PART 2: PHYSICAL LAYER

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PART 2 PHYSICAL LAYER

• Lecturer Tae-Hyong Kim (B201-4)
• thkim_at_cespc1.kumoh.ac.kr

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Physical Layer

• Position of Physical Layer

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Physical Layer

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

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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

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Ch.3 Signals

• Lecturer Tae-Hyong Kim (B201-4)
• thkim_at_cespc1.kumoh.ac.kr

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Contents

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

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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)

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Contents

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

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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

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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

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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

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Sine Wave

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

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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

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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

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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

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Time and Frequency Domain

• Relation

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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

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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

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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

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Composite Signals

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

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Composite Signals

• Fourier Analysis
• An example a square wave
• Frequency spectrum comparison

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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

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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
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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

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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.

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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

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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

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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

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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

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Properties of Digital Signals

• Intuitive Approach using One Harmonic
• Positive 1, Negative 0

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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Contents

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

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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

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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

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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)?

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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?

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Distortion

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

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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.

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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

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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

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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

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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

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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