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1
Dr. Tom HicksComputer Science Department
Trinity University
2
Some Material From Chapter 3
Some Material From Chapter 5I think it fits in here better!
3
Position of the Physical Layer
3.4
Communication at the Physical Layer
5
Services
6
Signals
7
To be Transmitted, Data
Must be Transformed to
Electromagnetic Signals.
8
Physical Layer
Concerns
9
Physical Major Concern
Major Concern of Physical Layer – Moving
Information, In The Form Of Electromagnetic Signals,
Across A Transmission Media.
Information
Voice
Image
Numeric Data
Character Data
Binary Data
Etc.
10
Physical Layer Must Encode
Information Usable To A Person Can Not Be
Directly Transmitted Across A Network; The
Information Must Be Encoded As 0’s and 1’s
11
Analog Signals
&
Digital Signals
12
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.
Note:
13
Analog/Digital
Analog – Continuous Signal – A Set Of Points
With All Points In-Between
Digital – Discrete Signal – A Set Of Points With
NoPoints In-Between
14
Graph Of An Analog Signal
The term Analog Data refers to information that
is Continuous
15
16
Digital Data refers to Information that has Discrete
States
Graph Of A Digital Signal
17
Graph Of A Digital Signal
Acceptable Values Might Range
From
-1 to 1
18
Analog vs. Digital Signals
19
Periodic Signals
&
Aperiodic Signals
20
In Data Communication, we
commonly use
Periodic Analog Signals
&
Aperiodic Digital Signals.
Note:
21
Sine Wave
A Periodic Signal completes a pattern within a
measurable time frame, called a Period, and
repeats that pattern over subsequent identical
periods.
The Sine Wave is a Periodic Signal. The graph
below has 3 Periods.
The Sine Wave is the Most Fundamental Form
of a Periodic Analog Signal.
22
Amplitude
23
Amplitude
Signal Amplitude – the value of the
signal at any point along the wave
24
Can Vary The Amplitude
Amplitude Can Be Changed by varying
the current! [volts, amperes, watts]
25
Two Signals – Two Amplitudes
26
Frequency and
Period are
Inverses of Each
Other.
Note:
27
Frequency
28
Periods In A Second Hz
29
Units of Period & Frequency
The voltage of a battery is a constant; this constant
value can be considered a sine wave, as we will
see later.
For example, the peak value of an AA battery is
normally 1.5 V.
Example 3.2
Express a period of 100 ms in microseconds.
Example 3.4
Solution
From Table 3.1 we find the equivalents of 1 ms (1 ms
is 10–3 s) and 1 s (1 s is 106 μs).
We make the following substitutions:
Period = 100 ms = _________________ μs105
The period of a signal is 100 ms. What is its frequency
in kilohertz?
Example 3.5
Solution
First we change 100 ms to seconds, and then we
calculate the frequency from the period (1 Hz = 10–3
kHz).
Frequency = _________________ kHz10-2
33
Periods
&
Cycles
34
Periodic Signals
Periodic Signal – completes a pattern
within a measurable time frame, called a
Period
35
Period
Period – the time required for a periodic
signal to complete one complete Cycle of
the pattern.
3.36
The power we use at home has a frequency of 60 Hz
(50 Hz in Europe). The period of this sine wave can be
determined as follows:
Example 3.3
Period = _________________ ms
This means that the period of the power for our
lights at home is 0.0116 s, or 16.6 ms.
Our eyes are not sensitive enough to distinguish
these rapid changes in amplitude.
16.6
37
Why The Sine
Wave?
38
39
Frequency
&
Periods
40
41
42
Units of Period & Frequency
43
Practice
Problems
44
45
46
47
48
49
50
High & Low
Frequency
51
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.
52
If a Signal Does Not Change at
all, its Frequency is Zero.
If a Signal Changes
Instantaneously, its Frequency is
Infinite.
53
Phase
54
Phase Describes the
Position of the Waveform
Relative to Time Zero.
55
Relationships Between Phases
56
About Phase
Phase Is
Measured
In
Degrees Or
Radians
Phase – describes the position of the
wave form relative to time 0.
57
Sine 3 Different Phases
3.57
A sine wave is offset 1/6 cycle with respect to time 0.
What is its phase in degrees and radians?
Example 3.6x
Phase = _________________ Degrees60
Solution
We know that 1 complete cycle is 360°.
A sine wave is offset 1/6 cycle with respect to time 0.
What is its phase in degrees and radians?
Example 3.6
Solution
We know that 1 complete cycle is 360°. Therefore,
1/6 cycle is
Phase = _________________ rad1.046
60
Time Domain &
Frequency
Domain Plots
61
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
Wavelength
Direction ofpropagation
63
An Analog Signal is
Best Represented in the
Frequency Domain.
64
65
66
The Frequency Domain is more compact and useful
when we are dealing with more than one sine wave.
Below, we see three sine waves, each with different
amplitude and frequency. All can be represented by
three spikes in the frequency domain.
Example 3.7
68
Decomposition
&
Fourier Analysis
Harmonics
69
A Single-Frequency Sine Wave is
Not Useful in Data
Communications
We Need to Change One or
More of its Characteristics to
make it useful.
70
When We Change One or More
Characteristics of a Single-
Frequency signal, it Becomes a
Composite Signal made of
many frequencies.
71
According to Fourier analysis, Any
Composite Signal Can be
Represented as a Combination of
Simple Sine Waves with Different
Frequencies, Phases, and
Amplitudes.
72
73
74
75
76
77
Square Wave
78
Three Separate Harmonics
79
Harmonics
80
Frequency Spectrum Comparison
81
Decomposition Of Signal - 1 The analysis of this signal below can give us a good
understanding of how to Decompose Signals.
It is Very Difficult to Manually Decompose this Signal into
a series of simple sine waves.
Time
Amplitude
• • •
There are Tools, both hardware and software, that can help
us do the job of Decomposing Signals.
I am not concerned about how it is done; I am only
interested in the result.
The Result of Decomposing this Signal in the Time
Domains
Decomposition Of Signal - 2
There are Tools, both hardware and software, that can help
us do the job of Decomposing Signals.
The Result of Decomposing this Signal in the Frequency
Domains
Decomposition Of Signal - 3
84
NonPeriodic
Composite Signal
Speaking a Word, or Two, into a Microphone,
or Telephone, would create a Nonperiodic
Composite Signal.
This composite signal Cannot Be Periodic; this
would only happen if we repeated the same
word or words with exactly the same tone.
Nonperiodic Signal
86
Signal
Corruption
87
Signal Corruption
88
Bandwidth
89
The Bandwidth is a property of a
Medium:
The Bandwidth is the Difference
Between the Highest and the
Lowest Frequencies that the
medium can satisfactorily pass.
90
91
92
If a Periodic Signal is decomposed into five sine waves with
frequencies of 100, 300, 500, 700, and 900 Hz, what is its
bandwidth? Draw the spectrum, assuming all components have
a maximum amplitude of 10 V.
Example 3.10
Solution
Let fh be the highest frequency, fl the lowest frequency, and B
the bandwidth. Then
B = _________________ Hz800
A Periodic Signal has a Bandwidth of 20 Hz. The highest
frequency is 60 Hz. What is the lowest frequency? Draw the
spectrum if the signal contains all frequencies of the same
amplitude.
Example 3.11
The spectrum contains all integer frequencies.
Solution
Let fh be the highest frequency, fl the lowest frequency, and B
the bandwidth. Then
B = _________________ Hz40
95
Digital Signals
Bit Rate
&
Bit Interval
96
97
98
99
100
Bit Rate & Bit Interval
101
Transmission Of
Digital Signals
Based on Fourier Analysis (See Appendix E), a
Digital Signal is a Composite Analog Signal.
With a Digital Signal, the Bandwidth is Infinite.
We can intuitively come up with this concept when we consider a digital
signal. A digital signal, in the time domain, comprises connected vertical
and horizontal line segments. A vertical line in the time domain means a
frequency of infinity: a horizontal line in the time domain means a
frequency of zero. Going from a frequency of zero to a frequency of
infinity implies all frequencies in between are part of the domain.
Digital Signal Is A Composite Analog Signal
3.103
Baseband Transmission
A Baseband Transmission means sending a
Digital Signal over a Channel Without Changing
the Digital Signal to an Analog Signal
A Baseband Transmission requires a Low-Pass
Channel with a Bandwidth that starts at 0.
Low-Pass Channel
Illustrated above is a Low-Pass Channel that has a Wide
Bandwidth and one that has a Narrow Bandwidth.
In order to preserve the Exact Form of a Nonperiodic
Digital Signal with vertical segments vertical and horizontal
segments horizontal, we need to Send The Entire Spectrum
the continuous range of frequencies between zero and
infinity.
Case 1 - Low-Pass Channel that has a Wide Bandwidth-1
This would require a Dedicated Medium with an Infinite
Bandwidth between sender & receiver.
NOT POSSIBLE!
Fortunately, the Amplitudes of the Frequencies at the
Borders of the Bandwidth are SO SMALL that they can BE
IGNORED!
Case 1 - Low-Pass Channel that has a Wide Bandwidth-2
If we have Coax of Fiber Optic Cables with a Very Wide
Bandwidth, two stations Can Communicate, by using Digital
Signals, with Very Good Accuracy!
An example of a Dedicated Channel where the
Entire Bandwidth of the Medium is Used as One
Single Channel is a LAN.
Almost every wired LAN today uses a Dedicated
Channel for Two stations Communicating with each
other.
Example – Dedicated Chanel – Entire Bandwidth Used - LAN
In a Bus Topology LAN with multipoint connections,
Only Two Stations Can Communicate with each
other at Each Moment in Time (timesharing); the
other stations need to refrain from sending data.
Dedicated Chanel – LAN – Bus Topology
In a Star Topology LAN the Entire Channel
Between Each Station & The Hub Is Used To
Communicate between these two entities.
Dedicated Chanel
Dedicated Chanel – LAN – Star Topology
Almost every Wired LAN uses a Dedicated Chanel
for Two Stations Communicating with each other.
Dedicated Chanel – LAN – Star Topology
If we wanted to send an Analog Signal that roughly
Simulate the Digital Signal We Would Have To
Consider The Worst Case
Maximum Number of Signal Changes
0 1 0 1 0 1 …
1 0 1 0 1 0 …
Positive Peak = 1
Negative Peak = 0
Digital Signal BitRate - N
B = N/2
112
A Digital Signal is a
Composite Signal with an
Infinite Bandwidth.
113
Bit
Rate
Harmonic
1
Harmonics
1, 3
Harmonics
1, 3, 5
Harmonics
1, 3, 5, 7
1 Kbps 500 Hz 2 KHz 4.5 KHz 8 KHz
10 Kbps 5 KHz 20 KHz 45 KHz 80 KHz
100 Kbps 50 KHz 200 KHz 450 KHz 800 KHz
Bandwidth Requirement
114
Digital vs. Analog
115
The Bit Rate and the
Bandwidth are
Proportional to Each Other.
116
The analog bandwidthof a medium is
expressed in hertz.
The digital bandwidth,
in bits per second.
Note:
3.117
Rough Approximation of a Digital Signal
118
Transmission
Impairment
119
3 Major Impairment Types
120
Attenuation
121
122
Suppose a signal travels through a transmission
medium and its power is reduced to one half. This
means that P2 = 0.5 P1. In this case, the attenuation
(loss of power) can be calculated as
Example 3.26
A loss of 3 dB (−3 dB) is equivalent to losing one-half
the power.
125
Distortion
126
127
Distortion
128
Noise
129
Signal to Noise Ratio shows the Ratio of
Signal Power to Noise Power
Power often expressed in Watts
S/N = Signal Power/Noise Power
Just a Simple Ratio
130
Signal to Noise Ratio (SNR or S/N)Bandwidth Requirement
Signal to Noise RatiodB shows the Ratio of Signal
Power to Noise Power in Decibels
S/NdB = 10 log10 (signal power/noise power)
Example 1: Signal power = 1000 watts, noise
power = 20 mw
Example 2: Signal power = 100 w, noise power =
0.000002w 131
Signal to Noise RatiodB
(SNRdB or S/NdB)
The loss in a cable is usually defined in decibels
per kilometer (dB/km). If the signal at the beginning
of a cable with −0.3 dB/km has a power of 2 mW,
what is the power of the signal at 5 km?
Solution
The loss in the cable in decibels is 5 × (−0.3) = −1.5
dB. We can calculate the power as
132
Signal to Noise RatiodB
(SNRdB or S/NdB)
133
Two cases of SNR: a high SNR and a low SNR
134
Shannon
Capacity
135
136
137
Media
Comparison
Criteria
138Consider The Future – CAT 6 or 7?
139