Introduction to Physical Layer Data and signals, periodic analog signals, digital signals, transmission impairment, data rate limits, performance.

1. CH-03
INTRODUCTION TO PHYSICAL
LAYER
-MEENAKSHI PAUL
1

2. OUTLINE
3.1 DATA AND SIGNALS
3.1.1 Analog and Digital
Signals
3.1.2 Periodic and Non-
periodic
3.2 PERIODIC ANALOG SIGNALS
3.2.1 Sine Wave
3.2.2 Phase
3.2.3 Wavelength
3.2.4 Time and Frequency
Domains
3.2.5 Composite Signals
3.3 DIGITAL SIGNALS
3.3.1 Bit Rate
3.3.2 Bit Length
3.4 TRANSMISSION IMPAIRMENT
3.4.1 Attenuation
3.4.2 Distortion
3.4.3 Noise
2

3. 3.1 DATA AND SIGNALS
3

4. 3.1.1 ANALOG AND DIGITAL DATA
• Analog data
• Data take on continuous
values
• E.g., human voice,
temperature reading
• Digital data
• Data take on discrete
values
• E.g., text, integers
4

5. 3.1.1 ANALOG AND DIGITAL
SIGNAL
• Analog signals
• have an infinite number of
values in a range
• Digital signals
• Have a limited number of
values
• Both signals can take one of two
forms: periodic or nonperiodic
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value
time
value
time

6. DATA AND SIGNALS
6
Telephone
Analog Data Analog Signal
Modem
Digital Data Analog Signal
Codec
Analog Data Digital Signal
Digital
transmitter
Digital Data Digital Signal

7. Analog Digital
Analog signal is a continuous
signal which represents physical
measurements.
Digital signals are discrete time
signals generated by digital
modulation.
Denoted by sine waves Denoted by square waves
Uses continuous range of values
to represent information
Uses discrete or discontinuous
values to represent information
Human voice in air, analog
electronic devices.
Computers, CDs, DVDs, and other
digital electronic devices.
Analog technology records
waveforms as they are.
Samples analog waveforms into a
limited set of numbers and records
them.
Can be used in analog devices
only. Best suited for audio and
video transmission.
Best suited for Computing and
digital electronics. 7

8. 3.1.2 PERIODIC AND NONPERIODIC
• A periodic signal completes a pattern within a
timeframe, 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.
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9. 3.2 PERIODIC ANALOG SIGNALS
9

10. 3.2 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.
10

11. 3.2.1 SINE WAVES
• It is most simplest form of periodic signal.
• A sine wave can be represented by three parameters:
• Peak amplitude
• Frequency
• Phase
11
period
T = 1/f
peak
amplitude
time
signal strength

12. 3.2.1.1 PEAK AMPLITUDE
• Its highest intensity,
proportional to the
energy it carries.
• For electric signals,
peak amplitude is
normally measured in
volts.
12

13. 3.2.1.2 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 one
second.
• Frequency and period are
the inverse of each other.
13

14. 3.2.1.3 PHASE
• Phase describes the
position of the waveform
relative to time 0.
• Phase is measured in
degrees or radians
14

15. VARYING SINE WAVES
15
-3
-2
-1
0
1
2
3
0 0.5 1 1.5 2 2.5 3
A = 1, f = 1,  = 0 -3
-2
-1
0
1
2
3
0 0.5 1 1.5 2 2.5 3
A = 2, f = 1,  = 0
-3
-2
-1
0
1
2
3
0 0.5 1 1.5 2 2.5 3
A = 1, f = 2,  = 0 -3
-2
-1
0
1
2
3
0 0.5 1 1.5 2 2.5 3
A = 1, f = 1,  = /4

16. 3.2.3 WAVELENGTH
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• The wavelength of a signal refers to the relationship between
frequency (or period) and propagation speed of the wave
through a medium.
• The distance between successive crests of a wave, especially
points in a sound wave or electromagnetic wave.
• It is represented by the symbol: Lambda

17. 3.2.4 TIME AND FREQUENCY
DOMAINS
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18. THE TIME-DOMAIN AND FREQUENCY-DOMAIN PLOTS OF A SINE WAVE
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19. 19
THE TIME DOMAIN AND FREQUENCY DOMAIN OF THREE
SINE WAVES

20. 3.2.5 COMPOSITE SIGNALS
20
• A composite signal is a combination of two or more simple sines waves with
different frequency, phase and amplitude.
• If the composite signal is periodic, the decomposition gives a series of
signals with discrete frequencies.
• If the composite signal is non-periodic, the decomposition gives a
combination of sine waves with continuous frequencies.

21. 3.2.6 BANDWIDTH
• The range of frequencies
contained in a composite signal
is its bandwidth.
• The band-width is normally a
difference between two
numbers.
• For example, if a composite
signal contains frequencies
between 1000 and 5000, its
bandwidth is 5000 − 1000, or
4000.
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22. 3.3 DIGITAL SIGNALS
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23. 3.3 DIGITAL SIGNALS
• A digital is a signal that has discrete values.
• The signal will have value that is not continuous.
• Information in digital signal can be represented in the form
of voltage levels.
• A 1 can be encoded as a positive voltage and a 0 as zero
negative voltage.
• A digital signal can have more than two levels
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24. FIGURE: TWO DIGITAL SIGNALS: ONE WITH TWO SIGNAL
LEVELS AND THE OTHER WITH FOUR SIGNAL LEVELS
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25. 3.3.1 BIT RATE
• It is number of bits sent in per second.
• It is expressed in bits per second (bps).
• Bit interval – duration of 1 bit
• 3.3.2 Bit Length
• The distance one bit occupies on the transmission medium.
• Bit length = propagation speed x bit duration
25

26. 3.3.3 DIGITAL SIGNAL AS A
COMPOSITE ANALOG 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
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27. 27
The time and frequency domains of periodic and nonperiodic
digital signals

28. TYPE OF CHANNELS
28

29. 3.3.4 TRANSMISSION OF DIGITAL
SIGNALS
• A digital signal by using one of two different approaches
• Baseband transmission
• Broadband transmission
• Baseband Transmission:
• Sending a digital signal over a channel without changing it to an
analog signal.
• Baseband transmission requires a low-pass channel
• LP is a channel with a bandwidth that starts from zero.
• Broadband Transmission:
• A digital signal cannot be transmitted directly through it.
• In broadband transmission we use modulation, i.e. we change the
signal to analog signal before transmitting it.
• Broadband transmission requires bandpass channel.
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30. BANDWIDTH
• A property of a medium
• Indicates the difference between the highest and the
lowest frequencies allowed to pass
• <highest freq allowed> – <lowest freq allowed>
• Also a property of a single spectrum
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Cutoff frequency
(half of power is lost)

31. 3.4 TRANSMISSION
IMPAIRMENT
31

32. 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
• Attenuation
• Distortion
• Noise
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33. CAUSES OF IMPAIRMENT
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34. 3.4.1 SIGNAL ATTENUATION
• Attenuation  Loss of energy
• Signal strength falls off with distance
• Attenuation depends on medium
• Attenuation is an increasing function of frequency
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Transmission medium

35. 3.4.2 SIGNAL DISTORTION
• Distortion  Change in signal shape
• Only happens in guided media
• Propagation velocity varies with frequency
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36. 3.4.3 NOISE
• Noise  Undesirable signals added between the
transmitter and the receiver
• Types of noise
• Thermal
• Due to random motion of electrons in a wire
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37. 3.4.3 NOISE (CONT..)
• Types of noise (cont’d)
• Crosstalk
• Signal from one line picked up by another
• Impulse
• Irregular pulses or spikes
• E.g., lightning
• Short duration
• High amplitude
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Wire 1
Wire 2

38. 3.5 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
• Noiseless Channel: Nyquist Bit Rate
• Noisy Channel: Shannon Capacity 38

39. NOISELESS CHANNEL: NYQUIST BIT
RATE
3.39

40. NOISY CHANNEL: SHANNON
CAPACITY
3.40

41. SUMMARY
• Data need to take form of signal to be transmitted
• Frequency domain representation of signal allows easier
analysis
• Fourier analysis
• Medium's bandwidth limits certain frequencies to pass
• Bit rate is proportional to bandwidth
• Signals get impaired by attenuation, distortion, and noise
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