N9 Home Routed

Description

N9 Home Routed (N9HR) refers to the use of the N9 interface in a home-routed roaming architecture for 5G networks, as defined in 3GPP Release 19 specifications 33.127 and 33.929. In this architecture, when a subscriber is roaming, the user plane traffic is routed from the visited network to the home network via the N9 interface between the Visited SMF (V-SMF) and the Home UPF (H-UPF). This allows the home network to maintain control over the user's data session, apply home-based policies, and provide services such as lawful interception, charging, and QoS enforcement.

The N9 interface is a reference point between the V-SMF and the H-UPF (or between V-UPF and H-UPF in some configurations). In a home-routed scenario, the V-SMF in the visited network communicates with the H-SMF in the home network via the N16 interface, while the user plane is anchored at the H-UPF. The N9 interface carries the user plane packets between the V-UPF (if present) and the H-UPF, or directly between the V-SMF and H-UPF in certain deployments. This setup ensures that all user traffic traverses the home network, enabling the home operator to apply its own policies and services.

Key technical aspects of N9HR include: support for lawful interception in the home network, as required by regulations; charging and policy control based on home network rules; and QoS enforcement according to the home network's configuration. The N9 interface must support encapsulation (e.g., GTP-U) and provide reliable transport for user data. It also needs to handle mobility events, such as handovers between visited networks, while maintaining the home-routed path.

N9HR is particularly relevant for operators that need to comply with legal requirements for lawful interception in the home network, or that want to offer value-added services (e.g., parental controls, content filtering) to their roaming subscribers. It contrasts with local breakout (LBO), where traffic is routed directly from the visited network to the data network, bypassing the home network. While LBO offers lower latency and more efficient routing, N9HR provides greater control and service consistency for the home operator.

The introduction of N9HR in Release 19 reflects the ongoing evolution of 5G roaming to support new services and regulatory requirements. It builds on the 5G roaming architecture defined in earlier releases (e.g., Release 15/16) and adds enhancements for security, interception, and policy enforcement. It is specified in 3GPP TS 33.127 and 33.929, which cover lawful interception and security aspects.

Purpose & Motivation

N9HR exists to provide a standardized mechanism for home-routed roaming in 5G networks, allowing home operators to retain control over user plane traffic when subscribers are roaming. It solves the problem of applying home network policies, lawful interception, and charging in a roaming scenario without requiring complex bilateral agreements or non-standard solutions. Before N9HR, home-routed roaming was possible but lacked clear specifications for interfaces and procedures, leading to interoperability issues and limited service offerings.

The motivation for N9HR includes regulatory requirements for lawful interception in the home country, the need for consistent QoS and policy enforcement, and the desire to offer differentiated services to roaming subscribers. It also enables home operators to monetize roaming traffic through value-added services and to ensure compliance with data protection laws.

Historically, roaming architectures have evolved from 2G/3G (where home-routed was common) to 4G (where local breakout became popular for efficiency). In 5G, both options are supported, and N9HR provides a robust, standardized way to implement home-routed roaming. Release 19 enhancements focus on security and interception aspects, ensuring that N9HR meets the stringent requirements of modern networks.