 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 like fixed to fixed, mobile to fixed, and mobile to mobile. When the user keeps on switching networks for a single usage record itself, the type is Roaming. SMS and call forward are grouped under value added service type. A part of the usage data collection the mediation system performs is a set of key activities: Polling for data: The mediation system continuously looks for data on usage records from network infrastructure. While polling is a pull based mechanism where the mediation system checks for new record availabil- ity and collects data, a push based mechanism can also be implemented, where the billing agents in the network infrastructure actively sends data to the mediation system when a new record is generated with details on the usage. To make the billing agents independent of the media- tion system implementation, it is a usual practice to implement a polling mechanism for data collection where responsibility to collect and per- form mediation to map to a format lies with the mediation system. b. Consolidation of data: Once data is collected by a "push or pull" mecha- nism, next the data across network elements needs to be consolidated. The Fundamentals of EMS, NMS and OSS/BSS information sent from the mediation system to the upper business layer modules needs 