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CHAPTER 4
Digital Transmission
Solutions to Review Questions and Exercises
Review Questions
1. The three different techniques described in this chapter are line coding, block cod-
ing, and scrambling.
2. A data element is the smallest entity that can represent a piece of information (a
bit). A signal element is the shortest unit of a digital signal. Data elements are
what we need to send; signal elements are what we can send. Data elements are
being carried; signal elements are the carriers.
3. The data rate defines the number of data elements (bits) sent in 1s. The unit is bits
per second (bps). The signal rate is the number of signal elements sent in 1s. The
unit is the baud.
4. In decoding a digital signal, the incoming signal power is evaluated against the
baseline (a running average of the received signal power). A long string of 0s or 1s
can cause baseline wandering (a drift in the baseline) and make it difficult for the
receiver to decode correctly.
5. When the voltage level in a digital signal is constant for a while, the spectrum cre-
ates very low frequencies, called DC components, that present problems for a sys-
tem that cannot pass low frequencies.
6. A self-synchronizing digital signal includes timing information in the data being
transmitted. This can be achieved if there are transitions in the signal that alert the
receiver to the beginning, middle, or end of the pulse.
7. In this chapter, we introduced unipolar, polar, bipolar, multilevel, and multitran-
sition coding.
8. Block coding provides redundancy to ensure synchronization and to provide inher-
ent error detecting. In general, block coding changes a block of m bits into a block
of n bits, where n is larger than m.
9. Scrambling, as discussed in this chapter, is a technique that substitutes long zero-
level pulses with a combination of other levels without increasing the number of
bits.
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