Session-based charging can be difficult in online charging, because the duration of a
call is not yet known, so there must be some means of authorizing the call if the subscriber
has enough credit in his or her account, yet it has to be a means for terminating
the call should the account be depleted. One method specified by 3GPP is the allotment
of credit in units.
174 Chapter 7
For example, a unit could be three minutes duration of time. When the call is originated,
and the account balance is checked, there could be nine minutes left in the
account. Using this principle, the charging function could authorize the call for six
minutes, allotting two units for the call.
If the call should use the reserved six minutes, the network element would then create
a new charging event, and the billing system would then allot another reservation
of units. The new reservation of units would be for the remaining three minutes left on
account. If there is no balance on the account, then the call is denied. This is referred
to as session-based charging with unit reservation.
Should the call exceed the reserved number of units, the call would be terminated
by the network element. Since the network element is in control of the session, it has
full control over the bearer path and is able to terminate the session without the use of
additional elements or messaging.
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