The Chinese government has become increasingly sophisticated at controlling the internet, taking a multilayered approach that contributes to precision in blocking political dissent, a report released Thursday finds.

The precision means that China's filters can block just specific references to Tibetan independence without blocking all references to Tibet. Likewise, the government is effective at limiting discussions about Falun Gong, the Dalai Lama, Tiananmen Square and other topics deemed sensitive, the study from the OpenNet Initiative finds.

Numerous government agencies and thousands of public and private employees are involved at all levels, from the main pipelines, or backbones, hauling data over long distances to the cybercafes where many citizens access the internet.

That breadth, the study finds, allows the filtering tools to adapt to emerging forms of communications, such as web journals, or blogs.

"China has been more successful than any other country in the world to manage to filter the internet despite the fast changes in technology," said John Palfrey, one of the study's principal investigators and executive director of Harvard Law School's Berkman Center for Internet and Society.

Saudi Arabia, for example, largely controls the internet by having all traffic flow through a central agency, where it can be monitored. Visitors trying to access a banned site get a message saying it has been blocked, Palfrey said.

"China is much more subtle than that," Palfrey said. "You don't know what you don't know. It's more effective than if you see it but know you can't access it."

With filters at multiple points, including some search engines, content is simply removed rather than replaced with a notice, he said.

Google has acknowledged its Chinese-language news service – introduced on a test basis last fall – leaves out results from government-banned sites, though the company says that is done so users won't end up clicking on links that lead nowhere because of the Chinese filters.

Palfrey added that Chinese filtering methods are effective because they constantly change, keeping its users off-balance.

China, which has the world's second-largest population of internet users behind the United States, promotes internet use for business and education, while trying to curb access to political dissent, pornography and other topics the communist government deems sensitive. Many users do find ways around the controls – for instance, using "proxy" servers that mask a site's true origin.

It is through similar proxy servers and long-distance calls that researchers outside China managed to test what users inside China see. The researchers also employed volunteers inside the country to conduct more extensive testing.

The researchers deployed software and physical equipment called packet sniffers to monitor the flow of traffic and try to gauge where content gets dropped. Palfrey would not elaborate on techniques, other than to say many internet systems have security flaws through which outsiders can sneak in software.

Funded by George Soros' Open Society Institute, the OpenNet Initiative is a collaboration of researchers at Harvard, the University of Cambridge and the University of Toronto working on issues of internet censorship and surveillance.

Their testing determined that:

Though some dissidents complain that e-mail newsletters sent in bulk are sometimes blocked, individual messages tend not to get filtered.

Much of the filtering occurs at the backbone, but individual internet service providers sometimes deploy additional blocking. Cybercafes and operators of discussion boards also control content proactively under threat of penalties.

Filtering tends to be triggered by the appearance of certain keywords, rather than a visit to a specific domain name or numeric internet address. The keyword-based filters also allow blogs to keep people from completing posts containing banned topics.

"You can filter much more precisely at a keyword level," Palfrey said. "China wants to be able to enable its citizens to use the internet and grow its economy. Shutting down all blog servers doesn't seem like a great idea, but it doesn't want to let through all forms of political dissent."

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