As more and more people use the mobile networks to access the Internet from smartphones, netbooks and laptops, a growing number of doom-mongers are warning that flat-rate data plans are unsustainable. They argue, with some justification, that operators are soon going to have to find new business models and sources of revenue to justify the investment in additional capacity and new technologies necessary to meet rising demand.

How should operators go about this? First they need to calculate “the production cost per gigabyte of data” based on the real cost of adding extra new capacity, rather than just using up the “sunk costs” of current radio assets, according to a new report from STL Partners.

STL suggests that operators then need to develop a broad range of tariff plans (based on the real production cost), ranging from long-term contracts to prepaid accounts to session-based services to bundles to mechanisms for enabling sponsored or advertising-supported connections. In some cases, operators will have to work with vendors and standards bodies to create the necessary technical enablers.

So far, so sensible. But some of STL’s other recommendations are more controversial. The consultancy cautions against marketing mobile broadband as a direct alternative to DSL or cable, as operators may soon be looking to offload traffic onto those wired broadband lines via femtocells or WiFi. “In dense areas, spectrum and network capacity is generally too valuable to waste on those users who are not truly mobile,” according to STL.

The report also argues that operators should look to add value to dongles, which many mobile users like, rather than pushing laptops and netbooks with embedded modules too hard.

So, should mobile broadband be reserved for the truly mobile and are dongles the way to go?