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PART 2: PHYSICAL LAYER

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Title: PART 2: PHYSICAL LAYER


1
PART 2 PHYSICAL LAYER
  • Lecturer Tae-Hyong Kim (B201-4)
  • thkim_at_cespc1.kumoh.ac.kr

2
Physical Layer
  • Position of Physical Layer

3
Physical Layer
  • Physical Layer Services
  • Transmission Media guided and unguided
    (wireless)
  • Networks and Technologies
  • Telephone Network voice and data
  • High Speed Access DSL, cable,

4
Physical Layer
  • Corresponding Chapters
  • Ch.3 Signals
  • Ch.4 Digital Transmission
  • Ch.5 Analog Transmission
  • Ch.6 Multiplexing
  • Ch.7 Transmission Media ? we will skip this
  • Ch.8 Circuit Switching and Telephone Network
  • Ch.9 High Speed Digital Access

5
Ch.3 Signals
  • Lecturer Tae-Hyong Kim (B201-4)
  • thkim_at_cespc1.kumoh.ac.kr

6
Contents
  • Introduction
  • Analog and Digital
  • Analog Signals
  • Digital Signals
  • Analog Vs Digital
  • Data Rate Limits
  • Transmission Impairment
  • More about Signals

7
Introduction
  • Information can be voice, image, numeric data,
    characters, code, picture, and so on
  • To be transmitted, data must be transformed to
    electromagnetic signals
  • information (analog / digital)
  • ? encoding
  • ? signals (analog / digital)

8
Contents
  • Introduction
  • Analog and Digital
  • Analog Signals
  • Digital Signals
  • Analog Vs Digital
  • Data Rate Limits
  • Transmission Impairment
  • More about Signals

9
Analog and Digital
  • Analog and digital data
  • Analog data real world information, ex. human
    voice
  • Digital data computer world information, ex.
    Data file
  • Analog and digital signals
  • Analog signal infinitely many levels of
    intensity ?continuous
  • Digital signal only a limited number of defined
    values ? discrete

10
Analog and Digital
  • Periodic and Aperiodic Signals
  • Periodic signal
  • The same signal pattern (cycle) repeats over time
    (period)
  • s(tT) s(t) -8 lt t lt 8
  • Aperiodic signal
  • Changes without a pattern that repeats over time
  • In data communication, periodic analog signals
    and aperiodic digital signals are used

11
Contents
  • Introduction
  • Analog and Digital
  • Analog Signals
  • Sine Wave
  • Phase
  • Time and Frequency Domain
  • Composite Signals
  • Bandwidth
  • Digital Signals
  • Analog Vs Digital
  • Data Rate Limits
  • Transmission Impairment
  • More about Signals

12
Sine Wave
  • Sine wave
  • s(t) Asin(2?ft ?)
  • s instantaneous amplitude
  • A peak amplitude
  • f frequency
  • ? phase
  • Period ?

13
Sine Wave
  • Period and Frequency
  • Period the amount of time (in sec) a signal
    needs to complete one cycle
  • Frequency the number of periods in one second
  • ? they are inverses of each other
  • T 1/f
  • Ex. 100 ms 10-1 s ? 1/(10-1) Hz 10Hz 10-2KHz

14
Sine Wave
  • More about Frequency
  • Frequency the rate of change wrt. time.
  • Change in a short span of time ? high frequency
  • Change over a long span of time ? low frequency
  • Two Extremes
  • A signal does not change at all ? frequency 0
    (DC)
  • A signal changes instantaneously ? frequency
    infinite
  • Phase
  • Position of the waveform relative to time zero

15
Time and Frequency Domain
  • Time-domain plot
  • instantaneous amplitude with respect to time.
  • Frequency-domain plot
  • maximum amplitude with respect to frequency
  • ? Analog signals are best represented in the
    freq. domain
  • Relation

16
Time and Frequency Domain
  • Relation

17
Composite Signals
  • Usage of a single sine wave
  • Carry of electric energy (power)
  • Send an alarm to a security center
  • ? single tone ? not useful in data communications
  • ? to make signals that can carry information, we
    have to add several different sine waves
    (composite signals)
  • Composite Signals
  • A periodic signal decomposed into two or more
    sine waves.
  • Fourier Analysis (Transform) is used to decompose
    a composite signal into its components

18
Composite Signals
  • Fourier Analysis
  • y(t) is a real function of time
  • We define the Fourier transform Y(f)
  • A complex function in frequency domain f
  • Y(f) is the spectral or harmonic representation
    of y(t), i.e., Frequency spectrum

19
Composite Signals
  • Fourier Analysis
  • Any composite signal can be represented as a
    combination of simple sine waves with different
    frequencies, phases, and amplitudes
  • s(t) A1sin(2?f1t ?1) A2sin(2?f2t ?2)
    A3sin(2?f3t ?3)
  • An example a square wave
  • s(t) 4A/ ? sin2?ft 4A/(3?) sin2?(3f)t
    4A/(5?) sin2?(5f)t
  • ? f fundamental frequency, nf the n-th
    harmonic

20
Composite Signals
  • Fourier Analysis
  • An example a square wave
  • First three harmonics f, 3f, 5f
  • Adding first three harmonics

21
Composite Signals
  • Fourier Analysis
  • An example a square wave
  • Frequency spectrum comparison

22
Composite Signals
  • Composite Signal and Transmission medium
  • A signal needs to pass through a transmission
    medium
  • A medium may pass some frequencies and may block
    or weaken others
  • Some harmonics of a composite signal may be
    blocked
  • depends on the bandwidth of the medium
  • An example

23
Bandwidth
  • Definition
  • The range of frequencies that a medium can pass
  • Without losing one-half of the power contained in
    that signal
  • ? 3dB point
  • the difference between the highest and the lowest
    frequencies

A
A/2
The media may pass some frequencies outside the
bandwidth
24
Bandwidth
  • Relation to composite signals
  • If the bandwidth of a medium does not match the
    spectrum of a signal
  • ? some of the frequencies are lost
  • Passing a square wave through any medium will
    always deform the signal

25
Bandwidth
  • Relation to composite signals
  • Bandwidth may be used wrt a signal
  • This signal has a bandwidth of 1000Hz
  • ? The signal has a spectrum with significant
    frequencies that span 1000Hz
  • ? We need a medium with a bandwidth of 1000Hz if
    we want to send this signal without losing a
    significant part of it
  • Example 1 If a period signal is decomposed into
    five sine waves with frequencies of 100, 300,
    500, 700, and 900 Hz, What is the bandwidth? Draw
    the spectrum, assuming all components have a
    maximum amplitude of 10V.

26
Bandwidth
  • Example 2 A signal has a bandwidth of 20 KHz.
    The highest frequency is 60 KHz. What is the
    lowest frequency? Draw the spectrum if the signal
    contains all integral frequencies of the same
    amplitude.
  • Example 3 A signal has a spectrum with
    frequencies between 1000 and 2000 Hz (bandwidth
    of 1000 Hz). A medium can pass frequencies from
    3000 to 4000 Hz (a bandwidth of 1000 Hz). Can
    this signal faithfully pass through this medium?
  • No

27
Contents
  • Introduction
  • Analog and Digital
  • Analog Signals
  • Digital Signals
  • Bit Interval and Bit Rate
  • Properties of Digital Signals
  • Digital versus Analog Bandwidth
  • Higher Bit Rate
  • Analog Vs Digital
  • Data Rate Limits
  • Transmission Impairment
  • More about Signals

28
Bit Interval and Bit Rate
  • Bit interval
  • The time required to send one single bit
  • Bit rate
  • the number of bit intervals per second (bps)
  • Ex. Bit rate 2000 bps, bit interval ?
  • 1/2000 0.0005 sec 500 ?sec

29
Properties of Digital Signals
  • Digital Signal as a Composite Analog Signal
  • A digital signal is a composite signal with an
    infinite bandwidth.
  • Digital Signal Through a Wide-Bandwidth Medium
  • Digital signals can be sent because enough
    frequencies are passed to preserve a decent
    signal shape
  • Ex. Coaxial cable
  • Digital Signal Through a Band-Limited Medium
  • The minimum required bandwidth B in hertz for
    sending n bps
  • Can be derived by Nyquist theorem and the Shannon
    capacity
  • Intuitive Approach for understanding the data
    transmission

30
Properties of Digital Signals
  • Intuitive Approach using One Harmonic
  • Positive 1, Negative 0

31
Properties of Digital Signals
  • Intuitive Approach using One Harmonic
  • Positive 1, Negative 0
  • bit rate 6, frequency 0 3 Hz
  • Medium needs to have a bandwidth of 3Hz (3-0)
  • ? To send n bps through an analog channel using
    the above approximation, we need a bandwidth B
    such that
  • B n / 2

32
Properties of Digital Signals
  • Using More Harmonics
  • To improve the shape of the signal for better
    communication, particularly for higher data
    rates, we need to add some (odd) harmonics.
  • With the 3rd harmonic, B n/2 3n/2 2n Hz
  • With the 3rd and 5th harmonics, B n/2 3n/2
    5n/2 9n/2 Hz
  • ? B gt n / 2, or n lt 2B
  • The bit rate and the bandwidth are proportional
    to each other

33
Properties of Digital Signals
  • Digital vs. Analog Bandwidth of a medium
  • Analog bandwidth the range of frequencies that a
    medium can pass (Hz)
  • Digital bandwidth the maximum bit rate that a
    medium can pass (bps)
  • Higher bit rate
  • Can we send data at higher bit rate than
    calculated above?
  • Ex. Traditional telephone line BW 34kHz ? 8kbps
    max?
  • Owing to modulation techniques (Ch.5), multiple
    bits can be represented in one single period of
    analog signal

34
Contents
  • Introduction
  • Analog and Digital
  • Analog Signals
  • Digital Signals
  • Analog Vs Digital
  • Low-pass vs. Band-pass
  • Digital Transmission
  • Analog Transmission
  • Data Rate Limits
  • Transmission Impairment
  • More about Signals

35
Low-Pass vs. Band-Pass
  • Should we use analog or digital signal?
  • Depends on the situation and on the available
    bandwidth
  • Filter effect of a channel
  • A channel or a link is either low-pass or
    band-pass
  • Low-pass channel - BW 0 f
  • Band-pass channel - BW f1 f2

36
Digital vs. Analog Transmission
  • Digital Transmission
  • Needs a low pass channel (BW 0 f (?8))
  • By limiting the number of harmonics)
  • Cannot be shared at the same time
  • Point-to-point, or shared in time
  • Ex. LAN with cables
  • Analog Transmission
  • Can use a band-pass channel (BW f1 f2)
  • BW of an analog signal can be shifted (w/ a
    carrier signal)
  • Ex. f1 f2 ? f3 f4 (f4 f3 (f2 - f1))
  • BW of a medium can be divided into several band
    pass channels to carry several analog
    transmissions
  • Ex. Cell phone (BW 30kHz), cable TV (BW 6MHz)

37
Contents
  • Introduction
  • Analog and Digital
  • Analog Signals
  • Digital Signals
  • Analog Vs Digital
  • Data Rate Limits
  • Noiseless channel Nyquist Bit Rate
  • Noisy channel Shannon Capacity
  • Using Both Limitsb
  • Transmission Impairment
  • More about Signals

38
How Fast can we send data?
  • The speed of data transmission depends on
  • The BW available
  • The levels of signals we can use
  • Digital 2 levels 1 bit, 4 levels 2 bits /
    period
  • Analog magnitude, frequency, phase ? modulation
    (Ch.5)
  • The quality of the channel
  • The level of the noise
  • Theoretical formulas for data rate calculation
  • Noiseless channel ? Nyquist Bit Rate
  • Noisy channel ? Shannon Capacity

39
Nyquist Bit Rate
  • Theoretical maximum bit rate
  • BitRate 2 ? Bandwidth ? log2L
  • L the number of signal levels
  • Ex.1 Consider a noiseless channel with a BW of
    3000Hz transmitting a signal with two signal
    levels. The maximum bit rate is
  • BitRate 2 ? 3000 ? log22 6,000bps
  • Ex.2 Consider the same noiseless channel,
    transmitting a signal with four signal levels.
    The maximum bit rate is
  • BitRate 2 ? 3000 ? log24 12,000bps

40
Shannon Capacity
  • In practical, the channel is always noisy.
  • Theoretical highest data rate for a noisy
    channel
  • Capacity Bandwidth ? log2(1SNR) (bps)
  • SNR Signal-to-Noise Ratio
  • SNR (dB) 10 log10 (signal power / noise power)
  • ? not be affected by the signal level
  • ? Shannon capacity defines the characteristics of
    the channel
  • Ex.1 What is the capacity of an extremely noisy
    channel in which the value of the SNR is almost
    zero?
  • CBlog2(10)Blog21 0
  • Ex.2 What is the theoretical highest bit rate of
    a regular telephone line? (BW3000 hz, SNR 3162
    (35 dB))
  • C3000log2(13162)3000log2316334,680bps
  • If we want to send data faster than this, we can
    either increase the BW of the line or improve the
    SNR

41
Using Both Limits
  • Ex.1 We have a channel with a 1MHz BW. The SNR
    for this channel is 63 what is the appropriate
    bit rate and signal level?
  • Upper limit by Shannon formula
  • Capacity B log2(1SNR) 106 log2 (163) 106
    log2 64 6Mbps
  • Lets choose 4Mbps for better performance
  • Then, the number of signal levels by Nyquist
    formula
  • 4M bps 2 ? 1 MHz ? log2L
  • ? L 4

42
Contents
  • Introduction
  • Analog and Digital
  • Analog Signals
  • Digital Signals
  • Analog Vs Digital
  • Data Rate Limits
  • Transmission Impairment
  • Attenuation
  • Distortion
  • Noise
  • More about Signals

43
Transmission Impairment
  • Transmission media are not perfect because of
    impairment in the signal sent through the medium
  • Signal at the beginning and end of the medium
    are not same

44
Attenuation
  • Definition loss of energy
  • When signal travels trough a medium, it losses
    some of it energy
  • To compensate for this loss, amplifiers are used
    to amplify the signal

45
Attenuation
  • Decibel (dB)
  • dB 10 log10 (p2/p1)
  • Pn the power of at point n
  • dB lt 0 attenuated
  • -3dB half the power
  • dB gt 0 amplified
  • 10 dB increased 10 times
  • Ex.1 If a signal travels through a transmission
    medium and its power is reduced to half, then
    what is the attenuation (loss of power)?

46
Attenuation
  • Ex.2 If a signal travels through an amplifier and
    its power is increased 10 times, what is the
    amplification (gain of power)?
  • Ex.3 Decibel number can be added when cascading,
    How come?

47
Distortion
  • Definition
  • signal changes its form or shape
  • Reason
  • the difference in propagation delay

48
Noise
  • Types of Noise
  • Thermal noise random motion of electrons
  • Induced noise from sources such as motors,
    appliances
  • Crosstalk the effect of one wire on the other
  • Impulse noise a spike that comes from power
    lines, lightning, and so on.

49
Contents
  • Introduction
  • Analog and Digital
  • Analog Signals
  • Digital Signals
  • Analog Vs Digital
  • Data Rate Limits
  • Transmission Impairment
  • More about Signals
  • Throughput
  • Propagation Speed
  • Propagation Time
  • Wavelength

50
Throughput
  • Concept
  • How fast can data pass through a point?
  • The number of bits that can pass the wall in one
    second
  • Measured in bps
  • Depends on the transmission speed
  • Sometimes represented as the proportion to the
    propagation speed ? link utilization

51
Propagation Speed and Time
  • Propagation speed
  • Distance per second (or bps)
  • Ex.1 light propagation speed 3?108m/s
  • Propagation time
  • Distance/propagation speed
  • Ex.1
  • Twisted pair 3.33 ?s/km
  • Coaxial or fiber optic cable 5 ?s/km

52
Wavelength
  • Distance a simple signal can travel in one period
  • Usually used for the transmission of light in an
    optical fiber
  • Wavelength(?) propagation speed(c) ? period(T)
  • propagation speed(c) / frequency(f)
  • Depend on both the frequency of a signal and the
    medium
  • ? cT c/f
  • Ex.1 the wavelength of red light (f 4?1014 Hz)
    in air
  • ? c/f (3?108)/(4?1014) 0.75?m

53
Assignment 2
  • Part 1 Exercises 50, 54, 63, 65, 66, 67, 72, 73,
    74, 75, 76, 77, 82, 83, 85
  • Part 2 Survey the characteristics of some
    popular media for the following points
  • Properties
  • Usage
  • Data transmission method
  • Bandwidth
  • Due Date
  • The same class next week
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