Billing systems collect, rate, and calculate charges and then invoice for the product(s) or service(s). Figure 1 depicts a simple billing flow:

Figure 1. Projected WLL Subscribers by Region

Figure 1 shows that after a call is made, a collector gathers data from the switch and builds a call-detail record (CDR). This CDR must contain the originating number, the terminating number, and the start and end times. The CDR is then stored until it can be rated. To rate the call, the CDR is examined to see if the call is, for example, an 800 number, a local call that is covered by a local-area calling plan, or a toll call. Information such as the time the call was placed and the distance between callers is also used to calculate the rate for the call. Once each call is rated, this information is stored until the invoice is run, usually once a month in North America. When the invoice is run, other nonusage charges can be applied to the bill, such as volume discounts or monthly fees. This information is then prepared so that it can be printed in a readable form. Finally, the envelope is printed, stuffed with enclosures, and mailed. Call data is also shared between companies to handle calls that originate, terminate, or are transported on another company's network.

Several options are available that change the nature and capability of the billing system. For example, if calls are rated and stored, then an accurate current bill total can be quoted to a customer. Unfortunately, this limits the option to provide flexible ratings based on volume of usage. Another option might be to delete all nonbillable (local) calls as early as possible, which would save significant storage costs. However, that call data might be useful for studying usage patterns.

Issues that must be addressed when managing the billing system are many; reliability, accuracy, and readability are basic needs. Data formats, especially among different billing system components, are a significant source of complexity and expense in any system. Data that is shared is particularly sensitive to compatibility issues. The ability to bill many different services (local and long-distance phone, Internet, wireless, cable TV, etc.) on a single bill has proved difficult; the logic for handling wireless-call billing is very different from the logic of regular fixed-line billing. Integrating this logic into a single application is quite complex and requires skills in all media. Increasingly, this capability will be a differentiator in billing solutions over the next few years.