id 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. Information in the database is accessed by the rating engine to rate the calls. The rating engine will apply the payment rules on the transactions. Discounts and promoting can also be applied. 5. The information from a rating engine is sent to the billing system for sending the invoice. Any bill related settlements can be done by the billing system. 6. Invoices prepared by the billing system are sent to the customer as post, e-mail, or uploaded to a secure Web page. Most service providers allow the customer to register in the service provider Web site and give them the capa- bility to securely access and pay their bills through the Internet. Some of the essential requirements in billing are: Multiple rating techniques: In a competitive market it i