LOCAL AND LONG-DISTANCE NETWORKS
Let us try another example. Assume that all four-wire connections have a 7-dB loss.
Figure 8.20 indicates that 7 dB permits an 11-msec round-trip delay. Again assume that
the velocity of propagation is 105,000 mi/sec. Remember that we are dealing with round-
trip delay. The talker's voice reaches the far-end hybrid and some of the signal is reflected
back to the talker. This means that the signal traverses the system twice, as shown in
Figure 8.20. Thus 7 dB of loss for the given velocity of propagation allows about 578 mi
(925 km) of extension or, for all intents and purposes, the distance between subscribers,
and will satisfy the loss requirements with a country of maximum extension of 578 mi
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
to 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--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
As networks evolve to all-digital, four-wire transmission is carried directly through the
local serving switch such that subscriber loops terminate through a hybrid directly to a
PCM channel bank. Hybrid return losses could now be notably improved by adjusting
the balancing network for its dedicated subscriber loop.
Example of echo round-trip delay (5
.5 + 5.5 = 11-msec round-trip delay).