, and will satisfy the loss requirements with a country of maximum extension of 578 mi (925 km). It is interesting to note that the talker's signal is attenuated only 7 dB toward the distant-end listener; but the reflected signal is not only attenuated the initial 7 dB, but attenuated by 7 dB still again, on its return trip. It has become evident by now that we cannot continue increasing losses indefinitely compensate for echo on longer circuits. Most telephone companies and administrations have set a 45- or 50-msec round-trip delay criterion, which sets a top figure above which echo suppressors are to be used. One major goal of the transmission-loss plan is to improve overall loudness rating or to apportion more loss to the subscriber plant SO that subscriber loops can be longer or to allow the use of less copper (i.e., smaller-diameter conductors). The question arises as to what measures can be taken to reduce losses and still keep echo within tolerable limits. One obvious target is to improve return losses at the hybrids. If all hybrid return losses are improved, the echo tolerance curve shifts; this is because improved return losses reduce the intensity of the echo returned to the talker. Thus the talker is less annoyed by the echo effect. One way of improving return loss is to make all two-wire lines out of the hybrid look alike-t that is, have the same impedance. The switch at the other end of the hybrid (i.e., on the two-wire side) connects two-wire loops of varying length, thus causing the resulting impedances to vary greatly. One approach is to extend four-wire transmission to the local office such that each hybrid can be better balanced. This is being carried out with success in Japan. The U.S. Department of Defense has its Autovon (auto- matic voice network), in which every subscriber line is operated on a four-wire basis. Two-wire subscribers connect through the system via PABXs (private automatic branch exchanges). As networks evolve to all-digital, four-wire transmission is carried directly through 