Notes

When an analog medium is used to transmit digital data (such as through the telephone system
with a modem), the digital data may be represented by various forms of a carriermodulated
signal. Two forms of carrier modulation are amplitude modulation and
frequency-shift keying. In amplitude modulation, the digital data controls the presence of a
fixed-frequency carrier signal. In frequency-shift keying, the 0s and 1s are assigned two different
frequencies, resulting in a shift in carrier frequency when the data changes from 0 to 1 or
from 1 to 0. A third method is called phase-shift keying, in which the digital data controls the
phase shift of the carrier signal.
When a digital medium is used to transmit digital data (between the serial communication
port, COM1, of two PCs, for example), some form of digital waveform is used to represent
the data. A digital waveform is a waveform that contains only two different voltages. Inside
the computer, these two voltages are usually 0 volts and 5 volts. Outside the computer, plus
and minus 12 volts are often used for digital waveforms. Refer again to Figure 1-3. The nonreturn
to zero (NRZ) technique simply uses a positive voltage to represent a 0 and a negative
voltage to represent a 1. The signal never returns to zero.
Another popular method is Manchester encoding. In this technique, phase transitions are used
to represent the digital data. A one-to-zero transition is used for 0s and a zero-to-one transition
is used for 1s. Thus, each bit being transmitted causes a transition in the Manchester
waveform. This is not the case for the NRZ waveform, which may have long periods between
transitions. The result is that Manchester encoding includes both data and a clock signal,
which is helpful in extracting the original data in the receiver