disclosed is a remote supervisory system in which slave supervisory apparatus for supervising network elements can be polled from a master supervisory apparatus , and yet each s - sv is able to send an alarm signal to the m - sv at any appropriate time . in this system , a first transmission path connected to the m - sv is connected via a switch to a second transmission path leading from each s - sv . the switch is controlled in accordance with the presence of a signal on the first transmission path and also with the presence of a signal on the second transmission path .