following a fault in a bidirectional optical communication system , the lasers which generate traffic - carrying channels and a supervisory channel are shut down to permit personnel to effect repairs safely . the low power supervisory channel is pulsed on at intervals to determine if the fault is still present , but if a receiver is able correctly to restore frame alignment from such a pulse , the reverse direction supervisory channel is enabled , and following frame alignment in both directions , the main lasers are safely turned on .