dynamic adjustment of QoS for ongoing sessions, such as reserving resources for high-priority flows. QoS enforcement occurs at bearer level through parameters like Allocation and Retention (ARP), which determines admission control and pre-emption during —higher ARP values allow pre-emption of lower-priority bearers to free resources. For GBR bearers, the Guaranteed (GBR) parameter ensures a minimum reserved , with Maximum Bit Rate (MBR) capping the upper limit to prevent over-utilization, applied end-to-end from radio access to core network. These mechanisms are implemented via signaling procedures, such as those in the EPS Session Management (ESM) part of the Non-Access Stratum (), for bearer setup and modification. Compared to earlier 3GPP packet-switched systems like GPRS/, which relied on aggregate QoS profiles per Packet Data Protocol () context with limited differentiation, SAE's per-bearer QoS model introduces standardized QCIs and dynamic for more precise, service-aware , enhancing support for multimedia applications.

Integration with Access Networks

Support for Multiple Access Technologies

The System Architecture Evolution (), through the Evolved Packet Core (), facilitates seamless integration of multi-mode (UE) across 3GPP Radio Access Technologies (RATs), including E-UTRAN (), UTRAN (), and GERAN (//). This support enables handovers in connected mode, ensuring continuity of packet-switched services during mobility between these RATs. Multi-mode UEs, capable of operating on multiple RATs, interact with the EPC via the S1 interface for inter-RAT handovers, while intra-E-UTRAN handovers utilize the X2 interface for efficiency. Handover procedures from E-UTRAN to UTRAN or GERAN are network-initiated and coordinated by the Mobility Management Entity (), which forwards relocation requests to the target Serving GPRS Support Node (SGSN) or Mobile Switching Center/Visitor Location Register (). For E-UTRAN to UTRAN , the source sends a Handover Required message to the , which issues a Forward Relocation Request to the SGSN; the target Radio Network Controller (RNC) then allocates resources and acknowledges, enabling forwarding and handover execution. Similarly, E-UTRAN to GERAN handovers involve PS Handover Request/Acknowledge signaling over S1, supporting packet-switched continuity with direct or indirect tunneling for user plane . Reverse handovers from UTRAN or GERAN to E-UTRAN follow analogous steps, with the source SGSN initiating relocation to the , ensuring bearer modification and context transfer for seamless reattachment to EPC bearers. These procedures, defined in Release 8 and enhanced in subsequent releases, prioritize minimal service interruption through preparation and execution phases. In idle mode, supports mobility via reselection policies that allow the to autonomously transition between RATs based on signal quality and network priorities, without frequent signaling to the core network. The evaluates suitability using criteria such as (RSRP) or quality (RSRQ) for E-UTRAN, and equivalent metrics for UTRAN (e.g., CPICH RSCP/Ec/No) and GERAN (e.g., received signal strength). Inter-RAT reselection to a higher-priority RAT occurs if the serving 's signal level (Srxlev) falls below a (e.g., ThreshServing,LowP) and the target RAT's exceeds ThreshX,LowP, after a TreselectionRAT expires. For equal or lower priorities, ranking based on measured quality minus offsets determines the best , with reselection to the highest-ranked suitable after camping for at least one second. Idle Mode Signalling Reduction () further optimizes this by allowing reselection between E-UTRAN and GERAN/UTRAN without immediate Tracking Area (TAU) or Routing Area Update (RAU), using a Temporary Identity (TIN) flag in accept messages to indicate ISR activation. The handles any necessary context synchronization during subsequent updates, as detailed in the Mobility Management Entity section. Combined attach procedures in SAE enable simultaneous registration for Evolved Packet System (EPS) services (e.g., IP connectivity) and non-EPS services (e.g., IMSI-based circuit-switched fallback or SMS), streamlining initial network access for multi-mode UEs. The UE initiates this with an ATTACH REQUEST message indicating "combined EPS/IMSI attach," including identifiers like GUTI or IMSI and UE network capabilities; the MME authenticates the UE, performs security setup, and activates a default EPS bearer within the ATTACH ACCEPT response, while signaling the MSC/VLR for non-EPS registration if supported. Upon success, the UE enters EMM-REGISTERED state, supporting both service types; failure for non-EPS (e.g., due to network congestion) results in EPS-only attach with an appropriate EMM cause. This single-procedure approach, applicable in S1 mode, reduces signaling overhead compared to separate GPRS/IMSI attaches in legacy systems. UE-assisted RAT selection relies on measurements and reporting to inform handover and reselection decisions across RATs. In connected mode, the eNodeB configures measurements via RRC signaling, prompting the UE to report inter-RAT events like B1 (better cell in target RAT) or B2 (serving becomes worse and target better), based on thresholds for UTRAN CPICH or GERAN BCCH quality relative to E-UTRAN RSRP/RSRQ. For idle mode, the UE autonomously measures neighboring RATs when criteria like SnonIntraSearchP/Q are met, reporting indirectly through reselection outcomes or TAU updates that include RAT type and bearer status to the MME. These reports enable the network to optimize RAT/frequency selection using RFSP indices from the MME, ensuring efficient mobility without excessive battery drain on the UE.

Interworking with Non-3GPP Networks

The (SAE), also known as the (EPC), enables integration with non-3GPP access networks such as and through defined reference points that facilitate connectivity while maintaining security and . This interworking supports both trusted and untrusted non-3GPP accesses, allowing () to connect to the EPC via gateways that anchor sessions at the Packet Data Network Gateway (PGW). The primary mechanisms involve the S2a, S2b, and S2c interfaces, which employ protocols like Proxy Mobile IP (PMIP) version 6, GPRS Tunneling Protocol (GTP) version 2, or Dual Stack Mobile IPv6 (DSMIPv6) to handle and bearer management between non-3GPP accesses and the PDN GW. For trusted non-3GPP accesses, the S2a connects the non-3GPP gateway (e.g., a trusted WLAN access gateway) directly to the PDN GW, using PMIP or GTP to establish IP-CAN sessions and manage bearers without intermediate tunneling for . In contrast, the S2b addresses untrusted non-3GPP accesses, such as , by through an Evolved Packet Data Gateway (ePDG) to the PDN GW; this setup employs PMIP or GTP over tunnels established via the SWu reference point using version 2 (IKEv2) for and . The S2c supports host-based mobility protocols like DSMIPv6, enabling direct UE-to-PDN GW communication for dynamic address allocation and binding updates, typically in scenarios requiring minimal network involvement. These interfaces ensure session continuity, with the PDN GW serving as the mobility anchor for preservation during transitions. The ePDG plays a critical role in securing untrusted Wi-Fi connections by encapsulating user plane traffic in IPSec Security Associations (SAs), with a single SA per PDN connection to optimize overhead; it handles IKEv2 signaling for tunnel establishment and maps packet filters to S2b bearers for QoS enforcement. Complementing this, the Access Network Discovery and Selection Function (ANDSF) assists UEs in identifying available non-3GPP networks and selecting appropriate ones (trusted or untrusted) through policies delivered over the S14 interface, including inter-system mobility policies (ISMP) and WLAN selection policies (WLANSP) to guide ePDG discovery via fully qualified domain names (FQDNs). For seamless mobility, SAE supports make-before-break handovers from LTE to Wi-Fi, where the UE initiates a handover attach procedure (using "Handover" attach type) while maintaining the existing 3GPP connection; this involves creating parallel sessions on S2a or S2b, followed by bearer modification requests to switch traffic without service interruption, preserving IP addresses and PDN connections. Authentication for non-3GPP accesses leverages the Home Subscriber Server (HSS) via the SWx interface to the AAA , enabling (EAP)-based procedures during initial attachment; the HSS provides subscription data, PDN GW identities, and static IP allocations if needed, ensuring unified authentication across accesses while integrating with the non-3GPP AAA server over SWa or interfaces. This framework supports scenarios, including PMIP-based chaining over S8/S2b for home-routed traffic, and allows for local breakout in trusted cases to reduce . Overall, these mechanisms enhance SAE's extensibility by integrating diverse accesses without compromising EPC's core principles of all-IP and flat architecture.

Evolution Across 3GPP Releases

Key Enhancements in Early Releases (Rel-8 to Rel-11)

The early releases of the specifications, from Release 8 to Release 11, established the foundational architecture for by introducing the and integrating it with radio access, enabling an all-IP network for enhanced mobility and data services. These releases focused on baseline capabilities, incremental optimizations, and preparations for advanced features, achieving key performance targets such as a downlink peak data rate of 100 Mbps in Release 8 while supporting flexible spectrum usage and seamless handovers. Release 8, frozen in December 2008, defined the baseline architecture with the comprising core elements like the Mobility Management Entity (MME) for functions and the Serving Gateway (S-GW) and Packet Data Network Gateway (P-GW) for user plane handling, facilitating basic including idle mode mobility and procedures between and legacy networks. It laid groundwork for voice services through integration with (IMS), enabling Circuit-Switched Fallback (CSFB) as an initial voice solution. The release emphasized low latency (targeting 5 ms user plane) and efficient , with peak data rates of 100 Mbps downlink and 50 Mbps uplink for a 20 MHz bandwidth deployment. Release 9, completed in December 2009, built on the foundation with refinements to support dual-layer for improved downlink performance at cell edges, enhancing signal quality through multiple antenna streams without increasing complexity significantly. It introduced (VoLTE) for IMS-based voice services, comprehensive Location Services (LCS) for LTE, including methods like Assisted (A-GPS), Observed Time Difference of Arrival (OTDOA), and Enhanced Cell ID (E-CID), enabling precise positioning for emergency services and location-based applications within the . It also introduced Home (HeNB) support for deployments, enabling secure broadband-connected indoor access points integrated with the . Additionally, enhancements to (MBMS) via evolved MBMS (eMBMS) optimized single-cell and multi-cell transmission modes for efficient delivery of broadcast content over bearers. These additions improved network efficiency and user experience while maintaining backward compatibility with Release 8 elements. Release 10, frozen in March 2011, marked the transition to LTE-Advanced, introducing to combine multiple component carriers (up to 100 MHz bandwidth) for higher throughput, achieving peak data rates of 1 Gbps downlink and 500 Mbps uplink, directly leveraging SAE's bearer management for aggregated flows. It further enhanced support through improved Home eNodeB (HeNB) integration, with better interference management and closed subscriber group (CSG) handling for indoor coverage. SAE-specific advancements included refined (SON) features for automated configuration and self-healing, reducing operational costs in heterogeneous deployments. Release 11, completed in December 2012, further refined LTE-Advanced capabilities with Coordinated Multi-Point (CoMP) transmission and reception to mitigate inter-cell , improving in dense deployments through joint scheduling across eNodeBs coordinated via the . It expanded to support up to five component carriers, including non-contiguous intra-band configurations and multiple timing advances for better flexibility in bearer establishment. impacts included enhanced bearer handling for CoMP and aggregation scenarios, such as prioritized QoS bearers for services and improved robustness during inter-eNodeB handovers. These developments solidified 's role in supporting advanced radio features without core network overhauls.

Advancements Toward 5G Integration (Rel-12 Onward)

Starting with Release 12 in 2013, the SAE architecture incorporated enhancements to support deployments, including improved integration for better urban coverage and capacity, advanced offload procedures via the IP Flow Mobility in Evolved Packet System (IFOM) and Access Network Discovery and Selection Function (ANDSF), and Coordinated Multi-Point (CoMP) operations to reduce inter-cell interference. These features optimized EPC handling of multi-RAT traffic and prepared the ground for denser, more efficient networks. Releases 13 and 14, spanning 2014 to 2016, extended capabilities with Licensed Assisted Access (LAA), allowing to opportunistically use unlicensed spectrum while maintaining anchoring for control and mobility management. Release 13 also introduced initial (V2X) support, enabling direct device-to-device communications for safety applications with integration for network-assisted coordination. Enhanced Coverage (EC) modes were added for , providing up to 20 dB gain in to serve devices in basements or rural areas, with optimizations for low-power, low-data-rate traffic. Building on this, Release 14 advanced with enhanced sidelink communications and further refinements, including multi-carrier support in EC for broader deployment flexibility. From Release 15 onward, beginning in , SAE evolved to directly support integration through the Non-Standalone (NSA) , where 5G New Radio (NR) air interface leverages the existing for initial deployments, enabling enhanced (eMBB) with peak data rates exceeding 10 Gbps, massive Machine-Type Communications (mMTC) for up to 1 million devices per square kilometer, and Ultra-Reliable Low-Latency Communications (URLLC) with latencies under 1 ms. This option utilized EPC's and SGW/PGW for control and user plane functions, allowing operators to boost capacity without immediate replacement. The parallel introduction of the (5GC) in Standalone () mode facilitated a gradual transition, with EPC serving as a bridge until full 5GC adoption. SAE received key extensions in these releases, such as virtualized EPC (vEPC) implementations compliant with ETSI NFV standards, deploying core functions like PGW and SGW on hardware for scalable, cloud-native operations. Precursors to emerged via enhanced Policy and Charging Control (PCC) in TS 23.203, enabling dynamic bearer-level QoS differentiation and service-based policies that anticipated slicing isolation. As of November 2025, widespread 5G SA deployments utilizing 5GC have diminished EPC's role, with many operators migrating legacy LTE traffic to unified 5G cores for efficiency. Release 18, with specifications frozen in 2025 (Stage 3 in September 2025), further advanced integration within the evolved architecture by enhancing non-terrestrial network (NTN) support for satellite- connectivity, introducing positioning for low-complexity devices with sub-meter accuracy, and improving coverage for personal networks through extended discontinuous reception and power-saving features. These updates, building on prior EC modes, ensure SAE-derived elements remain viable for hybrid 4G-5G ecosystems.

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