Comcast Corp.'s interference with Internet traffic has prompted a federal investigation and is at the center of calls for "Net Neutrality" laws, but another U.S. cable company appears to be doing the same thing without drawing scrutiny.

A study released Thursday found conclusive signs that file-sharing attempts by subscribers of Cox Communications were blocked, along with customers at Comcast and Singapore's StarHub.

Of the 788 Comcast subscribers who participated in the study, 62 percent had their connections blocked. At Cox, 54 percent of subscribers examined were blocked, according to Krishna Gummadi at the Max Planck Institute for Software Systems in Saarbruecken, Germany. The institute examined the network connections of 8,175 Internet subscribers around the world.

Philadelphia-based Comcast is the country's second-largest Internet service provider, with 14.1 million subscribers. Atlanta-based Cox Communications is the fourth-largest, with 3.8 million. It is part of privately held Cox Enterprises Inc.

Comcast's practice of interfering with traffic was brought to light by user reports last year and confirmed by an Associated Press investigation in October.

Consumer advocacy groups and legal scholars criticized the interference, saying that letting a service provider selectively block some connections makes it a gatekeeper to the Internet, violating the network's open principles. Their complaints prompted the Federal Communications Commission to launch an investigation, which is ongoing.

Legislation also has been introduced in Congress to guarantee "Net Neutrality," or equal treatment of traffic by Internet service providers.

"This research proves beyond a shadow of a doubt that consumers, Congress and the FCC must urgently pursue the complaints against network providers," said Ben Scott, policy director of Free Press, one of the groups that urged the FCC to fine Comcast.

File-sharing programs like BitTorrent, which let people exchange documents, songs, movies and other content, can be heavy users of Internet bandwidth.

Comcast maintains that hampering such programs helps ensure that traffic other than file-sharing is not impeded by a few big users. The company adds that is "delaying" file transfers rather than blocking them. Even that will end later this year, Comcast said in March, as it pledged to stop selectively targeting file-sharing.

Much of the FCC's attention to the matter has been focused on Comcast's secrecy. Before the AP's investigation, Comcast acknowledged only in the most general terms that it was managing traffic.

At least since 2006, Cox's subscriber agreement has noted that the company engages in "protocol filtering," which means it treats different types of Internet traffic, like Web surfing, e-mail and file-sharing, differently.

Cox said in a written statement Thursday that it takes such steps "to ensure the best possible online experience for our customers." But Cox denied that protocol filtering amounts to discrimination of any specific services.

The blocking observed by Gummadi's group occurs when a subscriber has downloaded a file using the BitTorrent application and tries to upload it, or share it with others, over the Internet. The main victims are the other Internet subscribers, who will not be able to download a file if a complete version is not available from someone else's computer.

Persistent attempts by file-sharing software to get through an Internet service provider's filtering may succeed after several minutes, as experienced in the AP's test last year. But Gummadi's test did not look at the duration of the traffic blocks.

The percentage of blocked connections for Comcast and Cox subscribers did not appear to correspond to periods of high congestion, despite Comcast's assertions to the FCC that the filtering only happens at certain times. Subscribers were roughly equally likely to be blocked at all times of day and night.

Robb Topolski, a former Intel Corp. engineer who noticed blocking on his home Comcast connection last year and brought it to notice, said Gummadi's work was the most authoritative study so far of this type of traffic interference.

Gummadi acknowledged that the test did not conclusively show that Cox or Comcast were blocking traffic, since neither company carried data all the way from their subscribers to his servers in Germany. In theory, Internet backbone carriers that take the traffic the rest of the way could be disrupting the connections, but there is no reason to believe they're doing so.

Apart from Comcast and Cox, Gummadi found signs of interference at seven other U.S. Internet providers, all of them cable companies. But the number of blocked connections was too low to conclusively say their subscribers are being targeted, and Gummadi withheld the companies' names.

StarHub is Singapore's dominant cable company. It did not reply to an e-mail to its press office.

Gummadi, who is the head of the network systems research group at the German institute, found no signs of interference by phone companies. Their DSL Internet connections aren't shared between neighbors the same way cable is, so they have less need to manage congestion.

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On the Net:

Glasnost study at Max Planck Institute for Software Systems: http://broadband.mpi-sws.mpg.de/transparency

http://www.cox.net

http://www.comcast.net