DCN 5th ed. slides ch03 Introduction to Physical Layer.pdf

PART II: PHYSICAL LAYER
INTRODUCTION TO PHYSICAL LAYER
Subject: Computer Networks
Tutor: Bilal Munir Mughal
1
Ch-3

Content Outline
 DATA AND SIGNALS
 PERIODIC ANALOG SIGNALS
 DIGITAL SIGNALS
 TRANSMISSION IMPAIRMENT
 DATA RATE LIMITS
 PERFORMANCE
2

DATA AND SIGNALS
One of the major functions of the physical layer is to
move data in the form of electromagnetic signals across
a transmission medium.
3

DATA AND SIGNALS - Analog and Digital Data
 The term analog data refers to information that is
continuous; digital data refers to information that has
discrete states.
For example, an analog clock that has hour, minute,
and second hands gives information in a continuous
form; the movements of the hands are continuous.
On the other hand, a digital clock that reports the
hours and the minutes will change suddenly from 8:05
to 8:06.
4

DATA AND SIGNALS - Analog and Digital
Signals
 Like the data they represent, signals can be either
analog or digital.
 An analog signal has infinitely many levels of
intensity over a period of time.
As the wave moves from value A to value B, it passes
through and includes an infinite number of values
along its path.
 A digital signal, on the other hand, can have only a
limited number of defined values.
Although each value can be any number, it is often as
simple as 1 and 0.
5

DATA AND SIGNALS - Analog and Digital
Signals
6

DATA AND SIGNALS - Periodic and Nonperiodic
 Both analog and digital signals can take one of two
forms: periodic or nonperiodic (sometimes referred to
as aperiodic)
 A periodic signal completes a pattern within a
measurable time frame, called a period, and repeats that
pattern over subsequent identical periods. The completion
of one full pattern is called a cycle.
 A nonperiodic signal changes without exhibiting a
pattern or cycle that repeats over time.
 In data communications, we commonly use periodic
analog signals and nonperiodic digital signals
7

PERIODIC ANALOG 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.
8

PERIODIC ANALOG SIGNALS - Sine Wave
 The sine wave is the most fundamental form of a
periodic analog signal.
When we visualize it as a simple oscillating curve, its
change over the course of a cycle is smooth and
consistent, a continuous, rolling flow.
 A sine wave can be represented by three
parameters: the peak amplitude, the frequency, and
the phase. These three parameters fully describe a
sine wave.
9

Peak Amplitude
 The peak amplitude of a signal is the absolute value of its
highest intensity, proportional to the energy it carries.
For electric signals, peak amplitude is normally measured in
volts.
10

Period and Frequency
 Period refers to the amount of time, in seconds, a signal
needs to complete 1 cycle.
 Frequency refers to the number of periods in 1 s.
 Note that period and frequency are just one characteristic defined in two
ways. Period is the inverse of frequency, and frequency is the inverse of
period, as the following formulas show.
 Period is formally expressed in seconds. Frequency is
formally expressed in Hertz (Hz), which is cycle per second.
11

Period and Frequency…
12

 Another way to look at frequency is as a measurement of
the rate of change.
 Electromagnetic signals are oscillating waveforms; that is,
they fluctuate continuously and predictably above and
below a mean energy level e.g.
 A 40-Hz signal has one-half the frequency of an 80-Hz signal; it
completes 1 cycle in twice the time of the 80-Hz signal, so each
cycle also takes twice as long to change from its lowest to its
highest voltage levels.
 Frequency, therefore, though described in cycles per second
(hertz), is a general measurement of the rate of change of a
signal with respect to time.
13

 If the value of a signal changes over a very short span of
time, its frequency is high.
 If it changes over a long span of time, its frequency is low.
 If a signal does not change at all, its frequency is zero.
 If a signal changes instantaneously, its frequency is infinite.
14

Phase
 The term phase, or phase shift, describes the position of the
waveform relative to time 0.
 If we think of the wave as something that can be shifted
backward or forward along the time axis, phase describes
the amount of that shift. It indicates the status of the first
cycle.
 Phase is measured in degrees or radians [360º is 2π rad;
1º is 2π/360 rad, and 1 rad is 360/(2π)].
 A phase shift of 360º corresponds to a shift of a complete
period; a phase shift of 180° corresponds to a shift of one-half
of a period; and a phase shift of 90º corresponds to a shift of
one-quarter of a period
15

Phase…
16

Phase…
 Looking at Figure 3.6, we can say that
a. A sine wave with a phase of 0° starts at time 0 with a
zero amplitude. The amplitude is increasing.
b. A sine wave with a phase of 90° starts at time 0 with a
peak amplitude. The amplitude is decreasing.
c. A sine wave with a phase of 180° starts at time 0 with
a zero amplitude. The amplitude is decreasing.
17

Phase…
 Another way to look at the phase is in terms of shift
or offset. We can say that
a. A sine wave with a phase of 0° is not shifted.
b. A sine wave with a phase of 90° is shifted to the left by
¼ cycle.
c. A sine wave with a phase of 180° is shifted to the left
by ½ cycle.
However, note that the signal does not really exist before
time 0.
18

Wavelength
 Wavelength is another characteristic of a signal
traveling through a transmission medium.
 Wavelength binds the period or the frequency of a
simple sine wave to the propagation speed of the
medium
19

Wavelength…
 While the frequency of a signal is independent of the
medium, the wavelength depends on both the
frequency and the medium.
 Wavelength can be calculated if one is given the
propagation speed (the speed of light) and the period of
the signal.
 If we represent wavelength by λ, propagation speed by
c (speed of light), and frequency by f, we get
20

Wavelength…
 The propagation speed of electromagnetic signals
depends on the medium and on the frequency of the
signal. For example, in a vacuum, light is propagated
with a speed of 3 × 108 m/s. That speed is lower in
air and even lower in cable.
 The wavelength is normally measured in micrometers
(microns) instead of meters.
 For example, the wavelength of red light (frequency = 4 ×
1014) in air is
 In a coaxial or fiber-optic cable, the wavelength is shorter
(0.5 μm) because of the decreased propagation speed.
21

Time and Frequency Domains
22

Time and Frequency Domains…
 The time-domain plot shows changes in signal
amplitude with respect to time (it is an amplitude-
versus-time plot).
 A frequency-domain plot is concerned with only the
peak value and the frequency. Changes of amplitude
during one period are not shown.
 The advantage of the frequency domain is that we
can immediately see the values of the frequency and
peak amplitude.
23

Time and Frequency Domains…
 The frequency domain is more compact and useful
when we are dealing with more than one sine wave.
24

Composite Signals
 A single-frequency sine wave is used in many aspects
of daily life e.g. energy transfer, alarm signals etc.
 However, 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.
 In the early 1900s, the French mathematician Jean-
Baptiste Fourier showed that any composite signal is
actually a combination of simple sine waves with
different frequencies, amplitudes, and phases.
(Fourier analysis - Appendix E.)
25

Composite Signals…
 A composite signal can be periodic or nonperiodic.
 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.
26

 Figure 3.10 shows
a periodic
composite signal
with frequency f.
27

 Figure 3.10 shows a periodic composite signal with
frequency f.
 The amplitude of the sine wave with frequency f is
almost the same as the peak amplitude of the
composite signal. It is called the fundamental
frequency, or first harmonic.
 The amplitude of the sine wave with frequency 3f is
one-third of that of the first (third harmonic), and the
amplitude of the sine wave with frequency 9f is one-
ninth of the first (ninth harmonic).
28

 Note that the frequency decomposition of the signal is
discrete; it has frequencies f, 3f, and 9f. Because f is
an integral number, 3f and 9f are also integral
numbers.
 There are no frequencies such as 1.2f or 2.6f.
 The frequency domain of a periodic composite signal
is always made of discrete spikes.
29

 Figure 3.12 shows a nonperiodic composite signal. It can
be the signal created by a microphone or a telephone set
when we speak. 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.
30

Bandwidth
 The range of frequencies contained in a composite
signal is its bandwidth. OR
The bandwidth of a composite signal is the difference
between the highest and the lowest frequencies
contained in that signal.
 For example, if a composite signal contains frequencies
between 1000 and 5000, its bandwidth is 5000 − 1000,
or 4000.
31

Bandwidth…
32

DIGITAL SIGNALS
 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.
 In general, if a signal has L levels, each level needs
log2 L bits.
 A digital signal has eight levels. Then
33

DIGITAL SIGNALS - Bit Rate
 The bit rate is the number of bits sent in 1s, expressed
in bits per second (bps).
 Assume we need to download text documents at the
rate of 100 pages per second. What is the required
bit rate of the channel?
Solution
 A page is an average of 24 lines with 80 characters
in each line. If we assume that one character requires
8 bits, the bit rate is
35

DIGITAL SIGNALS - Bit Length
 The concept of the wavelength for an analog signal is
the distance one cycle occupies on the transmission
medium.
 A similar concept for a digital signal is the bit length.
 The bit length is the distance one bit occupies on the
transmission medium.
Bit length = propagation speed x bit duration
36

DIGITAL SIGNAL as a Composite Analog Signal
 Fourier analysis can be used to decompose a digital
signal. (See Appendix E)
 If the digital signal is periodic, which is rare in data
communications, the decomposed signal has a frequency
domain representation with an infinite bandwidth and
discrete frequencies.
 If the digital signal is nonperiodic, the decomposed
signal still has an infinite bandwidth, but the frequencies
are continuous.
37

DIGITAL SIGNAL as a Composite Analog Signal
38

DIGITAL SIGNALS - Transmission
 Let us consider the case of a nonperiodic digital
signal.
 We can transmit a digital signal by using one of two
different approaches:
 baseband transmission
 broadband transmission (using modulation).
39

DIGITAL SIGNALS - Baseband Transmission
 Baseband transmission means sending a digital
signal over a channel without changing the digital
signal to an analog signal.
 Baseband transmission requires that we have a low-
pass channel, a channel with a bandwidth that starts
from zero.
 This is the case if we have a dedicated medium with a
bandwidth constituting only one channel.
For example, the entire bandwidth of a cable connecting
two computers is one single channel.
40

DIGITAL SIGNALS - Baseband Transmission
 Let us study two cases of a baseband
communication: a low-pass channel with a wide
bandwidth and one with a limited bandwidth.
41

DIGITAL SIGNALS – Broadband
Transmission(Using Modulation)
 Broadband transmission or modulation means
changing the digital signal to an analog signal for
transmission.
 Modulation allows us to use a bandpass channel, a
channel with a bandwidth that does not start from
zero.
 Bandpass channel is more available than a low-pass
channel.
42

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.
43

TRANSMISSION IMPAIRMENT - Attenuation
 Attenuation means a loss of energy.
 When a signal, simple or composite, travels through a
medium, it loses some of its energy in overcoming the
resistance of the medium. To compensate for this loss,
amplifiers are used to amplify the signal.
44

Decibel
 To show that a signal has lost or gained strength,
engineers use the unit of the decibel.
 The decibel (dB) measures the relative strengths of
two signals or one signal at two different points.
 Note that the decibel is negative if a signal is
attenuated and positive if a signal is amplified.
 Variables P1 and P2 are the powers of a signal at
points 1 and 2, respectively.
45

Decibel…
 Note that some engineering books define the decibel
in terms of voltage instead of power.
 In that case, because power is proportional to the
square of the voltage, the formula is dB = 20 log10
(V2/V1).
 In this text, authors express dB in terms of power.
46

TRANSMISSION IMPAIRMENT - Distortion
 Distortion means that the signal changes its form or
shape.
 Distortion can occur in a composite signal made of
different frequencies. Each signal component has its own
propagation speed through a medium and, therefore, its
own delay in arriving at the final destination. Differences
in delay may create a difference in phase if the delay is
not exactly the same as the period duration.
 In other words, signal components at the receiver
have phases different from what they had at the
sender. The shape of the composite signal is
therefore not the same.
47

TRANSMISSION IMPAIRMENT - Distortion
 Figure 3.29 shows the effect of distortion on a
composite signal.
48

TRANSMISSION IMPAIRMENT - Noise
 Several types of noise, such as thermal noise,
induced noise, crosstalk, and impulse noise, may
corrupt the signal.
 Thermal noise is the random motion of electrons in a
wire, which creates an extra signal not originally sent
by the transmitter.
49

 Induced noise comes from sources such as motors and
appliances. These devices act as a sending antenna,
and the transmission medium acts as the receiving
antenna.
 Crosstalk is the effect of one wire on the other. One
wire acts as a sending antenna and the other as the
receiving antenna.
 Impulse noise is a spike (a signal with high energy in
a very short time) that comes from power lines,
lightning, and so on.
50

Signal-to-Noise Ratio (SNR)
 To find the theoretical bit rate limit, we need to know
the ratio of the signal power to the noise power. The
signal-to-noise ratio is defined as
 We need to consider the average signal power and
the average noise power because these may change
with time.
51

Signal-to-Noise Ratio (SNR)…
 SNR is actually the ratio of what is wanted (signal) to
what is not wanted (noise).
 A high SNR means the signal is less corrupted by
noise; a low SNR means the signal is more
corrupted by noise.
 Because SNR is the ratio of two powers, it is often
described in decibel units, SNRdB, defined as
52

Signal-to-Noise Ratio (SNR)…
53

DATA RATE LIMITS
 A very important consideration in data
communications is how fast we can send data, in bits
per second, over a channel.
 Data rate depends on three factors:
1. The bandwidth available
2. The level of the signals we use
3. The quality of the channel (the level of noise)
 Two theoretical formulas were developed to calculate
the data rate:
one by Nyquist for a noiseless channel,
another by Shannon for a noisy channel.
54

DATA RATE LIMITS - Noiseless Channel: Nyquist
Bit Rate
 For a noiseless channel, the Nyquist bit rate formula
defines the theoretical maximum bit rate
 In this formula, bandwidth is the bandwidth of the
channel, L is the number of signal levels used to
represent data, and BitRate is the bit rate in bits per
second.
 Increasing the levels of a signal may reduce the
reliability of the system.
55

DATA RATE LIMITS - Noisy Channel: Shannon
Capacity
 In reality, we cannot have a noiseless channel; the
channel is always noisy.
 In 1944, Claude Shannon introduced a formula,
called the Shannon capacity, to determine the
theoretical highest data rate for a noisy channel:
 Where bandwidth is the bandwidth of the channel,
SNR is the signal-to-noise ratio, and capacity is the
capacity of the channel in bits per second.
56

DATA RATE LIMITS - Using Both Limits
 In practice, we need to use both methods to find the
limits and signal levels.
 The Shannon capacity gives us the upper limit; the
Nyquist formula tells us how many signal levels
we need.
57

PERFORMANCE
 The quality of service, an overall measurement of
network performance, is discussed in greater detail
in Chapter 30. However, we introduce terms that we
need for future chapters:
 Bandwidth
 Throughput
 Latency (Delay)
 Bandwidth-Delay Product
 Jitter
58

PERFORMANCE - Bandwidth
 In networking, we use the term bandwidth in two
contexts.
 The first, bandwidth in hertz, refers to the range of
frequencies in a composite signal or the range of
frequencies that a channel can pass e.g. a
subscriber telephone line bandwidth is 4 kHz.
 The second, bandwidth in bits per second, refers
to the speed of bit transmission in a channel or link
e.g. bandwidth of a Fast Ethernet is 100Mbps.
59

PERFORMANCE - Throughput
 The bandwidth is a potential measurement of a link;
the throughput is an actual measurement of how
fast we can send data.
 For example, we may have a link with a bandwidth of
1 Mbps, but the devices connected to the end of the link
may handle only 200 kbps.
60

PERFORMANCE - Latency (Delay)
 The latency or delay defines how long it takes for
an entire message to completely arrive at the
destination from the time the first bit is sent out from
the source.
 We can say that latency is made of four
components: propagation time, transmission time,
queuing time and processing delay.
61

 Propagation Time
 Propagation time measures the time required for a
bit to travel from the source to the destination. It is
calculated as:
62

 Transmission Time
 In data communications we don’t send just 1 bit, we
send a message which is composed of multiple bits.
 The first bit leaves earlier and arrives earlier; the
last bit leaves later and arrives later. The
transmission time of a message depends on the size
of the message and the bandwidth of the channel.
63

 Queuing Time
 The queuing time is the time needed for each
intermediate or end device to hold the message
before it can be processed.
 The queuing time is not a fixed factor; it changes
with the load imposed on the network. When there
is heavy traffic on the network, the queuing time
increases.
 An intermediate device, such as a router, queues the
arrived messages and processes them one by one.
64

PERFORMANCE - Bandwidth-Delay Product
 The bandwidth-delay product defines the number
of bits that can fill the link.
65

 The above two cases show that the product of
bandwidth and delay is the number of bits that
can fill the link.
 This measurement is important if we need to send
data in bursts and wait for the acknowledgment of
each burst before sending the next one.
66

 Example 3.48
 We can think about the link between two points as a
pipe. The cross section of the pipe represents the
bandwidth, and the length of the pipe represents the
delay. We can say the volume of the pipe defines the
bandwidth-delay product, as shown in Figure 3.34.
67

PERFORMANCE - Jitter
 We can roughly say that jitter is a problem if
different packets of data encounter different
delays and the application using the data at the
receiver site is time-sensitive (audio and video data,
for example).
 If the delay for the first packet is 20 ms, for the
second is 45 ms, and for the third is 40 ms, then the
real-time application that uses the packets endures
jitter. (detail in Chapter 28)
68

DCN 5th ed. slides ch03 Introduction to Physical Layer.pdf

More Related Content

Similar to DCN 5th ed. slides ch03 Introduction to Physical Layer.pdf (20)

More from Bilal Munir Mughal (10)

Recently uploaded (20)

DCN 5th ed. slides ch03 Introduction to Physical Layer.pdf