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TOBB ETU Bil557 Wireless Networks Lecture 03 January 24, 2007

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Title: TOBB ETU Bil557 Wireless Networks Lecture 03 January 24, 2007


1
TOBB ETU Bil557 Wireless Networks Lecture
03 January 24, 2007
  • Spring 2007
  • Wednesday 0830 1200
  • Room Number 212
  • Bülent Tavli
  • Office 169
  • btavli_at_etu.edu.tr

2
Reminders
  • Homework I
  • Due Today
  • ns-2
  • Installation / Tutorials
  • Homework II
  • Due February 07, 2007 in class
  • Following questions from the text book 3.6, 4.5,
    4.8, 5.3, 5.10, 5.14, 6.1, 6.9, 6.13, 6.14

3
Highlights of The Previous Lectures
  • Chapters 1, 2, 3, 4
  • (Part One Technical Background)

4
Protocol Stack
Application
Transport
Network
MAC
Physical
Channel
5
Techniques Used in Switched Networks
  • Circuit switching
  • Dedicated communications path between two
    stations
  • E.g., public telephone network
  • Packet switching
  • Message is broken into a series of packets
  • Each node determines next leg of transmission for
    each packet

6
Packet Switching
7
Datagram versus Virtual Circuit
8
Asynchronous Transfer Mode (ATM)
  • Also known as cell relay
  • Operates at high data rates
  • Resembles packet switching
  • Involves transfer of data in discrete chunks,
    like packet switching
  • Allows multiple logical connections to be
    multiplexed over a single physical interface
  • Minimal error and flow control capabilities
    reduces overhead processing and size
  • Fixed-size cells simplify processing at ATM nodes

9
TCP/IP Concepts
10
Comparison of OSI and TCP/IP
11
ns example TCP
set ns new Simulator set n0 ns node set n1
ns node
set ftp new Application/FTP ftp attach-agent
tcp ns at 0.2 "ftp start" ns at 1.2
exit" ns run
ns duplex-link n0 n1 1.5Mb 10ms DropTail
set tcp new Agent/TCP set tcpsink new
Agent/TCPSink ns attach-agent n0 tcp ns
attach-agent n1 tcpsink ns connect tcp
tcpsink
12
Part Two Wireless Communication Technology
  • Chapter 5 Antennas and Propagation
  • Antennas, propagation modes, Line-of-Sight
    Transmission, Fading in the mobile environment
  • Chapter 6 Signal Encoding Techniques
  • Signal encoding, digital data / analog signals,
    analog/analog, analog/digital
  • Chapter 7 Spread Spectrum
  • Spread spectrum concept, frequency hopping spread
    spectrum, direct sequence spread spectrum, CDMA
  • Chapter 8 Coding and Error Control
  • Error detection, block error correction codes,
    convolutional codes, automatic repeat request

13
Antennas and Propagation
  • Chapter 5

14
Introduction
  • An antenna is an electrical conductor or system
    of conductors
  • Transmission - radiates electromagnetic energy
    into space
  • Reception - collects electromagnetic energy from
    space
  • In two-way communication, the same antenna can be
    used for transmission and reception

15
Radiation Patterns
  • Radiation pattern
  • Graphical representation of radiation properties
    of an antenna
  • Depicted as two-dimensional cross section
  • Beam width (or half-power beam width)
  • Measure of directivity of antenna
  • Reception pattern
  • Receiving antennas equivalent to radiation
    pattern

16
Types of Antennas
  • Isotropic antenna (idealized)
  • Radiates power equally in all directions
  • Dipole antennas
  • Half-wave dipole antenna (or Hertz antenna)
  • Quarter-wave vertical antenna (or Marconi
    antenna)
  • Parabolic Reflective Antenna

17
(No Transcript)
18
Antenna Gain
  • Antenna gain
  • Power output, in a particular direction, compared
    to that produced in any direction by a perfect
    omnidirectional antenna (isotropic antenna)
  • Effective area
  • Related to physical size and shape of antenna

19
Antenna Gain
  • Relationship between antenna gain and effective
    area
  • G antenna gain
  • Ae effective area
  • f carrier frequency
  • c speed of light ( 3 108 m/s)
  • ? carrier wavelength

20
Propagation Modes
  • Ground-wave propagation
  • Sky-wave propagation
  • Line-of-sight propagation

21
Ground Wave Propagation
  • Follows contour of the earth
  • Can Propagate considerable distances
  • Frequencies up to 2 MHz
  • Example
  • AM radio

22
Sky Wave Propagation
  • Signal reflected from ionized layer of atmosphere
    back down to earth
  • Signal can travel a number of hops, back and
    forth between ionosphere and earths surface
  • Reflection effect caused by refraction
  • Examples
  • Amateur radio
  • CB radio

23
Line-of-Sight Propagation
24
Line-of-Sight Propagation
  • Transmitting and receiving antennas must be
    within line of sight
  • Satellite communication signal above 30 MHz not
    reflected by ionosphere
  • Ground communication antennas within effective
    line of site due to refraction
  • Refraction bending of microwaves by the
    atmosphere
  • Velocity of electromagnetic wave is a function of
    the density of the medium
  • When wave changes medium, speed changes
  • Wave bends at the boundary between mediums

25
Line-of-Sight Equations
  • Optical line of sight
  • Effective, or radio, line of sight
  • d distance between antenna and horizon (km)
  • h antenna height (m)
  • K adjustment factor to account for refraction,
    rule of thumb K 4/3

26
Line-of-Sight Equations
  • Maximum distance between two antennas for LOS
    propagation
  • h1 height of antenna one
  • h2 height of antenna two

27
LOS Wireless Transmission Impairments
  • Attenuation and attenuation distortion
  • Free space loss
  • Noise
  • Atmospheric absorption
  • Multipath
  • Refraction
  • Thermal noise

28
Attenuation
  • Strength of signal falls off with distance over
    transmission medium
  • Attenuation factors for unguided media
  • Received signal must have sufficient strength so
    that circuitry in the receiver can interpret the
    signal
  • Signal must maintain a level sufficiently higher
    than noise to be received without error
  • Attenuation is greater at higher frequencies,
    causing distortion

29
Free Space Loss
  • Free space loss, ideal isotropic antenna
  • Pt signal power at transmitting antenna
  • Pr signal power at receiving antenna
  • ? carrier wavelength
  • d propagation distance between antennas
  • c speed of light ( 3 10 8 m/s)
  • where d and ? are in the same units (e.g., meters)

30
Free Space Loss
  • Free space loss equation can be recast

31
(No Transcript)
32
Free Space Loss
  • Free space loss accounting for gain of other
    antennas
  • Gt gain of transmitting antenna
  • Gr gain of receiving antenna
  • At effective area of transmitting antenna
  • Ar effective area of receiving antenna

33
Free Space Loss
  • Free space loss accounting for gain of other
    antennas can be recast as

34
Categories of Noise
  • Thermal Noise
  • Intermodulation noise
  • Crosstalk
  • Impulse Noise

35
Thermal Noise
  • Thermal noise due to agitation of electrons
  • Present in all electronic devices and
    transmission media
  • Cannot be eliminated
  • Function of temperature
  • Particularly significant for satellite
    communication

36
Thermal Noise
  • Amount of thermal noise to be found in a
    bandwidth of 1Hz in any device or conductor is
  • N0 noise power density in watts per 1 Hz of
    bandwidth
  • k Boltzmann's constant 1.3803 10-23 J/K
  • T temperature, in kelvins (absolute temperature)

37
Thermal Noise
  • Noise is assumed to be independent of frequency
  • Thermal noise present in a bandwidth of B Hertz
    (in watts)
  • or, in decibel-watts

38
Noise Terminology
  • Intermodulation noise occurs if signals with
    different frequencies share the same medium
  • Interference caused by a signal produced at a
    frequency that is the sum or difference of
    original frequencies
  • Crosstalk unwanted coupling between signal
    paths
  • Impulse noise irregular pulses or noise spikes
  • Short duration and of relatively high amplitude
  • Caused by external electromagnetic disturbances,
    or faults and flaws in the communications system

39
Expression Eb/N0
  • Ratio of signal energy per bit to noise power
    density per Hertz
  • The bit error rate for digital data is a function
    of Eb/N0
  • Given a value for Eb/N0 to achieve a desired
    error rate, parameters of this formula can be
    selected
  • As bit rate R increases, transmitted signal power
    must increase to maintain required Eb/N0

40
Other Impairments
  • Atmospheric absorption water vapor and oxygen
    contribute to attenuation
  • Multipath obstacles reflect signals so that
    multiple copies with varying delays are received
  • Refraction bending of radio waves as they
    propagate through the atmosphere

41
Multipath Propagation
  • Reflection - occurs when signal encounters a
    surface that is large relative to the wavelength
    of the signal
  • Diffraction - occurs at the edge of an
    impenetrable body that is large compared to
    wavelength of radio wave
  • Scattering occurs when incoming signal hits an
    object whose size in the order of the wavelength
    of the signal or less

42
The Effects of Multipath Propagation
  • Multiple copies of a signal may arrive at
    different phases
  • If phases add destructively, the signal level
    relative to noise declines, making detection more
    difficult
  • Intersymbol interference (ISI)
  • One or more delayed copies of a pulse may arrive
    at the same time as the primary pulse for a
    subsequent bit

43
Types of Fading
  • Fast fading
  • Slow fading
  • Flat fading
  • Selective fading
  • Rayleigh fading
  • Rician fading

44
Rayleigh and Rician Fading
  • K 0 Rayleigh
  • K 8 AWGN

45
Distortions
Perfect channel
White noise
Phase jitter
46
Error Compensation Mechanisms
  • Forward error correction
  • Adaptive equalization
  • Diversity techniques

47
Forward Error Correction
  • Transmitter adds error-correcting code to data
    block
  • Code is a function of the data bits
  • Receiver calculates error-correcting code from
    incoming data bits
  • If calculated code matches incoming code, no
    error occurred
  • If error-correcting codes dont match, receiver
    attempts to determine bits in error and correct

48
Adaptive Equalization
  • Can be applied to transmissions that carry analog
    or digital information
  • Analog voice or video
  • Digital data, digitized voice or video
  • Used to combat intersymbol interference
  • Involves gathering dispersed symbol energy back
    into its original time interval
  • Techniques
  • Lumped analog circuits
  • Sophisticated digital signal processing algorithms

49
Diversity Techniques
  • Diversity is based on the fact that individual
    channels experience independent fading events
  • Space diversity techniques involving physical
    transmission path
  • Frequency diversity techniques where the signal
    is spread out over a larger frequency bandwidth
    or carried on multiple frequency carriers
  • Time diversity techniques aimed at spreading
    the data out over time

50
Signal Encoding Techniques
  • Chapter 6

51
Encoding and Modulation
52
Reasons for Choosing Encoding Techniques
  • Digital data, digital signal
  • Equipment less complex and expensive than
    digital-to-analog modulation equipment
  • Analog data, digital signal
  • Permits use of modern digital transmission and
    switching equipment
  • Digital data, analog signal
  • Some transmission media will only propagate
    analog signals
  • E.g., optical fiber and unguided media
  • Analog data, analog signal
  • Analog data in electrical form can be transmitted
    easily and cheaply
  • Done with voice transmission over voice-grade
    lines

53
Signal Encoding Criteria
  • What determines how successful a receiver will be
    in interpreting an incoming signal?
  • Signal-to-noise ratio
  • Data rate
  • Bandwidth
  • An increase in data rate increases bit error rate
  • An increase in SNR decreases bit error rate
  • An increase in bandwidth allows an increase in
    data rate

54
Factors Used to Compare Encoding Schemes
  • Signal spectrum
  • With lack of high-frequency components, less
    bandwidth required
  • With no dc component, ac coupling via transformer
    possible
  • Transfer function of a channel is worse near band
    edges
  • Clocking
  • Ease of determining beginning and end of each bit
    position
  • Signal interference and noise immunity
  • Performance in the presence of noise
  • Cost and complexity
  • The higher the signal rate to achieve a given
    data rate, the greater the cost

55
  • Sources Generate Sine Waves

Voltage
Time
Frequency
Voltage
Spectrum Analyzer
Oscilloscope
This is the ideal output most specs deal with
deviations from the ideal and adding modulation
to a sine wave
RF
Millimeter
Microwave
20-50 GHz
300 GHz
3-6 GHz
56
  • Modulation
  • ...Where the information resides

p
f
V A(t) sin2 f(t) (t)
AM, Pulse
PM
FM
q
V A(t) sin (t)
57
Why Carrier?
  • Effective radiation of EM waves requires antenna
    dimensions comparable with the wavelength
  • Antenna for 3 kHz would be 100 km long
  • Antenna for 3 GHz carrier is 10 cm long
  • Sharing the access to the telecommunication
    channel resources

58
Modulation Process
  • Modulation implies varying one or more
    characteristics (modulation parameters a1, a2, …
    an) of a carrier f in accordance with the
    information-bearing (modulating) baseband signal.
  • Sinusoidal waves, pulse train, square wave, etc.
    can be used as carriers

59
Modulation Demodulation
Carrier
Radio Channel
Carrier
Baseband Modulation
Synchronization/Detection/ Decision
Data out
Data in
60
  • Digital Modulation
  • ...signal characteristics to modify

Amplitude Frequency Phase
Both Amplitude and Phase
61
Basic Encoding Techniques
  • Digital data to analog signal
  • Amplitude-shift keying (ASK)
  • Amplitude difference of carrier frequency
  • Frequency-shift keying (FSK)
  • Frequency difference near carrier frequency
  • Phase-shift keying (PSK)
  • Phase of carrier signal shifted

62
Basic Encoding Techniques
63
Amplitude Shift Keying (ASK)
Baseband Data
1
0
1
0
0
ASK modulated signal
Acos(?t)
Acos(?t)
  • Pulse shaping can be employed to remove spectral
    spreading
  • ASK demonstrates poor performance, as it is
    heavily affected by noise, fading, and
    interference

64
Amplitude-Shift Keying
  • One binary digit represented by presence of
    carrier, at constant amplitude
  • Other binary digit represented by absence of
    carrier
  • where the carrier signal is Acos(2pfct)

65
Amplitude-Shift Keying
  • Susceptible to sudden gain changes
  • Inefficient modulation technique
  • On voice-grade lines, used up to 1200 bps
  • Used to transmit digital data over optical fiber

66
Frequency Shift Keying (FSK)
Baseband Data
1
0
1
0
BFSK modulated signal
f0
f0
f1
f1
where f0 Acos(?c-??)t and f1 Acos(?c??)t
  • Example The ITU-T V.21 modem standard uses FSK
  • FSK can be expanded to a M-ary scheme, employing
    multiple frequencies as different states

67
Binary Frequency-Shift Keying (BFSK)
  • Two binary digits represented by two different
    frequencies near the carrier frequency
  • where f1 and f2 are offset from carrier frequency
    fc by equal but opposite amounts

68
Binary Frequency-Shift Keying (BFSK)
  • Less susceptible to error than ASK
  • On voice-grade lines, used up to 1200bps
  • Used for high-frequency (3 to 30 MHz) radio
    transmission
  • Can be used at higher frequencies on LANs that
    use coaxial cable

69
Multiple Frequency-Shift Keying (MFSK)
  • More than two frequencies are used
  • More bandwidth efficient but more susceptible to
    error
  • f i f c (2i 1 M)f d
  • f c the carrier frequency
  • f d the difference frequency
  • M number of different signal elements 2 L
  • L number of bits per signal element

70
Multiple Frequency-Shift Keying (MFSK)
  • To match data rate of input bit stream, each
    output signal element is held for
  • TsLT seconds
  • where T is the bit period (data rate 1/T)
  • So, one signal element encodes L bits

71
Multiple Frequency-Shift Keying (MFSK)
  • Total bandwidth required
  • 2Mfd
  • Minimum frequency separation required 2fd1/Ts
  • Therefore, modulator requires a bandwidth of
  • Wd2L/LTM/Ts

72
Multiple Frequency-Shift Keying (MFSK)
73
Phase Shift Keying (PSK)
Baseband Data
1
0
1
0
BPSK modulated signal
s0
s0
s1
s1
where s0 -Acos(?ct) and s1 Acos(?ct)
  • Major drawback rapid amplitude change between
    symbols due to phase discontinuity, which
    requires infinite bandwidth. Binary Phase Shift
    Keying (BPSK) demonstrates better performance
    than ASK and BFSK
  • BPSK can be expanded to a M-ary scheme, employing
    multiple phases and amplitudes as different states

74
Phase-Shift Keying (PSK)
  • Two-level PSK (BPSK)
  • Uses two phases to represent binary digits

75
Differential Modulation
  • In the transmitter, each symbol is modulated
    relative to the previous symbol and modulating
    signal, for instance in BPSK 0 no change, 1
    1800
  • In the receiver, the current symbol is
    demodulated using the previous symbol as a
    reference. The previous symbol serves as an
    estimate of the channel. A no-change condition
    causes the modulated signal to remain at the same
    0 or 1 state of the previous symbol.

76
DPSK
  • Differential modulation is theoretically 3dB
    poorer than coherent. This is because the
    differential system has 2 sources of error a
    corrupted symbol, and a corrupted reference (the
    previous symbol)
  • DPSK Differential phase-shift keying In the
    transmitter, each symbol is modulated relative to
    (a) the phase of the immediately preceding signal
    element and (b) the data being transmitted.

77
Example BPSK Constellation Diagram
Q
I
?Eb
-?Eb
Constellation diagram
78
Phase-Shift Keying (PSK)
  • Differential PSK (DPSK)
  • Phase shift with reference to previous bit
  • Binary 0 signal burst of same phase as previous
    signal burst
  • Binary 1 signal burst of opposite phase to
    previous signal burst

79
QPSK
  • Quadrature Phase Shift Keying (QPSK) can be
    interpreted as two independent BPSK systems (one
    on the I-channel and one on Q), and thus the same
    performance but twice the bandwidth efficiency
  • Large envelope variations occur due to abrupt
    phase transitions, thus requiring linear
    amplification

80
QPSK Constellation Diagram
Q
Q
I
I
Carrier phases 0, ?/2, ?, 3?/2
Carrier phases ?/4, 3?/4, 5?/4, 7?/4
  • Quadrature Phase Shift Keying has twice the
    bandwidth efficiency of BPSK since 2 bits are
    transmitted in a single modulation symbol

81
Types of QPSK
Q
I
Conventional QPSK
?/4 QPSK
Offset QPSK
  • Conventional QPSK has transitions through zero
    (i.e. 1800 phase transition). Highly linear
    amplifiers required.
  • In Offset QPSK, the phase transitions are limited
    to 900, the transitions on the I and Q channels
    are staggered.
  • In ?/4 QPSK the set of constellation points are
    toggled each symbol, so transitions through zero
    cannot occur. This scheme produces the lowest
    envelope variations.
  • All QPSK schemes require linear power amplifiers

82
Phase-Shift Keying (PSK)
  • Four-level PSK (QPSK)
  • Each element represents more than one bit

83
Phase-Shift Keying (PSK)
  • Multilevel PSK
  • Using multiple phase angles with each angle
    having more than one amplitude, multiple signals
    elements can be achieved
  • D modulation rate, baud
  • R data rate, bps
  • M number of different signal elements 2L
  • L number of bits per signal element

84
Performance
  • Bandwidth of modulated signal (BT)
  • ASK, PSK BT(1r)R
  • FSK BT2DF(1r)R
  • R bit rate
  • 0 lt r lt 1 related to how signal is filtered
  • DF f2-fcfc-f1

85
Performance
  • Bandwidth of modulated signal (BT)
  • MPSK
  • MFSK
  • L number of bits encoded per signal element
  • M number of different signal elements

86
Quadrature Amplitude Modulation
  • QAM is a combination of ASK and PSK
  • Two different signals sent simultaneously on the
    same carrier frequency

87
Multi-level (M-ary) Phase and Amplitude Modulation
16 QAM
16 APSK
16 PSK
  • Amplitude and phase shift keying can be combined
    to transmit several bits per symbol.
  • Often referred to as linear as they require
    linear amplification.
  • More bandwidth-efficient, but more susceptible to
    noise.
  • For M4, 16QAM has the largest distance between
    points, but requires very linear amplification.
    16PSK has less stringent linearity requirements,
    but has less spacing between constellation
    points, and is therefore more affected by noise.

88
M256 M128 M64 M32 M16 M4

89
BER for Various Encoding Schemes
90
BER for MFSK and MPSK
91
Comparison of Modulation Types
92
Spectral Efficiencies - Examples
  • GSM Digital Cellular
  • Data Rate 270kb/s Bandwidth 200kHz
  • Bandwidth efficiency 270/200 1.35bits/sec/Hz
  • IS North American Digital Cellular
  • Data Rate 48kb/s Bandwidth 30kHz
  • Bandwidth efficiency 48/30 1.6bits/sec/Hz

93
Reasons for Analog Modulation
  • Modulation of digital signals
  • When only analog transmission facilities are
    available, digital to analog conversion required
  • Modulation of analog signals
  • A higher frequency may be needed for effective
    transmission
  • Modulation permits frequency division multiplexing

94
Basic Encoding Techniques
  • Analog data to analog signal
  • Amplitude modulation (AM)
  • Angle modulation
  • Frequency modulation (FM)
  • Phase modulation (PM)

95
Amplitude Modulation
  • Amplitude Modulation
  • cos2?fct carrier
  • x(t) input signal
  • na modulation index
  • Ratio of amplitude of input signal to carrier
  • a.k.a double sideband transmitted carrier (DSBTC)

96
  • Modulation Analog
  • Amplitude Modulation

Carrier
Important Signal Generator Specs for Amplitude
Modulation
Voltage
Time
  • Modulation frequency
  • Linear AM
  • Log AM
  • Depth of modulation (Mod Index)

Modulation
97
Spectrum of AM signal
98
Amplitude Modulation
  • Transmitted power
  • Pt total transmitted power in s(t)
  • Pc transmitted power in carrier

99
Single Sideband (SSB)
  • Variant of AM is single sideband (SSB)
  • Sends only one sideband
  • Eliminates other sideband and carrier
  • Advantages
  • Only half the bandwidth is required
  • Less power is required
  • Disadvantages
  • Suppressed carrier cant be used for
    synchronization purposes

100
Angle Modulation
  • Angle modulation
  • Phase modulation
  • Phase is proportional to modulating signal
  • np phase modulation index

101
  • Modulation Analog
  • Frequency Modulation

p
b
V A sin2 f t m(t)
c
  • Frequency Deviation
  • Modulation Frequency
  • dcFM
  • Accuracy
  • Resolution

Voltage
Time
102
  • Modulation Analog
  • Phase Modulation

b
c
b Df
peak
Voltage
Time
103
Angle Modulation
  • Frequency modulation
  • Derivative of the phase is proportional to
    modulating signal
  • nf frequency modulation index

104
Angle Modulation
  • Compared to AM, FM and PM result in a signal
    whose bandwidth
  • is also centered at fc
  • but has a magnitude that is much different
  • Angle modulation includes cos(? (t)) which
    produces a wide range of frequencies
  • Thus, FM and PM require greater bandwidth than AM

105
Angle Modulation
  • Carsons rule
  • where
  • The formula for FM becomes

106
Basic Encoding Techniques
  • Analog data to digital signal
  • Pulse code modulation (PCM)
  • Delta modulation (DM)

107
Analog Data to Digital Signal
  • Once analog data have been converted to digital
    signals, the digital data
  • can be transmitted using NRZ-L
  • can be encoded as a digital signal using a code
    other than NRZ-L
  • can be converted to an analog signal, using
    previously discussed techniques

108
Pulse Code Modulation
  • Based on the sampling theorem
  • Each analog sample is assigned a binary code
  • Analog samples are referred to as pulse amplitude
    modulation (PAM) samples
  • The digital signal consists of block of n bits,
    where each n-bit number is the amplitude of a PCM
    pulse

109
Pulse Code Modulation
110
Pulse Code Modulation
  • By quantizing the PAM pulse, original signal is
    only approximated
  • Leads to quantizing noise
  • Signal-to-noise ratio for quantizing noise
  • Thus, each additional bit increases SNR by 6 dB,
    or a factor of 4

111
Delta Modulation
  • Analog input is approximated by staircase
    function
  • Moves up or down by one quantization level (?) at
    each sampling interval
  • The bit stream approximates derivative of analog
    signal (rather than amplitude)
  • 1 is generated if function goes up
  • 0 otherwise

112
Delta Modulation
113
Delta Modulation
  • Two important parameters
  • Size of step assigned to each binary digit (?)
  • Sampling rate
  • Accuracy improved by increasing sampling rate
  • However, this increases the data rate
  • Advantage of DM over PCM is the simplicity of its
    implementation

114
Reasons for Growth of Digital Techniques
  • Growth in popularity of digital techniques for
    sending analog data
  • Repeaters are used instead of amplifiers
  • No additive noise
  • TDM is used instead of FDM
  • No intermodulation noise
  • Conversion to digital signaling allows use of
    more efficient digital switching techniques
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