When it comes to in-stadium wireless networks, it turns out that teams and wireless providers aren’t the only players in the game. ExteNet Systems, a Lisle, Ill. startup, is rapidly scoring points by playing the “middleman” role, building out stadium networks at no cost to teams and making money by renting network access to the carriers.

Last week ExteNet announced it was the builder of the new Distributed Antenna System (DAS) installation at the new Marlins Park in Miami, with Verizon Wireless and T-Mobile USA already signed up as customers. And it turns out that ExteNet was also behind Verizon’s DAS deployment in and around Lucas Oil Stadium for the recent Super Bowl XLVI. Those deployments are just part of a new stadium-business push from ExteNet, which also put in a DAS system at the “Big House,” the huge bowl stadium at the University of Michigan, last fall.

Though ExteNet also provides in-building networks to some more traditional markets — like health care and hospitality, where large groups of people are seeking network access inside a confined space — it’s looking to add more stadium customers in the near future. According to CEO Ross Manire, a veteran of the networking industry, the boom in powerful handheld devices has created a “perfect storm” of capacity consumption that can’t be addressed by the historical cellular antenna architectures.

“You used to just worry about voice coverage,” said Manire in a phone interview last week. “But phones are really data terminals now, and the question becomes how do you manage strain on networks [at stadiums]. “In the past you may have been able to get a voice signal from a cell tower several blocks away. That doesn’t work now.”

The privately held ExteNet, which received a $128.4 million funding round at the start of 2010, thinks it can help both carriers and venue owners by providing “middleman” network services. The basic business model, Manire said, has ExteNet building and owning the stadium network, which it deploys at no cost to the team or building owners. It then charges carriers a “node activation fee,” a sort of monthly rent to let their services ride on the network.

If you’re not familar with how a DAS works, it’s essentialy a bunch of small cellular antennas that are mounted on ceilings and walls inside buildings, instead of on huge towers like traditional cellular antennas. A DAS network administrator provides normal cellular connectivity to users, and then connects those signals to the appropriate cellular vendor on the network’s back end. DAS deployments can also add Wi-Fi antennas to their infrastructure mix. ExteNet, like many DAS operators, can connect its back end to any number of cellular networks, meaning that fans don’t have to be a customer of a certain provider to get better in-stadium service.

The openness of ExteNet’s design may make it a better choice for stadium operators than an internal network designed and owned by a single carrier, since fans wouldn’t have to be customers of a specific carrier to get better service. ExteNet runs a network operations center 24/7 to provide support to all its networks, and signs a service level agreement (SLA) with both the stadium owner and its cellular partners to give both parties what they really want — happier customers.

“We’re seeing an incredible amount of attention from stadium and team owners, who need to provide a good [wireless] experience,” Manire said. “Fans believe they should be able to connect wherever they go. Of course that’s easier said than done.”

But now ExteNet may be part of making that connectivity a reality for more fans, going forward.