Home Session Management Function

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.