rovided relevant billing data to the billing system. There is an account database that the bill- ing system will query to get information on customer account balance, credit limit, billing rules to be applied as per SLA, tax to be added as per regulations, additional charges for usage of specific service, and SO on that are associated with generating a final bill to the customer. Two important modules in the billing OSS are the mediation system and the rating system. All service providers have a mediation system that consists of several subsystems to capture usage information from the network and service infrastruc- ture and distribute it to upstream BSS modules like billing, settlement, and mar- keting (see Figure 20.14). The subsystems collect billing records from the various network infrastructures supported by the service provider and provide feed to the central mediation system that aggregates the records, performs correlation between records, repairs records with errors, filters the records based on predefined rules, and maps the data to a target format required by the business support systems. Some of the popular formats handled by the mediation systems include CDR, IPDR, and SDR (see Figure 20.14). Mediation functions can be classified into usage data collection and validation functions. OSS/BSS Functions Customer Billing Sales force Marketing relationship settlement automation management management Mediation Revenue warehouse system assurance Network specific APIs Collection probes and partner system ((a)) Wireless Application Circuit core Access Transport network servers network network network Figure 20.14 Mediation system. 1. Usage data collection: The usage records may be based on number of calls, messages sent, traffic utilization, or events based. A set of usage types can be defined based on originating/terminating point and the service being used. In fixed to mobile type the originating point is a fixed network while the terminating point is a mobile network. Similarly there can be other types lik