Rf (application id: 4)

The Rf interface is a component within the 3GPP network architecture, specifically designed for offline charging in the Packet-Switched (PS) domain. It is responsible for facilitating the transmission of Charging Data Records (CDRs) from network elements to the Charging Data Function (CDF) for subsequent processing, storage, and billing purposes. 
The primary purpose of the Rf interface is to ensure that charging data generated by various network elements, such as the Serving Gateway (S-GW), Packet Data Network Gateway (P-GW), Mobility Management Entity (MME), Trusted WLAN Access Gateway (TWAG), evolved Packet Data Gateway (ePDG), and Traffic Detection Function (TDF), is reliably collected and transferred to the CDF. This data is crucial for generating accurate billing records for subscribers based on their network usage.

Rf interface workflow

1. Event Detection and CDR Generation:
   • The CTF within a network element (e.g., S-GW, P-GW, MME) detects a chargeable event, such as the initiation, modification, or termination of a session. Based on this event, the CTF generates a CDR that includes details about the event, such as the session ID, timestamps, data volume, and other relevant information.
2. Charging Data Request Transmission:
   • The generated CDR is encapsulated within a Charging Data Request (CDR) message and transmitted from the CTF to the CDF over the Rf interface using the Diameter protocol. The message types typically used are "Start," "Interim," "Stop," and "Event" to indicate the nature of the charging data being sent.
3. CDR Processing at CDF:
   • Upon receiving the CDR, the CDF processes the data, stores it, and prepares it for further transmission to the CGF if necessary. The processing may include validation, aggregation, and formatting of the data according to billing requirements.
4. Charging Data Response:
   • After processing the CDR, the CDF sends a Charging Data Response (CDR) back to the originating CTF. This response acknowledges the successful reception and processing of the CDR.
5. Forwarding to Billing Domain:
   • The CGF receives the processed CDRs from the CDF and forwards them to the Billing System (BS) in the billing domain. The BS then uses this data to generate bills or usage reports for the subscribers.

For complete technical specification of Rf interface in Diameter protocol please refer to: [3GPP TS 29.251], [3GPP TS 29.299]