Gx (application id: 16777238)

Gx interface facilitates communication between the Policy and Charging Rules Function (PCRF) and the Policy and Charging Enforcement Function (PCEF), enabling the dynamic enforcement of Quality of Service (QoS) policies and charging rules for user sessions.
Gx interface allows for precise management of QoS parameters, ensuring that the user's experience meets the service level agreements (SLAs) defined by the network operator. The interface also enables the dynamic application of charging rules, allowing for flexible billing models such as time-based, volume-based, or service-based charging.
In a non-roaming scenario, the PCRF directly communicates with the PCEF within the same network. The Gx interface is used to enforce policies based on the decisions made by the PCRF.
In the Roaming with Home Routed Access scenario, the V-PCRF (Visiting PCRF) in the visited network communicates with the H-PCRF (Home PCRF) in the home network. The H-PCRF makes the policy decisions, and the Gx interface is used to enforce these decisions at the PCEF in the visited network.
In the Roaming with Visited Access scenario, V-PCRF in the visited network is responsible for making policy decisions. The Gx interface in the visited network is used to enforce the policies locally.

Gx interface workflow

1. Session Establishment:
   • When a user initiates a session (e.g., starting a video call or streaming content), the AF sends a request to the PCRF via the Rx interface, specifying the required QoS parameters for the session, such as bandwidth, latency, and priority levels.
2. Policy Decision by PCRF:
   • The PCRF evaluates the AF's request, considering factors such as user subscription data (retrieved from the Subscription Profile Repository or User Data Repository) and current network conditions. The PCRF then formulates a set of policies that include QoS settings and charging rules tailored to the session's needs.
3. Policy Enforcement via Gx:
   • The PCRF sends these policies to the PCEF using the Gx interface. The PCEF, located in the Gateway, implements these policies by configuring the network's data paths to match the specified QoS requirements. 
4. Real-Time Adjustments:
   • During the session, if the service requirements change (for example, if the user switches to a higher-definition video stream), the AF can send an updated request to the PCRF. The PCRF recalculates the policies and sends the updated instructions to the PCEF via the Gx interface. The PCEF adjusts the traffic management and charging rules accordingly in real-time.
5. Charging Interaction:
   • The PCEF interacts with the OCS and OFCS to apply the correct charging mechanisms as specified by the PCRF. This could involve real-time deductions from a prepaid balance or recording usage for postpaid billing.
6. Session Termination:
   • When the session ends, the PCRF notifies the PCEF to terminate the session-specific policies. The PCEF releases the resources allocated for the session and ensures that any final charging information is sent to the OCS or OFCS for billing purposes.

For complete technical specification of Gx interface in LTE networks please refer to: [3GPP TS 29.212]
For technical specification of Gx interface in pre-LTE 3G networks please refer to: [3GPP TS 29.210]