After much complaining from travelers, the Minneapolis -- St. Paul airport will have free wi-fi service by the end of the year. Currently, wi-fi is available for $7.95 per day at the airport, reports the Minneapolis Star-Tribune , which consulted a travel expert for an explanation of why free wi-fi is becoming so widespread at airports: "I think it's getting very close to the point where no airport can charge for wi-fi," said Joe Brancatelli, editor of Joe Sent Me , a business travel website. "These people just think wi-fi is in the air. They can't conceive of a place where there is no wi-fi, and they don't want to pay for it." "We know many travelers would love to see free wi-fi," says airport spokesperson Patrick Hogan. "The bottom line is that airports like MSP must generate the funds to cover all airport operating costs ... There is no such thing as free airport wi-fi. It's really just a question of who pays to cover the cost of providing the service." So who exactly is covering the costs of "free" wi-fi at airports today? Typically, providers such as Advanced Wireless Group (AWG) and Boingo use a tiered business model to generate revenue, in which all travelers have access to free basic wi-fi for a limited time (45 minutes or so) but must watch a sponsored ad before the session begins. A faster, unlimited, ad-free premium version of the service is available to those willing to pay for it. Even