| 摘要 |
PROBLEM TO BE SOLVED: To provide a line supervisory method and a communication unit in a supervisory terminal system with superior in economics that can obtain the security equal to that of a leased communication line. SOLUTION: Prior to communication, a logical channel is set between a terminal and a supervisory center, the terminal can transmit security information in terms of packets through logical channel (S1 to S6), the terminal returns an acknowledgement command denoting the communication consecutiveness to a communication network, in response to a confirmation command of confirming connection to the network periodically sent from the communication network to the terminal to maintain the logical channel (S8, S9) and the supervisory center continuously supervise whether or not a communication channel is in normal operation, by discriminating whether or not the logical channel is maintained.
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