Evolution of Data Networks

1
Abstract View of Data Transmission
Transmitter
Receiver
Communication channel
Communication Channel Properties -- Bandwidth --
Transmission and Propagation Delay -- Jitter --
Loss/Error rates -- Buffering
2
Analog vs. Digital Transmission

• (a) Analog transmission all details must be
  reproduced accurately

Received
Sent

• e.g. AM, FM, TV transmission

(b) Digital transmission only discrete levels
need to be reproduced
Received
Sent

• e.g digital telephone, CD Audio

3
A Typical Communication Channel
Transmission segment
Destination
Source
Repeater
Repeater
4
An Analog Repeater
Recovered signal residual noise
Attenuated distorted signal noise
Amp.
Equalizer
Repeater
5
A Digital Repeater
Decision Circuit. Signal Regenerator
Amplifier Equalizer
Timing Recovery
6
d meters
communication channel
0110101...
0110101...
7
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
8
Impact of Noise on Communication
signal noise
signal
noise
High SNR
t
t
t
noise
signal noise
signal
Low SNR
t
t
t
Average Signal Power
SNR
Average Noise Power
SNR (dB) 10 log10 SNR
9
Channel Characterization -Frequency Domain
Aincos 2?ft
Aoutcos (2?ft ?(f))
Channel
t
t
Aout Ain
A(f)
10
Signal Amplitude Attentuation
1
f
11
Signal Phase Modulation
?(f) tan-1 2?f
1/ 2?
0
f
-45o
-90o
12
A Pulse
1 ms
13
Output of Low-pass Communication Channel
14
Channel Characterization -Time Domain
h(t)
Channel
t
t
0
td
15
Signaling a Pulse with Zero Inter-symbol
Interference
s(t) sin(2?Wt)/ 2?Wt
t
T T T T T T
T T T T
T T T T
16
Digital Baseband Signal and Baseband Tx. System
1
1
1
1
0
0
A
2T
4T
5T
T
3T
0
t
-A
r(t)
Receiver
Transmitter Filter
Comm. Channel
Receiver Filter
Received signal
17
(a) 3 separate pulses for
sequence 110
t
T
T
T
T
T
T
(b) Combined signal for sequence 110
t
T
T
T
T
T
T
18
typical noise
4 signal levels
8 signal levels
19
Signal levels -- Error Probability
0 2 4 6
8
?/2?
?/2? A/(M-1) ? Channel Capacity W log (1
SNR)
20
0
1
0
1
1
1
0
0
1
Unipolar NRZ
Polar NRZ
NRZ-Inverted (Differential Encoding)
Bipolar Encoding
Manchester Encoding
Differential Manchester Encoding
21
Coding Methods -Properties

• Unipolar NRZ - power A2/2
• Polar NRZ - power A2/4
• 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

22
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23
f
f2
f1
0
fc
Figure 3.27
24
Amplitude, Frequency and Phase Modulation
Information
1
(a)
Amplitude Shift Keying
t
-1
1
(b)
Frequency Shift Keying
t
-1
1
(c)
Phase Shift Keying
t
-1
25
(a) Information
A
(b) Baseband Signal Xi(t)
t
2T
6T
T
4T
5T
3T
0
-A
t
2A
(d) 2Yi(t) cos(2?fct)
t
-2A
26
Modulator and Demodulator
(a) Modulate cos(2?fct) by multiplying it by Ak
for (k-1)T lt t ltkT
x
Ak
Yi(t) Ak cos(2?fct)
cos(2?fct)
(b) Demodulate (recover) Ak by multiplying by
2cos(2?fct) and lowpass filtering
Lowpass Filter with cutoff W Hz
x
Yi(t) Akcos(2?fct)
Xi(t)
2cos(2?fct)
2Ak cos2(2?fct) Ak 1 cos(2??fct)
27
QAM Modulator
Modulate cos(2?fct) and sin (2?fct) by
multiplying them by Ak and Bk respectively for
(k-1)T lt t ltkT
28
QAM Demodulator
Lowpass Filter with cutoff W/2 Hz
x
Y(t)
Ak
2cos(2?fc t)
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
x
Bk
2sin(2?fc t)
2Bk sin2(2?fct)2Ak cos(2?fct)sin(2?fct)
Bk 1 - cos(4?fct)Ak 0 sin(4?fct)
29
Signal Constellations
Bk
2-D signal
Ak
16 levels/ pulse 4 bits / pulse 4W bits per
second
30
Other Signal Constellations
4 levels/ pulse 2 bits / pulse 2W bits per
second
31
Electromagnetic Spectrum
Frequency (Hz)
106
108
1010
1012
1014
1016
1018
1020
1022
1024
102
104
power telephone
broadcast radio
microwave radio
gamma rays
infrared light
visible light
ultraviolet light
x rays
106
104
102
10
10-2
10-4
10-6
10-8
10-10
10-12
10-14
Wavelength (meters)
32
Twisted Pair - Attentuation vs. Frequency
26 gauge
30
24 gauge
27
24
22 gauge
21
18
Attenuation (dB/mi)
19 gauge
15
12
9
6
3
f (kHz)
100
1000
1
10
Figure 3.37
33
Coaxial Cable
34
Coaxial Cable Attentuation vs. Frequency
35
0.7/2.9 mm
30
25
1.2/4.4 mm
Attenuation (dB/km)
20
15
2.6/9.5 mm
10
5
0.01
0.1
10
100
f (MHz)
1.0
35
Cable TV Distribution Tree
Unidirectional amplifier
36
Hybrid Fiber-Coaxial System
37
(a) Current allocation
Proposed downstream
(b) Proposed hybrid fiber-coaxial allocation
750 MHz
550 MHz
38
(a) Geometry of optical fiber
(b) Reflection in optical fiber
39
(a) Multimode fiber multiple rays follow
different paths
reflected path
direct path
(b) Single mode only direct path propagates in
fiber
40
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41
Frequency (Hz)
106
1012
107
108
105
104
1011
109
1010
FM radio TV
Wireless cable
AM radio
Cellular PCS
satellite terrestrial microwave
LF
MF
HF
VHF
UHF
SHF
EHF
10-1
1
102
10-2
10-3
103
101
104
Wavelength (meters)
Figure 3.48