Analog and Digital Data transmission

Analog and Digital Data transmission

The terms analog and digital correspond, roughly, to continuous and discrete, respectively.
These two terms are used frequently in data communications in at least three
contexts: data, signals, and transmission.
Briefly, we define data as entities that convey meaning, or information. Signals
are electric or electromagnetic representations of data.Transmission is the communication
of data by the propagation and processing of signals. In what follows, we try
to make these abstract concepts clear by discussing the terms analog and digital as
applied to data, signals, and transmission.
Analog and Digital Data
The concepts of analog and digital data are simple enough. Analog data take on
continuous values in some interval. For example, voice and video are continuously

varying patterns of intensity. Most data collected by sensors, such as temperature
and pressure, are continuous valued. Digital data take on discrete values; examples
are text and integers.
The most familiar example of analog data is audio, which, in the form of
acoustic sound waves, can be perceived directly by human beings. Figure 2.6 shows
the acoustic spectrum for human speech and for music. Frequency components of
typical speech may be found between approximately 100 Hz and 7 kHz. Although
much of the energy in speech is concentrated at the lower frequencies, tests have
shown that frequencies below 600 or 700 Hz add very little to the intelligibility of
speech to the human ear. Typical speech has a dynamic range of about 25 dB;2 that
is, the power produced by the loudest shout may be as much as 300 times greater
than that of the least whisper.

Analog and Digital Signaling

In a communications system, data are propagated from one point to another by
means of electromagnetic signals. An analog signal is a continuously varying electromagnetic
wave that may be propagated over a variety of media, depending on
frequency; examples are copper wire media, such as twisted pair and coaxial cable;

fiber optic cable; and atmosphere or space propagation (wireless). A digital signal is
a sequence of voltage pulses that may be transmitted over a copper wire medium;
for example, a constant positive voltage level may represent binary 0 and a constant
negative voltage level may represent binary l.
The principal advantages of digital signaling are that it is generally cheaper
than analog signaling and is less susceptible to noise interference. The principal disadvantage
is that digital signals suffer more from attenuation than do analog signals.
Figure 2.7 shows a sequence of voltage pulses, generated by a source using two voltage
levels, and the received voltage some distance down a conducting medium.
Because of the attenuation, or reduction, of signal strength at higher frequencies,
the pulses become rounded and smaller. It should be clear that this attenuation can
lead rather quickly to the loss of the information contained in the propagated signal.
Both analog and digital data can be represented, and hence propagated, by
either analog or digital signals. This is illustrated in Figure 2.8. Generally, analog data
are a function of time and occupy a limited frequency spectrum. Such data can be
directly represented by an electromagnetic signal occupying the same spectrum. The
best example of this is voice data. As sound waves, voice data have frequency components
in the range 20 Hz to 20 kHz. As was mentioned, most of the speech energy is in
a much narrower range, with the typical speech range of between 100 Hz and 7 kHz.
The standard spectrum of voice signals is even narrower, at 300 to 3400 Hz, and this is
quite adequate to propagate speech intelligibly and clearly. The telephone instrument
does just that. For all sound input in the range of 300 to 3400 Hz, an electromagnetic
signal with the same frequency-amplitude pattern is produced. The process is performed
in reverse to convert the electromagnetic energy back into sound.
Digital data can also be represented by analog signals by use of a modem
(modulator-demodulator). The modem converts a series of binary (two-valued)
voltage pulses into an analog signal by modulating a carrier frequency. The resulting
signal occupies a certain spectrum of frequency centered about the carrier and may
be propagated across a medium suitable for that carrier. The most common modems
represent digital data in the voice spectrum and hence allow those data to be propagated
over ordinary voice-grade telephone lines. At the other end of the line, a
modem demodulates the signal to recover the original data.

In an operation very similar to that performed by a modem, analog data can
be represented by digital signals. The device that performs this function for voice
data is a codec (coder-decoder). In essence, the codec takes an analog signal that
directly represents the voice data and approximates that signal by a bit stream. At
the other end of the line, a codec uses the bit stream to reconstruct the analog data.
This topic is explored subsequently.