Remote controlled civil defence warning system - uses public switched telephone network to reach remote terminals to give alarm

摘要

The remotely-controlled civil defence warning system employs the public switched telephone network to reach remote terminals to give the alarm. All local switching centres as the first members of a return signalling chain send signals back to the relevant end switching centre on receipt of a signal block travelling forwards through the network. Each end switching centre checks whether the return signals from all local centres to which it is connected appear within a given time after sending the signal block.