Denver International Airport is betting that travellers will like getting something for free, and so far it looks like a good bet.

The airport, one of the busiest in the US, last month switched its public Wi-Fi offering from paid to advertising-supported. Within a week, and with no public notice of the change, Wi-Fi use grew tenfold, said Jim Winston, director of telecommunications for the airport. He expects the network to get even busier.

DIA is a large-scale case study of free Wi-Fi in airports. About 50 million passengers pass through the airport every year, with as many as 165,000 per day during busy times of the year, airport spokesman Jeff Green said. Now that Wi-Fi is free, there are 7,000 to 8,000 connections to the network per day, according to Winston. To link all those free users with the Internet, the airport at first bumped up its "backhaul" to 5Mbit/s but later found that wasn't enough. It now has a 10Mbit/s connection just for the Wi-Fi users.

The change was part of a complete revamping of the Wi-Fi service, which DIA first offered in 2002. For the first five years, the airport owned its own network but turned to AT&T to operate and maintain it in return for a concession fee. AT&T charged users on a variety of models, including one that cost US$7.95 per day. Now the airport has taken over the service and installed a whole new network with the latest technology.

Denver is probably the first airport in the world to announce a deployed public Wi-Fi network with IEEE 802.11n Draft 2.0 capability. The new technology, which the Wi-Fi Alliance is certifying for interoperability before the final 11n standard is approved, is designed for higher speed and longer range than previous versions. DIA's network also supports earlier versions of Wi-Fi.

The airport turned to Meru Networks for its infrastructure, partly for ease of management, Winston said. AT&T fixed and upgraded access points and bore that cost itself. As the carrier left the picture, DIA knew it would have to handle firmware upgrades and other changes to the approximately 60 access points in the 53-square-mile airport.

Even moving to the free model, "we would still have to visit each one of them," Winston said. The airport needed a new architecture that would let administrators modify each access point remotely, he said.

Meru's management software allows this - but more distinctively, DIA also liked the company's "single-channel" architecture. It runs all access points on the same Wi-Fi channel, treating interference as overlapping signals and automatically connecting users to another nearby access point if one gets overloaded. If that channel reaches its capacity limit, another one can be used. The airport has already seen 25 clients using one access point. Winston believes the system could handle as many as 45 at a time.

Public Wi-Fi providers such as DIA don't need to roll out Draft 11n access points yet, Gartner wireless analyst Ken Dulaney said. Few notebook PCs are even equipped with the technology today, and the bottleneck in most such setups is the shared backhaul to the Internet rather than the speed of the wireless LAN itself, he said.

However, one benefit of the 11n capability doesn't depend on legitimate customers demanding high speed, according to Meru. With its built-in 11n, Meru's network can easily detect unauthorised 11n access points in the area that could pose a security threat, said Rachna Ahlawat, vice president of strategic marketing at Meru. The alternative is a separate device just for detecting rogue networks, a significant added cost, she said.

Using a Wi-Fi network without 11n, administrators could detect a rogue hotspot, though they couldn't immediately tell it was an 11n system, Gartner's Dulaney said. He acknowledged the built-in 11n detection would make the IT department's security job easier.

Ease of management is important for the network because it still needs to make a return on its wireless offering even though travellers aren't paying for it directly, Winston said. DIA has construction bonds to pay off. So FreeFi Networks, a Wi-Fi advertising company, sells video ads that appear right before the user starts using the Internet and a persistent ad bar at the top of the computer screen, he said.

If the system keeps delivering on its promise, DIA will probably roll it out on another network it operates for airlines and concessionaires, which pay a monthly fee for a variety of data and voice services, Winston said.

Free, advertising-supported Wi-Fi has a chequered past, and some plans for municipal networks that would have used it fell through. Some other airports, such as Las Vegas International, have also adopted it. But among airports, like other hotspot venues, no one business model will fit all and a variety of approaches will remain for the foreseeable future, Gartner's Dulaney said.