The analysis, by Arbor Networks, is based on security, traffic and routing data gathered by over 100 of Internet service provider (ISP) customers in 17 countries. is that in the one week or so since the contentious elections, Web and video traffic and most forms of interactive communications have been severely impacted inside Iran.

Web traffic to and from Iran has dropped by 50%, suggesting that Internet administrators are blocking those within the country from accessing a large number of Web sites, said Craig Labovitz, chief scientist at Arbor.

While it's not possible to know which sites or the number of sites that are being blocked, such a drop in traffic usually means that sites are being blocked, he said. Even so, the drop-off in Web traffic is relatively low compared with other kinds of traffic.

Traffic using the secure shell (SSH) protocol for instance, has fallen off by nearly 85%, while video and Bittorrent traffic has also declined by more than 80% when compared with immediately before the elections. Webcam traffic is down nearly 70%, and e-mail is down 50%.

The numbers strongly suggest that instead of simply pulling the plug on the Internet, Iranian authorities are choosing to selectively block certain kinds of applications, Labovitz said. "The truth is Iran has very centralized control of the Internet. They could very easily turn off everything," he said.