Gx interface - sitting between PCRF and PCEF

After you are familiar with OFCS (Offline Charging System) and OCS (Online Charging System) time has come to be more up to date with interfaces that are being used to exchange signaling data. At first I will talk about Gx interface which is responsible for Offline Charging, but not only.

The Gx reference point is located between the Policy Control and Charging Rules Function (PCRF) and the Policy and Charging Enforcement Function (PCEF). The Gx reference point is used for provisioning and removal of Policy and Charging Control (PCC) rules from the PCRF to the PCEF and the transmission of traffic plane events from the PCEF to the PCRF. The Gx reference point can be used for charging control, policy control or both by applying AVPs relevant to the application.

As you probably know, it's always good to start from big picture. Here we go.

Fig. 1. Gx reference point at the Policy and Charging Control (PCC) architecture

So at first, few information about..

Offline Charging System (OFCS) - introduction to Gx interface

Last time I tried to familiarize You with Online Charging System (OCS), with was a introduction to Gy based on a diameter. But before I will go to the diameter, and all those CCRs and CCAs just I wanted to talk about Offline Charging System.

What is Offline Charging System (OFCS)?
According to TS 32.240 and TS 32.299 3GPP documentation:
Offline charging is a process where charging information for network resource usage is collected concurrently with that resource usage. The charging information is then passed through a chain of logical charging functions, which are described below. At the end of this process, CDR files are generated by the network, which are then transferred to the network operator's Billing Domain for the purpose of subscriber billing and/or inter-operator accounting (or additional functions, e.g. statistics, at the operator’s discretion). The BD typically comprises post-processing systems such as the operator's billing system or billing mediation device.

Generally offline charging is a mechanism where charging information does not affect, in real-time, the service rendered.
As it's always good to start with a high level design picture, let's start.

Fig. 1. Offline Charging high level architecture

Where:

CTF: Charging Trigger Function

CDF: Charging Data Function

CGF: Charging Gateway Function

BD: Billing Domain. This may also be a billing system/ billing mediation device.