 on duration of sessions the user is connected to the internet is an example of using duration-based billing. Content-based billing: This model is most common in IP billing. In this bill- ing, the type of content offered to the customer is the determining factor. It can be applied on services like video broadcast, multimedia services, gaming, and music download. Charging process can be: Off-line charging: In off-line charging the service rendered cannot be influ- enced by the charging information in real-time. Online charging: In online charging the service rendered can be influenced by the charging information in real-time. Online charging is becoming increasingly important with convergent services where a third-party con- tent provider will be paid by the service provider based on revenue sharing agreement for third-party content offered to a subscriber by the service provider. OSS/BSS Functions Switch Database Mediation Rating Billing engine system Figure 20.12 Steps in billing. Bill payment methods can be: Prepaid: In this payment system the customer has to pay an amount in advance for service usage. The service usage is tracked against the amount deposited and continuity of service is ensured only when customer credit is above a predetermined value. Postpaid: In this payment system the customer receives a bill for service already consumed. The payment is only after service usage and continuity of service is ensured when the previous usage bill is settled before a predetermined date. Let us next look into the steps (see Figure 20.12) involved in a typical billing process. The steps are: 1. Calls are made between mobile equipment through a switching center. The switch generates CDRs on the calls. 2. Mediation device collects call records from the switch. The mediation system can collect data from multiple switches handling call routing in the service provider network. Fundamentals of EMS, NMS and OSS/BSS 3. After processing of the call records, the mediation device stores relevant information to a database. 4.