The only biometric technology which does not measure the visual features of the human body. In voice recognition the sound vibrations of a person is measured and compared to an existing sample. The person to be identified is usually required to pronounce a designated password or phrase which facilitates the verification process. Voice recognition can be used on the phone, but has the weakness of technology is its susceptibility to interference by background noise.
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The Roman smoke signals network consisted of towers within visible range of each other and had a total length of about 4500 kilometers. It was used for military signaling.
For a similar telegraph network in ancient Greece see