Billing is generated when the Network Elements (e.g. GMSC/gsmSSF, MSC/gsmSSF, MSS, etc..) generates records that are associated with the call. These records are known as EDR – Event Data Records. The records are collected / processed by a Mediator. As a last step, the Mediator will send the consolidated records to the Billing system so that an Invoice can be generated. Standard billing processes are used.
For Realtime charging using Intelligent Networks (IN) a well defined protocol is used. As you have mentioned gsmSSF and gsmSCF, the CAMEL protocol is used. To help with Charging the ApplyCharging () and ApplyChargingReport() methods are used. This is built into the CAMEL (CAP V2) protocol.