Provide Subscriber Information

Description

Provide Subscriber Information (PSI) is a standardized service within the 3GPP core network architecture, defined as part of the Mobile Application Part (MAP) protocol and later the Diameter-based Sh and S6a/S6d interfaces. It is a network-initiated procedure where a requesting node, such as a Gateway Mobile Location Centre (GMLC), a Service Capability Exposure Function (SCEF), or a Lawful Interception node, queries the subscriber's home database – the Home Location Register (HLR) in 2G/3G or the Home Subscriber Server (HSS) in 4G/5G – to obtain specific information about a subscriber.

The procedure involves a well-defined request/response message exchange. The requesting entity sends a PSI message (e.g., MAP_PSI or Diameter PSI-Request) containing the subscriber's identifier (like MSISDN or IMSI) and an indication of the information required. The HSS/HLR processes the request, which typically involves checking the requester's authorization against subscriber privacy settings and regulatory requirements (e.g., for location services). Upon successful authorization, the HSS/HLR responds with a PSI-Answer message containing the requested data. This data can include a wide array of information: the subscriber's current serving network node (MSC, SGSN, MME, AMF), the subscriber's state (e.g., attached, detached), location information (if available and permitted), subscribed services, and IMSI-MSISDN mapping.

Key components in the architecture include the Requesting Node (the client of the service), the HSS/HLR (the server holding the data), and the relevant signaling interfaces. For 4G/5G, the primary interface is the S6a (between MME and HSS) and Sh (between Application Server/SCEF and HSS), using the Diameter protocol. The HSS acts as the central repository and policy enforcement point, ensuring that subscriber data is provided only to authorized network functions based on the subscriber's consent and network policies.

PSI's role is critical for enabling numerous value-added and regulatory services. It is the backbone procedure for network-based location services, allowing a GMLC to find out which Mobility Management Entity (MME) or Access and Mobility Management Function (AMF) is serving a target UE before issuing a location request. It is also used for restoring subscriber data in case of MME failure, supporting SMS delivery routing, facilitating lawful interception provisioning, and enabling service capability exposure for third-party applications. Its evolution across releases has seen it adapt from circuit-switched MAP to packet-switched Diameter, and its integration with new network functions like the SCEF and Network Exposure Function (NEF) in 5G, maintaining its position as a fundamental subscriber data access service.

Purpose & Motivation

The PSI service was created to address the fundamental need for authorized network entities to access centralized subscriber information in a standardized, secure, and controlled manner. In early cellular networks, different network nodes (like switches and service platforms) required subscriber data to perform their functions, but ad-hoc methods of sharing this data were inefficient and insecure. PSI provided a unified, protocol-based mechanism for querying the master subscriber database (HLR), solving problems related to service triggering, routing (e.g., for SMS or voice calls), and the nascent field of location-based services.

The historical motivation stems from the separation of the subscriber database (HLR) from the switching functions in GSM architecture. This separation created the need for a signaling service to connect them. PSI, as part of the MAP protocol, was one of the key services enabling this distributed architecture. It solved the limitation of having subscriber data siloed or replicated inconsistently across network elements. By centralizing data in the HLR and providing controlled access via PSI, network operators could ensure data consistency, simplify service deployment, and implement subscriber privacy controls.

As networks evolved through 3G, 4G, and 5G, the purpose of PSI expanded. It became essential for enabling regulatory services like lawful interception, where authorities require network attachment information. The drive towards service exposure and network APIs for third-party developers further leveraged PSI (via the SCEF/NEF) to provide subscriber status information to authorized applications, fueling the creation of new services. Thus, PSI has endured as a critical enabler, evolving from a basic network operations tool to a key component in service delivery, security, and innovation across the entire 3GPP system evolution.