Answer 1.4
The period for which the data stays at logic 1 can
be viewed as a constant dc voltage level on the line and for this period of time the
frequency content of the signal will become concentrated about 0 Hz. If the transmission
channel is ac coupled for some reason, then this information will not be faithfully passed
through the link. This problem is in fact evident in many practical communication links.
(See the section on the telephone channel in Section 4.4).

Techniques for overcoming this problem include
scrambling of the data to eliminate the occurrence of long strings of 1s or 0s, or some
form of line coding such as Manchester encoding (see Section 4.4).