cuts in Lower Manhattan all had disruptive effects on the Internet and other communications systems.

How did the Internet’s communications infrastructure in particular experience all these events? How much did the events in New York City, and in Washington, D.C., affect the movement of data throughout the Internet? How were ISPs affected by the events of September 11? How serious were the impacts? What actions did ISPs (and others) take in response?

This chapter sets out to answer those questions, as best they can be answered with the available information. Data pertaining to the Internet operations that day were of two types: quantitative data on the system as a whole and on the response of particular networks, and anecdotal reports from network operators, users, and news media that help provide context and possible explanations for the changes on the Internet, both at the macro and micro levels, that were deemed necessary after the attacks.

How comprehensive and authoritative is this information? Some of it—for example, data on changes in the Internet’s routing configurations— permit the overall impact on the Internet to be measured. Reports on specific incidents, on the other hand, do not allow generalizations about the whole system, though they do provide insights into the kinds of local problems that could arise in the future and the responses that may mitigate them. Still, the participation in this study of several national ISPs and one New York regional ISP, together with the anecdotal information obtained though informal information-sharing relationships within the Internet operator community, permit at least a reasonable sampling of the overall experience. In addition, user surveys taken by the Pew Internet and American Life project allowed the committee to relate reported user behavior to some ISP measurements.¹ However, in a number of instances, data that would inform the committee’s understanding of what transpired on and shortly after September 11 were lacking (a detailed discussion of Internet-measurement issues is presented in Chapter 5).

The terrorist attacks in New York City caused an immediate disruption in communications within the World Trade Center complex. Soon thereafter, the collapse of the Twin Towers damaged and destroyed equipment of several wireless providers and some data circuits serving the