The usage of RADIUS for accounting in 3GPP-defined systems is pretty much equivalent to the usage of RADIUS accounting in wireline networks, with the exception that there are some specific applications of RADIUS accounting that are particularly important in wireless networks. For instance, RADIUS interim accounting may be used to implement prepaid services (by triggering the sending of RADIUS Accounting Request interim updates based on the volume of time-based thresholds), as well as tariff time plans (by triggering the sending of RADIUS Accounting Request interim updates based on specific time of day or day of week).
This usage of RADIUS accounting may be adopted in wireline networks too, but it is particularly useful to adapt the service paradigm to the current prepaid service paradigm many GSM users are experiencing. When a user is roaming, the accounting server may record this based either on the 3GPP-SGSN-IP-ADDRESS VSA or the 3GPP-GGSN-MCC-MNC VSA (when a GGSN in the visited network was used) described in [3GPP TS29.061]. Note that in UMTS you can associate multiple bearers to a single session. As such, a session is up when one or more of these bearers are active. This requires the GGSN to indicate when the last existing bearer of a session is torn down, in order to enable, for instance, the release of the IP address assigned via RADIUS. 3GPP also defines the optional use of a uncommonly used DISCONNECT message [RFC2822]. This may be used to disconnect users exhausting their prepaid balance.