What is a 5G Access Gateway Function (AGF)?

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The 5G Access Gateway Function (AGF) definition is the result of a joint initiative between the Broadband Forum (BBF) and the GSMA’s 3rd Generation Partnership Project (3GPP). BBF technical report TR-456 specifies the AGF functional requirements, while the 3GPPs technical specification TS 23.316 is the companion document detailing wireless and wireline convergence (WWC) access support for the 5G system. 3GPP refers to this component as the Wireline Access Gateway Function (W-AGF) in its standards and covers not only WWC architectures defined within the BBF but also CableLabs (WR-TR-5WWC-ARCH).

The adoption of a common core infrastructure supporting both fixed and wireless access will benefit the majority of operators, who are now actively assimilating the resources and assets of these previously disparate networks. Moreover, with the advent of 5G new radio (5GNR), fixed wireless access (FWA) will increasingly complement or even replace copper alternatives in the last mile. From the perspective of the BBF, the AGF is an evolution of their technical and deployment specifications for ATM-centric Broadband Remote Access Servers (BRASs) and Ethernet-based Broadband Network Gateways (BNGs). These devices are essentially responsible for providing subscriber authentication, authorization and accounting (AAA) and traffic management in broadband wireline access networks.

An Access Gateway Function provides AAA services plus hierarchical traffic shaping and policing for fixed network (FN) and 5G residential gateways (RGs) being served from a standard 3GPP User Plane Function (UPF) within a common 5G Core (5GC). While policy and subscriber databases are distinct elements in today’s wireline broadband networks, the adoption of a 5G Service Based Architecture (SBA) enables resources, such as the policy control function (PCF) and authentication server function (AUSF) to be shared across mobile, fixed wireless and wireline access networks. It also easily supports shared supporting infrastructure, such as the IP Multimedia Subsystem (IMS) for rich multimedia service delivery.

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The Access Gateway Function (AGF) with a 5G wireline wireless convergence (5G-WWC) infrastructure

Taking into account differing and variable characteristics of each access technique, this approach guarantees that standardized services and service level agreements (SLAs) can be applied across all subscribers and when individual users connect over different access technologies. To facilitate this, the 3GPP QoS model defined within TS 23.501 is applied across the AGF as if it were an access network. When the AGF receives a QoS profile from the 5G core with a request for dedicated resources to support a specific protocol data unit (PDU) session, it is mapped to the AGF’s user plane-level QoS. As always, each QoS flow is identified by a QoS Flow Identifier (QFI).

Supporting modern control plane and user plane separation (CUPS) philosophies, the AGF comprises two functional components. The AGF-CP handles control plane data, exposing an N1/N2 interface towards the AMF, while the AGF-UP is responsible for handing user plane traffic with an N3 interface to a UPF. The specifics of AGF CUPS can be found within the complementary BBF WT/TR-458 specification. The N1, N2 and N3 5G reference interfaces are mandatory requirements on an AGF implementation, along with the V (or Va) interface to the wireline access network (wAN), defined within BBF’s TR-178 - Multi-service Broadband Network Architecture and Nodal Requirements. Wireline access network elements can include digital subscriber line access multiplexers (DSLAMs) and optical line terminals (OLTs). TR-456 also allows for UPF functionality to be combined with the AGF, extending the number of interfaces required to include N4, N6 and N9. This can be likened to the Intermediate UPF (I-UPF) edge aggregator, outlined within 3GPP TS 23.501.

Other key requirements include the ability to construct a concealed subscription permanent identifier (SUPI) - or subscription concealed identifier (SUCI) - based on user identification and authentication information extracted via PPPoE, DHCPv4 option 82 or DHCPv6 option 18. The SUCI registration request derived from these protocols is forwarded to the 5G Access and Mobility Management Function (AMF), which is then proxied to the AUSF and its supporting User Data Management (UDM) server.

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The AGF constructs a SUCI by extracting information from PPPoE or DHCP options

AMF selection is performed per 3GPP TS 23.501. With the AGF also participating in 5G network slicing, Network Slice Selection Assistance Information (NSSAI) must also be employed for AMF selection during the initial registration of an RG. In the case of a combined AGF and UPF, the AMF relays AGF identities parameters to the Session Management Function (SMF), enabling the SMF to determine if a session employs the integrated UPF and internal N3 interface. The SBA’s Network Resource Function (NRF) aids in this discovery process.

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