This is different than legacy prepaid billing systems
that use a service node in the bearer path. The service node is then responsible for controlling
the termination of the circuit should the account balance be depleted.
Service nodes provide an extra connection in the facilities, forming a form of loop
around so that the voice path can be controlled. The alternative would be some form of
communications to the switch itself, communicating back to the prepaid billing system.
The service node approach is popular in legacy networks because it is vendor agnostic
(it works with all switch types). The IMS online charging architecture eliminates the
need for service nodes in the call path by placing control back in the network element
itself and providing a standardized communications mechanism between the billing
system and the network element.
This represents a cost advantage to operators looking to deploy prepaid services. The
process is simplified within the IMS model by allowing the network element the means
of communicating directly with the charging system regarding an account, and in turn
controlling the bearer path.
Session-based charging requires that a CDR be opened as soon as the session begins,
and information is added to the CDR throughout the session. This means that interim
CDRs may be available but not forwarded to the billing system.
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