Home Application Function

Description

The Home Application Function (H-AF) is a core network element defined within the 3GPP Policy and Charging Control (PCC) architecture. It resides in the user's home Public Land Mobile Network (HPLMN). Conceptually, an Application Function (AF) is a network element that offers applications requiring dynamic policy and/or charging control over the user plane. The 'Home' qualifier specifies its location within the subscriber's home network, as opposed to a Visited AF (V-AF) which might be in a visited network during roaming.

The H-AF's primary role is to act as a service requestor towards the Policy and Charging Rules Function (PCRF in 4G) or Policy Control Function (PCF in 5G). It does this via the Rx reference point in 4G (between AF and PCRF) or the N5 reference point in 5G (between AF and PCF). The H-AF provides the PCC system with session-related information derived from the application layer. This information is essential for the PCRF/PCF to make informed policy decisions. For example, an H-AF for an IP Multimedia Subsystem (IMS) service would provide details about a VoIP call, such as the media type (audio/video), codec bandwidth, and packet filters describing the media flow's source/destination IP addresses and ports.

Upon receiving this application-level session information, the PCRF/PCF uses it, along with subscriber data from the SPR/UDR and network status, to create dynamic PCC rules. These rules are then provisioned to the Policy and Charging Enforcement Function (PCEF) in 4G (typically the PGW) or the Session Management Function (SMF) in 5G. The rules govern how the user plane traffic for that specific application session is treated—defining QoS parameters (like guaranteed bitrate), applying gating (blocking/allowing packets), and triggering appropriate charging actions. The H-AF may also receive notifications from the PCRF/PCF about events in the transport network, such as loss of bearer resources, allowing the application to adapt.

Architecturally, the H-AF is a logical function. Its most common physical instantiation is as part of an IMS network, such as the Proxy-Call Session Control Function (P-CSCF). However, it can be any server providing a service that requires guaranteed QoS from the underlying transport network, such as a gaming server or an enterprise application gateway. Its presence in the home network is crucial for maintaining consistent service policy and charging control, as the home operator retains authority over the subscriber's policy profile.

Purpose & Motivation

The H-AF was created to bridge the gap between the application layer (service layer) and the transport/control layer in mobile networks. Before the PCC architecture, network resources were allocated statically or based on simple Access Point Name (APN) configurations, with no dynamic interaction from the application. This was insufficient for emerging real-time, QoS-sensitive services like Voice over IP (VoIP) and video streaming, which require specific bandwidth guarantees and low latency that vary per session.

The PCC framework, introduced in 3GPP Release 7 and evolved through subsequent releases, was designed to solve this by enabling dynamic, application-aware policy control. The H-AF is a fundamental component of this solution. It exists to allow service providers (like IMS operators) to communicate their specific resource requirements to the network policy engine in real-time. This enables the network to treat application traffic on a per-flow basis with appropriate QoS, rather than applying a one-size-fits-all policy.

This addressed key limitations: it allowed for efficient network resource utilization by allocating high QoS only when needed, enabled new revenue-generating services with guaranteed quality, and provided a standardized interface (Rx/N5) for application providers to integrate with the network. The 'Home' aspect ensures that the subscriber's home operator, who owns the subscriber relationship and policy profile, maintains control over the policy decisions even when the user is roaming, through interaction between the home PCRF/PCF and the visited network's policy entities.