 billing system is used to manage the receivables invoicing, payment, collection, and crediting functions and the billing support systems also includes billing data collection, mediation, and rating (see Figure 20.18). The key players in billing can be segregated under the following heads: Billing products: Companies like Amdocs, Telcordia, and Convergys are among the top players developing billing products for the telecom industry. Billing services: Service bureaus for billing offer both tier 1 and tier 2 services. Amdocs, CSG, and Convergys are among the top tier 1 service companies. There are many players offering tier 2 service-a few notable companies being Alltel, VeriSign, Intec, and Info Directions. Solution integrators: Billing integration is another hot area and Oracle is one of the leaders in telecom billing integration. Legacy/custom solution development and support: Some of the service pro- viders opt for custom solution development or relay on their legacy applica- tion to meet the billing needs. This is mainly attributed to the high level of security in content associated with billing and the company specific billing process that may not easily be satisfied by an off the shelf product. In house developers or developers from service and consulting companies are employed by the service provider initially to develop custom solutions and later to sup- port the solution. OSS/BSS Functions Customer payment Billing system Recurring charges Charge calculation collection Rollup charges Invoicing Discounts and offers Rating Archival engine system Customer relationship management management system Figure 20.18 Billing system. 20.5 Conclusion OSS deals with the range of operations in a service provider space that is critical to the operator's success. Fulfillment, assurance, and billing (FAB) systems make up the major modules in operations and business support systems that are used by the service provider on a regular basis for seamless business continuity. A closer look into FAB reveals several core modules li