Accounting is the process of collecting information about resource consumption. While accounting data can be used for multiple purposes such as capacity and trend analysis or cost allocation, it particularly forms the basis for the charging and latter billing of a user. The IMS defines two charging modes: online and offline. Online charging is a process in which the charging information can affect in real time the service rendered and therefore directly interacts with session/service control. Prepaid services are applications that need online charging capabilities. Offline charging is a process in which the charging information does not affect in real time the service rendered. This is the traditional model in which the charging information is collected over a particular period . Both modes result in generated Charging Detail Records (CDRs) that are necessary for the purpose of billing a subscriber for the provided service. Offline charging is used over the Diameter-based Rf reference point which is specified between a Charging Collection Function (CCF) and either a CSCF, an Application Server (AS), a MRFC, a BGCF or a MGCF. In the roaming case it is verified during authentication phase that roaming to the visited network is allowed. IMS provides a means for
charging per media component, as IMS sessions may include multiple media components. This would allow a possibility to charge the called party, when a new media component is added in a session.
Quality of Service (QoS)
The policy-based QoS (Quality of Service) control architecture in the IMS is the key part to provide IP-based multimedia applications and services with end-to-end QoS guarantees. The IMS session setup is based on a clear separation between the IMS session signalling and the allocation of resources. This means the IMS session setup is
started but afterwards set on hold. At this time, both endpoints are responsible for requesting the required resources at least in their access network, where the IMS session setup is only successfully completed if both endpoints received sufficient resources.
The IMS Release 5 specifications specifies the policy architecture only to GPRS networks  and Release 6 describes improvements, e.g. the specification of the PDF as a stand alone element to allow not only IMS compliant technologies but also to control of the bearer traffic to other access technologies. ETSI TISPAN is extending the IMS interconnection with fixed networks in order to have better QoS management. The approach of resource monitoring can help to perform resource management in order to get mobility and roaming QoS functionalities
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