billing models. Even when advanced functionalities are offered, the billing system should still be scalable. An increased number of billing records should not affect the performance of the system. The bill- ing system should provide enough abstraction of information to make it easy for the operator to work on the system. Availability is the key requirement of a billing system, as a few seconds of discontinuity in billing can result in a major financial loss to the service provider. The billing module interacts with many other support system modules (see Figure 20.13). The main interactions at the business level happen with customer care and marketing and sales modules. At the service level, the interaction is with Customer Marketing and sales Billing Accounts Network database management Figure 20.13 Interactions with billing module. Fundamentals of EMS, NMS and OSS/BSS an accounts database and at the network level the interaction is with the network management module. The billing system can generate trend reports of services that are generating more revenue that can be used by the marketing and sales team for designing strate- gies to increase business. The billing reports help in planning for investments based on market trends. The billing data is to be used by a sales team in providing infor- mation to customers on what could be the best service plan for a customer based on trends in the specific customer segment that the user falls in. Impact of promotions and discounts on sales of a specific product can be computed from the billing sum- mary report for a specific product. The customer care modules will interact with billing to apply service order adjustments and any specific discounts that need to be offered to the customer on a need basis. For example, a customer might want to cancel a specific service before the end of the billing cycle. In this case the number of days for which the customer has to be billed has to be updated in the billing system based on information pro- vided to the customer support exec