The Gx interface is a crucial element within the Policy and Charging Control (PCC) architecture, serving as the communication bridge between the Policy and Charging Enforcement Function (PCEF) and the Policy and Charging Rules Function (PCRF). This interface enables dynamic policy and charging control for service data flows in mobile networks.

Specifically, the Gx interface facilitates:

1. Provisioning and removal of PCC rules: It allows the PCRF to dynamically deliver or revoke policy and charging control (PCC) rules to/from the PCEF.
2. Transmission of traffic plane events: The PCEF communicates event-driven information (e.g., usage thresholds, QoS changes) back to the PCRF.
3. Integrated policy and charging control: The Gx interface supports both charging and policy management by utilizing Attribute-Value Pairs (AVPs) that define specific application-level rules.

Figure 20-116: Gx Reference Point in the PCC Architecture

This figure provides a visual representation of the Gx interface’s role within the PCC framework.

Key Components of the Gx Interface

• Policy and Charging Enforcement Function (PCEF):
  The PCEF is responsible for enforcing PCC rules for service data flows. It ensures compliance with control and charging policies received from the PCRF and monitors service usage.
• Policy and Charging Rules Function (PCRF):
  The PCRF dynamically defines PCC rules based on service characteristics, user attributes, and network conditions. These rules are sent to the PCEF for implementation.

Note: A device can function solely as the PCEF, relying on an external PCRF for policy decisions.

Functions of the Gx Interface

The Gx interface delivers several critical functions to ensure seamless policy and charging control:

1. Provisioning PCC Rules:
   The PCRF delivers dynamic PCC rules to the PCEF in real time.
2. Deleting PCC Rules:
   The PCRF can instruct the PCEF to delete PCC rules when no longer required.
3. Event Transmission:
   The PCEF notifies the PCRF of specific traffic events, such as session starts or QoS changes.

PCC Rule Classification

• Dynamic PCC Rules:
  These rules are created on the PCRF based on real-time service data and user policies. The PCRF assigns a unique Charging-Rule-Name AVP to each rule and provides detailed parameters (e.g., QoS and charging thresholds). Rules can be modified or deleted dynamically using the Gx interface.
• Static PCC Rules:
  These rules are predefined on the PCEF but can be activated or deactivated by the PCRF as needed. Unlike dynamic rules, static rules are not modified dynamically and rely on the negotiation process between the PCRF and PCEF for activation using specific Charging-Rule-Name AVPs.

Diameter Protocol Stack on the Gx Interface

The Diameter protocol, defined in RFC 3588, serves as the backbone for information exchange over the Gx interface. It operates as an Authentication, Authorization, and Accounting (AAA) protocol and is designed for next-generation networks, offering advanced capabilities compared to its predecessor, RADIUS.

Key aspects of the Diameter protocol stack:

• Application Layer:
  Diameter operates here, with Gx applications being its specific extensions for the Gx interface.
• Transport Layer:
  The protocol stack employs TCP or Stream Control Transmission Protocol (SCTP) for reliable communication. The device supports IP but does not support SCTP or IPsec.

Figure 20-117: Diameter Protocol Stack Structure

This figure outlines the architecture of the Diameter protocol stack.

Figure 20-118: Gx Interface Protocol Stack

This figure illustrates the protocol stack as applied to the Gx interface.

Message Exchange on the Gx Interface

The Gx interface supports a structured message exchange process, as depicted in Figure 20-119. These messages enable bidirectional communication between the PCRF and PCEF for effective policy and charging management.

Key Messages

Table 20-37 details the six types of messages exchanged over the Gx interface, including their purpose and functional roles.