Visited Session Management Function

Description

The Visited Session Management Function (V-SMF) is a core network function within the 5G Service-Based Architecture (SBA), specifically defined for roaming scenarios. It is instantiated in the Visited Public Land Mobile Network (VPLMN) when a User Equipment (UE) roams outside its Home Public Land Mobile Network (HPLMN). The V-SMF is responsible for managing Protocol Data Unit (PDU) sessions for the roaming UE within the visited network. Its primary role is to interact with the Visited User Plane Function (V-UPF) to establish, modify, and release user plane connections, handling tasks like IP address allocation, QoS enforcement, and charging data generation for the local breakout traffic.

Architecturally, the V-SMF operates under the control of the Home SMF (H-SMF), which remains in the HPLMN. This split architecture is defined in 3GPP Release 16 and later for enhanced roaming. The V-SMF communicates with the H-SMF via the N16 interface (using the Nsmf_PDUSession service), receiving session policies and directives. Locally, it interfaces with the Visited Access and Mobility Management Function (V-AMF) via the N11 interface and controls the V-UPF via the N4 interface. This allows the V-SMF to manage the user plane path locally, enabling efficient data routing (Local Breakout - LBO) for internet access or access to local data network services without forcing all traffic through the home network.

The V-SMF's operation is crucial for implementing advanced roaming features like Home Routed (HR) and Local Breakout (LBO) PDU sessions. In an LBO scenario, the V-SMF selects a local V-UPF and manages the entire user plane path within the VPLMN, significantly reducing latency and backhaul costs. It ensures that session and service continuity (SSC) modes are maintained and applies QoS policies received from the H-SMF or the Policy Control Function (PCF). The V-SMF also collaborates with the Visited Charging Function (V-CHF) to generate charging records for the portion of the service used in the visited network, supporting converged charging systems.

Purpose & Motivation

The V-SMF was introduced to address the limitations of the initial 5G Release 15 roaming architecture, which primarily supported Home-Routed traffic. In Home-Routed roaming, all user plane traffic is tunneled back to the HPLMN, leading to increased latency, inefficient use of transport resources, and an inability to efficiently access local services or content in the visited country. The V-SMF enables Local Breakout (LBO) roaming, a more optimal routing method mandated for 5G standalone deployments.

Its creation was motivated by the need for more flexible and efficient 5G roaming, supporting low-latency use cases and reducing the cost and complexity of international data transport. By delegating session management control to a function in the visited network, the architecture reduces the signaling load on the home network and allows for the application of local network policies. This is essential for enabling advanced services like network slicing for roaming users, where a slice instance in the VPLMN can be directly managed.

Key Features

• Enables Local Breakout (LBO) PDU sessions for efficient roaming data routing
• Manages session establishment, modification, and release for roaming UEs within the VPLMN
• Interfaces with the Home SMF (H-SMF) for policy control and coordination
• Controls the Visited UPF (V-UPF) via the N4 interface for user plane management
• Supports converged charging by interacting with the Visited Charging Function (V-CHF)
• Facilitates the application of QoS policies and enforcement within the visited network

Evolution Across Releases

Defining Specifications