Terminology » LTE » CDF vs. CDR vs. CTF: LTE Charging Functions Explained

CDF vs. CDR vs. CTF: LTE Charging Functions Explained

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This article breaks down the functional differences between CDF, CDR, and CTF within the LTE (Long-Term Evolution) network architecture. Let’s dive in!

CDF: Charging Data Function

CDF stands for Charging Data Function.
It’s the key component responsible for creating or constructing CDRs (Charging Data Records). Think of it as the architect of billing information.
The CDF builds these CDRs based on received charging events. It’s constantly listening and gathering information about how services are being used.
These “charging events” are received by the CDF through a reference point called “Rf.”
Once the CDF has generated a CDR, it immediately transfers it to the CGF (Charging Gateway Function) via another reference point called “Ga.” The CGF is responsible for further processing and routing of the charging information.

offline charging architecture depicting CDF

CDR: Charging Data Record

CDR stands for Charging Data Record.
A CDR is essentially a formatted collection of information about chargeable events. This information is used for both billing and accounting purposes. It’s the record that tells the system how much to charge a user.
These “chargeable events” can include things like the time a call was set up, the duration of the call, the amount of data transferred, and more. Think of it as a detailed invoice for network usage.
Importantly, a separate CDR can be generated for each party that needs to be charged for parts or all of a chargeable event.

This means that for a long phone call, or in scenarios where multiple parties are responsible for charges, there might be more than one CDR. Consider a conference call, where multiple users might incur different charges.
A “charging event” refers to the set of charging information forwarded by the CTF (which we’ll discuss next) towards either the OCS (online charging system) or the CDF (offline charging).
Each charging event should correspond to exactly one chargeable event. It’s a one-to-one relationship ensuring accurate billing.

CTF: Charging Trigger Function

CTF stands for Charging Trigger Function.
The CTF is an integral part of the service element itself. It sits inside the network component delivering a particular service.
Its main job is to monitor service usage and generate charging events based on that usage. It’s the “eyes and ears” of the charging system, constantly watching what’s happening.

online charging architecture depicting CTF