NOTICE: The project that is the subject of this report was approved by the Governing Board of the National Research Council, whose members are drawn from the councils of the National Academy of Sciences, the National Academy of Engineering, and the Institute of Medicine. The members of the committee responsible for the report were chosen for their special competences and with regard for appropriate balance.

Support for this project was provided by the Defense Advanced Research Projects Agency and the National Institute of Standards and Technology. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the sponsor. Moreover, the views, opinions, and findings contained in this report should not be construed as an official Department of Defense position, policy, or decision, unless so designated by other official documentation.

Library of Congress Control Number: 2001093511