User Service Information

Description

User Service Information (USI) is a standardized information element within 3GPP specifications that carries details about the user's service profile and session context. It is transported across various reference points, notably between the Policy and Charging Rules Function (PCRF) and the Packet Data Network Gateway (PGW) as defined in 3GPP TS 29.212, and within IMS-related interfaces. The USI contains attributes that describe the service being invoked, such as the service identifier, media components, application identifiers, and subscriber-specific data. This information is crucial for the network to make informed decisions regarding policy enforcement, charging, and resource allocation.

Architecturally, USI is embedded within protocol messages, such as the Gx Diameter messages between the PCRF and PGW/PCEF. When a user initiates a data session or an IMS service, the relevant network function (e.g., Application Function (AF) in IMS) may provide service information to the PCRF. The PCRF then uses this information, potentially formatted as USI, to derive and install appropriate Policy and Charging Control (PCC) rules on the gateway. These rules dictate how user plane traffic is handled, including bandwidth limits, priority marking, and charging methods.

The content of USI can be quite granular, including parameters like the Flow-Description for identifying specific IP flows, QoS parameters such as QoS Class Identifier (QCI) and Allocation and Retention Priority (ARP), and charging-related information like the Rating Group and Service Identifier. In IMS, USI may also convey session details from the SIP signaling, allowing the PCRF to correlate the policy control with the specific IMS session. This enables dynamic policy control where network resources are allocated and managed in real-time based on the active service, improving network efficiency and user experience.

USI plays a vital role in enabling service-aware networks. By providing a structured way to communicate service context from the application layer to the core network transport layer, it allows for differentiated treatment of traffic. For example, a video streaming service can be granted higher priority and a specific charging model compared to best-effort web browsing. This capability is foundational for implementing advanced business models, such as zero-rating, sponsored data, and quality-of-service guarantees for premium services.

Purpose & Motivation

The creation of User Service Information (USI) was motivated by the need to move beyond simple, static subscriber profiles towards dynamic, service-aware policy and charging control in 3GPP networks. Early mobile data networks primarily used static QoS profiles based on the subscriber's APN. This was insufficient for the rise of diverse IP-based services like IMS, VoIP, and video streaming, which required real-time, application-specific resource management and charging.

USI was introduced as part of the broader Policy and Charging Control (PCC) architecture in 3GPP Release 7 and further refined in subsequent releases. It addresses the limitation of not having a standardized, extensible container to transport detailed service context from the application function (e.g., a P-CSCF in IMS) to the policy decision point (PCRF) and ultimately to the enforcement point (PCEF). Before such standardization, implementing service-specific policies often required proprietary extensions or was not possible, hindering the deployment of new, innovative services with specific network requirements.

By providing a defined structure for this information, USI enables operators to deploy sophisticated service differentiation, personalized charging plans, and network resource optimization. It is a key enabler for the monetization of services beyond simple data buckets, allowing for partnerships with content providers and the creation of tiered service offerings. Its evolution has been tied to the expansion of the PCC framework to support new service paradigms, including network slicing and edge computing in 5G.