As Boingo Wireless Inc.’s stock continues to slide, there’s a growing concern whether the Wi-Fi provider can overcome an obstacle that’s always loomed for the company: free public Internet.

The Westwood firm closed down another 6 percent today to $6.37, which totals a 20 percent drop from its price before last week’s earnings.

What seemed to shake up investors was not the recent quarter – in fact Boingo’s fourth quarter showed an increase in net income from that same period a year ago. Rather, investors are worried about the future.

Boingo’s primary revenue comes from people paying for access to a hotspot in one of the company’s many public locations, such as airports, restaurants and malls. Some customers, which the company called “retail,” pay for one-time use; others sign up for a monthly “wholesale” subscription. The retail customers have better margins and make up almost 40 percent of the company’s revenue.

But more and more venues partnered with Boingo are adopting a model that offers limited free Wi-Fi access. That trend has the company lowering its guidance for retail users and raising flags for analysts that the easy money could be drying up.

“The prospects on wholesale look very strong, but retail is almost half the company,” said Donna Jaegers, an analyst at an analyst with D.A. Davidson & Co. in Denver. “If that implodes the profitability doesn’t look as good.”

Boingo Chief Executive David Hagan deemed 2012 a transition year for the company as it begins to ramp up its role with cell carriers. The business of “data-offloading,” that is, working with the mobile companies to carry some of the burden that smartphone users put on the cell networks, could be a bright spot. Boingo has already partnered with some of the smaller domestic cell carriers to provide data offloading.

And as the number of smartphone owners and the amount of data they consume increases, the need for offloading will grow.