Visited AF

Description

The Visited Application Function (V-AF) is a logical network function within the 3GPP Policy and Charging Control (PCC) architecture, specifically relevant in roaming scenarios. An Application Function (AF) is generally an element that requires dynamic policy and/or charging control over a user's IP Connectivity Access Network (IP-CAN) session; examples include the Proxy-Call Session Control Function (P-CSCF) for IMS or a streaming server. When a subscriber is roaming outside their home network, a service application hosted in the visited network may need to interact with the PCC system. This visited network-located AF is termed the V-AF.

The V-AF operates by communicating with the Policy and Charging Rules Function (PCRF) in the visited network, known as the V-PCRF. This communication occurs over the Rx reference point. When the V-AF needs to influence the policy for a roaming subscriber's session (e.g., to request a guaranteed bitrate for a video call), it sends an AA-Request (AAR) Diameter message to the V-PCRF. The V-PCRF, in turn, engages in a dialogue with the home network's PCRF (H-PCRF) over the S9 roaming interface. The H-PCRF is the ultimate decision-maker, as it has access to the subscriber's profile and home policies. The H-PCRF makes the policy decision and provisions the relevant rules to the V-PCRF, which then forwards them to the visited network's Policy and Charging Enforcement Function (V-PCEF) for enforcement on the user plane.

Key components in this architecture are the V-AF itself, the V-PCRF, the H-PCRF, and the S9 interface. The V-AF's role is to act as the service layer's policy requestor within the visited domain. Its existence allows for efficient, localized service delivery. For instance, if a roaming user accesses a video service hosted in the visited country, that service's AF (the V-AF) can request the necessary QoS directly from the local V-PCRF, minimizing latency and allowing the visited network to manage its own resources while still respecting the home operator's overall policy decisions coordinated via the H-PCRF. This separation of concerns is vital for scalable and operator-friendly roaming.

Purpose & Motivation

The V-AF concept was introduced to address the complexities of applying dynamic policy control in roaming situations within the PCC framework defined in Release 7 and enhanced thereafter. Before its formalization, the interaction model for AFs in roaming was ambiguous. A key problem was determining which PCRF—the one in the home or visited network—should receive policy requests from an AF located in the visited network. The architecture needed to support scenarios where services are optimally delivered from the visited network (local breakout) while maintaining the home operator's control over subscriber policies and charging.

Its creation was motivated by the need for a clear, standardized roaming architecture for PCC. It solves the problem by defining a clean chain of responsibility: a locally hosted service (V-AF) interacts with the local policy controller (V-PCRF), which acts as a proxy to the home policy controller (H-PCRF). This ensures that the home operator retains ultimate authority (a key business requirement), while the visited network can efficiently process policy requests for services it hosts. This model enables advanced roaming features like optimized multimedia service delivery, where QoS for a locally hosted IMS application can be managed without all signaling traversing the home network, improving performance and reducing backhaul costs.