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 like order management, network manage- ment, and mediation system that interoperate to fulfill a customer order, assure service continuity, and bill the customer for the ordered service. The order management systems allow operators to perform the process of con- figuring the network to deliver the services ordered by a customer. Activating ser- vices on the allocated resource infrastructure is performed by the operator using provisioning systems. The allocation of resource and tracking the assets itself is performed by the operator using an inventory management system. After activa- tion, the task of monitoring the network to detect and isolate network failures and quickly restore services is performed using a network management system. A ser- vice management module works above the network manager to correlate network performance with service quality requirements, and appropriately communicate and compensate customers for any disruption. Finally, mediation systems allow operators to determine service usage. This information is taken utilized by a rating system that allows operators to determine how much to bill based on the service usage for a particular customer. Fundamentals of EMS, NMS and OSS/BSS Interaction between support system modules can be made simpler when service oriented architecture (SOA) is used, where the modules are developed as services with defined interfaces for interaction. Integration ac