It appears that cable TV giant Comcast blocked Internet traffic even when its network wasn't overloaded with traffic, the chairman of the U.S. Federal Communications Commission told Congress on Tuesday.

FCC Chairman Kevin Martin's testimony contradicted claims by Comcast. Martin told the U.S. Senate Committee on Commerce, Science, and Transportation that cable subscribers do not have the freedom to do what they want on the Internet.

"Specifically, based on the testimony we have received thus far, some users were not able to upload anything they wanted and were unable to fully use certain file sharing software from peer-to-peer networks," he said.

Martin also said that Comcast appears to have blocked Internet activities when traffic was low, instead of using network management techniques only to handle congestion during peak times as the company has claimed. He said the company apparently used equipment from Sandvine or a similar inexpensive and blunt technique that completely blocks certain traffic, like peer-to-peer file sharing.

"It does not appear that this technique was used only to occasionally delay traffic at particular nodes suffering from network congestion at that time," he said. "Indeed, based on the testimony we have received thus far, this equipment is typically deployed over a wider geographic or system area and would therefore have impacted numerous nodes within a system simultaneously. Moreover, the equipment apparently used does not appear to have the ability to know when an individual cable segment is congested. It appears that this equipment blocks the uploads of at least a large portion of subscribers in that part of the network, regardless of the actual levels of congestion at that particular time."

Martin added that Comcast's network management practices haven't been "content agnostic," despite claims to the contrary. He said Comcast's pledge to switch to content-agnostic network management concedes that the company's current practice isn't agnostic. Finally, he expressed caution regarding the company's announcement that it will deploy a new system by year's end.

"Indeed, the question is not when they will begin using a new approach but if and when they are committing to stop using the old one," he said.

Like other recent hearings on the issue, Comcast was absent and those opposing network neutrality -- or rules requiring content and provider agnostic methods of managing traffic on the networks -- were in the minority.

Comcast has maintained that it has managed its networks to accommodate traffic and relieve congestion as users increasingly demand rich media applications. The company recently said it would lead an effort to create industry standards on network management.

Kyle McSlarrow, president and CEO of the National Cable and Telecommunications Association, testified on behalf of Comcast and ISPs (PDF). He urged the federal government to not pass new laws or regulations regarding network management but to allow private companies and markets to determine the best way forward. Like other opponents of network neutrality, he said the Internet has boomed without government intervention. McSlarrow added that it would be impossible for the federal government to keep pace with evolving technologies.