In the last chapter we learned how to implement NAT traversal; now it is time to focus on the most important thing for a VoIP provider, the revenue. The accounting feature will allow you to determine the exact duration of each call. We will show you two methods. The first one is using MySQL and later using a RADIUS server. RADIUS is a de facto standard for AAA (Authentication, Authorization, and Accounting). Duration in minutes is not enough to bill the customers. You will also need a rating tool. This tool is able to convert minutes to whatever currency you will use to bill your customer.