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.
Classification
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (26 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 16, the V-SMF function was enhanced to support dynamic insertion, change, and removal procedures during key mobility and session management events. These new capabilities specifically enable V-SMF operations during a Service Request, UE Registration, and handovers between 3GPP and non-3GPP accesses. Furthermore, the release introduced detailed error handling, QoS rule provisioning, and charging identifier continuity for PDU sessions during a V-SMF change.
- Service Request with I-SMF insertion/change/removal or with V-SMF change TS 29.502CR0164
- ETSUN_Update Service Operation with I-V-SMF change TS 29.502CR0190
- Feature negotiation extension to support change of AMF, V-SMF or I-SMF TS 29.502CR0256
- V-SMF insertion or removal TS 29.502CR0304
- V-SMF and I-SMF service instance Id TS 29.502CR0322
- Error handling for PDU sessions with an I-SMF or V-SMF TS 29.502CR0373
+ 6 more changes
In Release 17, the V-SMF function saw enhancements in procedures for its insertion, change, and removal during UE Registration and Service Request scenarios. It also gained defined capabilities for handling NG-RAN tunnel information during mobility registration when a V-SMF change occurs. Furthermore, the specification detailed the inclusion of SMF URI attributes in the Create SM Context request for PDU sessions involving a V-SMF.
- I-SMF/V-SMF Restoration procedure TS 29.502CR0444
- Void I-SMF/V-SMF Restoration procedure from normal specification clauses TS 29.502CR0494
- UE Registration procedure - V-SMF insertion, change and removal TS 29.502CR0522
- SMF URI attributes in Create SM Context for PDU sessions with I-SMF/V-SMF TS 29.502CR0539
- NG-RAN tunnel information during mobility registration with I-SMF/V-SMF change TS 29.502CR0441
In Release 18, the V-SMF function was enhanced with specific procedures for handling Home Routed Sesssion BreakOut (HR-SBO) information during V-SMF insertion or change, including the H-SMF updating this information to the V-SMF. Furthermore, new capabilities were introduced for V-SMF change based on target DNAI and for including the S-NSSAI for the Serving PLMN during V-SMF insertion and inter-PLMN change, while the non-3GPP path switching capability between V-SMF and H-SMF was removed.
- HR-SBO information handling upon V-SMF insertion or change TS 29.502CR0635
- H-SMF updating the HR-SBO information to V-SMF TS 29.502CR0652
- Internal Group Identifier for HR-SBO subscription from V-SMF to AF TS 29.502CR0682
- V-SMF change based on target DNAI TS 29.502CR0688
- Removal of non-3GPP path switching capability between V-SMF and H-SMF TS 29.502CR0698
- Inclusion of S-NSSAI for Serving PLMN for V-SMF insertion and inter PLMN change TS 29.502CR0776
In Release 19, the V-SMF function was enhanced to support specific procedures for releasing an SM context, as indicated by the new "Support of V-SMF procedures for Release SM context operation." Furthermore, updates were made to correct the Information Element (IE) name for "V-SMF Rejected QoS Flows" within service procedures and to include "QoS Monitoring Info" specifically for scenarios involving mobility between an I-SMF and a V-SMF.
Explore further
Broader topics and technologies where V-SMF plays a role.
Defining Specifications
3GPP specifications that define or reference V-SMF, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 29.502 vj50 | 5G System; Nsmf Service Based Interface; Stage 3 | Rel-19 |
| TS 29.508 vj40 | 5G Session Management Event Exposure Service | Rel-19 |
| TS 29.512 vj40 | 5G Session Management Policy Control Service | Rel-19 |
| TS 29.542 vj30 | SMF NIDD Service Based Interface Stage 3 | Rel-19 |
| TS 29.561 vj30 | 5G Interworking with External Data Networks | Rel-19 |
