ime Warner Cable sent some of its New York City cable-modem subscribers letters last week warning that operating wireless networks and inviting others to freely share them violated their subscription agreements.

The company's action highlights a potential conflict between a small number of advocates of free, wireless networking and the broadband providers who supply their Internet connections.

Fewer than a dozen letters were sent, according to the company, a unit of AOL Time Warner. The letters cited a clause in the subscription agreement prohibiting redistribution of the company's Internet connection service.

Barry Rosenblum, president of Time Warner Cable of New York City, said he had no problem with users who share a wireless network within their own homes. What the company objected to, he said, were subscribers who used their networks to provide Internet access at no charge to others outside.

"We're trying to keep people from redistributing the service we sell them," Mr. Rosenblum said. "Our concern is when people specifically bolster the signal to share with others outside."

That is the aim of the so-called free wireless network groups that have emerged in many large American cities. These groups, including NYCWireless in New York, encourage individual users to establish, publicize and share wireless networks.

At the heart of the conflict lies a technology known as Wi-Fi, for wireless fidelity. Wi-Fi networks use radio signals to broadcast an Internet connection as far as 300 feet, permitting users with properly equipped computers to connect to the Internet at high speeds without wires.

Many Wi-Fi networks, intentionally or otherwise, allow passers-by to use the networks without any password. And there are tools that amplify the Wi-Fi radio signal, enabling it to be delivered over an even larger area, like a park.

Many broadband providers fear that every user of a free wireless network is one less paying customer. "Our goal is just to protect our customer base," said Mr. Rosenblum, adding that Time Warner Cable currently had no plans to extend this enforcement campaign to other areas that it serves.

Mr. Rosenblum acknowledged he had no way of knowing how many of these free wireless networks were being operated, or how much money, if any, they were costing the company. Among the sources Time Warner Cable consulted to track violators were public Web sites that promote the existence of these networks, including one operated by NYCWireless.

In at least one case a letter was sent to a user who said he had not actually set up a wireless network. "I don't actually have any wireless equipment; I've never had any wireless equipment," said Justin Cobb, a Manhattan resident who had indicated on the NYCWireless Web site that he was potentiality interested in some of the group's future projects.

Mr. Cobb said he understood Time Warner's need to prevent nonpaying users but was also "really bothered by the fact I'm being accused of criminal activity." He said he was considering switching Internet service providers.

For the moment, most publicly available wireless networks are limited to small areas such as sidewalk cafes and parks, but several groups have discussed finding ways to create a free wireless "cloud" that would offer Internet access to larger areas.

More immediately, broadband providers worry about situations in which one person pays for a broadband connection, then sets up a Wi-Fi network and shares it with a neighbor. Such an agreement would be illegal under the terms of Time Warner's current policy.

There are, however, some smaller Internet providers that have promoted themselves as friendly to free wireless in the hope that the customers gained will offset potential revenues lost through freeloading.

Arkady Goldinstein, chief executive of Acecape, a digital subscriber line provider based in New York, said it was "purely a cost-benefit analysis" to allow his customers to set up free networks. Mr. Goldinstein added that out of his firm's "several thousand" subscribers he believed "less than a dozen" have set up free networks.