Data and Signals.ppt

3.1
Chapter 3
Data and Signals
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

3.2
To be transmitted, data must be
transformed to electromagnetic signals.
Note

3.3
3-1 ANALOG AND DIGITAL
Data can be analog or digital. The term analog data refers
to information that is continuous; digital data refers to
information that has discrete states. Analog data take on
continuous values. Digital data take on discrete values.
 Analog and Digital Data
 Analog and Digital Signals
 Periodic and Nonperiodic Signals
Topics discussed in this section:

3.4
Analog and Digital Data
 Data can be analog or digital.
 Analog data are continuous and take
continuous values.
 Digital data have discrete states and take
discrete values.

3.5
Analog and Digital Signals
• Signals can be analog or digital.
• Analog signals can have an infinite number
of values in a range.
• Digital signals can have only a limited
number of values.

3.6
Figure 3.1 Comparison of analog and digital signals

3.7
3-2 PERIODIC ANALOG SIGNALS
In data communications, we commonly use periodic
analog signals and nonperiodic digital signals.
Periodic analog signals can be classified as simple or
composite. A simple periodic analog signal, a sine wave,
cannot be decomposed into simpler signals. A composite
periodic analog signal is composed of multiple sine
waves.
 Sine Wave
 Wavelength
 Time and Frequency Domain
 Composite Signals
 Bandwidth

3.9
Figure 3.3 Two signals with the same phase and frequency,
but different amplitudes

3.10
Frequency and period are the inverse of
each other.
Note

3.11
Figure 3.4 Two signals with the same amplitude and phase,
but different frequencies

3.12
Frequency
• Frequency is the rate of change with respect
to time.
• Change in a short span of time means high
frequency.
• Change over a long span of
time means low frequency.

3.13
If a signal does not change at all, its
frequency is zero.
If a signal changes instantaneously, its
frequency is infinite.
Note

3.14
Phase describes the position of the
waveform relative to time 0.
Note

3.15
Figure 3.5 Three sine waves with the same amplitude and frequency,
but different phases

3.16
Figure 3.6 Wavelength and period

3.17
A complete sine wave in the time
domain can be represented by one
single spike in the frequency domain.
Note

3.18
Signals and Communication
 A single-frequency sine wave is not
useful in data communications
 We need to send a composite signal, a
signal made of many simple sine
waves.
 According to Fourier analysis, any
composite signal is a combination of
simple sine waves with different
frequencies, amplitudes, and phases.

3.19
Composite Signals and
Periodicity
 If the composite signal is periodic, the
decomposition gives a series of signals
with discrete frequencies.
 If the composite signal is nonperiodic, the
decomposition gives a combination of
sine waves with continuous frequencies.

3.20
Figure 3.9 shows a periodic composite signal with
frequency f. This type of signal is not typical of those
found in data communications. We can consider it to be
three alarm systems, each with a different frequency.
The analysis of this signal can give us a good
understanding of how to decompose signals.
Example 3.4

3.21
Figure 3.9 A composite periodic signal

3.22
Figure 3.10 Decomposition of a composite periodic signal in the time and
frequency domains

3.23
Figure 3.11 shows a nonperiodic composite signal. It
can be the signal created by a microphone or a telephone
set when a word or two is pronounced. In this case, the
composite signal cannot be periodic, because that
implies that we are repeating the same word or words
with exactly the same tone.
Example 3.5

3.24
Figure 3.11 The time and frequency domains of a nonperiodic signal

3.25
3-3 DIGITAL SIGNALS
In addition to being represented by an analog signal,
information can also be represented by a digital signal.
For example, a 1 can be encoded as a positive voltage
and a 0 as zero voltage. A digital signal can have more
than two levels. In this case, we can send more than 1 bit
for each level.
 Bit Rate
 Bit Length
 Digital Signal as a Composite Analog Signal
 Application Layer

3.26
Figure 3.16 Two digital signals: one with two signal levels and the other
with four signal levels

3.27
Figure 3.17 The time and frequency domains of periodic and nonperiodic
digital signals

3.28
Figure 3.18 Baseband transmission

3.29
A digital signal is a composite analog
signal with an infinite bandwidth.
Note

3.30
Figure 3.19 Bandwidths of two low-pass channels

3.31
Figure 3.20 Baseband transmission using a dedicated medium

3.32
Baseband transmission of a digital
signal that preserves the shape of the
digital signal is possible only if we have
a low-pass channel with an infinite or
very wide bandwidth.
Note

3.33
In baseband transmission, the required bandwidth is
proportional to the bit rate;
if we need to send bits faster, we need more bandwidth.
Note
In baseband transmission, the required
bandwidth is proportional to the bit rate;
if we need to send bits faster, we need
more bandwidth.

3.34
Table 3.2 Bandwidth requirements

3.35
What is the required bandwidth of a low-pass channel if
we need to send 1 Mbps by using baseband transmission?
Solution
The answer depends on the accuracy desired.
a. The minimum bandwidth, is B = bit rate /2, or 500 kHz.
b. A better solution is to use the first and the third
harmonics with B = 3 × 500 kHz = 1.5 MHz.
c. Still a better solution is to use the first, third, and fifth
harmonics with B = 5 × 500 kHz = 2.5 MHz.
Example 3.22

3.36
We have a low-pass channel with bandwidth 100 kHz.
What is the maximum bit rate of this
channel?
Solution
The maximum bit rate can be achieved if we use the first
harmonic. The bit rate is 2 times the available bandwidth,
or 200 kbps.
Example 3.22

3.37
Figure 3.23 Bandwidth of a bandpass channel

3.38
If the available channel is a bandpass
channel, we cannot send the digital
signal directly to the channel;
we need to convert the digital signal to
an analog signal before transmission.
Note

3.39
Figure 3.24 Modulation of a digital signal for transmission on a bandpass
channel

3.40
An example of broadband transmission using
modulation is the sending of computer data through a
telephone subscriber line, the line connecting a resident
to the central telephone office. These lines are designed
to carry voice with a limited bandwidth. The channel is
considered a bandpass channel. We convert the digital
signal from the computer to an analog signal, and send
the analog signal. We can install two converters to
change the digital signal to analog and vice versa at the
receiving end. The converter, in this case, is called a
modem which we discuss in detail in Chapter 5.
Example 3.24

3.41
A second example is the digital cellular telephone. For
better reception, digital cellular phones convert the
analog voice signal to a digital signal (see Chapter 16).
Although the bandwidth allocated to a company
providing digital cellular phone service is very wide, we
still cannot send the digital signal without conversion.
The reason is that we only have a bandpass channel
available between caller and callee. We need to convert
the digitized voice to a composite analog signal before
sending.
Example 3.25

3.42
3-4 TRANSMISSION IMPAIRMENT
Signals travel through transmission media, which are not
perfect. The imperfection causes signal impairment. This
means that the signal at the beginning of the medium is
not the same as the signal at the end of the medium.
What is sent is not what is received. Three causes of
impairment are attenuation, distortion, and noise.
 Attenuation
 Distortion
 Noise

3.43
Figure 3.25 Causes of impairment

3.44
Attenuation
 Means loss of energy -> weaker signal
 When a signal travels through a medium it
loses energy overcoming the resistance of
the medium
 Amplifiers are used to compensate for this
loss of energy by amplifying the signal.

3.46
Distortion
 Means that the signal changes its form or shape
 Distortion occurs in composite signals
 Each frequency component has its own
propagation speed traveling through a medium.
 The different components therefore arrive with
different delays at the receiver.
 That means that the signals have different phases
at the receiver than they did at the source.

3.48
Noise
 There are different types of noise
 Thermal - random noise of electrons in the wire
creates an extra signal
 Induced - from motors and appliances, devices
act are transmitter antenna and medium as
receiving antenna.
 Crosstalk - same as above but between two
wires.
 Impulse - Spikes that result from power lines,
lighning, etc.

Data and Signals.ppt

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