Generic Public Subscription Identifier

Description

The Generic Public Subscription Identifier (GPSI) is a critical identifier in the 5G System (5GS) defined in 3GPP TS 23.501. It serves as a public-facing, globally unique address for a subscription, used by external Application Functions (AFs) and within the network to reference a user. There are two primary formats: an MSISDN (Mobile Subscriber Integrated Services Digital Network Number), which is a telephone number in E.164 format, and an External Identifier, which is a string formatted as 'username@realm'. The GPSI is stored in the Unified Data Management (UDM) as part of the subscription data and is associated with the SUPI (Subscription Permanent Identifier). Its key role is in service exposure via the Network Exposure Function (NEF) and in edge computing scenarios. When an external AF needs to interact with the network for a specific user (e.g., to influence traffic routing for low-latency applications), it uses the user's GPSI in its requests to the NEF. The NEF then translates this public GPSI into the internal SUPI for network operations, ensuring privacy. The GPSI is also used in UE policy configuration, SMS over NAS, and as a parameter in the Nnef_ services. Its handling involves strict privacy measures; for example, the GPSI is not used over the radio interface and is mapped to the SUPI within the secure core network domain.

Purpose & Motivation

GPSI was introduced in 5GS to fulfill the need for a stable, public, and user-friendly identifier that can be used by third-party application providers and within network service APIs, decoupling service addressing from internal network identifiers. Previous systems used MSISDN for this purpose, but this tied services to a specific numbering format (E.164) and was less flexible for non-telephony services or users without a phone number. The GPSI, with its support for both MSISDN and External Identifier (like an email address), solves this problem by providing a generic scheme. This is essential for 5G's service-based architecture and edge computing, where external applications need a consistent way to identify subscribers without being aware of their private SUPI or temporary 5G-GUTI. It addresses the limitations of using internal identifiers externally (a privacy and security risk) and the inflexibility of relying solely on MSISDN, enabling new business models and seamless integration with internet-based services and identity federations.