Steering Of Roaming Application Function

Description

The Steering of Roaming Application Function (SOR-AF) is a standardized network function introduced in 3GPP Release 16 as part of the 5G core network's Service-Based Architecture (SBA). It operates as a producer of Network Exposure Function (NEF) services or can have a direct service-based interface with the Unified Data Management (UDM) function. The primary role of the SOR-AF is to generate and supply Steering of Roaming (SOR) information, which includes lists of preferred Public Land Mobile Networks (PLMNs) for a subscriber. This information is securely transferred to and stored within the UDM.

Architecturally, the SOR-AF interacts with the UDM via the Nudm service-based interface, specifically using the Nudm_SDM (Subscription Data Management) service. When a subscriber's Steering of Roaming data needs to be provisioned or updated, the SOR-AF invokes the Nudm_SDM_Update service operation on the UDM. The UDM then stores this SOR information as part of the subscriber's profile. The SOR data is structured to contain transparent data (intended for the UE) and potentially a secure packet, which is encrypted and integrity-protected for the UE using keys derived from the subscriber's authentication credentials.

When a User Equipment (UE) registers in a visited network, the UDM includes the relevant SOR information in the subscription data sent to the Access and Mobility Management Function (AMF) during registration procedures. The AMF then forwards this SOR information to the UE via NAS signaling. Upon receiving the SOR data, the UE evaluates it, typically when it is in idle mode, and may initiate a PLMN selection procedure to camp on a network from the preferred list provided by the home operator. This mechanism allows the home operator to dynamically steer roaming subscribers towards partner networks with better commercial agreements or service quality, without requiring manual intervention on the UE.

Purpose & Motivation

The SOR-AF was created to provide a standardized, secure, and dynamic method for home network operators to steer their outbound roaming subscribers towards preferred partner networks. Prior to its standardization, steering of roaming was often implemented through proprietary, non-standardized means such as Over-The-Air (OTA) updates via the SIM card or device-based apps, which could be less secure, less reliable, and not integrated with core network provisioning systems. The lack of a network-based standard made it difficult to ensure timely updates and enforce steering policies consistently across all subscriber devices.

The motivation stems from the commercial and technical need for operators to manage roaming traffic efficiently. By steering subscribers to specific Visited Public Land Mobile Networks (VPLMNs), the home operator can control costs based on roaming agreements, improve the subscriber's quality of experience by directing them to networks with better coverage or service capabilities, and balance load across multiple roaming partners. The integration into the 5G SBA allows for automated, policy-driven steering that is part of the subscriber's core profile, enabling more sophisticated and real-time traffic management compared to legacy methods.