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
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (215 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the V-SMF function was newly introduced as a distinct entity for Home Routed roaming and Indirect Network Sharing, with its key role defined by the N16 reference point to the H-SMF. The release specified the V-SMF's service operations, such as Create, Update, and Release for PDU sessions, within the Nsmf_PDUSession service framework. It also detailed procedures involving the V-SMF, including its insertion and change during Xn handovers, service requests, and mobility between 3GPP and non-3GPP access.
- EBI Assignment for Home Routed PDU sessions TS 29.502CR0008
- Returning the H-SMF URI to the AMF TS 29.502CR0010
- Rejected PDU session during Xn handover TS 29.502CR0029
- URIs of created SM context and PDU session resources TS 29.502CR0030
- Alignments with NAS 5GS Session Management TS 29.502CR0047
- Roaming Charging Profile negotiation for Home Routed PDU sessions TS 29.502CR0051
+ 18 more changes
In Release 16, the V-SMF function was enhanced to support new procedures involving the I-SMF (Intermediate SMF), such as I-SMF insertion, change, and removal during service requests and handovers. It also gained explicit capabilities for V-SMF change procedures itself, as well as for managing Multi-Access (MA) PDU Sessions, including their establishment, update, and release over a single access. These updates were integrated into the Nsmf_PDUSession service operations like Create, Update, and Release SM Context.
- PDU Session Establishment support Control Plane CIoT Optimization TS 29.502CR0130
- ATSSS: MA-PDU Session Establishment TS 29.502CR0115
- MA PDU Session Update TS 29.502CR0136
- MA PDU Session Release over a Single Access TS 29.502CR0137
- Update SM context service operation for additional PDU Session Anchor and Branching Point or UL CL controlled by I-SMF TS 29.502CR0124
- PDU session establishment with I-SMF insertion - Create SM Context service operation TS 29.502CR0154
+ 50 more changes
In Release 17, key enhancements for the V-SMF included defining its role in the UE registration procedure for V-SMF insertion, change, and removal, and specifying the use of SMF URI attributes in the Create SM Context procedure for PDU sessions involving a V-SMF. The release also formalized the V-SMF's involvement in restoration procedures and in handling inter-PLMN mobility of PDU sessions.
- I-SMF/V-SMF Restoration procedure TS 29.502CR0444
- SMF Response in case of AN-Requested PDU Session Resource Release TS 29.502CR0471
- PDUSession API extensions for Redundant PDU Sessions TS 29.502CR0468
- Provisioning Server Information During PDU Session Establishment TS 29.502CR0485
- Void I-SMF/V-SMF Restoration procedure from normal specification clauses TS 29.502CR0494
- NSAC during change of Access-type of a PDU-Session TS 29.502CR0521
+ 57 more changes
In Release 18, the V-SMF's capabilities were enhanced to support Home-Routed Steering of Breakout (HR-SBO) information handling during its insertion or change, and to enable procedures for changing the network slice instance (S-NSSAI) of a PDU Session. Furthermore, new support was added for PDU session retention or release during a Network Slice Replacement Procedure and for V-SMF change based on a target DNAI, as defined within the Nsmf_PDUSession service operations.
- PDU Session for supporting HR-SBO in VPLMN TS 29.502CR0630
- 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
- Change of Network Slice instance for PDU Sessions TS 29.502CR0660
- PDU Session Release during Network Slice Replacement Procedure TS 29.502CR0671
- PDU Session Retention during Network Slice Replacement Procedure TS 29.502CR0672
+ 41 more changes
In Release 19, the V-SMF function was enhanced with new procedures to support the release of an SM context upon request from the network, as defined in "Support of V-SMF procedures for Release SM context operation". Furthermore, specific enhancements were introduced for I-SMF based Local Offloading Management, impacting the N11 and N16a interfaces, to improve the management of locally offloaded traffic.
- Establishing a PDU session in a SMF in a target PLMN TS 29.502CR0788
- N11 and N16a enhancements for I-SMF based Local Offloading Management TS 29.502CR0810
- Clarification to PDU session establishment with Indirect Network Sharing TS 29.502CR0816
- PDU Session Release due to AF Requested Slice Replacement TS 29.502CR0831
- N11 and N16a enhancements for I-SMF based Local Offloading Management TS 29.502CR0845
- N38 enhancements for I-SMF based Local Offloading Management TS 29.502CR0847
+ 19 more changes
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 |