## 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).