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Elements of Physical Layer
!
Source Transmitter
Receiver Destination
DCE DTE
DTE DCE Transmission Medium
•Data Terminating Equipment (DTE)
•Data Circuit Terminating Equipment (DCE)
2
1
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Periodic signal
t where T is the period of the signal
• s(t) = Asin(2ft + ) !
• Parameters: # Amplitude: A
Frequency: Phase :
”
• A signal s(t) is periodic if and only if s(t+T)=s(t) for all
”
• Example: sine wave s(t)
f
4
2
Continuous signal vs discrete signal
amplitude
amplitude
Continuous signal
(e.g. spe”ech) => 类比
Analog signal
time
!
Discrete #signal (e.g. binary 1s and 0s)
=> Digita
time
l signal
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Varying the sine wave parameters
Time domain vs frequency domain
!
!
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Frequency components
S1 S2
$S3 = S1 + S2
f 2f 3f
7
傅立叶级数
Fourier Series
g(t)
To t
gt a0
n 1
a cosn t n0n
2 0 T0
8
4
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Spectrum of Signal (1)
G (f)
f1 f2f
Spectrum of Signal: Range of frequencies it has energy/power
带宽
con# tent
Absolute Bandwidth: Width of its spectrum ( = f2 – f1) %
10
5
离散光谱
Discrete Spectrum
G (f)
fo 2fo 3fo 4fo 5fo f
fo =1/To
!
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Spectrum of Signal (2)
Band‐limited Signal&: A signal with finite Absolute Bandwidth
All band‐limited signals expand from ‐ infinity to + infinity in time domain ‐‐> No real signal can be band‐limited
Effective Bandwidth: Area of spectrum where
”
MOST of the signal bandwidth is contained
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6
通道容量
Channel capacity (without noise)
Nyquist Formula
C = 2Wlog2M
where
W = bandwidth in hertz
M = number of discrete signal levels
C = theoretical maximum capacity in bits per second
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7
Elements of Physical Layer
Transmission
Source Transmitter Medium Receiver Destination
Impairments
损害
Channel
Signal Impairments
!
Impact:
‐ d e g r a d e t h e s i g n a l q’u a l i t y f o r a n a l o g s i g n a l s
‐ introduce errors in digital signals (i.e. 0 may be
changed to 1 and vic
Types:
‐ signal a”ttenuation ‐ delaydistortion失真 ‐ noise ”
e‐versa)
衰减
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8
衰减
Attenuation
振幅 (
The amplitude of signal decreases with distance
over any transmission medium
repeaters and amplifiers are used to restore the signal to its original level
Attenuation is an increasing function of frequency
!
Signal impairments
Fact:
As a signal propagates along a transmission path there is
传播
loss, attenuation of signal strength.
”
transmitter
Solution: ” 补偿
receiver
Tocompensatetheattenuation, wecanusedevices inserted at various points to “boost” signal’s strength (amplifiers).
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Signal impairments
Pt
Attenuation is measured in dB
!
Pr
transmitter
receiver
Decibel (1)
• Gains and losses are expressed in decibels (dB)
• Definition: )
= power at destination
N
P
10log
= number of decibels
dB
10
r t
where: N dB
P
r
P
= power at source
log10 = logarithm base 10 (also noted log)
t
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Decibel (2)
• Important: the decibel is a measure of relative, not absolute difference.
• For example:
– alossfrom1000Wto500Wisalossof3dB.
!
– a loss from 10 mW to 5 mW is also a loss of 3 dB. • In other words, a loss of 3 dB halves the
”
”
strength; similarly, a gain of 3 dB doubles the
”
strength.
Amplifiers and Repeaters
放大
Amplifiers: Amplify the received signals (this includes useful signa&l plus noise).
Repeaters: Recover the digital information, and retransmit it.
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10
Amplifiers
amplifier
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Can be used with Analog and Digital Communication Systems
“&
22
transmitter
receiver
boosts the energy of the signal (also boosts the noise component)
21
Decibel (3)
• Useful to determine overall gain or loss in a system. This is done simply by adding or subtracting
amplifier
– gain of the amplifier is 30 dB
– loss of second portion of line is 40 dB
– theoveralllossis23dB(i.e.-13+30-40=-23dB)
station
station
if
#– loss on first portion of line is 13 dB
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Repeaters
Can be used with Digital Communication Systems r”epeater
transmitter
receiver
Function of Repeater
#
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Amplifiers
!
X
a X + N1
bX + (b/a) N1 +N2
cX + (c/a) N1 + (c/b) N2 + N3
X + (1/a) N1
X + (1/a) N1 +(1/b) N2
transmitter
receiver
Repeaters
!
X
bX + N2 a X + N1
cX + N3
X
transmitter
receiver
X
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Noise
!
• 4 categories
– therma*l noise – intermodulation noise
– crosstalk
– impulsive noise
Thermal Noise
搅动
• The amount of th!erma+l noise power in a BW of 1 Hz is given by:
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1-27
• Due to the thermal agitation of electrons in a conductor (uniformly distributed across the frequency spectrum)
N0 kT !
where:
– N0 = noise power density
– k = Boltzmann’ s
– T = temperature (oK)
28
ant = 1.3803 x E-23 J/oK
const
1-28
1
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x(t) y(t)
Output
!
Input
1-30
30
2
互调
• It is produced when different frequencies are passed through’the same non-linear
device (e.g. non-lin
Effect:
Inter-modulation Noise (1)
ear amplifier)
produces signals at frequencies that are the multiple, sum or dif’ference of the
frequencies the orig
inal signal contains.
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3
Inter-modulation Noise: Example
x(t) y(t)
y(t) = x(t) + a {x(t)}2
x(t) = cos(2f1t) + cos(2f2t)
f1 f2
f1 f2 2f1 2f2 f1+f2
1-31
y(t) = cos(2f1t) + cos(2f2t) + (a/2) + (a/2) {cos(2f1t) + cos(2f2t)}+
a {cos(2 f1+f2 ] t) + cos(2 f1-f2 ] t) }
f2-f1
串话
Crosstalk
• Unwanted coupling between signal paths
• Example: more than one conversation can be
heard.
,
!
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Power Spectr”al Density of Additive White Gaussian Noise (AWGN)
No/2
!
f
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4
浮躁
• Non-continuous no’ise consisting of irregular pulses or noise spikes of short duration and
Impulsive Noise
of relatively high amplitude.
Causes:
• external electromagnetic di
faults in the communication system
振幅
1-33
骚乱
sturbances and
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5
Delay distortion
传播
失真
The speed of propagation of a sinusoidal signal along a transmission line varies with the frequency
-!
The effect of delay distortion tends to increase with the size of the signal bandwidth (=> it increases with an increase in transmission ra”te)
1-35
正弦曲线
For a signal composed of more than one frequencies, the signal frequency components arrive at the receiver with different delays
from each other
This distorts the signal
the pulses become distorted, spread in time and can spill over to neighboring pulses, making their detection difficult
)!
符号间干扰 翻译
(Intersymbol Interference) => incorrect interpretation of the
received signal
大气 吸收
Atmospheric absorption
Strength of signal falls ‘off because the atmosphere absorbs some of its energy
衰减
Attenuation and delay are greater at higher frequencies, causing distortion
1.5 1 0.5
Signal 1
Signal 2
1.5
Signal 1
Signal 2
Sum
1-36
Sum 1
0.5
-0.5 -1 -1.5
-0.5 -1 -1.5
00 0 5 10 15
0 5 10 15
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Intersymbol Interference (1)
Intersymbol interference (ISI) occurs when a pulse spreads out in such a way that it interfe)res with adjacent at the sample instant.
Example: assume polar NRZ line code. The channel outputs are shown as “smeared” (width Tb becomes 2Tb) pulses (spreading due to bandlimited channel)
1-37
Intersymbol Interference (2)
Impact of ISI on received signal of binary communication system
!
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Illustration of knife-edge diffraction
!
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Causes of Impairments
Scattering – occurs when incoming signal hits an “‘!
39
Reflection – occurs when signal encounters a surface
that is large relative to the wavelength of the signal.
衍射
坚不可摧
Diffraction – occurs at the edge of an impenetrable
wave.
散射
body that is large compared to wavelength of radio
object whose size is in the order of the wavelength of the signal or less.
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Multipath Propagation
障碍
Obstacles reflect signals so that multiple copies of a signal may arrive at the receiver at different times, and therefore with different phases.
.%
破坏性的
If phases add destructively the signal level declines, and vice versa.
When the transmitter or the receiver moves even short distances (of the order of , which is a few centimeters for the frequencies used) the signal amplitude can vary
greatly.
厘米
Also produces ISI.
1-41
ISI due to Multipath
!
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Communication System
Transmission Transmitter Medium Receiver
Impairments
Channel
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9
Solution: Use of Equalizers
Transmitter
Transmission Medium
Impairments
Transmission Medium
Impairments
Receiver
Equalizer
均衡器
Channel
Transmitter
Equalizer
Receiver
1-44
Channel
!
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Adaptive Equalization
补偿
Used to compensate the distortion introduced by the channel
It basically tries to reverse the unequal response of the channel,
#
both in amplitude and phase, to the transmitted signal
the frequency components of
!’
It is commonly adaptive in the sense that the channel response
is periodically estimated and the equalizer adapts accordingly
战斗
It is useful to combat intersymbol interference
复杂的
It involves sophisticated digital signal processing algorithms
1-45
Channel capacity without noise
Nyquist Form/ula ‐ C = 2Wlog2M
where
‐ W = bandwidth in hertz
‐ M = number of discrete signal levels ‐ C = capacity in bits per second
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10
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Channel capacity with noise
Shannon‐Hartley f”ormula ‐ C = W log2(1+S/N)
!
where
0 ‐ W = bandwidth in hertz
‐ S/N = signal‐to‐noise ratio
‐ C = maximum theoretical capacity in bits per second
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类比
• Digital technology
– VLSI product became “low cost”
Digital vs. Analog Transmission (1)
”
• Data integrity
– Example: the use o”f repeaters guarantees the integrity of the data being transmitted
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Digital vs. Analog Transmission (2)
利用率
• Capacity utilization
– links can be shared (multiplexed)effectively
!””
多路复用
• Security and privacy
– encryption techniques can be applied easily to digital data
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Bit rate and baud rate
Bit rate:
number of bits transmitted per second
Baud rate:
number of signal changes per second
Relation
bit rate = baud rate * n
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!
n = numberofbitsperchange
!
where
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Transmission media
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Transmission media
引导的
Guided Transmission media
twist1ed pair 同轴电缆
coaxial Cable 光纤
optical Fiber
Wireless transmission micro1wave
radio 红外线
infrared and millimeter waves
light‐wave transmission
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双绞线
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Twisted pair (1) Conductor
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导体
• Widely used in the
W-ire telephone network 电介质
dielectric
Foil shield braid shield
jacked
箔盾
编织屏蔽顶
• Used for either analog and digital transmission
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Twisted pair (2)
Attenuation very strong with frequency
analog: amplifiers every 5‐6 km
digital: repeaters every 2‐3 km Low noise immunity
crosstalk is a problem
poor channel characteristics
Easy to install, repair, .. Low cost
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Coaxial cable (2)
Attenuation linear with frequency
Better noise immunity # Error rate:
baseband: 10‐7 broadband:10‐9
~ 1 Km Baseband Cable =?‐2 Gbps
~100 km Broadband cable => 300‐450 MHz Moderate cost
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Coaxial cable (1)
copper core in
braided outer
基带同轴电缆
铜芯
• Baseband coaxial cable • one channel
编织外导体
宽频 (
• Broadband coaxial cable
commonly used for analog transmission
conductor
protective plastic covering
TV, CD-quality audio.
• multiple
channels, e.g. analog dual cable system
模拟电视
• two types:
双
single cab”le system
1-55
”
绝缘
commonly used for digital
sulating material
transmission
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Optical Fibers (cont’d)
Jacket
Core
Cladding
• Idea: refraction principle
“#
light at less than critical angle is
absorbed in jacket
Angle of incidence
Angle of reflection
被困
• Utilization: light trapped by total internal reflection when…
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16
Optical Fiber
Physical Description
an optical fiber is a thin (2 to 125 m), flexible medium
.”
capable of conducting an optical ray
an optical fiber cable has a cylindrical shape and consists
of three concentric sections: the core, the cladding and
覆层
the jacket.
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Fiber optics(3)
Attenuation very low
2 High noise immunity
Error rate: 10‐15
~100 km of fiber => ~2 Gbps
Unfamiliar technology: high skills required
Lightweight
Expensive
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17
Fiber optics (2)
Three components: light1source (LED or
transmission medium (fiber)
dete
Two major t1ypes of fiber
multimode fiber (largely used)
single mode fiber (expensive but can be used for long distances)
1-59
光电二极管
ctor (photodiode)
激光二极管
laser diode)
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Optical Fiber (cont’d)
# Subscriber loop
fibers running from the central exchange to the subscriber
Local Area Networks
networks linking 100’s and even 1000’s of workstations Ethernet, ATM‐LAN,…
Applications (cont’d) 订户
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Optical Fiber (cont’d)
Applications ⻓途
Long‐Haul trunks
Rural‐exchange trunks +!
25 to 100 miles
less than 5,000 voice channels
average length 900 miles
20,000 to 60,000 voice channels 大都市
Metropolitan networks
average length 7.8 miles
100,000 voice channels 农村交流干线
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Optical Fiber (cont’d)
波分复用
Wavelength Division Multiplexing (WDM)
+ Send more than one wavelengths through the same fiber
Allows re‐use of fiber
1-63
Spectrum Allocation
1-64
有价值 商品
Spectrum is valuable commodity; needs to be shared
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Spectrum
”
Allocation
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Spectrum Allocation
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Spectrum Allocation
Microwave frequencies: range between 109 Hz (1 GHz) to 1000 GHz. Respective wavelengths: 30cm to 0.03 cm.
1-67
Narrow Wideband#Broadband
10 kbps
Fixed Wireless Access
Wireless LAN
Direct to Home Satellite
100 kbps 1 Mbps 10 Mbps 100 Mbps
,
1Gbps
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MSAT
2.5 G
3G
2G
68
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The m”ost valuable segment
Not Enough Bandwidth Poor Radio Coverage
2 GHz
Frequency
VHF 300 MHz UHF 3 GHz SHF 30 GHz EHF
Cellular/PCS/3G WLAN Fixed Wireless
Satellite
1-69
Licensed & Unlicensed Bands
Licensed
800-900 MHz 1800-1900 GHz 2 GHz
5 GHz
28 GHz
Cellular/PCS 3G/4G WiMax
Unlicensed
915 MHz 2.4 GHz
5 GHz
60 GHz ??
Wireless LAN ITS
IMS Applications
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Licensed
TACS/NMT/AMPS/TDMA 400 MHz AMPS/CDMA/TDMA 800 MHz GSM/TACS/NMT 850 MHz Subscriber Radio 1.4 GHz GSM/DECT 1.8 GHz
PCS/PHS/3G 1.9 GHz MMDS 2.5 GHz Point to multi-point 10.5 GHz LMDS 24/26/28/32/40 GHz
%
900 MHz General Application
60 GHz ??? New
Unlicensed
2.4 GHz IMS (802.11b/g/n, Bluetooth)
5 GHz UNII (802.11a), DSRC (802.11p)
1-71
Employed Frequencies
Wireless channel’s behaviour is dependent on the frequency band of the signal.
-3
Frequency band of operation depends on the 天线
availability of spectrum, antenna characteristics, propagation behavior, and technological preferences.
Licensed wireless systems operate at 150 MHz, 450 MHz, 800 MHz, 2 GHz, 28 GHz.
Unlicensed systems at 9400 MHz, 2.4 GHz, 5 GHz, [3.1 GHz to 10.3 GHz: UWB], 57 GHz t0 64 GHz, optical frequencies.
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24
无线电
Radio transmission
Radio waves
easy to generate, can trave’l long distances, penetrate buildings at 全向的
lower part of spectrum, omnidirectional (all directions from the source)
widely used for indoor and outdoor communication Low noise immunity
-!
interferencefromelectricalequipment multipath interference
Co/adjacent‐channel interference
Attenuation increases with distance “fast”
1-73
穿透
2021-01-18
Radio transmission
Radio waves
easy to generate, can travel long distances, penetrate buildings at lower part of spectrum, omnidirectional (all directions from the source)
widely used for indoor and outdoor communication Low noise immunity
interferencefromelectricalequipment multipath interference
Co/adjacent‐channel interference
Attenuation increases with distance “fast”
1-73
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1
Wireless transmission patterns
directional ‐
the transmitting antenna’puts out a focused electromagnetic 光束
beam
地面微波
example: terrestrial microwave, satellite
全向的
omnidirectional
the transmitted signal spreads out in all directions example: broadcast radio
!
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天线
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Multipath Propagation
Obstacles reflect signals so that multiple copies of a signal may arrive at the receiver at different times, and therefore with different phases.
If phases add destructively the signal level declines, and vice versa.
When the transmitter or the receiver moves even short distances (of the order of , which is a few centimeters for the frequencies used) the signal amplitude can vary greatly.
Also produces ISI.
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2
Microwave transmission
Microwave transmission is widely used
long‐distance communication, cellular phones, TV distribution,
”
Microwave transmissi”ons can be made easier
etc. directional
repeaters are needed (for 100‐m high towers, repeaters can be spaced 80 km apart)
Higher signal to noise ratio 穿透
They do not penetra”te deep into buildings 衰退
Multipath fading effect can occur !
1-76
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Terrestrial Microwave
Physical Description 抛物面
5!
78
parabolic dish
~ 10 feet in diameter 视线
line‐of‐sight transmission
maximum distance between 2 antennas (in km):
where
h1 = height of antenna one h2 = height of antenna two K = 4/3 (adjustment factor)
1-77
Terrestrial Microwave (cont’d)
Applications
long‐haul communications
TV and voice communications
transmission in small regions (radius < 10 km)
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3
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Terrestrial Microwave (cont’d)
Transmission Characteristics
attenuation is the major source of loss
" 4d2 L10log dB
where d is the distance and is the wavelength, expressed in the same units.
IMPORTANT:
loss varies as the square of the distance
!
1-79
80
4
Satellite Microwave
Physical Description
a satellite is a microwave'
中继
transmitters/receivers.
饶轨道的
a single orbiting satellite will operate on a number of frequency bands, called transponder channels.
Geostationary ‐ a sate4llite is required to remain stationary with respect to its position over the earth. This match occurs at ~36,000 km
relay station used to link 2 or more ground‐based microwave
Low Earth Orbit (LEO), Medium Earth Orbit (MEO) systems (satellite phones)
1-80
81 " Satellite Microwave (cont’d)
Applications:
television distribution (broadcast)
long‐distance telephone
private business networks (VSAT/USAT networks)
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Satellite Microwave (cont’d)
receives on one frequency (uplink)
transmits on another frequency (downlink)
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5
Satellite Microwave (cont’d)
Transmission characteristics
4 G H z / 6 G H*z , 1 2 G H z / 1 4 G H z , 1 5 G H z / 1 7 G H z
2021-01-18
below 1 GHz there is significant noise from natural sources, including galactic solar and atmospheric noise.
20 GHz to 30 GHz
Personal Satellite Communication Systems High‐Definition TV (around 23 GHz)
83
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Infrared, Optical and millimeter waves
They are widely used for short‐range communication
remote control on TV, VCRs, etc.
indoor wireless LANs Characteristics
#
relatively directional, cheap, easy to build
do not pass‐through solid object (e.g. wall)
mmWave and Visual Optical proposed for use in 5G Access networks
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Free Space Lightwave transmission
# An application: connect two LANs in two buildings via lasers
• high bandwidth, very low cost and easy to install • Characteristics
2 • laser beams cannot penetrate rain or thick fog • laser beams work well on sunny days, but ....
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7
Digital Data/Digital Signals
Modulation rate is t4he rate at which signal level is changed (rate at which signal elements are generated)
Digital signaling rate or just data rate of a signal is the rate, in bits per seconds, that data are transmitted.
Duration of length of a bit is the amount of time it 发出
takes for the transmitter to emit a bit. For a data date R, the bit duration, tB, is 1/R.
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Data Rate vs. Modulation Rate
data rate:
"R = 1/tB
wheretB isthebitduration
example: In the case of code Manchester, the maximum modulation rate, Dmax, is
2R
1-87
Data Encoding
1-88
88
8
89 " Square Pulse
90
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9
基带
Encoding onto a digital signal located at baseband !
Data Encoding - Baseband Transmission
x(t)
g(t)
digital x(t)
g(t)
or analog
digital
Encoder
Decoder
t
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Digital Data/Digital Signals
Definition
Bipolar signalling: one logic state is represented by a positive voltage level an the other by a negative voltage level
v(t)
t
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10
Digital Data/Digital Signals
Unipolar signal: all the signal elements have the same algebraic sign, all positive or all negative
v(t)
Definition 单极
"
!
t
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Digital Signal Encoding Schemes
Five evaluation factors:
1) signal spectrum &
lack of high‐frequency components means that less bandwidth is required for transmission
2) clocking
every bit being received needs to be identified
1-93
Digital Signal Encoding Schemes
3) error detection
useful to be able to detect errors at the physical level
4) signal interference and noise immunity 5) cost and complexit"y
!
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94
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Some encoding techniques
NRZ # NRZI Bipolar-AMI
Pseudoternary
Manchester
Differential Manchester
1-95
Digital Signal Encoding Schemes
3 main te(chniques:
Nonreturn to Zero (NRZ)
Multilevel Binary Biphase
1-96
96
12
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Digital Signal Encoding Schemes
Non‐return to zero (NRZ)
maintains a constant value for the duration of a bit
)!
time.
example 1: NRZ‐L (nonreturn‐to‐zero‐level)
during a bit interval there is no transition
two different levels for the two binary digits
binary 0 ‐ negative voltage binary 1 ‐ positive voltage
1-97
Some encoding techniques
NRZ
NRZI
Bipolar-AMI Pseudoternary Manchester
Differential Manchester
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98
13
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Digital Signal Encoding Schemes
Non‐return to zero (NRZ)
example 2: NRZI (non‐return to zero, invert on ones) !
the data is encoded as the presence or absence of a signal transition 6 at the beginning of the bit time.
this type is called differential encoding (the signal is decoded by comparing the polarity of adjacent signal elements)
1-99
Some encoding techniques
NRZ
NRZI
Bipolar-AMI Pseudoternary Manchester
Differential Manchester
1-100
100
14
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Digital Signal Encoding Schemes
Multilevel Binary
uses more than 2 signal levels
example 1:
b i p o l a r A M I ‐ t h e b i n a r y 1 )p u l s! e s a l t e r n a t e i n p o l a r i t y
example 2:
Pseudo‐ternary ‐ the binar
"
y0
pulses alternate in polarity
1-102
102
15
Digital Signal Encoding Schemes
Non‐return to zero (NRZ) Advantage:
"
make efficient use of bandwidth
Drawback: "
直流电 同步化
presence of a dc component and lack of synchronization
(used in digital magnetic recording)
"
1-101
103
2021-01-18
Some encoding techniques
NRZ
NRZI
Bipolar-AMI Pseudoternary Manchester
Differential Manchester
1-103
Digital Signal Encoding Schemes
Multilevel Binary adva)ntages:
1) since signal alternate in voltage, there is no net DC component
2) pulse alternation property provides a simple means of error detection
draw1back:
1) loss of synchronization
bipolar AMI ‐ if a long string of 0’s occurs pseudoternary ‐ if a long string of 1’s occurs
1-104
104
16
105
2021-01-18
Digital Signal Encoding Schemes
双相
Biphase:
there is a transition at(the middle of each bit period Example 1:
Manchester ‐ the mid‐bit transition serves as a clocking mechanism and also for transporting data.
Biphase: example 2:
Differential Mancheste%r ‐ mid‐bit transition provides clocking
binary 0 ‐ transition at the beginning of a bit binary 1 ‐ no transition at the beginning of a bit
1-105
Some encoding techniques
NRZ
NRZI
Bipolar-AMI Pseudoternary Manchester
Differential Manchester
1-106
106
17
107
2021-01-18
Digital Signal Encoding Schemes
Normalized Signal Transition Rate of Various Digital Signal
"
Minimum
0 (all o's or 1's) 0 (all 0's)
0 (all 0's)
0 (all 1's)
1.0 (1010...) 1.0 (all 1's)
Encoding Schemes
NRZ-L
NRZI
Binary-AMI Pseudoternary Manchester
Differential Manchester
101010... Maximum
1.0 1.0
0.5 1.0 (all 1's)
1.0 1.0
1.0 1.0
1.0 2.0 (all o's or 1's) 1.5 2.0 (all 0's)
1-108
108
18
Digital Signal Encoding Scheme
Biphase
ad#vantages:
a) synchronization ‐ predictable transition permit to the receiver to resynchronize.
b) absence of expected transition can be used to detect errors
c) no DC component
drawback
a) higher modulation rate than NRZ => higher BW
调制
1-107
2021-01-18
Evaluation
NRZ
lack of synchronization ca/pability; widely used for digital magnetic
recording but not for signal transmission Multilevel binary
long string of 0s (Bipolar‐AMI) and 1s (pseudoternary) cause synchronization problems (scrambling techniques are used to address this deficiency);
!7′
Biphase
it is easy to detect isolated errors; it is not as efficient as NRZ (three signal levels are used instead of 2 levels used in NRZ)
no synchronization problems; good error detection; more bandwidth is needed (as many as two transitions per bit time)
1-109
109
19
2021-01-19
Data Encoding
Modulation onto an analog signal
S(f)
m(t) digital or analog
fc
s(t) analog
m(t)
Modulator
Demodulator
fc
f
1-110
110 & Periodic signal
• A signal s(t) is periodic if and only if s(t+T)=s(t) for all t where T is the period of the signal
• Example: sine wave s(t)
• s(t) = Asin(2ft + ) • Parameters:
Amplitude: A
Frequency: Phase :
111
f
1-111
1
112
2021-01-19
Modulation Techniques
Amplitude modulation
-8
S(t)=[1+naX(t)]cos2fct, where cos2fct is the carrier and x(t) is the input signal, na is the modulation index (ratio of the amplitude of x(t) to the carrier)
simplest form of modulation Angle modulation
S(t)=Acos[2fct + (t)]
phase modulation : (t)=npm(t) where np is the phase modulation
index
frequency modulation: d(t)/dt=n modulation index
f
m(t) where n
f is the frequency
1-112
Modulation of a sine-wave carrier by a sine-wave signal
!
1-113
113
2
114
2021-01-19
Modulation of digital signal
Amplitude-shift keying
Frequency-shift keying
Phase-shift keying
1-114
Multileve”l modulation
Quadrature phase-shift keying Quadrature amplitude modulation (R=2*D) (R=4*D)
1-115
115
3
116
2021-01-19
Multiplexing
1-116
Spectrum Allocation
Spectrum is valuable commodity; needs to be shared
1-117
117
4
118
2021-01-19
多路复用
Multiplexing
• Objective: share a link between ‘several users (i.e., telephone companies transmit many conversations over a single physical
trunk)
• Two basic techniques: FDM (Frequency Division Multiplexing) and TDM (Time Division Multiplexing)
1-118
Frequency-d”ivision multiplexing
1-119
119
5
120
2021-01-19
m(t) digital or analog
FDM
s(t) analog
fc S(f)
fc !
m(t)
Modulator
Demodulator
ff f c
1-120
Frequency Division Multiple Access FDMA
bandwidth BW
guard band
!
f
1-121
121
6
FDM 3 System
Overview
1-122
2021-01-19
122
Crosstalk &
guard bands (separating two “adjacent” channels) should be
Frequency‐division multiplexing: problems
carefully chosen
a voice signal has an effective bandwidth of 3.1 kHz; a channel of 4 kHz is adequate to avoid crosstalk in analog voice transmissions
Intermodulation noise
Channels
More challenging and expensive RF technology (narrow
filters; large ) Inefficiency
ar effects of amplifiers on a signal in one channel can produce undesirable frequency components in other
nonline
123
Channels might be allocated to sources that are not using them all the time
1-123
7
124
2021-01-19
Consider the following samples of 3 users’ data to be multiple accessed
User 1 data
User 2 data User 3 data
1-124
Waveforms of Users 1,2 & 3 after FDM
%
1-125
125
8
126
2021-01-19
Input to the amplifier after 3 FDM signals are added
!
1-126
x(t) y(t)
Output
!
Input
1-127
127
9
2021-01-19
Nonlinear Effects in FDM
Received signal is sum of multiple carriers.
#Receiver power amplifiers are operated nonlinearly (near saturation) for maximum efficiency.
The nonlinearities cause intermodulation (IM) frequencies to appear in the amplifier output.
IM components can interfere with other channels in the FDMA system.
1-128
128
Inter-modulation Noise
x(t) y(t)
y(t) = x(t) + a {x(t)}2
x(t) = cos(2f1t) + cos(2f2t)
f1 f2
f1 f22f1 2f2 f1+f2
1-129
y(t) = cos(2f1t) + cos(2f2t)
a + (a/2) {cos(2f1t) + cos(2f2t) + a {cos(2 f1+f2 ] t) + cos(2 f1-f2 ] t)
!
f2-f1
129
10
130
2021-01-19
Synchronous Time Division Multiplexing
!
1-130
Time Division Multiple Access
TDMA systems divide the radio spectrum into time slots.
Only one user can t&ransmit or receive during one time slot.
Usually, each user may occupy the channel once during a time frame, where one frame comprises N time slots.
1-131
131
11
132
2021-01-19
TDM System ” Overview
1-132
Time‐division multiplexing: problems
Fram-e synchronization
use an identifiable pattern of bits at the beginning of each frame
Pulse stuffing
If user does not have data, the assigned slot needs to be staffed with dummy
bit
Inefficiency
many of the time slots are wasted; slots are allocated to inputs even these input are not sending any data
High Pick Transmission power
1-133
133
12
,
134
2021-01-19
Statistical TDM
in Synch TDM many slots are wasted
Statistical TDM allocates time slots dynamically based
line data rate lower than aggregate input line rates -!
on demand
multiplexer scans input lines and collects data until frame full
may have problems during peak periods must buffer inputs
1-134
Statistical time-division multiplexing
!
1-135
135
13
136
2021-01-19
TDMA Systems
TDMA systems tra&nsmit data in a buffer and burst method.
The transmission is non‐continuous.
Unlike FDMA systems which can transmit analog signals, TDMA must transmit data and digital modulation must be used.
1-136
TDMA
1-137
137
14
138
2021-01-19
TDMA
Slot 1
Slot 2
Slot 3
Slot N
1 frame
…
t
Guard time
t
trail bits
sync. bits
info bits
Each slot requires overhead bits. More overhead reduces efficiency.
1-138
User 1,2&3 dat”a before and after TDMA 1-139
139
15
140
2021-01-19
TDMA Features
Only one carrier. N’o intermodulation.
Number of time slots per frame depends on bandwidth, desired date rate, modulation technique.
Receiver must syn&chronize to each time slot, thus more synchronization bits are required in TDMA compared to FDMA.
It is possible to allocate more than one time slot per frame – bandwidth on demand.
140
Spread Spectrum
Also known as Code Division Multiple Access (CDMA)
Important encoding meth’od for wireless
communications
Can be used with analog & digital signal formats
Users share both time & frequency domains; their signals overlap, occupying a wide bandwidth
The separation is achieved by assigning different codes
to each user.
”
1-141
141
16
142
2021-01-19
Spread Spectrum
Makes jamming and interception harder
Initially used for mil”itary communications
Two approaches, both in use: Frequency Hopping (FH)
Direct Seque #
nce (DS‐SS)
Cellular radio (IS‐95, CDMA2000,WCDMA)
Wireless LANs (IEEE 802.11 b, g) ”
1-142
Spread Spectrum: Advantages/Disadvantages
Resistive to interference, multipath fading #Easy Encryption
Easy traffic multiplexing of discontinuous sources
Allows “soft” hand‐offs
Synchronization imposes a challenge
1-143
143
17
2021-01-19
General Model of Spread Spectrum System
!
1-144
144
Pseudorandom Numbers
generated by a deterministic algorithm
not actually rando”m
but if algorithm good, results pass reasonable tests of randomness
starting from an initial seed
need to know algorithm4and seed to construct the
sequence
hence only receiver can decode signal
1-145
145
18
146
2021-01-19
Frequency Hopping Example
!
1-147
147
19
Frequency Hopping Spread Spectrum (FHSS)
signal is broadcast ov&er seemingly random series of frequencies
receiver hops between frequencies in sync with
transmitter
窃听者
eavesdroppers hear unintelligible blips
jamming on one frequency affects only a few bits
1-146
148
2021-01-19
FHSS (Transmitter)
1-148
Frequency H”opping Spread Spectrum System (Receiver)
1-149
149
20
150
2021-01-19
Slow and Fast FHSS
commonly use mult#iple FSK (MFSK)
have frequency shifted every Tc seconds duration of signal element is Ts seconds S l o w F H S S h a s T c ’T s
Fast FHSS has Tc < Ts
FHSS quite resistant to noise or jamming
fast FHSS is giving better performance
1-150
Slow MFSK FHSS
!
1-151
151
21
152
2021-01-19
Fast MFSK FHSS
!
1-152
Direct Sequence Spread Spectrum (DSSS)
each bit is represented'by multiple bits using a spreading code
this spreads signal across a wider frequency band has performance similar to FHSS
1-153
153
22
2021-01-19
Direct Sequence Spread Spectrum System
"
1-154
154
Data
PN-1
Data spread by PN-1 PN-2
Data despread by PN-2
Data despread by PN-1
1 1001010
1001010 1011100 1101001 1
1-155
!
155
23
2021-01-19
DSSS Example Using BPSK
!
1-156
156
CDMA Example
157
!
157
24
158
2021-01-19
CDMA for DSSS
!
1-158
Direct Sequence Spread Spectrum Example
! 1-159
159
25
160
2021-01-19
Approximate Spectrum of DSSS Signal
1-160
Power of DS"‐SS Signals at Tx when square pulses are used
noise level
t
t
f
f
1-161
161
26
2021-01-19
Do all systems fall into only one of 3 categories?"
f
t
f, t
Answer: In practice NO.
!
162
162
Examples of their use in wireless mobile communications systems
FDM#/FDMA
1st generation: analog cellular, AMPS (each channel was occupying 60 KHz
bandwidth)
2nd generation North American digital cellular radio; IS‐41
2nd generation North American digital cellular radio (improved); IS‐136.
Hybrid architecture. TDMA multiplexing 4 users in each 60 KHz AMPS channel.
TDM/TDMA
2nd and 2.5 generation European cellular radio, GSM/GPRS/EDGE
Bluetooth (FH‐SS), ZigBee (DS‐SS), Home RF (FH & DS‐SS)
CDMA
2nd generation: IS‐95 based North American cellular radio (DS‐SS). 3rd generation CDMA2000 and WCDMA (DS‐SS)
IEEE 802.11 WLAN (FH & DS‐SS) +!
163
1-163
27
1
2021-01-25
1-1
Outline
Synchronization
Asynchronous and synchronous transmission Error detection/correction techniques
Line Configuration
1-2
2
1
3
2021-01-25
Digital Data/Digital Signals
v(t)
v(t)
t
t
1-3
Sender
Receiver
TB
SYNCHRONIZATION
Ts
sampling at the centre of each bit time
1-4
!
4
2
5
2021-01-25
Some encoding techniques
NRZ
NRZI
Bipolar-AMI Pseudoternary Manchester
Differential Manchester
1-5
6
3
SYNCHRONIZATION
• loss of synchronizat'ion
– In practice TB and TS are not equal. The result
is that the timing of the receiver may slowly
相对于
drift relative to the received signal.
1-6
同步化
7
2021-01-25
SYNCHRONIZATION
unpredictable time
11111001
1-7
SYNCHRONIZATION
• loss of synchronization
– Solution:
• data is sent in bit sequences called frames
!
• the receiving clock is started at the beginning of
each bit sequence
!
– Question:
!
Since synchronization needs to be kept only for the duration of the frame, what is the length of the frame that will allow us to avoid loss of synchronization?
1-8
8
4
9
2021-01-25
ASYNCHRONOUS TRANSMISSION
• Timing or synchronization must only be maintained within e'ach character; the receiver has the opportunity to resynchronize at the beginning of each new character.
idle state
start bit
11111001
unpredictable time
stop bit
1-9
SYNCHRONIZATION
• loss of synchronization
example:
#– a frame consists of 11 bits
– assume that the synchronization at the start of the first bit is late at most 10% of TB
We must fulfill the following 2 conditions:
(1012)TS 0.1TB 11TB and (1012)TS 10TB
These are satisfied if: Ts TB 3.8% TB
1-10
10
5
11
2021-01-25
ASYNCHRONOUS TRANSMISSION
Timing requirements are modest. Sender and receiver are synchronized at the beginning of every character (8 bits if ASCII)
high overhead
overhead control_bits total _ bits
!
1-11
12
6
SYNCHRONOUS TRANSMISSION
In this mode, blocks of characters or bits are 前言
)!
transmitted. Each block begins with a preamble and
ends with a postamble 2 types:
character‐oriented
bit‐oriented
后同步码
1-12
13
2021-01-25
SYNCHRONOUS TRANSMISSION
Character‐oriented
the frame consists of a sequence of characters
!
SYN
SYN
one or more SYN characters
control characters
control characters
SYN is a unique bit pattern that"signals the receiver the beginning of a block
1-13
data characters
SYNCHRONOUS TRANSMISSION
character‐oriented ‐ 2 approaches
the receiver having detected the beginning of the block
#
reads the information till it finds the postamble
SYN
SYN
PREAMBLE
POSTAMBLE
the receiver having received the preamble, looks for extra
starts to look
for next preamble
1-14
"
information regarding the length of the frame
SYN
SYN
LEN
14
7
15
2021-01-25
SYNCHRONOUS TRANSMISSION
Bit oriented
In this mode, the fram"e is treated as a sequence of bits. Neither data nor control information is
interpreted in units of
x‐bit characters
a special bit pattern indicates the beginning of a frame "
the receiver looks for
the occurrence of the flag
FLAG
FLAG
control fields
!
control fields
1-15
Dealing with presence of errors
#• Detect presence of errors (error detection)
• Try to correct them (error correction)
• If no correction have the mechanism to request retransmission (use of Automatic Repeat Request)
1-16
16
8
17
2021-01-25
Random Errors
Anerro"roccurswhena&bitisalteredbetween transmission and reception
Random, statistically uniformly spread errors
occurrence of an error does not increase the probability that
other bits, close to the one in error, while be in error white noise is producing such errors
For low BER and frames of “reasonable” length, most framers would experience no error or 1 error at most.
example: BER = 10^{‐6} and length of frame=[1000bytes*8=] 8,ooo bits
probability of receiving a frame correctly = [1‐10^{‐ 6}]^{8,000}>0.992
probability of a frame having a single error=8,000*receiving a
[1‐10^{‐6}]^{7,999}* {8,000}=0.007873 correctly = 10^{‐6}*
frame
Probability of having more than 1 error<[ 1‐0.992‐ 0.007873=]0.000127
1-17
18
9
Burst Errors
Occurrence of an error'having occurred in the sequence, means bits preceding/following the one in error have higher probability than the average
bit error probability to be in error
簇
impulsive noise
“slow” fading/shadowing in wireless (relevance of bit rate to average time/distribution channel attenuation remains below certain level)
Error strings (clusters of errored bits closely
located in the seq&uence) form 减损
Some channel related impairments producing error bursts
effect greater at higher data rates
1-18
2021-01-25
1-19
19
10
20
2021-01-26
Error detection and control
Objective
detect and correct errors that occur in the transmission of frames
Types of errors
lost frame: a frame that the receiver does not receive (e.g., !
because starting of frame/clock extraction is not achieved due to excessive signal attenuation, increased levels of noise...)
damaged frame: a frame that the receiver receives, but some of its
bits are in error
"
!
1-20
Impact of error location
The frame consists of a sequence of characters
SYN
SYN
one or more SYN characters
control control characters data characters
characters
SYN is a unique bit pattern that signals the receiver the beginning of a block
1-21
21
1
22
2021-01-26
Error Detection
1-22
Error Correction/Detection Process
!
1-23
23
2
2021-01-26
How Error Correction & Detection Works 冗余
# Adds redundancy to transmitted message 尽管
Can deduce original despite some errors
24
Example: block error correction code
map k bit input onto an n bit codeword
each distinctly different
if get error assume codeword sent was closest to that received
means have reduced effective data rate
most of work concerning error correction & detection is making use of Galois field algebra (Boolean algebra ‐ mod_2 arithmetic – is a case of it)
1-24
Code rate & minimum distance
Code rate r = # of information bits in a block k # of total bits in a block n
The bandwidth expansion is 1 / r = n / k "
The energy per channel bit (Ec )is related to energy perinformationbit(Eb)throughEc rEb
Minimum distance (dmin ): Minimum number of
positions in which any 2 codewords differ.
"
"
1-25
25
3
2021-01-26
26
A simple block code: (7,4) Hamming Code
Message 0000 1000 0100 1100 0010 1010 0110 1110 0001 1001 0101 1101 0011 1011 0111 1111
Codeword 000 0000 110 1000 011 0100 101 1100 111 0010 001 1010 100 0110 010 1110 101 0001 011 1001 110 0101 000 1101 010 0011 100 1011 001 0111
111 1111
27
•Error detection w/o error correction?
•Error detection with error correction?
•Minimum Humming distance? •Error Correction capability?
27
4
Correctable and detectable errors
A block code can correct at least errors if dmin > = 2 +1 => if dmin=3, then =1. If there is only one error in the block,
7! d o n o t c o r r e s p o n d t o a c o d e w’o r d .
it can be corrected.
A block code can detect any error pattern if the received n bits
If there are errors in the n‐bits codeword, the existence of errors is detected with certainly if < dmin.
However, even when >=dmin, many of the corrupted blocks can still be detected.
“&
Out of the 2n possible n‐bit combinations, only 2k code can be generated, thus, there are 2n ‐ 2k= 2k (2(n‐k)‐1) pro
combinations.
Above statement applies when no error correction is used.
1-26
words
禁止
hibited
28
2021-01-26
A simple block code: (7,4) Hamming Code
Message
0000
1000
0100 1100 0010 1010 0110 1110 0001 1001 0101 1101 0011 1011 0111 1111
Codeword
000 0000
110 1000
011 0100 101 1100 111 0010 001 1010 100 0110 010 1110 101 0001 011 1001 110 0101 000 1101 010 0011 100 1011 001 0111 111 1111
•Error detection w/o error correction? 3
•Error detection with error correction? 0
•Minimum Humming distance? 3
•Error Correction capability? 1
1-28
“Popular” Error Detection Techniques
Parity Checks
+ Longitudinal redundancy checks (LRC)
Cyclic redundancy checks (CRC)
1-29
29
5
30
2021-01-26
Simple Error Detection Scheme
奇偶性
Parity check
Value of parity bit is such that character has even (even
parity) or odd (odd parity) number of ones
Even number of bit errors goes undetected !
1-30
Parity Checks
1-31
31
6
32
2021-01-26
Longitudinal Redundancy Checks
1-32
Longitudinal Redundancy Checks
1-33
33
7
34
2021-01-26
Longitudinal Redundancy Checks
1-34
Longitudinal Redundancy Checks
1-35
35
8
36
2021-01-26
Longitudinal Redundancy Checks
36
Longitudinal Redundancy Checks
1-37
37
9
38
2021-02-03
Cyclic Redundancy Check
Based on cyclic error‐correcting codes
For a block&of k bits the transmitter generates n bit sequence
n insert redundancy in the codeword
Transmitter transmits the k+n bits
Receiver uses error detection process to decide if there were errors in the received sequence or otherwise
Cyclic Codes
• “Cyclic code is a block code, where the circular shifts of each codeword gives another codeword that belongs to the code”.
-!
• “They are error-correcting codes having algebraic properties
that are convenient for efficient error correction & detection”.
39
1
2021-02-03
Fundamentals of CRC coding
CRC codes tr&eat bit strings as representations of polynomials with coefficients of 0 and 1 only (modulo 2 arithmetic)
11001↔1∗𝑋 1∗𝑋 0∗𝑋 0∗𝑋 1∗𝑋 𝑋 𝑋 1
Polynomial arithmetic is done modulo 2
subtraction and addition are similar to EXCLUSIVE OR
division is similar to the one in decimal except the subtraction is done modulo 2
Make sure you are familiar with mod2 arithmetic/algebra
40
41
1-41
2
2021-02-03
1-42
42
2 The sender and receiver agree upon “a generator polynomial”, G(x), in advance.
CRC: Basic Idea
The sender appends a checksum (corresponds to the n redundancy bits) to the end of the (only data) frame, represented by the M(x) polynomial, in a way that the polynomial T(x), representing the {data + checksum bits} frame, is divisible by G(x).
Upon receipt of the frame, the receiver (generates and) divides H(x) by G(x) using modulo 2 division.
H(x) is the polynomial corresponding to the received sequence.
if there is a remainder, there has been transmission error. 43
3
44
2021-02-03
How to compute the checksum
If n‐1 is the degree of G(x), then append n zero to the low order end of the frame; the resulting frame corresponds to the polynomial X n M (x).
DivideG(x)into XnM(x) usingmodulo2division.
D(X): divisor; R(X): remainder
𝑋𝑀𝑋 𝐺𝑋𝐷𝑋𝑅𝑋
11001↔1∗𝑋 1∗𝑋 0∗𝑋 0∗𝑋 1∗𝑋 𝑋 𝑋 1=M(X)
110010001∗𝑋 1∗𝑋 0∗𝑋 0∗𝑋 1∗𝑋 0∗𝑋 0∗𝑋 0∗𝑋 𝑋 𝑋 𝑋=𝑋 ∗𝑀𝑋
→𝐷 𝑋 ;𝑅𝑋
How to compute the checksum
Subtracttheremainderfrom XnM(x) using modulo 2 subtraction/addition.
The result is the checksumed frame’s polynomial , T(x).
𝑇𝑥 𝑋𝑀𝑋𝑅𝑋
The frame corresponding to T(x) is transmitted.
T(X) XnM(X)R(x)[D(X)G(X)R(x)]R(X)
T ( X ) G(X) divides
T ( X ) D( X ) G( X ) O G( X ) D( X )perfectly T(X)
(remainder = O) !
45
4
46
2021-02-03
CRC: an Example
• Frame: 1101011011
• Generator: 10011
CRC Error Detection
Let us assume that some t’ransmission errors occur Instead of receiving T(x), the receiver will receive
H(x)=T(x)E(x)
If there are k “1” bits in E(x), (it is most probable that) k
errorswill bedetected.
single‐bit error means E(X ) X m1, where 0 𝑚 𝑛 𝑘
bit errors have occurred
the receiver computes (T(x)
& E(x))/G(x)=E(x)/G(x) If G(x) contains two or more terms, (i.e. n>1) all single
47
5
48
2021-02-03
Singleerrorand G(x)=Xn H(X)T(X)E(X) where
reversed (0 𝑚 𝑛 𝑘; the larger the value of m is, the more !’
E(X ) O if no bit errors occur
E(X) X m1 if only the m-th bit of the [k+n]-bit long frame is
significant the location of the bit within the frame is) E(X) L(X)G(X) F(X)
H(X)T(X)E(X)T(X)L(X)G(X)F(X)
H ( X ) D( X ) L( X ) F ( X ) G(X) G(X)
Error will be detected if F(X ) O G ( X”)
For m-1>=n,L(X ) X mn1 and F(X ) O . Error is not detected. F(X) G(X)
For m-1
12 bits ==> 4096 paths (NNI)
VCI ‐ Virtual Path Identifier 16 bits ==> 65,536 channels
Each VPI holds a bundle of circuits.
These are per physical medium, regardless of speed or bandwidth.
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Payload Type Identifier
Used to identify the following cells:
Network generated cells
used for maintenance & control of network
used for call set‐up, loopbacks and keep alives
Customer generated cells user information
Cell Loss Priority
CLP in the header
CLP=0: Highpriority,lastlikelytobediscarded
CLP=1: Lowpriority,maybediscardedduringcongested intervals
CLP can be set:
by the terminal
by the ATM switch
CLP determines the class of service or service contract
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Virtual Connections
Permanent Virtual Circuits (PVC) network operator connects endpoints
Switched Virtual Circuits (SVC) can be switched like PSTN
Call set‐up routine
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Service Categories (1)
Quality of Service (QoS) are parameters that are set for end‐to‐end network performance
Cell Transfer Delay (CTD): delay between start & finish of cell
Peak to Peak Cell Delay Variation (CDV): difference between
maximum CTD and minimum CTD
Cell Loss Ratio (CLR): % of cells lost
Sustained Cell Rate (SCR): average rate of transmitted cells
Bust Tolerance (BT): maximum burst size at PCR
Maximum Burst Size (MBS): maximum No. of cells sent at PCR
Minimum Cell Rate (MCR)
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Service Categories (2)
ATM is divided into (5 + 1=) 6 service categories
Constant Bit Rate (CBR).
CTD & CDV are tightly constrained, low CLR
Real ‐ Time Variable Bit Rate ( rt ‐ VBR) CTD & CDV are tightly constrained
Non‐Real ‐ Time Variable Bit Rate ( nrt ‐ VBR) CTV is tightly constrained
Available Bit Rate (ABR)
Minimize CTD, CDV and CLR
Unspecified Bit Rate (UBR)
No CTD, CVD or CLR constraints
Guaranteed Frame Rate (guarantees delivery of “x”% of frames to user)
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Available Bit Rate (ABR)
Itguaranteestothesourcesaminimumrate. Itis based on adaptation of the source rate to use resources when available and reduce transmission rate when resource are scarce, in order to avoid congestion.
Its primaryusewastocarryInternettraffic
Two main categories Rate Based Control
Credit Based Control
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ATM Bit Rate Services
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Traffic Shaping(1)
Traffic shaping is forcing your traffic to conform to certain specified behaviour
Each service has a contract
If the contract is violated, the network has the right
to discard the cells
The ATM switch monitors the traffic flow
The shaping and policing is based on the Generic Cell Rate Algorithm (Leaky Bucket)
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Traffic Shaping(2)
Traffic parameters
Mean Cell Rate (Sustained Rate) Peak Cell Rate
Burst Frequency
Burst Length
Cell‐loss Priority
Cell‐loss Rate
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Leaky Bucket
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ATM Layers
AAL adapts from other protocols to ATM.
ATM-L is responsible for routing and switching the cells.
PL is responsible for sending and receiving bits on the medium.
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Higher Layer
ATM Adaptation Layer (AAL)
ATM Layer (ATM-L)
Physical Layer (PL)
ATM Layers (2)
ATM Layer
Generic Flow Control (applied to UNI to alleviate short term overload)
Cell header generation/extraction
Cell Virtual Path Identifier(VPI)/Virtual Circuit Identifier
(VCI) translation
Cell multiplexing/demultiplexing.
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ATM Layers (3)
ATM Adaptation Layer is subdivided into: Segmentation And Reassembly (SAR)
Convergence Service Specific (CS)
CS – interfaces with the upper layer protocol information – provides padding and CRC checking.
SAR – generates the ATM payload.
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CS
SAR
ATM Adaptation Layers
Standardized ATM Adaptation Layers
AAL1 ‐ provides connection oriented Constant Bit Rate
services that have timing and delay requirements
AAL2 ‐ provides connection oriented Variable Bit Rate services that have timing and delay requirements
AAL3/4 ‐ provides connection‐oriented Variable Bit Rate services with no timing requirements (e.g., frame relay)
AAL5 ‐ provides connectionless Variable Bit Rate services with no timing requirements
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ATM Interfaces
User to Network Interface (UNI)
specifies how cells come to a public network
Broadband Inter‐Carrier Interface (B‐ICI)
specifies how two carriers interact their services defines the traffic contract between two carriers
ATM Data Exchange Interface (DXI) protocol between router and CSU/DSU
Network to Network Interface (NNI) connection between switches
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END
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