The IMS: IP Multimedia Concepts And Services, Second Edition

3.11. Charging

Flat-free and volume-based charging schemas are traditional charging models in current IP-based communication networks. The IMS enables new charging models which, in turn, enables different business models for IMS operators. The capability to charge based on session or event or service is one of the key advantages that the IMS enables for operators. There are foreseen benefits for end-users as well. For instance, an operator is able to offer peer-to-peer games as a pre-paid service (i.e., a user needs to have money in her account before consuming services) and other multimedia sessions as a post-paid service (i.e., a user pays for services periodically such as once per month), or instant messages could be available as a flat-free service and session based-messaging charged differently (e.g., based on duration of the session or based on transferred bytes).

In order to offer a post-paid service, the IMS needs to support a mechanism for offline charging. Offline charging is a charging process where charging information is mainly collected after the session and the charging system does not affect in real time the service being used. In this model a user typically receives a bill on a monthly basis, which shows the chargeable items during a particular period. A pre-paid service requires online charging support. This means that IMS network entities need to consult the Online Charging System (OCS) before allowing users to use services. The OCS is responsible for interacting ...