Lecture 1
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Data Communication and Media
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Concept and Model of Communications
Analogy Signal and Digital Signal
Signal Frequency, Spectrum and Bandwidth
System Frequency Response and Bandwidth
Transmission Media and Types
Transmission Modes
- Parallel & Serial Transmission
- Asynchronous & Synchronous Transmissions
- Simplex & Duplex Transmission
• Communication Standards: RS/EIA-232 & Others
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Concept and Model of Communications
General Communications: face-to-face conversation, write a letter, etc.
Electronic Communications: telephone, wireless phone, TV, radar, etc.
Our Focus  Computer Communication
General Communication Model
Source
S(t)
Microphone
Telephone
Computer
Scanner
Transmitter
Transformer
Encoder
Compress
Modulator
T(t) Transmission Tr(t)
Sd(t)
Receiver
Destination
System
Line/Cable
Fiber/Air
Satellite
Network
Transformer
Decoder
Uncompress
Demodulator
Speaker
Earphone
Computer
Printer
Basic Communication Criteria: Speed, Reliability, Security (SRS)
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Analogy Signal and Digital Signal
Information must be converted into
electrical energy, called signal, before transmission.
s(t) voltage
Text, voice
Video, etc
Digital
Text, voice
Video, etc
Converter
Encoder
Digital Signal
t
s(t) voltage
t
Analog
Analogy Signal
Input Signal s(t)
2
Signal Power: s (t)
Signal Energy:
ʃs 2(t)dt
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General
Communication
Component – H()
Digital-to-Digital
Analogy-to-Digital
Digital-to-Analogy
3
Analogy-to-Analogy
Output Signal o(t) =H[s(t)]
4 classes/types
of systems
- Input-to-Output
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Signal Frequency, Spectrum and Bandwidth
Signal in time domain
Wave
s(t)
Transformation
Periodic
cos2πf1t
Signal in frequency domain
Spectrum
S(f)
T=1/f1
t
T
period
f: frequency
S(f)
f1
A
s(t)=Acos2πf1t + Bcos2πf2t
t
Fourier Transform
Analogy Signal
S(f)=ʃs(t)e -j2πf df
s(t)
f2
f
S(f)
Aperiodic
Digital Signal
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B
T=LCM(1/f1, 1/f2)
f1
s(t)
f
t
B = F2 – F1
F1 Bandwidth F2
S(f)
f
f
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Time-Frequency Relation and Signal Bandwidth
General Relations:
Time Domain
Change Slow
Change Fast
Frequency Domain
Low Frequency
High Frequency
Signal Bandwidth
small
large
Frequency Unit: Hertz (Hz), Kilohertz (KHz), Megahertz (MHz), Gigahertz (GHz), Terahertz (THz)
• Earthquake wave: 0.01 ~ 10 Hz
• Nuclear explosion signal: 0.01 ~ 10 Hz
• Electrocardiogram (ECG): 0 ~ 100 Hz
• Wind noise: 100 ~ 1000 Hz
• Speech: 100 ~ 4000 Hz (4 KHz)
• Audio: 20 ~ 20000 Hz (20 KHz)
• NTSC TV: 6 MHz
• HDTV: > 10 MHz
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System Frequency Response & Bandwidth
Input Signal x(t)
Output Signal y(t) =H[x(t)]
System: H()
Input Spectrum: X(f)
Output Spectrum: Y(f)
System Frequency Response: H(f) = Y(f)/X(f)
H(f)
Transmission
Bandwidth
System Bandwidth
B = F2 – F1
F1
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Signal can pass
Signal can’t pass
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F2
f
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Transmission Media
A transmission medium: - a connection between a sender and a receiver
- a signal can pass but with attenuation/distortion
- a special system with a transmission bandwidth
Guided (Wired) Media
Unguided (Wireless) Media
(lines)
- Twisted pair (0~10MHz)
- Coaxial cable (100K~500MHz)
- Optical fiber (180~370THz)
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(air, vacuum, water, etc.)
LF (30~300KHz, Navigation)
MF/HF (300~3000KHz, AM/SW radio)
VHF (30~300MHz, TV & FM radio)
UHF (0.3~3GHz, TV, mobile phone)
SHF (3~30GHz, satellite, microwave)
EHF (30~300GHz, experimental com)
Infrared (no frequency allocation)
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Frequency and Spectrum
ISM band
902 – 928 Mhz
2.4 – 2.4835 Ghz
5.725 – 5.785 Ghz
LF
30kHz
10km
MF
300kHz
1km
VHF
HF
3MHz
30MHz
100m
10m
UHF
300MHz
SHF
3GHz
EHF
30GHz
300GHz
1cm
100mm
10cm
1m


X rays

infrared visible UV
1 kHz
1 MHz
1 GHz
1 THz
1 PHz
Gamma rays
1 EHz
Propagation characteristics are different in each frequency band
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Parallel Transmission and Serial Transmission
…011000110111010111…
Segment the 0/1
stream into
N bits groups
N
N
Sender
N
?
Receiver
N
… 01…00 01…10 11…10 10…11 …
Parallel Transmission
Sender
0
1
1
0
0
0
1
Receiver
Serial Transmission
Sender
0
1
1
0
0
0
1
0110001
P/S converter
7 (N) bits are sent together
7 (N) lines are needed
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0
1
1
0 Receiver
0
0
1
S/P converter
7 (N) bits are sent one after another
Only 1 line is needed
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Asynchronous and Synchronous Transmission
Timing or synchronization between a sender and a receiver is very important for data transmission
Asynchronous transmission:
1)
2)
3)
4)
A bit stream is segmented into small groups  characters (5~8 bits)
Add a start bit (0) and a stop bit (1) at the beginning and end of each character
Frame = start_bit + character + stop_bit (7~10 bits), but 2/9~2/10 no real data
Arbitrary long gap/interval/idle between two characters or frames
Sender
Frame4
1 0110001 0
Frame2
Frame3
Frame1
idle
1 1001100 0
1 0011101 0 1 1011100 0
Receiver
Independent clocks
Synchronous transmission:
1)
2)
3)
4)
A bit stream is segmented into relative large groups/blocks many characters or bytes
Add control bits at the beginning and end of each block
Frame = H_control_bits + characters (data_bits) + T_control_bits
No gap/interval/idle between two characters in a data block/frame
Trailer
Sender
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Con_bits 0110001
...
Frame
Header
0110001 1001100 0011101 1011100 Con_bits
Synchronized
10 clocks
Receiver
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Simplex Transmission and Duplex Transmission
Simplex
Transmission
Half Duplex
Transmission
Direction of data
Device A
Device B
One can send and the other can receive
Direction of data at time 1
Device A
Device B
Direction of data at time 2
Both can send and receive but in different time
Direction of data all the time
Full Duplex
Transmission
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Device A
Device B
Both can send and receive simultaneously
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Communication Standards and Related Organizations
Communications need standards for inter-operations of different devices
Standard Organizations:
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ISO (International Standards Organization): ISO number
ITU (International Telecommunication Union): V.num & X.num
EIA (Electronic Industries Association): EIA-num
IEEE (Institute of Electronics Engineers): IEEE.num
ANSI (American National Standards Institute): ASCII, etc.
ATM Forum and ATM Consortium
IETF (Internet Society and Internet Engineering Task Force): RFC num
W3C (World Wide Web Consortium): HTTP, HTML, XML, …
WAP Forum (Wireless Application Protocol): WAP-num
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Serial & Asynchronous Transmission Standards
Standards of transmission in short distance:
- EIA-232 or RS-232
- V.24
- ISO 2110
- EIA-449/RS-422/RS-423
- EIA-530
- X.21
Their common features
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Serial & asynchronous transmission
Transmissions of ASCII code, byte, char
Use twisted copper lines
Low speed: several Kbits ~ Mbits per second
Short distance: < several tens of meters
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EIA/RS-232 Standard
bit 0
Wave form of ‘+’, 2B or 0101101
Device B
Receiver
Device A
Sender
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•
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Transmit characters (7 or 8 bits)
Sender: 0  +15v and 1  -15v
Start bit (0) and stop bit (1) for every character  9/10 bits in total
A sender never leaves wire at 0v; when idle, puts –15v, i.e., 1
Receiver: 0  (+3v, +15v) and 1  (-3v, -15v), otherwise error
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EIA/RS-232 Standard (cont.)
• Agreement of transmission timing or rate/speed
bps  bits per second, bit rate or transmission speed
- 300bps, 2.4Kbps, 4.8Kbps, …, 19.2Kbps, 33.6Kbps, 56Kbps
• Setting bit rate (transmission speed) of devices/hardware
- switch (manually), software, auto-detection
• Either simplex or duplex
T: Transmitter R: Receiver G: Ground
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EIA/RS-232 and Other Standards
• EIA-232: rate<64Kbps; connection length< 15 meters; 25 pin connector
- pin 2: receive (RxD); pin 3: transmit (TxD); pin 7: groud
- other pins for transmission control
• EIA-449: rate<10Mbps; connection length< 12 meters; 37/9 pin connector
• EIA-530: same as the above; 25 pin connector
• X.21: 64/192 Kbps (N-ISDN rate); 15/8 pin connector
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Exercise 1
1. Two signals are given in the following figures. Whose bandwidth is large? Why?
s(t)
s(t)
(a)
t
(b)
t
2. Draw the RS-232 waveform diagrams of ASCII letters of R (1010010) and S (1110011).
3. Give at least one example for each of the following transmission/communication modes:
parallel transmission, serial transmission, simplex transmission and duplex transmission.
4. Suppose one sent 10000 7bit characters across an EIA-232 or RS-232 connection that
operated at 9600 bps (9.6Kbps). How long will the minimum transmission time be required?
(Hint: remember to add a start bit and a stop bit on each character.)
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