METHOD AND DEVICE FOR COMMUNICATION

PROBLEM TO BE SOLVED: To maintain a necessary minimum communication line when a fault takes place on a transmission line by setting only a preliminarly defined line between two lines set on a 1st communication line onto a 2nd communication line whose band is narrower when the 1st communication line whose band is broader becomes incommunicable. SOLUTION: A telephone line set between a telephone set T2 connected to communication equipment N1 and a telephone set T12 connected to communication equipment N2 and a video line set between a camera T3 connected to the equipment N1 and a monitor T13 connected to the equipment N2 are formed on a both up and down two-way wire transmission line (optical transmission line) for performing, e.g. 155 Mbps optical transmission. In the case a fault takes place on the optical transmission line and the telephone line and the video line constructed on the optical transmission line become incommunicable, the setting of the ATM switches of the ATM communication equipment N1 and N2 is changed so that only the telephone line can continue communication on a radio transmission line with the alarm generation by the fault detection at optical transmitting parts 101 and 102 to as a trigger.