Non-Seamless WLAN Offload Function

Description

The Non-Seamless WLAN Offload Function (NSWOF) is a network function introduced in the 5G System (5GS) architecture to manage the offloading of specific user data traffic from the 5G core to a trusted Wireless Local Area Network (WLAN) in a non-seamless manner. It operates as a specialized control plane function that interacts with the Policy Control Function (PCF) and the Session Management Function (SMF) to enforce offload policies. The NSWOF's primary role is to evaluate operator policies, user subscription data, and real-time access network information (like WLAN availability and quality) to make decisions on whether a particular Protocol Data Unit (PDU) Session or specific traffic within it should be routed via the WLAN access. These decisions are then translated into rules that are provided to the SMF, which ultimately configures the User Plane Function (UPF) to steer the designated traffic flows to the local WLAN network interface.

Architecturally, the NSWOF is part of the 5G policy framework and is defined to support the Access Traffic Steering, Switching and Splitting (ATSSS) features, though it specifically handles the 'non-seamless' offload component. It communicates with the PCF over the Nnswof service-based interface. When a PDU Session is established or modified, the PCF may consult the NSWOF (if the session is eligible for offload) to obtain specific traffic steering rules for WLAN. The NSWOF bases its decisions on pre-configured operator policies, which might consider factors such as the application type (e.g., streaming vs. background), the current load on the 3GPP radio access network (RAN), the quality of the available WLAN, and the user's subscription profile. The output is a set of Packet Filter Sets or steering rules that identify the IP flows to be offloaded.

How it works involves a coordinated sequence. During PDU Session Establishment, the SMF receives policy information from the PCF. If offload is applicable, the PCF may invoke the NSWOF. The NSWOF returns the specific offload policy rules. The SMF then generates N4 rules for the UPF, instructing it to match the specified traffic (e.g., based on 5-tuple) and forward it to a designated local area data network (LADN) or a specific N6 interface that connects to the WLAN. Crucially, this offload is 'non-seamless,' meaning there is no active mobility management between 5G and WLAN. If the UE loses WLAN connectivity, those offloaded flows are interrupted and must be re-initiated by the application or user over the 5G access. The NSWOF thus provides a centralized, intelligent control point for managing Wi-Fi offload in 5G, moving beyond the simpler APN-based method used in 4G.

Purpose & Motivation

The NSWOF was created to evolve and formalize non-seamless WLAN offload for the 5G era. While 4G's EPC used the NSWO-APN concept, the 5G core's service-based architecture and enhanced policy control demanded a more flexible and dynamic function. The NSWOF addresses the need for granular, real-time traffic steering decisions that can consider a wider array of inputs beyond a static APN, such as instantaneous network conditions, application awareness, and user-specific policies.

The motivation stems from the continued importance of Wi-Fi as a crucial complementary access technology in 5G deployments, especially for indoor coverage and traffic congestion management. Previous approaches were relatively static; the NSWO-APN essentially created a fixed pipe for offloaded traffic. The NSWOF, as a dedicated function, enables more adaptive and intelligent offload. It allows operators to implement complex policies that can, for example, offload only during times of 5G RAN congestion, or offload specific application traffic based on its QoS requirements. This solves the problem of inefficient or user-experience-degrading offload by making it a dynamic, policy-driven network decision rather than a simple static routing rule.