Editing IP Multimedia Subsystem (section)

=== Charging ===
Offline charging is applied to users who pay for their services periodically (e.g., at the end of the month). [[Online charging system|Online charging]], also known as credit-based charging, is used for [[Prepayment for service|prepaid]] services, or real-time credit control of postpaid services. Both may be applied to the same session.

''Charging function addresses'' are addresses distributed to each IMS entities and provide a common location for each entity to send charging information. ''charging data function'' (CDF) addresses are used for offline billing and ''Online Charging Function'' (OCF) for online billing.

* Offline Charging : All the SIP network entities (P-CSCF, I-CSCF, S-CSCF, BGCF, MRFC, MGCF, AS) involved in the session use the [[Diameter (protocol)|Diameter]] Rf interface to send accounting information to a CDF  located in the same domain. The CDF will collect all this information, and build a ''call detail record'' (CDR), which is sent to the billing system of the domain.<br/>Each session carries an ''IMS Charging Identifier'' (ICID) as a unique identifier generated by the first IMS entity involved in a SIP transaction and used for the correlation with CDRs. ''Inter Operator Identifier'' (IOI) is a globally unique identifier shared between sending and receiving networks. Each domain has its own charging network. Billing systems in different domains will also exchange information, so that [[roaming]] charges can be applied.
* Online charging : The S-CSCF talks to a ''IMS gateway function'' (IMS-GWF) which looks like a regular SIP application server. The IMS-GWF can signal the S-CSCF to terminate the session when the user runs out of credits during a session. The AS and MRFC use the [[Diameter (protocol)|Diameter]] Ro interface towards an OCF.
** When ''immediate event charging'' (IEC) is used, a number of credit units is immediately deducted from the user's account by the ECF and the MRFC or AS is then authorized to provide the service. The service is not authorized when not enough credit units are available.
** When ''event charging with unit reservation'' (ECUR) is used, the ECF (event charging function) first reserves a number of credit units in the user's account and then authorizes the MRFC or the AS. After the service is over, the number of spent credit units is reported and deducted from the account; the reserved credit units are then cleared.