Trusted Non-3GPP Access Network

Description

A Trusted Non-3GPP Access Network (TNAN) is a critical component in 3GPP's 5G architecture for supporting access network convergence. It refers to a non-3GPP radio access network, most commonly a Wi-Fi network or a fixed broadband network, which connects to the 5G Core (5GC) via a standardized, secure interface. The key distinction from an untrusted non-3GPP access is that the TNAN has a pre-established trust relationship with the 5G operator. This trust is validated through network authentication, allowing the 5GC to extend its services directly over this access without requiring an additional IPsec tunnel for all user traffic, as is the case with untrusted access.

Architecturally, a TNAN connects to the 5GC via the TNGF (Trusted Non-3GPP Gateway Function) in the case of wireline access, or the W-AGF (Wireline Access Gateway Function) for certain fixed scenarios. The TNGF acts as a control plane proxy and user plane anchor, presenting the non-3GPP access to the 5GC's AMF and SMF as a logical N3 interface. The connection between the User Equipment (UE) and the TNAN uses native access-specific security (e.g., WPA3 for Wi-Fi). The trust is established between the TNAN/TNGF and the 5GC operator, often based on the TNAN's credentials validated by the 3GPP AAA server or the N3IWF for the control plane. User plane traffic can flow directly from the TNAN to the UPF, secured by the underlying transport network's security.

How it works involves several steps. First, the UE discovers and associates with the TNAN using standard Wi-Fi or Ethernet procedures. For network access, the UE initiates 5G authentication and registration procedures. The TNGF relays these signaling messages to the AMF over the N2 interface. The 5GC authenticates the UE using 5G-AKA or EAP-AKA', and simultaneously validates that the TNGF/TNAN is a trusted entity. Once authenticated, the SMF establishes a PDU Session, and user data flows between the UE and the UPF via the TNAN and TNGF. The TNAN's role is to provide a trusted IP connectivity path, enabling seamless mobility and session continuity between 3GPP and non-3GPP accesses, and allowing the operator to apply consistent policies and charging across all access types.

Purpose & Motivation

TNAN was introduced in 3GPP Release 16 as part of the broader 5G System architecture to fully realize the vision of converged access. Prior to this, non-3GPP access integration (e.g., via ePDG in 4G) often treated all such access as 'untrusted,' mandating resource-intensive IPsec tunnels for every UE connection, which added overhead and complexity. The purpose of TNAN is to recognize that many non-3GPP networks, especially operator-managed Wi-Fi or partner fixed networks, are inherently trustworthy from the operator's perspective.

This concept solves the problem of inefficient resource usage and latency for trusted access scenarios. By establishing a network-level trust anchor (the TNGF), it allows the 5GC to bypass per-UE tunneling for the user plane, leading to more efficient data forwarding. It was motivated by the increasing importance of Wi-Fi 6 and fixed access in 5G deployment strategies, enabling true wireline-wireless convergence. TNAN allows operators to offer a unified 5G service experience regardless of the underlying access technology, simplifying network management and enabling new service models like 5G Fixed Wireless Access (FWA) with native 5G core integration.