Mobile Number Portability Function

Description

The Mobile Number Portability Function (MNPF) is a service-based network function introduced in 5G Standalone (SA) architecture to manage Mobile Number Portability (MNP). It operates within the 5G Core's Service-Based Architecture (SBA), exposing its capabilities as a producer of Network Repository Function (NRF)-discoverable services, primarily to the Unified Data Management (UDM) and other session management functions. When a network function needs to determine the current serving Public Land Mobile Network (PLMN) for a given mobile subscriber number (MSISDN)—for example, during the routing of an SMS-MT or an IMS voice session—it can invoke a service operation (e.g., Nnpf_NPResolution_Get) provided by the MNPF. The MNPF then acts as the centralized point of contact for number portability resolution, internally querying external Number Portability Databases (NPDBs) or caches to retrieve the routing information.

Architecturally, the MNPF is designed as a cloud-native, scalable microservice. It communicates with other 5G Core functions using HTTP/2-based service-based interfaces, specifically the Nnpf interface. Its key internal logic involves receiving a resolution request containing an MSISDN, applying any necessary number normalization, performing a lookup via a configured data source (which could be a direct query to an external NPDB, an internal cache, or a sync from a central data repository), and returning a response. The response includes the currently serving PLMN ID (Mobile Country Code and Mobile Network Code) for the queried number. This information is then used by the requesting function, such as the UDM, to determine the correct network for subsequent procedures like SMS routing or subscriber data retrieval.

How it works involves seamless integration with 5G procedures. For instance, when the Short Message Service Function (SMSF) needs to deliver an SMS to a ported-in subscriber, it may consult the UDM for routing info. The UDM, in turn, invokes the MNPF if the subscriber's data indicates a ported status or if a resolution is required. The MNPF's design emphasizes low latency and high availability, crucial for real-time communication services. It also supports features like batch queries and subscription to number portability data changes, allowing other functions to cache data efficiently. Its role is distinct from legacy MNP-SRF as it is not a signaling relay but a pure query service integrated into the control plane's service mesh.

Purpose & Motivation

The MNPF was created to modernize and integrate Mobile Number Portability into the 5G Service-Based Architecture. Previous solutions like the MNP-SRF were designed for legacy circuit-switched and early IMS networks, often acting as a signaling interception point. The 5G core, with its cloud-native, microservices-based design, required a new approach where MNP capability is provided as a consumable service to various network functions, aligning with the SBA principles of decoupling, reusability, and dynamic discovery.

It solves the problem of efficiently and scalably providing portability information in a 5G environment where services are decomposed. Without a dedicated MNPF, each network function (like UDM, SMSF) would need to implement its own complex logic to interface with external NPDBs, leading to duplication, inconsistent behavior, and increased operational complexity. The MNPF centralizes this capability, providing a single, standardized service endpoint.

The motivation for its introduction in Release 17 was to ensure full support for regulatory MNP requirements in 5G SA networks, covering all communication services including voice, SMS, and emerging services. It addresses limitations of relying on interworking with legacy networks for MNP resolution, enabling pure 5G deployments to be self-sufficient. It also facilitates easier deployment of advanced features like number portability for IoT and massive machine-type communication scenarios within the 5G framework.