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With a tightly focused beam (light is easy, I don't know about lower frequencies), you can prevent interception except by very obvious physical devices. (e.g. Someone in a cherry picker truck.) You may be able to avoid the need to encrypt the link (and all the paranoia about key management, advances in factoring etc. that that implies.)

Bill

The problem with this comes when you start creating links between much taller buildings like in San Fran. Any give building over 30 stories might sway a foot or so at any given time. Combine that with the other building and you might get a few feet of movement. (movement not including during an earthquake) :) (andr0id@midwest.net callsign: N9XLM) ( Computer Consulting & Management ) (P.O. Box 421 Cambria, IL 62915-0421) -----BEGIN PGP PUBLIC KEY BLOCK----- Version: 2.6.2 mQENAzCsIi4AAAEH/1hb5+tO/n99Nbppf0ImLJ6AaVZ3NlZP0ZHwRQor00uA129i d4zWixNXxc8t2auaqN+asV99LpIip3/nQzBnjydiumeBdGLF2PR9+6X8X/RrqKa1 dVIukxM5Agg2eM6ih+0J38hgKJ3qzKXSz6sjYmpaxvbXZoHHOLUk/ZtHUKvvEyPw hnJEYnut8NUnIeK56lqeqRw86yoeRKymbfCdjdpgeY2aRwK2FJts8sbb7Fs10s4y jgxWIxIipBznbGUTh1hb2XrLGPENwk3E/qqXQJEsrySbtwdl6VgTVQjhDDEJMitL DYeiQ3W5EgxfcdbM1j2FwYu3P/dM6Y0I8xLMYT0ABRG0NmFuZHIwaWRAb2ljdTgx Mi5jb20gKG9pY3U4MTIuY29tIHN5c3RlbSBhZG1pbmlzdHJhdG9yKYkBFQMFEDCs LO90C7R/GkJcSQEB01cH/0KC3sd+u4OxMku5378SJktoN6QIQYLJ7uVbuV4S51yK NAotCGf4Wl6wwjynzZvXKU0H87oDuMiq7FybgMNL2n+4bQIZi0iz0lIuzwoMDu63 NrHUW9Kz42pOnhrEhrdkHhHL9O5GgD1yc40fJ3qw5h7LQEjDxgypyw0IFILFc34u LeRLliNibxKp8JwAxXNHWSgxu28TQvmnkHi0AHP6tJ/uZYe+4dqJtrMMsYFjzZaz DPmxD+dzbTwlQKtJaP1ZkDI0Sr072wrZDv+G86GyGBMX2lpSafpRitnxuUttjU9o wsQ9Qo5xiH1nZRCs/bDzJe/gng+GHzevixDIITurtNA= =SgPT -----END PGP PUBLIC KEY BLOCK-----