Charging

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

• Offline Charging : All the SIP network entities (P-CSCF, I-CSCF, S-CSCF, BGCF, MRFC, MGCF, AS) involved in the session use the Diameter Rf interface to send accounting information to a CCF (Charging Collector Function) located in the same domain. The CCF will collect all this information, and build a CDR (Call Detail Record), which is sent to the billing system (BS) of the domain.
  Each session carries an ICID (IMS Charging Identifier) as a unique identifier. IOI (Inter Operator Identifier) parameters define the originating and terminating 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 an SCF (Session Charging Function) which looks like a regular SIP application server. The SCF 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 Ro interface towards an ECF (Event Charging Function).
  • When IEC (Immediate Event Charging) 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 ECUR (Event Charging with Unit Reservation) is used, the ECF 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.
    
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