Abstract: A clock receiving circuit provides network synchronization control of an exchange system. The clock receiving circuit includes a clock receiving unit for receiving a predetermined number of network synchronization reference clock signals exhibiting differential voltage levels from a trunk or a digital office time supply, and providing output of the network synchronization reference clock signals in corresponding logic levels. A clock determination unit determines whether the network synchronization reference clock signals received in the clock receiving unit are from a trunk or a digital office time supply.

Abstract: An optical data transmitting technique duplicates optical paths and includes: a divider for dividing electrical data transmitted to an optical data transmitting device of a counterpart device, into first and second output paths; a first optical data transceiver for converting first output path electrical data into optical data and transmitting it to a first receiver of the optical data transmitting device of the counterpart device, outputting first alarm data indicative of an abnormal existence or a nonexistence of the operation upon transmitting the data to the first receiver, receiving and converting optical data transmitted from a first transmitter of the optical data transmitting device of the counterpart device into electrical data, and supplying it to a first input path; a second optical data transceiver for receiving and converting optical data transmitted from a second transmitter of the optical data transmitting device of the counterpart device into electrical data, and supplying to a second input pa