Offline charging
is the charging for services after the service is delivered. This is also known as postpaid
charging. Online charging is the charging for services prior to service delivery.
This is known as pre-paid charging. The mechanisms used to support both online and
offline are different.
For online charging, there is an online charging system (OCS), while for offline charging
there is a charging data function (CDF) and a charging gateway function (CGF).
Both online and offline charging rely on a charging trigger function (CTF) residing in
each of the network elements to capture charging data.
The CTF is provisioned with the addresses of all of the CDFs and OCFs within
its own domain. It is to these addresses that the trigger function will forward charging
data using the DIAMETER protocol. Think of the trigger function as the software
within the network entity that provides the actual usage data.
For example, if an operator provides a video service, a voice service, and a messaging
service, there will be three distinctly different billing formats provided. For the voice
service, the media gateways will need to provide the minutes of use (if this is still supported)
as well as the number called. This information will be necessary for rating of
the call.
Online and Offline Charging in the IMS 163
For the video service, the video server will need to provide the billing system with
the video downloaded (or played using a streaming service) and how long the video was
watched.
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