First of all, offline charging requires the generation of CDRs in order for billing to be applied
at the end of a billing period. This is not the case with online charging because online charging
happens in real time.
As with online charging, the charging trigger function (CTF) within each network
element is the critical link in the process (see Figure 7.3). The CTF must collect all usage
data within the element, correlate the data and send it to the forwarding function
for processing using the DIAMETER protocol.
The CTF then interfaces to a charging data function (CDF). The CDF is responsible
for collecting charging data from all of the CTFs much as the OCF did in the online
charging architecture. The principle difference is that the CDF creates CDRs.
The CDF collects charging events from each of the CTFs in the network and uses
those events to create a single CDR or multiple CDRs. In other words, the CDF will create
a CDR for each individual event, or it will create a single CDR for multiple charging
events, provided all of those events occurred at the same network element. The events
can be from different types of sessions, for example, a voice session combined with a
video session (a video conference call) would be two different charging events but could
be recorded in one CDR. It would be up to the operator to configure the CDF rules.
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