Analyst firm Probe Research suggests that the deployment of new billing systems will be a key factor in driving the success of wireless data services. Currently in the U.S., most users pay a fee to their wireless operator for access to data services and then generally pay again per minute, regardless of the value or volume of the content downloaded. While this method is accepted because of its relative ease for existing billing systems, Probe suggests it is not a good long-term strategy because it is not suitable for the end customer. Liz McPhillips, a Probe analyst, suggests that the key to financial viability for carriers may lie as much in their ability to deploy wireless data-specific billing systems as in the need to manage the business and technology issues. Because there's a lot of uncertainty regarding which applications will be popular among users, McPhillips recommends against mobile operators building out networks to support specific applications. Rather, she supports the need to build extremely flexible and scalable infrastructures and then run applications as needed.