Abstract: A fault-location operating system for use with transmission networks using repeaters with loopback capabilities. A plurality of bidirectional transmission links L.sub.i, for i=1 through N, are used for transmission of data between the terminals. A test control unit is connected at the first terminal to the first end of the first transmission link L.sub.1, and controls transmission of control signals across the first transmission link L.sub.1. A plurality of repeaters R.sub.i, for i=1 through N-1, are used for interconnecting the plurality of bidirectional links. Each repeater R.sub.i is connected to the first end of one transmission link L.sub.i and to the second end of another transmission link L.sub.i+1. Each repeater includes a first circuit for repeating data transmitted from the one transmission link L.sub.i for transmission on the other transmission link L.sub.i+1, and a second circuit for repeating data transmitted from the other transmission link L.sub.