upport. Billing: Records are generated by the elements in the network that can be used for billing. In switches where calls are handled, call details records (CDR) are generated that have billing information. CDRs can have multiple call records. The CDR is parsed to for parameters like calling party number, destination number, duration of call, and SO on, and the customer is billed. A mediation module performs the activity of parsing and converting the infor- mation in CDR to a format that can be used by the billing system. Billing modules have a rating engine that applies the tariff, discounts and adjust- ments agreed upon in the SLA as applicable, and create a rated record on how the bill was calculated. This record is stored in a database and aggregated over the billing cycle. At the end of a cycle (like a month for monthly billing of calls), the aggregated records are used by modules like an invoicing system to prepare an invoice for the customer. There can be different billing models that can be applied based on the SLA with the customer. It can be flat bill where a fixed amount is billed for usage of service, or a usage-based bill where the bill reflects the customer's utilization. Another example along the same lines is real-time billing where the debit from a prepaid account or amount to be paid for on a postpaid account can be viewed as soon as a service is used. Now let us see an E2E flow of how the modules discussed in this section (see Figure 19.3) interact to offer a service like telephony. Every activity starts with a business requirement. So first the customer contacts the sales desk and uses a Web interface to request the service. Once the credit check is performed and the customer can place an order, the required customer details and service information is feed to the order management system. The order management system creates a work order and sends the details to the provisioning system. The provisioning system sends What Is OSS and BSS? Customer Customer Sales and Order relationship Billing cu