Traditional network operators have not tended to view WiFi as a major revenue-generating opportunity, but a San Diego-based startup called XCellAir is hoping to convince them otherwise. Spun out of wireless patents player InterDigital Inc. , from which it received a second tranche of funding in July, the 25-employee company is brandishing a new piece of research that suggests operators in the world's ten biggest financial centers alone could be missing out on an $18 billion opportunity as a result of poor WiFi management.