this fault recovery system determines the integrity of a duplex digital pcm switching network . the two copies of the switching network operate in synchronization and comparison of the outputs of each copy of the switching network takes place in order to detect network faults . the fault recovery mechanism is chiefly provided by inserting pcm samples from maintenance registers into the pcm data stream , through the digital switching network and trapping said pcm sample for analysis by the program . subsequently , the logical unit corresponding to each fault is determined and a count is recorded corresponding to each unit .