Monitoring a local station with first and second messages used when the

Abstract

In a communication network wherein a control station sends a communication test command once in each test cycle through a transmission path to a local station which sends a response signal back to the control station while kept in normal operation, the local station sends first and second response messages as the response signal while rendered idle and busy, respectively. Supplied with the first and the second response messages in one test cycle, the control station sends the communication test command to the local station in an immediately subsequent test cycle a normal and a longer test interval later, respectively. If supplied with no response signal, the control station sends the communication test command in the subsequent test cycle a shorter interval later and uses a shorter interval than a normal monitor interval in the subsequent test cycle in monitoring whether or not the response signal is received in response to the communication test command transmitted in this test cycle to the local station from which no response signal is received in the above-mentioned one test cycle. Usually, the local station is one of a plurality of local stations connected to the control station through the transmission path.