Inter-System Mobility Policy

Description

Inter-System Mobility Policy (ISMP) is a core network functionality defined in 3GPP specifications that governs how a User Equipment (UE) connects to and moves between different radio access networks. These networks can include various 3GPP technologies like E-UTRAN (LTE), NG-RAN (5G NR), and legacy systems like UTRAN, as well as non-3GPP accesses such as trusted and untrusted Wi-Fi networks. The ISMP consists of a set of operator-defined policy rules that are provisioned to the UE, typically via the Access Network Discovery and Selection Function (ANDSF) in the EPS or the Policy Control Function (PCF) in the 5G System.

The ISMP rules are structured as a prioritized list of selection criteria. Each rule contains conditions (e.g., location, time of day, roaming status, specific SSID for Wi-Fi) and associated actions that dictate the UE's access selection behavior. For example, a rule might state: 'If the UE is in the home network and detects both LTE and a specific enterprise Wi-Fi network, connect to Wi-Fi for all traffic.' The UE's mobility management entity evaluates these rules in order of priority to determine the preferred and allowed access types for establishing new connections or performing handovers.

Mechanically, the ISMP is delivered to the UE via OMA Device Management (DM) or over-the-air provisioning protocols. The UE stores these rules and applies them in real-time. When multiple accesses are available, the UE uses the ISMP to decide which one to register with or to use for a specific IP flow (in the case of Inter-System Routing Policy - ISRP, a related concept). The network can update these policies dynamically based on changing conditions like network load, subscriber tier, or application requirements. This allows operators to implement sophisticated traffic steering strategies, such as offloading data to Wi-Fi in congested areas or ensuring that mission-critical services always use the licensed 3GPP network for guaranteed quality of service.

Purpose & Motivation

ISMP was created to solve the problem of uncoordinated and potentially suboptimal access selection by UEs in heterogeneous networks. Early multi-mode devices relied primarily on simple signal strength measurements (e.g., selecting the strongest Wi-Fi signal) or user preferences, which often led to poor user experience, network congestion on preferred accesses, and inefficient use of overall network resources. Operators lacked control over how UEs utilized the available radio assets.

The introduction of ISMP, particularly with ANDSF in 3GPP Rel-8 and enhanced in later releases, gave operators a powerful tool for network-controlled mobility management. It addressed the need for intelligent traffic steering between 3GPP and non-3GPP networks to optimize capacity, improve user experience, and implement business policies (e.g., steering subscribers to Wi-Fi when roaming to reduce costs). Furthermore, as network slicing and application-aware QoS became critical in 5G, ISMP evolved to support more granular policies that could direct specific services or slices to the most appropriate radio access type, ensuring service level agreements are met across a multi-access edge environment.