Description
The Home Session Management Function (H-SMF) is a core network function within the 5G System (5GS) architecture, defined from 3GPP Release 16 onwards as part of the enhanced roaming and edge computing frameworks. It is a specific instance of the Session Management Function (SMF), which is responsible for session management (establishment, modification, release), IP address allocation, QoS control, and charging data collection. The 'Home' designation indicates that this SMF resides within the subscriber's Home Public Land Mobile Network (HPLMN). Its primary role becomes active during roaming scenarios, particularly when the roaming architecture does not employ Local Breakout (LBO).
In terms of architecture, the H-SMF interacts with numerous other Network Functions (NFs). During a roaming session with Home Routed (HR) traffic, the Visited Public Land Mobile Network (VPLMN) selects a Visited SMF (V-SMF). This V-SMF then establishes an N16 reference point with the H-SMF in the HPLMN. The H-SMF acts as the anchor SMF for the Protocol Data Unit (PDU) Session. It is responsible for interfacing with the Home Policy Control Function (H-PCF) for policy enforcement, the Home User Data Management (H-UDM) for subscription data, and the Home Charging Function (H-CHF). The H-SMF ultimately connects the session to the Home Data Network (e.g., the internet or an enterprise network) via a User Plane Function (UPF) in the HPLMN (the H-UPF).
The H-SMF operates by receiving session management requests from the V-SMF over the N16 interface. It authorizes the session based on home subscriber policies retrieved from the H-PCF and H-UDM. It allocates an IP address/prefix from the home network's pool and sets up the appropriate QoS policies. The H-SMF then instructs the H-UPF to establish the N9 tunnel towards the V-UPF in the visited network, creating a home-routed user plane path: UE <-> (R)AN <-> V-UPF <-> H-UPF <-> Data Network. All user plane traffic traverses the HPLMN, allowing the home operator to apply consistent policy enforcement, deep packet inspection, and charging. The H-SMF generates charging records based on home network tariffs and reports them to the H-CHF.
This architecture ensures that the home operator retains control over the session's policy and charging aspects even when the UE is roaming. It contrasts with the Local Breakout model, where a V-SMF in the visited network connects directly to a local data network, and the HPLMN only provides policy authorization via the H-PCF. The H-SMF model is crucial for services requiring access to home-network-specific services (like IMS) or for regulatory requirements where traffic must be routed through the home country. It is a foundational element for advanced roaming features like 5G Edge Computing, where the H-SMF can select an application function in the home network to influence traffic routing.
Purpose & Motivation
The H-SMF was introduced in Release 16 to formalize and enhance the Home Routed roaming architecture for the 5G Core network (5GC). In earlier releases (Rel-15), the 5GC roaming architecture was defined but the roles of the SMF in home and visited networks were not explicitly distinguished with separate terms like H-SMF and V-SMF. This led to potential ambiguity in implementation and specification. The explicit definition of H-SMF addressed this by clearly delineating responsibilities between the home and visited network SMFs, which is critical for interoperable roaming agreements, consistent policy enforcement, and accurate charging.
The primary problem it solves is maintaining home network control and service continuity for roaming subscribers. In 4G EPC, the PDN Gateway (PGW) in the home network served a similar anchor function. The 5GC's service-based architecture and separation of control and user plane functions required a new, more flexible model. The H-SMF ensures that even when a UE roams, it can access services anchored in its HPLMN (e.g., specific APNs, IMS services) with the same policies and charging as at home. It also allows the home operator to implement security functions, like firewalls or intrusion detection, on the home-routed traffic.
Furthermore, the H-SMF concept is vital for the evolution of 5G services like network slicing and edge computing. A roaming UE may request a network slice that is only instantiated in its HPLMN. The H-SMF is responsible for managing that slice instance for the roaming session. For edge computing, if an application requires a home-edge compute node, the H-SMF facilitates the connection to the appropriate application function in the HPLMN. Thus, the H-SMF enables complex, service-aware roaming beyond simple internet access, which was a limitation of less structured approaches.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (139 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the H-SMF was formally introduced as a distinct functional entity for Home Routed roaming, with its interactions defined through the N16 reference point to the V-SMF. The release specified key service operations for the H-SMF, such as Create, Update, and Release PDU session, as part of the Nsmf_PDUSession service. It also established procedures for H-SMF involvement in scenarios like PDU session establishment, handovers, and interworking with EPS during mobility.
- 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
+ 8 more changes
In Release 16, the H-SMF's role was expanded to support new procedures involving an Intermediate SMF (I-SMF), such as I-SMF insertion, change, and removal during PDU session establishment, update, and release. This included defining specific service operations like "Create SM Context" for I-SMF insertion and "Update SM Context" for modifying a PDU session with an additional PDU Session Anchor. Furthermore, the H-SMF's interactions were specified for new scenarios like Access Traffic Steering, Switching, and Splitting (ATSSS) for Multi-Access PDU sessions.
- 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
+ 39 more changes
In Release 17, new H-SMF capabilities included protocol support for restoration procedures for Home Routed PDU Sessions, enhancements for inter-PLMN mobility of PDU sessions, and procedures to notify when PDU session control is taken over by another SMF. The Nsmf_PDUSession service was extended to handle scenarios like V-SMF change during mobility registration and to provide user location information in session management messages. Furthermore, the release introduced mechanisms for PDU session release upon removal of DNN subscription and for handling network slicing (NSAC) during a change of access-type for a PDU session.
- 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
+ 17 more changes
In Release 18, the H-SMF's role was enhanced for Home Routed roaming, particularly with new procedures for handling HR-SBO (Home Routed Service Based Operations) information during V-SMF insertion or change and updating this information to the V-SMF. Key updates included refined handling of PDU sessions during network slice instance replacement, with specific procedures for session release and retention. Additionally, new capabilities were introduced for the V-SMF to change based on a target DNAI and for the H-SMF to support data collection for PDU session traffic analytics.
- 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
+ 28 more changes
In Release 19, key enhancements for the H-SMF included the introduction of H-SMF instance re-selection by the HPLMN for improved reliability and the specification of procedures for establishing a PDU session in a target PLMN's SMF. Furthermore, the release provided N11 and N16a interface enhancements to better support I-SMF based Local Offloading Management.
- 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
+ 17 more changes
Explore further
Broader topics and technologies where H-SMF plays a role.
Defining Specifications
3GPP specifications that define or reference H-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 29.542 vj30 | SMF NIDD Service Based Interface Stage 3 | Rel-19 |