e a major effect in real time on customer experiences with SIP trunks. Errors or slow processing of post-paid billing servers have a less immediate impact on customers as they only impact bills that are sent on a periodic basis. For services beyond basic SIP trunks for PSTN access, the SP must have a method for billing these alternative transactions that is normally beyond just per-minute billing. One idea that has not caught on is billing based on the number of packets that have been sent during a call. This enables customers and SP to introduce unique new features such as video calls or application sharing over SIP trunking while maintaining a steady billing mechanism. Unfortunately, the billing mechanism directly impacts the types of new fea- tures that are offered. Unless a SP can accurately bill for a new service and the service gen- erates more revenue than the cost to maintain it, there is little innovation that occurs at SPs. The features and interconnection of a SP Billing Server are shown in Figure 5-3. Billing Server Receives Transaction details Real-time billing servers may interact during call processing to allow, disallow, or redirect a call Call Agent Completed rating for calls based on Call Agent prepaid or post-paid billing information Sends/Receives Real-Time Billing Transaction Details Servers will interact with call processing during call setup time. Advanced feature billing may occur separately from Call Agent (for example, request / response generally occurs Updates bill and Video call. billing). Legal intercept billing flags, such as high invoice amount. via a feature server. Feature Customer Server Database Figure 5-3 Features and Interconnects of a SP Billing Server SP Network- Network Infrastructure One area that is often assumed and overlooked when examining components of a SIP trunking solution is the basic underlying IP Infrastructure. This infrastructure is often just shown as a "cloud" in architecture diagrams with the assumption that all devices can con- nect equally and with high 