Intermediate Session Management Function

Description

The Intermediate Session Management Function (I-SMF) is a core network function within the 5G System (5GS) defined from 3GPP Release 16 onwards. It is a specialized instance of the SMF designed to operate in conjunction with an Anchor SMF (A-SMF). The primary architectural role of the I-SMF is to provide localized session management when a User Equipment (UE) connects via an access network that is different from the one served by its A-SMF, or when the UE moves into a data network locality that requires a separate user plane path. In such scenarios, the I-SMF is inserted into the control plane path between the Access and Mobility Management Function (AMF) and the A-SMF.

Operationally, the I-SMF is responsible for managing the Protocol Data Unit (PDU) Session for the UE within its local domain. This includes interacting with the local User Plane Function (UPF) – often an Intermediate UPF (I-UPF) – for user plane management, handling local policy enforcement, and performing session-related signaling with the AMF. Crucially, the I-SMF relays certain session management messages to and from the A-SMF, which retains overall responsibility for the PDU Session, including interaction with the Unified Data Management (UDM) for subscription data and the Policy Control Function (PCF) for policy decisions. The I-SMF essentially acts as a proxy, allowing the A-SMF to remain the session anchor point while delegating local management tasks.

Key components of the I-SMF's functionality include its N11 interface to the AMF, its N16 interface to the A-SMF (using the Nsmf_PDUSession service), and its N4 interface to the locally deployed UPF. It performs local UPF selection, establishes, modifies, and releases N4 sessions, and enforces QoS rules provided by the PCF via the A-SMF. The I-SMF's role is vital in enabling efficient routing for scenarios like access to local area data networks (LADNs), non-3GPP interworking (e.g., with Wireline Access), and mobility events where inserting a local breakout point reduces latency and backhaul traffic. It decouples local access topology from the core anchor point, providing significant flexibility in 5G network design.

Purpose & Motivation

The I-SMF was introduced in Release 16 to address specific architectural challenges arising from the 5G Core's service-based architecture and the need for efficient support of edge computing, non-3GPP access, and complex mobility. Prior to its introduction, the SMF was a monolithic entity managing the entire PDU Session from a single logical point. This model became inefficient when a UE accessed services via a non-3GPP network (like WLAN) or moved into a localized service area (like a factory campus). Traffic would need to be backhauled to the anchor UPF controlled by the A-SMF, introducing unnecessary latency and load on the core network transport.

The I-SMF solves this by enabling a split SMF architecture. It allows network operators to deploy a lightweight SMF instance locally, close to the UE's point of attachment. This local I-SMF can manage the local user plane path (via an I-UPF) for low-latency breakout, while the A-SMF in the central cloud maintains the session's anchoring and overall policy context. This separation is particularly critical for supporting Ultra-Reliable Low-Latency Communications (URLLC) and efficient Mobile Edge Computing (MEC), where user plane functions must be deployed at the network edge. The I-SMF concept directly addresses the limitations of a single, centralized SMF by providing topological flexibility and enabling efficient data routing tailored to the UE's current location and access type.