| 摘要 |
Signalling between a first and second device (2,3) linked by a transmission line (8) and of sensing a security violation of the line (8) comprises the steps of (a) transmitting a signal from the first device to the second device, reflecting the signal back to the first device in a manner corresponding to a first bit sequence, receiving the signal thus reflected at the first device, and comparing this signal with the transmitted signal to determine whether there has been a security violation of the line and to extract the first bit sequence. This can be detected by measuring the round trip time of a reflected signal. A variation in trip times indicates a change in impedance which may be caused by a third party attempting to 'tap into' the communication line. |
