The Charging Trigger Function i.e. CTF generates charging events based on the observation of network resource usage. In every network and service element that provides charging information, the CTF is the focal point for collecting the information pertaining to chargeable events within the network element, assembling this information into matching charging events, and sending these charging events towards the Charging Data Function. The CTF is a mandatory, integrated component in all network elements that provide offline charging functionality.
Charging Data Function i.e. CDF receives charging events from the Charging Trigger Function via the Rf reference point. It then uses the information contained in the charging events to construct CDRs.
Reference: 3GPP 32240.XX