Access Gateway Function

Description

The Access Gateway Function (AGF) is a functional element within the 5G Core (5GC) network, specifically defined to support Wireline Access. Its primary role is to act as a gateway and control point for user equipment (UE) connected via fixed broadband networks, such as those using Digital Subscriber Line (DSL), cable, or fiber optics. The AGF terminates the access-specific protocols from the customer premises equipment (CPE) and presents a standardized interface (N2 and N3) towards the 5G Core's Access and Mobility Management Function (AMF) and User Plane Function (UPF), respectively. This allows the 5G core to manage sessions and mobility for these fixed-line devices as if they were connected via a 3GPP radio access network, enabling true fixed-mobile convergence (FMC).

Architecturally, the AGF sits at the boundary between the untrusted non-3GPP access network (the fixed broadband) and the trusted 5G Core. It contains both control plane and user plane functionalities. On the control plane, it interacts with the AMF over the N2 interface for registration, authentication, and mobility management procedures. It also communicates with the Session Management Function (SMF) for policy and session management, often via the AMF. On the user plane, the AGF connects to the UPF over the N3 interface, establishing the data path for user traffic. The AGF handles the encapsulation and decapsulation of user data, typically using protocols like Point-to-Point Protocol over Ethernet (PPPoE) or IP over the fixed link, and maps these flows onto the appropriate Packet Data Unit (PDU) sessions within the 5G system.

Key internal components of the AGF include the termination point for the fixed-line protocols (e.g., a Broadband Network Gateway (BNG) function), security functions for initial authentication and link encryption, and the logic to map access-specific parameters (like line identifiers) into 5G core identifiers (like SUPI and GPSI). It performs role similar to a Trusted Non-3GPP Interworking Function (TNGF) but is specifically optimized for the characteristics and requirements of wireline access, which is typically considered more stable and higher bandwidth than wireless non-3GPP access. The AGF's operation is governed by policies from the Policy Control Function (PCF), ensuring that service quality and access rules are enforced consistently across different access types.

By abstracting the details of the fixed access, the AGF enables operators to leverage their existing broadband infrastructure to deliver 5G services. This includes not only enhanced mobile broadband but also network slicing and low-latency services to fixed locations. The AGF ensures that features like seamless mobility between fixed and mobile access, consistent authentication, and unified policy enforcement become a reality, forming a cornerstone of the 5G converged core architecture.

Purpose & Motivation

The Access Gateway Function was created to address the industry's move towards Fixed-Mobile Convergence (FMC) within the 5G system. Prior to 5G, fixed and mobile networks largely operated as separate silos with distinct core networks (e.g., IMS for fixed voice, EPC for mobile). This separation led to operational inefficiencies, duplicated functions, and an inability to offer truly unified services that work seamlessly across both access types. The 5G core architecture was designed from the outset to be access-agnostic, but a standardized, efficient gateway was needed to integrate the vast installed base of fixed broadband networks.

The AGF solves the specific problem of how to connect legacy fixed access networks, which use protocols like PPPoE, DHCP, or IPoE, into the cloud-native, service-based architecture of the 5G Core. Without the AGF, operators would need complex and proprietary interworking solutions. The AGF provides a standardized 3GPP-defined method to authenticate fixed-line subscribers using 5G credentials (like 5G-AKA), establish secure PDU sessions for them, and apply 5G quality-of-service (QoS) and policy controls to their traffic. This allows operators to modernize their fixed networks by leveraging the advanced capabilities of the 5G core, such as network slicing for enterprise services or edge computing, without replacing the last-mile infrastructure.

Historically, fixed broadband was managed by Broadband Network Gateways (BNGs) in an IP Multimedia Subsystem (IMS) or simple IP service context. The AGF evolution recognizes that the intelligence and session management should reside in the mobile core for converged services. The creation of the AGF in 3GPP Release 16 was motivated by the desire to unify service delivery, reduce operational costs, and create new revenue streams through blended fixed-mobile offerings. It addresses the limitations of previous dual-stack approaches by providing a single, unified core for all access, enabling consistent user experience and streamlined management.

Key Features

• Terminates fixed broadband access protocols (e.g., PPPoE, IPoE) and interworks them to 5G N2/N3 interfaces
• Enables authentication of fixed-line devices using 5G-AKA and 5G subscriber credentials
• Supports the establishment and management of PDU sessions for UEs over non-3GPP wireline access
• Maps fixed access line identifiers to 5G subscription identifiers (SUPI/GPSI)
• Applies 5G QoS policies and charging rules received from the PCF to fixed access traffic
• Facilitates seamless service continuity and policy consistency across fixed and 3GPP mobile access

Evolution Across Releases

Defining Specifications