Professor Tarek Saadawi

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Local and Metroplitan Area Networks(I-7000)

• Professor Tarek Saadawi
• Rm 529
• X7263
• Office Hours Thursday 12 130
• Also random in Tuesday

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Introduction to Digital/Data Communications
Systems
x(t)
I(t)
I(t)
Transmitter
channel
Output Device
Receiver
r(t)
Input
Code A 1 0 0 0 0 0 1
Encode Modulation, amplification, Fitering
Fig 1 Basic Model For a Communication
System I (t) Analog
Digital X (t) Analog
Digital
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Transmitter Clock
Time
0
0
1
0
Transmitted signal
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x
x
x
x
x
x
Receiver with Fast Clock
0
0
0
1
1
x
x
x
x
Receiver with Slow Clock
0
0
1
Figure 2.6 Problems caused by Clock Drift
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ASYNCHRONOUS
SYNCHRONOUS 20 mA
current loop BISYNC EIA RS
232C HDLC Serial EIA RS-422, 423, 499

ISDN LAP-d EIA RS
485 IEEE 802 standards


_ Parallel
IEEE STD 488 1978 Microprocessor I
nterfaces


_ Figure 2.9 Typical data
transfer interfaces
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Bit stream to be transmitted b(t)
transmitted signal d(t)
b(t)
Encoder
Decoder
transmission link
Figure 2.10 bits are encoded digital signaling
Bit stream to be transmitted b(t)
transmitted signal a(t)
b(t)
Modulator
Demodulator
transmission link
Figure 2.11 bits are modulated into analog
signaling
11
v
0
RZ
-v
Leads to out of synch, DC voltage,
v
NRZ
-v
Differential encoding comparing the polarity of
adjacent bits, more reliable to detect a
transition than to compare a threshold
0
NRZ1
-v
Self-clocking, 2 symbols per bit (baud), 10 Mbps
means 20 MBauds, ?50
Manchester
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Alternating Mark Inversion
3-levels, binary zero is zero voltage, binary 1
alternates
Duo binary
1 previous 1 if even zeros, otherwise the
complement
Figure 2.12
Reasons for line coding Freq spectrum freq
spectrum, synchronization, better performance
under noisy environment
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0 1 0
0 1 0 1
0
b(t)
X

A1 cos(wt ? )
b(t) 1 A2 cos(wt ? )
b(t) 0
N
(a) ASK
A cos (2pf t ? )
frequency f1
(b) FSK
A cos (2pf1 t ? )b(t) A cos (2pf2 t ? )b(t)
frequency f2
1800 phase shift
(c) PSK
A cos (2pf1 t ? )b(t) A cos (2pf2 t 1800 )
b(t) 0
1800 phase shift
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Figure 2.14 Digital Signal Modulation Techniques
Q PSK
b(t) A cos (wt 0 ) 0 0 A cos (wt
90 ) 0 1 A cos (wt 180 ) 1
1 A cos (wt 2p ) 1 0 M-ary PSK
multiplication
x(t)
b(t)
X
N
Carrier
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Mary PSK x(t) A con ( 2pft 2pf2 / M ) ,
k 0,1,.., M. M 2
PSK M 4 QPSK
Type of signal transmitted
Anolog
Digital
Information to be transmitted
Anolog
AM, FM, PM Modulator (transmitter) Anolog
PCM, Delta Modulator
Digital
ASK, PSK, FSK, MSK (Moderns)
RZ, NRZ, NRZ1 codec, Digital transmitter
Figure 2.16 Example of Information Signaling
format and the devices used.
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G2 1
Input (10011)
G1 0
G0 1


A
B
C
G3 1
Shift Register G(X) X3 X2 1
XOR
r 3
0 0 0

0 1 1

1 0 1

1 1 0
Shift Register Content
Step Input A B
C 0 0 0
0 0 1 1
1 0 0 2
1 1 1 0 3
0 0 1 1 4
0 1 0 0
5 1 1 1
0 6 0 0
1 1 7 0
1 0 0 8
0 0 1 0
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