Transcript Evolution of Data Networks

ELEN602 Lecture 3
• Review of last lecture
– layering, IP architecture
• Data Transmission
Abstract View of Data Transmission
Transmitter
Receiver
Communication channel
Communication Channel Properties:
-- Bandwidth
-- Transmission and Propagation Delay
-- Jitter
-- Loss/Error rates
-- Buffering
Analog vs. Digital Transmission
(a) Analog transmission: all details must be reproduced accurately
Sent
Received
• e.g. AM, FM, TV transmission
(b) Digital transmission: only discrete levels need to be reproduced
Sent
Received
• e.g digital telephone, CD Audio
A Typical Communication Channel
Transmission segment
Source
Repeater
Repeater
Destination
An Analog Repeater
Attenuated & distorted signal
+
noise
Amp.
Repeater
Equalizer
Recovered signal
+
residual noise
A Digital Repeater
Decision Circuit.
& Signal
Regenerator
Amplifier
Equalizer
Timing
Recovery
0110101...
d meters
communication channel
0110101...
Characteristics of an Idealized Channel
(a) Lowpass and idealized lowpass channel
A(f)
A(f)
1
f
f
0
W
0
W
(b) Maximum pulse transmission rate is 2W pulses/second (Nyquist rate)
Channel
t
t
Impact of Noise on Communication
signal
signal + noise
noise
High
SNR
t
t
t
noise
signal
signal + noise
Low
SNR
t
t
Average Signal Power
SNR =
Average Noise Power
SNR (dB) = 10 log10 SNR
t
Channel Characterization -Frequency Domain
Aincos 2ft
Aoutcos (2ft + (f))
Channel
t
t
A(f) =
Aout
Ain
Signal Amplitude Attentuation
1
A(f) =
1
1+42f2
f
Signal Phase Modulation
(f) = tan-1 2f
0
1/ 2
f
-45o
-90o
A Pulse
10 0 00 001
...
...
t
1 ms
Output of Low-pass Communication Channel
0.625
0.75
0.875
1
0.625
0.75
0.875
1
0.75
0.875
1
0.5
0.375
0.25
0.125
0.5
0.375
0.25
0.125
0
0.625
0.5
0.375
0.25
(c) 4 Harmonics
0.125
1.5
1
0.5
0
-0.5
-1
-1.5
(b) 2 Harmonics
0
1.5
1
0.5
0
-0.5
-1
-1.5
(a) 1 Harmonic
0
1.5
1
0.5
0
-0.5
-1
-1.5
Channel Characterization -Time Domain
h(t)
Channel
0
t
t
td
Signaling a Pulse with Zero Inter-symbol
Interference
s(t) = sin(2Wt)/
2Wt 1.2
1
0.8
0.6
0.4
0.2
t
0
-7T
-6T
-5T
-4T
-3T
-2T
T
-1-0.2
0
-0.4
1T
2T
3T
4T
5T
6T
7T
Digital Baseband Signal and Baseband Tx. System
1
0
1
1
0
1
0
T
2T
3T
4T
5T
+A
t
-A
Transmitter
Filter
Comm.
Channel
Receiver
Filter
r(t)
Receiver
Received signal
(a) 3 separate pulses
for sequence 110
1
0
-2 T
-1T
0
1T
2T
3T
4T
t
-1
(b) Combined signal
for sequence 110
2
1
t
0
-2T
-1T
0
-1
-2
1T
2T
3T
4T
typical noise
4 signal levels
8 signal levels
Signal levels -- Error Probability
0
2
4
1.00E+00
1.00E-01
1.00E-02
1.00E-03
1.00E-04
1.00E-05
1.00E-06
1.00E-07
1.00E-08
1.00E-09
1.00E-10
1.00E-11
1.00E-12
/2 = A/(M-1) 
Channel Capacity = W log (1 +SNR)
6
8
/2
1
Unipolar
NRZ
Polar NRZ
NRZ-Inverted
(Differential
Encoding)
Bipolar
Encoding
Manchester
Encoding
Differential
Manchester
Encoding
0
1
0
1
1
1
0
0
Coding Methods -Properties
• Unipolar NRZ - power = A^2/2
• Polar NRZ - power = A^2/4
• Bipolar encoding reduces the low-frequency spectrum
– Timing Recovery is also easier, used in telephones
• NRZ Inverted -- A transition means 1, no transition is 0
– Errors occur in pairs
• Ethernet uses Manchester encoding
– A transition from + to - is 1, - to + is 0 (in the middle)
– Twice the pulse rate of binary coding
• Differential Manchester encoding -used in Token rings
– Every pulse has a transition in the middle
– A transition at the beginning is 0, no transition is 1
1.2
NRZ
Bipolar
0.8
0.6
0.4
Manchester
0.2
fT
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
-0.2
0.2
0
0
pow er density
1
0
f1
fc
f2
f
Figure 3.27
Amplitude, Frequency and Phase Modulation
Informatio
n
(a)
1
0
1
1
0
1
+1
Amplitude
Shift
Keying
0
T
2T
3T
4T
5T
6T
0
T
2T
3T
4T
5T
6T
0
T
2T
3T
4T
5T
6T
t
-1
+1
(b)
Frequency
Shift
Keying
t
-1
(c)
+1
Phase
Shift
Keying
-1
t
1
(a) Information
0
1
1
0
1
+A
(b) Baseband
Signal Xi(t)
T
0
2T
3T
4T
5T
t
6T
-A
+A
(c) Modulated
Signal Yi(t)
0
2T
T
3T
4T
5T
6T
t
6T
t
-A
+2A
(d) 2Yi(t) cos(2fct)
0
-2A
T
2T
3T
4T
5T
Modulator and Demodulator
(a) Modulate cos(2fct) by multiplying it by Ak for (k-1)T < t <kT:
Ak
x
Yi(t) = Ak cos(2fct)
cos(2fct)
(b) Demodulate (recover) Ak by multiplying by 2cos(2fct) and lowpass filtering:
Yi(t) = Akcos(2fct)
x
2cos(2fct)
Lowpass
Filter with
cutoff W Hz
Xi(t)
2Ak cos2(2fct) = Ak {1 + cos(2fct)}
QAM Modulator
Modulate cos(2fct) and sin (2fct) by multiplying them by Ak and Bk respectively for
(k-1)T < t <kT:
Ak
x
Yi(t) = Ak cos(2fc t)
cos(2fc t)
Bk
x
sin(2fc t)
+
Yq(t) = Bk sin(2fc t)
Y(t)
QAM Demodulator
Y(t)
Lowpass
Filter with
cutoff W/2 Hz
x
2cos(2fc t)
x
2sin(2fc t)
Ak
2cos2(2fct)+2Bk cos(2fct)sin(2fct)
= Ak {1 + cos(4fct)}+Bk {0 + sin(4fct)}
Lowpass
Filter with
cutoff W/2 Hz
Bk
2Bk sin2(2fct)+2Ak cos(2fct)sin(2fct)
= Bk {1 - cos(4fct)}+Ak {0 + sin(4fct)}
Signal Constellations
2-D signal
Bk
Bk
2-D signal
Ak
Ak
4 “levels”/ pulse
2 bits / pulse
2W bits per second
16 “levels”/ pulse
4 bits / pulse
4W bits per second
Other Signal Constellations
Bk
Bk
Ak
Ak
4 “levels”/ pulse
2 bits / pulse
2W bits per second
16 “levels”/ pulse
4 bits / pulse
4W bits per second
Electromagnetic Spectrum
Frequency (Hz)
102
10
10-2
10-6
Wavelength (meters)
x rays
10-4
10-8 10-10
gamma rays
1012 1014 1016 1018 1020 1022 1024
ultraviolet light
broadcast
radio
104
1010
visible light
108
infrared light
106
106
microwave
radio
104
power &
telephone
102
10-12 10-14
Twisted Pair - Attentuation vs. Frequency
26 gauge
30
24 gauge
27
Attenuation (dB/mi)
24
22 gauge
21
18
19 gauge
15
12
9
6
3
f (kHz)
1
10
100
1000
Figure 3.37
Coaxial Cable
Center
conductor
Dielectric
material
Braided
outer
conductor
Outer
cover
Coaxial Cable Attentuation vs. Frequency
Attenuation (dB/km)
35
0.7/2.9 mm
30
25
1.2/4.4 mm
20
15
2.6/9.5 mm
10
5
0.01
0.1
1.0
10
100
f (MHz)
Cable TV Distribution Tree
Hea
d
end
Unidirectional
amplifier
Hybrid Fiber-Coaxial System
Hea
d
end
Upstream fiber
Fiber
node
Fiber
Downstream fiber
Coaxial
distribution
plant
Bidirectional
Split-Band
Amplifier
Fiber
node
Fiber
Downstream
(a)
Current
allocation
500 MHz
54 MHz
Downstream
Upstream
750
MHz
550 MHz
500 MHz
54 MHz
42 MHz
5 MHz
(b)
Proposed
hybrid
fiber-coaxial
allocation
Proposed
downstream
(a) Geometry of optical fiber
light
cladding
core
(b) Reflection in optical fiber
c
jacket
(a) Multimode fiber: multiple rays follow different paths
reflected path
direct path
(b) Single mode: only direct path propagates in fiber
Electrical
signal
Modulator
Optical
source
Optical fiber
Receiver
Electrical
signal
Frequency (Hz)
104
105
106
108
107
109
1011
1010
1012
FM radio & TV
Wireless cable
AM radio
Cellular
& PCS
satellite & terrestrial
microwave
LF
10
4
MF
103
HF
102
VHF
101
UHF
1
SHF
10-1
EHF
10-2
10-3
Wavelength (meters)
Figure 3.48