Presence Information Data Format Location Object

Description

The Presence Information Data Format Location Object (PIDF-LO) is a specific usage of the IETF's Presence Information Data Format (PIDF), defined in RFC 4119, extended to carry location information. Within 3GPP architectures, primarily the IP Multimedia Subsystem (IMS), PIDF-LO serves as the standardized data structure for exchanging user location data between network entities and to application servers. It is an XML document that encapsulates location information, which can be expressed in either geographic coordinates (e.g., latitude, longitude, altitude based on the Geography Markup Language, GML) or civic address format (e.g., country, city, street).

The PIDF-LO document is carried within SIP (Session Initiation Protocol) messages, such as PUBLISH, NOTIFY, or INVITE, enabling location information to be an integral part of presence and session establishment procedures. A key architectural component that utilizes PIDF-LO is the Location Server (e.g., a Gateway Mobile Location Centre, GMLC, or a dedicated Location Information Server, LIS). This server is responsible for obtaining the user's location (e.g., from the RAN via control plane or user plane positioning methods), formatting it into a PIDF-LO document, and providing it to authorized requesters like an IMS Application Server (AS) for emergency services or a Service Capability Exposure Function (SCEF/NEF).

The format includes crucial metadata such as a timestamp indicating when the location was determined, an identifier for the located entity (e.g., a SIP URI or tel URI), and elements describing the method used to obtain the location and its estimated accuracy. This rich metadata allows consuming applications to assess the freshness and reliability of the location data. PIDF-LO's role is foundational for emergency call routing, where it is used to convey the caller's location to the Public Safety Answering Point (PSAP), and for commercial location-based services offered via IMS. Its standardization ensures interoperability between different vendors' network equipment and application servers, which is critical for large-scale, multi-vendor mobile networks.

Purpose & Motivation

PIDF-LO was created to solve the critical problem of standardizing how location information is represented and transported in IP-based networks, specifically within the 3GPP IMS framework. Prior to its adoption, proprietary or application-specific formats for location data hindered interoperability between network elements from different vendors and between networks of different operators. This was a significant barrier for essential services like emergency calling (E112/E911), where reliable and universally understood location delivery is a regulatory and life-saving requirement.

The motivation for adopting PIDF-LO stemmed from the convergence of telecommunications and internet technologies. As 3GPP defined IMS to deliver multimedia services over IP, it leveraged existing IETF protocols like SIP for session control and presence. PIDF was the IETF's standard for presence information. Extending it with a location object (PIDF-LO) provided a natural, XML-based, and extensible format that could integrate seamlessly with the SIP-based IMS architecture. This allowed location to become a first-class citizen in presence services, enabling not only emergency services but also a new generation of standardized, network-provided location-based applications.

Furthermore, PIDF-LO addresses the need for a common language between the cellular network's native location determination systems (e.g., control plane positioning) and IP-based service layer applications. It acts as the translation layer, abstracting the complex, technology-specific positioning measurements into a vendor-neutral, application-friendly format. This abstraction simplifies service development and deployment, fostering innovation in location-based services while ensuring compliance with privacy and security policies for handling personally identifiable location information.

Key Features

• Standardized XML schema for encoding geographic (GML) and civic address location data
• Transport within SIP messages (e.g., PUBLISH, NOTIFY) for integration with IMS services
• Includes metadata: timestamp, entity identifier, location method, and accuracy/uncertainty
• Supports location by-value (full data in message) and by-reference (a URI pointer) models
• Enables privacy and policy control through usage rules and authorization mechanisms
• Foundation for 3GPP emergency services (e.g., NAI, ESRD/ESRK derivation for routing)

Evolution Across Releases

Defining Specifications