ISG prepaid billing
is supported for IPv4, IPv6, and dual-stack subscribers. This feature allows
ISG to check the subscriber's available credit to determine whether to activate
a specified service and how long the session can last. The subscriber’s credit
is administered by a prepaid billing server as a series of quotas representing
either a duration of use (in seconds) or an allowable data volume (in bytes). A
quota is an
allotment, or fragment, of available credit. Allocating quotas in fragments
rather than providing all the credit at once enables ISG to support the use of
credit for multiple simultaneous prepaid sessions.
ISG uses the RADIUS
protocol to facilitate interaction between ISG and external authentication,
authorization, and accounting (AAA) servers and prepaid billing servers. A
single device can serve as the AAA server and the billing server.
To obtain the first
quota for a session, ISG submits an authorization request to the AAA server.
The AAA server contacts the prepaid billing server, which forwards the quota
values to ISG. ISG then monitors the session to track the quota usage. When the
quota runs out or a specified limit is reached, ISG performs re-authorization.
During re-authorization, the prepaid billing server may provide ISG with an
additional quota if there is available credit. If no further quota is provided,
ISG will log the user off from the service or perform some other specified
When a service is
deactivated, the cumulative usage is provided to the prepaid billing server in
an Accounting-Stop message.