U.S. cable operators generated 6.8 billion transactions from free video-on-demand programs from content suppliers such as Comedy Central and CBS in 2011, up 17 percent compared to 2010, Rentrak said Thursday.

The data could help Comcast, Charter Communications, Time Warner Cable and other cable MSOs sell advertising in VOD programming. Canoe Ventures, an interactive TV consortium backed by cable MSOs, shifted its focus earlier this year to building a national platform for VOD advertising.

Rentrak said the average VOD user watches five hours and 17 free on-demand TV shows or videos each month, noting that about 55 million U.S. homes have access to VOD content. If you include paid VOD movies, the average VOD home spends about eight hours monthly watching on-demand content.

Rentrak didn't name which TV series from the Big Four broadcast networks and cable networks generated the most VOD traffic last year. But in February, Comcast said the most viewed on-demand TV series last year on its cable systems was Comedy Central's Tosh.0, followed by CBS's NCIS and Comedy Central's South Park. Comcast said the most popular kids series was Nickelodeon's SpongeBob SquarePants.