Abstract: In a distributed system comprising a plurality of processors coupled to one another, each processor comprises a controllable clock circuit which indicates a local time. Clock synchronization methods are based on the idea in which randomly selected M processors out of the total N processors cooperate to adjust the controllable clock circuits of all processors in the distributed system. Three types of methods are described. In a first one of the methods, all processors randomly selects M processors, respectively, at time instants which are different from one another, and each processor adjusts its own controllable clock circuit to an average of the local times of the selected M processors. In a second method, each processor transmits its own local time to randomly selected M processors and adjusts its own controllable clock circuit to the average of received local times.

Abstract: A loop-back control apparatus for use in an optical fiber transmission system, wherein it is possible to compensate for faults in opto-electrical and electro-optical converters contained in the optical fiber transmission system. An electrical switch, an optical switch, and duplicate access controllers are provided, the electrical and optical switches being controlled by a control signal generator to enable selection from among the various electro-optical and opto-electrical converters in the event of fault in one of those devices.