Using tests conducted inside and outside China, researchers were able to identify censorship at many of those points.
Filters are placed on the main "backbone" networks that carry Internet traffic, the study said. A handful of licensed Internet providers also perform their own filtering. Major Chinese search engines filter out or block keywords that would enable surfers to see certain sites. Providers of Web log, or blogging, services block certain posts. Text messaging software has built-in forbidden lists of keywords, which halt service temporarily if used.
Chinese authorities perform these tasks largely using U.S. hardware and software.
For example, Cisco Systems Inc. routers, machines that move Internet traffic around, are capable of recognizing individual portions of data, a technology that helps battle worms and viruses. That same technology can be used to distinguish certain content.
Companies such as Cisco and Google Inc. have been accused of aiding China's censorship by tailoring their products to suit the government's needs. The study did not confirm those allegations, which the companies have denied.
Some reports on Chinese censorship also claim that the country has as many as 30,000 "Internet police" dedicated to the task, but the study did not confirm that estimate. Still, it identified 11 government agencies that share responsibility for controlling Internet use in the country.
Despite wholesale blocking of Web sites dedicated to news on Taiwan or Tibet, for example, Chinese surfers still can get access to many Western news and culture sites.
Researchers said the filtering efforts seem to shift regularly, so that at certain times a CNN site on Tiananmen Square was accessible, for example.
Rohozinski said the censorship efforts seem to primarily target sites written in Chinese.